Aluminium saturation in I- and S-type granites and the characterization of fractionated haplogranites

被引:1959
作者
Chappell, BW [1 ]
机构
[1] Australian Natl Univ, Dept Geol, ARC Key Ctr Geochem & Metallogeny Continents, Canberra, ACT 0200, Australia
关键词
granite; Lachlan; S-type granite; I-type granite; fractional crystallization;
D O I
10.1016/S0024-4937(98)00086-3
中图分类号
P3 [地球物理学]; P59 [地球化学];
学科分类号
0708 ; 070902 ;
摘要
Granites of the Lachlan Fold Belt resulted from partial melting of the crust. In most cases, fusion involved mainly quartz and feldspar, producing felsic melts. Varying degrees of separation of those melts from the unmelted source rock (restite) were responsible for much of the compositional variation seen in the granites of the belt. Less commonly, melting occurred at higher temperatures forming more mafic melts, such as for the I-type Baggy Plain Supersuite and the S-type Koetong Suite. Hence, the felsic haplogranites of the Lachlan belt dominantly formed initially as primary melts that separated from restite and less often by the fractionation of more mafic melts. Source rocks of the I- and S-type granites were undersaturated or oversaturated in Al, respectively, and the more mafic granites share that characteristic with their source. As the magmas of the Baggy Plain Supersuite evolved progressively by fractional crystallization, the rocks trended towards saturation in Al, to eventually form a mode close to Al saturation. Other felsic I-type magmas, formed directly by partial melting, were generally more oversaturated in Al, as were the corresponding S-type melts derived from peraluminous source rocks. In an unfractionated state, there are some overlaps in the degree of Al saturation in these magmas produced by partial melting. However, when extended fractional crystallization of these felsic partial melts took place, an almost complete separation in Al-saturation developed between I-type and more peraluminous S-type melts. Because apatite is soluble in peraluminous melts, P became progressively more abundant in the S-type melts as they fractionated, This led to contrasts in the abundances of P and of elements such as Y, the rare earth elements, and Th, between the strongly fractionated I- and S-type granites. Hence, such granites can easily be distinguished from each other. (C) 1999 Elsevier Science B.V. All rights reserved.
引用
收藏
页码:535 / 551
页数:17
相关论文
共 31 条