Sm-Nd in Garnet and U-Pb in Zircon Geochronology to Constrain the Age of Peak Metamorphism in the Northeastern Pamir
Stevens, Michael Kenneth 1989-
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The age of peak metamorphism of garnet bearing ortho-gneisses and schists in the Kongur Shan and Muztaghata gneiss domes of northeastern Pamir exposed is uncertain due to discrepancies in the recorded peak metamorphic age between different geochronometers from previous geochronologic studies. U-Pb analysis of detrital zircon record a predominately Jurassic metamorphic signature in the Muztaghata gneiss dome, while Th-Pb dating of monazite inclusions in garnet yield an Oligo-Miocene metamorphic signature in both the Kongur Shan and Muztaghata massifs. These results leave the question of when the peak metamorphic assemblages observed in these gneiss domes formed. To address the conflicting ages and resolve the timing of peak metamorphism in the northeastern Pamir; the timing of garnet growth related to the peak metamorphic assemblages was determined using the Sm-Nd isotope system and further investigated using U-Pb analyses of Zircon. Garnet whole-rock Sm-Nd geochronology from both the Kongur Shan and Muztaghata gneiss domes yields late Cenozoic ages, confirming that the late Miocene age recorded by the monazite is related to peak metamorphism. Laser ablation U-Pb analyses of zircon yield Carboniferous to Triassic crystallization ages along with numerous partially reset grains with ages through the late Mesozoic and early Cenozoic. In addition, a single grain with a distinct recrystallized rim yielded an age of 8.61±0.41 Ma. The analyses performed in this study, in combination with the previous monazite and detrital zircon data, provide a clear late Miocene age for peak prograde metamorphism in the gneiss domes of the northeastern Pamir. This age is interpreted to be related to under-thrusting and crustal thickening in the Pamir. My results also suggest that Th-Pb dating of monazite can be a more reliable geochronometer for addressing the timing of peak metamorphism in poly-metamorphic terranes.