Indian MORB Isotope Signatures in the Early Paleozoic Dongcaohe Ophiolite from the North Qilian Orogenic Belt, NW China

Details Indian MORB Isotope Signatures in the Early Paleozoic Dongcaohe Ophiolite from the North Qilian Orogenic Belt, NW China Indian MORB Isotope Signatures in the Early Paleozoic Dongcaohe Ophiolite from the North Qilian Orogenic Belt, NW China

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.t43c2052c&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #T43C-2052

Physics

1000 Geochemistry, 1021 Composition Of The Oceanic Crust, 1040 Radiogenic Isotope Geochemistry, 1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1065 Major And Trace Element Geochemistry

Scientific paper

Indian MORB is isotopically distinct from the Pacific and North Atlantic MORB. Indian MORB type isotope signatures also occur in oceanic crust from marginal seas at eastern Asia and some Tethyan ophiolites. Spatial distribution and age constraints from present and ancient Indian MORB type crust are major clues to the origin and evolution of such isotopic characteristics in the mantle regime. Exposed at the south of the North Qilian Orogenic belt (NQOB), the basalts, diabases, and gabbronorites from the Dongcaohe ophiolite (DCHO) show systematic depletions in incompatible elements with abundances readily reconciled by fractional crystallization. The basalts and diabases have similar 87Sr/86Sr and 143Nd/144Nd ratios of 0.70525-0.70674 and 0.51297-0.51308, respectively. However, the rather fresh gabbronorites show relatively depleted isotopic signatures with 87Sr/86Sr of 0.70285- 0.70323 and 143Nd/144Nd ratios of 0.51328-0.51341. Obviously, the 143Nd/144Nd ratio of the gabbronorites from the DCHO indicates derivation from depleted mantle whereas the Sr and Nd isotope compositions of DCHO basalts and diabases reflect postmagmatic alteration. Three fresh gabbronorite samples have Pb isotopes varying in limited ranges; specifically, 206Pb/204Pb of 17.39-17.43, 207Pb/204Pb of 15.44-15.45, and 208Pb/204Pb of 37.22-37.28, plotting above the NHRL. Zircon grains separates from the gabbronorites yield a U-Pb age of 497±7 Ma, which is inferred to represent crystallization age instead of emplacement age for the absence of zircon in the basalts and diabases. Consequently, the DCHO represents remnants of oceanic crust derived from early Paleozoic Indian MORB-type mantle. The composition and Sr-Nd-Pb isotope systematics of the DCHO indicate that the Indian MORB-type mantle might be of present global scale lasting for at least 500 Ma. However, the possibility that the DCHO represents spatially and temporally limited marginal sea cannot be completely precluded. Distinguish these two alternatives requires robust constraints on the tectonic affinity of all subunits in the NQOB.

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