东天山觉罗塔格构造带钙碱性侵入岩角闪石矿物学特征及其对区域找矿的启示

Mineralogical Characteristics of Amphibole in Calc-alkalic Intrusive Rocks from the Juelotage Tectonic Belt of the Eastern Tianshan and its Implication for Regional Prospecting

新疆东天山觉罗塔格构造带位于吐哈盆地和中天山地块之间,区内矿产资源丰富,是我国重要的金属矿产分布区,其成矿作用与岩浆活动密切相关。本文在详细岩相学观察的基础上,对角闪石进行了电子探针(EMPA)测试分析,限定了区内8个岩体结晶的温压条件、岩浆氧逸度和含水量等要素,为解析觉罗塔格构造带内钙碱性侵入岩的构造环境、演化过程和成矿作用提供矿物学方面的制约。测试分析表明,岩体中的角闪石富镁(MgO:5.82%~18.04%)、富钙(CaO:9.91%~12.56%)、富钠(Na_(2)O/K_(2)O>1.0)、贫钾(K_(2)O:0.06%~1.16%),属于钙角闪石族。角闪石化学成分特征揭示其大部分为中酸性侵入岩中的角闪石,寄主岩浆为钙碱性,岩浆物质来源具有壳幔混源的特征,是板块俯冲背景下由地幔楔与大陆地壳物质混熔形成。由角闪石温压计得出矿物的结晶温度为751.88~887.46,℃压力为62~248 MPa,相应侵位深度为2.35~9.4 km。通过角闪石成分计算出岩浆的氧逸度为ΔNNO+0.30至ΔNNO+2.48,并且在角闪石结晶时岩浆具有较高的含水量(H_(2)O_(melt)>5%)。通过分析可知觉罗塔格构造带中的钙碱性侵入岩具有高温、低压、高氧逸度、富水和侵位浅的特点,有利于Cu等元素在成矿流体中富集,符合区域斑岩型铜矿的成矿条件。因此推测觉罗塔格构造带内仍具有一定斑岩铜矿的找矿潜力。

The Juelotage tectonic belt lies between the Tuha Basin and the Central Tianshan Massif, within the Eastern Tianshan of XinJiang, where the geological structures are very complicated and rich in mineral resources, and the mineralization is closely related to the magmatic activity. Based on detailed petrographic observation, the authors analyzed the compositions of amphibole from the intrusive rocks by electron microprobe analysis (EMPA). The critical factors for calc-alkalic magmas such as temperature and pressure conditions, oxygen fugacity and water contents of the magmas are estimated, which provide the mineralogical constraints on the environments of magmatism, magmatic evolution and metallogenic processes in the Juelotage tectonic belt. The EMPA data show that the amphibole is rich in magnesium (MgO 5.82%-18.04%), calcium (CaO 9.91%-12.56%) and sodium (Na_(2)O/K_(2)O>1.0), belonging to calcic amphiboles. The chemical compositions of amphibole reveal that the host intermediate-acid intrusive rocks were derived from a hybrid of melts of the mantle wedge and the continental crust under subduction setting. The amphibole is characterized by crystallization temperatures of 751.88-887.46 , ℃ crystallization pressures of 62-248 MPa, with magma emplacement depth of 2.35-9.4 km. The oxygen fugacity of calc-alkalic magmas defined by amphibole components varies from ΔNNO+0.30 to ΔNNO+2.48, and H_(2)O contents of the melts when amphibole crystallized are higher than 5%. Through analysis, it is known that the calc-alkalic magmas in the Juelotage tectonic belt are characterized by high oxygen fugacity, water-rich and shallow emplacement, which is beneficial to the enrichment of Cu and other elements in the ore-forming fluids, and consistent with the metallogenic conditions for porphyry copper deposits. Therefore, it is suggested that this belt has great prospecting potential for porphyry copper deposits.