Paragenesis and petrogenesis of a corundum-bearing marble at Hunza (Kashmir)

A calcite-marble containing gem-quality ruby is exposed in the Hunza Valley, northwestern part of the Karakoram mountains, Pakistan zone of Kashmir. The marble forms concordant intercalations within sillimanite- and garnet-bearing biotite-plagioclase gneisses and mica schists. The metamorphic sequence is cut by discordant aplite and pegmatite dikes. The following mineral assemblages are recognized in the marble: 1) Calcite+corundum+phlogopite±margarite±sheridanite±Al-rich pargasite±anorthite (An 96.7), 2) calcite+spinel±corundum+phlogopite+sheridanite. Microprobe analyses are given for the essential minerals including corundum (ruby) and three different colour varieties of spinel. On the basis of recent experimental data, especially in the system CaO - Al2O3 - SiO2 - H2O -CO2 (and related subsystems), we assume that, during the regional metamorphism, temperatures of about 600 – 620°C and a water vapour pressure of about 6 kb were realized in part of the Hunza area. The gas phase must have contained roughly 20 mole-% of CO2. Thus the total fluid pressure may have reached about 7 kb. Presumably, temperatures increased in northwest direction, perhaps up to about 700°C. The estimated P-T conditions are consistent with a geothermal gradient of about 25°C/km.