松嫩地块东缘金山屯地区晚三叠世双峰式侵入岩年代学、地球化学及其对构造演化的制约

Geochronology,geochemistry and its constraint on tectonic evolution of Late Triassic bimodal intrusive rocks in Jinshantun area of eastern Songnen Massif

对采自松嫩地块东缘金山屯地区的两个晚三叠世侵入体进行了LA-ICP-MS锆石U-Pb年代学和全岩地球化学分析,以确定其形成时代、岩石成因以及构造意义。结果表明,黑云母二长花岗岩与角闪辉长岩样品中的锆石呈自形-半自形,具典型的震荡生长环带或条痕状吸收的特点,暗示其为岩浆成因;锆石测年结果分别为(216±1) Ma和(212±2) Ma;根据该区无同时代中性岩产出,SiO_2含量存在明显间断,因此其应为一套晚三叠世双峰式侵入岩。黑云母二长花岗岩具有高硅(SiO_2=74.15%~76.90%)、富碱(Na_2O+K_2O=2.01%~4.37%)、贫铁(TFe_2O_3=0.81%~2.68%)和镁(Mg#=12.61~23.90)的特点,为准铝质系列(A/CNK=1.04~1.08),具有明显的负Eu异常(δEu=0.06~0.48),同时锆石饱和温度较低(723℃~780℃),具有高分异的高钾钙碱性I型花岗岩属性。经过与区域上同时代酸性岩的对比,其应是伸展背景下,古俯冲残留板片中的流体参与到下地壳物质部分熔融过程的产物。角闪辉长岩由于亏损Nb、Ta、Zr、Hf、Ti等高场强元素,富集Ba、Sr等大离子亲石元素,并具有低的Ba/Nb (36.7~115.8)、Ba/La (25.1~39.0)、Zr/Hf (19.6~26.3)和Nb/Ta (12.0~16.0)比值,应来源于受到俯冲板片熔体交代后的岩石圈地幔的部分熔融。综合区域火成岩类型和分布特征,认为松嫩地块东缘在晚三叠世应处于古亚洲洋闭合后的伸展环境。

LA-ICP-MS zircon U-Pb dating and whole-rock geochemistry of two Late Triassic intrusions in Jinshantun area of the eastern Songnen Massif were carried out in order to understand their formation ages,petrogenesis,and tectonic implications.The results show zircon grains from biotite monzogranite and hornblende gabbro in the study area are euhedral-subhedral and display oscillatory growth zoning or striped absorption in CL images,suggesting their magmatic origin.The dating results indicate that the ages of these two samples are(216±1)Ma and(212±2)Ma,respectively.Due to no coeval intermediate rocks in the study area,and the content of SiO 2 is obviously discontinuous,it can be concluded that there are Late Triassic bimodal intrusive rocks.Geochemically,the biotite monzogranite samples are characterized by high silica(SiO 2=74.15%~76.90%),alkali-rich(Na 2O+K 2O=2.01%~4.37%),poor in iron(TFe 2O 3=0.81%~2.68%),low Mg#values(12.61~23.90),negative Eu anomalies(δEu=0.06~0.48),and low values of zircon saturation temperature(723℃~780℃),with the nature of metaluminous(A/CNK=1.04~1.08),suggesting that they belong to highly fractionated high-potassium calc-alkaline I-type granite,derived from an extensional setting by partial melting of lower crust that was mixed by fluids from the residual of subducted oceanic crust.The hornblende gabbro samples are depleted in high field strength elements(HFSEs;e.g.,Nb,Ta,Zr,Hf and Ti),enriched in large-ion lithophile elements(LILEs;e.g.,Ba and Sr),and also characterized by low values of Ba/Nb(36.7~115.8),Ba/La(25.1~39.0),Zr/Hf(19.6~26.3)and Nb/Ta(12.0~16.0),implying that they formed from partial melting of lithospheric mantle material that had been previously modified by the melts of subducted oceanic crust.Integrating the petrogenesis and distribution of igneous rocks in the eastern Songnen Massif,it is concluded that the eastern Songnen Massif was under an extensional setting during the Late Triassic after the final closure of the Paleo-Asian Ocean.