PETROLOGY AND GEOCHEMISTRY OF ENIGMATIC CA. 1.4 GA PLUTONS IN THE SOUTHERN FRONT RANGE OF COLORADO: IMPLICATIONS FOR PETROGENESIS AND TECTONIC SETTING

Granitic plutons of ca. 1.4 Ga age occur across much of North America, from the southwestern United States to Labrador. This study focuses on a group of these plutons in the southern Front Range of Colorado which have gone largely unstudied in terms of petrology and geochemistry. Major and trace elements were determined for 20 samples from the Elevenmile and Cripple Creek plutons and two other informally named pluton groups, the Firefly and Tarryall-Elkhorn. Textures and mineralogical features were first analyzed at hand specimen and thin section scale.

Petrographic and normative analyses show that most samples cluster in the monzogranite field near the minimum-melt-temperature composition for the granite system. Geochemical analysis shows that the samples are predominantly calc-alkaline or alkali–calcic, ferroan, and peraluminous, the latter predicted by modal muscovite and garnet. Two micas and myrmekitic intergrowths indicate hydrous magmas, as confirmed in the field by abundant pegmatites and hydrothermal alteration of sillimanitic schist in the wall rock.

The hydrous and peraluminous character of these plutons is consistent with S-type granites derived from the partial melting of metasedimentary rocks in thickened continental crust of collisional orogens. Concentrations of Nb, Y, Ta, Yb, Rb, Zr and Ga plotted on tectonic discrimination diagrams also suggest an orogenic, S-type petrogenesis. This contrasts strongly with plutons of similar age in northern and southern Colorado, which indicate the A-type features of anorogenic plutons produced in extensional tectonic regimes.

At 1.4 Ga these plutons were emplaced far inboard of the active southern margin of Laurentia, which had been expanded and thickened by arc accretion during the Yavapai and Mazatzal orogenies (1.71-1.60 Ga). Decompression melting during erosion of crust thickened by these previous contractional events may have been a factor in their genesis.