HR: 15:10h
AN: T23D-07    [Abstracts]
TI: High-Pressure/Low-Temperature Melanges in the Cycladic Blueschist Belt, Greece: Results of an Ionprobe (SHRIMP) U-Pb Study
AU: * Bröcker, M
EM: brocker@nwz.uni-muenster.de
AF: Institut für Mineralogie, Zentrallabor für Geochronologie, Corrensstr. 24, Münster, 48149 Germany
AU: Keasling, A
EM: keaslin@uni-muenster.de
AF: Institut für Mineralogie, Zentrallabor für Geochronologie, Corrensstr. 24, Münster, 48149 Germany
AU: Pidgeon, R T
EM: R.Pidgeon@curtin.edu.au
AF: School of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, W.A 6001 Australia
AB: The Cycladic blueschist belt (CBB) in the central Aegean Sea has experienced epidote blueschist to eclogite facies metamorphism in the Eocene (c. 50-40 Ma) during collisional processes between the Apulian microplate and Eurasia. The general geological framework is well established, but many details of the tectonometamorphic history still are not fully understood. Unresolved issues concern the importance of pre-Eocene HP metamorphism [1, 2] and the geochronological record that is preserved in melange sequences. These aspects are addressed in an ionprobe (SHRIMP) U-Pb zircon study focusing on block-matrix associations from the islands of Syros and Andros. Melanges comprise in variable mixtures eclogites, glaucophanites, meta-gabbros, ultrabasic rocks, meta-acidic gneisses and jadeitites in a serpentinitic and/or metasedimentary matrix. The origin of the block-matrix associations is controversial and in many cases it remains unclear whether these melanges record sedimentary or tectonic processes. A tectonic slab from Syros, consisting of an interlayered meta-acidic gneiss - glaucophanite sequence, yielded ²⁰⁶Pb/²³⁸U ages of c. 237-245 Ma, similar to ages determined for rocks collected from structurally coherent sequences on other Cycladic islands. On Syros, we have also studied zircons from a metasomatic alteration profile, which developed around a compound eclogite-jadeitite net-veined block enclosed in a serpentinite matrix. From the outside in, distinct blackwall alteration zones (c. 5-30 cm in thickness) can be distinguished, which predominantly consist either of actinolite- chlorite, glaucophane or omphacite. Zircon from the unaltered jadeitite and all reaction zones yielded ²⁰⁶Pb/²³⁸U ages of c. 80 Ma. Across this profile, systematic changes are observed in zircon morphology and CL patterns. U- and Th-concentrations in zircon decrease towards the peripheral rinds. These observations are difficult to reconcile with a magmatic origin of the zircons and instead we suggest a relationship of zircon formation to fluid infiltration and block-matrix interaction during subduction zone processes. Similar ages in meta-gabbros and associated felsic melts [2, this study] probably represent protolith ages. Thus, the Syros melange apparently records the coexistence of rocks documenting both magmatic and metamorphic processes of Cretaceous age. From Andros, we have dated a meta-gabbro and a meta-acidic gneiss that yielded Jurassic ²⁰⁶Pb/²³⁸U ages of 154.5 ± 0.7 Ma and 160.0 ± 0.5 Ma, respectively, which perfectly correspond to protolith ages reported for ophiolites from the Balkan peninsula and Crete [3]. Our study indicates that the Cycladic melanges comprise a heterogeneous age population of rocks derived from different sources. Up to now three magmatic (c. 240 Ma, c. 155-160 Ma, c. 75-80 Ma) and one metamorphic age group (c. 80 Ma) were recognized, but the number of dated blocks still is relatively small and the picture might be much more complex than implied by the available datasets. [1] Bröcker & Enders (1999): Geol. Mag. 136, 111-118. [2] Tomaschek et al. (2003): J. Petrol. 44, 1977-2002. [3] Koepke et al. (2002): Lithos 65, 183-203.
DE: 1100 GEOCHRONOLOGY
DE: 3616 Hydrothermal systems (0450, 1034, 3017, 4832, 8135, 8424)
DE: 3652 Pressure-temperature-time paths
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005