Early Proterozoic crustal evolution: Geochemical and Nd-Pb isotopic evidence from metasedimentary rocks, southwestern North America

• Published in: Geochimica et cosmochimica acta Vol. 59; no. 6; pp. 1153 - 1177
• Main Authors: McLennan, S M, Hemming, SR, Taylor, SR, Eriksson, KA
• Format: Journal Article
• Language: English
• Published: 01.03.1995
• Subjects: USA, New Mexico; USA, Colorado
• ISSN: 0016-7037
• DOI: 10.1016/0016-7037(95)00032-U

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.). Geochemical 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.