LEAD ISOTOPIC EVIDENCE FOR THE ORIGIN OF PALEO- AND MESOPROTEROZOIC ROCKS OF THE COLORADO PROVINCE, USA

Lead isotopic ratios of K-feldspars and whole-rocks from 1.7- and 1.4- Ga plutonic rocks of the Colorado Province are relatively non-radiogen ic for Pb-207/Pb-204, plotting below the average crust model curve of Stacey and Kramers (1975), indicating that the terrane was derived pri marily from juvenile, mantle material. Slightly more radiogenic ratios in the northern part of the terrane, near the Archean Wyoming Provinc e, suggest minor inclusion of an older component. The data from 1.7-Ga plutons plot in a broad field suggesting two episodes of re-equilibra tion with whole-rock Pb, probably related to heating events in the Mes oproterozoic (1.4 Ga) and Cretaceous (70 Ma). Possible differences in calculated whole-rock Th/U, coupled with slight Pb isotopic variations , along the north-south transect suggest either a terrane boundary thr ough central Colorado (near Salida and Gunnison), or fundamental diffe rences in source rocks (metasedimentary vs. metavolcanic). U-Pb analys es of multigrain splits of detrital zircons from quartzites throughout the Colorado Province have failed to identify Archean detritus. The o ldest Pb-207/Pb-206 ages found (in two samples of quartzite from north ern Colorado) are about 2.0 Ga (perhaps derived from rocks of the Tran s-Hudson orogen), in contrast to 2.75-Ga detrital zircon in a Paleopro terozoic quartzite from the southern part of the Wyoming Province. Whi le we are not yet able to discern if these ages are true provenance ag es or mixtures of Archean and Paleoproterozoic components, the absence of easily recognizable Archean zircons supports other isotopic data a nd a conclusion that most of the Paleoproterozoic crust of the Colorad o Province was ultimately derived from a juvenile (at 1.8 Ga) mantle r eservoir.