The Mogollon Mountains are a major volcanic source area in the southwestern Mogollon‐Datil volcanic field, where about 2000 m of middle‐Tertiary rocks are exposed. The volcanic sequence includes eight major (>100 km3) ash flow units, believed to represent four caldera‐related, compositionally zoned ash flow sequences. Each caldera‐related sequence shows normal compositional zonation from high‐silica rhyolite to rhyolite and dacite. The Mogollon caldera (34 Ma) is preserved only as a fragment in the wall of the Bursum caldera. The Emory caldera (34 Ma) is about 50 km east of the Mogollon Mountains, and only the high‐silica facies of its presumed outflow sheet (Fall Canyon Tuff) reaches the Mogollons. The Gila Cliff Dwellings caldera (30 Ma), earlier interpreted as the source of the Bloodgood Canyon Tuff, probably is a large remnant of a caldera or pair of calderas that were the source of the Davis Canyon and Shelley Peak Tuffs. The Bloodgood Canyon Tuff was most likely erupted from the Bursum caldera (29–28 Ma), but ponded in the Gila Cliff Dwellings caldera. The resurgent dome of the approximately 40‐km‐diameter Bursum caldera is the predominant topographic and structural feature of the Mogollon Mountains. The caldera probably collapsed initially in response to eruption of the Bloodgood Canyon Tuff, then was largely filled with Apache Spring Tuff prior to resurgent doming. The cyclical eruption, beginning about 34 Ma, of similar compositionally zoned ash flows from separate shallow magma chambers, suggests derivation from a common parent magma of batholithic proportions. A regional Bouguer gravity low of about 40 mGals has been interpreted to reflect the presence of such a batholith. Major ash flow eruptions in the Mogollon‐Datil field were followed 26–25 Ma by fundamentally basaltic volcanism, characterized mainly by andesite. About 22–21 Ma, high‐silica rhyolite, tholeiitic basalt, and alkali basalt were erupted as a bimodal suite roughly coincident with the beginning of basin‐range extensional faulting. Alkali olivine basalt as young as 5.5 Ma is interlayered with basin fill (Gila Conglomerate) adjacent to the Mogollon Mountains.

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Fifty-three whole rock Rb–Sr isotopic analyses are reported on two metasedimentary schist and four gneiss units in the Lewisian of the Loch Maree region. Isochron dates from the Gairloch metasediments–Ard gneisses (1980 ± 50 m.y.) and the Loch Maree gneisses (1965 ± 100 m.y.) date the main phase of metamorphism of the Laxfordian episode at 1975 ± 75 m.y. A 1745 ± 160m.y. whole rock isochron date on the Carnmore kyanite–biotite gneisses can be related to late tectonic pegmatite intrusions. Whole rock data points from the quartzofeldspathic gneisses from the Carnmore district do not define an isochron. A 1500 ± 95 m.y. isochron date for metasediments of the Loch Maree Group is interpreted in terms of final closure, during epeirogenic uplift, after a 200–300 m.y. period of isotopic exchange below an impervious cap of tectonically overthrusted hornblende schist and quartzofeldspathic gneiss. Initial 87 Sr/ 86 Sr considerations indicate both a crustal history for the gneisses of the Loch Maree and Carnmore districts going back to 2.7–2.8 b.y. and deposition of the Gairloch metasediments and Loch Maree Group after 2.2 b.y. ago, presumably unconformably on the older metamorphic complex. The Laxfordian orogenic cycle is defined, in the classical stratigraphic sense, to consist of a depositional episode 2.2–2.0 b.y. ago, an orogenic episode 2.0–1.7 b.y. ago and an epeirogenic episode 1.7–1.5 b.y. ago. There are marked similarities in the Laxfordian and Svecokarelian orogenic cycles, and the Lewisian chronology for the Loch Maree region shows striking parallelism with the Precambrian chronology of NW. Europe.

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