Compositional Zoning in an Aluminous A-Type Granite from South India: Major, Trace and REE Signatures

Abstract

The Proterozoic granulite facies terrain of the southern part of the Indian Peninsula is intruded by a suite of alkali granite and syenite plutons preserving evidence for a prominent Pan-African felsic magmatic event (Santosh and Drury, 1988; Rajesh et al., 1996). The Pan-African Ambalavayal (AL) granite intrude into the high grade metamorphic terrain of northern Kerala, South India, (Fig. 1) and is spatially associated to the Moyar and Calicut fault lineaments. The comparison of the pluton distribution with these map-scale lineaments show that the pluton was aligned nearly parallel to the NE-SW and E-W faults in the basement, consistent with magma ascent along pre-existing deep fault lineaments. It is a pink to red granite, owing to abundant brick red Kfeldspar, but cream white feldspar is also present particularly towards the margin of the pluton. In contention with the colour change there appears to be a slight variation in the mafic mineral (hornblende and biotite) contents from margin (appears to be more than the interior) to the interior of the pluton, although it was very difficult to delineate. Modal analyses of representative granitic samples plotted on a Quartz-Alkali feldspar-Plagioclase plot indicate that they range in composition from syenogranite to monzogranite. This is further supported by the R1-R2 multication diagram. CIPW norms for all the samples analysed by XRF, when projected onto the Quartz-Albite-Orthoclase phase diagram, indicate that granite compositions do not correspond to those of minimum melts in the pure quartz-albite-orthoclase system, but fall below the anhydrous ternary minima reflecting the effect of HzO undersaturation in the magma. Modal analysis and textural analysis indicate probable trends with slight overlaps and forms the basis of delineating two probable compositional zones within the pluton; outer and inner (Fig. 2). Modal proportion of plagioclase, K-feldspar and hornblende are more towards the outer zone of the pluton in comparison to