Characterization and origin of a compositionally zoned aluminous A-type granite from South India
Characterization and origin of a compositionally zoned aluminous A-type granite from South India
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DOI：
10.1017/s001675680000399x
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发表时间：
2000-05
影响因子：
2.3
通讯作者：
H. M. Rajesh
中科院分区：
文献类型：
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作者：
H. M. Rajesh
泛非Ambalavayal花岗岩侵入印度南部喀拉拉邦北部的高级变质地体，在空间上与Moyar和Calicut线形构造有关。该岩体几乎平行于基底中的东北-西南和东西向断裂，与伸展构造体制下已存在的深部地壳线形构造的岩浆上升一致。该岩体的特征是存在富铁的水合镁铁质矿物、原生磁铁矿、Ni-NiO缓冲层上方的fO2和1000°C附近的高初始侵位温度。莫代尔和结构分析揭示了岩体内可能存在两个成分带：外部和内部。主要元素变化支持这种分带，并指向过碱性到准铝质的外带和偏铝到略过铝的内带。两个带均具有A型花岗岩的主量元素和微量元素特征，外带为Eby的A1亚型，内带为A2亚型。从外带到内带观察到的微量元素趋势表明，在内带样品中，晶体分馏可能是产生高水平不相容元素的主要过程。较高的初始~(87)Sr/~(86)Sr比值(0.7135)和高Y/Nb比值(Y/Nb&Gt；1.2)处于地壳原岩岩石的预期范围内。提出了一种岩石成因模式，即在高级别岩浆房中，碳长花岗岩、镁铁质到中下地壳部分熔融，然后岩浆进行有限的分离结晶。在高硅质(SiO_2>；74%)岩浆室的结晶末期，高场强元素和稀土元素(除Eu外)在内带的富集被认为是结晶熔融和挥发活动的最终产物。
The Pan-African Ambalavayal granite intrudes the high-grade metamorphic terrain of northern Kerala, South India and is spatially associated with the Moyar and Calicut lineaments. The pluton was aligned nearly parallel to the northeast–southwest and east–west faults in the basement, consistent with magma ascent along pre-existing deep-crustal lineaments in an extensional tectonic regime. The pluton is characterized by the presence of iron-rich hydrous mafic minerals, primary magnetite, fO2 above the Ni–NiO buffer and high initial emplacement temperatures near 1000 °C. Modal and textural analyses reveal two probable compositional zones within the pluton: outer and inner. Major element variations support this zoning and point to a peralkaline to metaluminous outer zone and a metaluminous to slightly peraluminous inner zone. Both zones exhibit major and trace element characteristics of the A-type granites with the outer zone belonging to the A1 subtype and the inner zone to the A2 subtype of Eby. The trace element trends observed from outer zone to the inner zone suggests that crystal fractionation may have been the dominant process in the generation of high levels of the incompatible elements in the case of inner zone samples. The high initial 87Sr/86Sr ratio (0.7135) and high Y/Nb ratios (Y/Nb > 1.2) are in the range expected for rocks derived from crustal protoliths. A petrogenetic model involving partial melting of a charnockitic, mafic to intermediate lower crust followed by limited fractional crystallization of the magma in a high-level magma chamber is proposed. The enrichment of HFSE and REE (except Eu) in the inner zone is considered the ultimate product of crystal–melt and volatile activity during the final stage of crystallization in a highly silicic (SiO2 > 74%) magma chamber.