P-T-T PATH OF POLYMETAMORPHIC ROCKS FROM THE HINDU KUSH RANGE, NW PAKISTAN

Documenting the sequential growth of metamorphic index minerals in pressure-temperature (P-T) pseudosections for the interpretation of tectonic processes requires robust knowledge of textural relationships, microstructures, mineral composition zoning, and spatially referenced geochronology. This approach aims to reduce the quantitative presentation of metamorphic index mineral assemblages on P-T pseudosections to a well-defined, single, qualitative P-T-t path. Existing in situ (U-Th)/Pb monazite age data from the Lutkho valley, Chitral region NW Pakistan outline a protracted history of deformation and metamorphism spanning the Late Triassic to the Early Miocene. P-T pseudosections constructed for the same specimens to investigate the associated P-T-t paths were calculated in the MnNCKFMASHT system based on bulk compositional data obtained through XRF analyses. The chemical composition of zoned mineral phases was evaluated using a TESCAN Scanning Electron Microscope. Garnet end-member isopleth intersections outline distinct, prograde chemically isolated core and mantle regions on the P-T pseudosection. Moreover, the presence of garnet as inclusions in staurolite indicates amphibolite facies metamorphism and an isothermal prograde P-T-t path. Existing dates from monazite grains within garnet range between c. 201 and 211 Ma while those in staurolite range between c. 189 and 184 Ma. These age data and paired metamorphic P-T paths outline near continuous crustal thickening from the Late Triassic into the Early Jurassic. In addition to the Mesozoic record, the deflection of the late-state, elongate biotite and muscovite around garnet and staurolite porphyroblasts and young matrix (U-Th)/Pb monazite dates of c. 29 to 22 Ma are coeval with large-scale Cenozoic syntectonic plutonic intrusion, high temperature metamorphism, anatexis and deformation across the Himalaya, Karakoram, and southern Tibet.