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

Abstract

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 signi®cantly less Cr-rich (Cr/Cr + Al = 28±67 mol%) compared to those from the UPL (Cr# = 70±80). These features reect di erences in melt compositions between primitive LPL and the UPL, namely higher CaO and Al 2 O 3 and lower FeO* compared to the UPL at a given MgO. LPL parental melts (in equilibrium with Fo 92 ) had $10.5 wt% MgO and crystallization temperatures $1210°C, which are signi®cantly lower than those previously published for the UPL (14±15 wt% MgO and $1300°C for Fo 92 ). The fractionation path of LPL parental melts is also di erent from that of the UPL. It is characterized initially by olivine + clinopyroxene cotectic crystallization joined by plagioclase at $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-oceanic arcs, but broadly similar in its fertility to that of MidOcean Ridge Basalt (MORB) and Back Arc Basin Basalts (BABB). The higher degrees of melting during formation of the LPL primary melts compared to average MORB were caused by the presence of subduction-related components (H 2 O). 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 di erences between these sources ($150°C), their position in the mantle ($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 arc, propagation of backarc spreading into arc lithosphere) are discussed.

Boninites as windows into trace element mobility in subduction zones

“…Elevated 207 Pb/ 204 Pb in the Cyprus lavas ( Fig. 3c; see also Kö nig et al, 2008) confirm earlier observations arguing for the involvement of fluids derived from subducted pelagic sediments (e.g., Cameron, 1985;Rautenschlein et al, 1985;Portnyagin et al, 1997).…”

Fluid bubbles in melt inclusions and pillow-rim glasses: high-temperature precursors to hydrothermal fluids?

“…Backarc basin magmas in this study are characterised by olivine + clinopyroxene ± plagioclase-phyric basalts from the eastern Manus spreading ridge, near PACMANUS hydrothermal field (Binns and Scott, 1993;Kamenetsky et al, 2001), the New Hebrides Troughs, separating the Vanuatu island arc from the active North Fiji basin (Maillet et al, 1995), and the Okinawa Trough formed by extension of continental lithosphere behind the Ryukyu arc -trench system (Shinjo et al, 1999). A variety of primitive arc lavas (olivinebearing arc tholeiites and boninites) are from well known ophiolitic complexes, such as New Caledonia (Cameron, 1989) and Troodos (Sobolev et al, 1993;Portnyagin et al, 1997), and from the Hunter RidgeHunter Fracture Zone, an aborted embyonic intraoceanic arc along the southern margin of the North Fiji Basin (Sigurdsson et al, 1993). As an example of intraplate oceanic magmatism, a picritic tholeiite from Mauna Loa, HI, was selected.…”

Spheroidal textures in igneous rocks – Textural consequences of H2O saturation in basaltic melts

“…Figures 9A and B illustrate orbicular tholeiitic basalts from the Upper Pillow Lavas at Kalavassos and Kapileio (Cameron, 1985;Portnyagin et al, 1997;König et al, 2008) where glassy orbs are set in a glass-rich, micro-orbicular hyaloclastite matrix. Many textural parallels are noted with the orbicular gabbros illustrated in Fig.…”