Rocks of Montezuma stock and vicinity, Monzogranite porphyry

What the map says

Light-tan to gray, medium-grained, porphyritic, alkali-calcic monzogranite and granodiorite with euhedral to subhedral phenocrysts of orthoclase and quartz. Chemical composition indicates alkali-calcic granodiorite (Ed Dewitt, written commun., 2007). Orthoclase phenocrysts form as much as 20 percent of rock and are locally reported as long as 10 cm (Lovering, 1935). Quartz phenocrysts form as much as 10 percent of rock and are commonly partially resorbed. Matrix contains orthoclase, plagioclase, quartz, as much as 3 percent biotite, and trace amounts of hornblende, magnetite, and sphene. Locally includes light-pink aplite dikes (bma) not mapped separately. Includes several small intrusive bodies near Montezuma stock. Zircon fission-track ages between 39.8±4.2 and 34.8±3.7 Ma (Bookstrom and others, 1987); Cunningham and others (1994) reported a zircon fission-track age of 35.0±3.2 Ma; one K-Ar age is 37.0±1.4 Ma (Marvin and others, 1989)

1:100,000 sheet: Kellogg and others, 2008, USGS Geologic Investigations Series 3000 (9 crags). A contact line at that scale is about 100 m wide on the ground.

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