The metamorphic and structural evolution of the Barrovian Overprint, Naxos, Cyclades, Greece
The island of Naxos, which lies in the central-southern Aegean Sea, is part of the Attic Cycladic Massif. This, a group of islands, containing metamorphic complexes polymetamorphosed during the Tertiary, forms an arcuate belt between mainland Greece and Turkey. The present-day tectonic setting of this area is one of convergence between Africa and Europe, via a subduction zone situated to the south of the Attic Cycladic Massif. The back-arc region of this subduction zone is undergoing present-day extension. This study investigates the nature of Barrovian metamorphism on Naxos, and its relationship to the timing of compressional and extensional tectonics in the Attic Cycladic Massif. The earliest Tertiary phase of metamorphism is a blueschist event of Eocene age (M1), which is variably overprinted by an Oligo-Miocene (M2) metamorphism of Barrovian character. The M2 event is most strongly developed on Naxos, where it resulted in upper amphibolite facies metamorphism and associated partial melting. The high grade core of the metamorphic complex is divided into a Lower Series of meta-pelites and meta-carbonates, and a structurally underlying core of leucogneisses. The syn- to post- M2 structural evolution of the metamorphic complex is interpreted as resulting from a continuum of northerly-directed non-coaxial deformation. The earliest structures (D1), occur as NNE-SSW trending meso- and macro-scale recumbent isoclinal folds, which deform a series of pre-M2 (Hercynian?) granitic gneisses in the leucogneiss core. The exhumation of the metamorphic complex is characterised by the development and re-working of non-coaxial protomylonitic fabrics formed under successively lower grade metamorphic conditions. This non-coaxial deformation, termed D2, is interpreted as having occurred during low-angle ductile extension of the metamorphic complex. During the latter stages of D2 deformation, northwards-directed non-coaxial extension was accompanied by a component of W-E-directed sub-horizontal compression (D3), possibly related to large-scale rotation of the terminations of the originally rectilinear arc-system. The textural evolution of meta-pelite assemblages is related to divariant continuous equilibria in the simplified system KFMASH and to univariant equilibria in the system KTiFMASH, as appropriate for different bulk rock compositions. The mineral textures and inclusion relationships observed constrain the prograde M2 path as being one of heating and decompression, during which reaction products were continuously incorporated into the sub-horizontal composite foliation. Estimates of physical conditions during M2 metamorphism, made through garnet-biotite Fe-Mg geothermometry and the approach of internally consistent datasets, indicates (P, T,) conditions of ≈ (6-7±2 kbar, 560-660±50 deg. C) in the Lower Series, and ≈ (7-8±2 kbar, 640-690±50 deg. C) in the leucogneiss core. Geothermobarometry of zoned assemblages in meta-pelites from the leucogneiss core indicates a prograde P-T path of heating (60-100 deg. C) and decompression (1-3 kbar). (P, T) estimates and petrogenetic grids constrain equilibrium fluid compositions to be water-rich. Strontium isotopic studies suggest that anatexis in the leucogneiss core is not simply the result of large-scale partial melting of meta-pelites. Incipient anatexis on the margin of the leucogneiss core occurred by water-saturated melting of meta-pelites, whereas melting in the leucogneiss core occurred dominantly by re-melting of Models relating the exhumation of the metamorphic complex on Naxos to extensional shear zones of the type proposed for 'metamorphic core complexes' in the Basin and Range Province of North America are discussed. It is suggested that M2 ultrametamorphism on Naxos occurred in an island-arc setting, as the result of the emplacement of a basic intrusion below the present-day erosional surface. Ductile extension began before the M2 peak, possibly because of the southerly migration ('roll back') of the subduction zone to its present-day position.