喜马拉雅淡色花岗岩成因与稀有金属成矿潜力

喜马拉雅淡色花岗岩世界瞩目，具有重要的理论研究和找矿意义，但是其成因争议较大。本文统计了两千余件样品的全岩主微量地球化学、Sr-Nd-Pb-Hf同位素、锆石/独居石/磷钇矿等副矿物原位U-Pb年龄和锆石Hf同位素等，试图全面地总结喜马拉雅淡色花岗岩的研究进展和现状。喜马拉雅淡色花岗岩分为南北两带，北带花岗岩主要出露于特提斯喜马拉雅和片麻岩穹隆中，而南带花岗岩主要发育在高喜马拉雅顶部和东-西构造结中。从北往南，成岩时代逐渐变新；南北两带均以二云母花岗岩和(石榴石-电气石)白云母花岗岩为主，两期(始新世和中新世)中-基性岩脉和埃达克质岩主要在北带中发育。新生代岩浆活动分为5个阶段：49~40 Ma、39~29 Ma、28~15 Ma、14~7 Ma、6~0.7 Ma，分别主要与新特提斯洋壳板片断离、印度陆壳板片的低角度俯冲、断离或回撤、南北向撕裂(裂谷)和东西构造结的快速隆升有关。喜马拉雅淡色花岗岩起源于高喜马拉雅杂岩系的不一致(不平衡)部分熔融，并经历了矿物分离结晶的高分异演化。淡色花岗岩属于强过铝质岩石，具有高Si、K、Na，低Ca、Fe、Mg、Ti、Mn，高的Rb/Sr、Y/Ho值，低的Th/U、Nb/Ta、Zr/Hf、K/Rb值，稀土元素总量较低，负Eu异常明显的地球化学特征。随着成岩时代变新，Sr-Nd-Pb-Hf等同位素都指示岩浆源区中古老地壳物质的占比逐步增加。喜马拉雅淡色花岗岩/伟晶岩中Li、Be、W、Sn、Ta、Cs和Rb等稀有元素的富集系数大于10，伟晶岩属于典型的LCT型伟晶岩。喜马拉雅新生代淡色花岗岩带有望成为一条新的世界级的Li-Be-Sn-W-Ta稀有金属成矿带。

Genesis of Himalayan leucogranite and its potentiality of rare-metal mineralization

The Himalayan leucogranite attracts the attention of the world and has important theoretical and prospecting significances, but its genesis is controversial. In this paper, the geochemistry of whole rock main and trace elements, Sr-Nd-Pb-Hf isotopes, in-situ U-Pb ages of zircon/monazite/xenotime and other accessory minerals, and zircon Hf isotopes of secondary minerals from more than 2000 samples have been reviewed, in order to comprehensively summarize the research progresses and status of Himalayan leucogranites. The Himalayan leucogranites are divided into two zones.The northern zone is mainly exposed in the Tethyan Himalayas and gneiss dome, and the southern zone is mainly developed in the top of the Great Himalayan Copmlex and the Western-Eastern Himalayan Syntaxis. From north to south, the petrogenetic ages become younger gradually. There are two-mica granites and (garnet-tourmaline) muscovite granites in the northern and southern zones, and two stages (Eocene and Miocene) intermediate-basic dikes and adakite rocks are mainly developed in the northern zone. The Cenozoic magmatic activity can be divided into five stages: 49-40 Ma, 39-29 Ma, 28-15 Ma, 14-7 Ma and 6-0.7 Ma, which are mainly related to the separation of the New Tethyan oceanic plate, the low angle subduction, detachment or retraction, the north-south tearing (north-south-trending rift) of the Indian continental plate, and the rapid uplift of the Himalayan syntaxes, respectively. The Himalayan leucogranites originated from the incongruent (disequilibrium) partial melting of the Great Himalayan Complex and underwent highly differentiated evolution of mineral separation crystallization. The leucogranites are characterized by high Si, K, Na, low Ca, Fe, Mg, Ti, Mn, strong peraluminite, low total rare earth elements, obvious negative Eu anomaly, high Rb/Sr, Y/Ho values, and low Th/U, Nb/Ta, Zr/Hf, K/Rb values. As the petrogenetic ages become younger, the Sr-Nd-Pb-Hf isotopes show that the proportion of older crustal material in the magmatic source area increases gradually. The enrichment coefficients of rare elements such as Li, Be, W, Sn, Ta, Cs and Rb in the Himalayan leucogranite are greater than 10 relative to the total crustal value, and they belong to LCT-type pegmatite. The Cenozoic leucogranite belt of the Himalayas is expected to be a new world-class Li-Be-Sn-W-Ta rare metal metallogenic belt.