Page 20 - Geologic Investigations in the Lake Valley Area, Sierra County, New Mexico

with rhyolite flows making up most of the domes. Rhyolite breccia, dikes, and small, unmapped plugs were intruded along faults that bound the Town Mountain dome, near Lake Valley townsite. Mimbres Peak rhyolite and tuff collected from McClede Mountain has been dated at 34.32 ± 0.11 Ma and 34.22 ± 0.16 Ma by the 40Ar/39Ar method (W.C. McIntosh, written commun., 1999).
Brownish-pale-red to light-pinkish-gray pumice-and-lapilli tuff and breccia crop out on the slopes of hills cored by rhyolite domes. The tuff is bedded, vitric to altered, and crystal poor. Most beds are air-fall tuff, but unwelded ash flows, debris flows, and ground surge deposits are also present. On the north side of McClede Mountain, as much as 20 m of welded ash-flow tuff may indicate proximity to a vent. Typically, tuff of the Mim­ bres Peak Formation contains 5–10 percent broken crystals— mostly sanidine, oligoclase (An28–30), and quartz—with acces­ sory clinopyroxene, hornblende, biotite, and iron oxide miner­ als. Lithic fragments are sparse in air-fall tuff but abundant in individual breccia beds; north of McClede Mountain, blocks of Kneeling Nun Tuff as large as 1 m occur in tuff of the Mimbres Peak Formation. Tuff overlies and fills depressions in the pre- Mimbres Peak paleosurface. Thickness of the tuff varies from 6–12 m at Town Mountain to as much as 90 m at McClede Mountain.
Glassy to perlitic vitrophyre was mapped with overlying rhyolite; it represents the chilled basal zone of the rhyolite. Basal vitrophyre is typically lenticular and reaches as much as 12–15 m in thickness. Fresh surfaces of vitrophyre are vitreous with a conchoidal fracture and range in color from light bluish gray to dark gray; vitrophyre weathers light brown.
Rhyolite lava in domes is flow layered and foliated. Layers are locally folded and brecciated. Layers and foliation around the base of the Town Mountain dome dip gently toward the cen­ ter of the dome, but layers in overlying lava are contorted and dip steeply in various directions. The base of the McClede Mountain dome dips gently east on all sides; the uniform east- ward dip reflects regional tilting after Mimbres Peak time.
Rhyolite lava of the McClede Mountain dome is porphy­ ritic, containing about 15 percent phenocrysts of sanidine and bipyramidal quartz and accessory biotite, sphene, iron oxide, and calcite. Rhyolite of the Town Mountain dome is aphanitic, having only about 1 percent phenocrysts. An inferred rhyolite dike at Lake Valley townsite, which underlies a low hill between two strands of the Lake Valley fault (pl. 1), is intruded by several small aphanitic rhyolite dikes similar to the Town Mountain rhyolite.
The inferred rhyolite dike at Lake Valley townsite, which alternatively could be a faulted sliver of a porphyritic phase of the dome itself, is steeply layered and foliated; along with adja­ cent rhyolite intrusion breccia, it is interpreted to have been intruded along the Lake Valley fault zone. The rhyolite dike contains clasts of Rubio Peak Formation, Kneeling Nun Tuff, and, rarely, gneiss. Although not strongly mineralized, the dike contains veinlets of manganese oxide minerals and quartz. The dike adjoins rhyolite intrusion breccia to the northwest. Along its northern contact, with Rubio Peak Formation, the dike con­ tains vitrophyre.
Rhyolite intrusion breccia crops out adjacent to the rhyolite dike at Lake Valley townsite. The breccia contains large blocks
of pre-Mimbres Peak volcanic rocks, Paleozoic rocks, and Precambrian crystalline rock.
Trachydacite Porphyry
Phenocryst-rich dacite (Tdp) at the northwest corner of Sibley Mountain occurs as a shallowly emplaced sill locally overlain by a carapace of intrusion breccia (Tdpb) composed of fragments and blocks of Kneeling Nun Tuff and Rubio Peak Formation. The intrusion was partly exhumed prior to eruption of trachyandesite flows (Tta) that rest unconformably on the intrusive and carapace rocks. In thin section the trachydacite is seen to contain phenocrysts of sodic andesine (An 30–32) as much as 3 mm across set in a patchy felsitic groundmass that shows various stages of alteration to calcite, sericite, and sec­ ondary quartz. Andesine phenocrysts constitute 30–40 percent of the dacite, and oxyhornblende, biotite, and iron oxides consti­ tute less than 3 percent. Chemical analysis of the porphyry shows the rock to be a trachydacite (table 1, sample 5; fig. 3A).
40Ar/39Ar age determinations of the porphyry attempted during this study have yielded spurious results suggesting con­ tamination of the sill during emplacement. The sill has been interpreted to be older than Kneeling Nun Tuff (Seager and others, 1982; Hedlund, 1977a) and as Cretaceous in age by Kelley and Chapin (1997). Field relations observed in the map area (pl. 1) indicate that the sill is Oligocene in age; it intrudes the 34.9 Ma Kneeling Nun Tuff and is overlain unconformably by the 28 Ma andesite of Sibley Mountain.
Andesite of Sibley Mountain
Andesite flows (Tta) are a major, informal volcanic unit in the map area that underlies Sibley Mountain on the east and a ridge of hills that extend south from McClede Mountain on the west. The predominantly medium gray, aphanitic to slightly porphyritic flows characteristically have a platy weathering habit. In thin section, phenocryst content varies from 0 to 15 percent and includes about 10 percent zoned calcic oligoclase (An28), 3–4 percent oxyhornblende, 0–4 percent bipyramidal quartz, and 0–2 percent oxybiotite. The pilotaxitic to felted groundmass consists of sanidine and oligoclase microlites with interstitial iron oxides and clinopyroxene granules. Chemical analysis of the rock shows composition borderline between andesite and trachyandesite (table 1, sample 4; fig. 3A).
The andesite of Sibley Mountain was originally mapped as Pollack Quartz Latite by Hedlund (1977a) and later correlated with the Bear Springs Basalt by Elston (1989). The flows bear little resemblance to the Pollack Quartz Latite of Jicha (1954) at the type locality in the Lake Valley quadrangle and are probably too old to be correlative with the upper Oligocene Bear Springs Basalt. Whole rock K-Ar age determination of 28.1 ± 0.6 Ma has been reported by Seager and others (1982) and Seager (1986) from a series of similar flows in the Hillsboro 71/2′ quad­ rangle; the flows described by Seager are located about 3 km west of Hillsboro and are the northward extension of flows mapped at McClede Mountain in this report.
Geology of the Lake Valley Area 11