Zr-in-rutile thermometry in eclogite and vein from southwestern Tianshan, China

Trace element LA–ICPMS analyses of rutile from eclogites and veins from southwestern Tianshan, China, are carried out for testing the validity of Zr-in-rutile thermometry in low-T/UHP eclogites. Mineral inclusions in rutile and time-resolved mass spectrometer data are carefully examined to identify and reject contaminated data. The Zr contents vary between individual grains from the same sample; rutile inclusions in garnet, amphibole, and paragonite have a narrower range of Zr contents than matrix rutiles that always show the highest Zr contents, which reflects the progressive rutile growth in the eclogites. Average Zr concentrations in rutile from all six investigated eclogite samples statistically belong to one well-defined population (34 ± 5 ppm; 33 ± 11 ppm; 26 ± 7 ppm; 36 ± 17 ppm; 22 ± 6 ppm; 40 ± 17 ppm), while average Zr concentration in rutile from the three veins belong to a second well-defined population (55 ± 7 ppm; 66 ± 6 ppm; 55 ± 5 ppm). The peak temperature of eclogite can be estimated as 535–570 °C at a pressure of 2.7 GPa inverted from the discovery of well preserved coesite in an increasing number of rocks from southwestern Tianshan. The mineral assemblage of the veins suggests their retrograde formation in the epidote–amphibolite facies; the growth temperature of vein rutile is 530–540 °C at ∼1.0 GPa. A robust P–T path constructed by Zr-in-rutile thermometry in eclogites and veins indicates that the rocks underwent a rapid uplift involving an early isothermal decompression from eclogite to epidote–amphibolite facies. Zr in rutile from eclogites and veins can serve as a pressure indicator in study area that suggests the pressure has a significant effect to the Zr-in-rutile thermometry.

► Average Zr concentrations in rutile from eclogites and veins statistically belong to two well-defined populations. ► A robust P–T path for the Tianshan eclogites was derived from rutile thermometry in eclogites and veins. ► Zr content in rutile can serve as a pressure indicator in areas that show isothermal decompression.