Exhumation of early Tertiary, coesite-bearing eclogites from the Pakistan Himalaya

Abstract

Early Tertiary, coesite-bearing eclogites crop out in the internal zones of the Indian Plate in the Kaghan Valley of North Pakistan. Field and geochronological data suggest that UHP metamorphism, at c. 46 Ma, accompanied subduction of the leading edge of continental India beneath Kohistan. A new U-Pb rutile age of 44.1 AE 1.1 Ma can be interpreted as either a growth or a cooling age, given the uncertainty of the closure temperature of rutile for Pb, but clearly documents a very short-lived metamorphic peak, with cooling occurring between 44 and 40 Ma. Eclogitic assemblages are preserved in Permo-Triassic mafic rocks in both cover and basement sequences, with the best-preserved eclogites hosted in the anhydrous basement. The eclogite-facies rocks lie on the hanging wall of a major south-directed ductile thrust zone, which placed basement rocks that contain coesite eclogites southward onto lower-pressure cover metasediments. Shear criteria show that those sequences that contain eclogite-facies rocks are flanked above by a zone of pervasive north-directed extensional shearing. Mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite-to greenschist-facies transition. Amphibole Ar-Ar cooling history data suggest that this was at c. 40-42 Ma. As in the Alps this implies exhumation from the metamorphic peak to greenschist-facies conditions within a few million years. This short time scale has important implications, as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at UHP and brought the UHP rocks back to 'mid'-crustal levels. This late Eocene extensional shearing, and associated exhumation of HP or UHP rocks, has not previously been recorded in the Pakistan Himalaya. The thrusts and shears were later folded by major east-trending folds, themselves cut by Miocene-aged, north-vergent extensional faults that currently separate the Indian Plate from the structurally overlying Kohistan island arc.

High-pressure Tethyan Himalaya rocks along the India-Asia suture zone in southern Tibet

“…The HP to UHP thrust sheet in the Kaghan Valley metamorphic complex, located another ~400 km along strike to the northwest ( Fig. 1), consists of Proterozoic orthogneiss and overlying metasedimentary rocks including Neoproterozoic-Lower Paleozoic biotite metagraywacke with intercalated garnet and/or kyanite metapelites (Treloar et al, 2003) and Upper Paleozoic-Mesozoic calc-paragneiss and schist with intercalated Permian amphibolite (Spencer et al, 1995). Coesite-bearing eclogite experienced peak conditions of ~3.0 GPa and 770 ºC (O'Brien et al, 2001) whereas coesite-free rocks record peak conditions of ~2.4 GPa and 610 ºC (Lombardo et al, 2000).…”

Crustal redistribution, crust–mantle recycling and Phanerozoic evolution of the continental crust

“…Furthermore, the continental crystalline crust along the rifted margin is also at risk of being recycled. The presence of high-grade, eclogitic rocks in many orogenic belts indicates that some crust is underthrust to great depths (>80 km) and while some of this is returned to the surface some may also be lost (de Sigoyer et al, 2000;Leech and Stockli, 2000;Ratschbacher et al, 2000;Treloar et al, 2003).…”

LA-MC-ICPMS Pb–Pb dating of rutile from slowly cooled granulites: Confirmation of the high closure temperature for Pb diffusion in rutile

“…Smaller rutile grains can only be 207 Pb-206 Pb dated as our instrument set-up lacks the sensitivity to precisely measure the 204 ion beam in smaller grains. A brief review of rutile Pb contents, Pb isotope compositions, and l values ( 238 U/ 204 Pb values) from the literature indicates that with the probable exception of rutile from certain eclogites (e.g., Di Vincenzo et al, 1997;Treloar et al, 2003) rutile from a variety of metapelitic and metamorphosed igneous rocks (cf. Mezger et al, 1989aMezger et al, ,b, 1991Miller et al, 1996;Davis, 1997;Indares and Dunning, 2001;Timmermann et al, 2004) should be amenable to analysis by the LA-MC-ICPMS method described here.…”

“…It occurs widely as an accessory mineral, and it has been shown to yield precise U-Pb ages. Rutile has been used for nearly two decades as a relatively low-temperature geochronometer that can provide timing constraints on metamorphic cooling (e.g., Mezger et al, 1989aMezger et al, ,b, 1991Heaman and Parrish, 1991;Miller et al, 1996;Santos Zalduegui et al, 1996;Davis, 1997;Di Vincenzo et al, 1997;Indares and Dunning, 2001;Treloar et al, 2003; Timmermann et al, 2004). However, recent experimental results (Cherniak, 2000) place this historical use of rutile as low-temperature geochronometer in question.…”