P-T estimates of the calc-silicate rocks and the associated scheelite mineralisation from Miniki Gol, Chitral, N. Pakistan
Abstract
Co-existing calcic-amphibole and plagioclase have been used to obtain pressure-temperature estimates from the calc-silicate rocks at Miniki Gol, Chitral, N. Pakistan. The Miniki Gol calc-silicate rocks are located within the Hindu Kush range, approximately 50 km to the northwest of the Northern suture zone. Jurassic Arkari Formation hosts the calc-silicate rock, composed of clinozoisite, quartz, calcic-amphibole, plagioclase, chlorite, biotite, calcite, sphene, garnet and scheelite.
A pair of coexisting ferro-tschermakitic hornblende and anorthite from scheelite bearing calc-silicate quartzite and a pair of andesine and magnesio-hornblende from barren calc-silicate quartzite were selected for P-T estimates. These pairs have apparently formed under equilibrium conditions. The tschermakitic hornblende-anorthite pair yields a temperature range of 600-65oC whereas the andesine and magnesio-hornblende pair gives a temperature range between 530 ± 2oC and 490 ± 20oC. These temperatures are compatible with those of the upper amphibolite facies and greenschist facies metamorphism, respectively.
A general agreement was also observed when these estimates were checked with the criteria described for the correlation of temperature with increase in TiO2, Na2O and, Al2O3 from actinolite to tschermakite. The chemistry of the tschermakitic amphibole indicates that the calc-silicate rock has formed at a pressure below 5 kbar. Keeping in view the variation of chemical composition (A12O3 content) in calcic-amphibole associated with scheelite grains, it can be suggested that scheelite crystallised in a temperature range of 550oC-400oC.
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