Coexistence of two distinct mantle sources during formation of ophiolites: a case study of primitive pillow-lavas from the lowest part of the volcanic section of the Troodos Ophiolite, Cyprus

摘要

We present a detailed mineralogical, petrological and melt inclusion study of unusually fresh, primitive olivine + clinopyroxene phyric Lower Pillow Lavas (LPL) found near Analiondas village in the northeastern part of the Troodos ophiolite (Cyprus). Olivine phenocrysts in these primitive LPL show a wide compositional range (Fo(82-92)) and have higher CaO contents than those from the Upper Pillow Lavas (UPL). Cr-spinel inclusions in olivine are significantly less Cr-rich (Cr/Cr + Al = 28-67 mol%) compared to those from the UPL (Cr# = 70-80). These features reflect differences in melt compositions between primitive LPL and the UPL, namely higher CaO and Al2O3 and lower FeO* compared to the UPL at a given MgO. LPL parental melts (in equilibrium with Fo(92)) had similar to 10.5 wt% MgO and crystallization temperatures similar to 1210 degrees C, which are significantly lower than those previously published for the UPL (14-15 wt% MgO and similar to 1300 degrees C for Fo(92)). The fractionation path of LPL parental melts is also different from that of the UPL. It is characterized initially by olivine + clinopyroxene cotectic crystallization joined by plagioclase at similar to 9 wt% MgO, whereas UPL parental melts experienced a substantial interval of olivine-only crystallization. Primitive LPL melts were formed from a mantle source which was more fertile than that of tholeiites from well-developed intra-oceanicarcs, but broadly similar in its fertility to that of Mid-Ocean Ridge Basalt (MORB) and Back Are Basin Basalts (BABE). The higher degrees of melting during formation of the LPL primary melts compared to average MORE were caused by the presence of subduction-related components (H2O). Our new data on the LPL coupled with existing data for the UPL support the existing idea that the LPL and UPL primary melts originated from distinct mantle sources, which cannot be related by progressive source depletion. Temperature differences between these sources (similar to 150 degrees C), their position in the mantle (similar to 10 kbar for the colder LPL source vs 15-18 kbar for the UPL source), and temporal succession of Troodos volcanism, all cannot be reconciled in the framework of existing models of mantle wedge processes, thermal structure and evolution, if a single mantle source is invoked. Possible tectonic settings for the origin of the Troodos ophiolite (forearc regions of intra-oceanic island are, propagation of backarc spreading into are lithosphere) are discussed.