The role of MORB and SSZ magma-types in the formation of Jurassic ultramafic cumulates in the Mirdita ophiolites (Albania) as deduced from chromian spinel and olivine chemistry
Keywords:ophiolite, ultramafic cumulates, chromian spinel, olivine, Jurassic, Albania
Both the Western (WMO) and Eastern (EMO) Mirdita ophiolites include well-developed ultramafic cumulitic sequences. In the WMO, they mainly con- sist of dunites, plagioclase-dunites, plagioclase-wehrlites, and troctolites. In contrast, in the EMO they mainly consist of chromitiferous-dunites, dunites, lher- zolites and olivine-websterites, as well as minor plagioclase-bearing dunites and lherzolites, and troctolites. The main rock-forming minerals in ultramafic cu- mulates are cumulus olivine associated with various inter-cumulus minerals, as well as chromian spinel occurring as both cumulus and inter-cumulus phase. Mg#, FeO/MgO ratios and TiO2, Al2O3 concentrations in parental melts were calculated from olivine and spinel compositions. The inferred parental melt compositions indicate that the dunite-troctolite layered series at the mantle tectonite-ultramafic cumulate transition zone in the WMO was derived from a nor- mal-type mid-ocean ridge basaltic (N-MORB) parental melt. In contrast, the dunite-troctolite layered series from the top of the WMO ultramafic cumulitic se- quence was derived from a medium-Ti basaltic (MTB) parental melt originated, in turn, in a nascent forearc setting. The inferred composition of parental melts indicate that chromitiferous dunites, dunites, and lherzolites from the EMO were originated from primary boninitic melts, whereas olivine-websterites were originated from slightly fractionated boninitic liquids. The inferred parental melt compositions plagioclase- bearing dunites and lherzolites, as well as troctolites from the EMO are compatible with an N-MORB type parentage, being the only exception one plagioclase- dunite, which may have derived from a low-Ti tholeiitic melt. Comparison of the ultramafic cumulates from the WMO and EMO indicates that both these units, though to a different extent, have experienced similar and coeval magmatic events. The investigated WMO ultramafic cumulates record N-MORB and MTB magmatic episodes. The investigated EMO ultramafic cumulates testify for boninitic, N-MORB, and very subordinate low-Ti tholeiitic magmatic events. It is therefore concluded that both WMO and EMO were generated in a protoforearc-forearc setting likely initiated nearby an active mid-ocean spreading ridge.