HR: 14:50h
AN: V12F-05 INVITED     [PDF]
TI: Quantitative Prograde P-T Paths From Inclusion Assemblages in Eclogitic Garnets
AU: * Essene, E J
EM: essene@umich.edu
AF: Dept. Geological Sciences, Univ. Michigan, Ann Arbor, MI 48109-1063 United States
AU: Page, F Z
EM: fpage@umich.edu
AF: Dept. Geological Sciences, Univ. Michigan, Ann Arbor, MI 48109-1063 United States
AU: Mukasa, S B
EM: mukasa@umich.edu
AF: Dept. Geological Sciences, Univ. Michigan, Ann Arbor, MI 48109-1063 United States
AB: Many workers have used garnet inclusions as qualitative indicators of the early history of tectonically exposed eclogites. Assemblages indicative of crustal facies combined with standard P-T conditions of those facies and thermobarometry on the matrix eclogite constrain the prograde path. However, now that conditions of some blueschist, amphibolite and granulite facies rocks have been extended to much higher pressures, in the range of 10-20 kbar, those assignments are in need of review. While undertaking studies on eclogites from the Blue Ridge of North Carolina and the Franciscan in northern California, the authors have identified key inclusion assemblages of sphene-rutile-epidote-quartz and phengite-omphacite together with garnet that constrain the prograde P-T path based on univariant assemblages corrected for observed solid solutions. Equilibria that have proved most useful are those bounding epidote stability and two key reactions, one involving sphene/rutile: (1) clinozoisite + sphene = grossular + rutile + H$_{2}$O, and the second being the phengite barometer: garnet + Mg-celadonite = clinopyroxene + muscovite. Reactions (1) and (2) have negative slopes, intersecting with the Mg/Fe K$_{D}$ garnet-clinopyroxene thermometer and providing a reasonable estimate of pre- to syn-eclogite facies P-T. In the case of the Bakersville eclogite samples from North Carolina, the inclusion assemblage yields 10 $\pm$ 2 kbar and 500 $\pm$ 50$\deg$C with reaction (1) compared to the peak assemblage at P $>$ 15 $\pm$ 2 kbar and 700 $\pm$ 50$\deg$C. These data combined with evidence for a granulite facies overprint indicate a clockwise P-T path for those eclogites. A similar study on the Healdsburg eclogite samples from California yields about 500$\deg$C and 12 $\pm$ 1 kbar for the garnet cores, 14 $\pm$ 2 kbar for mantles and 16 $\pm$ 2 kbar for rim-matrix assemblages. The introduction of late glaucophane, epidote and chlorite partially replacing omphacite and garnet implies a retrograde return to the blueschist facies and a counterclockwise P-T path. The inclusion thermobarometry indicates that both eclogite bodies were continuously buried during prograde metamorphism. The geological evidence suggests that both eclogite bodies formed in subducted oceanic plates. The mechanism for a sudden pressure increase could be related to a change in the subduction angle or to oceanward underthrusting by another cold oceanic plate.
DE: 9609 Mesozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting