内蒙古温都尔庙地区早二叠世岛弧型花岗闪长岩的发现:对古亚洲洋闭合的时限约束

研究区位于西伯利亚板块与华北板块之间的关键部位，可能代表了古亚洲洋最终闭合的位置.在嘎顺地区首次识别出花岗闪长岩，通过对岩石地球化学、锆石U-Pb年代学和Lu-Hf同位素分析，获得LA-MC-ICP-MS锆石U-Pb年龄为294.8±2.7 Ma，形成于早二叠世.SiO₂含量介于64.69%~67.64%；Al₂O₃含量介于15.00%~15.52%；MgO含量介于1.14%~1.77%；Na₂O含量介于1.41%~3.61%；K₂O含量介于3.25%~3.98%.∑REE值介于157.89×10-6~174.43×10-6；LREE值介于147.86×10-6~149.55×10-6；HREE值介于9.34×10-6~11.8×10-6；右倾稀土配分曲线，呈轻度负Eu异常，轻重稀土元素分馏程度大，轻稀土相对于重稀土明显富集.明显富集Rb、K，亏损Ba、Sr大离子亲石元素（LILE）；富集Th，亏损Nb、Ta、Ti等高场强元素（HFSE）.锆石176Hf/177Hf值介于0.282 673~0.282 791；ε_Hf（t）值介于2.7~7.0，其二阶段模式年龄T_DM2介于861~1 129 Ma，176Lu/177Hf值介于0.000 486~0.000 734，结合岩石地球化学特征，认为该套岩石形成于岛弧岩浆作用，证实了早二叠世古亚洲洋仍未闭合. 

Discovery of Early Permian Island-Arc Type Granodiorites in Wenduermiao Area,Inner Mongolia: Constraints on Timing of Closure of PaleoAsian Ocean

The study area is located in the key part between the North China and the Siberia plates, which may represent the final closure of the Paleo-Asian Ocean. Granodiorites were recognized in the Gashun area for the first time. According to petrogeochemistry, zircon U-Pb geochronology and Lu-Hf isotope analysis, the granodiorites yield an LA-MC-ICP-MS zircon UPb age of 294.8±2.7 Ma, suggesting that they were formed in the Early Permian. The rocks have SiO₂=64.69%-67.64%, Al₂O₃=15.00%-15.52%, MgO=1.14%-1.77%, Na₂O=1.41%-3.61%, and K₂O=3.25%-3.98%. The content of ∑ REE varies from 157.89×10-6 to 174.43×10-6, the content of LREE varies from 147.86×10-6 to 149.55×10-6, and the content of HREE vary from 9.34×10-6 to 11.8×10-6. The rocks display rightly-inclined REE distribution curves, slightly negative Eu anomalies, highly fractionated LREE/HREE, and more enriched LREE relative to HREE. In the rocks, Rb and K were obviously enriched, and Ba and Sr (LILE) were depleted; Th was enriched and Nb, Ta and Ti etc. (HFSE) were depleted. Zircon 176Hf/177Hf changes from 0.282 673 to 0.282 791, and ε_Hf(t) varies from 2.7 to 7.0. The two-stage model ages (T_DM2) vary from 861 Ma to 1 129 Ma, and 176Lu/177Hf ranges from 0.000 486 to 0.000 734. Combined with petrogeochemical characteristics, this set of granodiorites is considered to be formed by island-arc magmatism, confirming that the Paleo-Asian Ocean was still open in the Early Permian.