岩浆演化对成矿作用的控制——以西藏冈底斯北缘铅锌矿带为例

与岩浆-热液或热液矿床有关的岩浆,大都经历了不同程度的演化(如结晶分异、同化混染等),其岩石学和地球化学特征只是岩浆演化终态的反映,针对这些岩浆岩本身的研究有时很难刻画出其详细的演化过程及该过程中关键成矿元素的地球化学行为。已有研究显示,同时代同区域大面积分布的火成岩可能记录着含矿岩浆演化早期或演化不同阶段的岩石学和地球化学信息,为解决上述难题提供了可能。基于此,本文以发育于大陆碰撞初期(即主碰撞)的西藏冈底斯北缘铅锌矿带为例,通过对主碰撞阶段冈底斯带大量火成岩地球化学数据的统计学研究,初步展示了岩浆演化过程及其对成矿的控制作用。研究结果表明,除了自身体系的分离结晶,陆壳混染作用是大陆碰撞早期岩浆由基性向中酸性演化的另一关键控制因素,主碰撞岩浆中普遍不具有高的含水量和氧逸度,但演化到中后期,岩浆的氧逸度会逐渐升高;富S和H2O的幔源岩浆底侵,诱发具有较高Zn含量的陆壳物质熔融,产生的岩浆再经历高度的演化(以达到Pb的富集),最终浅成就位形成岩浆-热液成矿系统。这是形成冈底斯北缘铅锌矿带的成矿动力学机制。

The control of magma evolution over metallogenesis: A case study of the northern Gangdise Pb-Zn ore belt in Tibet

In the magma-related deposits, the ore-forming plutons generally experienced high-degree evolution and differentiation, so its petrologic and geochemical characteristics can only reflect the final state of magma evolution. The evolution of ore-hosted magma and the geochemical behavior of main metallogenic elements in the evolution remain unclear. However, the study of contemporaneous and sympatric magmatic rocks provides a potential solution for this question. Here the authors made a case study of the northern Gangdise Pb-Zn ore belt in Tibet formed in the early period of continental collision. Based on the statistical study of lots of published geochemical analyses of magmatic rocks from the Gangdise belt, this paper presents the evolution of magma and its control of metallogenesis. The results suggest that, besides the crystallization differentiation of magma itself, crustal contamination was another factor controlling the evolution from basic magma to acid magma during differentiation. It is also found that most of the magmatic rocks formed in the early period of continental collision are less oxidized and less hydrous; nevertheless, when magma evolved to the middle and late period, the oxidation of magma increased with progressive evolution. Mantle-derived magmas are enriched in S and H₂O, which experienced underplating at the base of Zn-enriched crust materials and induced partial melting. Crust-derived magmas experienced high-degree evolution, and finally resulted in the formation of the northern Gangdise Pb-Zn ore belt.