HR: 17:35h
AN: V44A-06    [Abstracts]
TI: Metamorphic Evolution, Mineral Chemistry and Thermobarometry of Ultrahigh-Pressure Eclogites From the North Qaidam Metamorphic Belt, Western China
AU: * Menold, C
EM: cmenold@albion.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 49224, United States
AU: Manning, C
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 49224, United States
AU: Yin, A
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 49224, United States
AU: Chen, X
AF: Institute of Geomechanics, Chinese Academy of Geology Sciences, Beijing, 100081, China
AB: The North Qaidam UHP belt is composed of discontinuous exposures over 500 km of leucocratic gneisses containing mafic eclogite and local garnet-peridotite that experienced peak metamorphism 430-590 Ma. Although coesite has been reported in one locality, it remains uncertain whether all mafic eclogites experienced UHP conditions, and the P-T paths of exhumation are not well constrained. To address this issue a detailed study of paragenesis and petrology of mafic eclogite from the Luliang Shan was conducted. Four stages of mineral growth with can be identified: Stage I, anhydrous eclogite facies mineral assemblage; Stage II, early amphibole- eclogite facies assemblage; Stage III, amphibole-eclogite facies assemblage and Stage IV, epidote-amphibolite facies assemblage. Stage I records the high pressures of 25+/-2 kbar at 590+/-25 degC. Stage II minerals record growth during decompression through 24-18 kbar and associated increase in temperature to 650+/-25 degC. Stage III minerals record P-T conditions of 605+/-25 degC and 16+/-2 kbar. Stage IV minerals temperatures of 415-510 degC, with maximum pressure constrained by the epidote-amphibolite assemblage. Peak P-T conditions are within error of the coesite-quartz equilibria. The resulting P-T path is better constrained through decompression than previously published paths for the Qaidam belt revealing that the exhumation path is clockwise with slight heating during decompression. The identification of Stages II, III and IV assemblage allows construction of a more detailed retrograde P-T path, revealing that the Luliang Shan eclogites experienced heating during exhumation. Temperature at peak pressure was significantly lower than at other North Qaidam eclogite localities. The data also suggest that geothermal gradients varied significantly along strike in the belt, both at the maximum depth of lithospheric subduction and at middle crustal depths to which the eclogite-bearing lithologies were exhumed.
DE: 1090 Field relationships (3690, 8486)
DE: 3651 Thermobarometry
DE: 3652 Pressure-temperature-time paths
DE: 3656 Ultra-high temperature metamorphism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting