Abstract
The Neogene-Quaternary Harrat Rahat volcanic field is part of the major intercontinental Harrat fields in western Saudi Arabia. It comprises lava flows of olivine basalt and hawaiite, in addition to mugearite, benmorite, and trachyte that occur mainly as domes, tuff cones and lava flows. Based on opaque mineralogy and mineral chemistry, the Harrat Rahat volcanic varieties are distinguished into Group I (olivine basalt and hawaiite) and Group II (mugearite, benmorite and trachyte). The maximum forsterite content (approximate to 85) is encountered in zoned forsteritic olivine of Group I, whereas olivine of Group II is characterized by intermediate (Fo=50), fayalitic (Fo=25) and pure fayalite in the mugearite, benmorite and trachyte, respectively. The more evolved varieties of Group II contain minerals that show enrichment of Fe2+, Mn2+ and Na+ that indicates normal fractional crystallization. The common occurrence of coarse apatite with titanomagnetite in the benmorite indicates that P5+ becomes saturated in this rock variety and drops again in trachyte. Cr-spinel is recorded in Group I varieties only and the Cr# (0.5) suggests lherzolite as a possible restite of the Harrat Rahat volcanics. The plots of Cr# vs. the forsterite content (Fo) suggest two distinct trends, which are typical of mixing of two basaltic magmas of different sources and different degrees of partial melting. The bimodality of Harrat Rahat Cr-spinel suggests possible derivation from recycled MORB slab in the mantle as indicated by the presence of high-Al spinel. It is believed that the subcontinental lithospheric mantle was modified by pervious subduction process and played the leading role in the genesis of the Harrat Rahat intraplate volcanics. The trachytes of the Harrat Rahat volcanic field were formed most probably by melting of a lower crust at the mantle-crust boundary. The increase in f(O2) causes a decrease in Cr2O3, and Al2O3, and a strong increase in the proportion of Fe3+ and Mg# of spinel crystallizing from the basaltic melt at T approximate to 1200 degrees C. The olivine-pyroxene and olivine-spinel geothermometers yielded equilibrium temperature in the range of 935-1025 degrees C, whereas the range of <500-850 degrees C from single-pyroxene thermometry indicates either post crystallization re-equilibrium of the clinopyroxene, or the mineral is xenocrystic and re-equilibrated in a cooling basaltic magma.