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

Abstract

International audienceThe 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

Syn-kinematic emplacement of the Pangong metamorphic and magmatic complex along the Karakorum Fault (N Ladakh)

“…Thus, the most robust estimate is the average exhumation rate between 18 Ma and present is estimated at 1.0 ± 0.2 mm a À1 . We note that it is three times lower than in some areas of the Karakorum Metamorphic Complex of Pakistan (Dassu Dome: 3 mm a À1 , Rolland et al, , 2006a. A horizontal offset can be estimated from the 18 km exhumation using the 30°p lunge of lineations in the Pangong Range: the horizontal offset is calculated as 31 ± 3 km.…”

Combined thermobarometry and geochronology of peraluminous metapelites from the Karakoram metamorphic complex, North Pakistan; New insight into the tectonothermal evolution of the Baltoro and Hunza Valley regions

“…, 2001) and the Baltoro region in the central‐southeast (Searle & Tirrul, 1991; Searle et al. , 1992, 2010; Rolland et al. , 2001, 2006).…”

“…, 1989, 2010; Searle & Tirrul, 1991; Fraser et al. , 2001; Rolland et al. , 2001, 2006; and others) and, although a good understanding of the tectonothermal history of the region has improved over time as new data become available, some aspects are still unclear and require continued investigation.…”

Timing of granulite-facies metamorphism in the eastern Himalayan syntaxis and its tectonic implications

“…By ~24 Ma, doming occurs in the western Himalayan syntaxis and its adjacent area, involving the Indian crust and the Karakorum (Asian) margin (Rolland et al, 2001(Rolland et al, , 2006a(Rolland et al, , 2006b). In the core of domes, some granulites are found which have ages in the range of ~20- Ar amphibole dating by Rolland et al (2006b).…”