PROTEROZOIC OIB MAGMATISM IN THE CABORCA TERRANE, NORTHWESTERN MEXICO

Volcanic rocks of Proterozoic age are exposed in the Caborca terrane of norwestern Mexico. This terrane is made up of proterozoic metamorphic rocks, covered by a thick succession of Precambrian-Paleozoic sedimentary rocks. The Caborca terrane has been interpreted as a displaced terrane that was located near Nevada before Middle Jurassic time. The studied column is located in the Las Calaberas range, 50 km south from Pitiquito town, Sonora State. It belongs to the La Ciénega Formation and contains vesicular lava flows, breccia and volcaniclastics, interbedded with dolomite, cuartzite, and conglomerate that is made up of limestone, quartz and chert clasts. The studied succession does not contain fossils, but similar rocks are associated to Late Proterozoic stromatolites in other localities. The unit is overlain, on a parallel erosional unconformity by limestones with abundant Archaeocyathids of Early Cambrian age. The basalts have low silica (29 to 44%) and high K, ranging in composition from nephelinite to basanite. They are enriched in light rare earth elements (REE) (LaN 149 to 372), depleted in heavy REE (YbN 7 to 14) and do not have Eu anomaly. Their patterns are similar to those from OIB magmatism. Other elements, such as Ti, Zr, Y, and Nb plot in the Within Plate Basalt fields of different petrotectonic diagrams. They have ENdi(570) between 6.4 and 5.1, and model ages from 730 to 599 Ma. Lead isotopic ratios are within HIMU field. The geochemical and isotopic signatures suggest an intra continent rift to within plate tectonic setting for the La Ciènega volcanic rocks. Sedimentary environments suggest a minor uplift event during magmatism, evidenced by beach-type sedimentation. Regionally, the magmatic event is made up of two localized small-volume volcanic events. The short stratigraphic range, the lack of other igneous rocks, and the lack of stratigraphic changes that are typical of rift zones suggest that the lavas might have formed in a passive margin that has already evolved or might represent traces of a moving hot spot.