MORB-type rocks from the Paleo-Tethyan Mian-Lueyang northern ophiolite in the Qinling Mountains, central China:: implications for the source of the low 206Pb/204Pb and high 143Nd/144Nd mantle component in the Indian Ocean

Samples from a basalt and gabbro section of the Paleo-Tethyan (similar to350 Ma) Mian-Lue northern ophiolites (MLNO) in the Qinling Mountains of central China display sub-parallel and relatively smooth incompatible trace element-depleted patterns and have high E-Nd(350 Ma) (8.1-11.3) and low Pb-206/Pb-204((350 Ma)) (16.90-17.25). The MLNO basalts and gabbros are compositionally similar to normal mid-ocean ridge basalts (MORB), particularly to those from the Carlsberg Ridge and Indian Ocean Ridge Triple Junction. The basalts and gabbros also have high Delta7/4 and Delta8/4 isotopic values characteristic of the Dupal isotopic anomaly in the southern hemisphere. Although the MLNO is presently in the northern hemisphere, it was previously located within the southerly location of the Indian Ocean based on palcomagnetic data. Thus, assuming that the low Pb-206/Pb-204 ratio of the MLNO basalts and gabbros is not due to seawater alteration or continental contamination, the unique isotopic signature of both the Paleo-Tethyan oceanic igneous crust and the modern Indian MORB may have come from a very similar, if not identical mantle reservoir. This indicates that a portion of the modern Indian MORB mantle isotopic domain could have been in existence for at least similar to350 Ma. We propose that the low Pb-206/Pb-204 and high Nd-143/Nd-144 isotopic character of the MLNO basalts and gabbros as well as similar Indian MORB originated either from a low mu sub-domain of the depleted asthenospheric mantle in the southern hemisphere or due to contamination of the depleted asthenosphere by deep-rooted plumes carrying a low Pb-206/Pb-204 mantle component, In contrast, the origin of the more common Indian MORB with low Pb-206/Pb-204 and low Nd-143/Nd-144 is most probably associated with the delamination of the Gondwanan continental lithosphere during formation of the Indian Ocean. (C) 2002 Elsevier Science B.V. All rights reserved.