Petrology and Structure of the Vourinos Ophiolitic Complex of Northern Greece

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Abstract
Throughout the Tethyan mountain system, ultramafic rocks form lower portions of pseudostratiform sequences (ophiolites), overlain by mafic rocks, lava, and radiolarian chert. Many European geologists think these ophiolites represent submarine extrusion and differentiation of basaltic magma. A well-exposed ophiolite, the 200 km2 Vourinos Complex of northern Greece, was emplaced on lower Jurassic (?) sediments, and is overlain unconformably by Cenomanian Limestone. The complex comprises 75 to 85 percent dunite, peridotite, pyroxenite, and chromitite, and 15 to 25 percent gabbro, diorite, quartz diorite, and partly pillowed lava. Ultramafic lithologic units display layering and complex folds arranged in a fan-like manner. Pyroxenitic layering within peridotite, possibly formed during emplacement of the complex, commonly transects deformed pre-existing dunite-peridotite and chromitite-dunite layering. Comparatively regular, but isoclinally folded, ends of the ultramafic mass pass into a middle of complex divergent isoclinal folds with abundant pyroxenite pegmatites and derived gabbro dikes and bodies. This complex structure, probably formed by nearly solid penetrative flow, is limited to ultramafic rooks. Simpler layering structure of mafic rocks is probably of magmatic origin. Fo90–91 and En90–92 in dunites and peridotites indicate that these are “Alpine-type” rather than basaltic differentiates. En89–85 in pyroxenites and En75–65 in gabbros suggest fractionation during crystallization. Compositional similarity and complex gradational contacts between intrusive and extrusive mafic rocks imply common parentage. The Complex may have originated by partial fusion of lherzolitic mantle material (containing 3 to 4 percent alumina), and subsequent separation of approximately 80 percent ultramafic residue (< 1 percent alumina) and 20 percent mafic liquid (> 10 percent alumina) during emplacement on the ocean floor. True ophiolite complexes may represent the closest existing approach to ultramafic extrusives.

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