New Mexico Geological Society Annual Spring Meeting — Abstracts

Several geochronologic studies have been carried out on Precambrian rocks from the Sandia Hountains. Early studies, summarized by Brookins (1972), showed U-Pb, Th-Pb and Rb-Sr ages ranging from 1.47 to 1 BYBP, With a best estimate of 1.45 BYBP (error uncertain). More recent studies by the writers (DGB; DCB and AM) confirm that many of the granitic rocks crystallized at 1.44 ± .04 BYBP, based on a twenty-three data Rb-Sr whole rock isochron. In addition, detailed study of biotites from nine granite samples yields Rb-Sr ages of 1.33 ± 0.006 BYBP, which we interpret as a low grade thermal event affecting the granite at this time. This date is consistent with K-Ar mineral ages of muscovites from the metamorphic rocks of the Juan Tebo Series as well as from pegmatite minerals. Earlier work by Taggart and Brookins (1975) yielded a Rb-Sr whole rock age of 1.60 + 0.08 BYBP for gneisses of the Cibola Gneiss. We here report the first attempts at dating the Juan Teba Series by the Rb-Sr whole rock method, with an age of 1.62 ± 0.07 BYBP. In addition, folded pegmatites from the Rincon area of the Sandia Mountains yield an age of 1.47 ± 0.12 BYBP.

We have also investigated the proposed division of the Sandia granite into northern and southern plutons (Condie and Budding, 1979). Our chemical, geochronologic, petrographic and field work do not support such a division of the Sandia granite. However, Rb-Sr whole rock studies do suggest a younger granite in the Juan Tabo-La Cueva picnic areas of about 1.38 BYBP based on whole rock Rb-Sr analyses, which may possibly be correlated with the suggestion by Berkley and Callender (1979) that there were several pulses of granite formation on the northern part of the Sandia Mountains.