Abstract

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.

Immediately to the east of the fault scarp, a thin (0.7 to 1.0 km), uptilted slab of crust formed the roof of a high-level magma chamber (∼60 x 25 km) that dominated the eruptive history of the Abyad basalt, the youngest stratigraphic unit on Harrat Khaybar. The Abyad basalt occurs as seven stratigraphic subunits, the two youngest of which (Qb6 and Qb7) are "post-Neolithic" and historical in age. In contrast to the Jarad and Mukrash basalts, the Abyad basalt is dominated by AOB and hawaiite, with only minor OTB. No Abyad differentiated rocks occur on Harrat Ithnayn; however, fractional crystallization in the high-level magma chamber beneath Harrat Khaybar has resulted in an extreme compositional range of rock types, from mugearite to benmoreite, trachyte, and comendite. The vent distribution at Harrat Khaybar shows a distinct zonation indicative of a high-level, compositionally stratified magma chamber.

Whole-rock chemistry suggests that even the most primitive basalts are not primary magmas, but rather fractionated melts derived from primary magmas of variable melt fractions generated at differing mantle depths. The more undersaturated Harrat Kura basalts were derived from primary magmas representing the smallest degrees of partial melting, at the greatest depth. Crystal fractionation of most primary magmas appears to have taken place in reservoirs at the crust-mantle boundary. Chemical and petrographic data suggest that open-system magma recharge was an important process in the high-level magma chamber beneath Harrat Khaybar. The chemical and mineralogical heterogeneity of the Harrat Khaybar comendites, their Sr-isotope values, and their Zr/Nb ratios all indicate that they were derived by varying degrees of crystal fractionation (>95%), crustal melting, and halogen complexing.

The north-trending volcanic axes of Harrats Rahat, Khaybar, and Ithnayn form a single, 600-km-long linear vent system, designated the Makkah-Madinah-Nafud (MMN)volcanic line. The three transitional-to-mildly alkaline harrats extruded from this volcanic line differ from most Arabian harrats, which like Harrat Kura to the west, are significantly more undersaturated. The relatively large degrees of partial melting evident in Harrats Rahat, Khaybar, and Ithnayn suggest that the MMN volcanic line may overlie a north-south thermal lineation in the mantle. The petrochemical stratigraphic sequence in these three harrats is nearly identical; early voluminous extrusions of OTB were followed by later, less extensive extrusions of minor OTB and abundant AOB, hawaiite, and differentiated lavas. As this petrochemical stratigraphy evolved, it progressed northward along the MMN trend with time, beginning on Harrat Rahat about 10 Ma, on Harrat Khaybar about 5 Ma, and on Harrat Ithnayn about 3 Ma. The interplay of open-system magmatic processes beneath this migrating volcanic line has resulted in a classic example of a strongly fractionated, mildly alkaline basalt series.

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