Abstract:
The Chilas Complex is 40 km wide plutonic body present at the center of Kohistan
Island Arc sequence extending up to 300 km in east-west direction. The Kohistan arc
terrane has been considered to form due to the northward subduction of the Neo-
Tethyan oceanic lithosphere plate under the Eurasian plate during Cretaceous time. The
rocks of Chilas complex are exposed in Kiner Gah, east of Chilas town. These rocks
include gabbro, gabbronorite, tonalite, pyroxene quartz diorite, amphibolites and
granites. Field studies suggest that the complex has intrusive lower contact with the
Thak amphibolite and has a direct upper contact with granitic rocks of Kohistan
batholith. This research focuses on petrographic and geochemical study of Chilas
complex rocks to find out its origin. Major and trace elements study signify that these
rocks to belong to a one common magma source composition. The primary magma
seems to be basaltic in finally fractionating to granites. Negative Nb and P anomaly in
the rocks of the Kiner Gah area represented by spider diagrams indicate that the melt
was derived from metasomatized mantle, probably developed in island arc type back
arc environment. Kyanite bearing garnet tonalite, which is first time reported from the
Chilas complex in this research also serves as the evidence that the melt was derived
from metasomatized mantle.
