Πετρολογία και γεωχημεία των μεταμορφωμένων σχηματισμών της Αττικής

Abstract

In the present dissertation, we study, the metamorphic evolution and petrogenetic history of the protoliths of the metamorphic rocks of Eastern Attica. The metamorphic complex of Eastern Attica respresents the NW part of Attic-Cycladic Crystalline zone, comprised by the Upper Tectonic Unit (UTU) and the Lower Tectonic Unit (LTU). In this study we use thermobarometric methods (multi-equilibrium method, chlorite thermometry, chlorite-chloritoid thermometry, phengite barometry, pseudosections and graphite thermometry) geochemical plots have been used in order to evaluate the role of various petrological processes which induce the geochemical characteristics of the protoliths of the metabasites and gneisses. The zonation of blue amphiboles, #he glaucophane, pumpellyite and lawsonite inclusions in albite and epidote and the omphacite-glaucophane-actinolite-epidote assemblage in the metabasites show a prograde path with increasing pressure from pumpellyite-actinolite phase field towards lawzonite-blueschists. The temperatures are range from 310-3300C (Penteli area) and 330-3800C (Lavrion area) and the pressures range from 0.7-0.9 (Penteli area) and 0.8-0.9 GPa (Lavrion area). Petrological data from the LTU support the high pressure/low temperature character of the studied rocks. The maximum pressures are range from 1.2-1.3 GPa and 1.3-1.4 GPa for the area of Penteli and Hymettus-Lavrion respectively. Thermobarometric evaluation of the data of the UTU show that the exhumation path is followed by continuous cooling with the maximum P-T conditions of the UTU reached a depth >30 Km. The maximum temperatures are range from 450-4700C and 460-4900C for the area of Penteli and Hymettus-Lavrion respectively. The LTU rocks reached a maximum depth at about 45-50 Km. The exhumation path followed by temperature increase at a back-arc environment. The magmas that generate the protoliths of the metabasites from the UTU of Penteli area show enriched melts upwelling and mixing with depleted mantle melts. The magmas that generate the protoliths of the metabasites from the UTU of Lavrion area are the result of fractional crystallization. The magmas that generate the protoliths of the metabasites from the LTU of Penteli area came from the partial melting of a homogeneous source. The magmas that generate the protoliths of metamigmatites and orthogneisses show that plagioclase was the principal residual mineral phase during partial melting. The geotectonic environment of formation of the protoliths of the metamorphic rocks in Eastern Attica is generally displayed by a dual system volcanic arc-back arc basin. In UTU, the protoliths are settled in a N-MORB or evolved back-arc basin or to the internal part of an oceanic slab. In the LTU, the protoliths of the metabasites have been formed in an evolved tectonic environment from volcanic arc to back-arc basin. The gneissic protoliths of the LTU represent magmatic arc rocks either of pre-alpine or alpine age.