| The Beishan orogen is located in the joint areas among the Xinjiang, Gansu and Inner Mongolia Provinces. To its west is the east Tianshan mountain is to west and east is Alashan mountain. This orogen is key areas of the accretionary-collision and magmation in the southern of Central Asian Orogenic Belt. Regional geology show that the early Mesozoic is important period in the Central Asian Orogenic Belt. the study of granite could provide the new data for magmation evolvement and accretionary-collision process in the Central Asian Orogenic Belt.Combined investigations of field observations, petrology, zircon U-Pb, geochemistry, Sr-Nd-Hf isotopes, and spatio-temporal distribution of early Mesozoic granitoids in central and south CAOB, and taking available previous data into account, following results is achieved:1. Two granitic plutons(Huaniushan and Dahuoluoshan pluton) were confirmed as Mesozoic plutons by zircon U-Pb dating. The Huaniushan pluton SHRIMP zircon U-Pb dating yeilded an age of 221±3Ma, which is interpreted as the emplacement age. And the Dahuoluoshan pluton obtained 238±1Ma by LA-ICP-MS zircon U-Pb dating.2. The Huaniushan granitic pluton appears transitions from high K calc-alkaline of metaluminous-weakly peraluminous to alkali, and their rock type changes from I-type to A-type, showing the granitoids have I-A type feature. The Dahuoluoshan pluton belongs to high K calc-alkaline I-type.3. The Huaniushan granitic pluton is characterized by high alkali(AR=2.94～4.49,σ=1.94～6.37) content, high FeOT/MgO(2.8～9.3), high 10000xGa/Al(2.48～2.93) and differentiation index(DI=86.2～95.5) values. The granites shows a little rich of LREE and large iron lithophile elements(Rb, Th, K), an intense depletion in Ba, Sr, P, Ti and Eu, and a low depletion in Ta and Nb. The value ofεNd(t) is -1.5 to-2.1 and TDM 0.90 to 1.04Ga, andεHf(t) is 2.94 to 5.63 and TDM2(Hf) 896.8 to 1068.8Ma, indicating that the granitoids was probably formed by partial melting of younger continental crust, due to underplating mantle-derived magma(maybe contaminated by continental crust). And crystallization differentiation is obvious in the formation of the Huaniushan granitic pluton. During the formation of the Huaniushan granitic pluton, there is a strong crystallization differentiation.The Dahuoluoshan granitic pluton is characterized by low Si(SiO2=62.62%-70.52%), low alkali (AR=3.99～5.05, NK/A>0.85) content, high mafic content. The granites show a rich of REE, Rb, Th, Zr, Hf, an intense depletion in Ba, Ta, Nb, Sr, P and Ti, and unconspicuous to no negative anomaly of Eu, showing post-collision feature. The values ofεNd(t) (-1.1) and TDM(0.99Ga) is similar to that of the Huaniushan pluton, suggesting that they have the similar origin. But, there is a weak crystallization differentiation.4. Based on all above features and the areal geologic evolvement, the early Mesozoic granitic plutons are interpreted as post-orogenic or late stage orogenic pluton and provide important information on the accretionary orogeny and tectonic evolution in Beishan orogen.5. Early Mesozoic granite occur in many magma belts. According to zircon U-Pb ages, the ages of granitic plutons outcropped in North Mongolia belt range during 210-230Ma, Central Mongolia 241-253Ma,227-230Ma and 207-217Ma, Altai 244-251Ma,225-232Ma,210-221Ma,196-202Ma and 182Ma, East Tianshan 246-253Ma,230-241 Ma and 209-222Ma, Beishan 237-239Ma,220-229Ma, Inner Mongolia-Jilin 234-238Ma,222-230Ma and 204Ma, northern North China block 234-251Ma,220-229Ma and 203-209 Ma. In summary, the granitic plutons with ages between 233Ma and 240Ma outcropped in the belts of East Tianshan, Beishan, Inner Mongolia-Jilin and northern North China block, otherwise those with ages 219-232Ma in all belts and 210-218Ma in Altai and Central Mongolia. |
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