The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan

Abstract

The Jijal complex, covering more than 150 sq. km in the extreme north of Pakistan, is a tectonic wedge of garnet granulites intruded in the south by a 10 × 4 km slab of ultramafic rocks. The granulites are divisible into plagioclase-bearing (basic to 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. The plagioclase-free rocks are composed mainly of two or three of the minerals garnet, amphibole, clinopyroxene and epidote. Orthopyroxene occurs in websteritic rocks devoid of epidote. Much of the amphibole and some epidote appear to be prograde products. Although variation diagrams do not reveal a genetic link between the two types of granulite, it is considered that they are comagmatic rather than the products of two or more unrelated magmas. The compositions of garnet (Py28–46 Alm 27–43Gro16–28), clinopyroxene (Mg44–34Fe5–17Ca51–49, Al2O3 3·0–9·9 per cent), orthopyroxene (with up to 5·5 per cent Al2O3), amphibole (with up to 16·3 per cent Al2O3 and high Alvi/Aliv), and the abundance of garnet suggest a high-pressure origin for the granulites. The rocks appear to have differentiated from a tholeiitic magma of oceanic affinity or they may be genetically related to the pyroxene granulites of Swat considered to have originally crystallized from a calc-alkaline magma of island arc or continental margin affinity. They probably crystallized in the ancient Tethyan crust/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 of garnet. They are dominated by diopsidites; dunites, peridotites, and harzburgites together form <50 per cent of the area of outcrop. 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 harzburgite subtype. It is not clear whether they represent a faulted slab of suboceanic crust/upper mantle, mantle diapirs in deep orogenic roots, or dismembered ultramafic rocks differentiated from a basaltic magma. They seem to have a complex history; their present mineralogy is suggestive of high grade metamorphism (800–850 °C, 8–12 kb). They are magmatically unrelated to the garnet granulites and were probably intruded into the latter as plastic crystalline material after both had been independently metamorphosed, but before the entire complex was carried tectonically into its present surroundings. The abundances of the diopsidites is in marked contrast to other alpine-type complexes and the possibility of Ca and Si metasomatism during or before their metamorphism should not be totally ruled out.