东天山尾亚杂岩体:同源还是异源?——来自黑云母的证据

Weiya complex, eastern Tianshan: Single-sourced or diverse-sourced? ——Evidence from biotite

呈中心环状的尾亚杂岩体由外环石英正长岩和内环石英闪长岩、黑云母二长花岗岩以及细粒花岗岩组成,但对两个环带是同源还是异源的认识仍存在较大分歧。鉴于黑云母是大多数中酸性火成岩中比较重要的一种镁铁质矿物,它能很好地反映寄主岩浆的属性和成岩时的物理、化学条件,因此,本文的目的在于,通过比较外环石英正长岩和内环石英闪长岩、黑云母二长花岗岩以及细粒花岗岩中黑云母的成分变化,来为两个环带是同源还是异源提供矿物学方面的佐证。结果表明,外环黑云母与内环黑云母相比,富Fe和Ti,贫Mg和Al,前者为铁质黑云母,寄主岩浆属碱性岩系;后者皆属镁质黑云母,寄主岩浆属钙碱性岩系。碱性岩系中铁质黑云母的形成主要与其晶格中Fe→Mg和3Fe2+→2Al双替代有关,而钙碱性岩系中镁质黑云母则主要与2Al→3Mg替换有关。外环与内环黑云母成分的差别,归根结底,与其寄主岩浆形成于不同的源区、产于不同的构造环境密切相关。内环各相带云母之间没有明显的演化趋势,只是细粒花岗岩中的云母比石英闪长岩和黑云母二长花岗岩中的云母富Al和F,贫Ti。结合岩石地球化学、全岩氧同位素和锆石晶形及微量元素地球化学研究成果,外环和内环之间黑云母的成分变化,排斥了外环和内环同源的可能性;内环之间黑云母成分演化趋势表明石英闪长岩、黑云母二长花岗岩以及细粒花岗岩非同一母岩浆分离结晶所致,而可能是不同期次的异源岩浆相继侵入的产物。

The concentric Weiya complex is composed of an outer zone of quartz syenite, and an inner zone of quartz diorite, biotite monzogranite and fine-grained granite, although it remains controversial whether the outer zone and the inner zone came from a single source or different sources. Biotite is an important ferromagnesian mineral in most intermediate and felsic igneous rocks. Composition of biotite is indicative of both the nature of the source, and the physicochemical conditions of the magma. The present contribution aims at providing mineralogical evidence with comparison in compositional variations of biotites from both the quartz syenite in the outer zone and quartz diorite, biotite monzogranite and fine-grained granite in the inner zone. The analytical results indicate that biotite in the outer zone is distinctly different in composition from those in the inner zone. The biotites in the outer zone are rich in Fe and Ti, while those in the inner zone are rich in Mg and Al. The biotite in the outer zone is iron-rich (close to annite) and was crystallized from a magma of the alkaline suite. In contrast, the biotites in the inner zone are moderately enriched in Mg and were precipitated from magmas of the calc-alkaline suite. Iron enrichment of biotites in the alkaline suite is realized at the expense of both Mg and Al via two substitution schemes of Fe→Mg and 3Fe2+→2Al. The substitution of 2Al→3Mg in the calc-alkaline suite, however, is remarkably vital. Variation in biotite composition between the outer and the inner zones can be ultimately attributed to different sources and tectonic settings where the host magmas were generated. No evolution trend of the biotites from the quartz diorite, monzonitic granite and fine-grained granite has been found. Compared to biotites in quartz diorite and biotite monzonitic granite, the biotite in fine-grained granite is rich in Al and F, but poor in Ti. In combination with investigation on geochemistry and oxygen isotopes of the host rocks, and typology and trace element geochemistry of the zircons, compositional variations of the biotites are likely to indicate that the magmas for the outer and inner zones were not derived from a single source, and that the evolution trend of the biotite from quartz diorite through biotite monzonitic granite to fine-grained granite inside the inner zone does not record crystal fractionation of the same primary magma, but rather varying sources for the multi-phase intrusion.