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PERSPECTIVE
OPHIOLITES: PERSPECTIVES FROM
FIELDWORK IN THE APPALACHIANS
Jean H. Bédard*
1811-5209/13/0010-087$2.50
DOI: 10.2113/gselements.10.2.87
I
love ophiolites. They contain a huge variety of rocks,
preserved and arrayed on a scale that makes every outcrop
a possible discovery site! Had I the option, I would spend the
rest of my career investigating the Bay of Islands Complex in
Newfoundland, where the rocks wait for someone who has the
patience to listen to their story.
Oceanic crust formed by seafloor spreading covers about 75% of Earth’s
surface but is difficult to access. Submersible expeditions and ocean
drilling yield valuable but expensive and necessarily limited views.
Ophiolites preserve oceanic lithosphere on land, which allows a more
complete perspective to be developed as the rocks can be mapped and
examined in the field. The Canadian Appalachians are richly endowed
with Ordovician ophiolites preserved by the closure of the Iapetus
Ocean. Research on these rocks not only helps us to understand how
Earth’s biggest magmatic system operates, but contributed to the development of plate tectonic theory by providing unambiguous evidence of
the existence of ocean basins destroyed by subduction. Since ophiolites
represent the only record of pre-180-million-year-old oceanic crust,
their presence in the deep past is eagerly sought, as recognition of
oceanic lithosphere formed by seafloor spreading could constrain when
plate tectonics began on Earth.
Metagabbro xenolith in a fresh, intrusive, boninitic wehrlite, Bay of
Islands Complex lower crust. The xenolith has corrugated edges,
suggesting magmatic resorption. The white and green veins in the xenolith were
originally prehnite and tremolite created by greenschist facies hydrothermal
alteration, but they are now composed of anorthite and diopside due to thermal
metamorphism. A DAPTED FROM FIGURE 19 IN BÉDARD (1991)
FIGURE 1
of crystallization (= dunite–troctolite–olivine gabbro), others formed
largely from boninitic melts (Kim and Jacobi 2002) with an olivine–
pyroxene–plagioclase crystallization sequence, yielding different
cumulate parageneses (= dunite–wehrlite/harzburgite–websterite–gabbronorite); boninitic melts are responsible for many chromitite deposits.
Church (1977) called these boninite-dominated ophiolites “oceanic
crust of Betts Cove type.” Mixed types are common: Thetford Mines
boninitic lavas overlie tholeiites (Schroetter et al. 2003), Bay of Islands
tholeiitic gabbros are underplated by boninitic cumulates (Bédard 1991,
1993), and Betts Cove tholeiitic lavas overlie boninites (Bédard 1999).
Ophiolites are challenging to work on because a huge diversity of
geological expertise is required. Every facet of igneous petrology and
geochemistry is needed to understand how magma is expelled from
its mantle source (melting reactions, melt-segregation physics), how
it passes through the crust (intrusive mechanisms), how it differentiates there (fractional crystallization, host assimilation, cumulate processes, crystal/melt segregation mechanisms), to fi nally erupt on the
seafloor where geochemists can sample and analyze the lavas so as to
better understand mantle and bulk-Earth evolution. But structural and
metamorphic geology are equally necessary, since ridge magmatism is
necessarily synkinematic. The brittle carapace is dissected by multiple
generations of faults, which act as conduits for circulating seawater
(Alt and Teagle 2000), locally generating economic Cu–Zn–Pb sulfide
deposits. The faults root into the deeper crust as ductile and brittleductile shear zones, which facilitate penetration of seawater into the
deep crust (Schroetter et al. 2003) but which also guide movement of
magma, providing opportunities for the assimilation and/or reheating
of hydrothermally altered host rocks (FIG.1; Bédard et al. 2000; Koepke
et al. 2007). The ductile plutonic crust of some ophiolites is plastically
deformed, generating textures that are more akin to metamorphic granulites than they are to continental layered-intrusion cumulates (FIG. 2;
Nicolas and Poliakov 2001). Obduction- and post-obduction-related
structures also need to be recognized and understood (e.g. Schroetter
et al. 2005), otherwise the geometry of pre-obduction rocks and fabrics cannot be reconstructed. Associated sedimentary rocks provide
age and paleogeographical constraints (e.g. Robertson 2002), and may
be the only record of nearby eroded terrains. In some ophiolites the
sedimentary rocks record obduction and erosion of the very ophiolite
upon which they were deposited, forming piggyback basins! It is the
integration of these different disciplines that makes the study of ophiolites so powerful, yet oh so challenging!
Because a given segment of crust typically requires many thousands of
years to drift away from the axial melt-delivery zone, it experiences multiple magmatic/deformation/hydrothermal events. As long as it is above
the axial zone, new magma is constantly arriving from below to rejuvenate the system, which is cooled by seawater circulation through the
brittle carapace. Ophiolites formed at slow-spreading ridges or through
episodic magmatism typically show intense and deeply penetrating
hydrothermal overprints (FIG. 1), since ephemeral melt delivery from
the mantle allows complete crustal rigidification between melt pulses.
