DEVELOPMENT OF A DETAILED ND ISOTOPE STRATIGRAPHY FOR MIOGEOCLINAL SEDIMENTARY ROCKS IN THE SOUTHWESTERN U.S

A detailed Nd isotope stratigraphy has been established for Neoproterozoic to Cambrian miogeoclinal sedimentary rocks in the Nopah Range in southern California in order to study the infilling and subsequent tectonic history of the southern portions of the Cordilleran miogeocline. Forty siliciclastic sedimentary rocks where obtained from the southern Nopah Range in SE California for this study. The Neoproterozoic Johnnie Fm., Stirling Quartzite and much of the Neoproterozoic-Cambrian Wood Canyon Fm. have similar eNd values (-18.3 to –22.1) and Nd model ages (TDM ~2.0 Ga), while the Cambrian Zabriskie Quartzite and Carrara Fm. have slightly higher eNd(~-15) and younger TDM (~1.7 Ga). All these sediments were likely derived from local Precambrian basement, either in “Mojavia” or adjacent portions of Nd “Province” 2&3. The exception is the arkosic middle member of the Wood Canyon Fm. This unit has significantly lower eNd (~-9) and younger TDM ages (1.2 Ga) that require a provenance in 1.0 to 1.2 Ga Precambrian crust (Farmer and Ball, GSA Bull., 1997). New Nd isotopic data from elsewhere in the western U.S. indicate that such detritus was deposited over a wide portion of the western margin of Laurentia, from central Nevada (Caliente) to southwestern California (San Bernadino Mtns.) in both cratonal (Marble Mountains) and in shallow and deep water (White-Inyo Mtns.) miogeoclinal settings. However, our data reveal that detritus from this ~ 1.0 Ga crustal source was not delivered to the miogeocline prior to the deposition of the middle member of the Wood Canyon in the early Cambrian. One possible interpretation is that the abrupt appearance of this detritus marks the time when the rifted margin of Laurentia had sufficiently subsided to allow riverine detritus derived from the Grenville terrane to southeast and/or east to have been delivered directly to the western continental margin. Alternatively, the high eNd detritus could have been derived from a local granitic source and only deposited in this portion of the miogeocline. In either case, the lack of an obvious component of high eNd siliciclastic detritus in correlative units in northern Sonora suggests that the latter sediments were not originally deposited in the present-day southern Great Basin.