Petrogenesis and tectono-magmatic significance of basalts and mantle peridotites from the Albanian-Greek ophiolites and sub-ophiolitic melanges. New constraints for the Triassic-Jurassic evolution of the Neo-Tethys in the Dinaride sector

摘要

The Albanide-Hellenide ophiolites and related ophiolitic melanges include eight different types of volcanic and subvolcanic rocks: 1) Triassic, within-plate alkaline rocks (WPB); 2) Triassic high-Ti mid-ocean ridge basalts showing enriched compositions (E-MORB); 3) Triassic and Jurassic high-Ti mid-ocean ridge basalts showing normal compositions (N-MORB); 4) Jurassic basalts with geochemical features between MORB and island arc tholeiites; hereafter defined as medium-Ti basalts (MTB); 5) Jurassic low-Ti, island arc tholeiitic (IAT) rocks; 6) Jurassic very low-Ti (boninitic) rocks; 7) Jurassic backarc basin basalts and basaltic andesites (BABB); 8) Triassic and Jurassic calc-alkaline rocks (CAB). The geochemical and petrogenetic features of these rock-types, as well as the results from REE modelling of mantle sources, primary melt generation, and mantle residua indicate that they have formed in distinct tectonic settings within an oceanic environment. Both Triassic and Jurassic N-MORBs primary magmas derived from ~10 to 20% partial melting of a primitive asthenosphere, whereas Triassic alkaline WPB basalts originated from low degrees of partial melting of an OIB-type mantle source and were most likely erupted in seamounts. Triassic E-MORBs originated from -12% partial melting of a primitive asthenosphere influenced by the OIB-type component. The residual MORB mantle is represented by depleted lherzolites, which are commonly found in the Albanide-Hellenide ophiolites. Mid Jurassic MTB and IAT primary magmas derived from ~10% and 10-20% partial melting of the MORB residual mantle, respectively with the variable addition of subduction components and were erupted in an intra-oceanic, supra-subduction zone setting. The residual mantle associated with these magmatic events is represented by harzburgites. Mid Jurassic boninitic primary magmas may have originated either from 10 to 20% partial melting of the MTB and IAT residual mantle or from ~30% partial melting of the MORB residual mantle. In both cases, the depleted mantle sources were enriched in light rare earth elements (LREE) by subduction-derived fluids. The extremely depleted harzburgites, which are widespread in the Albanide-Hellenide ophiolites, are interpreted as the residual mantle associated with boninite formation.Mid-Late Jurassic CABs originated from ~15 to 20% partial melting of a depleted peridotite mantle significantly enriched in Th and LREE by subduction-derived fluids, whereas BABBs originated from 10 to 20% partial melting of a primitive asthenosphere somewhat enriched in Th and LREE by a nearby subduction. Both these rock-types were erupted in a continental arc-backarc setting.The different rock-types of the Albanide-Hellenide ophiolites record the fundamental stages of the Triassic-Jurassic evolution of the Neo-Tethys in the Dinaride sector: from sea-floor spreading, after continental break-up, to intra-oceanic subduction initiation and supra-subduction zone (SSZ) lithospheric accretion.