Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites
Fingerprinting feldspar phenocrysts using crystal isotopic composition stratigraphy: implications for crystal transfer and magma mingling in S-type granites
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DOI：
10.1007/s004100000128
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发表时间：
2000-06
影响因子：
3.5
通讯作者：
T. Waight;R. Maas;I. Nicholls
中科院分区：
文献类型：
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作者：
T. Waight;R. Maas;I. Nicholls
澳大利亚东南部拉克伦褶皱带的S型花岗岩中厘米级长石晶体的微取样揭示了复杂的内部Sr和Nd同位素变化。所观察到的同位素环带的一部分被解释为记录长石结晶在一个动态混合岩浆系统，同位素组成的变化，在响应更多的镁铁质和同位素更地幔状岩浆的涌入，后者的阶段，现在代表在修改后的形式由微粒包体。从微粒包体和相邻的主机花岗岩的长石巨晶类似的核心边缘同位素的变化强烈建议在包体中的巨晶从花岗质岩浆在结晶过程中转移。长石边缘的87 Sr/86 Sr比值高于邻近的全岩分析值，而Nd（i）值低于邻近的全岩分析值，但与周围包体基质中的矿物分离物的87 Sr/86 Sr比值相匹配。这表明，巨晶生长的最后阶段发生在一个组件的存在下，以前与高87 Sr/86 Sr，低Nd（i）的成分，如变质沉积围岩相互作用。同位素的不均匀性也presererved在不同的矿物相的包体基质，表明不同的阶段之间的包体岩浆和它的寄主花岗质岩浆的平衡增长。我们的研究结果揭示了主要的同位素不均匀性的单晶和矿物间的规模在一个岩体，这表明很好的约束岩浆混合的证据。这些结果表明，适宜的辉长岩记录同位素变化的岩浆系统，即使是那些已经冷却缓慢的深成环境，并建议在深成系统的异质性可能会被忽略在整个岩石规模。
Microsampling of cm-scale feldspar crystals within an S-type granite from the Lachlan Fold Belt of southeastern Australia has revealed complex internal Sr and Nd isotopic variations. The observed isotopic zonations are in part interpreted as recording feldspar crystallisation in a dynamically mixing magma system, the isotopic composition of which was varying in response to the influx of more mafic and isotopically more mantle-like magmas, the latter stages of which are now represented in modified form by microgranular enclaves. Similar core to rim isotopic variations in feldspar megacrysts from a microgranular enclave and the adjacent host granite strongly suggest megacrysts in the enclave were transferred from the granitic magma during crystallisation. Feldspar rims have higher87Sr/86Sriand lower ɛNd(i)than adjacent whole rock analyses, but match those of mineral separates from the surrounding enclave matrix. This suggests that the final stages of megacryst growth occurred in the presence of a component that had previously interacted with a high87Sr/86Sr, low ɛNd(i)component such as metasedimentary wall rocks. Isotopic heterogeneities are also presererved within different mineral phases in the enclave matrix, suggesting that differing phases grew at differing stages of equilibration between the enclave magma and its host granitic magma. Our results reveal major isotopic heterogeneities on a single crystal and also inter-mineral scale in a pluton which shows well constrained evidence for magma mingling. These results indicate the suitability of feldspars as recorders of isotopic change in magmatic systems, even those which have cooled slowly in the plutonic environment and suggest that much heterogeneity in plutonic systems may be overlooked on a whole rock scale.