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Witkosky 1
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Understanding the Growth and Construction of Earth’s
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Continental Crust Through Pluton Emplacement in the
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Trinity Alps, Klamath Mountains, California
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Ryan D. Witkosky1, advisor: Dr. Richard Heermance1
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Nordhoff Street, Northridge, California, 91330-8266
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ABSTRACT
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Department of Geological Sciences, California State University, Northridge, 18111
Plutons make up a significant volume of the Earth’s crust, and their emplacement
can hydrothermally alter surrounding basement, which is often related to the genesis of
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economic minerals. Despite their importance and relevance to various disciplines (i.e.
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historical and economic geology), there is debate over how plutons are assimilated into
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the crust. The paradigm suggests that a pluton forms in a singular episode as a large,
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unified body of magma, intruding into country rock and forcing the alignment of mineral
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grains along its boundary. In contrast, more recent models propose that the
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emplacement process is a series of multiple intrusions; a network of dikes that are
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integrated over time to make up the totality of the structure. Each model has unique
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implications, which I test on the Canyon Creek pluton from the Trinity Alps region of the
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Klamath Mountains in northern California. Geologic mapping and structural
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measurements show that this pluton contains an abundance of interconnected dikes,
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with a 50-100 m wide zone of brittle deformation along its boundary. This diffuse zone of
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brecciation along the margin has recorded a history of fracturing and diking during
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emplacement, with enclaves up to 30 m in diameter that have been isolated while
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maintaining an orientation parallel to that of the wall rock. Wall rock foliation is cut
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obliquely by the plutonic contact, which discredits the notion of forceful emplacement
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(i.e. balloon-like diapiric rise of a large, buoyant magma chamber). Also, enclaves in the
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contact zone do not show any evidence of partial melting or recrystallization caused by
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contact with hot, intruding magma. My field data lacks evidence for any ductile
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deformation in rocks along the contact zone, and supports a hypothesis that this pluton
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was emplaced at a shallow depth (roughly 5-10 km, the brittle realm) in a series of
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small, incremental molten injections. Furthermore, petrographic thin section analyses do
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not show alignment of mineral grains in rock samples collected along the margin, which
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would be consistent with the forceful emplacement of a single magma body. Future
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work will use radiometric zircon dating on rock samples collected from the dikes and
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main body of the pluton. If the dikes yield crystallization ages within several million
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years of the main plutonic body, then this will further support the multiple stage-building
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hypothesis.
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INTRODUCTION
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Continental crust has developed over geologic time, in part by pluton
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emplacement where magma cools slowly at depth. Large igneous intrusions account for
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the creation of continental crust in the sub-surface realm, and are often associated with
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ore deposits and other resources. Thus, understanding how plutons are emplaced is a
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fundamental aspect to both historical and economic geology. The process of
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emplacement, however, is poorly understood, mainly for two reasons. The first reason is
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that the process takes place deep underground where direct observations cannot be
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made in documenting the complete sequence of events that occurs during
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emplacement. The second reason is that the timeline of emplacement is on a scale
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much greater than the lifespan of human beings, which makes a complete step-by-step
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record impossible for attempts at documenting the mechanisms involved. Nevertheless,
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we can still use evidence from past emplacement episodes to make interpretations
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regarding the deformation that takes place during this process. Plutonic intrusions are
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also without a doubt associated with hazardous volcanic and seismic activity, which
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means that understanding emplacement processes can have benefits in assessing
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contemporary igneous activity along with its effects on civilization.
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The emplacement of large molten bodies in the Earth’s crust is a process that
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has been studied by geologists for many centuries. Throughout history, field evidence
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has been the main mode of guidance used in deductive interpretations about the
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emplacement process. Studying images of a contact zone can also be used to carry out
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a qualitative analysis for depth of emplacement by noting whether the primary
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interactions between host rock and the intrusive body show signs of brittle or ductile
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behavior. With an understanding of the established geothermal gradient, structural clues
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in the contact zone can generally lead to one of two interpretations: either a pluton is
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emplaced in shallow crust, where conditions are cold and rocks tend to be brittle (at a
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depth of roughly 5-10 km), or in the deeper, ductile realm (>10 km) where higher
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temperatures and pressures increase malleability, and intrusive forces cause rocks to
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deform plastically. In this study, I followed in the footsteps of my predecessors by
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rigorously documenting the contact zone between an intrusive body and its host rock.
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My field notes, accompanying sketches, and photographs were then used to infer the
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depth of emplacement for the pluton in question.
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The classic paradigm for pluton emplacement suggests that a large magma
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chamber buoyantly rises into the upper crust where it cools and solidifies as a unified
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body in a geologically short time period (<1 million years; please see Figure 1). This
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fundamental model seen in most geology textbooks has engendered unresolvable
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debates over how a massive amount of space, or room, can be accommodated for the
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emplacement of a pluton into solid crust (the so called “room problem”). Moreover,
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contemporary seismological investigations, have failed to locate the presence of large,
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liquid (i.e. magma) bodies in mid-ocean ridge volcanic hot spots, zones that typically
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have been believed to harbor the presence of diapirs (Detrick et al., 1990). Additionally,
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geochronologic data from radiometric analyses within individual plutons show ages
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varying up to 10 million years, giving rise to a more recent but alternative viewpoint,
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which suggests that plutons may form incrementally by dike emplacement over millions
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of years (Glazner et al., 2004). Field studies have since shown more evidence that the
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notion of incremental building episodes spanning several million years is a plausible
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mechanism for the formation and emplacement of plutons (Belcher and Kistners, 2006;
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Burchardt et al., 2012).
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The purpose of this study is to view the classic paradigm and the modern
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incremental building model as two separate but testable hypotheses. In order to be
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objective, I searched for evidence that might support either hypothesis. I tested the
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validity of the paradigm by searching for a discrete contact between a pluton and its
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host rock. If the paradigm is correct, then measuring structural attitudes in the host rock
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should yield some type of consistent regional pattern that changes and becomes
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parallel to a sharp plutonic contact (as you approach the contact from the exterior).
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Following the paradigm, this abrupt contact should also be characterized by plutonic
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rocks in situ near the margin that have a preferred grain orientation parallel to the sub-
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planar contact surface between the pluton and its host rock. The abrupt change in
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structural attitude and preferred grain orientation are contributed to forces created by
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the upwelling of a large liquid magma chamber as it rises into the upper crust,
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pervasively heating up and deforming its host rock (Figure 1: diapirism). As the diapir
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swells upward and needs room for accommodation, mineral grains along the boundary
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are forced into a preferential alignment by the balloon-like action of hot, buoyant, liquid
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magma, and one would expect to find a thick contact aureole (on the scale of tens of
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meters), where the transfer of heat as a pluton cools has caused extensive partial
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melting and recrystallization of the host rock.
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Any evidence of ballooning or diapirism would support the paradigm, but in order
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for this study to be neutral with respect to the competing hypotheses, I also tested the
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validity of the incremental building hypothesis by looking for an abundance of dikes
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within the body of the pluton that disrupt the notion of a massive, homogeneous rock
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mass. If the structural emplacement shows evidence that favors an interpretation of
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multiple-stage construction, then the body of the pluton is expected to be composed of
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many large dikes that have amalgamated to make up the totality of the structure. Hot
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magma from dikes might also affect the host rock, but would cool and solidify very
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rapidly when compared to the latent heat dissipated from a kilometer scale magma
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body, resulting in the absence of a thick contact aureole. Locating and documenting the
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orientation of abundant dike swarms within the body of a pluton would give merit to the
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more recent incremental building hypothesis, and radiometric dating of rocks found in
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the dikes can further test the validity of this new model, by showing that the dikes have
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crystallization and emplacement ages within several million years of the main body of a
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pluton.
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Understanding which model of pluton emplacement is correct, be it the classic
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paradigm or the incremental building concept, is a fundamental aspect of Earth science
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and has implications for future research. If the incremental building concept is found to
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be viable, then dikes and other small, discrete magma chambers should be more
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carefully considered while doing fieldwork and making geologic maps. Workers should
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include the locations and orientations of any dikes found, while also collecting samples
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for dating analyses. This will be relevant because dikes may play an important role in
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the emplacement process, and should not tacitly be assumed to represent a much
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younger, shorter-lived intrusive event. It is also beneficial to know whether diking may
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be occurring if attempting to make geophysical images while documenting a portion of
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the emplacement process in an actively evolving igneous system. Current knowledge
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on the limits of seismic wave resolution can then be used to test hypotheses on sub-
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surface magmatic deformation taking place and make accurate predictions regarding
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how long recent or present-day igneous activity may last.
