OIB signatures in basin-related lithosphere-derived alkaline basalts from the Batain basin (Oman) — Constraints from 40Ar/39Ar ages and Nd–Sr–Pb–Hf isotopes

Abstract

Tertiary rift-related intraplate basanites from the Batain basin of northeastern Oman have low SiO2 (<45.6wt.%), high MgO (>9.73wt.%) and moderate to high Cr and Ni contents (Cr >261ppm, Ni >181ppm), representing near primary magmas that have undergone fractionation of mainly olivine and magnetite. Rare earth element systematics and p-T estimates suggest that the alkaline rocks are generated by different degrees of partial melting (4–13%) of a spinel-peridotite lithospheric mantle containing residual amphibole. The alkaline rocks show restricted variations of 87Sr/86Sr and 143Nd/144Nd ranging from 0.70340 to 0.70405 and 0.51275 to 0.51284, respectively. Variations in Pb isotopes (206Pb/204Pb: 18.59–18.82, 207Pb/204Pb: 15.54–15.56, 208Pb/204Pb: 38.65–38.98) of the alkaline rocks fall in the range of most OIB. Trace element constraints together with Sr–Nd–Pb isotope composition indicate that assimilation through crustal material did not affect the lavas. Instead, trace element variations can be explained by melting of a lithospheric mantle source that was metasomatized by an OIB-type magma that was accumulated at the base of the lithosphere sometimes in the past. Although only an area of less than 1000km2 was sampled, magmatic activity lasted for about 5.5Ma with a virtually continuous activity from 40.7±0.7 to 35.3±0.6Ma. During this period magma composition was nearly constant, i.e. the degree of melting and the nature of the tapped source did not change significantly over time.