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

Description

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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