The biotite + muscovite ± garnet-bearing Texas Canyon (TC), Presumido Peak (PP), and Gunnery Range (GR) granites are members of an enigmatic suite of Eocene age granites in southern Arizona. The late orogenic granites intrude Precambrian through Jurassic metasediments and metavolcanics. The major and minor element geochemistry of the high silica, weakly peraluminous granites is rather uniform. However, trace element concentrations, REE patterns, and isotopic compositions imply gross similarities between the TC and GR granites when both are compared to the PP granite. The TC and GR granites were derived from a depleted Precambrian lower crustal source area with low Rb/Sr, while the PP granite was the result of anatectic melting of an enriched Precambrian mid-to-upper crustal source. The PP granite is an integral part of a metamorphic core complex, and this tectonic setting accounts for the geochemical differences between it and both the TC and GR granites.

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The Del Bac Hills are a northeast-trending chain composed of mid-Tertiary Turkey Track porphyry dikes and flows, overlain by potassic basaltic andesites intercalated with sediments. These hills are considered the southernmost extension of the Tucson Mountains and lie along a northeast-trending basement scarp which extends across the Tucson Basin. Turkey Track porphyry intrudes the San Xavier conglomerate as two parallel dikes, closely associated in time but with different magnetic orientations. Potassium-argon dating of a flow associated with the older dike yielded a mean of 26.8 m.y. for plagioclase phenocrysts and groundmass. Plagioclase of the younger dike sampled 50 feet from the contact with the San Xavier conglomerate and at a depth of approximately 200 feet yielded an anomalous age of 46.4 m.y., representing 0.29 x 10^-10 moles per gram excess Ar4° in comparison with the extrusive. The 21 basaltic andesite flows are essentially contemporaneous at 24-25 m.y. Tight grouping of K/Rb and Rb/Sr for the basaltic andesites confirms the field inference of two separate sources. Specific gravity and initial Sr^87/Sr^86 determinations for the porphyry and basaltic andesites do not preclude a comagmatic origin for the two rock types. Post-extrusive faulting along three principal directions has established the present configuration of the hills.

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