北山地区白云山花岗闪长岩岩石地球化学、年代学特征及其地质意义

GEOLOGICAL SIGNIFICANCE AND CHARACTERISTICS OF GEOCHEMISTRY AND CHRONOLOGY OF BAIYUNSHAN GRANODIORITE IN THE BEISHAN AREA OF INNER MONGOLIA

花岗闪长岩位于白云山蛇绿岩带北侧,公婆泉岛弧带南部,由细粒花岗闪长岩和中粒花岗闪长岩组成。细粒花岗闪长岩和中粒花岗闪长岩均具有具高Si(63.46%~67.68%,65.5%~66.98%)、低Al(14.27%~15.52%,15.72%~16.42%)特征,为亚碱性系列岩石,稀土含量较低(105.40~116.59×10^-6,114.21~132.69×10^-6),配分曲线具右倾特征,轻、重稀土分异程度略高(LREE/HREE为9.38~10.42,9.90~12.81),HREE分布较为平坦,岩石具Nb,Ta负异常,为I型花岗岩,具埃达克岩特征。岩石为底侵下地壳部分熔融成因,岩浆源区残留矿物主要为角闪石和石榴石。细粒花岗闪长岩锆石U-Pb谐和年龄为363.9±0.94Ma,加权平均值为363.8±1.8Ma,形成于晚泥盆世。中粒花岗闪长岩U-Pb谐和年龄为348.7±1.0Ma,加权平均值为348.5±2.0Ma,形成于早石炭世。细粒花岗闪长岩和中粒花岗闪长岩岩体形成于后碰撞构造环境。

The Baiyunshan granodiorite is located in the north of the ophiolite belt of Baiyunshan Mountain and in the south of the Gongpoquan island arc belt.It is composed of fine granodiorite and medium grained granodiorite with high content of Si(63.46%-67.68%,65.5%-66.98%)and low content of Al(14.27%-15.52%,15.72%-16.42%),thus classified as sub-alkaline series of rocks with low rare earth content(105.40-116.59×10^-6,114.21-132.69×10^-6)and right incline partition curve.The degree of differentiation of light and heavy rare earth elements is slightly higher than average(LREE/HREE is 9.38-10.42,9.90-12.81),the HREE distribution is relatively flat with negative Nb and Ta anomalies,belonging to I-type granite.The fine grained granodiorite shows adakite characteristics and formed from partial melting of the lower crust.The remaining minerals in the magmatic source area are mainly amphibole and garnet.Zircon U-Pb dating of fine grained granodiorite yields a concordant age of 363.9±0.94Ma and a weighted average value of 363.8±1.8Ma(late Devonian period),while the zircon U-Pb dating of medium grained granodiorite yields a concordant age of 348.7±1.0Ma and a weighted average age of 348.5±2.0Ma(early Carboniferous period).Both types of granodiorite rock masses were formed because of a post-collision tectonic environment.