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FIELD METHODS
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A major component of this study is thorough geologic mapping along the contact
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between a pluton and its host rock. Along with geologic mapping, I performed three
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separate transects along the contact zone. The purpose of these transects were not
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only to provide even further scaled documentation and measurements along the
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boundary between pluton and host rock, but also to collect oriented hand samples of
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rocks in the contact zone. At least three oriented samples were collected from each
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transect, in order to note, if present, a progressive sequence of deformation and
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metamorphism as one travels from the interior of the pluton, through the contact zone,
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and ending in host rock. Any planar surface on in-situ rocks were used to measure the
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geographic orientation of the face, then the sample was carefully removed and
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catalogued to be brought back to CSUN’s rock lab for thin section billet cutting. Billets
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were cut parallel and perpendicular to the contact zone, and microprobe polished thin
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sections were made for petrographic and scanning electron microscope (SEM)
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analyses. In addition to features found within a pluton itself, any structural fabric (or lack
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thereof) within or proximal to the contact zone can also shed light on the style of
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emplacement, revealing whether the interaction between pluton and host rock was
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warm and ductile or cold and brittle during emplacement. Detailed drawings along with
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photographs and documentation of the contact zone provide for a qualitative analysis to
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guide interpretations on depth of emplacement.
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REGIONAL GEOLOGIC SETTING
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The Klamath Mountains in northern California and Oregon are classified as the
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orogenic product along a convergent margin between oceanic and continental
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lithosphere (Irwin and Wooden, 1999). At least several hundred million years of east-
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directed subduction and accretion along the boundary between the Pacific and North
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American tectonic plates are recorded in four major belts of metamorphic rock that trend
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roughly north-south and parallel to the tectonic boundary (Davis et al., 1965). The
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metamorphic belts, or terranes, are pockmarked with numerous plutonic intrusions,
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most of which have an exposed area <100 km2, and therefore further categorized as
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stocks. Middle to Late Jurassic igneous activity is responsible for many of the plutons,
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which along with relationships to regional deformation, are believed to have formed in
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an arc system above an east-dipping subduction zone (Wright and Fahan, 1988).
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This study focuses on the northwestern contact between the Central
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Metamorphic Belt (or Central Metamorphic terrane) and the Canyon Creek Pluton in the
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Trinity Alps Wilderness area, located approximately 60 miles northeast of Eureka and
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45 miles northwest of Redding in northern California (please see Figures 2A and 2B).
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The Central Metamorphic terrane is interpreted to have formed by subduction of
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oceanic lithosphere along the Bully Choop thrust fault (Irwin and Wooden, 1999), and
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may correlate with the Feather River terrane of the Sierra Nevada (Hacker and
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Peacock, 1990). The Canyon Creek Pluton is part of a larger set of plutons that Allen
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and Barnes believe were derived by partial melting of its metabasic host rock, the
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Central Metamorphic terrane (2006). Glaciation during the Wisconsin period (roughly
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110-10 Ka) has exposed bedrock in the Grizzly Creek stream valley (Sharp, 1960),
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providing for excellent exposure of the plutonic contact in question.
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The majority of plutonic rocks in the Klamath Mountains are Mesozoic in age,
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ranging from approximately 174-136 Ma (Allen and Barnes 2006). The Canyon Creek
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Pluton is included in a suite of 5 plutons that are collectively referred to as the Post-
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western Klamath tonalite-trondhjemite-granodiorite (ttg) suite, with early Cretaceous
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ages ranging from approximately 142-136 Ma (Allen and Barnes, 2006). These ttg
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plutons, located in the Trinity Alps Wilderness, are of interest because they represent
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the second youngest plutonic episode in the Klamath Mountains (post accretionary
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plutonism according to Irwin and Wooden, 1999), yet are located further east than older
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metamorphic terranes with separate groups of older (Late Jurassic) plutons.
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Furthermore, inconsistent older ages have been reported for the Canyon Creek pluton
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(“z 160 – 170” in Figure 2, from Wright and Fahan, 1988), an aspect that will be
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discussed in a later section of this paper.
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ROCK UNITS
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Host Rock: Salmon Hornblende Schist of the Central Metamorphic Belt
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Davis et al. originally divided rocks that make up the Central Metamorphic Belt in
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the Klamath Mountains Province into the Grouse Ridge Formation and the Salmon
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Hornblende Schist (1965). In my study area, the host rock for the Canyon Creek Pluton
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is the Salmon Hornblende Schist. In hand sample, the Salmon Hornblende Schist is
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melanocratic, mafic, fine-grained (<1 mm) to cryptocrystalline, with a well-developed
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foliation and subtle lineation from the amphibole needles present. The foliation creates a
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slaty cleavage, with millimeter to centimeter thick felsic ribbons of plagioclase and
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quartz exploiting foliation planes. The felsic ribbons are also frequently isoclinally folded
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and boudinaged (please see Figure 3).
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In thin section, hornblende grains are predominantly hypidioblastic, exhibiting a
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dimensional preferred orientation and lattice preferred orientation with c-axes lying in
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the foliation plane. Quart grains exhibit undulose extinction, with sizes averaging
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approximately 0.1 mm. Hornblende makes up about 60% of the modal percentage, with
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plagioclase at 30%, quartz at 5% and opaque magnetite at 5% of the total (this and any
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other subsequent modal percentages noted were calculated using the percentage
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diagrams for estimating composition by volume in Appendix 3 of Compton, 1985). The
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plagioclase composition is An60, or labradorite (this and any other subsequent
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plagioclase compositions noted were determined using the Michel Levy method).
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According to Barnes et al. (1992), a tholeiitic igneous protolith was thrust beneath
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the eastern Klamath Belt in Devonian time to initiate lower amphibolite facies
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metamorphism in what is now the Central metamorphic Belt. Hacker and Peacock
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(1990) agree with the Devonian age for the Central Metamorphic Belt, adding that the
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protolith was mafic oceanic lithosphere, deformed under temperature-pressure
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conditions ranging from 500° to 650 ± 50°C, and 500 ± 300 MPa, respectively, in an arc
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basement/subduction zone couple (values are based on observed mineral
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assemblages, mineral chemistry, and limited thermometry). These metamorphic
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conditions, along with mineral assemblages and chemistry, guided the aforementioned
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correlation between the Central Metamorphic Belt and the Feather River terrane of the
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Sierra Nevada, which is also Early to Middle Paleozoic in age. Irwin and Wooden
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(1999), also report ages for the Central Metamorphic Belt (although they refer to it as
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the Central Metamorphic terrane) ranging from >400-380 Ma, Late Silurian to Middle
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Devonian (older ages are from K-Ar methods on hornblende, younger ages from Rb/Sr
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methods on whole rock).
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Davis et al. (1965) and Cox (1967) reported Late Paleozoic ages (286-270 Ma,
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Early to Middle Permian) for regional metamorphism of the Salmon Hornblende Schist
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from K-Ar methods on hornblende (these ages were reported in Cox’s 1967 manuscript
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as Pennsylvanian or pre-Permian, likely from an outmoded Geologic Time Scale used
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at the time). Another conflicting age for this unit comes from Barrow and Metcalfe
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(2006), who give two new Early Permian ages of 274 ± 2 Ma for Central Metamorphic
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terrane amphibolite near the Trinity Thrust Fault. These ages agree with Davis et al.
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(1965) and Cox (1967), and are much younger than the Devonian ages reported by
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others, but Barrow and Metcalfe have an interpretation that reconciles the conflicting
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ages. Barrow and Metcalfe believe that these ages represent Early Permian uplift and
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cooling during a proposed supra-subduction extensional episode that reactivated the
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Devonian subduction margin (Trinity Thrust fault). With this thermal event yielding
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younger ages for the Salmon Hornblende Schist, reactivation of the Trinity thrust fault
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as an extensional structure responding to upper plate extension in a Late Paleozoic
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subduction zone is a viable interpretation, and coincides with the notion of so-called
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“accordion tectonics,” where the predominate tectonic style in the Klamath Mountains
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has cycled between subduction (compression) and rift (extension) zones throughout the
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province’s genesis (D. Yule, personal communication, 2013). Wright and Fahan also
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mention cyclic tectonic periods in the orogenic history of the Klamath Mountains to
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explain the presence of various ophiolite terranes formed during extension in an
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otherwise compressive regime (1988). Thus, the maximum Devonian age was taken for
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the Salmon Hornblende Schist when compiling the geologic map of the study area in
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Plate 1.
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The Canyon Creek Pluton
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In my study area, the Canyon Creek pluton is composed of leucocratic, felsic,
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hypidiomorphic, medium-grained hornblende biotite tonalite. In hand sample, the
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tonalite is phaneritic, with 1-3 mm grains of hornblende, biotite, quartz, and plagioclase
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(listed in order of increasing modal abundance). Under thin section, some plagioclase
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grains show seritization, and others contain hornblende inclusions. Plagioclase
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composition is An39, or andesine, and accessory minerals present are apatite and
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zircon. A population of zircon has been separated and mounted for U-Pb radiometric
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age analysis on tonalitic rocks in my study area for the Canyon Creek Pluton.
