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

Rb-Sr and Ar-39-Ar-40 phengitic muscovite dating of the transitional bluesc hist-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, Ar-39-Ar-40 age spectra of sodic amphiboles were unsuitable for prec ise age determination. Mass-balance calculations show that the K concentrat ion 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 are) stretching lineation defined by pre ferred orientation of amphiboles was accompanied by synkinematic growth of riebeckite rims on the edges of microboudins. Evolution of the pressure con ditions based on the Al2O3 content in sodic amphibole shows that this defor mational stage was accompanied by a decrease in pressure from ca. 700 to ca . 400 MPa. This, together with our dating results, strongly suggests greate r than or equal to 10 km late Cretaceous exhumation of the Shangla blueschi sts, The exhumation may have been associated with the increase of are paral lel displacement rate due to changes of are obliquity. This led to thinning caused by are parallel extension and to exhumation of the Shangla blueschi sts. A lack of strong Eocene metamorphic overprint implies that after the p eak metamorphism, blueschists were accreted to the Kohistan are, which help ed to prevent burial and metamorphism during continental collision. The fin al emplacement of the Shangla blueschists most likely occurred during the I ndia-Kohistan are collision and was accompanied by the weak, locally develo ped greenschist facies overprint. (C) 2000 Elsevier Science B.V. All rights reserved.