Chemical interrelationships in a low-pressure granulite terrain in Namaqualand. South Africa, and their bearing on granite genesis and the composition of the lower crust
Chemical interrelationships in a low-pressure granulite terrain in Namaqualand. South Africa, and their bearing on granite genesis and the composition of the lower crust
复制标题
纳马夸兰低压麻粒岩地形中的化学相互关系。
DOI：
10.1016/0016-7037(76)90047-8
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发表时间：
1976
影响因子：
5
通讯作者：
T. McCarthy
中科院分区：
文献类型：
--
作者：
T. McCarthy
本文报道了南非纳马夸兰一套低压麻粒岩相岩石的化学研究结果。该区域下方为眼球状片麻岩和石英岩，其中含有由片麻岩广泛部分熔融形成的层间颗粒状石英-长石岩（称为“麻粒岩”）。在70%的熔融间隔内，片麻岩的K/Rb比从140增加到250：增加的速率受到黑云母的存在的影响。同时，K/Ba和Rb/Sr分别从80降至25和从4降至0.3。部分熔体（麻粒岩）部分反映了堆晶特征，其K/Rb比值与母片麻岩相似（175），但由于Ba和Sr保留在残余物中，K/Ba（238）和Rb/Sr（5）比值较大。其中之一是由非常先进的部分熔融的片麻岩。化学成分的连续性表明，其余两个花岗岩，虽然空间上分开，同源岩浆，并在上升过程中长石分馏演化。相对于围岩，这些花岗岩的母岩浆中Sr ~（87）/Sr ~（86）比值较低，Ba、Sr、Rb富集，排除了局部来源的可能性，表明它们是一种中等成分的下地壳源岩。再加上反映所研究地区下地壳改造的侵入花岗岩，支持了下地壳发育的部分熔融模型，该下地壳的成分逐渐变得更加钙基化。
The results of a chemical study of a suite of low-pressure granulite facies rocks in Namaqualand, South Africa, are reported. The area is underlain by augen gneisses and quartzites, which contain interlayered granular quartz-feldspar rocks (termed ‘granulites’) derived by extensive partial melting of the gneiss. The K/Rb ratio of the gneiss increases from 140 to 250 over a melting interval of 70%: the rate of increase being influenced by the presence of biotite. Simultaneously K/Ba and Rb/Sr decrease from 80 to 25 and from 4 to 0.3, respectively. The partial melts (granulites), which reflect, in part, a cumulate character, have similar K/Rb ratios to the parent gneiss (175) but larger K/Ba (238) and Rb/Sr (5) ratios, due to the retention of Ba and Sr in the residue.Three granites intrude the gneisses. One of these was produced by very advanced partial melting of the gneiss. Continuity of chemical composition suggests that the remaining two granites, although spatially separate, are comagmatic, and evolved by feldspar fractionation during ascent. Lower Sr87/Sr86ratio coupled with enrichment of Ba, Sr and Rb in the parent magma of these granites relative to the country rocks precludes local derivation and indicates a lower crustal source rock of intermediate composition.The progressive increase in cafemic character of the gneisses, which is similar to that observed in world granulite terrains as a whole, coupled with intrusive granite which reflects reworking of the lower crust in the area studied, supports a partial melting model for the development of a lower crust of progressively more cafemic composition.