Preliminary Petrochemical Study of the Chilas Complex, Kohistan Island Arc, Northern Pakistan
Abstract
The Chiles complex, streaching more than 300 kms reaching 40 km in width, consists predominantly of gabbronorites with minor pyroxenites, anorthosites and quartz diorites containing labradorite/andesine. Within these are found upto 4 km2, seemingly intrusive, masses of dunite-perodite-troctolite-anorthosite association characterised by the presence of bytownite/anorthite. Sixty four new chemical analyses reflect a genetic relationship among the various rock types. The data suggest crystallization from an arc-related parent magma of calcalkaline character. The oxides vs. D.I. and CMAS plots indicate olivine, orthopyroxene, and plagioclase as the dominant liquidus phases for the development of rocks varying from ultramafic through mafic to intermediate compositions. It is concluded that crystallization occurred possibly at a depth of <25 km followed by pyroxene grattulite facies metamorphism. This was followed by widespread amphibolitisation during uplift. The masses of ultramafic and related rocks appear to be early cummulates that were remobilized and emplaced into the main gabbronorite association.
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