滇西梁河勐养花岗闪长岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及其构造意义

勐养花岗闪长岩体位于滇西腾冲地块梁河县南勐养镇一带。用LA-ICP-MS技术测得花岗闪长岩锆石U-Pb年龄为115.2±1.0Ma,该年龄被解释为花岗闪长岩的形成年龄,表明该区花岗闪长岩体的形成时代为早白垩世。岩石地球化学特征表明,花岗闪长岩中SiO_2含量为62.39%~67.97%,Na_2O+K_2O为6.26%~7.80%,K_2O为1.98%~4.11%,具有贫钾(K_2O/Na_2O值为0.45~1.11)、低P2O5(0.09%~0.38%)的特征;MgO为1.78%~1.98%,Mg~#为34.90~48.40,属于准铝质-弱过铝质钙碱性系列。微量元素具有相对富集大离子亲石元素(U、Th、Rb、Ba)、亏损高场强元素(Tb、Nb、Zr、Hf)的特点。该花岗闪长岩具有I型花岗岩的特征,同时兼有S型花岗岩之特点,具有明显的岩浆混合作用特征。岩石具有岛弧或活动陆缘岩系的微量元素分布特征。地球化学特征和微量元素构造判别图解揭示,勐养早白垩世花岗闪长岩形成于碰撞后岩浆弧环境。花岗闪长岩为幔源岩浆与高黎贡山群古老地壳部分熔融的岩浆混合的产物。该区早白垩世花岗闪长岩是腾冲地块早白垩世侵入岩与班公湖-怒江-泸水-瑞丽洋盆的闭合、洋壳向南西俯冲及板块间的碰撞造山作用的产物。

LA-ICP-MS zircon U-Pb age and geochemistry of Mengyang granodiorite intrusion in Lianghe area, western Yunnan Province

Mengyang granodiorite intrusion is located in Mengyang area of Tengchong block, western Yunnan Province. LA-ICPMS zircon dating indicates that the granodiorite was emplaced at 115.2±1.0Ma. The granite mass was formed in the Early Creta-ceous. Geochemical data show that the granodiorites has the following data:SiO₂=62.39%~67.97%, Na₂O+K₂O=6.26%~7.80%, K₂O/Na₂O=0.45~1.11, rich in Si, Na, poor in P, MgO=1.78%~1.98%. Mg#=34.90~48.40,thus belonging to metaluminous to weakly per-aluminous, calc alkaline series. It has both the characteristics of I-type granite and S-type granite in the granodiorite and also has the characteristic of magma mixing. It has Sm/Nd ratios of 0.12~0.20, is enriched in Th, Rb, Ba, Pb and U, and depleted in Ta, Nb, Zr, Hf and Ce, thus similar to features of calc-alkaline rocks in island arc or active continental margin. According to geochemistry and trace element discrimination diagrams for tectonic interpretation, the Early Cretaceous granodiorites are considered to be of the post-collision type formed in a island arc tectonic setting. The granodiorite was from the mixing of mantle-derived magma and par-tial melting crust. The early Cretaceous intrusive rocks in the Tengchong block were intimately related to the closure of the Bangong Co-Nujiang-Longling-Ruili ocean basin and were formed by the subduction of ocean crust and the collision between the Tengchong block and the Zuogong-Baoshan block.