Selective entrainment of peritectic garnet into S-type granitic magmas: Evidence from Archaean mid-crustal anatectites
Selective entrainment of peritectic garnet into S-type granitic magmas: Evidence from Archaean mid-crustal anatectites
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DOI：
10.1016/j.lithos.2010.08.015
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发表时间：
2010-12
期刊：
影响因子：
3.5
通讯作者：
J. Taylor;G. Stevens
中科院分区：
文献类型：
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作者：
J. Taylor;G. Stevens
残余岩的夹带常被用来解释相对镁铁质花岗岩和花岗闪长岩的起源，以及花岗岩岩浆与其来源之间的化学联系。这一概念已经与岩浆迁移出源的模型联系在一起，因为当透辉石源移动时，被认为会发生残余岩夹带。这与通常出现的相对镁铁质花岗岩不一致，这些花岗岩是地壳中的高水平侵入体或其喷发等价物，这些侵入体必须由通过狭窄通道上升的明显水不饱和的岩浆形成。我们调查泥质混合岩从Mkhondo谷变质套房（MVMS）在斯威士兰，中地壳加热事件产生的变质混合岩与最小的后重熔重结晶。在这些岩石中，所有的石榴子石都是包晶的，它们是通过黑云母不含流体的熔融作用产生的，这种熔融作用产生了以熔融物、石英和黑云母的内含物为特征的石榴子石。代表熔体转移位点的白小体携带类似的、较小的（通常<1 mm）夹带的石榴石破磷粒，其能够合并以形成较大的复合颗粒。在深熔结构中，熔体存在的时间更长，夹带石榴石广泛重结晶，通过溶解-沉淀过程，采取更多的岩浆性质。在泥质源的石榴子石似乎已经长大的平衡与长石和HREE丰富的辅助相，而重结晶的石榴石在较大的熔体填充结构逐渐变得更好地平衡与这些矿物。因此，源中的橄榄榴石生长得足够快，以防止微量元素与大块岩石成分的平衡，同时快速的岩浆分离阻止了透辉石源条件的发展。分异岩浆由熔体、包体组合（主要为石榴石）及副矿物独居石和锆石组成。这些岩石表明，镁铁质花岗岩可能纯粹是熔体和由不一致熔融反应产生的包岩组合的混合物。重要的是，在产生MVMS深熔岩的环境下，橄榄榴石石榴石被夹带为<1 mm的辉石，这表明镁铁质花岗岩岩浆可以迁移出源岩，而源岩不会变成透辉石。
Entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites, as well as the chemical connection between granite magmas and their sources. This concept has become linked to models for magma migration out of the source, as restite entrainment is considered to take place when diatexitic sources mobilise en masse. This is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water-undersaturated magmas that ascended through narrow conduits. We investigate pelitic migmatites from the Mkhondo Valley Metamorphic Suite (MVMS) in Swaziland, where a mid-crustal heating event produced metatexitic migmatites with minimal post-anatectic recrystallisation. In these rocks all the garnet is peritectic, having arisen through biotite fluid-absent melting, which produced garnet poikiloblasts characterised by inclusions of melt, quartz and biotite. Leucosomes that represent sites of melt transfer carry similar, smaller (typically <1mm), entrained garnet poikiloblasts, which were capable of amalgamating to form larger composite grains. In anatectic structures where melt was present for longer, entrained garnet was extensively recrystallised, via a dissolution-precipitation process, to adopt a more magmatic character. The peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and HREE-rich accessory phases, while the recrystallised garnet in the larger melt-filled structures became progressively better equilibrated with these minerals. Thus, peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk-rock composition, and, concurrent rapid magma segregation prevented the development of diatexitic source conditions. The segregated magma consisted of melt, the peritectic assemblage (principally garnet) and the accessory minerals monazite and zircon. These rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction. Importantly, under the circumstances which produced the MVMS anatectites, peritectic garnet is entrained as <1mm poikiloblasts, demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic.