A Sillimanite Gneiss Dome in the Yukon Crystalline Terrane, East-central Alaska

Résumé du livre

A topographically high, nearly circular area of sillimanite gneiss crops out over approximately 600 km2 in the Big Delta quadrangle, Yukon crystalline terrane, east-central Alaska. Foliation is generally horizontal or subhorizontal in the center of the gneiss body and dips outward in all directions. Felsic dikes, some of which are pegmatitic, occur in the central area of gneiss. Quartzite and marble are locally infolded in the gneiss, particularly near its margins. The gneiss is partly bordered on the north and east by pelitic schist interlayered with lesser amounts of quartzite, marble, and amphibole schist. The ages of the protoliths of these rocks are believed to be Paleozoic or possibly Precambrian; major metamorphism probably took place between Mississippian and Middle Triassic time. Two isograds within the pelitic schist are concentric to the gneiss body and indicate an increase in metamorphic grade toward the gneiss. The outermost isograd is defined by the appearance of staurolite with biotite, at the expense of chlorite and muscovite. Upgrade of this isograd, several combinations of the aluminum silicate polymorphs (andalusite, kyanite, and sillimanite)+garnet+staurolite are present in quartz-biotite-muscovite schist. Textural relations suggest that these three polymorphs all formed during the same prograde metamorphic event in which pressure-temperature conditions were near the triple point. The innermost isograd, defined by the disappearance of staurolite, closely coincides with the schist-gneiss contact. Increase in metamorphic grade in the gneiss (quartz+biotite+sillimanite+orthoclase+plagioclase± muscovite± garnet and locally quartz+biotite+sillimanite+cordierite+orthoclase +plagioclase +muscovite) is indicated by an increase in modal orthoclase and decrease in muscovite toward the center of the body, consistent with the breakdown of muscovite+quartz. Migmatitic textures, although rare, may be evidence of local partial melting of the gneiss. The relative distribution of Fe and Mg between garnet and biotite indicates equilibration at about 535° to 600°±30°C for pelitic schist north of the gneiss body and 655° to 705°±30°C for sillimanite gneiss. Petrographic data from poorly exposed rocks south and west of the gneiss also suggest an increase in pressure and (or) temperature conditions toward the gneiss body; such an increase is indicated by an isograd which separates andalusite-sillimanite-bearing schist and gneiss to the southwest from sillimanite gneiss to the northeast. Evidence that the sillimanite gneiss is a dome consists of: (1) the topographic expression and radial drainage pattern, (2) the outward dip of foliation, (3) an increase in metamorphic grade toward the center of the body, and (4) an increase in garnet-biotite temperatures inward. Minor intrusion of synmetamorphic dikes or small granitic bodies is suggested by the presence of equigranular, aluminum-silicate-bearing rocks at several localities in the proposed dome which contain textural features indicative of both igneous and metamorphic processes. The staurolite-out and andalusite-sillimanite isograds define the northern and eastern and the southwestern margins, respectively, of the proposed gneiss dome.