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The Canyon Creek Pluton is included in an intrusive suite of ttg plutons that are
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believed to have formed as a result of arc magmatism above an east dipping subduction
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zone in the Middle to Late Jurassic (Wright and Fahan, 1988). This interpretation on
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magma genesis in the area is based on synchronous regional metamorphism and thrust
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faulting, and discredits the idea that the Klamath Mountain metamorphic terranes are
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allocthonous fragments that have traveled far before colliding with and being sutured
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onto the western boundary of the North American continental plate. Davis et al.,
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however, pointed out that the thrust faults do not represent paleo-tectonic plate
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boundaries, and doubt that the actual subduction interface has been preserved (1980).
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In 1988, Wright and Fahan published radiometric zircon ages for the Canyon
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Creek Pluton that spanned over 30 million years. Two separate samples, collected from
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a location approximately three miles from my study area (in a south 30° east direction)
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yielded the following ages (in Ma, uncertainties are two-sigma): 206Pb/238U = 143.7 ±
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0.4, 140.7 ± 0.4; 207Pb/235U = 144.9 ± 0.4, 141.7 ± 0.4; 207Pb/206Pb = 169 ± 9, 159 ±
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8. Wright and Fahan attributed the discordant ages to either recent Pb-loss or possible
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zircon inheritance, but regardless of the reason, these ages warranted further analysis.
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In 1992, Barnes et al. suggested that the older ages calculated by Wright and
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Fahan for the Canyon Creek Pluton resulted from inherited zircon, but they also further
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expanded on their interpretations, adding that the rocks showed geochemical signatures
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that were consistent with partial melting of low-K tholeiitic crust experiencing
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amphibolite grade metamorphism (i.e. the host rock, or Salmon Hornblende Schist). It
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wasn’t until 2006 that Allen and Barnes did another geochronologic study on plutons in
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the area that had previously yielded problematic age data. Samples from the Canyon
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Creek Pluton gave radiometric zircon ages of 140.0 ± 1.3 Ma, with an uncertainty of
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one-sigma (Allen and Barnes, 2006. Take careful note: this age is from their own study,
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with the data found in Table 2 of the article. Table 1 also has age data for the Canyon
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Creek pluton, but the numerics are not identical to data in Table 2, because the data in
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Table 1 is referenced from several other papers). The 2006 study by Allen and Barnes
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refined the previous ages by Wright and Fahan (1988), and showed that the previous
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discordance could be a result of inheritance.
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Dikes
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An abundance of dikes were identified and mapped in the study area. The dikes
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fell into one of 3 main compositions: andesite hornblende porphyry, dacite plagioclase
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porphyry, and greenstone (listed in order of decreasing abundance; please see Plate 1).
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There are also dikes that are gradational between 2 end members of the 3 main
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classes. These dikes make up approximately 6% of the surface area expression of the
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Canyon creek Pluton. The percentage of surface area calculated can be used as a
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proxy for the volume percentage of the pluton made up by dikes because the
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orientations of the dikes were measured as predominantly sub-vertical, meaning that
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the true thickness of the dikes are projected onto the surface expression. The
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contribution of the dikes to the structural emplacement of the Canyon Creek Pluton will
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be discussed in a later section.
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Andesite Hornblende Porphyry
The most abundant composition for dikes located in the study area is a dark grey
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porphyry of ~3 mm needle-shaped hornblende phenocrysts set in a fine-grained matrix
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of quartz, plagioclase, and hornblende (please see Figure 4). The hornblende needles
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are oriented in a random, felted manner, and the rock is strongly magnetic. The strong
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magnetism is attributed to 7% of the modal percentage being opaque magnetite in thin
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section. Other textures seen in thin section include skeletal hornblende with plagioclase
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growing in the voids, and microscopic views show that some of the larger hornblende
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phenocrysts are actually aggregates of many smaller anhedral grains. Plagioclase
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makes up about 60% of the modal percentage, with a composition of An58, or
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labradorite (from this measurement, a mafic origin is interpreted for this dike set).
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The porphyritic texture for these dikes leads to an interpretation of hypabyssal
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emplacement depths at roughly 5 km. Hornblende phenocrysts initially formed in a slow-
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cooling magma chamber, followed by injection into fracture systems of a growing pluton,
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where increased cooling rates created the finer-grained matrix of interstitial plagioclase.
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Zircon populations from this dike set have been separated and mounted, and currently
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await radiometric U-Pb dating analysis to determine emplacement age.
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Dacite Plagioclase Porphyry
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Although not the most abundant, the dacite plagioclase porphyry dikes are the
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largest found in the study area, some measuring up to 25 m in thickness (please see
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Plate 1). As previously stated, this measurement taken from the surface projection is
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very close to the true thickness because of the dike’s sub-vertical orientation. Rocks in
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these dikes are light gray, felsic, with 3-5 mm plagioclase and quartz phenocrysts in a
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fine-grained matrix of quartz, plagioclase, and hornblende needles up to 1 mm. There
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are rare 5mm hornblende and biotite grains, and some of the plagioclase is weathered
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and altered to clay minerals. In thin section some phenocrysts are sieved and embayed,
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with seritization of plagioclase, hornblende with actinolite rims, and secondary chlorite
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and epidote. Plagioclase composition is An46, or andesine.
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The genesis of the dacite plagioclase porphyry dikes closely resembles that of
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the andesite hornblende porphyry set, with initial slow cooling followed by rapid
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hypabyssal emplacement (i.e. porphyritic texture). The dacitic dikes are speculated to
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have been emplaced after the andesitic set, though, from an evolved, more siliceous
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melt. Likewise, zircon grains have been separated and mounted from the dacitic dikes,
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and a future radiometric age analysis can confirm the speculated chronology of
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emplacement. It is worth noting that zircons from the dacitic dikes have the largest grain
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sizes out of any population separated from rock samples in the study area. It is also
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worth noting that a similar dacite porphyry dike set was noted by Davis (1963) in the
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northwestern part of the Caribou Mountain Pluton, which is a tonalitic intrusion located
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immediately to the northeast of the Canyon Creek Pluton (at a distance of roughly 2
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km), leading to the notion that these dacite porphyry dike sets in separate plutons may
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be correlative. The Caribou Mountain Pluton is in the same Post-western Klamath
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tonalite-trondhjemite-granodiorite suite as the Canyon Creek Pluton (Allen and Barnes,
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2006), and has yielded U-Pb radiometric zircon ages of 139.2 ± 1.9 Ma, which is
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synchronous within error to the 140.0 ± 1.3 Ma age calculated for the Canyon Creek
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Pluton.
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Greenstone
The greenstone dikes record varying degrees of hydrothermal alteration to other
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dike sets in the study area, and are therefore further subdivided into 2 categories. The
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first type (type 1) is dark green, medium-grained (1-2 mm), mafic rock with amphibole
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needles and pistachio green epidote crystals in a random, felted texture. Subtle cm-
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scale bands are present with amphibole needles in a white, aphanitic groundmass
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(please see Figure 5). There are open vesicles (roughly 1 mm in diameter), and rare 5
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mm subhedral quartz megacrysts. In thin section, this first type of greenstone dike is
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predominantly made up of hornblende, bladed actinolite, epidote, and chlorite. Radiating
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masses of tremolite and pumpellyite (or possibly piemontite) are seen in the cm-scale
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white bands. Amphiboles are heavily sieved with quartz and feldspar intergrowth, and
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dark reddish-brown anhedral opaque minerals are in the process of breaking down to
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oxides.
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The second type of greenstone dike (type 2) is porphyry consisting of 2-3 mm
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blocky and needle-shaped skeletal remains set in a light bluish-green aphanitic
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groundmass (please see Figure 6A and 6B). The phenocrysts are so completely
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obliterated that the occupancy sites can be mistaken for vesicles (please see Figure 7).
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Under thin section, some of the skeletal phenocrysts can be identified as subhedral
362
amphibole in various stages of complete alteration to mica. The groundmass seems to
363
be composed mainly of chlorite and sericite, but this is difficult verify with the
364
cryptocrystalline nature of the grains.
365
Both the first and second types of greenstone dikes are interpreted to be
366
hydrothermally altered products of the andesite hornblende porphyry, with the second
Witkosky 17
367
type being a more advanced stage of alteration. The abundance of hydrous mineral
368
phases in these greenstone dikes can be attributed to the effects of steam and fluid
369
intrusion during emplacement. A hydrous injection episode coupled with magmatic
370
emplacement seems to follow emplacement of the other dike sets chronologically, as a
371
very small number of the dikes show alteration, and dikes that do show alteration have
372
phase assemblages that appear to be derivatives of other dike sets in the study area
373
(making other dike sets protoliths). The 2 types of greenstone dikes are also generally
374
found together in close proximity in the southern portion of the study area (please see
375
Plate 1). Unfortunately, attempts at zircon separation did not yield a population for
376
radiometric age analysis on the greenstone dike set.
