New Mexico Geological Society Annual Spring Meeting — Abstracts

Preliminary results show Los Lunas volcano is a polygenetic center composed primarily of intermediate lavas with subordinate pyroclastic lithologies. Los Lunas is one of six principal Pliocene Pliestocene volcanic centers erupted within the Albuquerque basin -an extensionally fonned depression within the Rio Grande rift system. Los Lunas volcano is currently being mapped under the U.S. Geological Survey's National Cooperative Geologic Mapping Program. Approximately 70 samples have been collected for geochemistry and ⁴⁰Ar/³⁹Ar geochronology.

Based on volcanic stratigraphy, landscape morphology and two previous ⁴⁰Ar/³⁹Ar dates, Los Lunas volcano is considered to have at least 2 main periods of eruptive activity. The older center, a lowlying jumbled mass SW of the 5955-ft central summit, consists of 4 volcanic units. The youngest of these units is an extensive flow complex of dacitic lavas dated at 3.88±0.01 Ma (Love and others, 1994). These flows are massive and generally faulted into large blocks. Many of the flows have a lower medium dark gray base that is heavily fractured into thin (-5 cm) lensoids, possibly a texture resulting from cooling and/or shear stress. In other locations, the lava bases exhibit thick (up to 4 meters) flow breccias. Underlying this flow to the south is a pyroclastic breccia and lapillistone and an underlying, co eruptive flow. Both flows are tilted and faulted along with underlying sediments and are, in places, overlain by eolian and fluvial sand and gravel lithofacies. A black dacitic ash, up to 2 m thick, which is overridden and defonned by the extensive upper flow, is exposed within a prominent NW-SE canyon separating the old and young centers. Several dacitic intrusives occur as isolated hills to the south and southeast of the volcano and are believed to be age-correlative with the older center.

The younger volcano of the Los Lunas complex fonns the central edifice with two prominent lava lobes that extend to the E and SE from the summit. The volcano consists of at least 5 eruptive units (4 lavas and pyroclastic deposit) and has been faulted and uplifted 150 m above the surrounding topographic top of basin fill. All of the units are trachyandesite in composition and one lava has been dated at 1.22±0.0 1 Ma (Love and others, 1994). The oldest unit is a massive chocolate-brown flow that fonns a laterally continuous (but faulted) lava and eruptive edifice along the top of the pre-eruptive surface on the west, north, and northeast sides of the young volcano. The flow is generally less than 8 m thick but is ponded to the north (18 m) and on the SW edge (20 m) of the volcano. Crustal xenocrysts are common within the chocolate-brown flow. It is overlain by a light grayish-red flow on the east side. The next youngest lava fonns a small isolated outcrop to the west of the main summit. A 2-5 m thick red scoriaceous pyroclastic fall, consisting of individual bombs and lapilli agglutinate, mantles both chocolate brown and high western lava. All three units are cut by a prominent N-S-trending fault exposed north of the summit and which displaces the units as much as 30 m down to the east. The youngest unit is a light gray to brown flow(s) originating from a central vent near the summit. This unit forms extensive flow lobes to the E and SE of the summit.

Preliminary results from major and trace elements (XRF analysis) reveal some important differences between the two volcanoes. Although the dacitic compositions of the older volcano appear more evolved (12-18% Q-nonn) relative to the younger trachyandesite volcano (3-8% Q-norm), the range in Mg#'s (100*Mg/Mg+Fe) are roughly equivalent (44-51). In addition, highly incompatible trace elements such as Ba, Th and Nb have significantly lower concentrations in the dacites relative to trachyandesites -all of which suggest a fundamental petrogenetic difference between the two volcanoes.