The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan |
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| Authors: | JAN M QASIM; HOWIE R A |
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| Institution: | 1Dement of Geology, University of Peshawar Pakistan
2Department of Geology, King's College Strand, London WC2R 2LS |
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| Abstract: | The Jijal complex, covering more than 150 sq. km in the extremenorth of Pakistan, is a tectonic wedge of garnet granulitesintruded in the south by a 10 x 4 km slab of ultramafic rocks.The granulites are divisible into plagioclase-bearing (basicto intermediate) and plagioclase-free (ultrabasic to basic)types, the two types reflecting differences in bulk chemistry.Garnet + plagioclase + clinopyroxene + quartz + rutile ±hornblende ± epidote is the most common assemblage. Theplagioclase-free rocks are composed mainly of two or three ofthe minerals garnet, amphibole, clinopyroxene and epidote. Orthopyroxeneoccurs in websteritic rocks devoid of epidote. Much of the amphiboleand some epidote appear to be prograde products. Although variationdiagrams do not reveal a genetic link between the two typesof granulite, it is considered that they are comagmatic ratherthan the products of two or more unrelated magmas. The compositions of garnet (Py28–46 Alm 27–43Gro16–28),clinopyroxene (Mg44–34Fe5–17Ca51–49, Al2O33·0–9·9 per cent), orthopyroxene (with upto 5·5 per cent Al2O3), amphibole (with up to 16·3per cent Al2O3 and high Alvi/Aliv), and the abundance of garnetsuggest a high-pressure origin for the granulites. The rocksappear to have differentiated from a tholeiitic magma of oceanicaffinity or they may be genetically related to the pyroxenegranulites of Swat considered to have originally crystallizedfrom a calc-alkaline magma of island arc or continental marginaffinity. They probably crystallized in the ancient Tethyancrust/upper mantle (or less likely in a continental margin),later to be metamorphosed to granulites (670–790 °C,12–14 kb) during the collision of the Indian-Asian landmasses,and carried upwards during later Himalayan orogenic episodes. The ultramafic rocks are alpine-type in nature and devoid ofgarnet. They are dominated by diopsidites; dunites, peridotites,and harzburgites together form <50 per cent of the area ofoutcrop. The chemistry of the rocks, and their olivines (Fo92–89)and clinopyroxenes (Mg49.5–48Fe2.8–5.2Ca47.4–46.8)are similar to those of alpine complexes of the harzburgitesubtype. It is not clear whether they represent a faulted slabof suboceanic crust/upper mantle, mantle diapirs in deep orogenicroots, or dismembered ultramafic rocks differentiated from abasaltic magma. They seem to have a complex history; their presentmineralogy is suggestive of high grade metamorphism (800–850°C, 8–12 kb). They are magmatically unrelated to thegarnet granulites and were probably intruded into the latteras plastic crystalline material after both had been independentlymetamorphosed, but before the entire complex was carried tectonicallyinto its present surroundings. The abundances of the diopsiditesis in marked contrast to other alpine-type complexes and thepossibility of Ca and Si metasomatism during or before theirmetamorphism should not be totally ruled out. |
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