Neogene cooling and exhumation of upper-amphibolite-facies 'whiteschists' in the southwest Pamir Mountains, Tajikistan

M. S. Hubbard, E. S. Grew, K. V. Hodges, M. G. Yates, N. N. Pertsev

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Abstract

The Pamir Mountains of Central Asia are the product of accretion events during the Paleozoic and Mesozoic and have been further impacted by the continental collision between India and Asia during the Cenozoic. Deformation continues today in what is now a mountain belt within a continental interior. The geology of the Pamir is characterized by several arcuate-shaped, structurally bound regions with a general east-west trend. The southwest Pamirs are noteworthy for the extensive development of 'whiteschists' (talc-kyanite rocks) and other ultramagnesian metamorphic assemblages in the Goran Series. Our interpretation of the Goran Series is that upper amphibolite facies metamorphism at temperatures above 650°C and pressures near 700 MPa (kyanite + talc, kyanite + potassium feldspar) was followed by decompression to below 400 Mpa (sillimanite, cordierite). Late-stage andalusite is found only at Kuhi-lal. Available U-Pb, K-Ar and Rb-Sr data gave 2130 Ma as a minimum age for the amphibolite-facies metamorphism. Our 40Ar/39Ar cooling ages for biotite, phlogopite, and hornblende from Kuhi-lal, Darai Stazh and Mulvoj indicate Miocene cooling for the Goran Series. Biotites from mica schist gave good plateau ages of 8.9 ± 0.3, 10.2 ± 0.3, and 18.3 ± 0.2 Ma, and phlogopite associated with sapphirine, forsterite and enstatite gave a 16.9 ± 0.3 Ma plateau. Hornblende from amphibolite gave a plateau age of 22.2 ± 1.3 Ma and an age of ~17 Ma from a saddle-shaped release spectrum. Our 17-22 Ma ages are comparable to previously reported 18-23 Ma Rb-Sr isochron ages on the Pamir-Shugnan complex of gneissic granite exposed 15 km northeast of the northernmost locality (Kuhi-lal), and thus could be related to heating associated with this plutonism. Our preferred interpretation is that the ages represent regional cooling due to exhumation related to tectonic activity in the Miocene.

Original languageEnglish (US)
Pages (from-to)325-337
Number of pages13
JournalTectonophysics
Volume305
Issue number1-3
DOIs
StatePublished - May 10 1999
Externally publishedYes

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Keywords

  • Cooling ages
  • Exhumation
  • Neogene
  • Pamir Mountains
  • White schists

ASJC Scopus subject areas

  • Geophysics
  • Earth-Surface Processes

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