PETROGENESIS OF ALBITE-RICH SCHIST IN THE UHP TERRANE OF CHINESE WESTERN TIANSHAN

In Chinese western Tianshan, meta-sedimentary rock suit make up >80% of the UHP terrane in volume, which mainly consist of schist and marble. Some schist is poorly foliated and is characterized by high content of porphyroblastic albite, with certain contents of quartz, white mica and garnet. In some cases, albite occurs as vein at an outcrop scale. Such albite-rich schist shows pronouncedly high contents of SiO₂ (> 70 wt.%) and Na₂O (4-6 wt.%), low CaO and K₂O, and high Na₂O/K₂O and Fe₂O₃/K₂O ratios. The protolith is assumed to be Fe-rich sandstone or shale at active continental margins and was subject to albitization. Albite occurs as porphyroblast with inclusions of quartz, white mica, amphibole and titanite/rutile, or as interstitial phase in between quartz grains, indicating the presence of silicate-rich fluids favoring albite growth. The grain size and inclusion abundance of garnet porphyroblast varies significantly between different samples. Inclusion-rich garnet usually is larger in size (>2mm in diameter) and the inclusion is dominated by quartz, with minor amounts of white mica, rutile, graphite, lawsonite (pseudomorph) and omphacite. Compared with the pelitic equivalent, garnet of albite-rich schist has similar Mg content but uniformly higher Ca content (X_Ca = 0.12-0.16). Using THERMOCALC 333 in the system NCKFMASH, pseudosection calculations point to peak conditions of 22-25 kbar and 530-570°C. This result is inconsistent with the presence of coesite. Based on such sophisticated thermodynamic modelling method, peak UHP garnet is supposed to have lower Ca content (X_Ca < 0.10). The measured enrichment of Ca in garnet probably suggests that garnet growth mechanism in a sodium-rich system is different to those in typical mafic and pelitic systems in suduction zone. One scenario is that the system is open to metasomatic fluids locally during or after garnet growth. Another is that compared with the pelitic equivalent, crystallization kinetics of garnet (e.g., nucleation rate) is distinct that results in poikilolastic or porphyroblastic garnet grains with small polulation. In both cases, the reasonable estimation of effective bulk composition for thermodynamic modeling may be hindered and thus caution must be taken while applying garnet isopleth thermobarometry to some ‘unusual’ rock types