MESOPROTEROZOIC (1.46 GA) MAGMATISM AND EXHUMATION DURING CRUSTAL EXTENSION IN THE MAZATZAL PROVINCE: THE BURRO MOUNTAINS GNEISS DOME, SOUTHERN NEW MEXICO

Large granitic plutons in the Burro Mountains, SW New Mexico include leucogranite and granodiorite. These intrusions are part of the vast ~1.4 Ga granite/rhyolite province. U-Pb zircon dating of 5 samples of the leucogranite yielded ages from 1469 ± 12 to 1455 ± 11 Ma (2s uncertainty). 3 samples of granodiorite range from 1470 ± 16 Ma to 1459 ± 14 Ma. All of the ages are within error and have a weighted mean age of 1462 ± 3 Ma. Granite intrudes granodiorite. Amphibolite from the country rock has a protolith age of 1680 Ma and a similar sample has granite segregations dated at 1458 ± 9 Ma. Quartzofeldspathic gneiss and gabbro both yield ~1630 Ma U-Pb zircon ages. ⁴⁰Ar/³⁹Ar dates from granite and amphibolite form two groups. The older group includes four hornblende ages of 1477-1467 Ma and biotite and white mica ages from 1472-1459 Ma. These are interpreted as cooling ages of the plutons and country rock soon after intrusion. The younger group of ages has a mean of 1228 ± 3 Ma and is close in age to the adjacent Redrock granite that intruded at 1225 Ma. Microprobe monazite U-Pb dates from metapelite country rock range from 1500-1400 Ma with a peak at 1474 Ma. The dominant strong foliation in the country rock is interpreted to have formed >1633 Ma based on static porphyroblast growth in the aureole of a 1633 Ma pluton and likely formed around 1650 Ma during the Mazatzal orogeny (Amato et al., 2008). Quantitative P and T estimates from the metapelites, the first from southern NM, are 4.5 ± 1 kbar and 670 ± 50 °C, consistent with their mineralogy and textures. Decompression textures on garnets indicate exhumation following peak conditions reached during plutonic heating at 1460 Ma. The 1460 Ma plutons are not pervasively deformed but the granodiorite has strong augen gneissic foliations and mylonitic fabrics, particularly near the contacts with the country rock. This fabric formed prior to the Burro Mountain granite intrusion and thus is synmagmatic within geochronologic uncertainty. We interpret the ~1.46 Ga plutonism, deformation with steep fabrics, exhumation, and rapid cooling as having formed during gneiss dome development driven by mafic, mantle derived magmatism within an extensional tectonic setting. This model explains many of the features observed regionally for ~1.46 Ga magmatism, particularly where country rock deformation has been reported.