EARLY PROTEROZOIC CRUSTAL EVOLUTION - GEOCHEMICAL AND ND-PB ISOTOPIC EVIDENCE FROM METASEDIMENTARY ROCKS, SOUTHWESTERN NORTH-AMERICA

Early Proterozoic (1.8 - 1.7 Ga) metasedimentary rocks in northern New Mexico and southern Colorado, USA, can be divided into turbidite successions (commonly volcanogenic) associated with mafic/felsic metavolcanic successions (e.g., Irving Fm.) and stable shelf quartzite-pelite successions of shallow marine origin (e.g., Hondo Gp.). Metapelites from the turbite successions reported here have low K2O/Na2O, low Th/U (<3.0), low to moderate Th/Sc (0.1-0.6), and slight negative Eu-anomalies, although regionally, negative Eu-anomalies in such rocks are common. At the time of sedimentation (ca. 1.7-1.8 Ga), epsilon(Nd) values were in the range +3 to +7, indistinguishable from associated metavolcanic and plutonic rocks. Similarly, lead isotopic data scatter about a 1.7 Ga reference isochron. Low kappa (Th-232/U-238) values for the Irving Formation are consistent with derivation from crustal sources similar to the southern Colorado/northern New Mexico lead isotope crustal province. These data are further consistent with a volcanic arc related origin. In contrast, stable shelf metapelites have high K2O/Na2O, variable but commonly high Th/U (2.0-7.0), moderate to high Th/Sc (0.5-1.4), and substantial negative Eu-anomalies. Although compositions are rather variable, they are typical of post-Archean shales. Neodymium isotopes are surprisingly radiogenic with epsilon(Nd) (1.7 Ga) in the range -0.2 to +4. Lead isotopic data for the least radiogenic samples also are consistent with a dominantly juvenile source and on a Pb-207/Pb-204 vs. Pb-206/Pb-204 diagram, data scatter slightly above the 1.7 Ga reference isochron, suggesting minor components of significantly older material. Lead isotopic systematics suggest that a major component of the provenance was derived from the immediately associated metavolcanic-plutonic terranes, consistent with suggestions of a first-cycle origin, but with an Archean component. Isotopic data restrict the Archean component to about 10%, on average, and no more than 25% in any sample. This older crustal component may be derived either by direct erosion of Archean rocks, such as the Wyoming Province, or indirectly through assimilation into Early Proterozoic igneous rocks. Although the stable shelf sedimentary rocks are derived from a provenance with similar ages as the volcanogenic turbidites, the geochemical characteristics of the provenance are significantly different. Accordingly, these data are consistent with especially rapid and widespread crustal growth and evolution in southwestern North America during the period 1.9-1.7 Ga. Several samples from the Hondo Group and Uncompahgre Formation have REE patterns that are rotated to LREE depletion and perhaps HREE enrichment. The change in REEs correlate with Mo, U, and V abundances and Pb isotopic characteristics suggesting sedimentary processes similar to those operating in black shales affected these REE patterns. REE patterns and Th/U ratios of Early Proterozoic volcanogenic turbidites examined in this and other studies differ on average from turbidites found in Archean greenstone belts. Negative Eu-anomalies are common, HREE-depletion is seen but comparatively rare, and Th/U ratios are commonly below 3.0. Accordingly, these data are consistent with models suggesting that the upper crust had a different composition in the Archean.