POTASSIUM-RICH FLUID INDUCED PARTIAL MELTING IN THE LOWER CONTINENTAL CRUST BENEATH NORTHERN ARABIA

Alkaline basalt in Harrat Ash Shaam has sampled the upper mantle and lower crust components beneath the region. Gabbroic (>30% mafics) and noritic (<30% mafics) granulites and one spinel-garnet clinopyroxenite xenoliths from Abu Tarboosh (ABT) and Ashqaf Usefir (AS) volcanos and their host basalt have been studied. All xenolith samples contain abundant plagioclase and orthopyroxene (excepting the spinel-garnet clinopyroxenite) with variable amount of spinel (low Cr#), cpx, ilmenite, and apatite, which are termed as “primary phases” in this study.

Segregations of “melt pockets” are interstitially distributed between the primary phases in these xenoliths. The size of melt pockets ranges from hundreds of micrometers to a few millimeters. Next to the melt pockets, the primary cpx forms peritectic texture along grain edge; the primary plagioclase forms reverse compositional zoning with Ca rich rim and resorbed boundary; the primary opx forms symplectite with dendritic oxides. In gabbroic granulites, the micron-size crystals in the peritectic zone are mainly cpx with variable chemistry and generally have lower Na₂O and SiO₂ and higher TiO₂ contents compared to the primary cpx. Olivine and bytownite coexist with cpx in the peritectic area. Microcrystalline amphibole exists in the melt pocket in some gabbroic granulites. K-rich hydrous phase (KHP: K-Ca-Al-Si-O-H) is interstitially distributed among the microcrystalline crystals and the resorption areas. In noritic granulites, all cpxs form peritectic texture. The melt pocket forms a zoning pattern, from the resorbed primary phase, to a recrystallized mantle, and the spherical shape KHP in the center. The recrystallization zone consists of microcrystalline cpx, amphibole, plagioclase, dendritic oxides and glass. KHP also exists in the melt pockets in spinel-garnet clinopyroxenite. KHP distributes in basalt as irregular shape lithic grains and no KHP exists in the matrix of basalt.

Based on the appearance of the KHP in both lower crust xenolith and basalt, the melt pockets in the xenoliths may generate from the partial melting induced by the movement of K-rich fluid in the lower crust. Since the alkaline basalt should be the provider of the K-rich fluid in this lower crust region, this alkaline basaltic magma might form from a high volatile-flux condition.