DEEP OROGENIC PROCESSES CONSTRAINED BY PRESSURE AND TEMPERATURE ESTIMATES OF HIGH-PRESSURE GRANULITES

The Dulan ultra-high-pressure (UHP) rocks of the North Qaidam UHP terrane in Western China represent continental crust that has been subducted 80-100 kilometers (above 25 kbars). High-pressure (HP) granulites found within the Dulan UHP rocks indicate a relationship between deeply subducted continental crust and HT rocks in the overriding crust. Pressure-temperature estimates from these HP granulites explain the exhumation history associated with continental subduction.

Petrographic analysis of granulite samples shows a primary mineral assemblage of grt + cpx + pl, representing HP granulite facies, along with zo(czo) +qtz + rt ± ky. A Grt-Cpx Fe-Mg exchange thermometer and GASP and GADS barometers constrain metamorphic conditions to 750-850 ˚C and 14-16 kbars. Zircon U-Pb ages of granulite conditions span 450-419 Ma. Ti-in-zircon analyses indicates temperatures around 700-800 ˚C in zircon cores within this time-span with zircon rims in the 419 Ma sample showing an increase in temperatures to around 900 ˚C. A ky – bt gneiss present in the same area has a mineral assemblage of qtz + pl + kfs + bt + grt + ky + rt. A Grt-Bt Fe-Mg exchange thermometer was combined with a GASP barometer yields 750-820 ˚C and 14-16 kbars. The adjacent UHP paragneiss contains qtz + pl + ms ± bt ± kfs ± grt, and very rarely ky or st, meaning formation occurred at lower temperatures compared to the granulite gneiss. Prior work has constrained the UHP conditions to 610-750 ˚C and 27-34 kbars.

High-pressure granulite formation overlapped UHP formation (~419-450 Ma). This combination of HP-HT and UHP rocks supports high temperatures at 50 km in the overriding crust, but lower temperatures at 80 km in the subducting plate. Also seen in one granulite sample is an indication of an increase in temperature at 419 Ma. This P-T history can be explained through formation and exhumation of UHP rocks. Crustal thickening during collision explains the HT in the granulites and not the UHP rocks. Slab break off and related mantle upwelling could transfer heat into the overriding plate, explaining the increase in granulite temperature at 419 Ma. Slab break off would be followed by UHP exhumation, during which the UHP rocks and HP granulites were juxtaposed, explaining the field relationship seen today.