The Dir-Utror metavolcanic sequence, Kohistan arc terrane, northern Pakistan

The Dir-Utror volcanic series forms a NE-SW trending belt within the northw estern portion of the Kohistan island are terrane in the western Himalayas of northern Pakistan. The Kohistan are terrane comprises a diverse suite of volcanic, plutonic, and subordinate sedimentary rocks of late Mesozoic to Tertiary age, developed prior to and after suturing of the Indo-Pakistan an d Asiatic continental blocks. The Dir-Utror volcanic series near Dir is dom inated by basaltic-andesite and andesite, with subordinate basalt, high-MgO basalt, dacite, and rhyolite. Porphyritic textures are dominant, with less common aphyric and seriate textures. Plagioclase is the dominant phenocrys t in mafic to intermediate rocks, K-feldspar and quartz phenocrysts predomi nate in the dacites and rhyolites. Chlorite, epidote, albite, and actinolit e are the most common metamorphic phases; blue-green amphibole, andesine, m uscovite, biotite, kaolinite, sericite, carbonate, and opaques are widespre ad but less abundant. Phase assemblages and chemistry suggest predominant g reenschist facies metamorphism with epidote-amphibolite facies conditions a ttained locally. Whole rock major element compositions define a calc-alkaline trend: CaO, Fe O, MgO, TiO2, Al2O3, V, Cr, Ni, and Sc all decrease with increasing silica, whereas alkalis, Rb, Ba, and Y increase. MORE-normalized trace element con centrations show enrichment of the low-field strength incompatible elements (Ce, La, Pa, Rb, K) and deep negative Nb, P, and Ti anomalies-patterns typ ical of subduction related magmas. Mafic volcanic rocks plot in fields for calc-alkaline volcanics on trace element discrimination diagrams, showing t hat pre-existing oceanic crust is not preserved here. All rocks are LREE-en riched, with La = 16-112 x chondrite, La/Lu= 2.6-9.8 x chondrite, and Eu/Eu * =0.5-0.9. Dacites and rhyolites have the lowest La/Lu and Eu/Eu* ratios, reflecting the dominant role of plagioclase fractionation in their formatio n. Some andesites have La/Lu ratios which are too high to result from fract ionation of the more mafic lavas; chondrite-normalized REE patterns for the se andesites cross those of the basaltic andesites, indicating that these l avas cannot be related to a common parent. The high proportion of mafic lavas rules out older continental crust as the main source of the volcanic rocks. The scarcity of more evolved felsic vol canics (dacite, rhyolite) can be explained by the nature of the underlying crust, which consists of accreted intra-oceanic are volcanic and plutonic r ocks, and is mafic relative to normal continental margins. Andesites with h igh La, La/Lu, K2O, and Rb may be crustal melts; we suggest that garnet-ric h high-pressure granulites similar to those exposed in the Jijal complex ma y be restites formed during partial melting of the crust. (C) 1999 Elsevier Science Ltd. All rights reserved.