Download Provenance of plutonic detritus in cover sandstones of Nicoya

Provenance of plutonic detritus in cover sandstones of Nicoya
Complex, Costa Rica: Cretaceous unroofing history of a
Mesozoic ophiolite sequence
Claudio Calvo†
Anna-Peters-Strasse 51/C, 70597 Stuttgart, Germany
ABSTRACT
This study presents new petrologic and
sedimentologic data from northwestern Costa Rica concerning the provenance of Cretaceous forearc sandstones that contain plutonic detritus. Plutonic rock fragments are
important accessory particles in pyroxenebearing sandstones overlying the ophiolite
named the Nicoya Complex. Through the
use of modal analysis of the framework
grains, I studied three sandstone suites of
the El Viejo and Rivas Formations that include both shallow- and deep-water deposits, ranging from late Campanian to Maastrichtian in age. In terms of primary
framework components, the sandstones resemble those derived from magmatic arcs.
Two modal parameters are introduced to
evaluate detrital plutonic contributions and
affinity of source rocks: the ratio of plutonic to total lithic fragments [(Lp 1 iQF)/Lt],
and the ratio of uralitized pyroxene to total
pyroxene grains (uralPx/Px). Modal values
for (Lp 1 iQF)/Lt indicate that plutonic
fragments comprise up to 9% of total lithic
fragments. A strong correlation between
these two parameters suggests that uralitized pyroxene grains were also derived from
intrusive rocks of probably basic and intermediate compositions. In particular, significant concentrations of lithic fragments exhibiting micrographic textures and
uralitized pyroxene grains are interpreted
to have been derived predominantly from
eroded plagiogranites. Sandstone suites
containing plutonic detritus signal an unroofing of deeper levels of the Mesozoic
ophiolitic sequence as a consequence of
strong uplift of the Costa Rican arc in late
Senonian time. This tectonic event began in
E-mail: [email protected].
†
the Campanian at ca. 75 Ma, ;9 m.y. after
the intrusive magmatic activity on Nicoya
Peninsula, and is consistent with the onset
of the Laramide orogeny.
Keywords: provenance, modal analysis,
plagiogranites, ophiolite complex, Costa
Rica.
INTRODUCTION
The ophiolitic basement of northwestern
Costa Rica and its sedimentary cover strata
constitute the most thoroughly studied on-land
rock assemblage of the southern Central
American forearc. This study focuses on the
origin and provenance of plutonic detritus in
both deep-sea channel and shallow-water basal sandstones from the Cretaceous cover units
overlying the ophiolite named the Nicoya
Complex. In these forearc sandstones, detrital
grains of igneous plutonic origin are important
accessory constituents, making up as much as
9% of total lithic framework grains.
Previous studies on petrology and provenance of forearc sandstones from northwestern
Costa Rica have suggested—on the basis of
primary framework modes—that Cretaceous
sandstones were derived from a magmatic arc
(Lundberg, 1991; Calvo, 1998). Compositionally, both those sandstones within the Nicoya
ophiolite complex and many of the unconformably overlying sandstones are commonly
referred to as basaltic sandstones, derived
from erosion of basaltic basement (e.g., Kuijpers, 1979; Baumgartner et al., 1984; Gursky,
1989; Lundberg, 1991). Although basaltic
sandstones dominate in some sequences, there
are also volcaniclastic sandstones that contain
abundant nonbasaltic framework components.
For example, volcaniclastic wackes from the
Loma Chumico Formation of the upper Nicoya Complex exhibit a bimodal composition
GSA Bulletin; July 2003; v. 115; no. 7; p. 832–844; 10 figures; 3 tables; Data Respository item 2003095.
832
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q 2003 Geological Society of America
consisting of basaltic fragments and differentiated pyroclastic materials (Calvo and Bolz,
1994). Similarly, Cretaceous cover sandstones
of the Nicoya Complex contain a wide spectrum of framework grains from other sources:
radiolarites (including radiolarian cherts), pelagic limestones, shallow-marine carbonates,
andesitic lavas, differentiated ejecta, and igneous intrusive rocks (Calvo, 1998). This
spectrum of grain types suggests that these
forearc sandstones were derived predominantly from shallow levels of an intraoceanic arc,
but it also reflects a significant contribution
from uplifted ophiolitic basement areas partly
fringed by shallow-water carbonate deposits
(Calvo, 1998). The plutonic detritus particularly records an ophiolitic provenance and
deeper unroofing of the ophiolitic basement
than previous workers have suggested.
The main objectives of this study are (1) to
analyze mineral composition and textural features of plutonic detritus in order to determine
likely source rocks and provenance relationships; (2) to establish detrital plutonic contributions and affinity of source rocks by using
modal analysis of secondary framework parameters; and (3) to integrate these new data
into a model for the Cretaceous unroofing history of the ophiolitic sequence of southern
Central America.
TECTONIC SETTING
Southern Central America (Costa Rica and
Panama) represents an island arc that formed
over an intraoceanic subduction zone situated
at the western margin of the Caribbean plate
(e.g., Kuijpers, 1979; Lundberg, 1982; Wildberg, 1984; Calvo and Bolz, 1994). At the
present time, the Cocos plate is being subducted beneath Costa Rica at 90 mm/yr (Minster and Jordan, 1978). Bathymetric swath
mapping of the convergent margin offshore
CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA
Costa Rica shows an oceanic plate covered
with numerous seamounts (von Huene et al.,
1995). The most prominent feature is the Cocos Ridge subducting opposite the Osa Peninsula (Fig. 1).
The study area is located in northwestern
Costa Rica. This region is characterized by
three major morphotectonic elements that reflect an evolved arc edifice (Fig. 1): (1) the
emerged outer arc, forming the Nicoya and
Santa Elena Peninsulas; (2) the inner forearc
trough (Tempisque basin), comprising the
Gulf of Nicoya and lowlands of the Tempisque River; and (3) the active volcanic cordilleras of Guanacaste and Tilarán, representing the inner magmatic arc. Behind the
cordilleras are the Guatusos lowlands, representing the southern terminus of the extensional Nicaraguan Depression.
