Combined rutile-zircon thermometry and U-Pb geochronology: New constraints on Early Paleozoic HP/UHT granulite in the south Altyn Tagh, north Tibet, China

HP granulite and garnet peridotite constitute a HP metamorphic unit in the south Altyn Tagh, northern Tibet. Previous data suggest that HP granulites experienced high pressure, ultrahigh-temperature metamorphism (870-1050 °C, 19-25 kbar), followed by a medium pressure granulite facies overprint (780-820 °C, 9.5-12 kbar). Previous zircon U-Pb geochronology of HP granulites gave an age of ca. 500 Ma, but it is unclear if this age reflects the metamorphic peak, or the post-peak overprint. In this contribution, new SHRIMP analyses indicate that the zircons associated with the peak metamorphic assemblage in HP felsic granulite are characterized by flat heavy REE patterns and 30-62 ppm Ti, yielding a temperature of 892-1002 °C by Ti-in-zircon thermometry, suggesting that the growth of the zircons occurred under or close to peak ultrahigh-temperature conditions rather than under post-peak conditions. These zircons gave a SHRIMP U-Pb age of 495 ± 4 Ma, representing the age of peak metamorphism. In contrast, oscillatory zoned zircon cores yield magmatic REE patterns, Ti-in-zircon temperatures of 720-820 °C, and Neoproterozoic U-Pb ages (936 ± 22 Ma). This indicates that the protoliths of the felsic granulites were formed in the early Neoproterozoic and were taken to extreme temperatures and pressures during the early Paleozoic orogenic event. In situ LA-ICPMS and EMP analyses indicate that rutiles are characterized by broad scatter and bimodal distribution in both Zr concentration and U-Pb age, which are interpreted as a result of recrystallization or growth rather than volume diffusion. The first rutile U-Pb age cluster is at ca. 485 Ma with a Zr concentration of ca. 3000 ppm, yielding ca. 950 °C at 20 kbar, implying that they grew at or near the peak metamorphic conditions. The second rutile U-Pb age cluster is at ca. 450 Ma with a Zr concentration of ca. 1000 ppm, corresponding to a temperature around 780 °C at 10 kbar, which is related to post-peak reheating under medium pressure/medium temperature granulite facies conditions. These results demonstrate that HP granulites in the south Altyn Tagh experienced a peak ultrahigh temperature and high pressure metamorphism at ca. 495 Ma, followed by medium pressure/medium temperature granulite facies overprint before ca. 450 Ma.