Syn- to post-orogenic exhumation of metamorphic nappes: Structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)

Abstract

International audienceThe Lavrion peninsula is located along the western boundary of the Attic-Cycladic metamorphic complex in the internal zone of the Hellenic orogenic belt. The nappe stack is well exposed and made, from top to bottom, of (i) a non-metamorphic upper unit composed of an ophiolitic melange, (ii) a middle unit mainly composed of the Lavrion schists in blueschist facies, (iii) and a basal unit mainly composed of the Kamariza schists affected by pervasive retrogression of the blueschist facies metamorphism in greenschist facies. The middle unit is characterized by a relatively steep-dipping foliation associated with isoclinal folds of weakly organized axial orientation. This foliation is transposed into a shallow-dipping foliation bearing a N-S trending lineation. The degree of transposition increases with structural depth and is particularly marked at the transition from the middle to the basal unit across a low-angle mylonitic to cataclastic detachment. The blueschist facies foliation of the Lavrion schists (middle unit) is underlined by high pressure phengite intergrown with chlorite. The Kamariza schists (basal unit) contains relics of the blueschist mineral paragenesis but is dominated by intermediate pressure phengite also intergrown with chlorite and locally with biotite. Electron probe micro-analyzer chemical mapping combined with inverse thermodynamic modeling (local multi-equilibrium) reveals distinct pressure–temperature conditions of crystallization of phengite and chlorite assemblages as a function of their structural, microstructural and microtextural positions. The middle unit is characterized by two metamorphic conditions grading from high pressure (M1, 9–13 kbar) to lower pressure (M2, 6–9 kbar) at a constant temperature of ca. 315 °C. The basal unit has preserved a first set of HP/LT conditions (M1–2, 8–11 kbar, 300 °C) partially to totally transposed-retrogressed into a lower pressure mineral assemblage (M3, 5–8.5 kbar) associated with a slight but significant increase in temperature (∼350 °C)

Kinematic reconstructions and magmatic evolution illuminating crustal and mantle dynamics of the eastern Mediterranean region since the late Cretaceous

“…~9 kbar, 550-570 °C) followed by a greenschist-facies retrograde metamorphism observed in the Andros, Tinos and Kea islands, in Lavrion (figure 6) [Altherr et al, 1982; Bröcker et al, 2004;Jolivet and Brun, 2010;Grasemann et al, 2012;Huet et al, 2014;Scheffer et al, 2015] and in the Olympos, Ossa and Pelion tectonic windows [Schermer, 1993;Lips et al, 1999;Lacassin et al, 2007]. In the lower Miocene, HT-MP metamorphism (i.e.…”

CO 2 flow during orogenic gravitational collapse: Syntectonic decarbonation and fluid mixing at the ductile-brittle transition (Lavrion, Greece)

“…Marbles and metapelites exposed in the Lavrion area represent a former carbonate platform involved in the Hellenides orogenic belt at the boundary between the Africa and Eurasia converging plates (Dewey et al, 1973;Jolivet and Brun, 2010). This sedimentary sequence has been successively (i) dragged with the downgoing slab, (ii) tectonically accreted after decoupling from the slab, and (iii) tectonically exhumed to the surface by the emplacement of low-angle detachment which accommodates gravitational collapse (Scheffer et al, 2016). This last exhumation event is accommodated by the development of a mylonitic to cataclastic low-angle detachment mainly localized in marble layers (Scheffer et al, 2016).…”

“…This sedimentary sequence has been successively (i) dragged with the downgoing slab, (ii) tectonically accreted after decoupling from the slab, and (iii) tectonically exhumed to the surface by the emplacement of low-angle detachment which accommodates gravitational collapse (Scheffer et al, 2016). This last exhumation event is accommodated by the development of a mylonitic to cataclastic low-angle detachment mainly localized in marble layers (Scheffer et al, 2016). Major ore deposits took place in this particular context, with the development of a porphyry system associated to the emplacement of the Plaka granodiorite (Voudouris et al, 2008a,b;Bonsall et al, 2011).…”

“…The Lavrion schists are rich in muscovite, quartz, albite, chlorite, glaucophane and epidote indicating greenschist to blueschist facies metamorphism (Baziotis et al, 2009). Two successive deformation-metamorphism phases at blueschist facies conditions have been distinguished by two chlorite-phengite assemblages using X-ray mapping on the Lavrion schists (Scheffer et al, 2016). D 1 M 1 is characterized by a steepdipping S 1 schistosity underlined by a first chlorite-phengite assemblage indicating 0.9-1.3…”

“…The Kamariza schists are rich in muscovite, quartz, albite and chlorite with granoblastic to lepidoblastic texture (Photiades and Carras, 2001). The lower unit is characterized by pervasive retrogression of the blueschist facies paragenesis into greenschist facies evidenced by two distinct chlorite-phengite assemblages of the Kamariza schists (Scheffer et al, 2016). Relics of early steep-dipping schistosity (S 1-2 ) reflecting a D 1-2 M 1-2 event are locally preserved and show P-T conditions in blueschist facies (0.8-1.1 GPa, 280-320 °C) almost similar to the ones of the Lavrion schists.…”