Page 21 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

Basalt, Dacite, and Rhyolite of McClede Spring
Younger Intrusive Rocks
A series of discontinuous, basaltic and dacitic flows and rhyolite ash-flow tuff are exposed at McClede Spring southeast of McClede Mountain (fig. 6). The deposits rest directly on the andesite of Sibley Mountain. North of Tierra Blanca Creek basaltic andesite (Tsa) is separated from the underlying andesite of Sibley Mountain (Tta) by a lens-shaped dacitic ash-flow tuff (Tsd). Overlying the mafic flow north of Tierra Blanca Creek is a light-brown, crystal-poor ash-flow tuff (Tvp) that contains abundant, weakly welded, devitrified ash shards and minor amounts of pumice. The crystal fragments are generally less than 2 percent of the rocks and include sanidine and bipyramidal quartz. The rhyolite is lithologically and chemically similar to the crystal-poor, high-silica Vicks Peak Tuff erupted from the Nogal Canyon cauldron in the southern San Mateo Mountains to the north (fig. 2). W.C. McIntosh (written commun., 1999) reported a new mean 40Ar/39Ar spectra age of 28.4 ± 0.11 Ma for Vicks Peak Tuff at McClede Mountain. Chemical analyses of these volcanic units are shown in table 1, samples 6–9; alkali- silica variation plots of these rocks are shown in figure 3A.
South of Tierra Blanca Creek the mafic flow (Tsa) is underlain by a thin interval of tuffaceous sandstone and con­ glomerate correlated with the basal Thurman Formation (Tt) of Oligocene age.
Trachyandesite also occurs as two small intrusive bodies in the east-central part of the map area. A basaltic trachyandesite plug (Ta2) intrudes the Rubio Peak Formation about 0.8 km east of State Highway 27 where it crosses Jaralosa Creek. A second trachyandesite body (Ta1) intrudes Sugarlump and younger vol­ canic flows as well as alluvial-fluvial deposits included with the Santa Fe Group rocks along Jaralosa Creek at the eastern mar- gin of the map area.
The age of these intrusive rocks or even that they are the same or similar age is uncertain. Because the composition of these igneous bodies (table 1, samples 10 and 11, and fig. 3A) is similar to Eocene and Oligocene Mogollon-Datil volcanic rocks, we would not be inclined to include them with upper Ter­ tiary alkali basalt flows and sills produced during younger Rio Grande rift igneous activity. However, the easternmost igneous body intruded alluvial-fluvial deposits that were mapped as Santa Fe Group rocks by us (pl. 1) and Seager (1982), and thus the intrusive could not be older than Miocene. We submit that misidentification of older, lithologically similar Thurman Formation at this locality as Santa Fe Group rocks is a distinct possibility; the Oligocene Thurman Formation is present above the andesite of Sibley Mountain in the extreme northeast corner of the map area where it clearly underlies, with angular
McClede Mountain
Tmr
Tmt
Tk
Qpy
Tvp Tta Tsa
Tk
Tsd
Northward-thinning flows of upper Oligocene andesite of Sibley Mountain (Tta) and Vicks Peak Tuff, a rhyolite ash-flow tuff (Tvp) along east flank of McClede Mountain. Sibley Mountain flows rest on Kneeling Nun Tuff (Tk) in middle ground and on Mimbres Peak Formation (Tmt and Tmr) in left background. View looking north across Tierra Blanca Creek at gravel-capped (Qpy) landscape underlain by Vicks Peak Tuff east of McClede Mountain. Tsa, basaltic andesite of McClede Spring; Tsd, dacite flows of McClede Spring.
Figure 6.
12 Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico