花岗岩混合问题:与玄武岩对比的启示——关于花岗岩研究的思考之一

最近,花岗岩混合成了花岗岩研究的热点,国内外许多学者探讨了花岗岩混合问题,并尝试用不同端元组分不同比例的混合来解释花岗岩的地球化学变化。本文从花岗岩与玄武岩的对比出发,探讨了花岗岩混合的可能性和局限性。作者认为,花岗岩混合的现象是普遍存在的,但是次要的和局部的。岩浆混合的能力或能干性(competence of mixing)主要取决于岩浆的黏性和温度,而黏性又与硅氧四面体有关。相对于玄武岩,花岗岩的SiO_2含量高,温度低,因此,花岗质岩浆的混合能干性很低。玄武质岩浆的混合是mixing(以化学混合为主),而花岗质岩浆的混合通常只是mingling(以机械混合为主),只有在少数情况下才能达到mixing的程度,例如,埃达克岩与地幔混合形成的高镁安山岩或高镁埃达克岩。许多人认为,花岗岩中的暗色微粒包体是花岗质岩浆混合作用最显著、最直接证据。研究表明,花岗岩中的暗色微粒包体大多是闪长质成分的,其初始成分大多是玄武质的。因此,暗色微粒包体不是花岗质岩浆混合作用最显著、最直接证据,而是玄武质岩浆混合能力强过花岗质岩浆的证据。与玄武质岩浆的起源比较,花岗质岩浆从一开始熔融就是不均一的,这源于源区的不均一及熔融过程的复杂性。花岗质岩浆原始均一性的假定是不可能的。花岗岩成分的变化以及在哈克图解中成分点的"连续谱系",主要是由源区不均一性引起的,混合和分异可能有一定的作用,但毕竟是次要的。花岗质岩浆从源区生成、迁移、直至在地表喷出或在浅部定位的全过程,是一个不断均一化和不均一化的过程。但是,由于花岗质岩浆的黏性大,上述过程及岩浆演化的程度和规模都受到限制,也限制了岩浆混合的程度和规模。许多人仅从花岗岩地球化学成分的变化来研究花岗岩的成因,而很少考虑花岗岩物理性质对岩浆演化的制约。对比玄武岩与花岗岩,我们认为,地球化学方法在花岗岩中应用的范围和程度可能远远不及玄武岩,我们应当重新考虑花岗岩的地球化学应用问题。

Granitic magma mixing versus basaltic magma mixing:New viewpoints on granitic magma mixing process:some crucial questions on granite study(1)

How granitic magmas are mixed has become an interesting topic in recent years.Many workers are trying to interpret the variation of the composition of mixing granites with the different proportion of several end-members.However,we argue that such kind of mixing process for granites is uncommon and is unlikely happened in the field on the basis of a comparison of the properties of granite and basalt.The ability or competence that two granitic magmas can be mixed mainly depends on the viscosity and temperature of the magmas.The viscosity is related to the structure of SiO_4 tetrahedron in the rocks.Granites in comparison with basalts have relatively high SiO_2 contents and low temperature,and therefore granitic magmas have low competence to be mixed with each other.Unlike the mixing process of basaltic magmas,granitic magmas are more likely mingling,rather than mixing,with each other.There is rare example for the mixing of granitic magmas,the only case is high-Mg andesite or high-Mg adakite formed by mixing of adakite and upwelling mantle.Many workers argued that mafic microgranular enclaves(MME)in granites are the direct evidence for the mixing of granitic magmas.However,we argue that it is an exact evidence for that granitic magmas are difficult to be mixed with these mafic enclaves. Granitic rocks are actually heterogeneous in compositions because they have derived from heterogeneous sources with complicated melting processes.It is thus unlikely to assume the original granites are homogeneous.The variation of compositions of granites and the correlation of elements in Harker diagrams are mainly the results of heterogeneity of sources,rather than mixing or differentiation processes.Usually magmas tend to be homogeneous by mixing in magma conduits on the way upwelling to emplacement and at the same time they may become heterogeneous with the crustal contamination.However,grantic magmas are hard to be as homogeneous as basaltic magmas in this process because of their higher viscosity than that of basaltic magmas.Therefore,not only geochemical compositions of granites but also their physical properties should be considered in the study of the origin of granites,and it should be cautious when using geochemical data to discriminate the origin and tectonic environment of granites.