Petrology and geochemistry of the Late Jurassic calc-alkaline series associated to Middle Jurassic ophiolites in the South Apuseni Mountains (Romania)

In the South Apuseni Mountains, Romania (SAM) Jurassic calc-alkaline magmatic series occur in association with Jurassic ophiolites within a narrow belt that marks the boundary between the Eurasian and Adria Paleozoic continental margins. This association of magmatic series has been previously reported as a single ophiolitic sequence by many authors. Calc-alkaline rocks include volcanic, subvolcanic and intrusive rocks and, along with associated ophiolites, occur in a composite nappe system in the central and NE part of the SAM. Volcanic rocks directly overly ophiolites and include basalts, basaltic andesites, andesites, dacites, and rhyolites, while dykes intrude both calc-alkaline volcanics and ophiolites, and are mainly represented by andesites and dacites, locally showing high-K calc-alkaline affinity. Intrusive rocks mainly include granites and granodiorites. The geochemical characteristics of the studied rocks correspond to those generated in island arc settings, with a depletion of high field strength elements relative to large ion lithophile elements, and a marked enrichment of light rare earth elements. Mineral-melt exchange equilibria, as well as bulk-rock major and trace element abundances indicate that fractional crystallization in closed systems played a major role in controlling the magmatic evolution of the SAM calc-alkaline series. Geothermometric and geobarometric estimations indicate that temperature along the fractionation trend varied from 1216°C for the less evolved rocks down to 860°C for the more evolved rocks, at pressures included between 1.8-2.3 kbar. SAM calc-alkaline lavas are derived from a MORB-like depleted source modified by slab-derived fluids in an intra-oceanic island arc setting that developed during the closure of the Vardar oceanic basin. These island arc series were formed by both intrusion and extrusion of calc-alkaline rocks into and onto a pre-existing oceanic crust now represented by SAM ophiolites. Unlike some previous interpretations, data presented in this paper demonstrate that no genetic relationship between the calc-alkaline series and the underlying ophiolitic rocks exists. Data presented in this paper may provide significant constraints for further geodynamic reconstruction of the northernmost border of the Vardar ocean and its related continental margins.