SPATIAL VARIATION IN SAMAIL METAMORPHIC SOLE PROTOLITHS: A REGIONAL-SCALE ISOTOPIC (HF, ND, U-PB) AND TRACE-ELEMENT PERSPECTIVE

Abstract

Metamorphic soles beneath ophiolites contain important records of subduction initiation; protoliths of these sole rocks preserve information about tectonic processes occurring prior to and during ophiolite emplacement. The Samail Ophiolite – the largest and best exposed ophiolite on Earth – exposes metamorphic soles at its base spanning hundreds of kilometers along its length, but there are a limited number of sole geochemical data from few outcrops. Here, we present a trace-element, isotopic (Hf and Nd), and geochronological (zircon U-Pb) dataset spanning the entire length of the ophiolite, which we use to interpret the tectonic setting and thermal mechanisms driving ophiolite formation and thrusting. To constrain the earliest perspective on ophiolite emplacement, we obtained whole-rock geochemical information from n=36 garnet- and clinopyroxene-bearing sole amphibolites at the top of each sole section. Most of these high-grade sole rocks have affinities toward enriched mid-ocean ridge basalt (E-MORB) compositions, nominally matching earlier inferences for derivation of the metamorphic sole from the Haybi Complex that immediately underlies the sole. This interpretation is also supported by detrital zircons from one mixed sole amphibolite-metasediment that yield a Carboniferous maximum depositional age, as well as a compiled sole-Haybi Carboniferous Sm-Nd whole-rock isochron. However, a subset of the sole amphibolites from south Oman and the UAE exhibit N-MORB-like elemental and isotopic characteristics. This observation suggests either that there is an unidentified N-MORB-like member of the Haybi Complex; that there is another, unidentified Indian Ocean MORB source for the Samail sole; or that the V1 volcanics (Geotimes unit) of the ophiolite may have supplied some of the rocks now forming the metamorphic sole. The first two of these would suggest ≥150 Ma protolith ages at the time of subduction initiation, while the latter of these would indicate mixing between protoliths of variable ages, despite the absence of variations in peak sole P-T along the ophiolite. Regardless of protolith age, the data presented in this study reveal multiple scales of heterogeneity for the basaltic protolith of rocks composing the uppermost Samail sole.