Geothermobarometric investigation of the zigana granitoid, eastern pontides, Turkey

In the Pontides, mineralogically similar, calc-alkaline, zoned I-type granitoids were emplaced into a volcanic-arc environment during the Late Cretaceous and Eocene. The granitoids contain an equilibrium assemblage of quartz, plagioclase, K-feldspar, hornblende, biotite, pyroxene, magnetite, and ilmenite. Large hornblende crystals (1-3 mm) are magnesiohornblende. The average magnesium number is 0.74; Si cation contents range between 6.64 and 7.32; and the average Altot is 0.86. The most common substitution in hornblende is pargasitic. The exchange vector is A□ + VIMg + 2IVSi = ANa + VIAl + 2IVAl. The Altot content of magmatic hornblende in granitoid rocks changes linearly with the crystallization pressures of intrusion; pressures, which were evaluated based on Altot in hornblende, have been estimated using the equation P(kbar) = 4.76Altot - 3.01. The crystallization pressures of granitoid rocks range from 2.4 ± 0.4 to 0.6 ± 0.2 kbar. For estimating temperatures, the hornblende-plagioclase thermometer, T = 0.667P - 48.98 + Y/-0.0429 - 0.008314 InK, was used, and yielded temperatures ranging from 684 ± 13 to 642 ± 7°C. The pressures and temperatures of these zoned intrusions are higher in the granitic and tonalitic rocks of the outer zones than in the quartz monzodioritic and monzonitic rocks of the core.