Tectonic setting for the genesis of oceanic plagiogranites: evidence from a paleo-spreading structure in the Oman ophiolite

Abstract

In the Oman ophiolite, detailed structural and lithological mapping of the 60 × 45 km Maqsad area has revealed several plagiogranite intrusions, ranging from metre scale dykes to hectometre scale bodies. These plagiogranites are spatially related to kilometre scale mafic plutons, displaying generally pegmatitic textures and locally deformed in amphibolite facies conditions along shear zones up to a few hundred metres thick. Isotope data demonstrate the mantle origin of these pegmatites and plagiogranites: ϵNd ranges from 6.3 to 9.9 and has an average value (26 samples) of 8.1, within the field of present-day MORBs, and similar to the average ϵNd (7.8) of the Oman ophiolite primary igneous sequence. These intrusions are not restricted to a given structural level: their paleovertical extent exceeds 5–6 km, from 2 km below the Moho up to the base of the sheeted dyke complex. Their modal composition evolves up section, from predominantly pyroxenites and gabbro norites in the mantle harzburgites to gabbro norites, diorites and quartz diorites at crustal level. Petrography, whole rock and mineral chemistry show that this vertical succession of lithologies can be simply explained by low pressure fractional crystallization of a hydrated basaltic melt. Plagiogranites, although more abundant at upper crustal level, are not restricted to this horizon. Plagioclase composition in plagiogranites presents a remarkable evolution with paleodepth, from highly calcic (70–95% an) in the mantle section, intermediate and highly scattered in the lower crust (10–85% an), to quite sodic at upper crustal level (5–20% an). Highly calcic plagioclases in quartz-bearing rocks is a puzzling character, inconsistent with fractional crystallization. Even if high degrees of fractional crystallization of a hydrated basaltic melt remain, the simplest way to account for the chemistry of the upper crustal plagiogranites, processes such as fluid-induced remelting or assimilation of country rocks must be invoked to account for the chemistry and field characteristics of the deeper plagiogranites.Mantle and crustal structures in the Maqsad area indicate that the pegmatites and plagiogranites emplaced in an ocean ridge setting, when a mantle diapir, soaked with basaltic melt, intruded a cool, hydrothermally altered, lithosphere. They were eventually transposed in an off-axis setting as the ascension of the diapir progressed up to Moho level. It is concluded that the formation of large volumes of buoyant leucocratic rocks is possible at ocean ridges, provided periods of amagmatic spreading are long enough to induce the growth and alteration of an axial lithospheric lid. This is likely more frequent at slow spreading centres where mantle upwelling is discontinuous in space and time.