In contrast, higher and more constant melt delivery to fast-spreading
ophiolites like Oman (Nicolas et al. 2003) may limit water penetration and keep parts of the lower crust at near-solidus temperatures for
much longer, until the ridge segment has drifted away from the axial
melt-delivery zone.
Where multiple intrusive and deformation events are preserved, older
metamorphosed relics adjoin less-perturbed younger intrusions. At
Annieopsquotch (central Newfoundland), the undeformed sheeted
gabbroic sills of the middle crust are the cumulate counterpart of the
lavas (Lissenberg et al. 2004), but are underlain by a heterogeneous
domain where strongly deformed older rocks, some with a boninitic
parentage, occur as basalt-metasomatized enclaves embedded in undeformed tholeiitic gabbros (Bédard 2014). The study of such complex
ophiolites becomes much like the study of an orogen, with relative
chronologies needing to be developed for the different intrusive and
deformation events.
Ophiolites come in various styles, reflecting variations of magma type,
relative rates of magma delivery versus tectonic extension, and a fluctuating and heterogeneous crustal rheology (e.g. Harper 1985). Although
many well-studied ophiolites formed from tholeiitic melts, yielding
cumulates with a typical olivine–plagioclase–clinopyroxene sequence
Geochemical studies of oceanic basalts are much used to infer mantle
composition and evolution, yet few oceanic-ridge basalts are in equilibrium with plausible mantle compositions, requiring a correction for
intracrustal differentiation. Ideal fractional crystallization is implicit in
most such inversions. The study of ophiolitic magma chambers provides
a sobering wake-up call here. The Bay of Islands plutonic crust records
pervasive reworking of older cumulates by younger melt pulses; this
results in the common development of hybrids between host gabbros
* Geological Survey of Canada
490 rue de la Couronne, Québec, QC, Canada GIK 9A9
E-mail: [email protected]
E LEMENTS
87
A PR IL 2014
PERSPECTIVE
Contact between a synkinematic wehrlite sill
(right) and foliated host gabbronorite (left)
at Bay of Islands. A pyroxenite reaction rim formed between
the intrusion and its host. This low-porosity pyroxenite
behaves brittly, fracturing and spalling off gabbro fold
noses to add pyroxene to the intrusive wehrlite (a viscous
liquid–crystal slurry), while simultaneously being
infolded into the gabbronorite to form pyroxene-rich
schlieren as a result of plastic deformation. So on a
scale of 5 cm, there are 3 different modes of
synchronous deformation: viscous, plastic, and
brittle! Black = olivine, green =
clinopyroxene, white = plagioclase.
A DAPTED FROM FIGURE 16 IN BÉDARD (1991)
FIGURE 3
Photomicrograph showing mortar texture
in lower-crustal gabbroic rocks from near
the figure 1 photo. Note the development of neoblasts
between the larger feldspar porphyroclasts, recording a
phase of high-temperature subsolidus deformation.
The width of the field of view is about 5 mm.
FIGURE 2
and intrusive olivine-saturated magmas and of
monomineralic reaction products (anorthosite,
chromitite, pyroxenite) at intrusive contacts,
as Bowen’s reaction series is put into reverse
(FIG. 3; Bédard 1991, 1993; Bédard et al. 2000).
The overall process resembles the operation
of a cationic exchange column, with melts
mixing, reacting, and equilibrating with older
host cumulates en route to the surface. This
implies that lava chemistry cannot be inverted
unambiguously since differentiation paths are
non-unique.
How mantle melts differentiate within the oceanic crust is much debated. Some believe that
ascending primary melt pools below the brittle
carapace and that cumulates formed from
this high-level axial melt lens founder like
a gabbroic glacier into a deforming substrate
to form the middle and lower crust (Nicolas
et al. 2009). Another class of models posits
distributed sill emplacement into a dynamic
REFERENCES
Alt JC, Teagle DAH (2000)
Hydrothermal alteration of upper
oceanic crust formed at a fastspreading ridge: mineral, chemical,
and isotopic evidence from ODP
Site 801. GSA Special Paper 349:
272-282
Bédard JH (1991) Cumulate recycling
and crustal evolution in the Bay
of Islands ophiolite. Journal of
Geology 99: 225-249
Bédard JH (1993) Oceanic crust as a
reactive fi lter: Synkinematic intrusion, hybridization, and assimilation in an ophiolitic magma
chamber, western Newfoundland.
Geology 21: 77-80
Bédard JH (1999) Petrogenesis of
boninites from the Betts Cove
Ophiolite, Newfoundland, Canada:
Identification of subducted source
components. Journal of Petrology
40: 1853-1889
Bédard JH (2014) Ophiolitic magma
chamber processes; a perspective
from the Canadian Appalachians.
