Mantle peridotites from the Dinaridic ophiolite belt and the Vardar zone western belt, central Balkan: A petrological comparison

摘要

The nearly parallel Dinaridic ophiolite belt and the Vardar zone western belt are assumed to be the remnants of two distinct oceanic basins, constituting different parts of the Tethys Ocean that separated the Gondwana and Eurasia continents during Mesozoic time. These belts comprise numerous large peridotite massifs and small bodies whose petrology was poorly known. This paper presents a large set of internally consistent analytical data for peridotites, including primary mineralogy, major-element chemistry and clinopyroxene geochemistry for massifs of both the ophiolitic belts. We propose, discuss and apply a set of mineralogical, geochemical and petrologic criteria that allow a recognition of the probable geodynamic setting of formation of the ultramafic massifs. Ultramafic massifs of the Vardar zone western belt gradually change in composition northwards from depleted spinel lherzolites (Banjska massif) to depleted harzburgites (Maljen massif); these bodies originated in the same geodynamic setting, probably a back-arc spreading center. By contrast, the Dinaridic belt ultramafic massifs include several different types that formed in different geodynamic environments. Orogenic lherzolites, interpreted as subcontinental peridotites (Kozara, Cavka, Borja, Sjenicki Ozren and Bistrica massifs) are dominated by fertile spinel and plagioclase lherzolites with subordinate amounts of depleted spinel lherzolite, spinel harzburgite, rare dunite and very rare vein garnet clinopyroxenite. The inferred subcontinental peridotites of these massifs are not co-magmatic with neighboring basalts and cannot be considered as members of a single ophiolitic assemblage. Massifs of two other types are less common in the Dinaridic ophiolite belt. These are composed of spinel lherzolite-harzburgite (Zlatibor and possibly Bosanski Ozren massifs) and depleted harzburgite (Tuzinje and Brezovica massifs); both probably originated in a suprasubduction environment. The available data suggest that the studied ultramafic rocks formed in two different oceanic basins, probably marginal seas. We also suggest that the orogenic lherzolites that dominate the central-northern part of the Dinaridic ophiolite belt formed as a result of continental lithosphere extension after closure of the Dinaride oceanic basin and were later tectonically incorporated into the ophiolitic melange.