STRATIGRAPHIC FRAMEWORK
The forearc basement is exposed along the
Pacific coast of Costa Rica and comprises a
Mesozoic ophiolitic sequence, the Nicoya
Complex (Dengo, 1962). It is principally composed of peridotites, basalts (massive and pillowed flows), basaltic breccias, dolerites, radiolarites, limestones, volcaniclastic rocks,
and igneous intrusive rocks (plagiogranites
and gabbros). Differentiated pyroclastic deposits of Albian to Campanian age within the
Nicoya Complex record the earliest volcanic
activity of the Costa Rican arc orogen (Calvo
and Bolz, 1994; Calvo, 1998). Overlying the
Nicoya Complex is the sedimentary cover that
includes both shallow- and deep-marine strata
ranging from Upper Cretaceous through Pliocene age. The forearc rock succession is partly covered by Quaternary volcanic rocks
erupted from stratovolcanoes of the northwestern cordilleras (Fig. 1). Seismic reflection
data from the Pacific margin offshore Costa
Rica suggest the seaward continuation of the
Nicoya Complex to the middle slope (Hinz et
al., 1996).
The cover sandstone suites studied are stratigraphically grouped into the El Viejo and Rivas Formations (Table 1). The El Viejo Formation (late Campanian–Maastrichtian) is a
shallow-marine clastic and related carbonate
reef unit a few meters thick consisting of rudistid framestones, bioclastic grainstones, and
sandstones that rest unconformably on rocks
of the Nicoya Complex (Schmidt-Effing,
1975; Ulloa, 1977; Seyfried and Sprechmann,
1985; Calvo, 1987). The Rivas Formation
(late Campanian–Paleocene) comprises a turbiditic slope sequence ;1500 m thick composed of volcaniclastic sandstones, mud-
stones, and minor conglomerates and breccias
(Dengo, 1962; Protti, 1981; Lundberg, 1982;
Rivier, 1983; Baumgartner et al., 1984; Calvo,
1998). This unit overlies conformably hemipelagic limestones and volcaniclastic rocks of
the Sabana Grande Formation of Campanian
age.
MATERIALS AND METHODS
Detritus derived from igneous intrusive
rocks in Cretaceous cover sandstones was first
detected during analysis of main framework
grains (Calvo, 1998). On the basis of depositional environment and age, 20 sandstone
samples were selected for this study. They
were collected from three different locations
in Cretaceous sandstone suites of the inner
forearc: La Tigra, Quebrada Pilas, and Calle
Codornices (Fig. 1). The sampled units form
part of the sedimentary cover of Nicoya Complex, recording shallow- and deep-water clastic sedimentation in the forearc region of
northwestern Costa Rica during the late Senonian. Their principal stratigraphic and sedimentologic characteristics are summarized in
Table 1. Sandstone ages were determined on
the basis of their contained foraminiferal assemblages. Samples were examined petrographically in thin section, cut normal to bedding. For analysis of sandstone frameworks,
an average of 500 grain points were counted
in each section. Significant concentrations of
plutonic detritus in these sandstones make
possible the modal analysis of detrital plutonic
contributions using the Gazzi-Dickinson
point-counting method (Gazzi, 1966; Dickinson, 1970). Point-count results are listed in Table DR1.1 Plutonic lithic fragments and mafic
mineral grains were counted as separated
grains. In order to establish plutonic contributions, fragments of plutonic rock were included in tabulations of total lithic grains. Because of the scarcity of total counts of mafic
mineral grains, additional 100 point counts for
total pyroxene grains were necessary to improve the statistical reliability of the values for
the uralitized pyroxene ratio. Petrologically,
the general correspondence of ratios calculated here is considered to be significant because
it reflects compositional tendencies observed
on the basis of the detailed optical analysis of
framework grains, made prior to the point
counts. Following Dickinson’s (1970) convention, carbonate fragments were not included in
1
GSA Data Repository item 2003095, sedimentary petrography of forearc sandstones from northwestern Costa Rica, is available on the Web at http:
//www.geosociety.org/pubs/ft2003.htm. Requests
may also be sent to [email protected].
Geological Society of America Bulletin, July 2003
the modal analysis, because of their possible
intrabasinal origin and generally minor importance for constraining ophiolitic provenance. Sandstone suites derived from different
depositional environments were also analyzed
to evaluate dispersal patterns of detrital sediment in the marine forearc basin.
In this study, the timing of unroofing of the
ophiolitic sequence was established from the
relative ages of foraminiferal assemblages
identified in sandstone samples. Radiometric
age determinations of intrusive rocks from the
Nicoya Peninsula by Sinton et al. (1997), coupled with the biostratigraphic data presented
here, constrain the age of crystallization and
earliest erosion of Nicoya Complex intrusive
rocks in northwestern Costa Rica.
RESULTS
Detailed petrographic examination of
framework grains reveals that Cretaceous
forearc sandstones from northwestern Costa
Rica contain abundant ophiolitic grains, ranging from basic volcanic fragments and igneous
intrusive-derived detritus to cherty rock fragments of sedimentary origin (Fig. 2). This
study focuses on plutonic rock fragments and
radiolarian chert grains, which are thought to
represent useful distinctive provenance indicators reflecting unroofing of deeper levels of
the ophiolitic sequence. Petrographically, analyzed sandstones comprise dominantly lithic
and feldspatholithic wackes, litharenites, and
hybrid arenites as defined by Zuffa (1980), including arenites bearing Sulcoperculina.
Plutonic Grains
Detrital grains of plutonic origin comprise
a diverse assortment of grain types, including
both lithic fragments and monocrystalline
grains. Figures 2A through 2D show photomicrographs of representative plutonic framework grains found in Cretaceous cover sandstones of the Nicoya Complex. A unique grain
type consists of crystalline lithic grains that
exhibit micrographic textures distinctive of igneous intrusive rocks (Tröger, 1967). These
micrographic grains are composed of plagioclase feldspar (albite) and quartz intergrowths
(Figs. 2A, 2C). Because of the instability of
feldspars in the sedimentary environment relative to quartz, micrographic intergrowths
generally exhibit differential weathering: In
plane-polarized light, albite is seen to have
been altered to clay minerals, whereas quartz
remained essentially unaltered. Other rock
fragments include polycrystalline quartz
grains and crystalline lithic fragments. Poly-
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C. CALVO
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Geological Society of America Bulletin, July 2003
CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA
TABLE 1. STRATIGRAPHIC AND SEDIMENTOLOGIC CHARACTERISTICS OF THREE CRETACEOUS FOREARC-SANDSTONE SUITES FROM NORTHWESTERN
COSTA RICA
Sandstone suite
Formation
Age
Foraminiferal assemblage
Thickness
Type of facies
Depositional environment
La Tigra
Quebrada Pilas
Calle Codornices
Rivas
Rivas
Late Campanian to early Maastrichtian
Maastrichtian†
Sulcoperculina dickersoni (Palmer);
Gansserina gansseri (Bolli); Orbitocyclina
Sulcoperculina globosa de Cizancourt;
minima (H. Douvillé); Sulcoperculina sp.
