New Mexico Geological Society Annual Spring Meeting — Abstracts

The Garrel Peak (1665 Ma), Twin Mountain (1705 Ma), Crampton Mountain (1705 Ma) and Phantom Canyon (1666 Ma) plutons intrude early Proterozoic amphibolite-grade supracrustal rocks in the northern Wet Mountains and southern Front Range near Canon City,
Colorado. Contact zones are complex and range from 0.75 to 2 km wide. The Garrel Peak pluton is somewhat discordant and contains two distinct xenolith populations along its southwestern margin. The other three plutons have gradational contacts, which include late syn-to early post-tectonic granitic sills and dikes injected parallel to country rock foliation, numerous xenoliths, and migmatites. Granitization is locally important in country rocks adjacent to these pluton contacts, with it most pronounced in the Phantom Canyon pluton. Numerous folded and nonfolded granitic pegmatites and aplitic dikes crosscut pluton contacts and country rock foliation. All four plutons appear to be compositionally zoned with the Garrel Peak, Crampton Mountain and Twin Mountain plutons becoming more felsic and porphyritic toward their centers and the Phantom Canyon pluton becoming less felsic toward its center.

The Wet Mountains' plutons range from granodiorite to granite in composition and from peraluminous to slightly metaluminous. They have geochemical characteristics of I-type granitoids, such as Ta-Nb and Sr depletion on primitive mantle normalized plots, and are LREE-enriched with variable negative Eu anomalies. On Nb-Y, Ta-Yb and related trace element plots, the granitoids exhibit arc to within-plate characteristics. Ti, Y, Zr and REE contents are greater and Eu anomalies are larger in the two younger plutons. Comparison of major and trace element distributions shows the Garrel Peak pluton to be homogeneous whereas the other plutons are quite heterogeneous.

Of numerous models tested for the origin of the granitoid magmas, a two-stage model best accommodates geochemical data. During the first stage, granodiorite magmas are produced by partial melting of a felsic granulite source, probably at mid-crustal depths. Fractional crystallization of these magmas produces more evolved compositions with the two younger plutons more fractionated than the two older ones.