大兴安岭北段新林战备村地区早白垩世火成岩年代学、地球化学及大地构造意义

大兴安岭北段新林战备村地区广泛发育晚中生代火山岩及花岗岩体。LA-ICP-MS锆石U-Pb测得战备村花岗岩体侵位年龄为121.0±0.8 Ma,其周围出露的白音高老组流纹岩喷发年龄为136.9±1.3 Ma,二者均形成于早白垩世。地球化学特征上,二者均属于过铝质高钾钙碱性-钾玄质系列,高SiO_2,富K_2O、Na_2O,低Mg、Ti,微量元素富集大离子亲石元素Rb、K、Ba及LREE,亏损高场强元素Nb、Ta、Ti及HREE,轻重稀土明显分异((La/Yb)_N=11~23.2),具中等-弱的Eu负异常(δEu=0.35~0.82),二者具同源岩浆演化的特征。结合前人研究结果,新林战备村地区早白垩世火成岩形成于后造山或碰撞后的构造环境,代表了伸展的大地构造背景,这些火成岩产出可能与蒙古—鄂霍茨克洋中侏罗世至早白垩世闭合造山后的伸展作用有关。

Chronology,geochemistry and tectonic implications of Early Cretaceous igneous rocks in Xinlin Zhanbeicun area,northern Great Hinggan Mountain

Late Mesozoic volcanic rocks and intrusive rocks widely occurred in Xinlin Zhanbeicun area of northern Greater Hinggan Mountain. Zircon U-Pb dating using the LA-ICP-MS method gives the emplacement age of 121.0±0.8 Ma for Zhanbeicun intrusive rocks and 136.9±1.3 Ma for Baiyingaolao Formation rhyolites distributed around the Zhanbeicun intrusive rocks. The results suggested that the intrusive rocks and rhyolites were formed in Early Cretaceous. Moreover the rocks belong to high K calc-alkaline-shoshonite series in geochemistry, and exhibit high SiO2, K2O, Na2O and low Mg, Ti. The trace elements generally have a similar variation trend, e.g. enrichment of LILE(Rb, K, Ba)and LREE, and depletion of HFSE (Nb, Ta, Ti)and HREE, with significant fractionation of LREE and HREE ((La/Yb)N=11~23.26) and weak-medium Eu anomalies (δEu=0.35~0.82). This indicates that the intrusive rocks and rhyolites have the same sources. Combined with previous studies, it is suggested that the Early Cretaceous igneous rocks in Zhanbeicun area are originated from partial melting of the lower crust and formed in a post-orogenic extensional tectonic setting related to the closure of Mongolia-Okhotsk ocean during Middle Jurassic-Early Cretaceous.