内蒙古科尔沁右翼前旗地区满克头鄂博组火山岩年代学、地球化学特征及其地质意义

Chronology,geochemistry and geological significance of volcanic rocks of the Manketouebo Formation in Horqin Right-Front Banner area,Inner Mongolia

对内蒙古科尔沁右翼前旗地区满克头鄂博组火山岩的形成时代与地球化学特征进行了研究。对2件典型流纹岩样品进行了LA-ICP-MS锆石U-Pb测年,结果分别为145.9±1.0 Ma和146.0±1.3 Ma,为晚侏罗世。该区满克头鄂博组火山岩属于过铝质、高钾钙碱性系列,具有高Si O_(2)(63.28%~77.40%,平均为71.06%)、高全碱(K_(2)O+Na_(2)O)含量(7.52%~10.14%,平均为8.66%),富集大离子亲石元素Rb、K,亏损高场强元素Nb、Ta、Ti、P等特征,显示出壳源岩浆的典型特征。满克头鄂博组流纹质和粗面岩在主微量元素上具有良好的演化趋势,推测是壳源物质部分熔融之后,经历了不同程度的分离结晶作用后喷发形成。同时,该组火山岩具有造山后岩浆作用地球化学特征,结合大兴安岭地区满克头鄂博组时空分布上存在自西南向东北年龄逐渐变新的特点,认为该期火山作用与蒙古-鄂霍次克缝合带闭合后伸展作用密切相关。

This paper studies the formation age and geochemical characteristics of the volcanic rocks of the Manketouebo Formation in the Horqin Right-Front Banner area,Inner Mongolia. The LA-ICP-MS zircon U-Pb dating of two typical rhyolite samples reveals that the eruption occurred in the Late Jurassic( 145. 9 ± 1. 0 Ma and 146. 0 ±1.3 Ma,respectively). The volcanic rocks of the Manketouebo Formation in the study area belong to the peraluminous,high-potassium calcium-alkaline series,with high SiO_(2)( 63.28% ~77.40%,with average of 71.06%) and total alkalis( 7.52% ~ 10.14%,with average of 8.66%). The trace element patterns are characterized by enrichment of large ion lithophile elements( e.g.,Rb,K) and depletion of high field strength elements( e. g.,Nb,Ta,Ti,P),showing the typical characteristics of crust-derived magma. The rhyolite and trachyte in Manketouebo Formation have a good evolution trend in the major and trace elements. It is speculated that the crust-derived material was partially melted and erupted after different degrees of fractional crystallization. Besides,the volcanic rocks have similar geochemical characteristics of post-orogenic magmatism. Combined with the temporal and spatial distribution that the ages of the Manketouebo Formation in the Da Hinggan Mountains increase from southwest to northeast,it is proposed that the volcanism of this period is closely related to the post-orogenic extension after closure of the Mongol-Okhotsk Ocean.