Age and geochemistry of contrasting peridotite types in the Dabie UHP belt, eastern China: Petrogenetic and geodynamic implications

摘要

The Dabie ultrahigh-pressure (UHP) metamorphic belt, central China, contains two contrasting types of ultramafic-mafic complex. The Bixiling peridotite in the southern Dabie terrane contains abundant garnet (21-32 vol.%) and thus has high CaO+ Al2O3 (9.94-15.3 wt.%). The peridotite also has high REE contents with flat REE patterns, high contents of S and other incompatible trace elements, together with low-Mg-# olivine and pyroxene and low Ni and PGE contents. Zircons from this peridotite mostly have low Th/U ratios, interpreted to reflect a metamorphic origin, and give dominantly Triassic ages (ca. 210 Ma). Other zircons with high Th/U ratios give upper intercept ages of 745 Ma. Most zircons have positive epsilon(Hf)(+-3.6 similar to +-8.1) values with depleted-mantle model ages (T-DM) of 0.6-1.0 Ga (mean 0.8 Ga) and crustal model ages (T-crust) of 0.8-1.4 Ga (mean 1.1 Ga). We interpret that the Bixiling complex was formed as cumulates in a Neoproterozoic asthenosphere-derived magma chamber in the continental crust, and was later carried to garnet-stable depths (ca. similar to 4.4 GPa) during the subduction of the Yangtze Craton in the Triassic. In contrast, the Raobazhai peridotite in the northern Dabie terrane was metamorphosed at lower P/T conditions (i.e. 15 Kb and 1000 C). All zircon grains from the peridotite yield Triassic ages (ca. 212 Ma) and have negative eHf values (-16.6 similar to -3.2), Mesoproterozoic model ages (T-DM=1.0-1.5 Ga) and Paleoproterozoic crustal model ages (T-crust = 1.5-2.3 Ga). The peridotite is enriched in LREE ((La/Yb)(n) approximate to 3.5), has high-Mg-# olivine and pyroxene, high Ni and PGEs but low Pd/Ir (mean 3.0). It represents a highly refractory residue of partial melting (up to 18%) of primitive mantle, and is similar to the cratonic mantle xenoliths in Phanerozoic igneous rocks from the eastern North China Craton (e.g. Mengyin and Hebi areas). Negative Ce, Eu and HFSE anomalies in the peridotites suggest that their protolith was derived from the shallow part of the mantle wedge (e.g. plagioclase-stable field) of the North China Craton, and was pulled to a deeper level (e.g. spinel-stable field) during the subduction of the Yangtze Craton. The mantle wedge, like peridotitic xenoliths in the Jurassic Xinyang diatremes at the southern edge of the North China Craton, was metasomatised by fluids/melts released from the subducted continental crust. The fragments of this modified mantle wedge were incorporated into the Yangtze crust during its subduction. (C) 2007 Elsevier B.V. All rights reserved.