Zircon U-Pb-Hf isotopes and whole-rock geochemistry of granitoid gneisses in the Jianping gneissic terrane, Western Liaoning Province: Constraints on the Neoarchean crustal evolution of the North China Craton

摘要

The Jianping gneissic terrane (JPGT), Western Liaoning Province (WLP) at the northwestern margin of the Eastern Block of the North China Craton is one of the major Neoarchean high-grade metamorphic terranes. The region is composed dominantly of metamorphosed volcanic and pyroclastic rocks and granitoid gneisses. Geological and petrological features together with geochemical data suggest that the magmatic precursors of these granitoid gneisses are tonalite, trondhjemite, granodiorite (TTG), diorite and monzogranite. Whereas the TTGs show strong gneissic fabric, the monzogranitic rocks are only weakly gneissic and generally display massive structures. LA-ICPMS zircon U-Pb isotopic analyses reveal that the dioritic and TTG gneisses formed at 2506-2532. Ma, followed by the emplacement of the monzogranitic gneisses at ～2496. Ma. These granitoid gneisses can be classified into two groups: a low silica group (LSG) and a high silica group (HSG). Detailed petrogenetic studies suggest that the dioritic gneisses (LSG) were derived from the partial melting of an enriched mantle source metasomatized by slab fluids, whereas the tonalitic and granodioritic gneisses of the LSG were produced by the partial melting of subducted oceanic slabs and the melts were contaminated by mantle peridotites during their ascent. In comparison, the HSG granitoid gneisses were generated by the partial melting of the lower continental crust composed mainly of metamorphosed basaltic (garnet amphibolites) and pelitic rocks.Comprehensive zircon U-Pb dating and Lu-Hf isotopic data indicate that the JPGT and the North Chaoyang-Fuxin-Yixian granite-greenstone belt (NCFY-GGB) experienced nearly the same crustal evolution during Neoarchean to early Paleoproterozoic time and major crustal growth at ～2.5 and ～2.7. Ga was recorded in the WLP. Collectively, the WLP preserves a complex geodynamic history from the mid-ocean ridge, through intra-oceanic arc to Andean-type active continental margin.