深熔成因伟晶岩初始岩浆中稀有金属富集特征

Enrichment characteristic of rare metals in the initial pegmatiteforming melt formed by anatexis of metasedimentary rocks

本文以新疆阿尔泰切木尔切克地区的混合岩和伟晶岩为例,开展区内混合岩、伟晶岩与周围的变沉积岩(二云母片岩)的野外地质、全岩地球化学以及云母类矿物化学组成研究,目的在于揭示深熔作用形成的熔体中稀有金属富集特征。研究结果显示,阿尔泰切木尔切克地区的伟晶岩与混合岩中浅色体有相似的矿物组合和主要化学组成,且伟晶岩与暗色体呈互补的微量元素组成,表明切木尔切克伟晶岩为变沉积岩深熔成因。从二云母片岩到混合岩中浅色体和伟晶岩,白云母中Li、Be、Nb、Ta、Rb、Cs显示增加趋势,表明深熔作用形成浅色体及其汇聚成伟晶岩脉过程中可促进稀有金属的富集,尤其是Be和Ta,富集程度达3倍以上。低温条件下白云母脱水熔融,导致黑云母作为残留相,明显制约Li、Rb、Cs等稀有金属在熔体中的富集,但对Be的影响非常有限。结合阿尔泰伟晶岩广泛的Be成矿作用,推断阿尔泰伟晶岩很可能是深熔成因的。

In this study,we examine the pegmatite and migmatite exposed in the Qiemuerqieke area to investigate the enrichment characteristics of rare metals in the initial pegmatite-forming melt derived from the anatexis of metasedimentary rocks.We analyzed the geology,whole-rock geochemistry,and mineral chemistry of micas from the migmatite,pegmatite,and surrounding metasedimentary rock(two-mica schist).The results show similarities in mineral assemblages and chemical compositions between the pegmatite and leucosome,along with complementary trace elements between the pegmatite and melanosome.This observation supports the anatexis origin of the Qiemuerqieke pegmatite.Furthermore,the increasing content of Li,Be,Nb,Ta,Rb,and Cs in muscovites from two-mica schist to leucosome and finally to pegmatite indicates progressive enrichment of rare metals during anatexis and aggregation processes from the leucosome to the pegmatite.Notably,Be and Ta showed a significant increase,with their concentrations rising by at least three times.Additionally,low-temperature dehydration melting of muscovite and the residual phase dominated by biotite appear to restrain the enrichment of Li,Rb,and Cs in the melt,but have no effect on Be concentrations.Given the extensive Be mineralization observed in the Altay pegmatite,it is reasonable to deduce that the Altay pegmatite probably originated from the anatexis of metasedimentary rocks.