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 seafl oor spreading covers about 75% of Earth’s surface but is diffi cult 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 fi eld. 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 devel-opment 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 seafl oor spreading could constrain when plate tectonics began on Earth.

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 differenti-ates there (fractional crystallization, host assimilation, cumulate pro-cesses, crystal/melt segregation mechanisms), to fi nally erupt on the seafl oor 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 sulfi de deposits. The faults root into the deeper crust as ductile and brittle-ductile 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 gran-ulites 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 fab-rics 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 ophio-lites so powerful, yet oh so challenging!

Ophiolites come in various styles, refl ecting variations of magma type, relative rates of magma delivery versus tectonic extension, and a fl uctu-ating 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

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–gab-bronorite); 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).

Because a given segment of crust typically requires many thousands of years to drift away from the axial melt-delivery zone, it experiences mul-tiple magmatic/deformation/hydrothermal events. As long as it is above the axial zone, new magma is constantly arriving from below to reju-venate 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 rigidifi cation between melt pulses. In contrast, higher and more constant melt delivery to fast-spreading ophiolites like Oman (Nicolas et al. 2003) may limit water penetra-tion 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 unde-formed 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.

Geochemical studies of oceanic basalts are much used to infer mantle composition and evolution, yet few oceanic-ridge basalts are in equi-librium 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 Canada490 rue de la Couronne, Québec, QC, Canada GIK 9A9 E-mail: JeanH.Bedard@RNCan-NRCan.gc.ca

FIGURE 1 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. ADAPTED FROM FIGURE 19 IN BÉDARD (1991)

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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 oce-anic 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

or static host to produce a sheeted sill archi-tecture (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 trapped-melt 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, gravitation-ally driven, top-down crystal sedimentation (Wager et al. 1960) may involve short-range homogeneous percolation followed by segrega-tion of melt into open conduits (Bédard 2014). In these heterogeneous media, processes may change rapidly on short timescales and spa-tial 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 assimi-lation by- products (FIG. 3; B édard 1991, 1993).

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.

FIGURE 2 Photomicrograph showing mortar texture in lower-crustal gabbroic rocks from near

the fi gure 1 photo. Note the development of neoblasts between the larger feldspar porphyroclasts, recording a phase of high-temperature subsolidus deformation. The width of the fi eld of view is about 5 mm.

REFERENCESAlt JC, Teagle DAH (2000)

Hydrothermal alteration of upper oceanic crust formed at a fast-spreading 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 intru-sion, hybridization, and assimi-lation in an ophiolitic magma chamber, western Newfoundland. Geology 21: 77-80

Bédard JH (1999) Petrogenesis of boninites from the Betts Cove Ophiolite, Newfoundland, Canada: Identifi cation 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

Bédard JH, Berclaz A, Hébert R, Varfalvy V (2000) Syntexis and the genesis of the oceanic crust. GSA Special Paper 349: 105-119

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, Scientifi c Results 118: 439-538

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

Kim J, Jacobi RD (2002) Boninites: characteristics and tectonic constraints, northeastern Appalachians. Physics and Chemistry of the Earth 27: 109-147

Koepke, J, Berndt J, Feig ST, Holtz F (2007) The formation of SiO2-rich melts within the deep oceanic crust

by hydrous partial melting of gab-bros. Contributions to Mineralogy and Petrology 153: 67-84

Korenaga J, Kelemen PB (1997) Origin of gabbro sills in the Moho transition zone of the Oman ophio-lite: Implications for magma trans-port in the oceanic lower crust. Journal of Geophysical Research: Solid Earth 102: 27729-27749

Lissenberg CJ, Bédard JH, van Staal CR (2004) The structure and geo-chemistry 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

Nicolas A, Poliakov A (2001) Melt migration and mechanical state in the lower crust of oceanic ridges. Terra Nova 13: 64-69

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

Nicolas A, Boudier F, France L (2009) Subsidence in magma chamber and the development of magmatic folia-tion 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 seafl oor 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 ophi-olitic belt. Tectonics 24: doi: 10.1029/2003TC00160

Wager LR, Brown GM, Wadsworth WJ (1960) Types of igneous cumu-lates. Journal of Petrology 1: 73-85

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.

FIGURE 3 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. ADAPTED FROM FIGURE 16 IN BÉDARD (1991)

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• Economic geology

• Geochemistry

• Geochronology

• Marine geology

• Mineralogy

• Paleontology

• Petrology

• Sedimentology

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