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

Abstract

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 TiO 2 , Al 2 O 3 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.