GEOCHEMICAL AND PETROLOGICAL CHARACTERIZATION OF MIGMATITE AND RESTITE IN THE WET MOUNTAINS, COLORADO
Gneissic rocks collected from the margins of the 1.44 Ga Oak Creek pluton record variable degrees of partial melting and melt extraction. Two samples from the northern margin of the pluton have distinct mineralogy, textures and major and trace element geochemisty, despite coming from the same outcrop. The first sample is a garnet-biotite gneiss. The second sample is a garnet-sillimanite-cordierite gneiss. The garnet biotite gneiss has trace element characteristics consistent with a sedimentary protolith, is rich in silica, and lacks aluminous minerals. The garnet-sillimanite-cordierite gneiss shows strong heavy rare earth element enrichment, has low silica, and contains abundant sillimanite and cordierite. Both rocks are highly oxidized, consistent with sedimentary protoliths for both.
The textures in both rocks are consistent with partial melting, and include differentiated gneissic layering, abundant perthite, high temperature deformation textures in feldspar and quartz, and mineral assemblages consistent with anataxis. We interpret the difference in mineralogy between the two samples to be the result of differing degrees of melt removal. The garnet-sillimanite-corderite gneiss is interpreted as a restite. Melt removal, with garnet in the residue, gives the sample its strong heavy rare earth element enrichment and allowed preservation of the peak mineral assemblage. The garnet biotite gneiss, in contrast, retained its melt. The absence of aluminous minerals is interpreted to result from back reaction between the peak solid mineral phases and melt phase. Back reaction appears to have been nearly complete leaving behind a garnet-biotite gneiss with trace element composition typical of metasedimentary rocks. Our results indicate that melt extraction can be heterogeneous at the scale of a single outcrop, and differing mineralogy does not imply different protoliths. Similar protoliths can produce distinctly different metamorphic rocks depending on degree of melt retention.