Geochemical aspects of volcanic rocks of the Valley of México

Abstract

Three periods of volcanic activity connected with tectonic events form the geological history of the Valley of Mexico. An igneous rock suite from rhyodacites to andesites can be observed in each period, lacking real rhyolites and basalts. During the Tertiary -in the Oligo-Miocene and Upper Miocene-Pliocene periods- volcanism is more dacitic, and in the Quaternary epoch more andesitic. This result was verified by calculating the average of all available and stratigraphical datable chemical analyses. The average chemical composition of Oligo-Miocene, Upper Miocene-Pliocene and Quaternary products was computed in addition to the corresponding igneous rocks in the classification by the Rittmann norms in the Streckeisen double triangle with the following results:
Quaternary: Quartz-la ti te-andesite
Upper Miocene-Pliocene: Leuco-quartz-latite-andesite
Oligo-Miocene: Leuco-quartz-latite-andesite
The similar average composition of Tertiary volcanic rocks seems to support the theory of a uniform primary andesite magma apart from the two possible theories of petrogenesis. The computed average trace element abundances show high Cr and Ni- values which speaks for an involvement of mantle material, if one considers the Tertiary products as partial melting products of the lower crust. A more elegant hypothesis seems to be the model of Gunn and Mooser (1971), who consider these volcanic rocks as partial melting products of oceanic tholeiites or their high pressure derivatives.