HR: 1340h
AN: V13A-1141    [Abstracts]
TI: The Origin and Significance of Eclogite and Metagabbronorite Knockers from the Bughea Complex, South Carpathians
AU: * Negulescu, E
EM: elinegu@yahoo.com
AF: Geological Institute of Romania, 1 Caransebeş St., Bucharest, 02271, Romania
AU: Sàbàu, G
EM: g_sabau@yahoo.co.uk
AF: Geological Institute of Romania, 1 Caransebeş St., Bucharest, 02271, Romania
AU: Massonne, H
EM: h-j.massonne@mineralogie.uni-stuttgart.de
AF: Institut für Mineralogie und Kristallchemie, Azenbergstr. 18, Stuttgart, 70174, Germany
AB: The metamorphic basement of the Leaota Massif, South Carpathians, consists of a flat-lying sequence of concordant units displaying internal lithologic and metamorphic contrasts. The Bughea Complex (BC), a mélange containing HP-rock lenses hosted in a semipelitic matrix, is one of these units situated between gneisses and units of pelitic/meta-igneous origin. Based on trace element patterns, three HP rock-types could be distinguished among the knockers: continental pelitic fragments which underwent VHP-metamorphism, N-MORB - derived eclogites, and E-MORB eclogites and metagabbronorites bearing a distinctive LILE enrichment and HFSE depletion. These features together with the decoupling of Nb and Ta as well as the incompatible behavior of Pb reveal a supra-subduction geotectonic context. The HP overprint is uneven. Prograde features are well documented by garnet zoning in eclogites and successive stages of textural-mineralogic overprint of igneous textures and assemblages (plagioclase, pyroxene, biotite). Calculated PT conditions are different among studied HP blocks ranging between 550-780°C, 1.7- 3.2 GPa. The water activity constantly increased during prograde eclogitization. The rocks from the matrix reached only 0.85-0.95 GPa and 555-585°C. The uneven preservation of mineral and textural relics in metagabbronorites as well as variable PT-conditions of the HP-overprint suggest that these rocks had experienced different burial-exhumation histories, returning to surface at different moments as the subduction progressed and being enclosed in a subduction-accretion complex. U-Pb zircon dating of eclogites constrained a Lower Cambrian age for the subduction event (537±13 Ma). The gneissic footwall complex displays a polymetamorphic history overprinted along localized concordant shear- zones by metamorphic assemblages similar to those in the BC matrix. This overprint is connected with the tectonic stacking over an active plate-margin, closely post-dating the emplacement of late collisional granites (479.1±6.5 Ma). The units on top of the BC document successively shallower evolution and emplacement dephts.
DE: 1115 Radioisotope geochronology
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3652 Pressure-temperature-time paths
DE: 3660 Metamorphic petrology
DE: 8104 Continental margins: convergent
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting