西昆仑大红柳滩花岗岩中黑云母地球化学特征及地质意义

Geochemical characteristics and geological significance of biotite in granite of Dahongliutan area in the West Kunlun Orogen

西昆仑大红柳滩岩体被认为与伟晶岩锂(铍)矿化密切相关。为约束大红柳滩岩体形成时的物理化学条件、岩石成因类型及源区性质,本文选取大红柳滩岩体作为研究对象,对花岗岩中的黑云母进行背散射观察(BSE)和电子探针分析(EPMA)。研究结果表明:大红柳滩二云母花岗岩与正长花岗岩中的黑云母属铁质黑云母,具有富硅、铝、铁和钾,贫镁、锰、钠和钙的特征;黑云母花岗岩中黑云母为铁质黑云母和镁质黑云母,具有富镁,贫铝、铁的特征。二云母花岗岩和正长花岗岩中黑云母具有高的AlⅥ值(0.7~0.9),与电气石、石榴子石和白云母等过铝质矿物共存。黑云母花岗岩中的黑云母具有高的MgO含量(9.30%~10.11%),黑云母与角闪石共存。通过黑云母成分估算二云母花岗岩、正长花岗岩及黑云母花岗岩中黑云母结晶温度分别为661℃~673℃、621℃~669℃、685℃~714℃,固结压力分别为332~404 MPa、356~377 MPa、248~270 MPa,对应侵位深度分别为12.46~14.98 km、13.18~14.38 km、9.18~10.01 km,氧逸度分别为-15.8~-15.0、-16~-15、-14.9~-14.0。综合研究结果表明,大红柳滩地区二云母花岗岩与正长花岗岩为壳源物质重熔的产物,具有典型强过铝质S型花岗岩的特点,而黑云母花岗岩是具有幔源物质混入成因的I型花岗岩。本研究成果为大红柳滩岩体不同岩性花岗岩成岩时的物理化学条件、岩石成因类型以及源区性质等方面提供了新资料。

The West Kunlun Dahongliutan rock mass is thought to be closely related to pegmatite type lithium(beryllium)mineralization.In order to constrain the physical-chemical conditions,petrogenesis and source,backscattering observation(BSE)and electron probe analysis(EPMA)are performed on biotite collected from the Dahongliutan granite.The results show that the biotite from two mica granite and syenogranite is ferrugaceous biotite,and featured by high rich silicon,aluminum,iron and potassium,but low magnesium,manganese,sodium,and calcium.Biotite in biotite granite is ferrugaceous biotite and magnesia biotite,which is magnesium rich,but poor in aluminum and iron.The biotite from two mica granite and syenogranite has a high AlⅥ(0.7~0.9),and biotite coexists with tourmaline,garnet,and muscovite.Biotite from biotite granite has high MgO content(9.30%~10.11%),and coexists with hornblende.The biotite composition further constrains crystallization temperatures of 661℃~673℃for two mica granite,621℃~669℃for syenogranite,and 685℃~714℃for biotite granite.Corresponding consolidation pressures 332~404 MPa,356~377 MPa,248~270 MPa,respectively,with estimated intrusion depths of 12.46~14.98 km,13.18~14.38 km and 9.18~10.01 km.Estimated oxygen fugacity fall into the range of-15.8~-15.0 for two mica granites,-16~-15 for syenogranite,-14.9~-14.0 for biotite granite.The comprehensive research results indicate that the two-mica granite and syenogranite in the Dahongliutan area are products of crustal source material re-melting,exhibiting typical characteristics of strongly peraluminous S-type granite.In contrast,the biotite granite is an I-type granite formed by the involvement of mantle source material.The results of this study provide new data for the physical-chemical conditions,rock genesis types and source of different lithological granites from the Dahongliutan rock mass.