Quartz Latite Breccia
A quartz latite breccia is found to rest either con
formably or unconformably upon the Cretaceous (?) complex in the western part of the area. In one exposure along the jeep trail in the east half of section 21, T. 17 S., R. 12 E.,
the contact between the arkose and the quartz latite breccia can be observed. A black siltstone approximately eight feet in thickness separates the two formations. This siltstone bed may represent an old soil zone, but it is believed to have been deposited as a silt bed since similar units occur throughout the arkose complex. The black siltstone is also found in drill cores separating the quartz latite breccia and arkose. Several exposures in the western part of the area also show that the quartz latite breccia is overlain by later andesite flows. If this quartz latite breccia is a forerunner of the granodiorite intrusion which occurs farther to the west, it is probably upper Cretaceous in age.
In hand specimen the quartz latite breccia is light gray to grayish-pink. It consists of angular to sub-angular, tabular fragments of whitish feldspar, quartz and lithic
material set in a microcrystalline groundmass. The smaller
fragments average .5 inch along their long axes and by their arrangements reveal flow structure. Small books of biotite are dispersed throughout the matrix and can occur as
small clots.
The larger fragments average 2 inches in diameter, but inclusions reaching eight inches in diameter were noted.
These fragments appear to have the same lithology as the enclosing rock, and are believed to be pieces of the same unit which solidifying earlier, were torn apart by continued flow.
In thin section the rock shows some characteristics of a welded tuff. Angular fragments of andesine-oligoclase, quartz, sanidine and small books of biotite are set in a matrix of glass shards that are partially divitrified.
Boundaries, between the large fragments and enclosing rock, that can be easily seen in hand specimen, can not be found in thin section.
Andesite
Andesite which appears to be partly intrusive and
partially extrusive is found intruding and lying unconformably on top of the Cretaceous (?) complex and the quartz latite breccia in the western and northern parts of the area (PI. 1).
This relationship is found in drill cores as well as in out
crop. The andesite is brecciated in part and appears both aphanitic and porphyritic. The colors of the fresh rock range from light green to purple. Age relationships are not certain, but field evidence suggest that the andesite intrudes the quartz latite breccia. It is younger than the quartz la- tite breccia, but is pre-sole faulting. The rock is probably upper Cretaceous in age.
Granodiorite
Granodiorite forms a large intrusive body to the west of the Ruby Star Ranch area, and its eastern exposures are mapped in sections 27 and 28, T. 17 S., R. 12 E. (PI. 1).
In the thesis area the rock is a medium grained, equigranular, oligoclase-biotite granodiorite, but other varieties are found in the western part of the district.
The Geochemical Laboratory of the University of Arizona dated the granodiorite by the K-Ar method and obtained an age of 58.7 + 1.9 million years (Damon, e t . al., 1964, p. AIII-2).
Another granodiorite intrusion is located between Foy Ridge and Twin Buttes (PI. 1), and it is probably the same age as and related to the granodiorite pluton in the western part of the district. It is intrusive into the Paleozoics and
forms the core of the northwest plunging Foy Ridge-Twin Buttes anticline. Drilling information indicates that north of
Twin Buttes the granodiorite breaks through the Pennsylvanian and Permian sediments as a large, shallow dipping, dike-like body 200 feet thick. In the vicinity of diamond drill hole T-47 the dike is overlain by 200 feet of gravel.
The Foy Ridge-Twin Buttes intrusion is an equigranular, medium grained, oligoclase-biotite granodiorite containing
small euhedral crystals of honey colored sphene.
Quartz Monzonite Porphyry
Small bodies of quartz monzonite porphyry intrude the sole fault and the overlying brecciated formations in the north
western part of the area (PI. 1). In section 15 and the
western half of section 22, T. 17 S., R. 12 E., unbrecciated, highly altered quartz monzonite porphyry intrudes the brecciated arkose and the quartz latite breccia. In the brecciated arkose a baked zone several feet wide surrounds the intrusion, and blocks of the arkose can be found incorporated in the quartz monzonite porphyry. In the baked zone the brecciated arkose
is healed by recrystallization and where not healed the fragments are held together by stringers of quartz monzonite penetrating around them.
Drilling in the same area shows that the quartz mon- zonite porphyry is post-sole, but nowhere are the intrusions found cutting the Helmet Fanglomerate. Thus the age of the intrusions must be post-sole and pre-Helmet. This confines the age to the Eocene or Oligocene epochs. On the geological map (PI. 1) it is designated Oligocene (?).
Hand specimens of the unaltered quartz monzonite porphyry are pinkish-gray and contain phenocrysts of plagio- clase, quartz and potash feldspar in a fine grained, gray groundmass. Small books of biotite and prisms of hornblende are present. Usually the feldspars are altered to sericite and clay.
Andesite Flows And Dikes
Andesite flows with plagioclase phenocrysts attaining one half inch in length are interbedded with the Helmet
Fanglomerate. The Geochemical Laboratory at the University of Arizona attempted to date the plagioclase by the K-Ar
method and obtained 37 + 17 million years. A very high error was assumed because of the extreme atmospheric air correction
(Damon, e t . al., 1964, p. AIII-1).
Andesite flows, which are petrologically similar to the andesite flows in the Helmet Fanglomerate, are found in
the southern part of the Tucson Mountains. The Geochemical Laboratory at the University of Arizona obtained an age of 28 + 2.6 million years for these flows (Damon, et. al., 1964, p. AII-7). This age probably approximates the true age for the andesite flows in the Helmet.
An andesite dike cuts the Helmet and is later than the deposition of the Helmet. A similar andesite dike cuts the Paleozoics on the northeast flank of Foy Ridge.
These two dikes appear the same petrographically, but whether they are of the same age is debatable. Creasy and Kistler
(1962) dated the dike cutting the Helmet at 24 million years.