377
STRUCTURE
378
In this section, two main aspects will be discussed regarding the structural
379
emplacement of the Canyon Creek Pluton. First, a section is devoted to describing the
380
contact between the pluton and its host rock, with emphasis on detailed transects
381
documented along the contact zone. Observations from the interactions between the
382
pluton and host rock are then used to make an interpretation on depth of emplacement.
383
A second section then covers observations made within the body of the pluton, mainly
384
concerning the abundance of dikes present and their potential contribution to
385
emplacement with the notion of a several million year incremental building hypothesis.
386
For both sections, continuous reference will be made to Plates 1 and 2, the geologic
387
map of the study area and the central transect, respectively.
388
Witkosky 18
389
390
Contact Transects and Depth of Emplacement
While making a geologic map of the contact zone, approximately 13% of the
391
contact between the Canyon Creek Pluton and its host rocks were mapped in detail,
392
with three separate transects made along the contact zone (the locations of the
393
transects can be seen in Plate 1). For the purpose of compiling this manuscript, only the
394
central transect was digitized and included, because the other two did not show any
395
new or otherwise different features than that of the central one.
396
The central transect was performed on a west facing wall of the Grizzly Meadows
397
stream valley (please see Figure 8A and 8B), where the bedrock is well-exposed and
398
not highly weathered, providing for a well-preserved historical view of the interactions
399
that took place between the pluton and its host rock during emplacement. The central
400
transect began in the Canyon Creek Pluton and proceeded north, ending in the Salmon
401
Hornblende Schist. A first order observation made is that the contact is not a discrete,
402
sharp boundary as predicted by the paradigm, but actually a broad, diffuse zone, 50 m
403
wide, that documents a sequence of fracturing and diking as the plutonic magma
404
intruded into the host rock (other transects recorded a contact zone up to 100 m wide).
405
One of the first features noted at the 5 m mark is the abundance of centimeter to
406
decimeter scale angular fragments of host rock that have been broken off and
407
incorporated as enclaves or xenoliths into the pluton (please see Figure T1 in Plate 2).
408
Another feature that seems to refute the paradigm is that the contact obliquely cuts
409
foliation in the Salmon Hornblende Schist, showing that the structural fabric in the host
410
rock is not pervasively parallel to the contact margin (please see Figure T2 in Plate 2).
411
This oblique feature was also noted by Davis et al. (1965), where in outcrop or map
Witkosky 19
412
scale it appears that pluton contacts are parallel to host rock foliation, but a closer view
413
reveals that the contacts cut the foliation at low angles. At 10-20 m into the transect,
414
many dikes and veins intrude into what is soon recognized to be a mega-enclave of
415
host rock. This mega enclave measures at least 30 m in diameter. Cox (1967) also
416
noted that along the western contact zone of the Canyon Creek Pluton (or Canyon
417
Creek stock, as referenced in his paper), the tonalite solidified in the process of stoping
418
large angular fragments of amphibolite. Strike and dip measurements of foliation in the
419
mega enclave, however, are parallel to those measured in host rock that makes up the
420
actual wall of the contact. This shows that the mega-enclave is not a stoped block, but a
421
large chunk of wall rock that was never granted the allowance to move and rotate freely
422
in a large, liquid magma body. The mega-enclave maintained its original orientation
423
during plutonic intrusion, owing its current isolated appearance to a series of small dike
424
injections occurring around it. Belcher and Kisters (2006) and Glazner et al. (2004) also
425
noted non-rotational wall rock translation in their interpretations on incremental plutonic
426
construction. As one moves northward in the transect, this multiple episode, smaller
427
scale diking is documented, with centimeter scale aplitic veins exploiting weak foliation
428
planes in the host rock (please see Figure T3 in Plate 2). Finally, the last instance of
429
plutonic rock is seen intruding into the host as a large tonalite apophysis (please see
430
Figure T4 in Plate 2). From distal views, this apophysis (large feeder dike) is seen to
431
have channeled magma into other large dikes that run parallel along the contact margin
432
(Figures 8A and 8B).
433
434
Upon return to California State University Northridge, thin sections of oriented
samples from the contact zone were made perpendicular and parallel to the trend of the
Witkosky 20
435
contact. This was done not only to study the petrography of the rock units, but also in
436
order to verify whether any alignment of mineral grains can be seen at the microscopic
437
level. Again, following the paradigm, an alignment of mineral grains parallel to the
438
contact margin should be present, produced by a forceful emplacement episode. After a
439
careful and extensive field analysis, I concluded that there is no obvious alignment of
440
mineral grains in hand samples collected from the pluton while performing the central
441
transect, and as previously mentioned, foliation in the host rock is not pervasively
442
parallel to the contact. Likewise, even when viewed on CSUN’s scanning electron
443
microscope, plutonic rocks from the contact zone show a random grain orientation
444
(please see Figure T6 in Plate 2). This lack of order in mineral grains is contrary to the
445
predictions of the paradigm, and seems to refute the notion of hot, buoyant, balloon-like,
446
forceful plutonic emplacement. Furthermore, there is no contact aureole present in or
447
near the contact zone, which leads to the following interpretation: emplacement of the
448
Canyon Creek Pluton occurred at a depth of 5-10 km, where cold, brittle behavior
449
dominates the probable style of rock deformation.
450
With no contact metamorphism present, the interpretation on shallow
451
emplacement depth is also supported by a graph showing relationships between
452
temperature, pressure, depth, and what metamorphic facies are found in different
453
combinations of these variables (Figure 9). In this graph, amphibolite grade
454
metamorphism begins to occur with temperature and pressure conditions present at a
455
depth of approximately 8-10 km. Thus, I make the argument that this puts a maximum
456
emplacement depth on the Canyon Creek Pluton, based on the youngest age for its
457
amphibolite grade host rock, the Salmon Hornblende Schist of the Central Metamorphic
Witkosky 21
458
Belt. If emplacement had occurred at a depth greater than 10 km, metamorphic
459
recrystallization would also have been taking place in the host rock during plutonic
460
intrusion, which means that crystallization ages for the amphibolite grade host rock
461
should be at maximum synchronous with the Early Cretaceous ages for crystallization of
462
the Canyon Creek Pluton. At depths greater than 10 km, the blocking temperature for K-
463
Ar dating method in hornblende (approximately 500° C) is not likely to be achieved,
464
which means that the radiometric clock could not have began ticking until the host rock
465
was at a depth that allowed for lower temperatures and further solidification of
466
hornblende crystals. Furthermore, heat conduction involved in a large molten body
467
coming into contact with the host rock would only further delay cooling, which again
468
supports the idea of a cold, brittle, and overall passive emplacement episode.
469
Dike Emplacement and the Incremental Building Hypothesis
470
In my study area, a large number of dikes are located in the main body of the
471
Canyon Creek Pluton (please see Figure 10 for one instance). While mapping, I
472
recorded the thickness and orientations of these dikes, in order to answer the following
473
questions: 1. Do the dikes make up a significant volume percentage of the pluton?; and
474
2. Do the trends of the dikes record a stress/strain regime that was present during
475
emplacement? Question 1 directly tests the incremental building hypothesis by showing
476
that it is possible to view the Canyon Creek Pluton as an amalgamation of many small
477
magma chambers, in this case, dikes (as suggested in Glazner et al., 2004). Question 2
478
aims to build on Question 1, by statistically analyzing structural data tallied on the
479
internal makeup of the pluton, and how tectonic forces may allow for geologically slow
480
(>1 million years) assimilation of large magma bodies into the Earth’s crust.
Witkosky 22
481
The thicknesses of the dikes mapped are shown in Plate 1. By first calculating
482
the surface area dominated by dike swarms around Grizzly Lake, and then calculating
483
the amount of surface area composed of dike exposures, the percentage of surface
484
area composed of dike exposures is approximately 6%. This is taken as a good proxy
485
for volume percentage because the dikes are for the most part oriented sub-vertically
486
(dike orientations can be viewed in the stereonet of Figure 14, but this figure will be
487
discussed in more detail soon). Postulation continues regarding the ages of the dike
488
swarms, as a zircon analysis that yields dates synchronous with or soon after
489
crystallization of the main plutonic body would help to corroborate the incremental
490
building hypothesis, barring any arguments over inheritance issues. Some final pieces
491
of physical evidence can be seen in Figures 11A and 11B, where it appears that at least
492
a portion of the plutonic and dike magmas may have concurrently been molten during
493
emplacement. Figure 11A shows dark, dike magma thoroughly mixing and mingling with
494
lighter-colored tonalitic magma, and in Figure 11B, it appears that a piece of hot,
495
Canyon Creek Pluton tonalitic magma was smeared up against a pre-existing dike, after
496
incorporating many small inclusions of the dike material into its matrix. These images
497
provide evidence for a hot, unsolidified pluton during dike development, which supports
498
the interpretation of synchronous emplacement.
