PRECISE DATING OF THE BURRO MOUNTAIN GRANITE AND ASSOCIATED METAMORPHISM, SOUTHERN NEW MEXICO; SYN-DEFORMATIONAL MAGMATISM AND RAPID COOLING OF A GNEISS DOME

The Burro Mts. in the southern Mazatzal Province exposes Proterozoic rocks that provide insight into the magmatic and tectonic evolution of the region from 1.65 Ga–1.1 Ga. The plutonic complex is part of the vast ~1.4 Ga granite province. The plutons are in fault contact with low-grade ~1.65 Ga metasedimentary rocks and intrude high-T schists. The complex includes the Burro Mt. granite, a granite/granodiorite body cut by felsic dikes. Our previous SHRIMP U-Pb zircon dates were 1470–1455 Ma. Here we present high-precision TIMS U-Pb zircon dates from 3 of the same samples, which provide a more detailed chronology of magmatism and deformation. A deformed granodiorite yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of ~1470 Ma. An undeformed granite with sharp contacts cuts the granodiorite at the same locality and yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of ~1463 Ma. A granite from 25 km south yielded 3 discordant analyses with an upper-intercept age of ~1460 Ma. Field mapping indicates numerous textural variations of the Burro Mt. Granite. The early phase of magmatism at 1470 Ma was accompanied by deformation resulting in foliation and mylonitic shear zone formation prior to intrusion of granite dikes at 1463-1460 Ma.

To constrain the timing of metamorphism, we dated monazite from 2 samples of bi-gt-sil schist. Previous EMP analyses of one of the samples yielded dates of ~1470 and ~1415 Ma. SHRIMP analyses of this sample yielded a single population with a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of ~1470 Ma. A second sample from the same area yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb date of ~1460 Ma. These overlap with our previously published ⁴⁰Ar/³⁹Ar dates (hb, mu, bi), and, combined with our new U-Pb dates, suggest that syndeformational magmatism at 1470 Ma was followed by rapid cooling, consistent with gneiss dome development within an extensional tectonic setting.

The nearby Redrock granite intruded the ~1.65 Ga low-grade rocks and caused contact metamorphism and resetting of ⁴⁰Ar/³⁹Ar dates in the country rock. LA-ICPMS zircon ²⁰⁷Pb/²⁰⁶Pb dates for the granite cluster around ~1230 Ma. A younger phase of magmatism from the Little Hatchet Mts. yielded ²⁰⁷Pb/²⁰⁶Pb dates of ~1080 Ma, coeval with the Pikes Peak pluton.