New Mexico Geological Society Annual Spring Meeting — Abstracts

Paleomagnetic and whole-rock geochemical data suggest that the El Calderon (EC) cinder cone in the Zuni-Bandera Volcanic Field (ZBVF) is polygenetic. The main flow, Laguna B, is tholeiitic. Laguna B contains 18-22% modal olivine, 16-41% modal clinopyroxene and 21-35% modal plagioclase. A representative sample from the flow contains 0.80% K₂O, 1.55% TiO₂, 0.25% P₂O₅ 200 ppm Ba, 343 ppm Sr, 185 ppm V, 324 ppm Cr, 138 ppm Zr and 20 ppm Nb. Laguna B yields a highly distinctive, west and shallow negative paleomagnetic direction (declination =273°, inclination = -19°; group mean for six sites, α₉₅=4.5°, K=221). Thus, Laguna B erupted during a high-amplitude excursion or aborted reversal of the Earth's magnetic field. This unusual direction was noted in one EC site by Champion et ale (1988, JGR, 93, 11,667-11,680) and interpreted as a transitional field. An ⁴⁰Ar/³⁹Ar date of 79+/-23 ka for Laguna B suggests the flow is latest Quaternary; a correlation with the Blake event, (about 100 to 114 ka; Herrero-Bervera and Helsley, 1993, SEPM Spec. Pub. 49, p. 71-87) is permissible.

Paleomagnetic and geochemical data distinguish Laguna B from an underlying tholeiitic flow, Laguna A, previously mapped as part of the El Calderon flow (Maxwell, 1986, USGS map I-595). A representative sample from Laguna A contains 0.52% K₂O, 1.16% TiO₂, 0.15% P₂O₅ 107 ppm Ba, 243 ppm Sr, 176 ppm V, 294 ppm Cr, 84 ppm Zr and 13 ppm Nb. Laguna A yields a north, moderate positive paleomagnetic direction (declination = 359°, inclination =59°; group mean for three sites, α₉₅=10.1°, K=149.8), clearly distinguishing it from the overlying Laguna B flow.

Paleomagnetic data are permissive of, but not conclusive for, a correlation of the Laguna A flow with tholeiites at Laguna Pueblo, 45 km east of the nearest exposed Laguna A flows. Laguna Pueblo flows yield a north, moderate positive paleomagnetic direction (declination = 353°, inclination =57°; group mean for six sites, α₉₅=3.9°, K=289.8) that is statistically indistinguishable from the Laguna A direction. Laguna Pueblo flows contain 6-14% modal olivine, 11-17% clinopyroxene, and 62-67% modal plagioclase. Major, minor and trace element data from Laguna Pueblo flows are indistinguishable from Laguna A data.

Late stage scoria eruptions at the EC cinder cone were alkalic. A representative sample from EC scoria contains 1.10% K₂O, 1.90% TiO₂, 0.37% P₂O₅ 256 ppm Ba, 483 ppm Sr, 209 ppm V, 368 ppm Cr, 178 ppm Zr and 26 ppm Nb. Thus, EC cinder cone is polygenetic. It has certainly undergone two stages of eruption (Laguna B and late stage alkalics) which may have been preceded by an earlier eruptive stage (Laguna A). Chemical variation from the Laguna B flows to the late stage alkalic scoria represents a reasonable basaltic fractionation trend. The source of Laguna A is uncertain, but minor oxide and trace element chemical differences between Laguna A and B may also represent a reasonable fractionation trend between erpptions from the EC vent.