西藏神公地区典中组火成岩锆石U-Pb年龄及地球化学特征

新特提斯洋俯冲消减、印度-亚洲板块碰撞导致西藏冈底斯构造带分布大量火成岩,其形成年龄尚存在一定争议,形成时代及构造意义对揭示板块运动信息具有重要意义。对神公地区典中组火成岩样品分别进行了锆石U-Pb同位素测年和元素地球化学分析,结果表明,典中组底顶部火成岩锆石U-Pb年龄值分别为65.37±0.58Ma和57.42±0.20Ma;该套火成岩主要为钙碱性系列岩石,稀土元素配分曲线表现为轻稀土元素富集、重稀土元素亏损的右倾形态,且负Eu异常明显,K_2O/Na_2O值平均为1.57,微量元素Rb/Sr值平均为1.78,Nb/Ta值平均为13.28,Rb、Th、U、Pb等大离子亲石元素富集,Nb、Ta、Ti亏损。由此认为,典中组火成岩是在俯冲碰撞背景下,来自地幔和地壳的岩浆以不同比例混合形成的,可能指示了新特提斯洋关闭引起的洋壳俯冲作用。

Zircon U-Pb age and geochemical characteristics of igneous rocks from the Dianzhong Formation in the Shengong area of Tibet

The subduction of the Neo-Tethyan Ocean and the collision of the India-Asia plate resulted in the distribution of large amounts of igneous rocks in the Gangdise tectonic belt in Tibet, whose age remains in dispute, and the exact age and tectonic significance are important for revealing the information of plate movement. In this study, the U-Pb isotopic age of zircon samples from the Dianzhong Formation in Shongong area was determined, and the geochemical analysis of major elements, trace elements and REE was carried out. The result indicates that the age of igneous rocks at the bottom and top of the Dianzhong Formation is 65.37±0.58Ma and 57.42±0.20Ma, respectively. The distribution curve of REE shows the rightward form of LREE concentration and HREE loss, with obvious negative Eu anomalies. The average ratio of K₂O/Na₂O, Rb/Sr and Nb/T is 1.57, 1.78 and 13.28, respectively. The Dianzhong Formation igneous rocks in Shongong area are enriched in Rb, Th, U and Pb and depleted in Nb, Ta and Ti. The authors hold that the igneous rocks of the Dianzhong Formation were formed by magmatic mixing of the mantle and the crust in different proportions in the background of subduction collision, which may indicate the subduction of oceanic crust caused by the closure of the Neo-Tethyan Ocean.