Origin of an A-type granite: Experimental constraints
Origin of an A-type granite: Experimental constraints
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发表时间：
1986
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通讯作者：
J. D. Cr-nlrnNs
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作者：
J. D. Cr-nlrnNs
对于化学性质独特的A型或非造山花岗岩的成因，人们已经提出了许多岩石成因模式。来自澳大利亚东南部A型花岗岩/流纹岩与玄武岩双峰组合的准铝质沃特古姆斯花岗岩已经过实验研究，以便对其成因加以限定。沃特古姆斯岩浆以几乎完全熔融的状态侵位于地壳的极浅部位。因此，将1千巴压力下的晶 - 液相关系与岩石学确定的结晶顺序进行了比较。数据表明岩浆的最低温度为830℃；它可能高于900℃。所推断的熔体含水量介于2.4% - 4.3%（重量百分比）之间。较高的岩浆温度意味着这种花岗岩的成因不同于该地区稍早形成的I型花岗岩。岩石学、地球化学和实验数据支持其成因是下地壳中熔体亏损的I型源岩在高温下直接部分熔融。地幔派生的岩浆（玄武岩）为部分熔融提供了热源。最有可能的熔融反应涉及到之前一次形成I型花岗岩的事件中残留的、富含卤素的云母和角闪石在无流体参与下的分解。A型花岗岩，尤其是经过分异的，由于其温度高、水含量适中以及氟含量较高，是低粘度熔体。这与A型流纹岩和黑曜石流的常见现象是一致的。相比之下，S型和I型火山岩一般都是火山灰流凝灰岩。
Numerous petrogenetic schemes have been proposed for the origin of the chemically distinctive A-type or anorogenic granites. The metaluminous Watergums Granite, from a bimodal association of A-type granite/rhyolite with basalt in southeastern Australia, has been experimentally studied so that constraints can be placed on its origin. The Watergums magma was emplaced at a very high level in the crust, in an almost entirely molten state. The crystalJiquid phase relations at I kbar are therefore compared with the petrographically determined crystallization sequence. The data indicate a minimum magma temperature of 830.C; it may have been > 900'C. The inferred water content of the melt lies between 2.4 and 4.3 wto/o. The high magma temperature implies that this granite had an origin distinct from the slightly older I-type granites in the region. Petrological, geochemical, and experimental data support an origin by direct, high-temperature partial melting of a meltdepleted I-type source rock in the lower crust. Mantle-derived magmas Oasalts) provided the heat source for partial melting. The most likely melting reactions involve fluid-absent breakdown of somewhat halogen-enriched micas and amphiboles that were residual from a previous I-type-producing event. A-types, especially when fractionated, are low viscosity melts owing to their high temperature, moderate HrO contents, and elevated F contents. This is consistent with the common occurrence of A-type rhyolite and obsidian flows. In contrast, both Sand I-type volcanics are generally ash-flow tuffs.