阿吾拉勒东部巩乃斯林场橄榄玄粗岩岩石地球化学特征及地质意义

Geochemical Characteristics and Geological Significance of the Gongnaisi Shoshonite,Eastern Awulale Mountains

西天山阿吾拉勒地区东部巩乃斯附近晚石炭世大哈拉军山组中分布着典型的橄榄玄粗岩,该岩石具高K2O(3.09%~4.26%)、K2O/Na2O(0.67~1.20)和高Al2O3(20.30%~20.50%)特征,但是其成因还没有得到很好的限制。巩乃斯橄榄玄粗岩由具较富集的Sr-Nd同位素组成((87Sr/86Sr)I=0.704471~0.704784;εNd(t)=3.76~3.83),说明其源自于富集地幔源区。通过地球化学数据模拟发现该橄榄玄粗岩起源于尖晶石二辉橄榄地幔,相对于岩石圈地幔具较低的Ba/Rb和高Rb/Sr,结合其较高Th/Nb(1.93~1.99)比值和Th(11.63×10-6~12.18×10-6)含量,指示岩石富钾的特征很可能与再循环俯冲沉积物的加入有关;该岩石的高铝含量特征是由于岩浆中水含量较高导致斜长石结晶较晚形成的,而岩浆富水的特征可能与俯冲板片后撤导致板片脱水加强有关。结合该岩石地球化学特征和区域地质背景,认为巩乃斯橄榄玄粗岩形成于俯冲-碰撞转换阶段,是西天山地区构造体制转换阶段岩浆作用的产物。

The Late Carboniferous Gongnaisi shoshonite is distributed in the Gongnaisi area,the eastern Awulale Mountains,Chinese Western Tianshan.The Gongnaisi shoshonite belongs to Dahalajunshan Formtian and is characterized by high K2O(3.09%~4.26%),Al2O3(20.30%~20.50%)contents and high K2O/Na2O ratios(0.67~1.20),but its petrogenesis is still unclear.The Gongnaisi shoshonite has relatively enriched Sr-Nd isotopic composition((87Sr/86Sr)I=0.704471~0.704784,εNd(t)=3.76~3.83),indicate it derived from an enriched mantle source.Its relatively lower Ba/Rb but higher Rb/Sr than these of the subcontinental lithosphere mantle,combined with its high Th/Nb(1.93~1.99)and Th(11.63~12.18 ppm),implying some subducted sedimentary has been input in its mantle source.Based on our batch partial model also indicate that the shoshonite has a spinel lherzolite source.Besides,its high Al2O3 can be attributed to the high H2O content in the primary magma.Combined with the geochemical characteristics and regional geological background,it is considered that the shoshonite in Gongnaisi was formed in the subduction collision transition stage,which is the product of magmatism during the tectonic regime transformation stage in the Western Tianshan area.