吉林南部晚三叠世蚂蚁河岩体的成因:锆石U-Pb年代学和地球化学证据

Petrogenesis of the Late Triassic Mayihe Pluton in Southern Jilin Province:Evidence from Zircon U-Pb Geochronology and Geochemistry

蚂蚁河岩体主要由辉长闪长岩-花岗闪长岩-二长花岗岩构成。辉长闪长岩和二长花岗岩中的锆石呈自形晶,发育细微振荡生长环带,显示高的Th/U值(0.14～2.56),表明了锆石的岩浆成因。锆石LA-ICP-MSU-Pb定年结果表明,辉长闪长岩中锆石206Pb/238U年龄集中于219～227Ma,其加权平均年龄为(224±4)Ma(n=8);二长花岗岩中锆石年龄为217～241Ma,206Pb/238U加权平均年龄为(226±3)Ma(n=19)。岩石地球化学分析结果表明,蚂蚁河岩体具有双峰式岩石组合特征,辉长闪长岩具有较低的SiO2,较高的TiO2、MgO和CaO,以及富集大离子亲石元素(LILEs)和轻稀土元素(LREEs),亏损高场强元素(HFSEs)特征,其87Sr/86Sr初始值和εNd(t)值分别为0.7037和3.45～3.82。上述特征暗示岩浆起源于亏损岩石圈地幔;相反,花岗闪长岩和二长花岗岩具有较高的SiO2,较低的TiO2、MgO和CaO,以及强烈富集Rb、Th、U,亏损Nb、Ta、P、Ti、Ba、Sr的地球化学属性,其87Sr/86Sr初始值和εNd(t)值分别为0.7053～0.7058和-0.73～-3.40,暗示了岩浆的壳源成因。蚂蚁河岩体双峰式岩石组合的存在反映了伸展的构造背景,结合区域构造研究成果,其成因应与扬子和华北克拉通之间俯冲碰撞后的伸展环境有关,华北克拉通东部中生代岩石圈减薄或破坏始于晚三叠世,减薄或破坏空间范围应包括吉南地区。

The Mayihe pluton, located in southern Jilin Province, consists mainly of gabbro-diorite, granidiorite, and monzogranite. Such characteristics of zircons from gabbro-diorite and monzogranhe imply their magmatic origin, as euhedral shape, typical oscillatory zoning and high Th/U ratios （0.14 - 2.56） . LA-ICP-MS zircon U-Pb dating indicates that the ^206 Pb/^238 U ages of zircons from gabbro-diorite range from 219 to 227 Ma, yielding a weighted mean age of （224±4） Ma （n=8）, and those of zircons from monzogranite from 217 to 241 Ma, yielding a weighted mean age of （226±3） Ma （n=19）. The petrogeochemistry of Mayihe pluton shows its bimodal rock association. Gabbro-diorites are characterized by lower content of SiO2, higher content of TiO2, MgO and CaO. Moreover, they are rich in both large ion lithophile elements （LILEs） and light rare-earth elements （LREEs）, depleted in high field strength elements （HFSEs）. Their initial ^87 Sr/^86 Sr ratios and εNd （t） values are 0. 703 7 and 3. 45 to 3.82, respectively, suggesting a depleted lith0spheric mantle source. In contrast, grandiorites and monzogranites have higher SiO2 and lower TiO2, MgO and CaO. They are strongly rich in Rb, Th, U, and depleted in Nb, Ta, P, Ti, Ba, Sr. Their initial ^87 Sr/^86 Sr ratios and εXd （t） values range from 0. 705 3 to 0. 705 8 and from -0. 73 to -3.40, respectively. Characteristics above suggest that both grandiorite and monzogranite be derived from the partial melting of the lower continental crust. The bimodal rock association of the Mayihe pluton, together with the research on regional tectonics, demon-strates that the pluton were resulted from the post-collision extensional environment between the North China craton and Yangtse craton. It can be concluded that thinning or destruction of the lithosphere in the North China craton could be initiated in the Late Triassic, and the southern Jilin Province belongs to this lithosphere-thinning region.