The Origin of the Early Proterozoic Kuandian Complex: Evidence from Geochemistry

The Kuandian Complex is scarcely preserved Early Proterozoic volcanic suite, formed2.3-2.4 Ga ago. It is located in an Early Proterozoic mobile belt bounded by the ArchaeanRangrim and Ryonggang Blocks of the northeastern Sino-Korean Craton. The Complex ismainly made up of amphibolites, gneisses, leucoleptite, leptite and layered granite. Petrologicaland geochemical studies show that the protoliths of the Complex are mainly assoctations ofbimodal 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 toNd and Pb; La/Nb ratios are higher than 1(1.75 to 5.18). The trace element patterns of theamphibolites are similar to continental flood basalts formed by the Gondwana break-up, suchas those in South Karoo and Tasmania, which shows continental contamination. ε_(Nd) valuesranging from 0.70 to 1.94 of the Kuandian amphibolites and the relationships between Nb/Yband La/Yb suggest that contamination of basaltic magma happened in the mantle, rather thanalong the conduit. Isotope ratios of ~(208)Pb/~(204)Pb, ~(207)Pb/ ~(204)Pb, ~(206)Pb/~(204)Pb, ~(143)Nd/~(144)Ndand ~(87)Sr/~(86)Sr indicate that the magma was derived from a contaminated mantle source likeDMM or a mixture of DMM and EM2. The Kuandian Complex has Dupal anomaly, as is thecase 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 camefrom the same parental magma, being products of strong fractional crystallization. Protoliths ofthe Kuandian Complex were formed in extensional tectonic setting during the transition fromcontinental crust to oceanic crust. The formation of the Kuandian Complex indicates that 2.3or 2.4 Ga ago tectonic evolution of the Sino-Korean craton was different from that of otherwell-studied Precambrian cratons, e.g. the North American shield, European platform andAustralian continent in that strong volcanic eruption resulted in its accretion. Besides, the con-taminated magma source with a Dupal anomaly for the Complex indicates that crust-mantleconvection whose scale was similar to that of the present plate tectonics had occurred at leastbefore the formation of the Kuandian Complex (2.3-2.4 Ga B.P).

The Origin of the Early Proterozoic Kuandian Complex: Evidence from Geochemistry

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.).