Geochemistry and Petrogenisis of the Ambela Granitic Complex, NW Pakistan
Abstract
The Ambela granitic complex comprises three groups of rocks. Group I, the product of the first magmatic episode, consists of granites and alkali granites which occupy ~70% of the 900 km2 area of the batholith. Group II, following the granites sequentially, comprises quartz syenites, syenites, feldspathoidal syenites and related rocks. Following these, the area was invaded by delerite dykes which occupy ~5% of the complex. These (Group III rocks) are not discussed in this paper. The granitic rocks range from dominantly peraluminous to metaluminous to mildly alkaline. These were derived from melts produced by crustal anatexis due to crustal thinning and rifting. Different degrees of partial melting and fractional crystallization led to variation in the composition of these Group I rocks. With deepening of the zone of magma generation in the crust, the underlying mantle was activated, resulting in the influx of volatiles and alkalis. This led to the generation of magma batches which were successively more SiO2-undersaturated and alkaline, resulting in the production of Group II rocks. The Ambela batholith appears to have developed during a major phase of crustal thinning and rifting towards the end of Palaeozoic.
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