499
Evidence supporting the interpretation of a shallow emplacement in cold, brittle
500
crust has already been presented by a detailed transects along the contact zone, and
501
this interpretation is now extended to include the tectonic stress/strain regime present
502
during the time of emplacement. To consider the possibility of synchronous diking
503
during pluton emplacement, I draw from a stress analysis presented with the stereonet
Witkosky 23
504
in Figure 14. Poles to planar surfaces of dikes, faults and fractures show very good
505
statistical grouping, allowing for a best-fit line to be placed in the center of the stereonet.
506
By also including shear sense indicators that mirror faulting documented in the study
507
area (mainly strike-slip, please see Figures 12 and 13), an analogy is made to a rock
508
cylinder crushing experiment, where the critical angle of failure is, on average,
509
approximately 30°, measured counter-clockwise from σ1, the direction of greatest
510
principal stress (or principal direction of shortening). The principal direction of
511
shortening thus represents the east-directed subduction recorded throughout most of
512
the history of Klamath Mountains metamorphic terrane accretion. Fracture sets that
513
follow the best-fit line on the stereonet of Figure 14 can be seen as a pervasive pattern
514
in the Canyon Creek Pluton tonalite of Figure 12. The chronology of emplacement is
515
then interpreted to follow a path beginning with tectonic compression, fracturing,
516
opening of the fracture sets, and subsequent filling with magma, creating the abundant
517
dike sets that have amalgamated to make up the totality of the plutonic structure.
518
Faulting also occurred during and post-emplacement, as evidenced by displacement of
519
select dikes (Plate 1). I draw these interpretations from discussion on pluton
520
emplacement through dike amalgamation in Glazner et al. (2004), and magma transport
521
mechanisms in Petford et al. (2000), which include magma ascent through narrow
522
conduits (dikes), and dilatational faulting, and also see the overall emplacement process
523
as episodic, with discrete pulses that result in field evidence showing the inner plutonic
524
structure as a composition of internal sheets, varying in thickness from decimeter to
525
kilometer scale.
Witkosky 24
526
A final issue to address is the previous interpretation on emplacement style for
527
the Canyon Creek Pluton presented in Davis et al. (any reference made to Davis et al.
528
in this paragraph concerns the 1965 paper). In their study, the predominant
529
emplacement style was interpreted as forceful shouldering aside of the host rock, and
530
they based this interpretation on the domical presentation of the pluton in map view
531
(please see Figure 15). The domical presentation can be attributed to a pervasive
532
foliation measured throughout the body of the pluton, and appears to resemble a bird’s
533
eye view of the model for the paradigm seen in Figure 1 of this paper. Davis et al. also
534
note that the pluton appears to have been intruded as a single mass, and interpreted its
535
geometric form from measurements made on planar foliation and other various
536
structural alignments located well inside the body of the intrusion that run parallel to the
537
contact. No foliation is present in the body of the pluton in my study area, which is
538
consistent with the map produced by Davis et al. (Figure 15). It is possible that the
539
fracturing in my study area represents cleavage along foliation planes that correlate with
540
those mapped by Davis et al., but they did report seeing joint sets and andesitic dikes
541
trending northeast in the body of the pluton. With respect to the stress analysis already
542
presented regarding dike emplacement, I expand on the idea of incremental
543
construction under a specific tectonic regime by noting that sequential dike injections
544
would have plenty of time to cool and solidify in a long-lived, several-million-year
545
emplacement process, meaning that at any given time during emplacement, a large
546
portion of the pluton would be solid. This follows my previous interpretations on shallow
547
emplacement in the cold, brittle realm, with fracturing as a main mechanism that guides
548
magmatic injections. It is possible that the foliation documented by Davis et al. could
Witkosky 25
549
have been formed in a solid state, similar to that mentioned by another interpretation on
550
incremental pluton emplacement (Belcher and Kisters, 2006; although a batholith scale
551
pluton in their case). The foliation documented by Davis et al. can then be explained by
552
drawing a finite strain ellipsoid on their map of the Canyon creek Pluton, where foliation
553
planes, along with fold axes mapped, are perpendicular to the principal direction of
554
shortening, which is again an artifact of the east dipping subduction zone that ultimately
555
generated the conditions required for magma generation. The dikes in the Canyon
556
Creek Pluton represent incremental sheeted intrusions of this magma source into
557
existing fracture planes. Sheeted intrusions are also a style of incremental injections
558
interpreted by Belcher and Kisters (2006).
559
An alternative interpretation for the field data presented in this paper is that the
560
portion of the Canyon Creek Pluton in my study area may represent a discrete lobe of
561
magmatic generation, detached from emplacement of the main body in the intrusive
562
mass. A detailed study using zircon geochronology on the Tuolumne intrusion of the
563
Sierra Nevada range in California showed that such magmatic lobes provide snapshots
564
of pluton growth (Memeti et al, 2010), and is still consistent with the idea of incremental
565
stage pluton construction, albeit not through the dike sheeting mechanism as proposed
566
by Glazner et al. (2004). Again, a geochronologic analysis on zircon separated from the
567
aforementioned tonalite samples from my study area would help guide interpretations,
568
and possibly verify that the lack of foliation in my study area is a result of a discrete,
569
shorter-lived magmatic pulse, independent in genesis from that of the main body of the
570
pluton.
571
Witkosky 26
572
573
DISCUSSION AND IMPLICATIONS
A discussion on historical geology is now in order, due to the repetitious hints at
574
a pending U-Pb radiometric age analysis on zircon separated from plutonic and dike
575
rocks collected in the study area. The main concern is to emphasize the importance in
576
methodology described by Memeti et al. (2012), regarding single zircon grain age
577
analyses. If radiometric dating is performed on rocks from my study area, I will classify
578
zircon grains into one of three categories: xenocrysts, antecrysts, and autocrysts.
579
Following Memeti et al. (2012), xenocrysts are inherited from the host rock, antecrysts
580
are recycled from older parts of a pluton, and autocrysts give the true representative
581
ages, as they are the grains that actually grew during crystallization of the sample being
582
dated. The discordant ages for the Canyon Creek Pluton calculated by Wright and
583
Fahan (1988) pay tribute to the loss of precision in performing multiple grain analyses,
584
as xenocrystic cores likely gave erroneous results in their ages (that spanned 30 million
585
years) for the Canyon Creek Pluton. With a highly controversial interpretation such as
586
the incremental building hypothesis presented in my results, great care must be taken to
587
rule out the possibility of inheritance or recycling in any dike ages calculated, as there is
588
a large possibility that zircons separated from dike rocks could be xenocrysts or
589
antecrysts from an older mass of the plutonic tonalite.
590
With respect to economic geology, and as noted by Ernst et al. (2008), calc-
591
alkaline activity in the Jurassic Klamath Mountain province produced numerous plutonic
592
intrusions that, coupled with subduction zone dewatering, allowed for the rise of metallic
593
aqueous solutions into fracture zones, forming ore bodies in the Earth’s crust (Fig. 9 in
594
their manuscript shows a striking geographic correlation in northern California between
Witkosky 27
595
various gold mines and Mesozoic and Paleozoic plutonic and metamorphic rocks).
596
Likewise, gold bearing quartz veins have been mined in the Salmon Hornblende Schist
597
(Cox 1967), and according to Begnoche (2002), mining of precious metals such as
598
copper, zinc, gold, and silver played an important role in the development of the early
599
northern Californian economy (in the late 1800’s to early 1900’s). To this end, I present
600
a new model for pluton emplacement, based on field evidence in this study and the
601
contemporary ideas regarding the incremental building hypothesis for pluton
602
emplacement, where plutons form over a several million year time span and are not
603
believed to ever exist as a large, singular magma body (please see Figure 16). If
604
incremental, or multiple stage construction is found to be viable, this could have
605
important implications for metallic resource exploration. Fluids that are heated deep in
606
the crust rise and travel along fractured or unconformable surfaces as they attempt to
607
de-gas and escape into the atmosphere. Metallic minerals often precipitate in these
608
weak zones as vapors exploit a path of least resistance. If the paradigm (Figure 1) is
609
correct, then there is only one major surface that could potentially harbor mineralization:
610
the main contact between a pluton and its host rock. If many dikes are injected over
611
time, however, to form an integrated body, then every molten injection serves as a
612
branch-like finger of available path, thus drastically increasing the total amount of
613
surface area available for the growth of minerals that are economically important. My
614
new model is referred to as a “dendritic style” of pluton emplacement, because the
615
network of dikes grows slowly upward like a group of tree branches (Figure 16). Finally,
616
the dendritic style of pluton emplacement could also be a potential solution to the room
617
or space problem, because large volumetric displacement in the solid, dense
Witkosky 28
618
subsurface realm can be resolved by considering smaller incremental injections over a
619
longer time period.
620
CONCLUSIONS
621
Field mapping and subsequent calculations show that the Canyon Creek Pluton
622
is composed of dike rocks that make up nearly 6% of its total volume in the study area.
623
Some of the dikes also show evidence for mixing and mingling with tonalitic magma,
624
meaning that both rock units may have been molten at the same time, and diking could
625
have been a significant mechanism that aided in the process of plutonic emplacement.
