CORONA TEXTURES INVOLVING HERCYNITE AND HÖGBOMITE IN GEDRITE-CORDIERITE GNEISSES FROM EVERGREEN, COLORADO

Proterozoic gedrite-cordierite gneisses interlayered with hydrothermally altered sillimanite-biotite-garnet gneisses from Evergreen, Colorado, which have been metamorphosed to the upper amphibolite facies, contain aluminous enclaves consisting of sillimanite (sil), staurolite (st), corundum (crn), hercynite (hc), ilmenite (ilm), and högbomite (hg). A moat of cordierite, which separates the aluminous enclave from an intergrowth of gedrite (ged), cordierite (crd), quartz (qtz), and almandine garnet (gt) formed via the reaction: 6 sil + 2 ged + 7 qtz=5 crd + 2 H₂O. Garnet found only in contact with gedrite and as inclusions in cordierite may have formed by the reaction: 2 ged + sil + qtz=crd + gt + H₂O. Reactions between gedrite and sillimanite are also responsible for the development of a symplectic intergrowth between hercynite and cordierite and for the assemblage cordierite±hercynite±corundum±staurolite. Corroded corundum was a likely precursor phase to euhedral staurolite, in which it is occasionally found as inclusions. Högbomite (with up to 8.6 wt. % ZnO) occurs along the contact between ilmenite and hercynite (with up to 19.8 wt. % ZnO), in association with corundum, and formed by the reaction ilm + hc + 3 crn=hg. The corona textures were products of diffusion controlled-reactions that formed along a decompressive trajectory during uplift of the gneisses into the upper crust. Phase relations in the system ZnO-FeO-MgO-Al₂O₃-SiO₂-H₂O, garnet-cordierite thermometry, and textural relationships suggest that the aluminous enclaves formed at 630-665^oC and approximately 5 kb. Cross-cutting biotite textures and the garnet-biotite thermometer indicates subsequent reequilibration during a retrograde stage of metamorphism between 456^o and 473^oC.