15 Tectonic, magmatic & geomorphic evolution of high plateaus. Part 1
Monday, 4 October 2010
2:30 PM - 4:50 PM
Salon C
EARLY CRETACEOUS SUBDUCTION-RELATED ADAKITE-LIKE ROCKS FROM THE SOUTHERN MARGIN OF THE LHASA TERRANE, TIBET: PRODUCTS OF SLAB MELTING AND SUBSEQUENT MELT–PERIDOTITE INTERACTION?
Paper number: 15-3
Program: Scientific Sessions
Time: 3:10 PM – 3:30 PM
Authors
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Zhidan ZhaoChina University of Geosciences, Beijing, N/A
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Gui-Tang PanChengdu Institute of Geology and Mineral Resources, Chengdu, N/A
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Hao-Yang LeeNational Taiwan University, Taipei, N/A
Abstract
Conflicting geodynamic models have been proposed for the generation of the widespread Cretaceous igneous rocks in the central and northern Lhasa subterranes. To contribute this issue, we present SHRIMP U-Pb zircon data and geochemical and Sr-Nd-Pb-Hf isotopic data for the Mamen andesites from the southern margin of the Lhasa Terrane, Tibet. The Mamen andesites, emplaced at 136.5 Ma, are sodic (Na2O/K2O = 1.2–2.3) and have geochemical characteristics typical of adakites (i.e., high Al2O3, high La/Yb ratios and Sr contents, low Y and HREE contents, and positive Eu anomalies), except for high Cr, Ni, and MgO contents. The andesites have initial (87Sr/86Sr)t ratios of 0.70413–0.70513, positive eNd(t) values of 3.7–5.8, and (206Pb/204Pb)t ratios of 18.37–18.51, (207Pb/204Pb)t ratios of 15.59–15.65, and (208Pb/204Pb)t ratios of 38.43–38.72. In situ Hf isotopic analyses of zircons that had previously been dated by SHRIMP yielded positive initial eHf(t) values ranging from +11.0 to +15.5. A model calculation using trace element and Sr-Nd-Pb isotopic data indicates that several percent of subducted sediment is required to generate the Mamen andesites, which were derived via the partial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid) and subsequently hybridized by peridotite in the mantle wedge. Our data indicate that the Neo-Tethyan ocean floor was subducted northward beneath the Lhasa Terrane during the Early Cretaceous at a high angle. Our results are inconsistent with a tectonic model that advocates the low-angle or flat-slab subduction of Neo-Tethyan ocean floor in generating the widespread Cretaceous magmatism documented in the Lhasa Terrane.