FIRST GEOCHRONOLOGY DATA FROM THE CAMBRIAN-PRECAMBRIAN ULTRAMAFIC FLOWS IN THE CABORCA REGION, NW SONORA, MEXICO

An Upper Proterozoic and Cambrian sequence is exposed in the localities surrounding the Bamori Ranch in the Caborca region (NW Mexico). The sequence consists of sedimentary rocks from a miogeoclinal prism containing, among other fossils, Cloudina and T. pedum (Sour-Tovar et al., 2007). Considering the proven ages of these fossils, the Ediacaran/Cambrian boundary should be located between the La Cienega Fm. and the Unit 1 (base) of the Puerto Blanco Fm. Our recent investigations from the base of the Puerto Blanco Fm. shows that it consists of volcanic rocks that includes agglomerates, tuffs, lapillistones and alkaline flows that were geochemically characterized as mafic and ultramafic lava flows; among other sedimentary rocks. Until this study, no radiometric ages have been published on these flows. Results from our ongoing investigations provide the first radiometric age for the ultramafic flows. A 40Ar/39Ar age was systematically obtained from different materials, as well as groundmass concentrates. The most concordant age was obtained from vapor phase phlogopite encased in quartz from an ultramafic lava flow from the C. Clemente locality. This mica yielded a plateau age of 491.47 ± 2.4 Ma. In general, all of the ultramafic lava flows are significantly altered only allowing for the analysis of the groundmass and phlogopite. However, incremental heating of clinopyroxene (the only remaining phase to date) was obtained for several of the lava flows, generally yielded mixed results. For one samples, we combined the age data for 3 phases (clinopyroxene, groundmass, and phlogopite) that yield an Inverse Isochron age of 557.39 ± 51.71 Ma. Considering that the lava flows are interbedded in the lower Cambrian sequence, we suggest that the age of the ultramafic lava flows are around 557 Ma, but more work needs to be conducted to see if similar ages are obtained. The upper Cambrian age obtained may be the result of a metamorphic overprint partially resetting the phlogopite or a combination of argon loss and thermal overprint. This thermal overprint may have resulted from the intrusion of Laramide granites. We believe the ages for two groundmass samples from C. Los Apaches (56.86 ± 0.36 Ma and 55.89 ± 0.60 Ma), and one from C. Calaveras (57.31.0.57 Ma), could be considered as a metamorphic print at the Paleocene-Eocene boundary.