Metamorphic Evolution of the Elekdag Eclogites (Central Pontides)

Abstract

The topic of the thesis is focused on the metamorphic and tectonic evolution of high-pressure metamorphic eclogites and by extension, the surrounding meta-lherzolite and serpentinite of the Elekdag Ophiolite within the Central Pontide Mountains of Turkey. The goal of this study is to continue the work of previous authors in better understanding the timing and conditions of metamorphism and their associated tectonic events through field mapping, detailed petrographic and microstructural analysis, investigation of stable equilibrium assemblages with wavelength-dispersive x-ray, energy-dispersive, angle-selective backscatter spectroscopies coupled with isochemical phase equilibria diagrams and conventional cation exchange geothermobarometry analysis. The Elekdag Ophiolite and other related ophiolites in the Central Pontide Mountains of Northern Anatolia are interpreted as being mantle and crust accretionary prisms which formed along the southern edge of the Laurasian Supercontinent prior to the closure of the Paleotethys Ocean and the Formation of Pangea together with the Gondwana Supercontinent. Elekdag is closely associated with the larger Cangaldag Island Arc and the larger Domuzdag HP mélange. During subduction in the Cretaceous the Elekdag Complex experienced up to eclogite-facies metamorphic conditions. Metamorphic rocks present in the study area include typical ophiolite assemblages with the addition of HP-LT blueschists, greenschist facies mineral assemblages, serpentinite, mica schists and HP-MT eclogites. The HP-LT units are contained within lenses along the boundary of the serpentinite body and are also in contact with the neighboring Domuzdag Complex. Typical mineral assemblages of the eclogites are garnet + omphacite + glaucophane + clinozoisite + white mica ± lawsonite ± tourmaline ± rutile. Blueschist mineral assemblages consist of garnet + glaucophane + clinozoisite + white mica + chlorite ± quartz ± lawsonite ± rutile. Most primary mineral assemblages have been heavily overprinted with lower grade hydrous mineral assemblages. Retrogression occurred due to lower pressures and temperatures during exhumation and the extensive infiltration of hydrothermal fluids within the rocks characterized by chlorite. Typical retrograde phases are glaucophane (for eclogites), as well as the common greenschist facies assemblage of chlorite + albite + clinozoisite ± stilpnomelane. The metabasite samples investigated show mid-ocean ridge basalt affinities (MORB) with at least one protolith being a cumulate. Constraints on maximum pressures and temperatures of metamorphism, and by extent depth of subduction and subsequent exhumation were inferred based upon geochemical and petrographic analysis. Two distinct samples were modeled in isochemical phase diagrams. One, a cumulate metabasite yielded maximum PT conditions of 21±2 kbar at 360 ± 50 °C based upon garnet and phengite compositional isopleths as well as a maximum temperature of metamorphism of 400 °C according to Fe/Mg cation exchange thermometry between garnet and clinopyroxene. A second modeled sample yielded lower pressure ranges of 10±2 kbar at a higher temperature of 500 °C which is in contrast to similarly thermometry results which give maximum temperatures of ~450 °C.