内蒙古赤峰地区早二叠世花岗岩LA-ICP-MS锆石U-Pb年龄及地球化学特征

LA-ICP-MS zircon U-Pb geochronology and geochemical characteristics of the Early Permian granite in Chifeng area,Inner Mongolia

以赤峰地区铭山复式岩体、尖山子岩体为研究对象,通过锆石U-Pb年龄、地球化学研究,确定了其形成时代,探讨了岩石成因和岩浆源区性质及其形成的构造背景。LA-ICP-MS锆石^(206)Pb/^(238)U测年结果表明,铭山复式岩体中灰白色斑状黑云二长花岗岩、尖山子岩体中浅肉红色细粒正长花岗岩分别形成于284.4±7.9Ma、294.7±8.5Ma,为早二叠世。根据地质体间接触关系可知,铭山复式岩体中的二长花岗岩岩体和尖山子岩体均为复式岩体,有待进一步解体。斑状黑云二长花岗岩SiO_2含量较高,K_2O+Na_2O含量较高,A/CNK<1,属于高钾钙碱性I型花岗岩。细粒正长花岗岩Al_2O_3含量为14.32%~15.14%,Na_2O/K_2O=0.71~0.99,A/CNK=1.17~1.20,在标准矿物中出现刚玉分子。二者微量元素特征相似,富集大离子亲石元素,亏损高场强元素Nb、Ta,具Sr、P、Ti的负异常,表现出岛弧岩浆岩类特征。结合岩石地球化学、区域地质特征,认为赤峰地区早二叠世岩体形成于岛弧/活动大陆边缘构造背景,其形成与古亚洲洋向南俯冲有关。

In this paper, the authors studied the geochronology and geochemistry of the Mingshan granitic complex and Jianshanzi granite in Chifeng area, with the purpose of providing constraints on the formation time, petrogenesis and tectonic background on the northern margin of the North China Block. The zircon U-Pb age data obtained by using laser ablation ICP-MS technique are 284.4±7.9Ma and 294.7±8.5Ma, indicating that the Mingshan granitic complex and Jianshanzi granite were formed in Early Permian.The major elements characteristics of the porphyritic biotite monzogranite are high in SiO2 and （K2O＋Na2O）, with A/CNK lower than 1, belonging to high K calc-alkaline I-type granite. The syenogranite has Al2O3 14.32%-15.14%, Na2O/K2O 0.71-0.99 and A/CNK 1.17-1.20, with corundum molecule in CIPW standard mineral calculation. The porphyritic biotite monzogranite and syenogranite are similar in trace elements, with enrichment of large ion lithophile elements, loss of high-field strong elements, and obvious negative Sr, P and Ti anomalies, thus having the characteristics of island-arc magma. In combination with the geochemistry and regional geological characteristics, it is suggested that the Early Permian granites in Chifeng area was generated in an island-arc/ active continental margin tectonic setting.