Abstract

Volcanic and plutonic rocks and associated sediments in the Santa Rita Mountains, southeast Arizona, represent one of the most complete Mesozoic sections in the Cordillera. Zircon from the lower member of the Mt. Wrightson Formation, the oldest of the dated units, has lost radiogenic lead and contains a component of older zircon. Accordingly, only an imprecise maximum age of ∼220 Ma could be obtained for an andesite from near the middle of this unit. The lower member is intruded by the Piper Gulch granodiorite, which gave a concordant zircon U-Th-Pb age of 188 ± 2 (2σ) Ma. Complex isotopic systematics of zircon from dacite in the middle member of the Mt. Wrightson Formation allow its age to lie between 172 and 190 Ma. Because the Piper Gulch is interpreted to also cut the base of the middle member, a hiatus of no longer than 30 m.y. is likely to separate the lower and middle members. This stratigraphic break marks the change from early predominantly basaltic and andesitic volcanism to later dacitic to rhyolitic lavas and tuffs. The silty to conglomeratic red beds and minor volcanic rocks of the Gardner Canyon Formation, for which we obtained a nearly concordant U-Th-Pb age of 200 ± 5 Ma on an abraded fraction of zircon, may identify a rift-dominated tectonic regime and mark the transition to felsic volcanism. A second cycle of andesite and rhyolitic volcanism and sedimentation, represented by the Temporal and Bathtub Formations, was dated at 151 ± 5 (2σ) Ma by the whole-rock Rb-Sr isotopic method.

Compared to previous age estimates, our results suggest an earlier initiation of arc magmatism in this part of the Cordillera. Apparently, by the Late Triassic, this area was already an uplifted continental arc margin, analogous to the present-day Andes, shedding volcanic material to what is now the Colorado Plateau. The volcanic rocks we dated fall within the range of ages measured for cobbles of similar lithology from the Upper Triassic Chinle Formation in the Colorado Plateau. Therefore, it is not necessary to invoke the tectonic removal of an early Mesozoic arc terrane along the proposed Mojave-Sonora megashear.

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