The evolution of the Eagle Peak volcano — a distinctive phase of middle miocene volcanism in the western Mogollon-Datil volcanic field, New Mexico

Abstract

The andesitic to dacitic Eagle Peak volcano represents a distinctive phase of Middle Miocene, post-caldera volcanism in the western part of the Mogollon-Datil volcanic field in southwestern New Mexico. Erupted during Basin and Range extensional tectonism, rocks of the Eagle Peak volcano are chemically and isotopically distinct from the bimodal suite, extension-related basalt and rhyolite that also erupted in this area from the Early Miocene to the Pleistocene. Instead, they have close petrogenetic affinities to the early post-caldera (~ 27–23 Ma) calc-alkaline, Bearwallow Mountain Andesite erupted from shield volcanos aligned along prominent Basin and Range fault structures. Geologic mapping and detailed petrographic and chemical studies of the Eagle Peak volcano has enabled the distinction of five different flow units, a central plug and a feeder dike. The flows were erupted from a central vent and two subsidiary “satellitic” centers on the western and southwestern flanks of the volcano. 40Ar 39Ar age-spectrum and paleomagnetic studies indicate that the Eagle Peak volcano was active between 12.1 and 11.4 Ma; its activity spanned at least one magnetic polarity reversal. With exception of late satellitic eruptions on the northwestern margin of the volcano, central vent and satellitic flows were erupted in rapid succession and have an average age of 11.7 Ma. The central plug yielded a plateau age of 11.4 Ma, which is a minimum of 90,000 years (2σ) younger than the 11.7 Ma average age of the central vent and satellitic flows. Major-oxide, trace-element and isotope geochemistry define two distinct magmatic series: a central vent and a satellitic series. Rocks of the satellitic series, although similar in modal mineralogy and rare earth element patterns, are slightly more alkaline and relatively enriched in the high field strength elements Nb, Ta, P and Ti compared to the central vent eruptives. Sr and Nd isotopes further demonstrate these differences; a sample of the satellitic flows exhibits lower ( 87Sr 86Sr ) i (0.7084) and higher ϵ Nd values ( ϵ Nd = −4.8) relative to an upper flow of the central vent series [ ( 87Sr 86Sr ) i = 0.7096), ϵ Nd = −8.5 ]. Major- and trace-element data support petrogenetic models based on periodic tapping of the central vent and satellitic series magmas, which both evolved by crystal fractionation. Central vent magmas evolved mainly by a modified process of filter pressing that accompanied the transfer of magma from a deep into a higher-level reservoir. Thus, a portion of the melt with some of the original crystals was extracted during this transer changing the resulting bulk magma chemistry but not affecting the major phenocryst compositions. In contrast, crystal fractionation within the satellitic magmas accompanied a progressive evolution in both phenocryst composition and bulk magma chemistry. Although temporally associated with bimodal basalt-rhyolite volcanism (< 21 Ma to Holocene), Eagle Peak rocks differ markedly from the suite of mafic bimodal rocks in trace-element and Sr and Nd isotope chemistry. Mafic rocks of the bimodal suite have Sr and Nd isotopic initial ratios that range from depleted mantle to bulk earth values [ ( 87Sr 86Sr ) i = 0.7030-0.7057, ϵ Nd = 0 to +9.12 ], and are substantially more primitive than Eagle Peak rocks [ ( 87Sr 86Sr ) i = 0.70839-0.70958, ϵ Nd = − 8.4 to − 4.9 ]. In contrast, Eagle Peak volcanics are geochemically more similar to the 27-23 Ma post-caldera Bearwallow Mountain Andesite, which is characterized by ( 87Sr 86Sr ) i = 0.7070–0.7102 and ϵ Nd = −8.15 to −5.95. The Eagle Peak volcanics, like the Bearwallow Mountain Andesite, assimilated a significant component of crustal material and were both likely derived from a similar lithospheric mantle source beneath the western Mogollon-Datil volcanic field.