吉林中部烟筒山早白垩世辉长闪长岩的年代学和地球化学

吉林中部烟筒山地区辉长闪长岩中锆石均呈自形-半自形, 具有均匀的震荡生长条带和较高的 Th /U比值(0.63～1.49) , 暗示其具有岩浆成因。锆石的LA-ICP-MS U-Pb定年结果表明, 206 Pb /238U 年龄为(118 ±1)～(125 ±1) Ma ( n = 24) , 其加权平均年龄为( 122 ±1) Ma (MSWD = 415) , 代表了岩体的侵位结晶年龄-早白垩世。岩体的SiO2含量为52.41% ～53.38% , TiO2 = 1.27% ～1.95% , MgO、TFe₂O₃含量分别为3.34% ～4.27%和8.42% ～9.48% , Mg#值为44～47。Na₂O含量为3.99% ～4.31% , K₂O含量为1.54% ～1.97% , Na₂O /K₂O为2.05～2.75, Al₂O₃ = 16.87% ～19.60% , 具有相对富钠、高铝的特点。稀土元素球粒陨石标准化曲线显示其具有富含轻稀土元素(LREE) 、贫重稀土元素(HREE) 以及微弱的Eu的正或负异常的特点。微量元素原始地幔标准化蛛网图上, 岩体具富含大离子亲石元素(Rb、Ba、Th) , 明显亏损高场强元素(Nb、Ta、Zr、Hf和Ti) 等特点。烟筒山辉长闪长岩的原始岩浆起源于受俯冲流体交代的地幔楔的部分熔融, 其形成应与古太平洋板块俯冲作用形成的类似弧后盆地性质的伸展环境有关。

Chronology and geochemistry of Early Cretaceous gabbro-diorite in Yantongshan area of central Jilin

Gabbro-diorite zircons from Yantongshan in central Jilin Province are euhedral-subeuhedral in shape, and display homogeneous growth zoning and high Th/U ratios (0.63～1.49), suggesting their magmatic origin. LA-ICP-MS zircons U-Pb dating indicate that ~(206)Pb/~(238)U ages of 24 spots ranging(118±1) to (125±1) Ma with a weighted mean age of (122±1) Ma (MSWD=4.5), implying the crystallization of this rock body as Early Cretaceous in age. The contents of SiO_2 are 52.41%～53.38%, TiO_2 1.27%～1.95%, MgO 3.34%～4.27% (Mg~#=44～47), TFe_2O_3 8.42%～9.48%, Na_2O 3.99%～4.31%, K_2O 1.54%～1.97%, Na_2O/K_2O 2.05～2.75 and Al_2O_3 16.87%～19.60%, characterizing high Na and Al. The gabbro-diorites have high light rare earth elements (LREEs), low heavy rare earth elements (HREEs) and weakly positive Eu anomalies in chondrite-normalized rare earth element curves, and with high LILEs(Rb, Ba and Th)and relative low HFSEs(Nb,Ta,Zr, Hf and Ti) in primitive mantle-normalized trace element spider diagram. The primary magma of gabbro-diorite in Yantongshan derived from the partial melting of mantle wedge which metasomatized by subduction fluids, and formed under an extensional environment similar to back-arc basin during the subduction of the Paleo-Pacific Plate.