CONTRASTING PLUTONIC STYLES IN THE PALEOPROTEROZOIC OF COLORADO BETWEEN CA. 1.725 TO 1.690 GA: RESULTS FROM SHRIMP U-PB ZIRCON GEOCHRONOLOGYAND MAJOR AND TRACE-ELEMENT GEOCHEMISTRY
Six samples of gabbro to monzogranite from the Rawah batholith, composed mainly of calc-alkalic diorite to alkali-calcic granite in north-central Colorado, range in age from 1715 to 1724 Ma (mean age = 1718 ± 4 Ma) and appear to be slightly older than calc-alkalic tonalite to alkali-calcic granodioritic rocks of central Colorado (e.g., Boulder Creek batholith) that are 1705 to 1721 Ma (mean age = 1713 ± 4 Ma; N = 15). Zircons from the Rawah-Boulder Creek samples have Th/U from 0.35-0.50, 600-700 ppm U, and some contain inherited cores. Geochemically, these rocks belong within a subduction-produced, evolved-arc environment.
These batholith-size older rocks can be distinguished from a suite of Fe-rich gabbro (~10%) to granite (~90%) that is concentrated in central Colorado, west of Denver. The gabbroic to dioritic rocks were emplaced during a very short time span at 1705 ± 3 Ma and typically are undeformed and cut by pegmatite and granite.
The granite suite was emplaced over a longer time period between 1707 and 1692 Ma (mean age = 1696 ± 4 Ma; N = 15). The greatest concentration of granite is found in intrusive complexes, where the volume of granite can be high. Many granite bodies are sills, emplaced as crystal-rich mushes into previously-deformed schist and gneiss. The crystals orient with flow emplacement and causes "layering" in the granite and development of slightly mafic-enriched and felsic-enriched layers. Most of the fabric in the rock is related to flow during emplacement. A tectonic fabric was superimposed on some of these bodies after emplacement. Zircons within this group are more U rich (avg 1200 ppm), have lower Th/U (~0.2), some contain 1.70- to 1.9-Ga and minor late Archean inheritance, similar ages are found in the schist and gneiss they intrude.
The bimodal nature of this 1.7-Ga plutonism is extensional in nature, and not subduction-related. The lack of intermediate-composition bodies is strong evidence that subduction played no role in the development of these rocks. The 1705 Ma age and style of this plutonic rock is not found north of ~40°N latitude.