A method is developed to precisely determine Mg/Ca and Sr/Ca ratios in biocarbonates by inductively coupled plasma atomic emission spectrometry (ICP-AES). This method precision (RSD%) is 0.52% for Mg/Ca and 0.28% for Sr/Ca, respectively. The precision suggests that ICP-AES is satisfactory for supplying good quality Mg/Ca and Sr/Ca data of biocarbonates for paleo-reconstruction. This ICP-AES technique was applied to 51 continuous coral sub-samples, and the results show annually periodical variations in coral Mg/Ca and Sr/Ca ratios, which are consistent with previous findings.
Mesozoic basaltic magmatism in the North China craton (NCC) is a key for probing the nature of the Mesozoic lithospheric mantle, including the process of lithosphere thinning over Phanerozoic time. Early Cretaceous calc-alkaline basalts and mafic dikes are widespread in the Fangcheng, Mengyin, Jimo, and Jiaodong regions of the southeastern NCC. Both rock types have similar geochemical signatures; are high in SiO2 content; moderately high in MgO, CaO, and Al2O3; highly enriched in LREE ((Ce/Yb)N > 10) and LILE (Rb, Ba, U, Th); and depleted in HFSE (Ti and Nb). These rocks are highly enriched in Sr-Nd isotopes (87Sr/86Sri > 0.7072, εNd(t) = -11.5 ∼ -17.5) and depleted in Pb isotopes (206Pb/204Pb < 17.6, 207Pb/204Pb < 15.6, 208Pb/204Pb < 38.1). Their geochemistry demonstrates that these basalts and mafic dikes were derived from enriched lithospheric mantle, which differs considerably from the Paleozoic and Cenozoic lithosphere of the region. Calculations indicate that the Mesozoic lithosphere was extensively modified by silicic melt, derived from the partial fusion of subducted silicic crust. Tectonic underplating of the Yangtze continental crust beneath the base of the NCC lithosphere is well established. Extensive melting of the enriched lithosphere and tectonic collapse of the continental root provide a possible mechanism for lithosphere thinning linked to the Dabie collision.
Abstract: The Trans‐North China Orogen (TNCO) was a Paleoproterozic continent‐continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton (NCC). Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan‐type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward‐directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550‐2520 Ma, the deep subduction caused partial melting of the medium‐lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low‐ to medium‐grade granite‐greenstone terranes. At 2530‐2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back‐arc and/or intra‐arc basins in the orogen. At 2520‐2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic‐trondhjemitic‐granodioritic (TTG) magmatism. At this time following further extension of back‐arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, ∼2250 Ma 2110–21760 Ma and ∼2050 Ma granites in the orogen. Contemporary volcano‐sedimentary rocks developed in the back‐arc or intra‐arc basins. At 2150‐1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium‐ to high‐pressure mafic granulites. At 1880‐1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the closing of the ocean led to the continent‐arc‐continent collision, which caused large‐scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks.
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