Petrography of the Jijal Complex, Kohistan
Abstract
This paper presents a detailed petrography of the garnet granulites and garnet-free ultramafic rocks which constitute a > 150 sq. km. tectonic block at Jijal, Indus Kohistan. The granulites are classified into (1) those with essential plagioclase, and (2) those with little or no plagioclase; the two types are a consequence of bulk chemical rather than environmental differences (Jan and Howie, in prep.). A total of 17 mineral assemblages have been identified, with garnet + clinopyroxene + plagioclase + quartz + - rutile +- hornblende+ - epidote being the most widespread. The plagioclase-free granulites are generally composed of two or three of the minerals garnet, clinopyroxene, hornblende, and orthopyroxene or epidote. The granulites were produced by high-pressure metamorphism of a series of hypersthene gabbros with some more and some less silicic rocks. The ultramafic rocks have alpine-type features but, unlike most alpinetype ultramafic rocks, they are dominated by diopsidites (with or without olivine); dunites, harzburgites aud websterites together make less than 50% of the > 40 km slab. The granulites and ultramafic rocks are considered to have originated in the Tethyan lower crust-upper mantle before they were metamorphosed during the coIlision of the Indian-Asian masses. The granulites were intruded in the granulites as plastic material after the main metamorphism but before the two were tectonically brought in their present surroundings.
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