东昆仑金水口泥盆纪层状花岗岩成因和找矿意义

东昆仑造山带是青藏高原与冈底斯带相媲美的巨型岩浆岩带, 但该带是否具有形成硬岩型稀有金属矿床的潜力还不清楚。最近在东昆仑金水口地区发现了锂铍矿化伟晶岩, 暗示其有可能成为以锂铍为主的稀有金属成矿潜力区。本文以矿化伟晶岩空间上紧密共生的金水口层状花岗岩体为研究对像, 揭示其地球化学特征和形成时代, 探讨其成因和找矿意义。金水口层状花岗岩下部为弱过铝高钾钙碱性黑云母二长花岗岩, 上部为高分异强过铝二云母正长花岗岩、石榴子石碱长花岗岩和钠长花岗岩。花岗岩LA-ICP-MS锆石U-Pb年龄为396~395Ma, LA-ICP-MS独居石U-Pb年龄为396~391Ma。黑云母二长花岗岩、二云母正长花岗岩、石榴子石碱长花岗岩和钠长花岗岩具有相似的Nd同位素组成, ε_Nd(t)为-6.52~-4.48, 黑云母二长花岗岩与二云母正长花岗岩具有相似的全岩Pb同位素组成和锆石Hf同位素组成, ²⁰⁸Pb/²⁰⁴Pb(t)为38.187~38.219, ²⁰⁷Pb/²⁰⁴Pb(t)为15.627~15.652, ²⁰⁶Pb/²⁰⁴Pb(t)为18.299~18.527, ε_Hf(t)为-6.25~+0.58。下部花岗岩和上部高分异花岗岩均源于元古代岩石和少量新生年轻地壳物质组成的下地壳, 为I型花岗岩, 形成于碰撞后伸展构造环境。金水口花岗岩岩浆演化过程中Li、Be、Nb、Ta和Sn富集, 与其东侧跃进山花岗岩共同构成了潜在的稀有金属矿化伟晶岩的母体花岗岩, 推断跃进山花岗岩南部和东部具有伟晶岩型Li-Be矿的找矿潜力。东昆仑造山带出现泥盆纪Li-Be矿成矿作用, 暗示青藏高原北部除了中生代外, 古生代也存在Li-Be矿重要成矿潜力。

Petrogenesis of layered Devonian granites in the Jinshuikou area of East Kunlun and its prospecting significance

The East Kunlun orogenic belt is a giant magmatic belt which is comparable to the Gangdese belt in the Qinghai-Tibet Plateau, however, it is still unclear whether it holds any potential for forming hard-rock rare-metal deposits. Recently, lithium-beryllium mineralized pegmatites have been discovered in the Jinshuikou area of East Kunlun, suggesting that it may become a potential rare-metal metallogenic area dominated by lithium-beryllium. This paper reports the geochemical characteristics and ages of layered granite intrusions in the Jinshuikou area that are spatially closely associated with the mineralized pegmatites and discusses their petrogenesis and prospecting significance. The lower part of the layered granite intrusions in the Jinshuikou area is composed of weakly peraluminous high-potassium calc-alkaline biotite monzogranites, while the upper part consists of highly fractionated and strongly peraluminous two-mica syenogranites, garnet alkali feldspar granites, and albite granites. LA-ICP-MS zircon and monazite U-Pb ages of the granites are 396~395Ma and 396~391Ma, respectively. Furthermore, these granites share similar Nd isotopic compositions with the εNd(t) values of -6.52~-4.48. Biotite monzogranites and two-mica syenogranites have similar whole-rock Pb isotopic compositions (208Pb/204Pb(t)=38.187~38.219, 207Pb/204Pb(t)=15.627~15.652, 206Pb/204Pb(t)=18.299~18.527) and zircon Hf isotopic compositions (εHf(t)=-6.25~+0.58). The lower and upper granites were both derived from a lower crustal source composed of Proterozoic rocks with minor juvenile crustal material. They are I-type granites formed in a postcollisional extensional tectonic environment. Li, Be, Nb, Ta and Sn were enriched during the magmatic evolution of the Jinshuikou layered granites. These granites, together with eastern Yuejinshan granites, constitute parent granites for potential rare-metal mineralized pegmatites. It is inferred that the southern and eastern parts of the Yuejinshan granite intrusion are potential areas for pegmatite-type Li-Be mineralization.The occurrence of Devonian Li-Be mineralization in the East Kunlun orogenic belt indicates that in addition to the Mesozoic era, the Paleozoic era also has significant mineralization potential for Li-Be ores along the northern Qinghai-Tibet Plateau.