Abstract

Harrat Kishb is a 5,892 km2 lava field in western Saudi Arabia with an overall K-Ar age range from 2 Ma to prehistoric (at least 4,500 to 2,000 yr B.P.). It contains three stratigraphic units: the Diakah, Nafrat, and Hil basalts. Harrat Kishb differs from the coeval, mildly alkaline harrats along the 600-km-long Makkah-Madinah-Nafud volcanic line to the west in that its lavas are nodule-bearing and considerably more silica-undersaturated. The nodules are most abundant in basanite from the central vent zone and include both Type I and Type II mantle xenoliths.

Harrat Kishb is fundamentally a bimodal lava field that is dominated, at one end, by alkali olivine basalt (AOB) and basanite, with subordinate hawaiite and olivine transitional basalt (OTB), and at the other end, by phonolite. The few intermediate phonotephrites are compositionally and texturally heterogeneous, and regarded as hybrid lavas of basalt and phonolite.

Although none of the basaltic lavas is a primary melt, their chemistry was controlled largely by partial melting. From bottom to top, the stratigraphic units become less voluminous and increasingly more undersaturated, reflecting decreasing degrees of partial melting with time. Fractional crystallization of basanites (the smallest-degree partial melts) probably occurred by the plating of pyroxene and spinet (± olivine) along the walls of narrow conduits during their ascent through the subcontinental mantle lithosphere (flow crystallization). Fractional crystallization of OTB and AOB (the larger-degree partial melts) may have occurred in reservoirs at the crust-mantle boundary, a density filter for rising magmas. The relatively higher volatile content of the basanites, a consequence of smaller degrees of partial melting, may have allowed many of them to accelerate through the crust-mantle density barrier carrying their load of mantle nodules rapidly to the surface.

Major-element mass-balance calculations, and trace-element enrichment factors, demonstrate that the phonolites were probably derived from basanitic magmas via about 62% fractional crystallization of a clinopyroxene-dominated, feldspar-bearing mineral assemblage. There is a lack of evidence for fractional crystallization in static, high-level magma chambers, and a preferred model for the basanite-to-phonolite link involves the variable removal of feldspar by continued "flow crystallization" during the ascent of basanitic magmas through the crust. Such a model requires that these parental basanites had slower flow rates than did the nodule-entrained basanites extruded at the surface.

Field evidence for the ascent of basaltic and phonoltic magmas in shared conduits is consistent with periodic magma recharge during "flow crystallization." Although there is little evidence for the mixing of these diverse magma types at depth, they were mixed at the vent by clastogenic processes to produce hybrid lavas of phonotephrite composition.

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