Abstract:
The key to understanding the generation of mantle-derived magma and the formation of new continental crust is the recognition of the granulite-facies metamorphism, associated partial melting, and coeval magmatism that are documented in active continental margins. We firstly present a combined analysis of petrography, mineral chemistry, zircon U-Pb ages and hafnium (Hf) isotopes, in addition to whole-rock geochemistry on the garnet-biotite gneisses (Grt-Bi gneisses) and mafic granulites from the Aketashitage area, southeastern Tarim Craton. The implications of this analysis are discussed with respect to the thermal and tectonic evolution of the Tarim Craton. The Grt-Bi gneisses and mafic granulites are strongly peraluminous in composition and have flat REE patterns with negative Eu anomalies that are identical to those from the middle crust but higher than lower continental crust. Both the Grt-Bi gneisses and the mafic granulites have negative Nb-Ta anomalies, slightly positive Zr-Hf anomalies, and pronounced positive spikes in U and Pb. The characteristics in whole-rock geochemistry and mineral compositions indicate that the Grt-Bi gneisses and mafic granulites have the same source locality, which is most likely the garnet-bearing middle crust. The retrograde metamorphic P-T estimates for the Grt-Bi gneisses and mafic granulites are T = 574 °C; P = 7.0 kbar and T = 715 °C; P = 7.0 kbar, respectively. SIMS zircon U-Pb dating results reveal a large spread in metamorphic ages (2036–1962 Ma) for these granulite-facies rocks, interpreted to be the consequence of a large amount of magmas being added to the crust in an uninterrupted manner. Zircon Hf isotopes are characterized by variable but mostly negative εHf (t) values (−18.2 to −0.6), reasonably uniform Hf isotopic compositions, and highly variable TDMC model ages between 2628 and 3599 Ma, representing a long-lived reworking of the preexisting crust. Our data, along with available geological evidences, lead us to propose a model for the evolution of the Dunhuang Block in the Paleoproterozoic. That is, the southeastern margin of the Tarim Craton, the Dunhuang Block was possibly an active continental margin during the Paleoproterozoic. The steep subduction of oceanic slab resulted in the thickening of the continental crust, which in turn caused the crust anatexis. The increase in asthenosphere high heat flow and the intrusion of mantle-derived magma induced extensive heating, which preceded the granulite-facies metamorphism and coeval magmatism in the middle crust of the southern Tarim Craton.
