Journal of Petrology Advance Access originally published online on January 21, 2005
Journal of Petrology 2005 46(4):829-858; doi:10.1093/petrology/egi002
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Evidence for a Widespread Tethyan Upper Mantle with Indian-Ocean-Type Isotopic Characteristics
1 SCHOOL OF OCEAN AND EARTH SCIENCE AND TECHNOLOGY, UNIVERSITY OF HAWAII, HONOLULU, HI 96822, USA
2 CHINA UNIVERSITY OF GEOSCIENCES, BEIJING 100083, P. R. CHINA
3 GEOLOGY DEPARTMENT, GEORGIA STATE UNIVERSITY, ATLANTA, GA 30303, USA
4 INSTITUTE OF MINERALOGY, UNIVERSITY OF FERRARA, 44100 FERRARA, ITALY
5 CENTRE FOR ORE DEPOSIT STUDIES, UNIVERSITY OF TASMANIA, HOBART, TAS. 7001, AUSTRALIA
The mantle sources of Tethyan basalts and gabbros from Iran, Tibet, the eastern Himalayas, the seafloor off Australia, and possibly Albania were isotopically similar to those of present-day Indian Ocean ridges and hotspots. Alteration-resistant incompatible element compositions of many samples resemble those of ocean-ridge basalts, although ocean-island-like compositions are also present. Indian-Ocean-type mantle was widespread beneath the Neotethys in the Jurassic and Early Cretaceous, and present beneath at least parts of the Paleotethys as long ago as the Early Carboniferous. The mantle beneath the Indian Ocean today thus may be largely ‘inherited’ Tethyan mantle. Although some of the Tethyan rocks may have formed in intra-oceanic back-arcs or fore-arcs, contamination of the asthenosphere by material subducted shortly before magmatism cannot be a general explanation for their Indian-Ocean-ridge-like low-206Pb/204Pb signatures. Supply of low-206Pb/204Pb material to the asthenosphere via plumes is not supported by either present-day Indian Ocean hotspots or the ocean-island-like Tethyan rocks. Old continental lower crust or lithospheric mantle, including accreted, little-dehydrated marine sedimentary material, provides a potential low-206Pb/204Pb reservoir only if sufficient amounts of such material can be introduced into the asthenosphere over time. Anciently subducted marine sediment is a possible low-206Pb/204Pb source only if the large increase of U/Pb that occurs during subduction-related dewatering is somehow avoided. Fluxing of low-U/Pb fluids directly into the asthenosphere during ancient dewatering and introduction of ancient pyroxenitic lower-crustal restite or basaltic lower-arc crust into the asthenosphere provide two other means of creating Tethyan–Indian Ocean mantle, but these mechanisms, too, have potentially significant problems.
KEY WORDS: Indian Ocean; mantle geochemical domains; ophiolites; Tethyan Ocean
* Corresponding author. Telephone: 808-956-8705. Fax: 808-956-5512. Email: jmahoney{at}hawaii.edu
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