HR: 1340h
AN: V13E-0583    [Abstracts]
TI: Compatible P-T evolutions for eclogites and surrounding metapelites in the Yuka area of the north Qaidam Mountains, western China
AU: * Zhang, J
EM: zjx66@yeah.net
AF: Institute of Geology, Baiwanzhuang, Beijing, 100037 China
AB: In the Yuka area of the north Qaidam Mountains, eclogite lenses are enclosed within metapelites. Eclogite is composed of garnet and opmhacite with varying amounts of amphibole, phengite, clinozoisite (epidote), rutile and quartz. Inclusions in prograde-zoned garnets from eclogite record pre-eclogite facies conditions of 10.6-11.1 kbar, 560-577°C. The phengite-bearing matrix assemblage records peak conditions of 23.5-25.1 kbar and 610-680°C, Amphibolitized eclogite in the boudin margin records retrograde conditions of 10.9­_1.2 kbar, 630­_44°C. The metapelites around eclogites contain mineral assemblages of garnet + kyanite + chloritoid + phengite + quartz + rutile. The petrographic observations and textural relations testify to the following sequence of mineral assemblages connected to three metamorphic stage: 1œc Grt + ChlI + CldI + PheI ­_ StI + Qtz; 2œcGrt + Ky + PheII ­_ CldII + Qtz; 3œcGrt + CldIII + ChlII + PheIII + StII ­_ ky + Qtz. Applying THERMOCALC Program, Grt - Phe thermometer and Grt-Ky-Phe-Qtz barometry, P-T conditions for three metamorphic stages were obtained: P=1.07­_0.31GPa, T=564­_22°C (prograde stage); P=2.3-3.1GPa, T=615-700°C (peak stage); and P =1.22 ­_ 0.26GPa, T=581­_20°C (retrograde stage). A hairpin shape P-T path similar to that of adjacent eclogite is inferred. Calculations of the P-T conditions in the metapelites are in good agreement with analogue calculations of the eclogite samples. In combination with eclogitic mineral relics in marbles and orthogneisses enclosing eclogites, these results support a single P-T loop for this area. The Yuka eclogite-bearing unit might represent a coherent HP metamorphic terrane and the relationship between eclogites and country rocks is `in situ' rather than `tectonic emplacement'.
DE: 8108 Continental tectonics: compressional
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005