Pseudorbitoides sp.; Sulcorbitoides pardoi
Brönnimann
;150 m
;30 m
Channel sandstones
Channel sandstones
Inner forearc–trough (Tempisque Basin):
Lowstand submarine slope
Arc platform–related submarine slope
El Viejo
Late Campanian to early Maastrichtian
Sulcoperculina globosa de Cizancourt;
Sulcoperculina sp.; Pseudorbitoides cf.
israelsky Vaughan & Cole
;5 m
Basal sandstones
Island-arc platform:
Transgressive bioclastic shoals
Age also supported by paleontologic determinations of macrofauna (Fischer and Aguilar, 1994)
†
crystalline quartz grains resemble those derived from granitoid source rocks, because
they are composed of relatively few crystals
of equant shape and have few or no intracrystalline sutured contacts. Crystalline lithic fragments containing polysynthetically twinned
plagioclase and uralitized clinopyroxene are
also present (Fig. 2B). Some of them display
subophitic textures. Monocrystalline grains
include plagioclase, quartz, and pyroxene.
Both plagioclase and quartz grains are, however, difficult to interpret, because in many
cases they do not exhibit diagnostic petrographic features of a plutonic or volcanic origin. The most important mafic mineral grain
is pyroxene, which often appears partly uralitized (Fig. 2B).
The predominance of coarse-grained plutonic fragments showing little evidence of
chemical weathering, in conjunction with angular grain textures exhibiting remains of intercrystalline boundaries (Fig. 2A), indicates
that intrusive rocks in the source area were
broken mechanically along intercrystalline
boundaries and that detrital grain size is, in
part, controlled by original textures of coarsegrained source rocks. Although tropical
weathering conditions prevailed during denudation, as suggested by alterite grains identified in samples analyzed, the spectrum of plutonic detritus apparently reflects original
compositions of igneous source rocks, an interpretation that implies significant relief in
the source area.
Cherty Rock Fragments
Siliceous rock fragments of sedimentary origin, including radiolarian mudstones and
cherts, are typical framework grains found
with plutonic detritus in studied Cretaceous
forearc sandstones (Fig. 2). In thin section, radiolarite grains exhibit a dark rusty red to
brown color and bear silica-filled blebs that
are likely microfossil (radiolaria) remains
(Fig. 2E). Radiolarian chert grains are polycrystalline quartz fragments, commonly exhibiting hematitic clusters and vestiges of radiolaria (Fig. 2F). Such grains are of particular
interest for provenance of cover sandstones,
because they probably result from erosion of
ribbon cherts within the Nicoya Complex that
are interpreted to form part of a remnant accretionary unit (Baumgartner, 1990), i.e., the
Punta Conchal Formation (Gursky and
Schmidt-Effing, 1983).
Modal Analysis of Plutonic Detritus
In terms of primary framework grains
(QFL, Qp-Lvm-Lsm), Cretaceous sandstones
from northwestern Costa Rica containing plutonic detritus have a composition similar to
those derived from magmatic arcs (Fig. 3;
Calvo, 1998). Ophiolitic and especially plutonic provenances are commonly obscured in
these sandstones by abundant volcanic fragments, but both sources are recognizable in
channel sandstones and transgressive basal
sandstones from the slope and neritic sequences, respectively, selected for this analysis.
This study focused on the semiquantitative
modal analysis of detrital parameters sensitive
to plutonic provenance, defined in Table 2.
Recalculated modal values are summarized in
Table 3 and presented graphically in Figures
4 and 5. Original point data for primary and
secondary parameters are available in Table
DR1. In order to establish detrital plutonic
contributions and affinity of source rocks, two
new modal parameters are introduced. They
are defined as the ratio of plutonic to total lithic fragments [(Lp 1 iQF)/Lt] and the ratio of
uralitized pyroxene to total pyroxene grains
(uralPx/Px). Uralitization, the alteration of clinopyroxene to amphibole, is characteristic of
plutonic terranes (Tröger, 1967). Modal values
for (Lp 1 iQF)/Lt indicate that plutonic fragments average up to 9% of total lithic fragments. Values between 4% and 9% are only
found in sandstones of the Calle Codornices
and La Tigra suites (Table 3). Modal content
of plutonic materials calculated with this
method represents only a minimum contribution, because (Lp 1 iQF)/Lt provides data
only on the contribution of lithic fragments.
Quartz and feldspar grains were not included
in tabulations of plutonic grains, because their
provenance remains in many cases unclear.
Because many monocrystalline grains are independent of lithic fragments, but some were
surely derived from plutonic rocks, it is not
possible to calculate the true total contributions of detritus from plutonic rocks.
Ternary Diagrams
The ternary diagrams of Figure 4 display
the detrital compositions of sandstones in
terms of their populations of lithic fragments
and mafic mineral grains. The [(Lp 1 iQF)/
Lt]–Lvm–(rC 1 Lsm) diagram shows modal
values of plutonic, volcanic, and sedimentary
rock fragments. Radiolarian chert grains are
included in tabulations of lithic grains of sedimentary origin. This diagram demonstrates
that the lithic grain fraction in all analyzed
samples is strongly dominated by volcanic
Figure 1. Geologic map of northwestern Costa Rica showing locations of studied Cretaceous forearc sandstone suites: La Tigra, Quebrada
Pilas, and Calle Codornices. Place names discussed in text are also included. From Calvo and Bolz (1994). Inset map shows the regional
tectonic framework of the study area (MPF—Motagua-Polochic Fault; MAT—Middle America Trench; ND—Nicaraguan Depression;
PFZ—Panama Fracture Zone).
