ASSESSING WHOLE ROCK GEOCHEMISTRY TO CONSTRAIN HIGH TEMPERATURE CONDITIONS USING ZR-IN-RUTILE THERMOMETRY FROM HP GRANULITES OF THE DULAN AREA, WESTERN CHINA

The Dulan area of the North Qaidam Terrane, Western China has undergone ultrahigh pressure (UHP) metamorphism and hosts UHP eclogites and localized high pressure (HP) granulites.

The integration of petrographic analysis, whole rock geochemistry, Zr-in-rutile thermometry, and phase equilibria modeling constrain metamorphic conditions for ten HP granulites.

Petrographic analysis of the granulites yields a mineral assemblage of Grt + Cpx ± Pl + Czo ± Ky ± Qtz ± Rt, and previous studies estimate pressure and temperature conditions of 14-18kbar and ~950°C. Major minerals display compositional zoning, and all samples contain Amp from various degrees of amphibolite facies overprinting.

Whole rock XRF and ICP geochemistry yields compositions ranging from basaltic to rhyolitic. Granulite samples with lower (<0.2wt%) TiO₂ and higher (>20ppm) Zr concentrations yield Zr/Ti ratios suitable for Zr-in-rutile thermometry. Samples with low Zr/Ti ratios will underestimate temperatures because Zr concentrations will become depleted with respect to Ti, and Zr can no longer increase with temperature. Two granulite samples (D136B and D143) and one amphibolite sample (D220C) contain adequate Zr/Ti ratios and rutile abundance for Zr-in-rutile thermometry.

Zr-in-rutile thermometry yields temperature estimates ranging from 543-815°C (calculated at 15kbar). Zr concentrations can re-equilibrate to lower temperatures in small (<20µm) rutile grains so peak temperature estimates are produced from the highest 10% of Zr-in-rutile concentrations. Rutile grain size ranges from 10-300µm in D143, 10-80µm in D136B, and 20-80µm in D220C. Samples D143 and D220C exhibit well constrained temperatures and yield peak temperature estimates of 803°C and 641°C respectively. Sample D136B shows poorly constrained temperatures and a peak temperature estimate of 774°C.

In granulite samples, we interpret the highest 10% of Zr-in-rutile concentrations to be in equilibrium and reflect temperatures during peak metamorphic conditions. We interpret the low temperature in sample D220C to record amphibolite facies retrogression.