喜马拉雅碰撞造山带新生代地壳深熔作用与淡色花岗岩

自从印度-欧亚大陆碰撞以来,伴随着构造演化和温度-压力-成分(P-T-X)的变化,喜马拉雅造山带中下地壳变质岩发生不同类型的部分熔融反应,形成性质各异的过铝质花岗岩。这些花岗岩在形成时代、矿物组成、全岩元素和放射性同位素地球化学特征上都表现出巨大的差异性。始新世构造岩浆作用形成高Sr/Y二云母花岗岩和演化程度较高的淡色花岗岩和淡色花岗玢岩,它们具有相似的Sr-Nd同位素组成,是碰撞早期增厚下地壳部分熔融的产物。渐新世淡色花岗岩主要为演化程度较高的淡色花岗岩,可能指示了喜马拉雅造山带的快速剥露作用起始于渐新世。早中新世以来的淡色花岗岩是喜马拉雅造山带淡色花岗岩的主体,是变泥质岩部分熔融的产物,包含两类部分熔融作用——水致白云母部分熔融作用(A类)和白云母脱水熔融作用(B类)。这两类部分熔融作用形成的花岗质熔体在元素和同位素地球化学特征上都表现出明显的差异性,主要受控于两类部分熔融作用过程中主要造岩矿物和副矿物的溶解行为。这些不同期次的地壳深熔作用都伴随着高分异淡色花岗岩,伴随着关键金属元素(Nb、Ta、Sn、Be等)的富集,是未来矿产勘探的重要靶区。新的观测结果表明:在碰撞造山带中,花岗岩岩石学和地球化学性质的变化是深部地壳物质对构造过程响应的结果,是深入理解碰撞造山带深部地壳物理和化学行为的重要岩石探针。

Cenozoic crustal anatexis and the leucogranites in the Himalayan collisional orogenic belt

Since the India-Eurasian continental collision, the Himalayan orogenic belt has experienced major tectonic transitions from earlier crustal compression and thickening to later extension and rapid exhumation, which induced pronounced changes in the pressure-temperature-composition (P-T-X) of high-grade metamorphic rocks. Consequently, the mid to lower crustal rocks has undergone correspondingly different partial melting processes and produced a wide spectrum of melts of leucogranitic compositions. Such granites show substantial differences in ages of crystallization, mineral compositions, whole-rock element as well as radiogenic isotope (Sr and Nd) compositions. The earliest anatexis is represented by the Eocene (~43Ma) high Sr/Y granites from melting of mafic rocks under thickened crustal conditions, followed by the melting of metasedimentary rocks in the Oligocene time, possibly induced by the initiation of rapid exhumation of the Himalayan high grade basements. A majority of leucogranites formed from ~25Ma to ~10Ma were derived either from fluxed melting of muscovite (A-type) or from muscovite dehydration melting (B-type) of metasediments. These two modes of crustal anatexis, possibly from vastly similar source rocks, generated granitic melts with substantial differences in major and trace element as well as Sr isotope geochemistry due to the coupled differences in the melting behavior of the major minerals (muscovite, feldspar) and accessory phases (zircon and monazite) during different modes of crustal anatexis. Each phases of leucogranite production are accompanied by various degrees of differentiation and the formation of highly fractionated leucogranites. Some of such leucogranites are highly enriched in key metal elements (Sn, Nb, Ta, and Be) and thus could be potential targets for future exploration of precious metals. Data summarized in this contribution suggests a strong coupling of granite chemical compositions with the changes in tectonic regimes, which in turn implies that leucogranites, by sorting out their original melt compositions, could serve as a valuable probe to investigate the physical and chemical behavior of deep crustal rocks in collisional orogenic belts worldwide.