New Mexico Geological Society Annual Spring Meeting — Abstracts

The Mazatzal province is thought to be a juvenile arc terrane that accreted onto southern Laurentia during the ~1.65 Ga Mazatzal orogeny (Karlstrom et al., 2004). Thermal overprinting and deformation during the emplacement of A-type granites at ~1.46 Ga has made it difficult to date Proterozoic fabrics. U-Pb ages were obtained by LA-MC-ICPMS from four localities in southern New Mexico. These dates will be used to constrain the timing of deformation and the relationship of these rocks to the arc magmatism.

A weakly deformed quartzite from the Caballo Mountains yields the youngest maximum depositional age (MDA) of 1629 ± 27 Ma (2s); a similar quartzite in the San Andres Mountains yielded an MDA of 1657 ± 11 Ma. Phyllites from the San Andres yielded MDAs of 1664 ± 19 Ma and 1669 ± 16 Ma. Schist from the San Andres yielded an MDA of 1651 ± 14 Ma. Quartz-rich amphibolite (possibly sedimentary) from Mud Springs Mountain yielded a MDA of 1655 ± 17 Ma and a sphene U-Pb metamorphic age of 1427 ± 20 Ma. A low-grade conglomerate from the Kingston District yielded a MDA of 1661 ± 21 Ma, and a metasedimentary rock from there yields the oldest MDA of 1670 ± 30 Ma.

A gneissic granite from the Caballo Mountains intruded at 1667 ± 27 Ma. An undeformed granophyre from the Kingston District yielded an age of 1659 ± 21 Ma. Granites from the San Andres yielded ages of 1654 ± 14 Ma and 1667 ± 30 Ma. A boudinaged amphibolite intruding orthogneiss yielded an age of 1629 ± 13 Ma; a granitic dike that cuts the amphibolite had zircons with ages of 1463 ± 44 Ma and 1646 ± 16 Ma. These are interpreted as representing inherited zircons at 1.65 Ga and igneous zircons at 1.46 Ga.

These new ages combined with previously published ages from the Mazatzal province in New Mexico yield MDA’s ranging from 1629–1670 Ma and igneous ages of 1623–1684 Ma. Thus, deposition seems to be synchronous with magmatism in the province, suggesting it occurred prior to arc-continent collision. In the Kingston District, rocks have apparently escaped significant deformation, whereas ~30 km east in the Caballo Mountains rocks are highly deformed. In the San Andres Mountains, rocks range from undeformed to weakly deformed quartzite to highly deformed orthogneiss. Contact metamorphism near ~1.6 Ga plutons has yielded large garnet growth. The pluton that likely caused the garnet growth is mostly undeformed but has discrete shear zones. The age of these shear zones is unknown. Presence and intensity of deformation does not appear to be related to age of the protolith or location within the Mazatzal province. We suggest that the end of Mazatzal arc magmatism occurred at 1645 Ma, prior to final collision and the subsequent onset of bimodal, possibly extensional magmatism that occurred around 1630 Ma in southern New Mexico.

References:

1. Karlstrom et al., 2004, NMGS Special Publication 11, p. 1-34