The Mineral Evolution And Chronology Studies On The Progressive Metamorphic Belts In The Gem Mine Area,Xinjiang Altay

Progressive metamorphic belts occurred extensively in the Altai orogen were traditionally divided into Kyanite-type and Aandalusit-type. Investigation on the mineral evolution of metamorphic belts is critical to the understanding of the origin of the metamorphic belts and even the related geological evolution. A typical Kyanite-type metamorphic belt outcropped in the Gem mine region to the west of the Altay City was studied by integrating petrological observation, phase equilibrium modelling with geochronological analysis. Field and petrological observation shows the metamorphic belt involves a zonal sequence of biotite, garnet, staurolite, kyanite, sillimanite zones. P-T pseudosection was calculated for the typical pelitic composition in the metamorphic belt in the model system NCKMnFMASHTO（Na₂O-CaO-K₂OMnO-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-O）. Based on the observed mineral assemblage and the calculated P-T pseudosection, the P-T conditions are constrained to470℃⁵20℃ at 0.35Gpa⁰.64 Gpa for biotite zone, 510℃⁶60℃ at 0.6Gpa¹.0Gpa for garnet zone, 595℃⁶60℃ at 0.61 GPa⁰.98 GPa for staurolite zone; 0.66 Gpa⁰.98 Gpa,648℃⁶63℃for kyanite zone;0.67 GPa⁰.74 GPa,653℃⁶86℃for sillimanite zone.Systematic observation on cathodoluminescence（CL） and optical images shows that in some samples metamorphic rim overgrown on the inherited cores. In general, more zircon grains have rims from the low grade biotite zone to the high grade sillimanite zone. The width of the rims generally increases from low to high grade. Except the temperature, the growth of the rims could also be affected by other factors including fluid, zirconium abundance.Based on petrological analysis, LA-ICP-MS U-Pb analyses were conducted for detrital zircons of quartzite narrowly interbedded in metamorphic belt and yielded 100 concordant or nearly concordant ages with a range from（443±5） Ma to（2682±19） Ma. Combining the youngest detrital zircon age with the emplacement time of the tonalite directly intruded in the metamorphic belt, it could be inferred that the protolith to the quartzite deposited between the Early Silurian and the Early Devonian and metamorphosed later. The protolith of the metasedimentary sequence should have formed in the active continental margin setting, and metamorphosed to form the kyanite-type progressive metamorphic belt overprinted by Andalusite metamorphism during later geological event.