ON THE LIMITS AND LIMITATIONS OF THE OPHIOLITE - MID-OCEAN RIDGE ANALOGY: OMAN VS THE EAST PACIFIC RISE

Since Ian Gass first proposed in 1968 that ophiolite complexes formed by a process directly analogous to seafloor spreading their study has been central to the development of conceptual models for the generation of oceanic lithosphere at mid-ocean ridges. Akiho Miyashiro's seminal observation in 1973, that the geochemistry of ophiolites requires their formation above subduction zones rather than at ‘proper’ open-ocean mid-ocean ridges (MOR), has therefore long been problematical in essentially questioning the analogy. To a first order it is reasonable to assume that the physical processes of crustal formation at an open-ocean MOR and at a supra-subduction spreading centre should be closely comparable: the presence of sheeted dyke complexes in each is convincing evidence for seafloor spreading. But – beyond the first order – can we really assume the detailed processes of crustal accretion are identical? What effects do the differences in thermal regime, chemical compositions and volatile contents have on the physical behaviour of magmas and hence resultant ocean lithosphere? Exactly how far can we press the analogy?

We explore this conundrum by comparing the ‘crown jewel’ of ophiolites, Oman, with the East Pacific Rise to test the widely-held belief that Oman represents a direct analogue for a fast-spreading (open-ocean) MOR. We have previously shown (MacLeod et al. 2013) that whereas the mantle source of the axial volcanics in Oman is very similar to that of MORB, the magmas were generated in the presence of water, hence that the ophiolite was most likely formed by a short-lived (fast) spreading episode following subduction initiation.

We show here that – contrary to general belief – the Oman ophiolite crust differs subtly but significantly in composition and internal structure from what we know of the EPR. We however demonstrate that the differences are explicable and may be successfully modelled as resulting primarily from the elevated water contents in Oman magmas, here showing the consequential effects on chemical fractionation, physical properties and melt transport, and ultimately on crustal architecture. The supra-subduction zone ophiolite - mid-ocean ridge analogy should always be applied with an understanding that such caveats are likely.