Late Archean crust-​mantle interactions: geochemistry of LREE-​enriched mantle derived magmas. Example of the Closepet batholith, southern India

摘要

The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Gyr ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3-​3.0 Gyr)​. In the field, it appears as a composite batholith made up of at least two groups of intrusions. An early SiO2-​poor group (clinopyroxene quartz-​monzonite and porphyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr​/86Sr ratio (Sri, 0.7017-​0.7035) and εNd (-​0.9 to -​4.1)​. A later SiO2-​rich group (equigranular gray and pink granites) is located along the interface between the SiO2-​poor group and the Peninsular Gneisses. They progressively grade into migmatized Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (Sri = 0.7028-​0.7336 and εNd values from -​2.7 to -​8.3, at 2.52 Gyr)​. Field and geochronol. evidence shows that the two groups are broadly contemporaneous (2.518-​2.513 Gyr) and mech. mixed. This observation is supported by the chem. data that display well defined mixing trends in the εSr vs εNd and elemental variation diagrams. The continuous chem. variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-​members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses)​. It is proposed that the intrusion of mantle-​derived magmas into mid-​crustal levels occurred along a transcurrent shear zone; these magmas supplied addnl. heat and fluids that initiated anatexis of the surrounding crust. During this event, large-​scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-​derived magmatism is clearly assocd. with granulite-​facies metamorphism 2.51 ± 0.01 Gyr ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.