The origin of the early Proterozoic Kuandian Complex: evidence from geochemistry

Abstract

The Kuandian Complex is scarcely preserved Early Proterozoic volcanic suite, formed 2.3–2.4 Ga ago. It is located in an Early Proterozoic mobile belt bounded by the Archaean Rangrim and Ryonggang Blocks of the northeastern Sino–Korean Cratoa The Complex is mainly made up of amphibolites, gneisses, leucoleptite, leptite and layered granite. Petrological and geochemical studies show that the protoliths of the Complex are mainly associations of bimodal volcanics and anorogenic granites. The Kuandian amphibolites are depleted in Nb, Ta, P and Ti, and enriched in LILE, e.g. K, Rb and Cs, with pronounced depletion of Sr relative to Nd and Pb; La/Nb ratios are higher than 1 (1.75 to 5.18). The trace element patterns of the amphibolites are similar to continental flood basalts formed by the Gondwana break–up, such as those in South Karoo and Tasmania, which shows continental contamination. Nd values ranging from 0.70 to 1.94 of the Kuandian amphibolites and the relationships between Nb/Yb and La/Yb suggest that contamination of basaltic magma happened in the mantle, rather than along the conduit. Isotope ratios of 208Pb/204Pb, 207Pb/204Pb, 206Pb/204Pb, 143Nd/144Nd and 87Sr/86Sr indicate that the magma was derived from a contaminated mantle source like DMM or a mixture of DMM and EM2. The Kuandian Complex has Dupal anomaly, as is the case with some continental basalts in the south hemisphere, e.g. in South Karoo and Tasmania. Petrochemical modelling proposes that the Kuandian gneiss, granite, and amphibolite came from the same parental magma, being products of strong fractional crystallization. Protoliths of the Kuandian Complex were formed in extensional tectonic setting during the transition from continental crust to oceanic crust. The formation of the Kuandian Complex indicates that 2.3 or 2.4 Ga ago tectonic evolution of the Sino–Korean craton was different from that of other well–studied Precambrian cratons, e.g. the North American shield, European platform and Australian continent in that strong volcanic eruption resulted in its accretioa Besides, the contaminated magma source with a Dupal anomaly for the Complex indicates that crust–mantle convection whose scale was similar to that of the present plate tectonics had occurred at least before the formation of the Kuandian Complex (2.3–2.4 Ga B.P.).