N
Geological Society of America Bulletin, July 2003
835
C. CALVO
Figure 2. Photomicrographs of ophiolitic detritus found in Cretaceous forearc sandstones from northwestern Costa Rica, including (A–
D) plutonic fragments and (E and F) cherty rock fragments. (A) Lithic fragment composed of monocrystalline quartz (Qm) and micrographic intergrowth of quartz and albite (iQF); albite at extinction. Irregular grain outline (right) is interpreted to be a relict intercrystalline contact. Calle Codornices sandstone, cross-polarized light. (B) Uralitized pyroxene grain (urPx) showing cores of unaltered
pyroxene (Px). Calle Codornices sandstone, plane-polarized light. (C) Lithic fragment composed of altered plagioclase (P) and intergrowth of quartz and plagioclase exhibiting micrographic texture (iQF); quartz at extinction. Quebrada Pilas arenite, cross-polarized
light. (D) Outsized feldspar grain (F) of probably plutonic origin showing differential weathering and thin coatings of hematite. It is
composed of fresh albite and altered anorthite-rich zones, replaced by calcite and sericite minerals. Calle Codornices sandstone, planepolarized light. (E) Radiolarite fragment (Rf), bearing microfossil remains (radiolaria) filled with silica. La Tigra sandstone, crosspolarized light. (F) Radiolarian chert grains (rC) showing vestiges of radiolarian microfossils and hematitic clusters; tests of Sulcoperculina sp. (S), a late Senonian larger foraminifera species, also occur. Calle Codornices sandstone, plane-polarized light.
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Geological Society of America Bulletin, July 2003
CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA
Figure 3. QFL and Qp-Lvm-Lsm plots of mean framework values and their standard
deviations for analyzed Cretaceous sandstone suites from northwestern Costa Rica containing plutonic detritus. Provenance fields from Dickinson and Suczek (1979) and Dickinson (1985). Mean values and standard deviations are represented by distinctive symbols
and polygons, respectively. Magmatic arc provenance field on diagram QFL is divided
into dissected arc (P . V) and undissected arc segments (P , V) (P—plutonic detritus,
V—volcanic detritus); n—number of samples. Because plutonic fragments are accessory
constituents in cover sandstones, plutonic contributions cannot be reflected directly by the
primary framework parameters on both diagrams. Note that the QFL plot for the Quebrada Pilas sandstone suite within the P . V subfield, in particular, is caused by strong
contents of radiolarian chert fragments in this suite (Table DR1). This origin is demonstrated by a second plot (depicted by a dashed polygon) where these rock fragments are
added to the lithic grain populations L and Lsm. The detailed petrographic examination
reveals that the Quebrada Pilas sandstones have relatively low contents of identifiable
plutonic detritus. This example shows that the use of secondary parameters is indispensable to determine the real provenance relationships in cover sandstones. In general, all
three cover sandstone suites record volcanic sources derived from the Cretaceous arc
activity, in addition to an ophiolitic provenance derived from exposed plagiogranite bodies
and radiolarian chert units of the Nicoya Complex.
TABLE 2. MODAL PARAMETERS AND CLASSIFICATION OF FRAMEWORK GRAINS
Primary parameters (after Dickinson and Suczek, 1979)
Q 5 total quartz grains
F 5 total feldspar grains
Lt 5 total lithic fragments (including polycrystalline grains, Qp)
Lvm 5 volcanic and metavolcanic rock fragments
Lsm 5 sedimentary and metasedimentary rock fragments
Secondary parameters sensitive to plutonic provenance (defined in this study)
rC 5 radiolarian chert grains
Lp 5 plutonic lithic fragments exhibiting granular and subophitic textures, composed of plagioclase, quartz,
and pyroxene
iQF 5 grains showing micrographic intergrowth of quartz and plagioclase feldspar
Hbl 5 hornblende grains
Px 5 total pyroxene grains
Bt 5 biotite grains
Ratios:
uralPx/Px, where uralPx 5 uralitized pyroxene grains
(Lp 1 iQF)/Lt, where (Lp 1 iQF) 5 total lithic grains demonstrably of plutonic origin
fragments. Sandstone suites of the Rivas Formation display, however, divergent trends.
Sandstones from the La Tigra suite show significant values of plutonic and volcanic fragments, very similar to those for Calle Codornices arenites. Arenites from Quebrada Pilas
are a particular petrofacies, because they are
characterized by extraordinarily high values of
sedimentary lithic fragments and especially of
radiolarian chert grains. In some samples,
these dominate the lithic grain fraction. The
predominance of radiolarian chert materials in
this forearc suite is interpreted to suggest a
subduction-complex provenance, in addition
Geological Society of America Bulletin, July 2003
to a magmatic arc source (Calvo, 1998). On
the QFL diagram of Figure 3, in particular,
such a compositional trend is expressed by a
mean plot within the subfield for dissected-arc
provenance. In contrast, identifiable plutonic
detritus in these sandstones occurs only in
trace amounts.
The Bt-Hbl-Px diagram of Figure 4 displays modal values of the mafic mineral grain
population. Two main types of sandstones
can be distinguished: pyroxene- and pyroxenehornblende–bearing sandstones. Most of these
samples contain clinopyroxene and green
hornblende. Arenites from La Tigra are predominantly pyroxene-rich, whereas in sandstones from Quebrada Pilas, hornblende dominates. The Sulcoperculina-bearing sandstones
from Quebrada Pilas are extraordinarily rich
in hornblende, which accounts for 87% to
93% of the mafic mineral grain fraction. One
volcaniclastic sandstone sample from La Tigra
contains a similar high abundance of hornblende. Arenites from Calle Codornices contain exclusively clinopyroxene, partly uralitized, and are quite similar to most of La Tigra
sandstones. The enrichment of detrital pyroxene grains is strongly related to significant
contents of plutonic fragments within the lithic grain fraction (Fig. 4). Biotite grains are
rare in most sandstones. Only a few samples
from Quebrada Pilas and one sample from
Calle Codornices contain trace amounts of biotite fragments.
Plutonic Lithic Fragments vs. Pyroxene
Grains
The (Lp 1 iQF)/Lt vs. uralPx/Px diagram
shows that abundance of uralitized pyroxene
grains can be correlated with increasing
amounts of plutonic rock fragments (Fig. 5).
In sandstone samples from Calle Codornices
and La Tigra that contain .4% of identifiable
plutonic lithic fragments, these two detrital
parameters increase in parallel. This observation allows the following conclusions. First, it
indicates that uralitized pyroxene grains also
are derived from eroded intrusive rocks. Second, because both uralitized pyroxene and intergrowths of albite and quartz are characteristic mineral phases and textures of
plagiogranites (Wildberg, 1984), the strong
correlation between these parameters records
a plutonic provenance, probably resulting
from erosion of plagiogranitic bodies. It also
documents the predominantly intermediate
composition of igneous source rocks (Wildberg, 1987; Sinton et al., 1997).
This relationship can also be recognized indirectly, by comparing modal values of rock
fragments with those of mafic mineral grains
837
C. CALVO
(Table 3; Fig. 4). Note that significant values
of plutonic lithic fragments only occur in
those sandstones that contain a mafic mineral
grain population strongly dominated by pyroxene grains. This statement is true for both
La Tigra and Calle Codornices sandstone
suites. In samples from Calle Codornices, plutonic contributions are, in part, documented by
primary detrital modes (Table DR1). Some
modal QmPK values fall in Dickinson’s
(1985) volcanoplutonic suite of circum-Pacific
sandstones (Calvo, 1998).
DISCUSSION
Source Rocks
The stratigraphic context and framework
grain compositions of the analyzed sandstones
indicate that the source of plutonic material
likely lies within the underlying Nicoya Complex. Plutonic rocks occur in the lower part of
the ophiolitic sequence, where they generally
have intruded massive basaltic flows (Kuijpers, 1979; Wildberg, 1987). The transgressive sandstones from Calle Codornices provide unequivocal sedimentologic evidence for
this interpretation, because they were deposited directly on exposed basement. Thus, noncarbonate detritus reworked during the transgression was derived from basement rocks
and/or from volcanic eruptions. The composition of framework grains that include radiolarian mudstones and cherts, pelagic limestones, and basalts records basement source
lithologies that are all found in the ophiolitic
sequence. Nonophiolitic grains are biogenic
carbonate fragments and volcanic grains, principally andesitic grains and ejecta fragments
including glass shards and fresh individual
crystals. On the other hand, the composition
of plutonic fragments clearly indicates that the
dominant plutonic source was basic to intermediate in composition. Conglomerate deposits within the La Tigra sequence containing
gabbro and dolerite clasts support this interpretation. Plagiogranites in the Nicoya Complex constitute a likely specific source rock of
plutonic coarse-grained lithic fragments and
related pyroxene grains.
Plagiogranitic Source Rocks
According to Wildberg (1987), plagiogranites on the Nicoya Peninsula occur within intrusions of isotropic gabbros and dolerites.
They form dikes up to several meters in thickness and intrusive bodies of considerable size
with exposures several hundred meters across.
On the basis of the extent of outcrops, plagiogranites probably constitute no more than 5
838
Figure 4. Ternary diagrams displaying the lithic and mafic mineral grain populations of
forearc sandstones from the El Viejo and Rivas Formations (upper Campanian–
Maastrichtian). Mean values indicated by distinctive symbols; n—number of samples.
vol% of the Nicoya Complex. Plagiogranites
are composed of plagioclase, quartz, pyroxene, opaque minerals, apatite, and zircon.
Most of them display subhedral granular textures. Graphic intergrowth of plagioclase and
quartz is a further typical feature (Figs. 6A,
6C). Secondary amphibole resulting from
complete or partial uralitization of clinopyroxene is often present (Fig. 6C). Gabbros, dolerites, and diorites also contain accessory mineral phases of uralitized pyroxene and
micrographic intergrowths (Kussmaul, 1980)
(Fig. 6D). However, the detrital abundance of
micrographic grains of up to 9% of total lithic
Geological Society of America Bulletin, July 2003
fragments in some sandstone samples requires
igneous source rocks to contain abundant micrographic intergrowth textures, rather than
only accessory amounts. Such sources are the
plagiogranites. Kuijpers (1979) called attention to the abundance of micrographic intergrowths of albite and quartz in some plagiogranites of the Nicoya Complex (e.g., Playa
El Ocotal; Fig. 1). Petrographic examination
of these rocks confirms a wide occurrence of
micrographic textures with intergrowths of up
to 10 mm across, displaying differential
weathering recognizable in plane-polarized
light (Fig. 6A).
CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA
entiated intrusive rocks displaying a mature
island-arc affinity, previously detected in this
region by Wildberg (1984).
Provenance Areas
Figure 5. Correspondence of proportion of lithic fragments that are plutonic and proportion of clinopyroxene grains that are uralitized in pyroxene-bearing sandstones from the
El Viejo and Rivas Formations (upper Campanian–Maastrichtian); n—number of
samples.
TABLE 3. RECALCULATED MODAL VALUES OF DETRITAL PARAMETERS FOR 20 FOREARC
SANDSTONES OF EL VIEJO AND RIVAS FORMATIONS (UPPER CRETACEOUS) FROM
NORTHWESTERN COSTA RICA
Lithic fragments†
Sample
Lvm
Lp 1
iQF
Calle Codornices sandstones
BB-14/1
81
6
BB-14/2
81
5
BB-14/4
80
5
BB-1414a
86
6
BB-14/7
78
9
La Tigra sandstones
LT-1/1
86
8
LT-1/4
87
5
LT-1/5
80
6
LT-2/2
94
1
LT-2/6
83
3
LT-4/1
83
,0.5
LT-4/1a
89
1
Quebrada Pilas sandstones
BB-4/1b
43
3
BB-4/2b
49
2
BB-4/5b
56
0
BB-4/1a
49
,0.5
BB-4/2a
42
,0.5
BB-4/5a
47
1
BB-4/6b
70
0
BB-4/6a
56
2
Mafic mineral grains†‡
Ratios†§
rC 1 Lsm
Px
Hbl
Bt
(Lp 1
iQF)/Lt
uralPx/Px
13
14
15
8
13
96
100
100
100
100
2
0
0
0
0
2
0
0
0
0
5.5
4.4
5.0
5.1
9.0
9
3
5
12
25
6
8
14
5
14
17
10
100
97
100
21
100
67
100
0
0
0
79
0
33
0
0
3
0
0
0
0
0
8.2
5.6
6.0
0.9
3.4
0.3
0.6
22
15
8
0
0
0
0
54
49
44
51
58
52
30
42
33
53
33
40
63
31
5
13
62
47
67
57
37
54
93
87
5
0
0
3
0
15
2
0
2.2
0.9
0
0.3
0.5
1.2
0
2.0
0
0
0
0
0
0
0
0
Values in percent plotted on diagrams of Figure 4 and Figure 5.
