DefinePK

Abstract

The Kohistan island arc terrane in the northwestern Himalayas of N. Pakistan is sandwiched between the Indian and Karakoram plates. The base of the arc is occupied by a major stratiform ultramafic-gabbroic complex (the Sapat-Babusar complex), which overrides the crust of the Indian plate along the Indus suture (i.e., the Main Mantle Thrust; MMT). It was intruded into- the base of a thick pile of metavolcanics (the Kamila belt), which comprise a tectonic collage of MORB-type tholeiitic basalts, island-arc tholeiites and calc-alkaline andesites. The Chilas complex, comprising ultramafic and gabbronorite rocks, is also intrusive into the Kamila belt, it is emplaced onto the top rather than the base of the Kamila belt. A sizeable proportion of granitoid rocks are present in the south-eastern part of Kohistan, which intruded the Kamila amphibolites. These are predominantly dioritic in composition, but include gabbros, granodiorites, granites and trondhjemites. The granitoids occur in two types: (l) large sheet-like lenticular masses, and (2) minor intrusives in the form of veins, sills or dykes. Three large sheets like bodies are mapped. All these bodies are composite, comprising gabbros, diorite/tonalite, granodiorite and granite. The. minor intrusions of granitic and trondhjemitic composition are abundantly present in the form of veins, sills and dykes; and are characterized by variation in distribution. Strong shearing transformed the rocks into blastomylonite gneisses. The mineral assemblage consists of quartz, plagioclase, amphibole, epidote, chlorite, biotite, muscovite, sphene, magnetite and apatite. The granitoids show a wide variation in SiO2 content. Spidergrams show three distinctive patterns;(i) flat patterns showing small enrichment in HFSE with lack of Nb anomaly, (ii) enrich in LILE sloping towards right with Nb anomaly and showing coherent trends with increasing LILE component from diorite through granodiorite to granite and (iii) the highly spiked patterns sloping towards right with more depletion in Nb, P, Zr, Ti and Y. The HFSE enriched granitoids are comparable with the host metavolcanics of MORB type Kamila amphibolite belt. This supports the petrogenetic link between this group of granitoids and the host Kamila amphibolites. The HFSE depleted granitoids include rock types gabbro, diorite, tonalite, granodiorite and granite, which show enrichments in LILE relative to HFSE, and coherent behavior. Majority of the rocks have typical subduction-related chemistry, with characteristics such as low TiO2 and high A12O3 contents, high LIL/HFS element ratios, variable interalkali ratios and distinct Nb depletion anomalies, all consistent with magma derivation from a metasomatized mantle wedge above a subduction zone. The trondhjemites, characterized by enriched HFSE, are in intimate association with Kamila amphibolite belt and the HFSE depleted trondhjemites with highly spiked patterns with very low incompatible trace element abundances are considered (l) to be unrelated with gabbro-diorite-tonalite-granodiorites-granite series, and (2) to have resulted from partial melting of Kamila amphibolites. The melt producedfor these trondhjemites whether due to the process of subduction or due to the intrusion of 90-80 Ma Chilas Complex. and crustal shortening accompanying the Jal shear zone of 80 Ma is not clear.