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Metamorphic zoning and geodynamic evolution of an inverted crustal section (Karakorum margin, N Pakistan), evidence for two metamorphic events

Abstract : The Karakorum Range comprises a crustal section of marbles and metapelites providing an opportunity to study the extent of high-temperature metamorphic reequilibration in an active orogen. Metamorphism culminated during the Mio-Pliocene, at 6-7 Ma. Peak metamorphic conditions increased from south to north, i.e. from (1) the Upper Anchizone grade (lawsonite, chlorite-smectite) to (2) lower granulite migmatite grade (HT∼800°C) conditions along strike of a 30-km section perpendicular to the structural fabric of the rocks. The metamorphic section can be separated into two domains: 1. A domain with low to transitional metamorphic conditions, with respect to the HT zone, where initial bedding is preserved. These moderate PT conditions prevailed during the main tectonic stacking event (50-37 Ma), prior to the Mio-Pliocene event. In this domain, metamorphism is governed by fluid-assisted grain-scale diffusion, as suggested by the progressive coarsening of minerals with increasing metamorphic grade and the preservation of sedimentary δ13C signatures in carbonates. A low thermal gradient (17°C/km) is derived from P-T estimates of the prograde metamorphic sequences. 2. A higher-grade metamorphic domain where, by contrast, the metamorphic style is dominated by advection of heat with magmatic intrusions involving mantle melts during the recent tectonic history (20-3 Ma). Strong devolatilisation of CO2 and partial melting of the metapelites triggered mixing up of carbonated and pelitic lithologies resulting in ultramafic restites and calc-silicate mobilisates. The δ13C isotopic composition of carbonates is widely modified, though locally preserved.
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Submitted on : Thursday, April 2, 2009 - 1:13:51 PM
Last modification on : Monday, October 12, 2020 - 11:10:19 AM

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Yan Rolland, Elizabeth Carrio-Schaffhauser, Simon Sheppard, Arnaud Pecher, L. Esclauze. Metamorphic zoning and geodynamic evolution of an inverted crustal section (Karakorum margin, N Pakistan), evidence for two metamorphic events. International Journal of Earth Sciences, Springer Verlag, 2005, 95 (2), pp.289 à 305. ⟨10.1007/s00531-005-0026-x⟩. ⟨insu-00372768⟩

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