New Mexico Geological Society Annual Spring Meeting — Abstracts

Basaltic andesite, andesite, dacite, and rhyodacite of the Pliocene Polva-dera Group on La Grulla Plateau display smooth continuous major element trends. The northeast margin of La Grulla Plateau is the locus for structurally controlled basaltic andesite volcanism; a 100m section of ol-bearing basaltic andesite flows of the Lobato Fm. increase from 53% to 60% SiO₂ from base to top with sym-pathetic Fe enrichment of ol, opx, and cpx phenocrysts. Uppermost flows are much reduced in SiO₂ (53-54%). The main vent is characterized by 100+ m of pyroclastic ejecta intruded by an endogenous dome of more highly evolved composition (64% SiO₂) which may represent a link to younger dacitic magmatism.

Dacite and rhyodacite domes (64% to 70% SiO₂) of the younger Tschicoma Fm. are also structurally controlled by high-angle, rift-marginal faults and are younger than the Tschicoma andesite (61% to 64% SiO₂) on which they sit. These dacites and rhyodacites are enriched in incompatible trace elements and radiogenic Sr relative to other similar rocks previously described from the Polvadera Group.

Although major element trends in the basaltic andesite, and possibly more evolved rocks, could easily be accounted for by fractionation of known phenocryst assemblages, trace element data, in addition to heterogeneous ⁸⁷Sr/⁸⁶Sr measurments ranging from .7048 to .7067, suggest a more complex history involving mixing of parental basaltic magma(s) (e.g. Lobato basalt) with mid-to-upper crustal rocks in varying proportions. Combined assimilation-fractional crystallization could produce the trace element and Sr isotopic trends in all rock types. This
interpretation is supported by mineralogical evidence for contamination (such as resorbed quartz in basalts and basaltic andesites) and chemical evidence in favor of fractionation (as above).