APPLYING MULTI-METHOD P-T ESTIMATION TECHNIQUES TO CONSTRAIN DEEP CRUSTAL TECTONIC PROCESSES: AN EXAMPLE FROM HP GRANULITES, NORTH QAIDAM TERRANE, CHINA

High pressure (HP) granulites form within collisional and subduction zone tectonic environments. Due to high temperature and presence of melt, it is difficult to recover their pressure and temperature (P-T) history by conventional methods. An integration of multi-method P-T estimations provides insight on these deep crustal processes.

We apply petrographic analysis, Zr-in-Rt thermometry, phase equilibria modeling, and conventional thermobarometric calculations to constrain metamorphic conditions of Early Paleozoic HP granulite samples formed during late stage continental subduction/collision from the North Qaidam terrane.

HP granulites contain the mineral assemblage Grt + Pl ± Cpx + Qtz + Ky + Rt ± Czo. Compositional banding is common throughout samples and is interpreted to record migmatization. All samples exhibit various degrees of amphibolite facies overprinting. Whole rock compositions range from basaltic (dark bands) to rhyolitic (light bands).

Phase equilibria models (produced using XRF data and Perple_X) of two granulite samples reproduce the observed mineral assemblages + melt at 16-19kbar (at 775°C). Zr-in-rutile thermometry from the highest 10% of EMP Zr measurements yield peak temperature estimates of 774-803°C (calculated at 15kbar). Thermobarometric calculations using the Ky-Cpx-Grt-Qtz, Ky-Grt-Pl-Qtz, Grt-Pl-Cpx-Qtz, and Ab-Jd-Qtz systems of mafic bands yield pressure estimates of 13-17kbar (at 775°C). Thermobarometric calculations within compositionally felsic bands are not coherent.

We interpret the mineral assemblage to record higher P-T conditions than the mineral chemistries, and the mineral chemistry to record lower P-T conditions due to later stage reequilibration. We interpret the P-T discrepancy within felsic bands to stem from an inaccurate bulk composition as a result of melt loss during metamorphism. We conclude that the alteration of bulk composition and mineral chemistry through deep crustal migmatization and exhumation respectively obscure the results of single-method thermobarometry, but an integration of thermobarometric methods provides a segment of the P-T path showing high-T, melt-present decompression late in the continental subduction/collision process.