In: Charlier B, Namur O, Latypov
R, Tegner C (eds) Layered
Intrusions. Springer Verlag, in press
or static host to produce a sheeted sill architecture (Bédard 1991, 1993; Korenaga and
Kelemen 1997; Lissenberg et al. 2004), which
then requires a mechanism to separate melt
from crystals so as to explain low trappedmelt fractions in the intrasill cumulates. One
proposal on how to do this is shear pumping
of ductile cumulates in deforming crustal
domains with plastic–viscous rheologies (Dick
et al. 1991). Pore melt from thick cumulate
piles produced by more classic, gravitationally driven, top-down crystal sedimentation
(Wager et al. 1960) may involve short-range
homogeneous percolation followed by segregation of melt into open conduits (Bédard 2014).
In these heterogeneous media, processes may
change rapidly on short timescales and spatial scales, as when viscous magma (crystal–
liquid slurry) is emplaced into a deforming
plastic host (subsolidus, slow strain rates) and
develops stiffer, brittle reaction rims as assimilation by-products (FIG. 3; Bédard 1991, 1993).
Jean Bédard has been
studying ophiolites since
1988, the year he joined
the Geological Survey of
Canada. He works on
magmatic differentiation
processes, trace elements,
large igneous provinces,
and Archean tectonics.
by hydrous partial melting of gabbros. Contributions to Mineralogy
and Petrology 153: 67-84
Bédard JH, Berclaz A, Hébert R,
Varfalvy V (2000) Syntexis and the
genesis of the oceanic crust. GSA
Special Paper 349: 105-119
Korenaga J, Kelemen PB (1997)
Origin of gabbro sills in the Moho
transition zone of the Oman ophiolite: Implications for magma transport in the oceanic lower crust.
Journal of Geophysical Research:
Solid Earth 102: 27729-27749
Church WR (1977) The ophiolites of
southern Quebec: oceanic crust of
Betts Cove type. Canadian Journal
of Earth Sciences 14: 1668-1673
Dick HJB, Meyer P, Bloomer S, Kirby
S, Stakes D, Mauwer C (1991)
Lithostratigraphic evolution in an
in situ section of oceanic layer 3.
In: von Herzen RP, Robinson PT et
al. (eds) Proceedings of the Ocean
Drilling Program, Scientific Results
118: 439-538
Lissenberg CJ, Bédard JH, van Staal
CR (2004) The structure and geochemistry of the gabbro zone of
the Annieopsquotch ophiolite,
Newfoundland: implications for
lower crustal accretion at spreading
ridges. Earth and Planetary Science
Letters 229: 105-123
Harper GD (1985) Tectonics of slow
spreading mid-ocean ridges and
consequences of a variable depth
to the brittle/ductile transition.
Tectonics 4: 395-409
Nicolas A, Poliakov A (2001) Melt
migration and mechanical state in
the lower crust of oceanic ridges.
Terra Nova 13: 64-69
Kim J, Jacobi RD (2002) Boninites:
characteristics and tectonic
constraints, northeastern
Appalachians. Physics and
Chemistry of the Earth 27: 109-147
Nicolas A, Mainprice D, Boudier F
(2003) High-temperature seawater
circulation throughout crust of
oceanic ridges: A model derived
from the Oman ophiolites. Journal
of Geophysical Research 108(B8):
doi 10.1029/2002JB002094
Koepke, J, Berndt J, Feig ST, Holtz F
(2007) The formation of SiO2 -rich
melts within the deep oceanic crust
E LEMENTS
In the lower ophiolite crust, boundaries
between igneous, metamorphic, and structural
geology necessarily blur, and cross-fertilization
between disciplines is the name of the game.
The variety and dynamic nature of Earth
materials is what makes ophiolites such superb
natural laboratories. They will undoubtedly
yield many more important and surprising
discoveries about the processes by which
oceanic lithosphere is formed, deformed,
and preserved.
88
Nicolas A, Boudier F, France L (2009)
Subsidence in magma chamber and
the development of magmatic foliation in Oman ophiolite gabbros.
Earth and Planetary Science Letters
284: 76-87
Robertson AHF (2002) Overview of
the genesis and emplacement of
Mesozoic ophiolites in the Eastern
Mediterranean Tethyan region.
Lithos 65: 1-67
Schroetter J-M, Pagé P, Bédard JH,
Tremblay A, Bécu V (2003) Forearc
extension and seafloor spreading
in the Thetford Mines Ophiolite
complex. Geological Society Special
Publication 218: 231-251
Schroetter J-M, Bédard JH, Tremblay
A (2005) Structural evolution of
the Thetford Mines Ophiolitic
Complex, Canada: Implications
for the southern Québec ophiolitic belt. Tectonics 24: doi:
10.1029/2003TC00160
Wager LR, Brown GM, Wadsworth
WJ (1960) Types of igneous cumulates. Journal of Petrology 1: 73-85
A PR IL 2014
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