Zeolitic basanite from southeastern California

In a belt of Quaternary volcanism within the Mojave Desert, California, lavas of alkali-olivine basalt have been extruded from many centers. Also three small volcanoes erupted alkaline lavas similar to basanite, except that the zeolites phillipsite and chabazite crystallized in the groundmass instead of the usual feldspathoid. These zeolite-bearing lavas, more alkaline than the olivine basalts, brought many peridotitic nodules to the surface. Two of the volcanoes built cones 500 to 600 feet high and extruded several short lava flows. The third volcano is smaller and mostly covered by younger alluvium. All three cones are composed of cinders, bombs and blocks, and lava spatter. The lava spatter strengthened the cones, making it possible for one or two flows to pour over the rims. Short flows also issued from fissures cutting the flanks of the cones. The two larger volcanoes each concluded with a flow that began issuing from a fissure that enlarged until one side of the cone was rafted away. Even these final flows are small. The basanitic lavas have an average composition ofSiO ₂>=44.5;TiO ₂=3.4;Al ₂ O ₃=14.5;Fe ₂ O ₃=5.0;FeO=7.3;MnO=0.2;MgO=7.4;CaO=9.0;Na ₂ O=4.1;K ₂ O=2.3;P ₂ O ₃=1.1;CO ₂=0.1; andH ₂ O=1.0. The lavas are very fine-grained and hypocrystalline with microphenocrysts of olivine (Fo ₈₅). The groundmass consists of olivine (Fo ₇₀), plagioclase (An ₄:), titanaugite, titanmagnetite, ilmenite, glass (less than 10%), and the zeolites-phillipsite and chabazite. The zeolites occur in small irregular patches only in the nearly holocrystalline centers of the flows. The basanites contain abundant peridotitic nodules, xenoliths of partially fused granite, and xenocrysts (possibly cognate) of titanaugite and kaersutite. The minerals in the inclusions, except for olivine and clinopyroxene, have all reacted with the alkaline melt. In several flows the nodule enstatite has been partially replaced by titaniferous clinopyroxene; this reaction drew so much lime and silica from the surrounding melt that zeolite formed a halo around the reaction rim. Three chemical analyses show about 15% normative nepheline, even though there is no modal nepheline. The feldspathoid is represented instead by the undersaturated zeolites phillipsite and chabazite. A high water content in the magma probably accounts for the occurrence of zeolite. The peridotite nodules do not occur in the nearby, less alkaline lavas, and they appear to be cognate. The separation of olivine and enstatite under high pressures can produce the basanite liquid from the same magma that yields the alkali-olivine basalt liquids by olivine crystallization under low pressures.