New Mexico Geological Society Annual Spring Meeting — Abstracts

Field relationships, mineralogical, and geochemical studies are used to explain the origin of a complex network of light-colored, igneous veins that intrude dark, fine-grained basanite at EI Porticito and the adjacent Tejana Mesa, located approximately 8 km NNW of Quemado, N. M. Subhorizontal veins, ranging in thickness up to 2 m, form anastomosing patterns across the faces of EI Porticito. In places, subvertical veins suggest 'tap roots' appearing to feed the higher level networks of subhorizontal veins. Veins similar to those in EI Porticito intrude the nearby lowermost Tejana Mesa lava. Thin (≤ 0.2 mm), subhorizontal, veinlets are common in host rock at both locations, and occasionally penetrate host rock/vein interfaces. Beneath the Tejana Mesa lavas, a few hundred meters SE of EI Porticito, scoria and bombs are exposed.

The veins are holocrystalline and coarse-grained. Titaniferous pyroxene, the dominant phenocryst in veins, displays two crystal habits. The first consists of euhedral, fractured, sometimes offset, grains that apparently grew to near their final size (up to about 2 cm long) prior to final vein emplacement, as indicated by the absence of further growth after fracturing. Many other pyroxene phenocrysts nucleated on vein walls. Vein textures suggest that entrainment of these elongate phenocrysts into flowing vein magma was common. The elongate habit of these pyroxene phenocrysts and the skeletal and dendritic habits of other phenocrystic phases, particularly magnetite and alkali feldspar, are consistent with growth during rapid cooling of veins. Vein groundmass consists of Ba-rich alkali feldspar, analcite, leucite, and minor phases. The mineralogy of the thin, subhorizontal veinlets is similar to vein groundmass. In contrast to the veins, the host rock exhibits a fine-grained igneous texture typical of an extrusive mafic rock. It consists of small, sparse to common phenocrysts of olivine, augite, and magnetite enclosed in a groundmass consisting primarily of euhedral clinopyroxene crystals. Acicular apatite is ubiquitous in both veins and host rock. Virtually all phenocrysts >1 mm long, except apatite, in both host rock and veins are fractured.

The lavas of EI Porticito and nearby Tejana Mesa have similar compositions. Major element compositions of bombs, host lavas, and veins form a coherent basanitic trend. Bombs contain about 40% SiO₂, and 2-3% total alkalis, host lavas 41-43% SiO₂ and 2-4.5% total alkalis, and veins 41-47% SiO₂ and 4.5-8.5% total alkalis. Various component pairs, notably TiO₂ vs. MgO, V vs. Ni, and K₂O vs. Ni, form a second trend within the veins. These trends in host and vein rocks are tentatively interpreted to be the result of fractional crystallization, first within the magma source, and then within the veins as they were emplaced.

Lithologic relations seen in outcrop indicate that at least some of the veins were intruded into fractures in the host rock. Possibly the host rock was passing through the 'critical crystallinity' cooling phase (about 50% crystals), and thus exhibited brittle deformation in response to stress. The rarity of volcanic exposures that exhibit the structure of EI Porticito is tentatively attributed to the low probability that significant strain occurs as the magma passes through the range of critical crystallinity. When this does occur, development of brittle fractures in the cooling magma mush allows segregaton, transport, and/or emplacement of more evolved magma to form veins. This evolved magma may be derived entirely in-situ, or it may be differentiated from a hidden magma chamber and intruded into the present location. In either case, the formation and emplacement of veins may be a consequence of the stress field that alI lowed the crystallizing mush to fracture.