Paleoproterozoic Gneisses of the San Luisito Hills in NW Sonora: Extending the Mexican Yavapai Province of SW Laurentia

The San Luisito Hills area is located in NW Sonora, approximately 60 km NW of the town of Caborca and 40 km to the SSE of Quitovac. Three gneiss samples from this area have been dated by U-Pb zircon geochronology (SHRIMP-RG) and have shown complexities associated with different degrees of Pb-loss present in the zircons. We have obtained upper concordia intercept ages at 1712 ± 32 Ma, 1707 ± 20 Ma, and 1676 ± 10 Ma for these samples that we interpret as the best approximation of their time of crystallization. Consistent lower concordia intercepts at ~400 Ma (±170) may record the time of extensive Pb-loss in response to a yet unrecognized geological event in the region.

These Paleoproterozoic gneisses have equivalent major- and trace-element characteristics, as do similar rocks from the Quitovac area. However, the San Luisito Hills rocks are more enriched overall, a fact that might be explained by their intraplate affinities (i.e. Y vs Nb). The Quitovac samples also appear to be slightly older (up to ~1.78 Ga) suggesting that these younger gneisses may have intruded into the pre-existing Quitovac basement. Whole rock epsilonNd initial values between +1.13 and +2.87, and corresponding Nd model ages (TDM) between 1.76-1.91 Ga, further suggest that these rocks are similar to the ones reported in Quitovac that have been classified as Yavapai-type based on earlier studies. Rocks from both areas may have been formed after juvenile magmas (mantle source) incorporating some degree of crustal contamination.

This new occurrence of Yavapai-type gneisses in San Luisito Hills in NW Sonora contributes to our current efforts to delineate the basement provinces in NW Mexico. In addition, this more detailed delineation of provinces of the SW margin of Laurentia should help in reducing the uncertainties associated with current reconstructions of the Rodinia supercontinent.