SUPRA-SUBDUCTION ZONE OPHIOLITES ALONG THE APPALACHIAN-CALEDONIDE OROGENIC BELT

The spontaneous subduction initiation model of Whattam & Stern (2011) predicts that subduction initiation should produce a MORB-like forearc section that, with time and development of subduction, is overprinted first by boninites and then by calc-alkaline igneous rocks. Drilling and diving expeditions in the Izu-Bonin-Mariana (IBM) forearc have revealed a ‘supra-subduction zone’ (SSZ) chemostratigraphy that is consistent with the spontaneous subduction initiation model hypothesis, and which has been associated with subduction initiation in the Western Pacific at c. 51 Ma (Ishizuki et al., 2011). Some ophiolites (e.g., Oman, Betts Cove) have similar chemostratigraphy to the IBM forearc and have also been interpreted as SSZ sections that formed during subduction initiation (Dilek & Furnes, 2011).

Magmatic compositions from subduction initiation show increasing SiO₂and MgO and decreasing TiO₂ as lavas evolve from MORB-like forearc basalts to calc-alkaline volcanic arc lavas. Additionally, trace element patterns vary due to source mantle modification with evolution of subduction processes. Decompression melting occurs during initial extension, leading to a depletion of light rare earth elements (LREEs) and a lack of high field strength elements (HFSEs). Hydrous fluids released from the subducting slab modify the mantle wedge resulting in enriched LREEs and large ion lithophile elements (LILEs) relative to HSFEs. As the mantle source becomes progressively more depleted, geochemical characteristics will record a pattern of increasing HFSE depletion and LILE enrichment.

Using the chemostratigraphic sequence obtained from ‘subduction initiation’ SSZ ophiolites, we test ophiolites and mafic-ultramafic complexes (partial or hypothesized ophiolites) along the entire Appalachian–Caledonian orogenic system. With geochemical evidence to distinguish SSZ Appalachian–Caledonian ophiolites linked to subduction initiation, dating of these ophiolites may provide insights into the geodynamics of the Iapetus Ocean and earliest stages of its closure.

References

Whattam, SA & Stern, RJ, 2011. Contrib Mineral Petrol v. 162, 1031–1045.

Dilek, Y & Furnes, H, 2011. GSA Bull v. 123, 387–411.

Ishizuka, O. et al., 2011, Earth and Planetary Science Letters, v. 306, p. 229–240.