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Suprasubduction-Zone Ophiolites: Is There Really an Ophiolite Conundrum?

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Suprasubduction-Zone Ophiolites: Is There Really an Ophiolite Conundrum? The Geological Society of America Special Paper 438 2008 Suprasubduction-zone ophiolites: Is there really an ophiolite conundrum? Rodney V. Metcalf† Department of Geoscience, University of Nevada–Las Vegas, Las Vegas, Nevada 89154-4010, USA John W. Shervais‡ Department of Geology, Utah State University, Logan, Utah 84322-4505, USA ABSTRACT Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphi- cally, show evidence for synmagmatic extension, and contain lavas with geochemical characteristics of arc-volcanic rocks. They are now inferred to have formed by hinge retreat in the forearc of nascent or reconfi gured island arcs. Emplacement of these forearc assemblages onto the leading edge of partially subducted continental margins is a normal part of their evolution. A recent paper has challenged this interpretation. The authors assert that the “ophiolite conundrum” (seafl oor spreading shown by dike complexes versus arc geochemistry) can be resolved by a model called “historical contingency,” which holds that most ophiolites form at mid-ocean ridges that tap upper-mantle sources previously modifi ed by subduction. They support this model with examples of modern mid-ocean ridges where suprasubduction zone–like compo- sitions have been detected (e.g., ridge-trench triple junctions). The historical contingency model is fl awed for several reasons: (1) the major- and trace-element compositions of magmatic rocks in suprasubduction-zone ophio- lites strongly resemble rocks formed in primitive island-arc settings and exhibit distinct differences from rocks formed at mid-ocean-ridge spreading centers; (2) slab-infl uenced compositions reported from modern ridge-trench triple junctions and subduction reversals are subtle and/or do not compare favorably with either modern subduction zones or suprasubduction-zone ophiolites; (3) crystallization sequences, hydrous minerals, miarolitic cavities, and reaction textures in suprasubduction-zone ophiolites imply crystallization from magmas with high water activities, rather than mid-ocean-ridge systems; (4) models of whole Earth convection, subduction recycling, and ocean-island basalt isotopic compositions ignore the fact that these components represent the residue of slab melting, not the low fi eld strength element–enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; and (5) isotopic components indicative of mantle heterogeneities (related to subduc- †E-mail: [email protected]. ‡E-mail: [email protected]. Metcalf, R.V., and Shervais, J.W., 2008, Suprasubduction-zone ophiolites: Is there really an ophiolite conundrum?, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: Geological Society of America Special Paper 438, p. 191–222, doi: 10.1130/2008.2438(07). For per- mission to copy, contact [email protected]. ©2008 The Geological Society of America. All rights reserved. 191 192 Metcalf and Shervais tion recycling) are observed in modern mid-ocean-ridge basalts (MORB), but, in con- trast to the prediction of the historical contingency model, these basalts do not exhibit suprasubduction zone–like geochemistry. The formation of suprasubduction-zone ophiolites in the upper plate of subduction zones favors intact preservation either by obduction onto a passive continental margin, or by accretionary uplift above a sub- duction zone. Ophiolites characterized by lavas with MORB geochemistry are typi- cally disrupted and found as fragments in accretionary complexes (e.g., Franciscan), in contrast to suprasubduction-zone ophiolites. This must result from the fact that oceanic crust is unlikely to be obducted for mechanical reasons, but it may be pre- served where it is scraped off of the subducting slab. Keywords: ophiolite, suprasubduction zone, mid-ocean ridge, geochemistry mantle. INTRODUCTION Moores et al. (2000) challenged the suprasubduction interpre- tation of ophiolite genesis. These authors assert that the “ophio- Ophiolites are distinct assemblages of ultramafi c, mafi c, and lite conundrum” (seafl oor spreading shown by dike complexes felsic igneous rocks, commonly associated with siliceous pelagic versus arc geochemistry) can be resolved by a model called “his- sediments (cherts), that have long been recognized as important torical contingency,” which holds that most ophiolites are formed components of mountain belts worldwide (Steinmann, 1906; Hess, at mid-ocean ridges that tap upper-mantle sources previously 1955). In the 1960s, this assemblage was proposed to represent modifi ed by subduction. They support this model with examples oceanic crust formed at mid-oceanic spreading centers, a