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    Prolonged magmatism, juvenile nature and tectonic evolution of the Chinese Altai,NW China Evidence from zircon U–Pb and Hf isotopic study of Paleozoic granitoids [查看] KedaCai、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao、XiaopingLong、FuyuanWu
    Paleozoic granitoid magmatism played an important role in the tectonic evolution of the Chinese Altai,and zircon U–Pb and Hf isotopic compositions have been determined for samples from eleven granitic plutons/batholiths. The Jiadengyu gneissic granitic pluton yielded a zircon U–Pb age of 479 Ma, and thus it does not represent Precambrian basement as suggested previously. Our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30% area of the Chinese Altai during the period from 447 Ma to 368 Ma with a climax at ca. 400 Ma. Ages for zircon overgrowth rims demonstrate additional thermal events at ca. 360 and 280 Ma, respectively. Positive eHf(t) values (0 to +9) of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources. Xenocrystic zircon cores are 543–421 Ma old and also give positive eHf(t) values (+2.5 to +12), suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly-accreted meterials. The strong magmatism at ca. 400 Ma significantly changed the Hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period.Geophysical, geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca. 400 Ma and the above mentioned change of Hf isotopic composition in the magma source.
    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.
    Geochronology, petrogenesis and tectonic significance of peraluminous granites from the Chinese Altai, NW China [查看] KedaCai、MinSun、ChaoYuan、GuochunZhao、WenjiaoXiao、XiaopingLong、FuyuanWu
    The Paleozoic granites in the Chinese Altai are important for the study of tectonic evolution and crustal growth in the Central Asian Orogenic Belt (CAOB). Four representative peraluminous granitic intrusions were selected for systematic studies of zircon U–Pb and Hf isotopic compositions and whole-rock geochemical and Nd–Sr isotopic analyses. These rocks have high ASI (Alumina Saturation Index, Al2O3/(CaO+Na2O+K2O)=1.01–1.46 molecular ratios), with 0.6–5.6 wt.% of normative corundum, and are characterized by moderately negative Eu anomalies (Eu/Eu*=0.38–0.98) and strong depletion in Ba, Nb and Sr elements. Our data suggest that these intrusions were emplaced from 419 to 393 Ma, consistent with a period of intensive magmatic activities and high temperaturemetamorphismin the Chinese Altai.While in situ zircon Hf isotopic analyses for these granites give predominantly positive εHf(t) values (+0.8 to+12.8), a few inherited zircons yield negative εHf(t) values from −12.5 to −1.53. The U–Pb age and Hf isotopic data of these inherited zircons are similar to that of the widespread metasediments. In addition, the peraluminous granitic rocks have near-zero or negative εNd(t) values(−3.3 to−0.5) and relatively high initial 87Sr/86Sr ratios (0.7079–0.7266), distinct fromthose of the I-type granites in the study region, but similar to the Early Paleozoic Habahe sediments. These isotopic compositions suggest that the newly accreted metasediments of Habahe Group may be the major source rock of the peraluminous granites. The geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level (P≤5 kbar) and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672–861 °C. The peraluminous granitic magmas were generated by dehydration melting of newly accreted materials, which were possibly brought to at least middle crustal depth by subduction-related processes in an active margin, and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the Paleozoic.
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