Magmatism and metamorphism at 1.46 Ga in the Burro Mountains, southwestern New Mexico
Jeffrey M. Amato, Andre O. Boullion, and Amos E. Sanders


The Burro Mountain granite and biotite-hornblende granite in the Burro Mountains make up a large plutonic complex that cuts 1.65 Ga Mazatzal province metamorphic rocks. A foliation in the pluton consists of aligned biotite, is most prominent near the contact with the country rock, and is absent in the interior of the pluton. We interpret this fabric as being generated during magmatic emplacement. The Burro Mountain granite locally cuts the smaller volume biotite hornblende granite. U-Pb zircon dating of four samples of the Burro Mountain granite yielded ages ranging from 1463–1455 Ma. The weighted mean 207Pb/206Pb date from all of these samples is 1457 ± 4 Ma. The biotite hornblende granite was dated at 1459 ± 14 Ma. An amphibolite has a protolith interpreted to be coeval with 1.65 Ga metasedimentary rocks and a U-Pb zircon age of 1459 ± 9 Ma. This age reflects metamorphism of the amphibolite coeval with the intrusion of the granites. Timing of metamorphism of the metasedimentary rocks was investigated using electron microprobe dating of monazite. Most ages are around 1460 Ma with one sample yielding an age of 1411 Ma. The main episode of deformation in the Burro Mountains occurred between 1.65–1.63 Ga, and 1.46 Ga magmatism occurred in the apparent absence of regional tectonic activity. High-heat flow associated with the 1.46 Ga magmatic event reset both U-Pb monazite and 40Ar/39Ar thermochronologic systems and was apparently short-lived in the Burro Mountains on the basis of numerous dates within error of 1.46 Ga.


  1. Amato, Jeffrey M.; Boullion, Andre O.; Sanders, Amos E., 2008, Magmatism and metamorphism at 1.46 Ga in the Burro Mountains, southwestern New Mexico, in: Geology of the Gila Wilderness - Silver City area, Mack, Greg; Witcher, James, Lueth, Virgil W., New Mexico Geological Society, Guidebook, 59th Field Conference, pp. 107-115.

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