Paragenesis and petrogenesis of a corundum-bearing marble at Hunza (Kashmir) |
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| Authors: | M Okrusch T E Bunch H Bank |
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| Institution: | (1) Braunschweig, F. R. Germany;(2) Moffet Field, California, USA;(3) Idar-Oberstein, F. R. Germany |
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| Abstract: | 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. |
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