Geochemistry and Petrogenesis of Metavolcanic Rocks in Gawuch and Drosh Formations, Chitral, Northern Pakistan

Geo!. Bul!. Univ. Peshawar, Vol. 38, pp. 189-202,2005 Geochemistry and petrogenesis of metavolcanic rocks in Gawuch and Drosh formations, Chitral, northern Pakistan TAZEEM TAHIRKHELI, M. ASIF KHAN & M. TAHIR SHAH National Centre of Excellence in Geology, University of Peshawar. ABSTRACT: Metavolcanic rocks constitute an important lithology in the study area. It occurs as two stratigraphic entities i.e., Gawuch and Drosh Formations. The Metavolcanic of the Gawuch formation are both porphyritic and very fine-grained. The porphyretic volcanics vary from fresh to highly altered and are commonly dissected by quartz and calcite veins. Volcanic rocks of Drosh Formation are massive to weakly foliated, fine-grained and commonly altered . The detailed geochemistry, mainly based on major and trace element, suggests that the metavolcanic rocks of the Gawuch formation are calc -alkaline, and those of Drosh Formation are tholeiitic in character. It is, therefore, suggested that the two groups of metvolcanics, despite difference in their petrological character, originated with a strongly subduction component. INTRODUCTION viens, host the copper mineralization in the Drosh-Shishi area. The study area occurs as a narrow elongated belt within the Drosh-Shishi area in the Metavolcanic rocks constitute important vicinity of Shyok suture zone, at the north lithologies in the study area. It occurs as two western margin of the Kohistan Island-arc stratigraphic entities such as Gawuch terrane. According to Pudsey et aI., (1985), Formation and Drosh Formation). The age it is divided into three stratigraphic units, difference between these two formations is which from bottom to top, include Gawuch unknown, but according to Pudsey et al. Formation, Purit formation and Drosh (1985), these are separated from each other Formation. Metabasalts / meta-andesites with by Purit Formation of fluviatile origin. In the common intercalation of carbonate lithologies context of the Kohistan geology, two groups (limestone and marble) form the cover of volcanic rocks are also identified by sequence in this part of the Kohistan terran. Pudsey et al.(1985) in the Shamran (Shandur These lithologies, include in the Gawuch Pass) area, to the east of presently studied Formation (Pudsey et aI., 1985) are western area. The age relationship between the two equivalents of the better known Chalt-Yasin groups of volcanics in the Shamran area is Group of Yasin and Hunza valleys (Tahirkheli controversial; Pudsey et al. (1985) consider et aI., 1979; Petterson & Windley, 1991). these to be stratigraphically below the Atine The Gawuch Formation is abundantly Albian Yasin Group and thus lower intruded by sills of diorites and granodiorites Cretaceous in age, while Sullivian et al. composition belonging composition to the (1993) differentiate the fresh volcanics of Early Eocene Lowari pluton of the Kohistan andesite-dacite-rhyolite composition from batholith. These minor intrusions of diorites those metamorphosed to amphibolite and granodiorite in the Gawuch Formation, greenschist facies and assigned them an age 39 together with intimately associate quartz of 58Ma on the basis of 40Ar_ Ar on 189 hornblende, equating them with the volcanic 1985 ; Pudsey, 1986), termed as the Yasin rocks in the Dir area (Sullivan et al., 1993 ; Group. The Yasin Group contains Early Shah & Hamidullah, 1994). Cretaceous fauna and is marine in origin (Pudsey, 1986). Khan et al. (1994) have This study is conducted to understand identified a succession of parageisses, schists the geochemical and petrological characters and amphibolites (some containing pillow of the metavolcanic rocks of the Gawuch and structures) called the Gilgit Formation Drosh formations of Chitral region in the occupying the base of the Chait volcanics. context of their possible origin. The Kohistan batholith occupies the central spine of the Kohistan terrene and is intrusive REGIO;NAL AND LOCAL GEOLOGICAL into both of the above mentioned tectonic SETTING elements. The batholith is composite and consists of plutons of a wide range of In the northern Pakistan, the Shyok suture compositions from gabbros, through diorites, separates the Eurasian plate to the north tonalities and granodiorites to granites. The from the Kohistan terrene to the south igneous activity related with the Kohistan (Tahirkheli et al., 1979; Bard et al., 1980). batholith spans a time range of 102 to 29 Ma The northern part of the Kohistan terrene (Petterson & Windley, 1986). comprises three principal tectonic unites, which from north to south include: 1) Shyok The geology of the NW margin of the suture melange, 2) sedimentary-volcano Kohistan terrene in Chitral including the cover sequence and 3) Kohistan batholith. Drosh-Shishi