Stratigraphy south of the Main Mantle Thrust, Lower Swat, Pakistan

Abstract

The Lower Swat rock sequence, in northern Pakistan, is composed of greenschist and amphibolite facies Indian plate rocks that crop out in a dome directly south of the Main Mantle thrust zone and Kohistan arc terrane. The metamorphism is Eocene to Oligocene in age and records the collision of the Indian plate with the Kohistan arc. This paper presents mineral assemblage, mineral composition and garnet zoning data on the Lower Swat rock sequence in order to estimate the pressure-temperature conditions of metamorphism and to infer a possible pressure-temperature-time path. The stratigraphic sequence consists of Precambrian to Cambrian(?) Manglaur formation unconformably overlain by late Paleozoic to early Mesozoic Alpurai group. The Alpurai group is further subdivided into the Marghazar, Kashala, Saidu and Nikanai Ghar formations. The Manglaur formation, in the core of the dome, is at kyanite grade. Metamorphic grade decreases in the overlying Alpurai group where a garnet "isograd" is mapped in calcareous schist of the Kashala formation. Mineral composition geothermometers and geobarometers indicate final metamorphic equilibrium conditions of about 600ø-700øC and 9-11 kbar in the Manglaur and Marghazar formations. These conditions are associated with a phase of static recrystallization which postdates the main phases of deformation. Correlation of the structural-tectonic history with the pressure-temperature estimates, and with the garnet zoning analysis, suggests that in the Eocene the Lower Swat rock sequence was subducted to a depth of 35 to 45 km beneath the MMT suture melange. Subduction was followed immediately by exhumation. The initiation of exhumation is believed to have been triggered by a change from subduction to strike slip motion in the MMT, thereby allowing the relatively buoyant Lower Swat crust to rise. The entire metamorphic-deformational cycle lasted from 7 to 16 m.y., ending in the late Eocene about 38 Ma when the rocks cooled through the argon blocking temperature in hornblende (=550øC).

Structural evolution of the footwall of the Indus Suture in Malakand (N Pakistan) during the Himalayan collision

“…According to DiPietro et al (1999), there is an equivalent rock unit to the south of Jowar of lower metamorphic grade, where Late Triassic conodonts have been reported (Pogue et al, 1992b). The same authors interpret these amphibolite facies rocks to be in continuity with the lower grade Nikanai Ghar and Saidu Formations (DiPietro et al, 1993;DiPietro et al, 1999).…”

“…They have been intensively studied in surrounding regions by earlier authors (Lawrence et al, 1989;DiPietro, 1991;Pogue et al, 1992a;Pogue et al, 1992b;DiPietro et al, 1993;Chaudhry et al, 1997a;Kazmi and Jan, 1997;DiPietro et al, 1999;Ghazanfar and Chaudhry, 1999;Pogue et al, 1999). The orthogneisses, structurally at the base of the succession, are correlated with the Swat gneisses (Pogue et al, 1992a;Chaudhry et al, 1997a;DiPietro et al, 2000).…”

“…Although the thickness of the amphibolites is variable, ranging from a few metres to tens of metres, the amphibolites are traceable at regional scale and are known as the Marghazar Formation (DiPietro, 1991). In other parts of the Swat the base is regarded as discordant over older, most likely Proterozoic rocks (DiPietro et al, 1993). The schists constitute a rather monotonous sequence made up predominantly of calcareous and garnet-bearing schists, relatively rich in centimetre to several tens of metres thick carbonate layers (type locality Thana), known also as the Alpurai Schists (Anczkiewicz et al, 1998) or the Kashala Formation (DiPietro, 1991).…”

A cathodoluminescence-assisted LA-ICP-MS study of topaz from different geological settings

“…According to Jan (1979), the topaz-bearing veins were geologically related to the Lower Swat Buner Schistose Group, namely to the unit of Marbles and Calcareous Schists, which was classified by Martin et al (1962). However, after the studies carried out by DiPietro et al (1993), the mentioned group corresponds to the Alpurai Group, which is composes, along with others, the Kashala Formation. The topaz-bearing veins occur in the rocks of the Kashala Formation, which are essentially composed of a sequence of garnet-and/or epidote-bearing calcareous schist that grade into schistose marble and into a coarse grey to brown calcite marble (DiPietro et al 1993).…”

“…However, after the studies carried out by DiPietro et al (1993), the mentioned group corresponds to the Alpurai Group, which is composes, along with others, the Kashala Formation. The topaz-bearing veins occur in the rocks of the Kashala Formation, which are essentially composed of a sequence of garnet-and/or epidote-bearing calcareous schist that grade into schistose marble and into a coarse grey to brown calcite marble (DiPietro et al 1993). These veins crosscut the schistosity of the aforementioned marbles and are mainly composed of coarse sparry calcite, whose colors can be white, grey and brownish, quartz and, rarely, laths of muscovite (Jan 1979, Morteani & Voropaev 2007.…”