HR: 1340h
AN: V43A-1557    [Abstracts]
TI: Excess Argon in UHP Gneisses: A Field-based Experiment in North Qaidam, China
AU: * Menold, C A
EM: cmenold@ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567
AU: Manning, C E
EM: manning@ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567
AU: Grove, M
EM: marty@argon.ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567
AU: Yin, A
EM: yin@ess.ucla.edu
AF: University of California, Los Angeles, 595 Charles Young Dr., Los Angeles, CA 90095-1567
AB: ⁴⁰Ar/³⁹Ar thermochronology can provide important constraints on the tectono-thermal evolution of orogens; however, application to white mica from ultrahigh pressure (UHP) and granulite terranes commonly results in ages that are impossibly old, despite relatively flat plateau over most of the ³⁹Ar release spectrum. A fundamental assumption of the ⁴⁰Ar/³⁹Ar method is that radiogenic ⁴⁰Ar is instantaneously lost from the mineral and surrounding system when it is above the closure temperature for argon retention in the mineral of interest. The presence of excess ⁴⁰Ar in UHP rocks implies that this assumption is not valid at high pressure; however, mechanisms by which excess argon is generated remain poorly understood. To investigate the occurrence of excess argon in UHP terranes, eighteen samples were collected on a traverse across a lithologic contact between leucocratic gneiss and mafic eclogite in the UHP North Qaidam metamorphic belt, western China. Geochronologic and petrologic studies indicate that the two lithologies have experienced the same metamorphic history, culminating in a major amphibolite facies regional metamorphic event (T_m > T_c,phg) which caused extensive recrystallization of the gneiss. U/Pb ages on metamorphic zircon (495±3 Ma) obtained throughout the North Qaidam UHP metamorphic belt for the both the eclogite and gneiss (e.g. Menold et al. 2005; Zhang et al. 2005) and puts an upper bound the cessation of regional metamorphism. Bulk aliquots of white mica were analyzed with resulting apparent ages ranging from 419±13 to 824±12 Ma. All eclogite samples have significant excess ⁴⁰Ar yielding unreasonable cooling ages ranging from 611-750 Ma. The ages from the gneiss > 5 m from the eclogite yield an average age of 461±6 Ma which is approximately 35 Ma younger than the age of UHP metamorphism. Gneiss samples within 1 m of the contact with the eclogite have a similar average age of 454±5 Ma. However, gneiss samples 1-5 m from the contact and all eclogite samples have significant excess ⁴⁰Ar, based on unreasonable cooling ages ranging from 610-824 Ma. Electron probe analyses and point counting confirm that excess argon correlates with the presence of texturally earlier phengite, as noted by other workers. Within the eclogite, retrograde recrystallization increases toward the rims of the eclogite and phengite retains its high-pressure composition (3.45 Si p.f.u.) and contains excess ⁴⁰Ar. White mica from gneiss > 5 m away from eclogite, contains only muscovite (mean Si 3.08±03 per formula unit), whereas between 1 and 5 meters the gneiss contains phengite (3.33±03 Si p.f.u.) and muscovite (3.19±03 Si p.f.u.) and at the contact (0-1 m) the gneiss contains only low-silica muscovite (3.08 Si p.f.u.). In addition, average grain size in the gneiss decreases from ~1 mm to 0.25 mm within 5 meters of the contact. All samples with excess argon are fine-grained, samples without are both coarse- and fine-grained. A preliminary hypothesis is that excess argon correlates with the extent of retrograde recrystallization, which will be further tested by constructing pseudosections to predict the temperatures and pressures at which argon could not effectively leave the system, and by recontstruction of the fluid history using fluid inclusions in coexisting phases (e.g. K-feldspar, quartz, omphacite, hornblende).
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1140 Thermochronology
DE: 3625 Petrography, microstructures, and textures
DE: 3660 Metamorphic petrology
DE: 3690 Field relationships (1090, 8486)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005