SPATIAL AND TEMPORAL TRENDS IN JEMEZ MOUNTAINS VOLCANISM

New ⁴⁰Ar/³⁹Ar dates combined with recent 1:24:000 geologic mapping reveal previously unrecognized shifts in the loci of pre-caldera volcanic activity across the northern Jemez Mountains volcanic field (JMVF). Early activity in the field includes a pulse of basaltic to dacitic activity 10.8-9.2 Ma on Lobato Mesa in the northeastern JMVF and 12-9 Ma mafic to silicic activity in the southwestern JMVF. While 9-7 Ma activity persisted in the southern JMVF, a new 8.7-7.2 Ma eruptive center developed on the La Grulla Plateau in the northwestern JMVF. Older 8.7-7.8 Ma mafic lavas on Encino Point and younger 7.7-7.2 Ma trachyandesite and dacite on the La Grulla Plateau are assigned to a new stratigraphic unit called the La Grulla Formation. New major and trace element data from a 600 m thick stack of intermediate-composition lava flows on Cerro de la Garita reveal a subtle shift in chemistry through time across the Paliza Canyon Formation to La Grulla Formation transition. La Grulla Formation lavas have higher total alkalis and lower Sr, Y, Zr and Nb contents than underlying Paliza Canyon lavas, with the two uppermost Paliza Canyon Formation samples exhibiting transitional chemistry. New age constraints from a rhyolite intrusion in the southern JMVF and pumiceous deposits in the northern JMVF suggest an episode of localized 7.8-7.6 Ma rhyolitic volcanism in the central part of the JMVF. Younger Bearhead Rhyolite intrusions (6.5-7.1 Ma) are more widespread than previously documented, extending from the southeastern and southern JMVF into the northeastern JMVF. Tschicoma Formation dacite and rhyodacite erupted at 5 Ma in the Sierra de los Valles, then erupted throughout the northeastern JMVF from 5-2 Ma. Our more refined geochronology of the JMVF indicates that pre-caldera volcanic centers were characterized by geographically distinct, relatively short-lived episodes of volcanism. Volcanism generally migrated eastward through time in the southern JMVF, but the pattern in the northern JMVF had a more complex east (10-9 Ma) to west (9-7 Ma) to east pattern (5-2 Ma) that may reflect the timing of activity on faults.