626
The transects along the contact margin record a long history of brittle rock deformation,
627
with abundant fractures and yet even more dikes in a diffuse zone at least 50 meters
628
wide. There is no evidence for alignment of mineral grains (in either the pluton or host
629
rock) parallel to the contact zone, which detracts from the implications of the paradigm.
630
Through field evidence and stress/strain analyses, it has been shown how the
631
Canyon Creek Pluton could have been emplaced in incremental sheeted dike injections
632
over a very long time period. Tectonic forces helped to make space for magma
633
emplacement through compression, fracturing, faulting, and subsequent dilatational
634
filling. This style of brittle deformation indicates a shallow emplacement depth of 5-10
635
km, at epizonal to mesozonal depths. The wide, diffuse contact margins between pluton
636
and host rock show no signs of preferred grain orientation, and seem to refute the idea
637
of a hot, dome-like, forceful emplacement episode. Observations made while compiling
638
the transects point toward more passive interactions between the magma and host rock
639
during emplacement. Still, as some of the original workers, Arthur Snoke and Cal
640
Barnes, on geology of the Klamath Mountains suggest, zircon inheritance and
Witkosky 29
641
assimilation may present problems in future studies (2006). Therefore, the only
642
remaining unresolved issue is calculating precise ages for pluton and dike rocks from
643
the study area to constrain the timeline of emplacement, which might help corroborate
644
the several-million-year incremental building hypothesis.
645
ACKNOWLEDGEMENTS
646
During my time as an undergraduate at CSUN, I have truly received help from
647
every single one of my friends, faculty, and faculty who have become my friends in the
648
Department of Geological Sciences. Thus, narrowing down the list is tough, but here is
649
my best shot at expressing my gratitude to those who contributed to this thesis: Jose
650
Cardona and Joshua Graham for assisting in field work while camping in the study area;
651
Mrs. Marylin Hanna for providing the financial means necessary to embark on such an
652
amazing geological journey; Dr. Dick Heermance for coercing me into coming along on
653
graduate research, and his belief in my ability to execute my own project; Dr. Elena
654
Miranda for assisting in SEM work; Christine Rains for providing endless support in
655
discussion and editorials on this and other manuscripts; Dr. Joshua Schwartz for
656
assisting in petrography and zircon separation/radiometric age analyses; and last only
657
because this list is in alphabetical order, Dr. J. Doug Yule for inspiring my interest in
658
field mapping, introducing me to the geology of the Klamath Mountains, and how the
659
area can be used as an excellent natural laboratory for solving geologic problems.
660
Actually, I take that back, the final thank you would have to go to my mother, Anita
661
Weiss, who persuaded me to search for a brighter future by returning to college late in
662
life.
663
Witkosky 30
664
REFERENCES CITED
665
Allen, C. M., and Barnes, C. G., 2006, Ages and some cryptic sources of Mesozoic
666
plutonic rocks in the Klamath Mountains, California and Oregon, in Snoke, A. W.,
667
and Barnes, C. G., eds., Geological studies in the Klamath Mountains province,
668
California and Oregon: A volume in honor of William P. Irwin: Boulder, Colorado,
669
Geological Society of America Special Paper 410, p. 223-245.
670
Allmendinger, R. W., 2011-2012, Stereonet 8: A program for plotting and analyzing
671
structural field data that can be found and downloaded for free at:
672
http://www.geo.cornell.edu/geology/faculty/RWA/programs/stereonet-7-for-
673
windows/. Please also see the following listing for more reference information on
674
this amazing program.
675
Allmendinger, R. W., Cardozo, N., and Fisher, D., in press, Structural geology
676
algorithms: Vectors and tensors in structural geology: Cambridge University
677
Press (book to be published in early 2012).
678
Barnes, C. G., Petersen, S. W., Kistler, R. W., Prestvik, T., Sundvoll, B., 1992, Tectonic
679
implications of isotopic variation among Jurassic and Early Cretaceous plutons,
680
Klamath Mountains: Geological Society of America Bulletin, v. 104, no. 1,
681
p.117-126, doi: 10.1130/0016-7606(1992)104<0117:TIOIVA>2.3.CO,2.
682
Barrow, W. M., and Metcalfe, R. V., 2006, A reevaluation of the paleotectonic
683
significance of the Paleozoic Central Metamorphic terrane, eastern Klamath
684
Mountains, California: New constraints from trace element geochemistry and
685
40
686
Geological studies in the Klamath Mountains province, California and Oregon: A
Ar/39Ar thermochronology, in in Snoke, A. W., and Barnes, C. G., eds.,
Witkosky 31
687
volume in honor of William P. Irwin: Boulder, Colorado, Geological Society of
688
America Special Paper 410, p. 393-410.
689
Belcher, R. W., and Kisters, A. F. M., 2006, Progressive adjustments of ascent and
690
emplacement controls during incremental construction of the 3.1 Ga Heerenveen
691
batholith, South Africa: Journal of Structural Geology, v. 28, p. 1406-1421,
692
doi: 10.1016/j.jsg.2006.05.001.
693
694
695
Begnoche, D., 2002, Islands in Time: published by Don Begnoche, 178 p.,
http://books.google.com/books/about/Islands_in_Time.html?id=hAeeYgEACAAJ
Burchardt, S., Tanner, D., and Krumbholz, M., 2012, The Slaufrudalur Pluton, southeast
696
Iceland—An example of shallow magma emplacement by cauldron subsidence
697
and magmatic stoping: Geological Society of America Bulletin, v. 124, no. 1-2,
698
p. 213-227, doi: 10.1130/B30430.1.
699
700
701
Compton, R. R., 1985, Geology in the Field: New York, John Wiley and Sons, Inc.,
398 p.
Cox, D. P., 1967, Reconnaissance geology of the Helena quadrangle, Trinity County,
702
California: California Division of Mines and Geology, SR 92, Short Contributions:
703
Cox, p. 43-55.
704
Davis, G. A., 1963, Structure and Mode of Emplacement of Caribou Mountain Pluton,
705
Klamath Mountains, California: Geological Society of America Bulletin, v. 74,
706
no. 3, p. 331-348, doi: 10.1130/0016-7606(1963)74[331:SAMOEO]2.0.CO;2.
707
Davis, G. A., Holdaway, M. J., Lipman, P. W., Romey, W. D., 1965, Structure,
708
Metamorphism, and Plutonism in the South-Central Klamath Mountains,
709
California: Geological Society of America Bulletin, v. 76, p. 933-966, 8 figs., 3 pls.
Witkosky 32
710
Davis, G. A., Ando, C. J., Cashman, P. H., Goulladd, L., 1980, Geologic cross section of
711
the central Klamath Mountains, California: Summary: Geological Society of
712
America Bulletin, Part 1, v. 91, p. 139-142, 1 fig.
713
Detrick, R. S., Mutter, J. C., Buhl, P., and Kim, I. I.,1990, No evidence from multichannel
714
reflection data for a crustal magma chamber in the MARK area on the
715
Mid-Atlantic Ridge: Nature, v. 347, p. 61-64.
716
Ernst, W. G., Snow, C. A., Scherer, H. H., Mesozoic transpression, subduction and
717
metallogenesis in northern and central California: Terra Nova, v. 20, n. 5,
718
p. 394-413, doi: 10.1111/j.1365-3121.2008.00834.x.
719
Glazner, A. F., Bartley, J. M., Coleman, D. S., Gray, W., and Taylor, R. Z., 2004, Are
720
plutons assembled over millions of years by amalgamation from small magma
721
chambers?: GSA Today, v. 14, no. 4/5, doi: 10.1130/10525173(2004)014
722
<0004:APAOMO>2.0.CO;2.
723
724
725
Google Earth, 2013, “Grizzly Lake, CA.”: 41°00’26.28” N 123°03’08.02” W.
April 18, 2013.
Hacker, B. R., and Peacock, S. M., 1990, Comparison of the Central Metamorphic Belt
726
and Trinity terrane of the Klamath Mountains with the Feather River terrane of
727
the Sierra Nevada, in Harwood, D. S., and Miller, M. M., eds., Paleozoic and
728
early Mesozoic paleogeographic relations; Sierra Nevada, Klamath Mountains,
729
and related terranes: Boulder, Colorado, Geological Society of America Special
730
Paper 255, p. 75-92.
731
732
Irwin, W. P., and Wooden, J. L., 1999, Plutons and Accretionary Episodes of the
Klamath Mountains, California and Oregon: U.S. Geological Survey Open—File
Witkosky 33
733
Report 99—374, scale 1:950400,
734
http://geopubs.wr.usgs.gov/open-file/of99-374/of99-374.pdf,
735
accessed on April 5, 2012.
736
Memeti, V., Paterson, S., Matzel, J., Mundil, R., and Okaya, D., 2010, Magmatic lobes
737
as “snapshots” of magma chamber growth and evolution in large, composite
738
batholiths: An example from the Tuolumne intrusion, Sierra Nevada, California:
739
Geological Society of America Bulletin, v. 122, no. 11/12, p. 1912-1931, doi:
740
10.1130/B30004.1, 12 figures.