concept of subduction-zone reversal, which place oceanic spreading that became central to the new theory of plate tectonics (Gass, centers above lithosphere previously modifi ed by subduction 1968). A compelling aspect of this proposal was the recognition of (i.e., the Woodlark basin), with examples of modern mid-ocean sheeted dike complexes in some ophiolites that implied formation ridges where suprasubduction zone–like compositions have been by 100% extension (e.g., Troodos; Oman), consistent with the new detected (e.g., ridge-trench-trench triple junctions), with models concept of seafl oor spreading in ocean basins (Moores and Vine, of mantle convection that show recycling of oceanic lithosphere 1971). Dedicated campaigns of deep-ocean drilling, dredging, on grand scale, and with a discussion of the isotopic components and seismic-refraction surveys confi rmed the similarity of oceanic found in ocean-island basalts (OIBs) (Moores et al., 2000). crust to ophiolites, although there were differences in detail. As a Moores et al. (2000) also suggest that differences observed result, this paradigm became entrenched within the scientifi c com- in the structural preservation of ophiolites result from distinct munity—especially among those who did not work on ophiolites. spreading environments, not from their subsequent emplace- Suprasubduction-zone ophiolites have been recognized in the ment. Thus, ocean crust and ophiolites formed at slow spread- geologic record for over three decades (Miyashiro, 1973; Pearce ing centers are highly faulted and commonly have volcanic rocks et al., 1984; Shervais and Kimbrough, 1985). These ophiolites are juxtaposed against serpentine, whereas ocean crust and ophio- made up of plutonic rocks and lavas with the mineralogical and lites formed at fast spreading centers tend to be stratigraphically geochemical characteristics of arc-plutonic and arc-volcanic rocks, intact and lack the extreme structural attenuation found in slow and they are petrologically and chemically distinct from igneous spreading ocean crust (Moores et al., 2000). Examples of rocks formed at modern spreading centers in the major ocean slow spreading ophiolites would include those in the Western basins. In general, suprasubduction-zone ophiolites are intact Mediterranean (Apennines); examples of fast spreading ophio- structurally and stratigraphically and show evidence for nearly lites would include Troodos and Oman. 100% extension. Such ophiolites are now inferred to have formed We suggest that the historical contingency model is fl awed primarily by hinge retreat in the forearc of nascent or reconfi gured for several reasons: (1) the major- and trace-element composi- island arcs, a model derived from studies of Cenozoic subduc- tions of magmatic rocks in suprasubduction-zone ophiolites tion systems in the western Pacifi c (Fig. 1; Hawkins et al., 1984; strongly resemble rocks formed in primitive island-arc settings Stern and Bloomer, 1992; Bloomer et al., 1995; Hawkins, 2003). and exhibit distinct, consistent differences from rocks formed at Emplacement of these forearc assemblages onto the leading edge mid-ocean-ridge spreading centers; (2) slab-infl uenced composi- of partially subducted continental margins (Tethyan ophiolites) tions reported from modern ridge-trench triple junctions and sub- or exposure by accretionary uplift along an active plate margin duction reversals are subtle and/or do not compare favorably with (Cordilleran ophiolites) is a normal part of their evolution (e.g., either modern subduction zones or suprasubduction-zone ophio- Shervais, 2001). Several recent papers have discussed the develop- lites; (3) crystallization sequences, hydrous minerals (hornblende), ment of suprasubduction-zone ophiolite models, their genesis, and miarolitic cavities, and reaction textures in suprasubduction-zone tectonic implications, most notably Shervais (2001), Dilek (2003), ophiolites imply crystallization from magmas with high water Pearce (2003), Hawkins (2003), and Flower (2003). activities, rather than mid-ocean-ridge magmatic systems; (4) Suprasubduction-zone ophiolites: Is there really an ophiolite conundrum? 193 A oceanic crust the low fi eld strength element–enriched component found in active arc-volcanic suites and suprasubduction-zone ophiolites; LM LM and (6) the isotopic components are indicative of mantle hetero- geneities (related to subduction recycling) observed in modern AM AM mid-ocean-ridge basalts (MORBs), but, in contrast to the pre- B SSZ forearc spreading diction of the historical contingency model, these basalts do not exhibit suprasubduction zone–like geochemistry. In the following sections, we present a more comprehensive review of the historical contingency model and its implications,
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