新疆北部卡拉麦里地区黄羊山碱性花岗岩的岩石成因

Petrogenesis of the Huangyangshan alkali granites in Kalamaili area,northern Xinjiang

北疆地区晚古生代后碰撞岩浆活动以大面积分布碱性花岗岩为特征。侵位于晚石炭世的黄羊山碱性花岗岩体是北疆地区产于300Ma左右后碰撞伸展环境中的典型的花岗岩体之一,以正εNd(t)值(+5.06～+6.67)和年轻的tDM(501～878Ma)为特征。黄羊山碱性花岗岩包括钠铁闪石碱性花岗岩和角闪石碱长花岗岩,为过碱质到弱过铝质,岩石化学属钾玄岩系列和高钾钙碱性系列;稀土元素配分曲线为典型的"V"字形,具明显的负Eu异常;富集大离子亲石元素和高场强元素,显著亏损Ba、Sr、P2O5、TiO2。根据这种碱性花岗岩高εNd(t)值和tDM与区域内洋壳年龄吻合度较好的特征,推测其可能源岩应为洋盆闭合时洋壳残片转化而来的年轻地壳。模拟计算得出,黄羊山碱性花岗岩中幔源组分的比例可达88%～92%。在维宪末期-谢尔普霍夫末期的萨吾尔运动之后,软流圈物质上涌所携带的巨大热能加热了区域范围内的地壳,先期抵达的幔源岩浆所携带的热能进一步加热并熔融了下地壳镁铁质物质,形成北疆地区～300Ma的碱性花岗岩。同时,本文也说明了洋壳残片通过构造运动归并到陆壳当中亦可能是某些地区在某些地质时期大陆地壳增生的重要方式之一。

Late Paleozoic post-collisional magmatism in northern Xinjiang is characterized by extensive development of alkali A-type granites,Huangyangshan alkali granite is one of the typical post-collisional plutons in northern Xingjiang around 300Ma,which are characterized by positive εNd( t) values ( + 5. 06 ～ + 6. 67) and young tDM ages ( 501 ～ 878Ma) . The Huangyangshan alkali granite includes arfvedsonite alkali granite and amphibole alkali-feldspar granite. The composition ranges from peralkaline to weakly peraluminous,belonging to shoshonite series. The rocks display rare earth elements patterns with'V'shape,and evidently depleted in Eu. They are enriched in light rare earth elements and high strength field elements,significantly depleted in Ba,Sr,P2O5,TiO2. Based on their high εNd( t) values,and the coincidence between their tDM ages and the age of oceanic crust in the research area,the possible magma source could be the young crust transformed from oceanic fragment during the closure of ocean basin. Modeling calculation suggests high mantle component up to 88% ～ 92% for the Huangyangshan alkali granite. After the Sawuer orogeny,at the end of Visean to the end of Serpukhovian,a great amount of heat energy provided by the upwelling of asthenosphere heated the crust in the region,early arrived mantle derived magma further heated and melted the mafic lower crust,forming the ～ 300Ma alkali granites in northern Xinjiang. Meanwhile,it is concluded that the formation of new crust by the transformation from oceanic fragment in tectonic activities is one of the important mechanisms for the accretion of crust in certain area and period.