Rare earth element evidence concerning the origin of voluminous mid-Tertiary rhyolitic ignimbrites and related volcanic rocks, Sierra Madre Occidental, Chihuahua, Mexico

Abstract

The mid-Tertiary volcanic sequence of the central Sierra Madre Occidental in Chihuahua, Mexico, is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites. Basaltic andesite to dacitic lavas are interbedded with the rhyolites, but they are of minor volumetric importance. Rare earth element (REE) data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model. The REE patterns indicate that if the rhyolites were formed by direct crustal anatexis, the residue from partial melting could contain no more than a few percent garnet or about 20% hornblende. This eliminates residues with the mineralogy of amphibolite, eclogite, or garnet granulite, but melting of a garnet-free granulite source is permitted. The crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid-Tertiary geothermal gradient and the composition of the crust beneath the Sierra Madre Occidental. In contrast, the fractional crystallization model can be tested rigorously. Rayleigh fractionation calculations are used to closely model REE patterns in the basaltic andesite to rhyolite series. The minerals involved are those occurring as phenocryst phases, and the mineral proportions were generated by leastsquares major element calculations. The results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase-dominated crystal fractionation.