THE JEMEZ MOUNTAINS VOLCANIC FIELD (JMVF): A PETROLOGIC HISTORY

The Jemez Mountains consists of a sprawling, long-lived, episodically constructed volcanic field located at the intersection of the Rio Grande rift and the Jemez lineament, New Mexico. Decades of study have produced a wealth of data, allowing the development of a large continental volcanic system and its climactic caldera-forming phase to be reconstructed in detail. The JMVF owes its existence to the coincidence of a belt of fertile mantle marked by the Jemez lineament with the extensional Rio Grande rift. The volcanic field was largely built from 10.5 Ma onwards, during active eruptive episodes peaking at 9.5, 9.0, 7.8 and 6.8 Ma, plus extrusion of large dacite domes between 5 and 2 Ma. An increase in the frequency of eruptions in flanking basalt fields after 3.0 Ma may mark the thermal episode leading to eruption of the climactic caldera-forming Bandelier Tuff (two major events at 1.60 and 1.26 Ma; combined volume ~800 km³ DRE). The overall erupted magma flux was low at ~0.2 km³ per ka, rising to 0.5 – 0.6 km³ per ka during periods of peak productivity. The two broad periods of maximum activity, 10 – 6 Ma and post-3 Ma, coincide with episodes of extension on rift-related faults. The first period was dominated by eruption of andesites and dacites, the products of interaction between mantle-derived melts and complex regional crust at a variety of crustal depths. The second period is more bimodal, consisting of caldera-related rhyolites (Bandelier Tuff and associated units) and flanking mafic lavas ranging from hawaiites to tholeiites with subordinate intermediates. Isotopic data indicate that intrusions related to pre-caldera volcanics were major contributors to the Bandelier high-silica rhyolite magmas, with an additional direct contribution from Proterozoic country rock. Zircon ages indicate that catastrophic eruptions of silicic magma and caldera development in the JMVF critically depended on the growth rate of eruptible magma bodies. Both tuffs are systematically chemically zoned yet are overall strongly depleted in Sr, Ba and Eu, implying a large volume of complementary feldspathic cumulates left in the crust. Zoning is attributed to melting of these cumulates as a consequence of repeated recharge events culminating in eruption; the triggering recharge magma is preserved in the later tuff, but is obscure in the earlier.