USING PSEUDOSECTIONS TO HELP CONSTRAIN THE METAMORPHIC EVOLUTION OF UHP ECLOGITE IN THE NORTH QAIDAM METAMORPHIC BELT, WESTERN CHINA

In the Luliang Shan, ultrahigh-pressure (UHP) eclogite occur as boudins in felsic gneiss. Pseudosections for the host gneiss and thermobarometric calculations for the eclogite are in good agreement, and show that both units experienced similar conditions during peak and retrograde metamorphism. While the thermobarometry data allows for an accurate assessment of the peak and mid-crustal stages, it lacks the ability to predict the composition of the eclogite at all stages of its metamorphic history. This study analyzed the accuracy and reliability of pseduosections for low variance mafic rocks. Comparing the pseudosections to petrologic analysis as well as to traditional thermobarometric calculations allows for greater certainty that the results are accurate. The Luliang Shan eclogite preserves four distinct petrologic stages along the peak to retrograde P-T path: a peak eclogite facies stage (25 ± 2.5 kbar and 590 ± 25 ̊C), lower eclogite facies stage (16 ± 2.5 kbar at 650 ± 25 ̊C), continued exhumation to upper amphibolite facies (605 ± 25 ̊C and 13 ± 2.5 kbar) and finally a lower amphibolite stage (510-425 ̊C and and 5-12 kbar). The petrology and mineral chemistry of each stage can be compared to the pseudosection predictions for the same rocks. These stages fall within large phase fields in the pseudosection, which is problematic for determining an exact P-T path with only the pseudosection. Thus, additional modeling of garnet composition and metasomatic reactions aid in constraining a P-T path with pseudosections. With careful selection of parameters, pseudosections can aid in recreating the metamorphic history of mafic rocks.