Abstract

Large-volume granitic plutons in the Burro Mountains, southwestern New Mexico, cover an area of 1300 km2 and include biotite leucogranite and biotite-hornblende granodiorite. These intrusions are part of the ca. 1.4 Ga granite and rhyolite province stretching across Laurentia. U-Pb zircon dating of five samples of the biotite leucogranite yielded ages ranging from 1469 ± 12 to 1455 ± 11 Ma (2σ uncertainty). Three samples of granodiorite range from 1470 ± 16 Ma to 1459 ± 14 Ma. All of the zircon ages are within error and together have a weighted mean age of 1462 ± 3 Ma, but the granodiorite is older than the granite based on crosscutting relationships.

Pressure and temperature estimates from metapelitic country rock, the first from southern New Mexico, are 4–6.5 kbar and 575–650 °C, consistent with their mineralogy. Decompression textures on garnets indicate exhumation following peak conditions, interpreted to have been reached during magmatic heating at 1460 Ma. Decompression had a magnitude of 1–3 kbar.

An amphibolite from the country rock has metamorphic zircons dated at 1458 ± 9 Ma, with low Th/U and soccer-ball morphology enclosed within hornblende; this is the age of amphibolite-facies metamorphism. Electron microprobe monazite U-Pb dates from metapelite country rock mostly range from 1500 to 1400 Ma, with a peak at 1474 Ma. This is interpreted as indicating that the area experienced high-temperature metamorphism associated with pluton emplacement at this time. The 40Ar/39Ar dates from granite and amphibolite form two main groups. The older group includes four hornblende dates of 1477–1467 Ma and biotite and white mica ages from 1472 to 1459 Ma. These are interpreted as reflecting cooling of the plutons and their country rock soon after intrusion. The younger group of hornblende and biotite ages is at 1228 ± 3 Ma and is close in age to the adjacent Redrock granite, which intruded at 1225 Ma.

There is a dominant strong foliation in the country rock gneisses and metasedimentary rocks. Foliated granite and gabbro both yield ca. 1630 Ma U-Pb zircon ages. The Burro Mountain granite is not pervasively deformed, but the granodiorite has strong augen gneissic foliations and mylonitic shear zones with S-C fabrics. This fabric formed prior to intrusion of undeformed dikes of Burro Mountain granite and thus is synmagmatic within geochronologic uncertainty. The fabric is identical to the fabric in the country rock, and xenoliths of country rock are foliated with orientations parallel to the foliation in the granodiorite. The similar orientations and the timing of metamorphism suggest that most of the deformation in the Burro Mountains occurred at 1460 Ma and was synchronous with intrusion of the granodiorite.

We interpret the 1460 Ma plutonism, deformation with steep fabrics, exhumation, and rapid cooling as having formed during gneiss dome development within an extensional tectonic setting, although a transpressional setting is also permissible.

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