Includes independent monomineralic grains only.
Values for uralPx/Px based on additional 100 point counts for total pyroxene.
The geographic distribution of the sandstone suites records very different provenance
areas of plutonic detritus in northwestern Costa Rica. On the other hand, the common occurrence of angular, coarse sand-sized plutonic
particles requires limited transport and deposition near the source area. The sandstone
suite from La Tigra, located in the eastern Nicoya Peninsula, illustrates the relationship between the source area and site of deposition,
because this region exposes several intrusive
bodies. In the Quebrada Cuajiniquil, near the
locality of La Tigra where the clastic sequence
is exposed, Protti (1981) found a plagiogranitic sill. This rock obviously represents a potential source of plutonic detritus present in
the Cretaceous turbidite sandstones. In this
case, the distance from source rock to site of
deposition is only a few kilometers (Fig. 1).
Similarly, sandstones from Calle Codornices
and Quebrada Pilas record source areas situated on the northeastern margin of the Gulf of
Nicoya (Fig. 1). The intrusive rocks of Cerro
Barbudal, located 2.5 and 10 km from the outcrops of Quebrada Pilas and Calle Codornices,
respectively, are considered to be possible
sources. These intrusive rocks contain accessory mineral phases including uralitized clinopyroxene as well as myrmekitic and micrographic intergrowths of plagioclase and quartz
(Calvo, 1998). Trace amounts of plutonic lithic fragments are also present in Maastrichtian
bioclastic limestones within the rudistid reef
facies of Cerro Barbudal (Fig. 1). The limestones rest unconformably on eroded basaltic
flows laterally penetrated by the intrusions.
†
‡
Detrital Dispersal Patterns
§
Only very minor amounts of both potassium feldspar and biotite grains in analyzed
sandstones argue against granitic source rocks.
This inference is consistent with the fact that
no older rocks of continental affinity have
been identified in southern Central America
(Lundberg, 1991). The predominance of sodium-rich plagioclase feldspar within micrographic grains and the strong correlation between uralitized pyroxene grains and plutonic
lithic fragments (Fig. 5) also require a plutonic
source rock of basic to intermediate composition, preferably a plagiogranitic source.
Geochemical studies of potential plagiogranitic sources (Wildberg, 1987; Sinton et al.,
1997) suggest that plutonic rock fragments in
cover sandstones and megabreccia deposits on
Santa Elena Peninsula (Tournon and Azéma,
1980) were derived from intrusive rocks of
both island arc– and oceanic plateau–related
origin. Of particular interest is the occurrence
of arc-derived plutonic grains containing potassium feldspar in Pliocene sandstones from
the Montezuma Formation of the southern Nicoya Peninsula (Lundberg, 1991) (Fig. 1).
They probably record the erosion of differ-
Geological Society of America Bulletin, July 2003
Sandstone suites from northwestern Costa
Rica record the dispersion of plutonic rock
fragments into both shallow- and deep-water
environments during Late Cretaceous time.
However, sandstones deposited in the two different environments exhibit very divergent
dispersal patterns. The detrital dispersion appears to have been controlled mainly by a
combination of location and areal extent of
exposed source rocks, sedimentary environment, and relative sea-level changes.
In the shallow-water clastic rocks (Calle
Codornices sandstones) and related carbonate
reef deposits of the El Viejo Formation, plutonic detritus commonly appears in lithofacies
839
C. CALVO
Figure 6. Photomicrographs of representative intrusive rocks from the Nicoya ophiolite complex. (A) Plagiogranite exhibiting characteristic granophyric texture. The radiating intergrowth of quartz and albite (iQF) is arranged about a euhedral plagioclase crystal (P);
albite at extinction. Rock sample from Playa El Ocotal, cross-polarized light. (B) Dolerite with subhedral texture consisting mainly of
plagioclase laths and clinopyroxene crystals. Rock sample from Cerro Barbudal, plane-polarized light. (C) Plagiogranite exhibiting
graphic intergrowth of quartz and albite (at the right of the photograph), clinopyroxene (Px), partly uralitized (urPx), and plagioclase
phenocryst (P). Rock sample from Playa El Ocotal, cross-polarized light. (D) Accessory micrographic intergrowth of plagioclase and
quartz (iQF), albite at extinction, and zoned plagioclase crystals (P) in dolerite from Cerro Barbudal, cross-polarized light.
at the base of the sequence. Significant concentrations are found in clastic deposits that
immediately overlie the basement rocks (Fig.
7). Up section, the relative content of plutonic
detritus rapidly decreases in the reef framework and in grainstone facies overlying basal
strata. In terms of sequence stratigraphy, these
facies represent deposits of a transgressive
systems tract. Moreover, it is clear from the
presence of both plutonic detritus and weathered basaltic rocks that the unconformity at
the base of the Calle Codornices transgressive
sequence represents a subaerial erosional surface, a type 1 unconformity following the
classification of Posamentier and Vail (1988).
840
Transgressive systems tracts are deposited
during rapid rises in relative sea level when
little sediment is delivered to the shelf (Posamentier and Vail, 1988). This scenario coincides with the accumulation of basementderived detritus at the base of the Calle
Codornices sequence, proximal to the source
area, indicating moderate reworking during
the transgression. These observations suggest
that plutonic detritus was deposited in shallowwater environments during rapid marine incursions on exposed and previously emergent
source terranes of the Nicoya Complex.
In comparison with the shallow-water sequence, the slope sequences of La Tigra and
Geological Society of America Bulletin, July 2003
Quebrada Pilas of the Rivas Formation (Table
1) have an extensive vertical distribution of
plutonic detritus, but in generally lower concentrations. Identifiable plutonic fragments
commonly appear in levels of these sequences
that contain medium to coarse-grained channel
sandstones (Fig. 7). Texturally, they comprise
well-sorted, graded and massive sandstones
with grain support and without or little matrix
contents, showing erosional basal contacts.
