⁴⁰ Ar/ ³⁹ Ar chronostratigraphy and geochemistry of Harrats Ishara and Kura in the Hamd–Jizl Basin, Madinah, Saudi Arabia

Abstract

An extensive system of mafic dikes, volcanic centers, and provincial flood basalts (harrat) have erupted within the Arabian flank of the Red Sea rift since the early Oligocene. The age, spatial distribution, and composition of these features correlates with distinct changes in rift flank structural architecture as the margin progressed through successive deformational phases. A widespread pre-rift dike swarm in the proto-Red Sea region ca. 25 Ma created thermomechanical crustal conditions favorable for diffuse continental rifting and inboard extensional strain markers like the Hamd-Jizl Basin (HJB) originated amagmatically ca. 23 Ma. Six million years later, basalt flows from Harrat Ishara and Kura entered the HJB at several points along strike during a 4.5 million year-long eruptive phase. Two distinct volcanic units developed in Harrat Ishara: a predominantly alkali-olivine basalt with an age range of 18.0-14.3 Ma (Ishara A) and a mostly olivine transitional basalt with an age range of 16.6-12.1 Ma (Ishara B). The contemporaneous eruption of these units indicates that multiple magma sources operated simultaneously during the middle Miocene. Both units generally decrease in alkalinity and rare earth element abundances up section, signaling either ongoing melt fractionation processes or output from different magma chambers with time.