New Mexico Geological Society Annual Spring Meeting — Abstracts

Paleoproterozoic sedimentary rocks in the Burro Mountains, southern Mazatzal province, have an unknown depositional age, depositional setting, and source region, and they were subsequently metamorphosed by one or all of the magmatic events that occurred at 1.63 Ga, 1.46 Ga, and 1.25 Ga. The main goals of this project are to (1) use detrital zircons to evaluate the age and source regions for the protoliths of the Proterozoic metasedimentary rocks exposed in the Redrock area of the northern Burro Mountains, and (2) to determine the timing of magmatism that caused regional and contact metamorphism of these rocks.

Preliminary mapping has identified three main metasedimentary rock types in the Redrock area: (1) a phyllite found in the southern part of the map area, with white mica, biotite, and an aluminosilicate that is probably kyanite based on its blue color; (2) a resistant quartzite found at Lydian Peak that has white mica and biotite; and (3) a mafic quartzite with amphibole and mica, found mainly in the Ash Creek drainage. These rocks are intruded by 1.63 Ga gabbro, 1.46 Ga Jack Creek granite, and 1.25 Ga granitic rocks. 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, indicating more rapid cooling of the intrusive body at its margins. 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 observed in several locations along the boundary. The 1.25 Ga intrusive rocks, some of which were dated by Ramo et al. (2003), are the likely cause of contact metamorphism of the metasedimentary rocks.

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 contact with the Jack Creek granite was dated (n=21) and it yielded a weighted mean 207Pb/206Pb age of 1228 ± 6.7 Ma (MSWD=0.32). These results suggest that the ~1.2 Ga Redrock pluton extends much further to the east than previously mapped and are also in agreement with the hypothesis that the ~1.2 Ga magmatic event was the cause of the contact metamorphism in the Redrock area.