Trace element systematics in the plutonic section of fast-spread oceanic crust – evidence from the Wadi Gideah reference profile (Wadi Tayin Massif, Oman ophiolite)

Abstract:

The crystallization processes during the accretion of fast spreading oceanic crust are still not well constrained and a continous geochemical profile through recent oceanic crust that could provide deeper insights into such processes is still missing. We sampled a complete section of gabbros, dikes, and basalts along the Wadi Gideah in the Cretaceous ophiolite of Oman that is regarded as the best example of fast-spread oceanic lithosphere on land. Here we present trace element data displaying systematic compositional trends that are correlated with stratigraphic depth (distance to MOHO) in the plutonic suite. Concentrations of incompatible trace elements Th, Nb, Ta, REE, Zr, Hf show little variation in the layered gabbro section but display a continuous increase from foliated to varitextured gabbros. In contrast, Nb/Ta, Nb/La, Zr/Hf, La/Yb, La/Nd show significant fractionation in the layered and foliated gabbros suggesting in situ crystallization in a lower crustal filter layer and formation of small portions of highly evolved interstitial melt also favoring crystallization of zircon. Further up, fractional crystallization processes in a well-mixed magma chamber control the composition of isotropic gabbros and sheeted dikes where fractionation of HFSE element ratios is minor.