area related with this study, The Shyok suture melange, as the name contains all the three tectonic elements signifies, defines the collision zone between outlined above. The Shyok suture melange in Kohistan and Eurasian plates. and marks the this area comprises lenticular blocks of closure of the northern branch of the ultramafics, limestones etc. The volcano Neotethys, termed as the Shyok ocean (Khan sedimentary succession in this area is, et aI., 1994). It ranges from a razor-sharp however, much more complex than that of fault to a 4km wide zone and consists of the Yasin-Hunza segment. Pudsey et al. lenticular blocks of highly variable lithology (1985) recognized three formations, which including serpentinites, marbles, from south to north include I) Gawuch conglomerates, sandstones and basalts, Formation, 2) Purit Formation and 3) Drosh mostly set in a pellitic matrix comprising Formation. The Gawuch Formation slates of turbidite nature (Pudsey, 1986). In comprises rnetabasalts and limestones and is the Yasin area, limestone blocks are reported probably marine in origin while the Purit to contain fauna of Albian-Aptian (Early Formation comprising red conglomerates, Cretaceous) age. The volcano-sedimentary sandstones and shale and is fluvial in origin cover sequence in the Kohistan terrene (Pudsey et aI., 1985). Occurrence of a occurs as a lenticular belt of variable width succession of andesite/dacite volcanics of the squeezed between the Shyok suture in the Drosh Formation to the north and probably north and the Kohistan batholith in the south. over the Purit Formation points to the The sequence comprises a thick succession possibility of a phase of Eocene volcanic of metabasalts at the base (the Chait event similar to that of Dir-Utror (Shah & volcanics; Petterson & windley, 1991) Hamidullah, 1993: Shah & Shervias, 1999). overlain by a succession of quartzites, Towards south the Gawuch Formation, the limestones and turbidites (Pudsey et al., host of the copper mineralization, is in 190 - contact with the Lowari pluton belonging to formation contains meta volcanics together the Kohistan batholith . The contact, probably with common intercalations of intrusive in origin, is now strongly sheared metasediments like quartzites and marble. and is occupied by phyllites derived from Most of the volcanics are probably andesites metavolcanics of the Gawuch Formation in composition and are strongly foliated and through mylonitization. Much of the lower commonly altered. Tuffites are commonly half of the Gawuch Formation is occupied by associated with volcanics . They are fine metabasalts, which are locally strongly grained porcellaneous tuffites. Among the sheared and transformed into phyllites. The sediments, limestone occurs both as very upper half of the succession making the thick units and as thinly interbeded Gawuch Formation comprises commonly of limestone-phyllite-minor quartzite units. The intercalated metabasalts and limestone / Gawuch Formation is commonly intruded by marble. This part of the Gawuch Formation plutons of diorites, granodiorites and granite is additionally commonly intruded by sills of composition belonging to Kohistan batholith. diorite and granodiorite composition which Other intrusions, include basic and andisitic are themselves pervasively intruded by dykes , are also present. quartz veins . The contact between the Gawuch Formation and the overlying Purit Drosh Formation: The Drosh Formation Formation is occupied by a 10m thick band comprises a sequence of thickly bedded of marble . porphyritic andesites with phenocrysts of plagioclase, hornblend, and pyroxene. It is FIELD RELA nON very well exposed along the road , south of Drosh. Some lava flows are highly vesicular Metavolcanic rocks constitute important and few are brecciated, some red shales are lithologies in the studied area (Fig. I). These interbeded with the andesite flow. This metavolcanics occur as two stratigraphic Formation commonly overlies the Purit entities (i.e ., Gawuch Formation and Drosh Formation. Formation). The age difference between these two formations is unknown. However, these RESULTS AND DISCUSSION are separated from each other by Purit Formation of fluviatile origin. Field Fifteen representative metavolcanic rock observations during this study suggest that the samples from the Gawuch Formation and Gawuch Formation of the Drosh-Shishi area three from that of the Darosh Formation has lithological similarities with the chaIt were analyzed for major and trace elements. volcanics and the Yasin group combined, and The major elements have been determined by lower Cretaceous age may be assigned to the atomic absorption spectrometry, at the Gawuch Formation on the basis of this National Centre of Excellence in Geology, correlation. University

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