文档共享_Beijing SHRIMP Center

你好,请

Neoproterozoic (~900 Ma) Sariwon sills in North Korea Geochronology,geochemistry and implications for the evolution of the south-eastern margin of the North China Craton [查看] PengPeng、Ming-GuoZhai、QiuliLi、FuyuanWu、QuanlinHou、ZhongLi、TieshengLi、YanbinZhang

The Sariwon sills are distributed in the Pyongnam basin at the center of the Korean peninsula, eastern part of the North China Craton. These sills are up to 150 m in thickness and up to more than 10 km in length. Baddeleyite grains separated from a ~50 m thick sill give a SIMS 206Pb–207Pb age of 899±7 Ma(MSWD=0.34, n=14), which is interpreted to be the crystallization age of this sill. Zircon grains from the same sill gives a lower intercept U–Pb age of ~400 Ma, which is likely a close estimation of the greenschistfacies metamorphism of this sill. The Sariwon sills are dolerites and have tholeiitic compositions. They show enrichment of light rare earth element concentrations (La/YbN=1.4–2.8) and are slightly depleted in high field strength elements (e.g. Nb, Zr, and Ti), in comparison to neighboring elements on the primitive-mantle normalized spidergram. The whole rock εNdt (t=900 Ma) values are around −2, whereas in-situ εHft(t=900 Ma) values from zircon grains vary from−25 to+8. They are similar to the coeval sills in other parts of the North China Craton, e.g., the Chulan sills (Xu-Huai basin, Shandong peninsula) and the Dalian sills (Lv–Da basin, Liaodong peninsula). These sills possibly originated from a depleted mantle source (e.g., asthenosphere), rather than from the ancient lithospheric mantle of the North China Craton, and have experienced significant assimilation of lithospheric materials. The strata and sills in the Xu-Huai, Lv–Da and Pyongnam basins are comparable; moreover the three basins are geographically correlatable based on Neoproterozoic geographical reconstruction. We therefore propose that there is a Xu-Huai–Lv–Da–Pyongnam rift system along the south-eastern edge of the North China Craton during Neoproterozoic (~900 Ma), with the closure of the rift at ~400 Ma as a result of a continent-margin process. It is possible that this southeastern margin of the NCC did not face the inland in the configuration of the supercontinent Rodinia.

Ca.2.5 billion year old coeval ultramafic-mafic and syenitic dykes in Eastern Hebei Implications for cratonization of the North China Craton [查看] TieshengLi、MingguoZhai、PengPeng、LiangChen、JinghuiGuo

A group of extremely rare coeval ultramafic–mafic and syenitic dykes was discovered in the Eastern Hebei region of the North China Craton. These dykes intrude the 3.8–2.55 Ga old Caozhuang complex. An olivine gabbro dyke and syenite dyke yield, respectively, SHRIMP zircon U–Pb ages of 2516±26Ma and 2504±11 Ma, interpreted as the magmatic crystallization age. Their zircons have single-stage Hf model ages 2677Ma and 2705 Ma. The olivine gabbros have Mg# values of 59–63, similar to high-magnesian tholeiitic basalt. They show relatively LREE-enriched patterns without Eu anomalies (La/YbN = 9.28–9.78, Gd/YbN = 2.8, δEu = 0.96–1.00), with enrichment in LILEs and depletion in HFSEs. They also contain high Cr and Ni, and have Zr/Hf and Nb/Ta ratios similar to those of the primitive mantle. The syenites are alkaline in composition, with 8.67–8.88 wt.% Na2O+K2O, and show high total REE contents (543–854 ppm) and strong LREE-enriched patterns with minor Eu negative anomalies (La/YbN = 50–101, Gd/YbN = 5.2–5.3,δEu = 0.80–0.82). Trace element diagrams show strong LILE-enrichment relative to HFSE. Zr/Hf and Nb/Ta ratios are 47–49 and 26–76, respectively, which are much higher than those of primitive mantle and higher than the average Nb/Ta ratio of post-Archean continental crust. The petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source, which implies that the NCC probably had continental crust of considerable thickness at this time. Combined with evidence for ∼2.5 Ga granite intrusion and metamorphism in the Eastern Hebei region and adjacent areas, we propose that the NCC has been a present scale craton at the end of Archean.

© BJSHRIMP 2013 - bjshrimp.cn