TECTONO-MAGMATIC SETTING OF THE JURASSIC OPHIOLITES FROM THE SOUTH APUSENI MOUNTAINS (ROMANIA): PETROLOGICAL AND GEOCHEMICAL EVIDENCE

Abstract

In the South Apuseni Mountains (Romania), Jurassic ophiolites can be found in association with Upper Jurassic - Lower Cretaceous island-arc magmatic suites, within a narrow belt which marks the boundary between the Eurasian and Apulian Palaeozoic continental margins. These ophiolites are highly dismembered, and sections of the lowermost crust and upper mantle are not present. New data on the field occurrence, petrology, and geochemistry of the plutonic, subvolcanic, and volcanic ophiolitic units are presented. The plutonic rocks consist of both layered and isotropic gabbros, as well as subordinate Fe-gabbros, and quartz-diorites. Ultramafic cumulates are very scarce, and include plagioclase-dunites and plagioclase-wehrlites. The subvolcanic section is represented by typical sheeted dyke complexes, cropping out in several localities and including basalts and basaltic andesites. The volcanic section includes pillowed and massive lava flows, as well as subordinate volcanic breccias, all displaying high-Ti magmatic affinity. The geochemical characteristics of these volcanic rocks are very similar to those of basalts generated at Mid-Ocean Ridge (MORBs), as well as those of the high-Ti ophiolitic complexes of the Dinaride and Hellenide orogenic belts. The constituent minerals of the intrusives are olivine, plagioclase, clinopyroxene (rarely orthopyroxene), Cr-spinel, and Fe- Ti-oxides. The subvolcanic and volcanic rocks contain plagioclase, clinopyroxene, and Fe-Ti oxides. The clinopyroxene chemistry also confirms the high-Ti affinity.