SHRIMP-RG U-PB GEOCHRONOLOGY AND TRACE ELEMENT GEOCHEMISTRY OF ZIRCON FROM PROTEROZOIC GRANITIC PLUTONS IN THE SAWATCH RANGE, COLORADO, USA

Twenty-two new SHRIMP-RG U-Pb zircon ages are presented from Proterozoic granitic plutons in the Sawatch Range of Colorado that greatly refine previous geochronology and imply three distinct time periods of regional emplacement. The initial magmatic pulse thus far recognized in this area is the Roosevelt granite at 1765 ± 6 Ma. The Roosevelt pluton is likely a continuation of the 1770 – 1755 Ma suite of suspected extensional Na-granites found to the east in the central Colorado Front Range. A second pulse of magmatism, exhibited in eight of the plutons, is constrained between 1732 ± 4 (Salida dacite) and 1682 ± 18 Ma (Kroenke granodiorite). Other representative plutons formed during this time period (~1.7-Ga) include Denny Creek, Fairview Peak, and Mt. Yale. After an amagmatic period that lasted approximately 230-m.y., a third pulse of more widespread intrusive emplacement, recognized in thirteen plutons that include Hellgate and St. Kevin, is constrained between 1451 ± 13 (Taylor River granite) and 1415 ± 6 Ma (Mt. Harvard granodiorite).

Selected least magnetic zircons from all 22 samples are low in U (avg. 413 ppm; avg. Th/U ~0.51), well preserved, commonly exhibit euhedral to subhedral prismatic forms, are generally devoid of inherited cores, and produce concordant to near-concordant ages (>73% of the analyses are 5% discordant or less).

REE, Hf, U, and Th abundances of the Sawatch zircons, analyzed simultaneously with the U-Pb isotope analyses, reveal geochemical differences reflective of magma provenance. Although zircon REE patterns do not markedly differ between individual plutons, distinct differences can be recognized between different grains within individual samples, and in core/rim pairs within individual zircons. Zircons from the 1.7-Ga intrusions, which likely formed in an oceanic spreading or back-arc environment, are typically characterized by lower U and Th, higher Hf, slightly lower concentrations of LREEs, and a steeper HREE (Yb/Gd) pattern compared to their 1.4-Ga counterparts. The 1.4-Ga plutons likely derived through oceanic crust from partial melting of the 1.7-Ga basin forming events. The trace element data from all analyzed zircons denote a chemical signature (e.g., U/Yb ratio 0.26) more indicative of pluton formation from oceanic crust.