741
Petford, N., Cruden, A. R., McCaffrey, K. J. W., and Vigneresse, J. -L., 2000, Granite
742
magma formation, transport and emplacement in the Earth’s crust: Nature,
743
v. 408, December, p. 669-673, www.nature.com.
744
745
746
Sharp, R. P.,1960, Pleistocene glaciation in Trinity Alps of Northern California:
American Journal of Science, v. 258, May 1960, p.305-340.
Snoke, A. W., and Barnes, C. G., 2006, The development of tectonic concepts for the
747
Klamath Mountains province, California and Oregon in Snoke, A. W.,
748
and Barnes, C. G., eds., Geological studies in the Klamath Mountains province,
749
California and Oregon: A volume in honor of William P. Irwin: Boulder, Colorado,
750
Geological Society of America Special Paper 410, p. 1-29.
751
752
U. S. Geological Survey, 2012, Thompson Peak Quadrangle, California, 7.5-Minute
Series: United States Geological Survey, scale 1:24 000, 1 sheet.
753
Wright, J. E., and Fahan, M. R., 1988, An expanded view of Jurassic orogenesis in the
754
western United States Cordillera: Middle Jurassic (pre-Nevadan) regional meta-
755
morphism and thrust faulting within an active arc environment, Klamath
Witkosky 34
756
Mountains, California: Geological Society of America Bulletin, v. 100, p. 859-876,
757
11 figs., 6 tables
Witkosky 35
Magma is generated
in a large (kilometer
scale), unified body...
~100,000 years later,
mineral grains
become aligned as
the ductile, molten
body swells upward...
After ~500,000 years,
the pluton is almost
completely formed and
solidified.
Figure 1. The classic paradigm for pluton emplacement shows a profile view of magma rising as a diapir in
the Earth’s crust (timeline for solidification taken from thermal modeling in Glazner et al., 2004). Also referred
to as “forceful emplacement,” the arrows represent strong buoyancy and upwelling forces as the body
intrudes into a dense, solidified crust. Deformation is not limited to crystals in the magma mush of the
pluton, but also affects any structural fabric (i.e. metamorphic foliation or sedimentary layering) in the host
rock.
Witkosky 36
Figure 2A. Regional location of the study area: Grizzly Lake, Trinity National Forest, in Northern California
(from Google Earth, 2013). The exact location is the small red bubble behind the letter “F” in “Forest”. For
sense of scale, the study area is located approximately 60 miles northeast of Eureka and 45 miles northwest
of Redding. Note that the study area is in the northwest portion of a distinct, light-gray-colored semicircular
shape: this is the leucocratic igneous rock of the Canyon Creek Pluton, which can also be seen as a geologic
unit in Figure 2B.
Witkosky 37
Figure 2B. Zooming Sequence to the study area in the Klamath Mountains of northern California. The lightgray-colored rock mentioned in the previous figure (Figure 2A. Google Earth satellite image) is represented
as a yellow-colored geologic unit in the centerpiece of this figure: the Canyon Creek Pluton (labeled “Canyon
Cr”). “Z141 – 145” and “z 160 – 170” represent radiometric zircon ages (in millions of years) calculated for
the Canyon Creek Pluton from Wright and Fahan, 1988 (figure modified from Irwin and Wooden, 1999).
Witkosky 38
Figure 3. The Salmon Hornblende Schist is an amphibolite grade metamorphic rock with slaty cleavage and
abundant isoclinally folded ribbons that consist of labradorite plagioclase and plastically deformed quartz
(hammer and Brunton compass for scale). The Salmon Hornblende Schist is part of the Central Metamorphic
Belt and plays host to the Canyon Creek Pluton in my study area.
Witkosky 39
Figure 4. Close-up image of an andesite hornblende porphyry dike. Hornblende laths and needles
(phenocrysts) are randomly oriented in a fine-grained matrix of mainly plagioclase microlites (pencil for
scale).
Witkosky 40
Figure 5. First type of greenstone dike (type 1), representing the first stage of hydrothermal alteration to
other dike sets, and containing mostly actinolite, epidote, and chlorite. In the lower left hand portion a subtle,
1 cm thick white band of tremolite and punpellyite with reddish-brown oxides forming next to it (pencil for
scale).
Witkosky 41
Figure 6A. Half meter thick greenstone type 2 dike, showing light bluish-green color (hammer, pencil, and my
foot for scale).
Witkosky 42
Figure 6B. Zoomed in photo of Figure 6A, showing a relict porphyritic texture in type 2 greenstone dikes,
with skeletal outlines of hornblende laths (hammer head and pencil for scale) set in an aphanitic matrix.
Witkosky 43
Figure 7. Scanning electron microscope (SEM) image of a hornblende phenocryst that has been almost
completely obliterated by hydrothermal alteration in a type 2 greenstone dike. Also note that a significant
portion of the medium gray colored matrix is cryptocrystalline (massive). SEM working conditions are as
follows: low vacuum wih 20.00 Pa H2O; accelerating voltage- 20.00 kV; spot size- 7.0; working distance10.999 mm; filament current- 2.18 A; beam current 94 µA; scale bar shown in lower left hand corner of image.
Witkosky 44
Figure 8A. Annotated image showing the west-facing wall of the Grizzly Meadows stream valley, where the
central transect (Plate 2) was performed. A portion of the transect outline is shown in pink at the bottom of
the image. To the right, the light gray colored rock is that of the Canyon Creek Pluton, with the darker
Salmon Hornblende Schist (host rock) to the left. The contact appears to be subvertical, where a large
tonalite apophysis (labeled “Large Feeder Dike”) has channeled magma to several dikes that run parallel to
the contact. Mount Shasta can also be seen in the background (photo credit to R. Heermance).
Witkosky 45
Figure 8B. Approaching the location for the central transect on the morning of Thursday, August 9, 2012
(photo taken facing due east). In this image the complete central transect outline is shown in pink, again with
the “Large Feeder Dike”, or tonalite apophysis (photo credit to R. Heermance).
Witkosky 46
1600
1200
Eclogite
50
Blueschist
Amphibolite
Pressure
(MPa)
Granulite
30
Depth
(km)
800
Greenschist
400
10
200
400
Temp
(ºC)
600
800
TEMPERATURE-PRESSURE-DEPTH RELATIONSHIPS
Figure 9. This graph copied from Begnoche (2002, p. 101, or p. 125 if viewed in PDF format) shows that a rock
undergoing extensive amphibolite facies metamorphism must be at a depth of at least 10 km. With the
amphibolite grade host rock (Salmon Hornblende Schist of the Central Metamorphic Belt) yielding Late
Paleozoic metamorphic ages (at minimum), it is not likely that the host rock and its magmatic intrusion could
have been at a depth greater than 10 km during emplacement of the Canyon Creek Pluton in the Early
Cretaceous. The taupe-colored, shaded region shows the temperature-pressure conditions interpreted by
Hacker and Peacock for the host rock (1990): 500° to 650 ± 50°C and 500 ± 300 MPa, respectively. These
conditions are in general achieved at a depth much greater than 10 km.
Witkosky 47
~2 m
Figure 10. 2 m thick, sub-vertical dike of andesite hornblende porphyry located within the main body of the
Canyon Creek Pluton (photo taken facing roughly north 60° east). Some of the dikes in the study area have a
measured thicknesses of 20 m or greater (thickness measured is very close to true thickness, not apparent
thickness, due to the vertical orientation of most dikes mapped). (photo credit to R. Heermance).
Witkosky 48
Figure 11A. An example of magma mixing between a dark, mafic dike, and lighter-colored tonalite. This
mingling of magmas shows that the main body of the pluton was still very hot during dike development,
meaning that both rock units were may have been simultaneously molten during emplacement (ruler for
scale; photo credit to R. Heermance)
Witkosky 49
Figure 11B. This rock collected from the body of the Canyon Creek Pluton might possibly show
crystallization of tonalitic magma (top) following emplacement of dark, mafic dike material (bottom). The
lighter colored tonalite contains many small inclusions of the darker dike material, and directly above my
finger, the hot tonalite appears to have been smeared up against the dike. Both of these features show that
the darker material could have solidified first, with the liquid tonalite, albeit highly viscous, later coming into
contact with a cold, solid dike rock.
Witkosky 50
Figure 12. This photo of Grizzly Falls (located at the northernmost point of Grizzly Lake) is taken facing west,
and shows pervasive fracture sets and a major fault damage zone running approximately north 60° east and
subvertically through the Canyon Creek Pluton. The waterfall face is also a major fracture surface, roughly 30
meters high (photo credit to Joshua Graham).
Witkosky 51
Figure 13. Beautiful fault slickenlines on a slickensided surface, cutting and displacing a dike in the eastern
portion of the study area (photo taken facing roughly due north, with Jose Cardona for scale). The
orientations of the slickenlines show that the dominant style of faulting in the study area is strike-slip, with a
slight oblique component.
