Zirconium in rutile thermometry from garnet granulites of the Jijal complex of Kohistan arc, NW Himalaya

抄録

Zirconium in rutile thermometry data from the garnet granulites of the Jijal Complex of Kohistan arc, NW Himalaya are presented in this study. The garnet granulites are composed of garnet, clinopyroxene, plagioclase, quartz, symplectic augite/amphibole, rutile, ilmenite, zircon, and magnetite. Rutile grains range in size from 50 to 350 µm, occur as inclusion in garnet, clinopyroxene, and in plagioclase as well as along the grain boundaries. In total 19 rutile grains were analyzed for Zr contents using an X–ray Analytical Microscope (XGT–5000) by HORIBA. The Zr contents among the analyzed grains ranged between 450 and 920 ppm, where the analyzed spots with lower Zr contents (containing SiO₂ or Fe₂O₃), indicating some influence of host silicate or ilmenite, were removed from results. At the individual grain scale, most of the rutile grains exhibited homogeneous chemical compositions, regardless of their textural affinity. Temperature values, based on zirconium in rutile thermometry, ranged between 792 and 849 °C for rutile enclosed in garnet, 771 and 851 °C for rutile in clinopyroxene, and 784 and 862 °C for rutile in plagioclase whereas matrix rutile grains showed T values between 820 and 847 °C. Using the pressure–dependent zirconium in rutile thermometry, the T values were slightly lower (±50 to 100 °C). The maximum temperature values were consistent with the temperature data obtained from the conventional thermobarometry results (P; 1.2 ± 0.2 GPa and T; 818 ± 80 °C) whereas the lower values, likely, reflect chemical resetting of the analyzed grains during later stages of retrogression.