CORONAS, SYMPLECTITIC TEXTURES, AND REACTIONS INVOLVING ALUMINOUS MINERALS IN GEDRITE - CORDIERITE - GARNET GNEISS FROM EVERGREEN, FRONT RANGE, COLORADO

Abstract

Gedrite - cordierite - garnet gneisses metamorphosed to the upper amphibolite facies near Evergreen, Colorado, are interlayered with sillimanite - biotite - garnet gneisses and contain spectacular corona, symplectite, and pseudomorph textures. Corona textures are characterized by a core of aluminous minerals, including zinc-bearing hercynite (hc), staurolite (st), corundum (cm), cordierite (crd), and relict sillimanite (sil), associated with ilmenite (ilm) and later-formed högbomite (hög). The aluminous minerals ("corona" association) are separated from gedrite (ged), cordierite, garnet (grt), and quartz (qtz) ("matrix" association) by a shell of cordierite such that they are nowhere in contact with gedrite and garnet. Zincian hercynite - cordierite symplectites occur within the inner part of the cordierite shell, whereas staurolite - corundum - cordierite and hercynite - cordierite pseudomorphs mimic the shape of prismatic sillimanite crystals. Textural studies and phase relationships in the system FeO - MgO - Al2O3 - SiO2 ± MnO ± ZnO ± TiO2 ± S2 suggest the following reactions, derived from gedrite + sillimanite: ged + sil = crd + grt; ged + sil + sphalerite (sph) = grt + crd + hc + S2; ged + sil + sph + qtz = grt + crd + hc + crn + S2; sil + hc = st + crn; grt + sil + crd = st; grt + sil + crd + hc = st; and hc + ilm + crn = hög. These reactions were developed in response to changing physicochemical conditions and enhanced by low diffusivity of aluminum and a low fluid-to-rock ratio. Reactions involving zincian hercynite were developed owing to the presence of sphalerite and preserved by the low diffusivity of aluminum. Högbomite formed later along the retrograde cooling path. Minimum peak conditions of metamorphism were 680°C and 5 kbar on the basis of the stability field of ged + sil + grt. Conditions recorded by the garnet - cordierite geothermometer indicate temperatures of 629-673°C at 5 kbar and 616-659°C at 2 kbar. On the basis of the slope of the garnet- and staurolite-forming reactions, coronas, symplectites, and pseudomorphs formed after peak metamorphism during a clockwise path associated with post-collisional decompression and subsequent cooling of the Yavapai-Mazatzal provinces, probably related with rapid exhumation.