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Late Triassic granitoids in the southern part of the Songpan-Garze fold belt:Petrology,geochemical composition and petrogenesis

WAN Chuan-hui1,2,YUAN Jing1,LI Fen-xiang1 and YAN Sheng-wu1(1.Faculty of Earth Sciences,China University of Geosciences,Wuhan 430074,China;2.Central-South Research Institute of Metallurgical Geology,Yichang 443003,China)  
The Songpan-Garze fold belt(SGFB)on the eastern margin of the Qinghai-Tibetan Plateau covers a huge triangular area bounded by Yangtze(South China),North China and Qinghai-Tibetan Plateau blocks.It has gone through two orogenic phases,i.e.,Paleotethys and Neotethys,with the major deformation stages occurring in Late Triassic.Mesozoic intermediate-acid intrusives,composed of both normal calc-alkaline and high Sr low Y granitoids,are widely distributed in the Songpan-Garze fold belt.The investigation of their petrogenesis,characteristics of the sources and geodynamic significances is very important in understanding the evolution of SGFB characterized by poorly-exposed basement.The authors studied petrology,geochemical composition and zircon U-Pb chronological characteristics of Lanniba and Yangfanggou granitoids in the southern part of the Songpan-Garze fold belt as well as their petrogenesis and geological significance.The study of Lanniba and Yangfanggou granitoids formed during the post-collision period can shed light on such problems as regional tectono-magmatic events,basement nature and tectonic evolution.Cathodoluminescence(CL)images show that the zircons are big and idiomorphic and have clear magmatic oscillatory zoning,suggesting magmatic origin.U-Pb zircon LA-ICPMS dating shows that the Lanniba and Yangfanggou granitoids have magmatic crystallization ages of around 211 Ma.These granitoids are high-K calc-alkaline(even shoshonite),with intermediate to high SiO2(58.31%~68.02%)and metaluminous characteristics(A/CNK=0.68~0.99).Rocks in the southern part of the Lanniba granitoids contain high Al2O3(15.85%~16.27%)and K2O(3.29%~3.40%),intermediate MgO(1.16%~1.47%),high Sr(869×10-6 ~1 032×10-6),low Y(9.53 ×10-6~9.85×10-6)with fractionated REE[(La/Yb)N31]and also have some dark inclusions.These rocks are very similar to the K-adakite derived from partial melting of thickened lower crust.Yangfanggou and the northern part of the Lanniba granitoids are very similar to each other in major and trace elements.In contrast with the southern part of Lanniba granitoids,they have higher K2O(4.08%~5.96%)and Y(19.71×10-6~27.76-6),lower Sr(664×10-6~868×10-6),and obviously lower Sr/Y ratio(29.39~42.05),showing characteristics of high-K calc-alkaline I-type granitoids;thus they might have been derived from partial melting of thickened lower crust.Field geological and geochemical characteristics indicate that the granitoids contained mantle components,thus providing heat and part of source materials for the Mesozoic granites in Songpan-Garze fold belt.The involvement of mantle-derived magma and the existence of ultrahigh-K calc-alkaline I-type granitoids(211±2 Ma)indicate that the Songpan-Garze orogen was under an extensional scheme at least in Late Triassic period.
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