Mode of plagioclase twinning of two plutonic bodies in Kohistan terrane, northern Pakistan.

Abstract

Mode of plagioclase twinning is examined in the Chilas complex and Kohistan batholith which are large plutonic bodies in the kohistan terrane, northern Pakistan. The frequency of C twins, which are defined as all laws other than albite and pericline twins, is mostly 10 to 20% in the tonalite and granodiorite of the Kohistan batholith. These values characterize igneous rocks. Frequency of C twins in the Chilas complex is low (0 to 5%) in the eastern part and high (20 to 30%) in the central part. The former represents metamorphic features and the latter igneous features. These features result from the difference of the erosional surface of the complex and explain the petrological discrepancy that the complex is deduced to have been metamorphosed to the granulite facies, but that it locally intruded and gave thermally altered effect to the surrounding amphibolite. Frequency of pericline twin is almost the same in both bodies. This consistency may be developed by a result of the same tectonic event, i.e., collision between the Indian plate and the Kohistan terrane.