Late Cretaceous blueschist metamorphism in the Indus Suture Zone, Shangla region, Pakistan Himalaya

Abstract

Rb–Sr and 39Ar– 40Ar phengitic muscovite dating of the transitional blueschist–greenschist facies rocks from the Shangla region of Pakistan Himalaya resulted in a concordant ca. 80 Ma age interpreted as a time of metamorphic peak. 39Ar– 40Ar age spectra of sodic amphiboles were unsuitable for precise age determination. Mass-balance calculations show that the K concentration in glaucophane/crossite is ca. 25 ppm, and the K–Ar budget is dominated by inclusions of phengitic muscovite. E–W-oriented (parallel to Kohistan arc) stretching lineation defined by preferred orientation of amphiboles was accompanied by synkinematic growth of riebeckite rims on the edges of microboudins. Evolution of the pressure conditions based on the Al 2O 3 content in sodic amphibole shows that this deformational stage was accompanied by a decrease in pressure from ca. 700 to ca. 400 MPa. This, together with our dating results, strongly suggests ≥10 km late Cretaceous exhumation of the Shangla blueschists. The exhumation may have been associated with the increase of arc parallel displacement rate due to changes of arc obliquity. This led to thinning caused by arc parallel extension and to exhumation of the Shangla blueschists. A lack of strong Eocene metamorphic overprint implies that after the peak metamorphism, blueschists were accreted to the Kohistan arc, which helped to prevent burial and metamorphism during continental collision. The final emplacement of the Shangla blueschists most likely occurred during the India–Kohistan arc collision and was accompanied by the weak, locally developed greenschist facies overprint.