MAGMATISM, METAMORPHISM, AND DEFORMATION OF PROTEROZOIC MAZATZAL PROVINCE CRUST: A COMPREHENSIVE CASE STUDY FROM THE BURRO MOUNTAINS, SOUTHWESTERN NEW MEXICO

A diverse suite of Proterozoic rocks is present in the Burro Mtns. of SW New Mexico. Rock types include deformed granitic orthogneisses, amphibolite, and metasedimentary rocks, and undeformed gabbro, granodiorite and granite. U/Pb zircon geochronology was performed using multicollector LA-ICPMS. A granite orthogneiss yielded an age of 1672 ± 17 Ma (2s uncertainties) and a tonalite orthogneiss an age of 1625 ± 50 Ma. The granite orthogneiss has ε_Nd of -1 and a model age of 2.1 Ga. The tonalite orthogneiss has ε_Nd of +2 and a model age of 1.8 Ga. Model ages older than intrusive ages suggests involvement of older continental crust during petrogenesis. Detrital zircons from a gt-bi schist have a main peak at 1714 Ma, and the youngest grains yield a weighted mean age of 1684 ± 12 Ma. Zircons from two qtz-mica schists yield youngest peaks at 1649 ± 17 Ma and 1645 ± 5 Ma, and main peaks at 1753 Ma and 1673 Ma. Deformed undated amphibolite has xenoliths of metasedimentary rocks. Undeformed 1633 Ma gabbro (Rämö et al., 2003) cuts metasedimentary rocks, bracketing their deposition between 1645-1633 Ma. Undeformed granitic plutons have U/Pb zircon ages of 1449 ± 12 Ma, 1441 ± 41 Ma, 1431 ± 24 Ma, 1427 ± 23 Ma, and 1416 ± 18 Ma. Several have magmatic foliations that parallel their margins, decrease in intensity away from their contacts, and have different attitudes than adjacent gneisses.

Metamorphism reached peak conditions of 0.5 GPa and 725°C based on P-T analysis of gt-bi schists. These conditions were attained after 1645 Ma, the depositional age of the protoliths, but before 1633 Ma because metasedimentary rocks adjacent to the 1633 Ma gabbro have static low-pressure mineral growth overprinting the foliation. Metamorphic events at 1465 ± 5 Ma, 1420 ± 6 Ma, and ~1250 Ma as recorded by U/Pb monazite and ⁴⁰Ar/³⁹Ar hornblende ages were static and coeval with magmatism. PT conditions attained during the ~1.4 Ga event were 0.25 GPa and >550°C. The main fabric-forming event occurred between 1645-1633 Ma and is probably related to the Mazatzal orogeny. The main magmatic event occured between 1460-1420 Ma, occurred at high temperatures, and formed static textures in previously deformed metasedimentary rocks. Lack of pervasive ~1.4 Ga deformation indicates the rocks were located in the interior of a block that may have been deforming along shear zones at its margins.