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Evidence for a Widespread Tethyan Upper Mantle with Indian-Ocean-Type Isotopic Characteristics |
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| Authors: | ZHANG S-Q; MAHONEY J J; MO X-X; GHAZI A M; MILANI L; CRAWFORD A J; GUO T-Y; ZHAO Z-D |
| Institution: | 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 |
| Abstract: | The mantle sources of Tethyan basalts and gabbros from Iran,Tibet, the eastern Himalayas, the seafloor off Australia, andpossibly Albania were isotopically similar to those of present-dayIndian Ocean ridges and hotspots. Alteration-resistant incompatibleelement compositions of many samples resemble those of ocean-ridgebasalts, although ocean-island-like compositions are also present.Indian-Ocean-type mantle was widespread beneath the Neotethysin the Jurassic and Early Cretaceous, and present beneath atleast 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 theTethyan rocks may have formed in intra-oceanic back-arcs orfore-arcs, contamination of the asthenosphere by material subductedshortly before magmatism cannot be a general explanation fortheir Indian-Ocean-ridge-like low-206Pb/204Pb signatures. Supplyof low-206Pb/204Pb material to the asthenosphere via plumesis not supported by either present-day Indian Ocean hotspotsor the ocean-island-like Tethyan rocks. Old continental lowercrust or lithospheric mantle, including accreted, little-dehydratedmarine sedimentary material, provides a potential low-206Pb/204Pbreservoir only if sufficient amounts of such material can beintroduced into the asthenosphere over time. Anciently subductedmarine sediment is a possible low-206Pb/204Pb source only ifthe large increase of U/Pb that occurs during subduction-relateddewatering is somehow avoided. Fluxing of low-U/Pb fluids directlyinto the asthenosphere during ancient dewatering and introductionof ancient pyroxenitic lower-crustal restite or basaltic lower-arccrust into the asthenosphere provide two other means of creatingTethyan–Indian Ocean mantle, but these mechanisms, too,have potentially significant problems. KEY WORDS: Indian Ocean; mantle geochemical domains; ophiolites; Tethyan Ocean |
| Keywords: | : Indian Ocean mantle geochemical domains ophiolites Tethyan Ocean |
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