PALEOPROTEROZOIC METASEDIMENTARY ROCKS IN THE BURRO MOUNTAINS, NEW MEXICO: POTENTIAL ELUCIDATION OF DEPOSITIONAL SETTING AND CAUSES OF METAMORPHISM IN THE SOUTHERN MAZATZAL PROVINCE

We have initiated a study of low-grade metamorphosed Paleoproterozoic sedimentary rocks in the Burro Mountains, southern New Mexico with the goals of determining the timing of sedimentation, the tectonic setting of the basins, source regions for sediment, and the timing and cause of deformation and both regional and contact metamorphism. Preliminary mapping has identified three main metasedimentary rock types in the Redrock area: (1) a phyllite, with white mica, biotite, and kyanite; (2) a resistant quartzite with white mica and biotite; and (3) a quartzite with amphibole and white mica. These rocks are intruded by 1.63 Ga gabbro, 1.46 Ga Jack Creek granite, and 1.25 Ga Red Rock granite. There is an intrusive relationship between the 1.63 Ga gabbro and the metasedimentary rocks, as determined by a smaller grain size in the gabbro near the contact, and confirms that the depositional age of the sediment is >1.63 Ga. This contact has been modified by faulting in several locations, probably because of the strong rheological contrast between the rigid gabbro and the weak phyllites. Field observations near the eastern boundary of the study area have revealed an intrusive relationship between the Jack Creek Granite and the Lydian Peak quartzite. Evidence for this includes 2 cm wide dikes of Jack Creek granite cutting the quartzite observed in several locations along the boundary. The 1.25 Ga intrusive rocks were the likely cause of contact metamorphism of the metasedimentary rocks based on their proximity and based on the presence of porphyroblasts away from the gabbro. This contrasts with our previous interpretation that the gabbro may have been the heat source. Within the northern Burro Mountains, there are two phases of the Redrock Granite. The main phase of the granite on the northwestern part of the field area has not previously been dated. LA-ICP-MS zircon dating (n=23) from a granite sampled on the eastern edge of the Redrock area indicate a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1227 ± 2.3 Ma (MSWD=2.5). Another dike (~20 m wide) on the eastern edge of the metasedimentary rocks near the Jack Creek contact was dated (n=21) and it yielded a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1228 ± 6.7 Ma (MSWD=0.32). This is an unusual age as most of the late Mesoproterozic felsic magmatism in the southwest U.S. ranges from 1.15–1.08 Ga.