P-T evolution of a spinel + quartz bearing khondalite from the Highland Complex, Sri Lanka: Implications for non-UHT metamorphism

摘要

Here, we report a natural field example for the coexistence of spinel + quartz as a non-UHT assemblage in spinel- and cordierite-bearing garnet-sillimanite-biotite-graphite gneiss (khondalite) interbedded with orthopyroxene-garnet-biotite bearing intermediate granulites from the Highland Complex (HC) in Sri Lanka. The khondalite contains Zn-rich spinel mainly in four textural assemblages namely: (a) spinel co-existing with tiny quartz (ZnO = 12.67-12.85 wt%), (b) spinel surrounded by sillimanite moates and in intergrowth with skeletal sillimanites (ZnO = 9.03-9.17 wt%), (c) symplectitic spinels at the margin of sillimanite (ZnO = 4.09-4.28 wt%) and (d) spinel co-existing with ilmenite or as isolated grains (ZnO=7.61-7.97 wt% and Cr_2O_3 = 5.99-6.27 vrt%). Assemblage (a) and (b) occur within garnet while assemblages of (c) and (d) are present within cordierite moates after garnet in the matrix. Pseudosections calculated in the NCKFMASHTMnO system and conventional geothermobarometry suggest that the metamorphic peak conditions attained by the spinel + quartz bearing khondalites and associated intermediate granulites did not exceed T of 900 ℃ and P of 7.5-8.5 kbar. Post-peak evolution was characterized by a stage of nearly-isobaric cooling down to T of 770 ℃ and P of 7.5 kbar, followed by a late stage of isothermal decompression down to P ＜ 6.5 kbar and T of 770 ℃. We propose that the incorporation of large amount of Zn into spinel from exotic, metasomatic fluids and possibly incorporation of Fe~(3+) into spinel under high oxidizing conditions may have shifted the stabilization of co-existing spinel + quartz to T＜ 900 ℃. Hence, this study provides insights into the occurrence of spinel + quartz as a non- UHT assemblage suggesting that the coexistence of spinel + quartz should be treated with care and considered only as indicative, but not diagnostic of UHT metamorphism.