西秦岭格尔括合花岗闪长斑岩岩石成因:黑云母矿物学特征约束

Petrogenesis of Ge'erkuohe porphyry granitoid,western Qinling: Constraints from mineral chemical characteristics of biotites

西秦岭造山带位于松潘-甘孜地体、碧口地体、祁连造山带和东昆仑造山带等多个块体的结合部位,经历了古特斯洋北枝的构造演化,发育了巨量的印支期岩浆活动与显著的金、铜（钼）、铅锌成矿作用,为研究秦岭印支期造山作用与成矿过程提供了可靠物质记录,是理解古特提斯洋北枝构造演化及成矿系统的关键。本文通过对西秦岭南缘格尔括合花岗闪长斑岩进行系统的岩石学、岩相学研究与黑云母成分的电子探针原位分析,厘定了矿物形成的物理化学条件,探讨了岩石成因及形成的构造环境,为解析秦岭造山带印支期造山过程中壳幔演化、构造-岩浆活动提供了矿物尺度的制约。研究表明,格尔括合花岗闪长斑岩中黑云母Fe2＋/（Mg＋Fe2＋）比值介于0.22%~0.34%,并显示贫钙的特征,属于典型的岩浆成因黑云母。黑云母化学成分FeO T与MnO呈现正相关性,FeO T、MnO值与MgO值呈现负相关性,表明其结晶过程中可能发生了Mg2＋和Fe2＋的置换反应,而Mn则主要以类质同象替代Mg进入黑云母晶格。黑云母Al2O3介于16.15%~14.97%,TiO 2介于2.81%~2.05%,卤族元素（F、Cl）含量介于1.73%~0.84%。黑云母的氧化系数为0.27~0.56,MF值为1.33~1.57,Mg#为0.66~0.78,单位分子中阳离子数AlⅥ为0.08~0.18,以单位分子中Ti和Al阳离子数计算的黑云母结晶温度为629~684℃,结晶压力为131~181MPa,相应的侵位深度为5.0~7.1km。上述特征指示该岩体为壳幔混源成因的I型花岗岩,黑云母结晶于相对高温和高氧逸度的环境,岩浆结晶分异程度高。根据岩浆岩内黑云母成分构造环境判别,结合区域上同时期岩体的地质、地球化学特征,推测格尔括合岩体形成于西秦岭晚三叠世后碰撞伸展构造环境,可能是由底侵的幔源岩浆与广泛部分熔融的下地壳物质混合后上升至浅部岩浆房,并发生结晶分异而形成的具有壳幔混源成因的I型花岗岩。

Located in the binding sites among the Songpan-Ganze Terrane,the Bikou Terrane,the Qilian and East Kunlun Orogenic Belt,the West Qinling Orogenic Belt experienced the prolonged evolution of the northeastern branch of the Paleo-Tethys and is essential to understand the Indosinian tectonic evolution and ore-forming process of West Qinling and East Asia. West Qinling is characterized by extensive magmatism coeval with Middle to Upper Triassic gold,copper,molybdenum,lead and zinc mineralization which provide reliable material records for the Qinling Indosinian orogeny and mineralization process. Based on systematic fieldwork,petrography and the mineralogical composition of biotite occurring in Ge＇erkuohe porphyry granitoids on the southern margin of the West Qinling,the paper not only place constraints on the physicochemical conditions of the formation of mineral and the host rock,but also provide important insights into understanding the Qinling orogenic process on the mineral scale. The analyzed biotite from two samples display low Cao,high Mg O and Ti O2,low content of AlⅥand are rich in volatiles（ fluorine,chlorine）. These features suggest that the Mgrich biotite have not been modified by late-stage fluids and the substitution among Mg2 ＋,Fe2 ＋and Mn2 ＋maybe occurred during the crystallization of biotite. Mineral chemistry analysis reveals that the crystallization temperature for biotite is 629 ~ 684℃ and the solidification pressure is 131 ~ 181 MPa,corresponding to solidification depth 5. 0 ~ 7. 1km. The host pluton which was formed in a relatively high temperature and oxygen fugacity environment belongs to I-type granite derived from a crust-mantle mixing source.Considering the research results of discrimination diagrams for tectonic settings of biotite and the regional Triassic plutons,we conclude that Ge＇erkuohe pluton was emplaced in the Late Triassic post-collision extensional environment and the product of magma mixing and crust-mantle interaction.