西藏门巴地区早白垩世花岗岩地球化学特征及LA-ICP-MS锆石U-Pb年龄

门巴地区早白垩世花岗岩位于冈底斯弧背断隆带东段,是研究冈底斯演化的重要组成部分。以门巴地区黑云母花岗岩和二云母花岗岩为研究对象,运用岩石地球化学和锆石U-Pb测年方法,分析了该区域早白垩世花岗岩的地球化学特征及构造背景。样品锆石具有明显的振荡环带,属岩浆成因锆石;测得LA-ICP-MS锆石~(206)Pb/~(238)U年龄为139.4～140.6Ma,为早白垩世。岩石地球化学特征表明,门巴地区早白垩世花岗岩具有富硅、高碱,贫FeO、MgO、CaO的特征,为强过铝质的高分异花岗岩,属高钾钙碱性系列,并具同源岩浆演化的趋势;稀土元素配分模式为右倾型,具Rb、Ba、Th、Nb、Ce等富集,Hf、Zr、Sm、Y、Yb等亏损的特征;门巴地区花岗岩形成于陆-陆碰撞造山环境,构造背景由同碰撞逐渐过渡为造山期后的构造环境,具有后碰撞伸展构造性质,是软流圈上涌或玄武岩底侵的岩浆响应。

Geochemical characteristics and LA-ICP-MS zircon U-Pb age of Early Cretaceous granites in Menba area, Tibet

The Early Cretaceous granites in the Menba area are located in the eastern segment of the Gangdise belt, being an important part for the study of Gangdise evolution. With biotite granite and two mica granite in Menba area as the object of study, the author analyzed the geochemical characteristics and tectonic setting of the Early Cretaceous granites in this area by using the method of rock geochemistry and zircon U-Pb chronology. In this paper, the zircon samples for the test and analysis have obvious oscillation ring band, belonging to zircon of magma origin, and the LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U age is 139.4~140.6Ma, suggesting Early Cretaceous. The geochemical characteristics of rocks indicate that the Early Cretaceous granites in Menba area are rich in silicon, high in alkali, and poor in FeO, MgO, CaO, implying strong aluminum high differentiation granite and high potassium calcium alkaline series, with the trend of homologous magma evolution; the model of rare-earth element allocation is the right type with Rb, Ba, Th, Nb, Ce enrichment and Hf, Zr, Sm, Y, Yb depletion. The granite in Menba area was formed in the land-land collision orogenic environment, the tectonic background was gradual transition from "the same collision" to the tectonic environment after "orogenic period", which had the extensional tectonic characteristics of "post collision", and was the magma response to the soft flow ring upwelling or the basalt bottom invasion.