These sedimentologic features indicate that
plutonic detritus in this case was redistributed
into the deep-water environment by turbidity
currents and/or sandy debris flows. Typical
fining-upward trends in both sequences sug-
CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA
gest, moreover, a grain transport via submarine channels, probably during periods of relative sea-level lowstands.
Unroofing History of the Ophiolite
Sequence and Tectonic Implications
Basaltic sandstones directly overlying the
Nicoya Complex were derived from eroded
basaltic rocks (massive and pillowed flows).
Cover sandstones bearing plutonic detritus record the erosion of deeper levels of the ophiolitic sequence. Biostratigraphic age determinations of sandstones based on their
foraminiferal assemblages indicate that the
earliest unroofing of these levels took place
during late Campanian and Maastrichtian time
(Fig. 8). This finding implies that intrusive
bodies of the Nicoya Complex must have intruded before the late Campanian (ca. 75–71
Ma, Gradstein et al., 1994). In fact, radiometric ages of two intrusive rocks (a gabbro and
a plagiogranite) from the Nicoya Peninsula as
determined by the 40Ar-39Ar method indicate
that intrusive magmatic activity occurred at
ca. 84 Ma (Sinton et al., 1997). The unroofing
of plutonic rocks probably required long periods of subaerial exposure and concomitant
denudation of the forearc ophiolite complex
during evolution of the Costa Rican orogen,
as suggested by a marked Campanian unconformity. Relationships between crystallization
and earliest erosion ages of Nicoya Complex
intrusions suggest that unroofing had been effectively accomplished ;9 m.y. after the ca.
84 Ma intrusive activity on northern Nicoya
Peninsula (Fig. 8). On the other hand, geochemical studies of these plutonic rocks (e.g.,
Wildberg, 1987) suggest that plutonic detritus originated, in part, from intermediate arcrelated intrusions. Campanian sandstones and
tuff interbeds of the Sabana Grande Formation
in southern Nicoya Peninsula (Lundberg,
1982), as well as Albian to Campanian pyroclastic rocks of the Loma Chumico Formation
occurring within the Nicoya Complex (Calvo
and Bolz, 1994), indicate that the arc was active at the time of intrusive activity.
Like late Senonian carbonate reefs and platforms, which grew on both mafic and ultramafic substrates in northwestern Costa Rica
(Ulloa, 1977; Seyfried and Sprechmann, 1985;
Calvo, 1987), plutonic and radiolarian chert
detritus in cover sandstones points to strong
uplift of the Costa Rican orogen, which included the formation of emergent terranes of
ophiolitic basement (Fig. 9). Therefore both
grain types in these forearc sandstones can be
considered as detrital paleotectonic indicators
of the regional Campanian unconformity—the
Figure 7. Model suggested here for the dispersion of plutonic detritus in the marine environment during relative sea-level changes, based on detrital dispersal patterns observed
in (A) the neritic sequence of Calle Codornices, and (B) the slope sequences of La Tigra
and Quebrada Pilas. Dispersion of plutonic detritus succeeded periods of significant uplift
and subaerial erosion that likely cut deep into the basement sequence to expose the plutonic bodies (e.g., plagiogranites). U—Campanian unconformity representing an erosion
surface on exposed basement areas. See text for discussion.
Figure 8. Comparing crystallization and earliest erosion ages of intrusive rocks from the
Nicoya ophiolite complex of northwestern Costa Rica. Radiometric age determinations of
Nicoya Peninsula intrusive rocks from Sinton et al. (1997); the two rocks come from
outcrops located in the northwestern part of the peninsula. Biostratigraphic ages of sandstones bearing plutonic detritus established on the basis of their foraminiferal assemblages.
Time scale and planktic foraminiferal zones from Gradstein et al. (1994) and Erba et al.
(1995), respectively. See text for discussion.
most important Cretaceous unconformity in
southern Central America and one that marks
the boundary between the ophiolitic basement
rocks and their sedimentary cover strata. This
tectonic uplift is coeval with the onset of the
Laramide orogeny in the Late Cretaceous. In
terms of relative plate motions, the onset of
the Laramide orogeny at ca. 75 Ma coincides
with the beginning of rapid convergence
Geological Society of America Bulletin, July 2003
(.100 km/m.y.) of the Farallon plate with respect to North America (Engebretson et al.,
1985).
Two volcanic rock units of note are: (1) the
vesicular basaltic lavas intercalated within the
upper Campanian carbonate slope sequence of
Bahı́a Santa Elena, located in northern Santa
Elena Peninsula (Baumgartner et al., 1984),
and (2) the basaltic lavas and breccias inti-
841
C. CALVO
plagiogranites implies significant strong uplift
and subsequent deep erosion of the ophiolite
sequence. The Cretaceous megabreccia deposits containing plagiogranite boulders (Tournon
and Azéma, 1980) suggest a similar profile of
erosion on Santa Elena Peninsula. In contrast,
framework grains derived from shallow levels
of the Nicoya ophiolitic sequence principally
include basaltic and tachylite grains (partly
showing vesicular textures) as well as radiolarian mudstone, pelagic limestone, and tuffaceous lithic fragments. In general, the spectrum of ophiolitic grains in cover sandstones
points to an ophiolitic source-rock assemblage
that does not differ from that of the exposed
modern forearc basement (Fig. 10).
