新疆百口泉闪长岩中高An值斜长石的成因及岩石学意义

The origin of high-An plagioclase in diorite from Baikouquan,Xinjiang and its petrogenetic significance

岩浆成因的高An值斜长石（An80-100）是玄武质岩浆早期结晶的产物，由于其形成条件较为苛刻，它对于限定寄主岩浆条件和探讨岩石成因有重要的指示作用。在新疆西准噶尔百口泉地区发育含高An值斜长石（An80-90）的闪长岩，高An值斜长石呈不规则包裹体形式存在于闪长岩主体斜长石中（An40-60）。锆石SHRIMP定年显示百口泉闪长岩形成于316．9±2．9Ma（MSWD=1．5），闪长岩主、微量元素和Sr-Nd同位素与区域内同时期岩浆作用产物具有相似的演化趋势。根据闪长岩岩石学特征，结合高An值斜长石的实验研究成果，本文认为百口泉闪长岩中的高An值斜长石不是捕虏晶，也不是闪长岩的结晶产物，而是与闪长岩成岩过程相关的循环晶。依据角闪石铝压力计的估算，闪长岩结晶于1．2-2．9kbar，与实验条件下获得的高An值斜长石产出的压力范围相符。高An值斜长石的产出表明原始岩浆具有富水、亏损的特征，对于探讨整个岩浆系统的演化过程有重要指示意义。

Magmatic high-An plagioclase （An80-100 ） crystallizes at the early stage of basaltic meh evolution. Due to the restricted conditions under which high-An plagioclase can form and preserve, it can help constrain the crystallization environment and petrogenesis of the host magma. High-An plagioclase （ An80-90 ） bearing diorite outcrops in Baikouquan, West Junggar, Xinjiang. It was dated to be 316. 9 ± 2. 9Ma （ MSWD = 1.5 ） by zircon SHRIMP. The high-An plagioclase occur as irregular patches in the predominant plagioelase of diorite. The major and trace elements and Sr-Nd isotopic data match the trend of the contemporary intrusions in this region. Detailed petrological and mineralogical studies demonstrate the high-An plagioclase are antecryst which belong to the magmatic system producing the host but are not directly a consequence of the crystallization of the host. They came from an earlier pulse of magma and which are incorporated in a later pulse. Based on the Al-in-amphibole geobarometer, the diorite crystallized at 1.2 - 2. 9kbar, which is compatible with the condition for the high-An plagioclase formation. The crystallization of high-An plagioclase in Baikouquan diorite suggests a H2 O-rich and depleted primary magma, and has a profound implication on the magmatic system.