High-grade metasedimentary rocks (Gilgit Formation) in the vicinity of Gilgit, Kohistan, northern Pakistan

Abstract

A thick succession of metasedimentary and metavolcanic rocks has been reported for the first time in the Gilgit area. This succession (the Jaglot Group) comprises from bottom to top 1) the Gilgit Formation, 2) the Gashu Confluence Volcanics, and 3) the Thelichi Formation. The Gilgit Formation consists of thick turbiditic sediments interstratified with amphibolites and calc-silicates. The lower contact of the formation is not exposed, but presumably underplated by the Chilas Complex, and the upper contact is conformable with the Gashu-Confluence Volcanics. The Gilgit Formation contains more than 80% paragneisses and schists showing metamorphism up to the sillimanite grade. The paragneisses are strongly deformed and at places migmatized.
     The paragneisses of the Gilgit Formation are quartz rich, but also contain biotite, garnet, staurolite, kyanite, sillimanite, graphite, together with pyrite and magnetite. Petrographical and geochemical data show that they are metapelites, semi-metapelites and semi-metapsammites. Using geothermometry and geobarometry on garnet-biotite pairs, the estimated maximum temperature and pressure conditions are 640°C and 6.7 kbar, respectively. We assume that igneous intrusions, both the Chilas Complex and the Kohistan Batholith and crustal thickening accompanying arc accretion to the Karakoram plate might have ensued metamorphism in the Gilgit Formation and other lithological units of the Jaglot Group. As depicted by the chemical zonation profiles of garnets in two samples from Jutial, the grade of metamorphism in the metasediments of the Gilgit Formation seems to increase from top to bottom in a normal sequence. The field and laboratory studies show almandine amphibolite subfacies and sillimanite-almandine amphibolite subfacies metamorphic conditions for the Gilgit Formation, and ocean basin of a back-arc affinity for its deposition.