SUMMARY AND CONCLUSIONS
Figure 9. Cartoon depicting model of evolution for the Mesozoic ophiolite sequence of
Costa Rican arc orogen during the late Senonian, including (A) intrusive magmatic activity, probably arc-related, in the Santonian–early Campanian (at ca. 84 Ma, Sinton et al.,
1997), and (B) uplift and subsequent erosion in the late Campanian, resulting in unroofing
of ophiolitic intrusive bodies: dolerites, gabbros, and plagiogranites. Both episodes were
accompanied by andesitic arc and basic forearc volcanism. In late Campanian and Maastrichtian times, transgressive shallow-marine carbonate reef and platform deposits of the
El Viejo Formation prograded over exposed basement. (MAT—Middle America Trench).
mately commingled with hemipelagic limestones of Maastrichtian age, exposed on the
western coast of Nicoya Peninsula, between
Garza and Puerto Carrillo (Schmidt-Effing,
1979) (Fig. 1). These two volcanic units indicate that both uplift and unroofing of the
ophiolitic sequence in the late Senonian were
accompanied by mafic forearc volcanic activity (Fig. 9). These processes together attest to
coeval Cretaceous subduction and erosion,
probably related to rapid convergence of the
Farallon plate in southern Central America
also. In addition, the presence of alterite
grains, indicative of tropical weathering environments (Johnsson, 1990), in cover sandstones provides strong evidence for the residence of the Costa Rican arc in low latitudes
near the equator during Cretaceous time, as
constrained by paleomagnetic data (de Boer,
1979; Sick, 1989; Frisch et al., 1992).
Cretaceous Erosion Profile
Provenance relationships recorded by detrital compositions of cover sandstones allow the
842
reconstruction of a Cretaceous erosion profile
through the ophiolitic sequence (Fig. 10). On
the basis of the earliest assemblage of pseudorbitoidal foraminifera present in analyzed
samples, the late Campanian (ca. 75 Ma) is
assumed for this reconstruction. As can be deduced from framework grain compositions,
erosion at that time affected both the upper
and lower parts of the Nicoya ophiolite complex. Because intrusive rocks appear in the
lower part of the Nicoya Complex, plutonic
rock fragments in cover sandstones record the
unroofing of the Lower Nicoya Complex in
northwestern Costa Rica since at least ca. 75
Ma. Such unroofing is also well constrained
by the Maastrichtian cherty sandstones from
the Quebrada Pilas suite, whose detrital modes
(Q23F50L27 and Qp41Lvm46Lsm13; Table DR2)
plot close to Dickinson’s dissected-arc and
subduction-complex provenance fields, respectively (Calvo, 1998) (Fig. 3). Considering
the minimum depth of formation of 4 km observed for MORB-related intrusions (S. Foley,
2002, personal commun.), the unroofing of
Geological Society of America Bulletin, July 2003
Detrital plutonic grains, previously overlooked in earlier studies, comprise accessory
framework components of Cretaceous forearc
sandstones in northwestern Costa Rica. Integrated petrographic, sedimentologic, stratigraphic, and field evidence clearly indicate
that plutonic detritus was derived from eroded
intrusive rocks of the Nicoya ophiolite complex. Moreover, the plutonic detritus corresponds, compositionally and texturally, to
source rocks of basic and intermediate compositions. An important new petrologic result
is the parallel increase of plutonic lithic fragments [(Lp 1 iQF)/Lt] and uralitized pyroxene grains (uralPx/Px) identified in the framework population of pyroxene-bearing arenites.
This observation indicates that uralitized pyroxene grains probably also resulted from
eroded intrusive rocks; the observation also
reflects more clearly a predominantly intermediate composition of plutonic source rocks.
In particular, significant concentrations of lithic fragments exhibiting micrographic textures
and uralitized pyroxene grains are interpreted
to be predominantly derived from eroded plagiogranitic intrusions. This study demonstrates, moreover, that both newly introduced
parameters—i.e., (Lp 1 iQF)/Lt and uralPx/
Px—can be used as semiquantitative modal
parameters to determine affinity of source
rocks and detrital plutonic contributions in
sandstones derived from ophiolitic sequences.
Sedimentologically, dispersal patterns of
detrital sediment suggest that plutonic fragments, and all ophiolitic grains in general,
were deposited in shallow-water environments
during rapid marine transgressions on exposed
basement areas. The detritus was apparently
distributed into the slope and deep-water environments by turbidity currents and sandy debris flows, principally during relative sea-level
CRETACEOUS UNROOFING HISTORY OF A MESOZOIC OPHIOLITE SEQUENCE, COSTA RICA
Figure 10. Cartoon showing a hypothetical profile of erosion through the ophiolitic sequence of Nicoya Complex in the Late Cretaceous (at ca. 75 Ma), deduced from framework
compositions and biostratigraphic ages of forearc sandstones of the Rivas and El Viejo
Formations (upper Campanian–Maastrichtian). This ophiolitic source-rock assemblage is
very similar to that currently exposed in the forearc region of northwestern Costa Rica.
The erosion unroofed deeper levels of the sequence, where intrusions occur. The erosional
surface corresponds with the Campanian unconformity. No scale implied.
lowstands. Provenance areas were apparently
located on Nicoya Peninsula as well as in the
area of Cerro Barbudal, north of the Gulf of
Nicoya.
Tectonically, Cretaceous sandstone suites
containing plutonic detritus provide evidence
for the unroofing of deeper levels of the Mesozoic ophiolitic sequence of Costa Rica in late
Senonian time. Sandstone ages show that the
earliest erosion of Nicoya Complex intrusive
rocks began at least by late Campanian time
(ca. 75 Ma), ;9 m.y. after cessation of the
intrusive magmatic activity on northern Nicoya Peninsula. Geochemical signatures of
potential plutonic sources (Wildberg, 1987;
Sinton et al., 1997) suggest coeval erosion of
both island arc– and oceanic plateau–related
intrusive rocks. The unroofing of the ophiolitic sequence concomitant with the beginning of
neritic carbonate sedimentation records strong
uplift of the Costa Rican orogen, including the
formation of emergent ophiolitic basement areas in the forearc. This tectonic event marks
the onset of the Laramide orogeny in Late
Cretaceous time. In this context, plutonic and
associated radiolarian chert grains in Cretaceous cover sandstones are considered as detrital paleotectonic indicators of the prominent
Campanian unconformity separating the
ophiolitic basement from its sedimentary cover strata.
ACKNOWLEDGMENTS
I thank A. Bolz for determination of larger foraminifera in thin section and S. Kussmaul for providing representative samples of the plagiogranites
from Playa El Ocotal. M. Meschede and S. Foley
provided additional data on plutonic rocks. Reviews
by J. Mezger, A. Bolz, and H.-J. Gursky improved
the early manuscript version. I am also indebted to
R.J. Dorsey, N. Lundberg, and G.H. Girty for helpful reviews of the submitted manuscript.
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