Textures and chemistry of columbite-group minerals record magmatic–hydrothermal processes in the Baishitouquan pluton in the Eastern Tianshan, NW China

IF 3.2 2区 地球科学 Q1 GEOLOGY
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Abstract

Rare-metal granites result from a combination of magmatic and hydrothermal processes. The mineralogical, textural, and chemical characteristics of columbite-group minerals (CGMs), which are ubiquitous in rare-metal granites and pegmatites, can serve as records of the magmatic–hydrothermal processes responsible for the evolution of these granitic systems. The highly evolved Baishitouquan (BST) granite pluton, located in the Eastern Tianshan of NW China, hosts the large-scale Zhangbaoshan (ZBS) Rb deposit (Rb2O reserve of 67080 t), making it of economic significance. The BST pluton comprises a series of gradual lithological zones, which, from bottom to top, are leucogranite (Zone-a), amazonite-bearing granite (Zone-b), amazonite granite (Zone-c), topaz-bearing amazonite granite (Zone-d), topaz albite granite (Zone-e), and amazonite pegmatite veins. Based on textural and chemical characteristics, five types of CGMs were identified — CGMs with no zoning, and oscillatory zoned, normally zoned, hydrothermally overprinted, and patchy CGMs. The CGMs that exhibit no zoning, oscillatory zoning, and normal zoning mainly occur in Zones-a to -c, suggesting that the lithological and geochemical variations in these zones formed as a result of the magmatic evolution of the BST magma. Hydrothermally overprinted CGMs and those with a patchy texture mainly occur in Zones-d and -e, and the pegmatite veins, suggesting that the evolution of these zones involved hydrothermal processes. From Zone-a to Zone-c, the Ta# and Mn# of CGMs increase gradually, suggesting a gradual evolution of the BST magma. The CGMs are characterized by REE tetrad effect (TE1,3) that is consistently greater than 1.1 and increases from Zone-a to the pegmatite veins, indicative of increased melt–fluid interaction during evolution of the BST magma. It is, therefore, suggested that the BST magma evolved not only via high degrees of fractional crystallization, but also by the interaction of the melt with hydrothermal fluids, the latter of which likely originated by exsolution from the evolving melt. Based on the mineralogical, textural, and chemical characteristics of CGMs in the BST pluton, a petrogenetic model is proposed to explain the magmatic–hydrothermal evolution of the BST granitic magma.

Abstract Image

铌铁矿族矿物的纹理和化学性质记录了中国西北部东天山白石头泉岩浆-热液过程
稀有金属花岗岩是岩浆和热液过程相结合的产物。稀有金属花岗岩和伟晶岩中普遍存在的铌铁矿族矿物(CGMs)的矿物学、纹理和化学特征可以作为这些花岗岩系统演化过程中岩浆-热液过程的记录。高度演化的白石头泉(BST)花岗岩岩体位于中国西北部的东天山,是大型张宝山(ZBS)铷矿床(铷氧化物储量为 67080 吨)的所在地,因此具有重要的经济意义。BST岩体由一系列渐变岩性带组成,自下而上依次为白云母岩(a区)、含亚安山岩花岗岩(b区)、亚安山岩花岗岩(c区)、含黄玉亚安山岩花岗岩(d区)、黄玉白云母花岗岩(e区)和亚安山岩伟晶岩脉。根据纹理和化学特征,确定了五种类型的 CGM--无分带 CGM、振荡分带 CGM、正常分带 CGM、热液叠印 CGM 和斑状 CGM。无分带、振荡分带和正常分带的CGM主要出现在a区至c区,表明这些区域的岩性和地球化学变化是BST岩浆演化的结果。热液叠加CGM和具有斑状纹理的CGM主要出现在d区和-e区以及伟晶岩脉中,表明这些区域的演化过程涉及热液过程。从a区到c区,CGMs的Ta#和Mn#逐渐增加,表明BST岩浆是逐渐演化的。CGMs的REE四元效应(TE1,3)持续大于1.1,且从a区到伟晶岩脉的TE1,3持续增大,表明BST岩浆在演化过程中熔液相互作用增强。因此,这表明 BST 岩浆的演化不仅是通过高度的分馏结晶,而且还通过熔体与热液的相互作用,后者很可能源自演化熔体的外溶。根据 BST 柱岩中 CGM 的矿物学、纹理和化学特征,提出了一个岩石成因模型来解释 BST 花岗岩岩浆的岩浆-热液演化过程。
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来源期刊
Ore Geology Reviews
Ore Geology Reviews 地学-地质学
CiteScore
6.50
自引率
27.30%
发文量
546
审稿时长
22.9 weeks
期刊介绍: Ore Geology Reviews aims to familiarize all earth scientists with recent advances in a number of interconnected disciplines related to the study of, and search for, ore deposits. The reviews range from brief to longer contributions, but the journal preferentially publishes manuscripts that fill the niche between the commonly shorter journal articles and the comprehensive book coverages, and thus has a special appeal to many authors and readers.
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