Geologic Investigations Series I-2431
Online version 1.0
Lithologic Age-Group, Magnetopolarity, and Geochemical Maps of Springville Volcanic Field, East-Central Arizona
Christoper D. Condit, Larry S. Crumpler, and Jayne C. Aubele
The Springerville volcanic field is one of the many late Pliocene to Holocene, mostly basaltic, volcanic fields present near the Colorado Plateau margin (fig. 1, in pamphlet). The field overlies the lithospheric transition zone between the Colorado Plateau and the Basin and Range Province (Condit and others, 1989b). Establishing relations in time, space, and composition of the rocks of these plateau-margin fields offers the possibility to integrate more fully into a regional synthesis the detailed geochemistry of these fields now being examined (for example, Perry and others, 1987; Fitton and others, 1988; Menzies and others, 1991). The work also provides baseline information for understanding mantle properties and processes at different depths and locations. Because the Springerville field is the southernmost of the plateau-margin fields, and because it contains both tholeiitic and alkalic rocks (tables 1 and 2, in pamphlet), it is a particularly important location for establishing these patterns in time, space, and composition.
Our four thematic maps of the Springerville field were compiled by using digital mapping techniques so that associated petrologic and chemical data could be conveniently included in a geographic information system for one of the plateau-margin fields. Parts of these maps have been included in Condit (1995), a stand-alone Macintosh2 computer program that takes advantage of their digital format.
In contrast to other plateau-margin volcanic fields, including the San Francisco, Mormon Mountain, Mount Baldy (White Mountains), and Mount Taylor fields (fig. 1, in pamphlet; Moore and others, 1976; Lipman and Mehnert, 1979; Crumpler, 1980, 1982; Holm and others, 1989; Nealey, 1989), the Springerville field contains no coeval silicic centers or large composite volcanoes (Condit and others, 1989a); it consists dominantly of monogenetic cinder cones and their associated flows. The field within Arizona encompasses about 3,000 km2 and has a volume of about 300 km3; it contains approximately 400 cones (Condit and others, 1989b). An estimated 100 km2 of the field extends eastward into New Mexico.
The geology of the field's 2,166 km2 of volcanic outcrop in Arizona was mapped at 1:24,000 scale, compiled at 1:50,000 scale, and reduced to a scale of 1:100,000. The south-central part of the field (fig. 5, sheet 1) was not mapped because of access problems, and detailed mapping in the central part of the field extends as far north as about lat 34°27' N. Reconnaissance suggests that an additional 50 km2 to the north is also covered by flows, a large part of which are diktytaxitic; sampling by Cooper and others (1990) shows that the northern end of this area (Volcanic Mountain, fig. 5, sheet 1) is composed of tholeiitic lavas; a sample from Volcanic Mountain has an age of 5.31 Ma (Cooper and others, 1990).
The mapped units of the Springerville field range in age from 2.1 to 0.3 Ma, with the exception of six older flows around the periphery of the field. The oldest two of these six flows, found on the southwest edge of the field and dated at 8.66 and 8.97 Ma (table 3), have a source on Mount Baldy (fig. 1; Condit, 1984). Two northern units have ages of 7.6 and about 6.6 Ma (two aliquots have ages of 6.52±0.12 and 6.66±0.12 Ma); the last two of the older flows, on the southeast margin of the field, are dated at 2.94 Ma and 3.1 Ma. Sources for these last four units are unidentified.
The lithologic types and chemical classes of the map area, as defined in this report, are summarized in table 1A. The areal data were obtained directly from digital map images. The most common rock is olivine phyric basalt (lithologic types b, c, and d); these lithologies make up about 46 percent of the volcanic outcrop area. Olivine phyric lithologies most commonly belong either to chemical class alkali olivine basalt (AOB, table 1A), or to a transitional (TRANS) chemical class between AOB and tholeiite (THOL). The next most abundant lithology is diktytaxitic basalt (types f and g, which together cover about 32 percent of the volcanic area); most flows of this lithology are tholeiitic. The only other lithologic type to cover more than 3 percent of the outcrop area is aphyric basalt (type h), which makes up about 11 percent of the volcanic area; most of these flows are hawaiitic.
Eighty-seven percent of the outcrop area (1,887 km2) and 60 percent of the mapped units (267 of 449) are mildly alkalic to alkalic, largely basaltic rocks (table 1A); these rocks compose about 72 percent of the area described chemically. Of this alkalic group, about one-third (27 percent of the area described chemically) is hawaiite, mugearite, and benmoreite (evolved alkalic rocks, or EAR). Vents for most of these flows are cinder cones. The rest of the rocks (about 28 percent) are tholeiitic and emanate from 16 vents; where exposed, these vents appear to have been fissures initially but are now elongate spatter mounds.
The geochemical evolution of the field, as expressed in percent of outcrop area with chemistry, is summarized in table 1B, where rocks are subdivided first by chemical class and then by age group. Geochemical modeling by Condit (1984) and additional work involving isotopic considerations by Cooper (1986, 1991) and Cooper and others (written commun., 1992) suggest that the transitional (TRANS) rocks are chemically similar to the alkalic rocks; many apparently owe their transitional chemistry (low alkali content) to their olivine-rich character (for example, picrites of lithologic type b). For this reason, in table 1B these rocks have been included in the alkalic basalt (ALK) chemical group, which also includes basanite and alkali olivine basalt. A third geochemical group shown in this table consists of evolved alkalic rocks (EAR) listed above. Early volcanism (age group 1) is represented by six units (1.3 percent of the volcanic area) dominated by tholeiitic chemistry; if the flows of Volcanic Mountain were included, this bias would be even greater (about 4 percent). Tholeiitic basalts of this group differ from younger tholeiites of the field in that they plot in a distinct group having lower alkali content with respect to silica (Cooper, 1991; Cooper and others, written commun., 1992). Rocks of age groups 2 and 3 (about 20 percent of the volcanic field) are dominantly tholeiitic; only about 5 percent of the field is alkalic basalt of these age groups. Volcanism was greatest during the time of age group 4 formation (47 percent of the field emplaced); during this interval tholeiitic eruptions declined, alkalic basalt volcanism peaked (about 30 percent), and evolved alkalic rock eruptions started to increase in volume. The youngest period of volcanism, that of age group 5, produced about 19 percent of the field. This period marks the peak of evolved alkalic rock eruptions; alkalic basalt eruptions declined to levels about half that of evolved alkalic rock production, and tholeiitic volcanism is represented by a single small flow.
A vent-distribution map and a quantitative analysis of the geographic distribution of the 409 vents of the field (including those in the unmapped south-central part) have been made by Connor and others (1992). Their analyses show regional cinder cone alignments in an arcuate pattern subparallel to the Mogollon Rim (the Colorado Plateau/Transition Zone boundary in the map area, fig. 1). The additional coincidence of these alignments with aeromagnetic lineaments suggests that the vent alignments are a reflection of the structural margin of the Colorado Plateau. While the structures implied by these vent alignments are regional in extent, they appear to differ significantly from those in other plateau-margin fields in that they cannot clearly be related to major reactivated Precambrian structures, which are lacking around the Springerville field.
Note to users: Only the pamphlet and sheets 2, 3, and 4 are available for download here. Sheets are not fully digital and, therefore, are not available here.
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Sheet 1 contains the lithologic and geochemical maps, and sheet 5 contains the magnetopolarity and age maps, but these sheets are not available on the web at this time.
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Last modified: 8-3-00 (cad)