METAMORPHIC PRESSURE-TEMPERATURE ANALYSIS OF THE GANDAF FORMATION, INDUS SYNTAXIS, PAKISTAN HIMALAYA

The Indus Syntaxis is a large south-plunging anticline situated at the northern edge of the Indian plate in the Pakistan Himalaya. The anticline is cored with Early Proterozoic schist, amphibolite, and granitic gneiss, and flanked with Late Proterozoic to Triassic (and younger?) schist, marble, amphibolite, and granitic gneiss. The rock units are truncated on their north side by the Kohistan fault and the Indus suture zone but extend southward without structural break into lower-grade rocks.

The Early Proterozoic Gandaf formation extends almost continuously the length of the Indus syntaxis from the Indus suture zone southward to the Main Boundary thrust, a distance of about 140 km. The rock unit is metamorphosed to amphibolite facies in the north near the suture zone, and greenschist facies in the south near the Main Boundary thrust. We analyzed nine rocks from the Gandaf formation along a north to south traverse of about 75 km in order to determine if or how equilibrium temperature and pressure conditions vary with distance from the suture zone within rocks at the same stratigraphic level and of similar composition. All six rocks contain the assemblage garnet-plagioclase-biotite-muscovite-quartz with variable accessory minerals that include rutile, ilmenite, tourmaline, graphite, apatite, and chlorite.

Preliminary data suggest that temperature and pressure decrease from about 660 ±25 °C and 9 to 11 kbar near the suture zone to 600 ±25 °C and 6 to 8 kbar at a distance of 75 km south of the suture zone. The Gandaf formation is overlain by about 5 km of Proterozoic and younger Indian plate stratigraphy which, in turn, is overlain by ophiolitic mélange of the Indus suture zone. We attribute metamorphism to underthrusting beneath the suture zone mélange during Late Cretaceous-Eocene Himalayan orogeny prior to collision of the Indian plate with the Kohistan arc. Pressures indicate that the Gandaf formation was buried (subducted) to a depth of at least 22 km at a distance of about 75 km south of the present-day location of the suture zone.