New Mexico Geological Society Annual Spring Meeting — Abstracts

We initiated a petrogenetic study of the 25-26 Ma Cieneguilla basanites at Cerro Seguro and another unnamed vent in the southern Española Basin, Santa Fe County, to improve our understanding of early Rio Grande Rift magmatism. These small-volume poorly bedded monogenetic centers erupted through Eocene sedimentary rocks, Oligocene intrusive stocks, and Oligocene volcanic rocks, some of which are related to a north trending chain of volcanic centers of monzonitic-latitic activity that includes the Cerrillos Hills and Ortiz Mountains. The Cieneguilla basanites are olivine porphyritic, magnetic, slightly vesicular, and relatively unweathered. These samples contain mildly to moderately altered olivine phenocrysts in a fine-grained intergranular olivineorthopyroxene-magnetite-plagioclase matrix. Rock magnetic experiments conclude that two distinct Fe-Ti oxide phases are responsible for the rocks’ magnetic character. The Cieneguilla basanites are mildly alkaline and fall within the sodic series of the alkali olivine basalt scheme. They have low silica values (42.4-44.1 weight percent) and high MgO values (11.5-13.5 weight percent) and are some of the most primitive eruptive products in the central Rio Grande Rift. They are sufficiently mafic that their incompatible trace element compositions are controlled mainly by their mantle source region(s). The basanites are enriched in the light rare earth elements relative to the heavy rare earth elements (La/Yb)N=18-26. Chondrite-normalized trace element patterns show a strong enrichment in most incompatible elements, but marked depletions in K and Rb relative to light rare earth elements, Nb, Ta, and Th. These are characteristics of strongly silica-undersaturated ocean island basalts (OIB). The basanites have (La/Nb)N values 1.2-1.5, which are also characteristic of OIB. The Cieneguilla basanites have high Ta/Ba and Nb/Ba ratios which are interpreted as originating from either an OIB-like asthenospheric mantle or an OIB-modified lithospheric mantle.