A garnet-two-mica granite, Coyote Mountains, southern Arizona: Geologic setting, uranium-lead isotopic systematics of zircon, and nature of the granite source region

Abstract

The 58-m.y.-old Pan Tak Granite, considered representative of the widespread early Tertiary garnet-two-mica granites of south-central Arizona, consists of biotite and muscovite-biotite granite intruded by garnet-muscovite and garnet-biotite granite and associated pegmatite. Textural evidence indicates that muscovite is a primary, magmatic mineral. The granite was intruded into Jurassic dioritic to granitic rocks and Paleozoic metasedimentary rocks during a latest Cretaceous and early Tertiary regional metamorphic episode. Part of the pluton was subsequently mylonitized during an early or middle Tertiary deformational episode of “metamorphic-core-complex” aspect. The fabrics produced by these two metamorphic episodes differ in distribution, style, orientation, and, probably, age. The zircon population of the Pan Tak Granite consists of a mixture of small euhedral crystals and larger, rounded crystals with or without overgrowths. Uranium-lead isotopic ratios of five size fractions of discordant zircon define a chord with a lower concordia intercept age of 58 ± 2 m.y. and a late Precambrian upper intercept. 207 Pb*/ 206 Pb* ages range from 214 m.y. for the finest fraction to 434 m.y. for the coarsest fraction. The Pan Tak discordia is interpreted as a mixing line between 58-m.y.-old magmatic zircon and an older zircon component largely or entirely of late Precambrian age. Geologic and isotopic evidence indicates that this older zircon component was not assimilated from the country rocks but, rather, was inherited from the source material of the granite. The presence of this inherited zircon indicates that the Pan Tak magma was produced by melting of crustal rocks containing late Precambrian zircon. This source material was probably the 1.7- to 1.4-b.y.-old crystalline basement of south-central Arizona. Generation of the Pan Tak and related granites by crustal anatexis was one aspect of a latest Cretaceous to early Tertiary orogenic episode in south-central Arizona.