Constraining Peak Metamorphism in the Shala-Tala Thrust Hanging Wall of the Northeastern Pamir via Zircon U-Pb Depth Profile Geochronology and REE Analysis

The timing of peak metamorphism of Triassic garnet-bearing metapelitic schists from the hanging wall of the Shala-Tala Thrust Fault in the northeastern Pamir is not well constrained due to differences in recorded peak metamorphic ages from monazite from different samples in the same metamorphic terrane. In a previous study, in situ Th-Pb geochronology results of monazite inclusions from the hanging wall showed three distinct age populations ranging from the Late Triassic to the Early Cretaceous, leaving the timing of prograde metamorphism uncertain. Further, geologic relationships suggest the possibility of multiple metamorphic events within the terrane. These conflicting results call into question the reliability of monazite as a geochronometer. To address the conflicting ages and better constrain the timing of peak metamorphism in the northeastern Pamir, U-Pb and rare-earth element (REE) depth profile analyses of detrital zircon grains from two metapelitic schists of the Shala-Tala Thrust hanging wall were performed. The rim and core portion of each depth profile analysis from individual zircon grains were isolated and compared to investigate possible metamorphic overgrowths. Laser ablation U-Pb analyses of zircon display a decrease in U/Pb age and increase in U/Th value from of the rims relative to the cores of both samples. My analyses yield metamorphic signatures ranging from ~190 Ma to ~120 Ma with a strong population of eight Early Cretaceous ages ranging from ~132 to ~110 Ma that overlaps with the youngest monazite age population. Additionally, REE analyses from both samples reveal seven Early Cretaceous rim ages with high U/Th values that display a notable depletion of the heavy rare-earth elements in relation to corresponding core analyses; a sign of coeval garnet growth. However, six of these ages come from a single sample indicating a variable metamorphic response between the two samples. In combination with the previous monazite ages, my analyses indicate an Early Cretaceous garnet growth event corresponding to crustal thickening in the Pamir within the hanging wall of the Shala-Tala thrust. Further, the monazite age populations suggest mixing of two distinct metamorphic populations recorded by the variability of older Th-Pb ages.