The lithosphere beneath the northwestern part of the Arabian plate (Jordan): evidence from xenoliths and geophysics

The Cenozoic alkali basaltic lavas of the northwestern part of the Arabian plate contain a wide variety of crustal and upper mantle xenoliths. The compositional diversity of the xenolith suite indicates a heterogeneous upper mantle beneath the northwestern part of the Arabian plate. It mainly consists of depleted and later metasomatized spinel Iherzolite injected with magmatic veins of pyroxenites. Thermobarometry of the lherzolites and pyroxenites yield temperatures of 900-1030°C at 12-18 kbar and 940-1020°C at 11-13 kbar respectively. The estimated δ log f_O2 varies between -1.7 and -2.3. Calculated temperatures of the granulitic xenoliths vary between 775° and 800°C at 5-11 kbar. The geotherm derived from the modelled xenolith predicts a higher surface heat flow than the measured surface heat flow and suggest a widespread regional elevation of the geotherm below the Cenozoic basalt provinces in the Arabian plate. Correlation of xenolith data and available geophysical studies suggests a cross section of the lithosphere below the northwestern part of the Arabian plate consisting of about 18-21 km thick granitic upper crust underlain by a mafic lower crust of gabbroic composition to a depth of about 37 km. The Moho appears to be a 5-8 km thick transition zone of spinel lherzolite intercalated with mafic rocks.