gems, analytical, geology, Red Sea, Saudi Arabia, olivine basalts, Harrat Kishb

Three coalesced basaltic lava fields, Harrats Khaybar, Ithnayn, and Kura, constitute the largest contiguous area of Cenozoic basalt in Saudi Arabia, similar in extent (20,564 km²) and volume (1,850 km³) to Harrat Rahat, which is situated only 25 km to the south. Harrat Kura, with an age range from about 11 to 5 Ma, is the oldest of these harrats; it contains a single stratigraphic unit, the Kura basalt, primarily composed of alkali olivine basalt (AOB) with subordinant basanite and hawaiite and a single cluster of four phonolite domes. In contrast, Harrats Khaybar (5 Ma to Present) and Ithnayn (3 Ma to Present) are composed of more mildly alkaline basalt types. The two oldest units on Harrat Khaybar, the Jarad and Mukrash basalts, are dominated by dictytaxitic-to-ophitic olivine transitional basalt (OTB) flows, which were extruded primarily from major arterial lava tubes to form unusual morphological features, here designated “whaleback lava flows.” The lava tubes emanate from a 50-km-long, north-trending fault scarp along the western edge of the central vent zone, which marks the site of voluminous outwelling of OTB lava. The fault scarp ceased to be a major site of extrusion at the end of Mukrash time.