40ar/39ar geochronology of Middle to Late Cenozoic ignimbrites, mafic lavas, and volcaniclastic rocks in the Quemado Region, New Mexico

WILLIAM C. MCINTOSH and RICHARD M. CHAMBERLIN

New Mexico Geological Society Guidebook 45, p165-185, 1994

Abstract - In order to better address stratigraphic problems identified during geologic mapping the Quemado, New Mexico area, ⁴⁰Ar/ ³⁹Ar dating methods were applied to 33 samples of volcanic and volcaniclastic rocks. Single-crystal laser-fusion techniques were used to determine eruptive ages of rhyolitic ignimbrites and of volcanic pumice and ash in sandstones. Mafic lavas were analyzed by step-heating of bulk samples. Sanidine phenocrysts from Quemado area ignimbrites range in age from 34.98 + 0.19 to 27.23 + 0.09 Ma. Age and K/Ca results allow these distal-facies outflow sheets to be confidently correlated with 11 well-known regional ignimbrites erupted from calderas in the Mogollon-Datil volcanic field. Ages for the Datil Well Tuff (34.98 + 0.19 Ma) and Rock House Canyon Tuff (34.17 + 0.08 Ma) are significantly younger than published values, probably reflecting xenocrystic contamination in previously analyzed bulk samples. Lithic-rich ignimbrites within the South Crosby Peak Formation contain phenocrystic plagioclase that indicates an eruptive age near 29.67 Ma. These units lack phenocrystic sanidine but contain sanidine xenocrysts derived from older rhyolitic ignimbrites. Sparse sanidine and abundant plagioclase from syneruptive volcaniclastic facies suggest depositional ages of 35 to 33 Ma for the volcaniclastic unit of Cañon del Leon and approximately 37 Ma for ash-rich beds in the Dog Springs Formation. Phenocrystic plagioclase from the Cerrito Viejo dacitic intrusion yielded low-resolution results indicating emplacement near 30.6 + 1.5 Ma. Step heating of groundmass concentrates from Oligocene and Pliocene mafic to intermediate lavas gave disturbed spectra and approximate isochron ages for the andesite of San Antone Canyon (29.2 Ma, new unit), Squirrel Springs Andesite (28.8 Ma), Bearwallow Mountain Andesite (26.1 Ma), and basalts of Blue Mesa (5.7-4.5 Ma). In combination with recent mapping and stratigraphic data, ⁴⁰Ar/ ³⁹Ar dating results provide a valuable chronostratigraphic framework for interpretation of stratigraphy, sedimentation and tectonic history of the Quemado area.