Pressure-temperature-time constraints for exhumation of ultrahigh-pressure metamorphic rocks, North Qaidam terrane, Western China

 
Brittany Kristine Fagin
March 2015

Abstract

Ultrahigh-pressure rocks of a Paleozoic continental suture zone are exposed in the southeastern North Qaidam terrane (Dulan region). Garnetite sample D119 contains minor Czo+Qtz+Chl+Ttn; rutile inclusions in titanite suggest that titanite replaced rutile during decompression. Pressure-temperature estimates of sample D119 are 16.1-18.4 kbar and 485-520 °C. Sample D130B is a mafic band in calc-silicate gneiss, with garnet porphyroblasts in a fine-grained Hbl-Cpx-Pl-Qtz symplectite that is interpreted as former omphacite. D130B symplectite pressure-temperature estimates are 7.7-9.4 kbar and 623-708 °C. Titanite U-Pb ages of 419.7±3.1 Ma and 415.9±4.2 Ma are interpreted to date retrogression of D119 and D130B, respectively. Average exhumation and cooling rates from ultrahigh-pressure to retrograde conditions are 6.4±6.0 mm/yr and 30±27 °C/Ma (D119) and 7.9±6.1 mm/yr and 3.9±8.0 °C/Ma (D130B). These data are compatible with early, rapid exhumation of a coherent block large enough to experience nearly-isothermal decompression in its insulated interior (D130B), while colder crust conductively refrigerates the block's margins (D119).

Full Thesis (PDF format)