Geochemistry, geochronology and evolution of Paleoproterozoic granitoid gneisses in the Khondalite Belt, North China Craton

摘要

The studied granitoid gneisses from the Daqingshan complex in the Khondalite Belt of the North China Craton consist of tonalitic, trondhjemitic, and granodioritic (TTG) gneisses, as well as monzogranitic and granitic (MG) gneisses. The respective melts forming these granitoid gneisses are geochemically similar. The granitoid gneisses are generally enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, Nd, La, Ce and depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. Chondrite-normalized rare earth elements (REE) patterns indicate that both TTGs and MG gneisses illustrate various fractionation with (La/Yb)(N) ratios of 7.43-58.29 and 3.59-87.82, respectively. However, most TTG gneisses show positive Eu anomalies, and relatively negative Th and positive Sr anomalies, while MG gneisses have opposite characteristics. These geochemical characteristics suggest that TTG gneisses were derived from a magma source at higher pressures than that to have formed MG gneisses. Some TTG gneisses yield high Sr/Y (36-139) and (La/Yb)(N) ratios, indicating a subduction-related affinity. The whole series of granitoid gneisses from the Daqingshan complex belong to fractionated calc-alkaline I-type granitoids, suggesting their igneous protoliths akin to island arcs and continental magmatic arcs. LA-ICP-MS U-Pb dating of zircons from four representative granitoid samples indicates that tonalite and granodiorite exhibit weighted mean ages of 2416 +/- 12 Ma and 2348 +/- 10 Ma, respectively, whereas trondhjemite provides two groups of zircon ages yielding weighted mean ages of 2437 +/- 15 Ma and 1876 +/- 3 Ma, respectively. The studied granitic gneiss reveals an upper intercept of 1947 +/- 45 Ma. Integration of cathodoluminescence images, Th/U ratio, and REE patterns of zircons indicates that all zircons of granitoid gneisses have a magmatic origin. Therefore, three major episodes of granitic magmatism at ca. 2.35-2.42, 1.95, and 1.88 Ga can be inferred. The ages of 1.95 and 1.88 Ga are coeval with widespread metamorphic events recorded by mafic and pelitic granulites in the Khondalite Belt. The Khondalite Belt could have undergone long-term arc-continent accretion along the southern margin of the Yinshan Block during early to late Paleoproterozoic time, and subsequent continental collision between the Yinshan and Ordos Blocks occurred at ca 1.95-1.88 Ga.