Witkosky 52
σ3
σ1
30˚
σ1
σ3
Figure 14. Stereonet shows poles to planar surfaces measured in the study area: blue circles = dikes; purple
diamonds = fault planes; and orange squares = fracture sets. Field data was compiled digitally using
Stereonet 8 (Allmendinger 2011-2012). The predominant subvertical orientations of these structures result in
pole clusters falling on the boundary of the stereonet. The best-fit line is in black in the center, and is well
representative of the predominant vertical nature of structural features. Shear sense indicators show right
lateral strike slip faulting (evidenced by slickenlines and displaced dikes in the study area), and the direction
of greatest principal stress (σ1) represent tectonic compression for an east dipping subduction zone. The
resulting principal stress directions, along with the best fit line for faults and fracture surfaces, is analogous
to a rock cylinder crushing experiment, where the critical angle of failure is measured 30°, counter clockwise,
from σ1. Emplacement of the Canyon Creek Pluton can then occur in a shallow brittle regime, beginning with
compression, fracturing, dilational opening of fracture sets, and subsequent filling with magma.
Witkosky 53
Figure 15. A portion of a previous map drafted in my study area (borrowed and modified from Davis et al.,
1965). Davis et al. interpreted the overall geometry of structural features located within the Canyon Creek
Pluton to represent a dome like structure, having been emplaced in an episode that involved forceful
shouldering of the host rock as the predominant emplacement mechanism (the foliation symbols parallel the
contact, meaning that this is essentially a bird’s eye view of the paradigm model that I present in Figure 1 of
this paper). My contemporary interpretation on incremental construction through dike amalgamation
involves superimposing a finite strain ellipse over the map, to show that the foliation pattern and fold axes
drawn by Davis et al. could have resulted from a tectonic stress/strain regime, where S3 represents the
principal direction of shortening (roughly parallel to σ1, the direction of greatest principal stress seen in
Figure 14) caused by east directed paleo-subduction (the only caveat is that Irwin and Wooden, 1999, report
a significant amount of rotation, meaning that a study devoted to reconstructing blocks of continental crust
would require a paleomagnetic component). Note that in my study area, the foliation is not documented,
which leads to one of two interpretations: either the fracture sets and subsequent diking in my study area
exploited subtle, weak foliation planes that remain obscured to the naked eye, or the portion of the pluton in
my study area could have been formed in a discrete pulse, forming a shorter-lived magmatic lobe that was
just one of several emplacement episodes that built up the Canyon Creek Pluton. Regardless of which of
these two interpretations is favored, incremental construction is implied, either by sheeted dike
amalgamation, or multiple lobe injections.
Witkosky 54
Figure 16. The new model for pluton emplacement, based on contemporary notions of incremental plutonic
building episodes, the abundance of dikes and wide contact zone documented in the study area. The timeline
of emplacement is hypothetical, and requires some verification through age analyses on dike rocks collected
from the study area. This model could have implications on resource exploration, as every branch-like
intrusive finger drastically increases the total amount of surface area available for precipitation of metallic
minerals. Additionally, this cartoon provides a potential solution for the room problem, as large volumetric
displacement of dense, solidified crust can be resolved by considering many small incremental injections
over a very long time period.
PLATE 1. GEOLOGIC MAP OF THE GRIZZLY LAKE AREA,
TRINITY ALPS, KLAMATH MOUNTAINS, CALIFORNIA
Map location
BY RYAN D. WITKOSKY, AUGUST 2012
Topographic base, Thompson Peak Quadrangle, California,
7.5 Minute Series (U.S. Geological Survey, 2012)
Legend
60
Hornblende Biotite Tonalite (pluton)- leucocratic, felsic, medium-grained, phaneritic, plutonic
rock containing mostly plagioclase and quartz,
with biotite and minor hornblende
Kt
00
{ {
41
52
Cretaceous
G rizzly B utte
84
MN
U
D
60 0 0
00
60
64 0 0
0
0
68 0
76 0 0
65
0
r
20
Kt
*
*
1
1
10
72 0 0
79
66
15
35
20 81
1
47
1
1
20
3
84
8000
L ois
66 66
L ak e
6
3
+
*
U
G r izzl y
L ak e
1
2
$
5
14
1
1
$
10
15
0.3
$
72 0
0
3
0.3
$
2
2
3
$
0.3
0.3
3
3
20
76 0
0
10
D
10
5
10
25
76 0 0
66
7
3
1
56
86
Dhs
10
G rizzly M eadows
0
Andesite Hornblende Porphyry
Dacite Plagioclase Porphyry
Greenstone
3
3
3
0
70
Glaciation
68 0
72 0
+
C
8000
0
ly
60
81
$ Hand sample location
56 0 0
0
76
G rizz
METERS
64 0 0
72
67
*+ Location of base camps
Oriented sample location
0.5
3000
FEET
Strike of vertical foliation
Dikes- dashed where inferred, dotted where concealed. Numerics on terminal ends are thickness
in meters, and terminal ends do not necessarily
represent termination of the dike, but the extent to
which they were exposed and/or mapped.
0.5
0
14
Detailed transect location (not to scale; each
transect contains at least 3 oriented samples with
exact locations not shown on this map)
71
CONTOUR INTERVAL = 80 FEET
Strike and dip of metamorphic foliation71 if located within the pluton, represents a
measurement made in a wall rock enclave (small
arrow indicates trend and plunge of fold hinge
lineation measured in quartz ribbons)
Fault- with shear sense indicators, or U/D
indicating up/down apparent displacement
(dashed where inferred, dotted where
concealed). Single arrow indicates trend
and plunge of slickenlines.
SCALE 1:8000
68 0 0
1000
60
Contact margin- a broad, diffuse
zone up to 100 m thick
61
1000
Dhs
Explanation of Symbols
15°
2000
Devonian
G r izzl y C r
79
{ {
Salmon Hornblende56Schist
(host rock)- mafic,
00
amphibolite grade metamorphic rock with pervasive slaty cleavage planes, and isoclinally folded,
boudinaged quartz ribbons
48 0 0
N
Paleozoic
0
64 0
59
Mesozoic
51
{
65
84 0 0
2
88
76 00
00
0.3
Caes ar Peak
7600
84 0 0
T hom ps on Peak
$2
72 0 0
NORTH
PLATE 2. CENTRAL TRANSECT ALONG THE CONTACT BETWEEN
THE PLUTON AND HOST ROCK FROM PLATE 1
BY RYAN D. WITKOSKY, AUGUST 2012
(the same legend holds true as that of the geologic map in Plate 1;
border numerics = scale in meters)
Figure T3. Small aplitic
veins initially intrude perpendicular or oblique to
the highlighted foliation
16
50
planes of the brittle host
rock, then gradually
follow a path of least resistance, exploiting foliation planes in the schist 45
(Big Blue for scale).
Figure T4. Arm of large
feeder dike. This tonalite
apophysis is the last instance of plutonic rock
found intruding into host
rock in the central transect
(photo credit to R. Heermance).
12
8
4
0
4
40
8
89
12
16
50
Dhs
35
45
30
Dhs
Kt
25
Mega-enclave
35
20
72
30
Figure T2. Contact obliquely 15
cuts foliation in host rock.
The structural fabric
(foliation) has been highlighted to show that it is 10
not parallel to the
contact margin, as
predicted by the
5
paradigm in an
episode of forceful
plutonic emplacemnent.
0
16
40
88
PRE-EXISTING FOLIATION
IN HOST ROCK...
25
*
20
44
Kt
12
8
4
0
*
*
71
SEM Sample
5
4
Figure T1. Brecciated zone of
wall rock enclaves. In the contact
zone, numerous brecciated angular fragments of dark host rock
are found as enclaves in the
lighter colored plutonic rock (ruler
for scale seen below center of
image; photo credit to R. Heermance).
8
12
16
Figure T5. SEM image showing preferred orientation in mineral grains of the host rock. Grains are
15 deformed and forced into alignment so their long
axes form horizontal bands across the image, but
not parallel to the contact, as seen in Figure T2. This
shows that the structural fabric of the host rock was
created in a previous deformational event, and not a
10
result of pluton emplacement, as predicted by rthe
paradigm. SEM working conditions are as follows:
low vacuum with 20.00 Pa H20; accelerating voltage20.00 kV; spot size- 7.0; working distance- 19.999
mm; filament current- 2.68 A; beam current- 95 µA;
scale bar shown in lower left hand corner of image.
PLUTONIC ROCK FROM
0 THE CONTACT ZONE...
Figure T6. SEM image
showing random grain orientation in plutonic rock.
No microstructural or ductile
deformation, and mineral
grains are randomly oriented,
contrary to the predictions of
the paradigm. SEM working
conditions are same as in
Figure T5, except working
distance is 10.002 mm, and
scale bar in lower left hand
corner is larger.