EXCEPTIONALLY RAPID EXHUMATION OF THE KAGHAN UHP TERRANE, PAKISTAN FROM U-TH-PB MEASUREMENTS ON ACCESSORY MINERALS

The Kaghan Valley ultrahigh-pressure (UHP) terrane contains eclogite and other felsic granitic and metasedimentary lithologies. The tectonic unit lies at the leading edge of the Indian plate continental crust, in the immediate footwall of the Main Mantle Thrust in northern Pakistan, and was subducted to depths approaching 100 km following continent-continent collision in the Eocene, as indicated by peak pressure metamorphic conditions of at least 25-26 kbar and 750-780C recorded in a coesite-bearing mafic eclogite. We have addressed the P-T and chronological history by combining petrological measurements with previous U-Pb dating of coesite-bearing zircon (Kaneko et al., 2001) from granite gneiss, new U-Pb zircon dates from gneiss and eclogite, new U-Th-Pb dates from chemically-zoned allanite in eclogite, new U-Pb dates from retrogressive titanite, and recently published U-Pb rutile dates from eclogite (Treloar et al., 2003). The REE chemical zoning of allanite suggests growth during prograde, peak and retrograde portions of the P-T path. The ages of allanite, zircon, titanite and rutile are 46.4+/-1.0 Ma, 46.3+/-0.2 Ma, 46.4-45.5 Ma, and 44.1+/-1.0, respectively. In that titanite replaces rutile and dates on rutile are younger than titanite, rutile dates are interpreted as the age of cooling through approximately 400-500C. The Kaghan UHP terrane was at >100km depth 46.3+/-0.2 Ma ago, and from retrograde assemblage P-T estimates and titanite dating, was exhumed to pressures less than 10kb (35km depth) before 45 Ma. The very limited duration of the passage of the terrane from coesite-stable depths to less than 10kb implies an extremely rapid exhumation rate of >40mm/a, but it may have been as rapid as 90mm/a in travelling from mantle to middle crustal depths. This may be the most rapid exhumation rate documented as yet. The initial exhumation rate appears to have decreased once crustal depths were attained, presumably due to a change in exhumation mechanism. This may be the result of a decrease in buoyancy force once the less-dense crustal material was removed from the mantle. Kaneko, Y., et al., 2001, UHPM Workshop, Waseda University, 121-123. Treloar, PJ, et al., 2003, J Geol. Soc, v. 160, 367-376.