花岗伟晶岩成因探讨:岩浆分异与深熔

Overview of magmatic differentiation and anatexis: Insights into pegmatite genesis

花岗伟晶岩成因研究是探索花岗伟晶岩成岩和稀有金属成矿作用的重要内容。花岗伟晶岩成因主要为岩浆分异和深熔作用,即伟晶岩来自花岗质岩浆的结晶分异或者小比例部分熔融(深熔)。花岗伟晶岩分类以及特征矿物组合可为花岗伟晶岩成因探讨提供初步依据。岩浆分异成因伟晶岩由母体花岗质岩浆派生而成,明确母体花岗岩是判别岩浆分异成因的关键。母体花岗岩与伟晶岩通常具有密切的时空关系(近同期,相距不超过10 km),存在连续的分异演化趋势,并且来自同一物质来源。通过瑞利分馏方程模拟(主量-微量-稀土元素和稳定同位素等)查明结晶分异程度,解析分异成因伟晶岩岩浆的形成过程。根据区域构造-变质事件,区域变质作用特征,伟晶岩与变质岩的空间关系、成分关系和一致的同位素组成,伟晶岩产出的特色矿物组合和矿物包裹体,以及伟晶岩与部分熔融熔体相近的化学组成等,来明确伟晶岩来自深熔作用。通过岩石特征元素含量、同位素示踪和微量元素模拟判别源岩,运用Rb/Sr-Ba图解等明确熔融方式,根据源岩矿物组成特征查明熔融条件和熔体产出情况,结合特征元素在熔体-矿物相间的分配行为探讨熔融过程和熔体抽提汇聚史,最终阐明深熔成因伟晶岩熔体形成过程。稀有金属伟晶岩岩浆来自高分异花岗质岩浆的极度结晶分异,少数情况下可由成熟沉积岩/变质沉积岩低程度部分熔融形成。没有母体花岗岩出露的稀有金属伟晶岩群也可来自岩浆分异作用,或是深熔熔体通过进一步分异产生。深入理解部分熔融和结晶分异对伟晶岩岩浆的控制作用,探索伟晶岩岩浆形成过程中相关的物理化学过程,尤其是熔体逃离迁移机制及其对稀有金属元素富集的影响,并建立花岗伟晶岩成因的判别标志,是花岗伟晶岩成因探讨亟待开展的研究工作。

The origin of granitic pegmatites is significant for the understanding of their formation processes and rare-metal metallogenesis.Granitic pegmatites are mainly formed by fractional crystallization of granitic magmas or by anatexis.In discussing pegmatite genesis,pegmatite classification and its mineral assemblage characteristics can provide the preliminary evidence,whilst parental granite plutons provide the final proof of origin.Studies have shown that granite pluton and its pegmatite swarm are nearly coeval,with less than 10 km apart in location and continuing to differentiate,and they have a common material source.To determine the degree of fractional crystallization and to decipher the formation process of granite magmas major element/trace element/REE/stable isotope Rayleigh fractional crystallization models have been used.Current evidences for an anatexis origin include regional metamorphic-tectonic events;metamorphism features;close spatial and chemical compositional relationships and consistent isotopic compositional relationships between pegmatites and metamorphic rocks;formations of unique mineral assemblages and mineral inclusions in pegmatites;and similar chemical components between the parental granite magma and partial melt.The following research approaches have been used to discuss the partial-melting process and melt-extraction history and to clarify the pegmatite-forming process via anatexis:determining protolith by elemental comparison,isotope tracing,and trace-element simulation;clarifying major melting model using Rb/Sr Ba diagram;and determining the melting condition and melt production based on mineral composition of protolith and element partitioning between mineral and silicate melt.Researchers have found that rare-metal pegmatites mainly formed from extreme fractional crystallization of highly evolved granitic magmas and,in rare cases,from low-degree partial melting of fertile metasediments.Besides,rare-metal pegmatite swarm could be formed from magmatic differentiation without a parental pluton,or from further differentiation of anatectic granite.Future researches need to gain a deeper understanding of the partial melting and fractional crystallization controls on granitic magmas,explore the physical and chemical processes during the formation of granitic magmas—especially melt escaping and migration mechanisms and their affects on rare-metal enrichment,and establish petrogenetic discrimination criteria for granitic pegmatites.