Suprasubduction-zone ophiolites: Is there really an ophiolite conundrum?

Abstract

Suprasubduction-zone ophiolites have been recognized in the geologic record for over thirty years. These ophiolites are essentially intact structurally and stratigraphically, 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 compositions have been detected (e.g., ridge-trench triple junctions). The historical contingency model is fl awed for several reasons: (1) the majorand trace-element compositions of magmatic rocks in suprasubduction-zone ophiolites 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

Rock Magnetic, Petrography, and Geochemistry Studies of Lava at the Ijen Volcanic Complex (IVC), Banyuwangi, East Java, Indonesia

“…Based on Figure 2b, the magma characteristics in the study area are associated with an affinity series of moderate-K calc-alkaline to high-K calc-alkaline. Besides that, basalts from the study area have a titanium oxide (TiO 2 ) content less than 1.4, indicating volcanic rock that derived from a subduction system [38]. Handley et al [27] obtained the same results.…”

Lithogeochemistry of the meta-igneous units from Arroio Grande Ophiolitic Complex, southernmost Brazil

“…dolomites and dolomitic marls). In general, the meta-mafics have major element composition consistent with the average oceanic basalt composition of Metcalf and Shervais (2008). Average Cr, Ni, V, Co and Zr contents are consistent with gabbroic and basaltic compositions of Coleman (1977).…”

“…(J) Th-Ta-Hf/3 ternary diagram (modified from Wood et al 1981), N-MORB: normal MORB; E-MORB: enriched MORB; WPB: within-plate basalts; SSZ: supra-subduction zone. (K) N-MORB normalized (Sun & McDonough 1989) spidergram: average ocean island basalts (OIB), E-MORB and supra-subduction zone tholeiites (SSZT) from Metcalf and Shervais (2008). In the normal MORB (N-MORB) normalized spidergram (Fig.…”

Autochthonous v. accreted terrane development of continental margins: a revised in situ tectonic history of the Antarctic Peninsula

“…Similarly, on the Peninsula this magmatism records a period of lithospheric extension and consequent upwelling of fertile nonmetasomatized mantle during the resumption of subduction after a Late Jurassic pause (Macdonald et al 1999). This is supported stratigraphically by the presence of contemporaneous OxfordianKimmeridgian radiolarian cherts associated with pillow basalts and volcanic tuffs within the oldest LeMay Group outcrops (Burn 1984;Holdsworth & Nell 1992), a typical feature of suprasubduction-zone ophiolite sequences (Metcalf & Shervais 2008), and the appearance of volcanolithic material higher up the LeMay Group stratigraphy indicating a proximal mafic arc source (Willan 2003). A similar association of Late Jurassic to Early Cretaceous OIB, E-MORB and arc-like mafic dykes are recorded in the Central Domain in NW Palmer Land (Scarrow et al 1998) and the Eastern Domain in NE Palmer Land (Wever & Storey 1992), further supporting a shared tectonic history of the three terranes.…”