内蒙古巴林右旗建设屯埃达克岩锆石U-Pb年龄及成因讨论

Zircon U-Pb age and petrogenetic discussion on Jianshetun adakite in Balinyouqi,Inner Mongolia

本文对内蒙古东南部建设屯花岗闪长岩进行了岩石学、地球化学、锆石U-Pb定年及原位Hf同位素分析研究。LA-ICP-MS锆石U-Pb定年结果显示了该岩体的形成时代为248.5±2.3Ma,为早三叠世早期岩浆岩侵入体。岩石学和地球化学研究表明,该岩体铝饱和指数A/CNK为1.02,属于钙碱性I型花岗岩;SiO2≥56%,Al2O3≥15%,Na2O和K2O含量分别为4.34%～4.41%和2.03%～2.07%,Na2O/K2O介于2.10～2.17之间;Sr>400×10-6,Y≤18×10-6,Yb≤1.9×10-6,岩石轻重稀土分馏明显,(La/Yb)N介于16.92～17.38之间,无明显的Eu负异常,显示了埃达克岩的地球化学特征。锆石Hf同位素分析显示εHf(t)值介于11.2～14.4之间,反映了源区亏损的特征。综合考虑区域岩浆岩和地层古生物资料,本文认为该岩体是晚二叠世古亚洲洋沿西拉木伦河缝合带闭合后,加厚的新生下地壳部分熔融作用的产物。

In this paper petrology, geochemistry, zircon U-Pb dating and in-situ Hf isotopic analyzing for Jianshetun granodiorite in SE Inner Mongolia are reported. LA-ICP-MS zircon dating gives the emplacement age of Early Triassic （248.5 ±2. 3Ma） for the pluton. Petrological and geochemical study indicate that the aluminum saturation index （A/CNK） of the rock is 1.02, belonging to calk-alkaline I-type granite. The rock has high SiO2 （≥56% ） and A1203 （≥ 15% ） contents, and the Na20 and K20 contents range between 4. 34% -4. 41% and 2. 03%±2. 07% respectively, with Na20/K20 ratio varying from 2. 10 to 2. 17; the rock has high content of Sr （ 〉 400 ×10^-6）, low content of Y （≤18 ×10^-6） and Yb （≤ 1.9×10^-6）, and has notable fractionation between LREE and HREE, with （La/Yb）N varying between 16. 92 N 17.38. Besides, there is not negative Eu anomaly in the chondrite-normalized REE pattern. All of the geochemical characteristics above indicate that the Jianshetun pluton belongs to adakite. The εHf（t） of in-situ zircon Hf isotopic analysis is between 11.2 - 14.4, showing the magma originated form depleted source. Considering regional magmatism, stratigraphy and palaeontology data, it is thought that the pluton is the product of partial melting of the thickened newborn lower crust after the Late Permian closure of the Paleo-Asian Ocean along Xar Moron River suture belt.