Abstract
Quaternary volcanism in southeastern California has produced scattered cinder cones and lavas of trachybasalt. Typically the lavas contain magnesian olivine and titanaugite which becomes progressively enriched in Ca, A1 and Ti towards the crystal margins. Oscillatory and hourglass zoning with abrupt changes in Ti, A1 and Si (inversely) have been observed in some titanaugites. Plagioclase may zone from labradorite to anorthoclase, and discrete sanidine is present in the coarsest grained lavas; the iron-rich residual glass is enriched in normative nepheline which is absent modally. Titanomagnetite (micropheno-cryst and groundmass phases) is notably rich in MgO and Al2O3 in contrast to equivalent tholeiitic assemblages. Analyses show that the lavas are not simply related to one another by fractionation of their phenocryst phases; the controlling factor remains obscure as typically the most silica-poor lavas are the most alkali rich.
