花岗岩结晶分离作用问题——关于花岗岩研究的思考之二  

Does fractional crystallization occur in granitic magma? Some crucial questions on granite study (2)

作　　者：张旗[1] 潘国强[2] 李承东[3] 金惟俊[1] 贾秀勤[4] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]南京大学地球科学系,南京210093 [3]中国地质调查局天津地质矿产研究所,天津300170 [4]中国科学院高能物理研究所,北京100039

出　　处：《岩石学报》2007年第6期1239-1251,共13页Acta Petrologica Sinica

基　　金：国家"973"项目(2002CB412601);国家"973"项目(G1999043206-05);中国科学院知识创新工程基金(KZCX 1-07和 KZCX2-SW-119)资助的项目.

摘　　要：岩浆结晶分离作用是一个古老的话题,很早就有学者指出,地球内部生成的岩浆大多是玄武质岩浆,大多数花岗岩是由玄武岩结晶分离形成的。本文在考察了岩浆结晶分离作用的制约因素、比较了不同性质岩浆结晶分离作用的特征之后指出:玄武质岩浆可以发生结晶分离作用,因为有与其相关的堆晶岩产出;安山质岩浆也可以发生结晶分离作用,因为也有与其相关的堆晶岩产出。但是,花岗质岩浆似乎不大可能发生结晶分离作用,因为,很少见到有与(富硅的)花岗质岩浆相伴的堆晶岩产出。花岗质岩浆之所以不大可能发生结晶分离作用的原因在于:(1)岩浆的黏性大,它不仅阻滞了矿物的结晶作用(使斜长石不能发育为自形晶),而且阻止了密度大的矿物(例如角闪石)下沉;(2)主要造岩矿物(例如斜长石)的密度与花岗质岩浆的密度相差无几,使结晶分离作用难以进行。本文详细考察了花岗质岩浆中斜长石的行为,指出在花岗质岩浆中斜长石结晶分离几乎是不可能的。那么,文献中大量充斥的花岗岩结晶分离作用的说法是依据什么呢?作者认为,文献中的许多说法可能主要是根据哈克图解得出的,而不是根据实际观察和理论研究得出的。作者认为,玄武岩和花岗岩不仅来源不同,成分不同,而且解释也不同。哈克图解中许多适合玄武岩的解释未必适合花岗岩。由于鲍文反应原理是结晶分离作用的理论基础,因此,文中也对鲍文反应原理进行了评述,并指出文献中存在的一些需要认真对待的问题,例如,从玄武岩-安山岩-英安岩-流纹岩的连续演化序列是不可能的;单元-超单元填图方法是不科学的;中国东部中生代大规模花岗岩不可能是玄武质岩浆结晶分离形成的等等。本文还以 Ajaji el ai.(1998)报道的摩洛哥 Tanncherfi 花岗岩为例,指出结晶分离作用的解释是不可能的。作者认为,花岗�It has been well accepted that basaltic magmas are derived from the mantle and most granites are generated by fractional crystallization of basaltic magmas. The evidence for the fractional crystallization of basaltic magmas and even andesitic magmas is there is corresponding cumulate, however, cumulate is rarely associated with Si-rich granitic magmas, therefore, it is unlikely that fractional crystallization happened in granitic magmas. This is probably because （1） granitic magmas have higher viscosity such that the crystallization of minerals （i. e. , plagioclase ） have been obstructed to form euhedral crystals and high density minerals （i. e. , hornblende） have been blocked to settle down; and （2） primary minerals （e. g. , plagioclase） have similar density to that of granitic magmas. We argue that plagioclase is unlikely fractionally crystallized from granitic magmas although the process was reported in numerous literature on the basis of the Harker diagrams. We argue that the Harker diagrams are more suitable for basalts rather than granites because granites are totally different from basalts in magma source and composition. We also argue that the Bowen reaction series, the principal of the Harker diagrams, is unlikely happened to form a continuous sequence from basalt through andesite and dacite to rhyolite. The unit and super-unit mapping method of granites is not suitable for regional geological mapping. Mesozoic granites in East China and Tanncherfi granites in Morocco are taken as two examples to indicate the fractional crystallization of granitic magmas is impossible. Variable compositions of granites mainly result from the composition of magma source, temperature, pressure, volatile composition, degrees of partial melting, mixing and differentiation process. Among them the composition of magma sources is the most crucial to the diverse compositions of granite whereas fractional crystallization of granitic magmas has rarely effect on the compositional variation of granites. Therefore

关 键 词：花岗质岩浆 玄武质岩浆 闪长质岩浆 结晶分离作用 哈克图解 鲍文反应系列 

分 类 号：P588.111[天文地球—岩石学]