LA GRULLA PLATEAU, AN EARLY ISOLATED CENTER OF MAFIC TO INTERMEDIATE VOLCANISM, NORTHWESTERN JEMEZ MOUNTAINS, NEW MEXICO

La Grulla Plateau (LGP), forming the NW part of the Jemez Mountains, extends from the rim of the Valles caldera approximately 15 km to Encino Point. The 7-km-wide subalpine mesa is bounded on the east by Cañones Canyon and on the west by Encino escarpment. Rocks underlying LGP include lavas, domes and shallow intrusives of basaltic to dacitic composition formerly assigned to the Polvadera Group (Bailey et al., 1969), a rock assemblage that regionally comprises the northern part of the Jemez Mountains volcanic field (JMVF). Based on recent mapping and geochronology and geochemical data, we proposed new stratigraphic terminology (Kelley et al., 2007) that recognizes that LGP mafic to intermediate rocks are temporally and geochemically distinct from other volcanic rocks in the northern JMVF. Data suggest that a sequence of basaltic to dacitic eruptions resulted in the local accumulation and development of an isolated volcanic field between 8.7 and 7.2 Ma. Collectively, this sequence is informally termed the La Grulla Formation. The formation includes early (8.7 to 7.8 Ma) fine-grained basaltic-andesites, basalts and andesites erupted from a volcanic center at Encino Point where a series of stacked lavas is exposed in the escarpment wall. Remnants of these flows occur atop northerly outlying mesas and in Cañones Canyon. Early mafic lavas were succeeded by voluminous eruptions of coarsely porphyritic, 2-pyroxene andesitic lavas from widely distributed centers on LGP (Cerro Pelon, Cerro del Grant and Four Hills) between 7.7 to 7.4 Ma. Subsequently, a series of dacitic to rhyodacitic domes and flows of limited aerial extent erupted between 7.4 and 7.2 Ma along projected N-S aligned faults delineating the east plateau margin. Additionally, an endogenous dome formed of fine-grained dacite that intrudes and uplifts mafic lavas at Encino Point, has an age younger than 7.8 Ma (Singer, 1985).