NEW 40AR/39AR AGES FROM THE JEMEZ VOLCANIC FIELD, NM: PERIODIC VOLCANISM CULMINATING IN TWO MAJOR SILICIC PHASES

⁴⁰Ar/³⁹Ar dating of 42 samples, representing key volcanic units that span the entire pre-caldera eruptive history of the Jemez volcanic field, NM, provide new insight into how the long-lived continental volcanic center evolved, and ultimately produced the caldera-forming eruptions of the Bandelier tuff. The new ⁴⁰Ar/³⁹Ar ages imply that eruptions throughout the volcanic field's history occurred as a series of discrete pulses rather than in a cyclic or continuous manner. Furthermore, ⁴⁰Ar/³⁹Ar analysis has revealed that excess argon is ubiquitous in nearly all samples dated, implying that numerous K/Ar dates from the Jemez volcanic field (JVF) may have over-estimated the timing of eruptive events and, ultimately, the age of the volcanic field. Eruptive activity in the JVF may have began ~11Ma, (~ 2-3 Ma later than previously documented) and consisted of Keres and Polvadera Group basalt. The first major eruptive phase ocurred ~9 Ma, when large volumes of Keres and Polvadera Group basalt through rhyolite (dominantly basalt and andesite) were erupted. After a ~1 Ma period of quiescence, the JVF's first major phase of rhyolitic volcanism occurred ~6-7 Ma, when the Bearhead Rhyolite was erupted. After a lull in volcanic activity from ~5-6 Ma, minor volumes of Keres and Polvadera Group dacite and basalt, respectively, were erupted along the western and northen periphery of the JVF ~4-5 Ma. The third major pulse of volcanic activity ocurred ~2.8 Ma and is characterized by basaltic eruptions of El Alto, Santa Ana Mesa, and Cerros del Rio, accompanied by minor volumes of Polvadera Group dacite and rhyolite. After a ~300 k.y. period of quiescence, the caldera forming eruptions of the Bandelier tuff began ~2 Ma. This refined chronology implies that the JVF may have evolved in two phases. The first is characerized by an initial interval of volcanism that included significant volumes of all magma compositions and culminated in a predominantly silicic phase represented by the Bearhead Rhyolite. The second phase began after a ~1 Ma lull in eruptive activity and is characterized by predominantly basaltic volcanism followed by the culminating silicic phase of the Bandelier tuff.