EMPLACEMENT MECHANISMS OF THE 1.4 GA CURECANTI PLUTON, GUNNISON, COLORADO

The 1.4 Ga Curecanti pluton, exposed in the Black Canyon of the Gunnison, Colorado, is a 6-km-long discordant sheet of undeformed fine-grained quartz monzonite. The pluton was emplaced as an east-west-trending tabular body into amphibolite facies quartzofeldspathic gneiss, schist, and migmatite. Its floor dips 10-25º to the northeast, its roof undulates between subhorizontal and vertical as it tapers towards the intrusion's margins, and nearly all contacts are highly discordant with the steeply dipping foliation in the host rock. Floor and roof contacts are characterized by an agmatic zone where wall rocks are injected with leucosome along preexisting foliations, leading to paleosomes entirely surrounded by leucosome, some of which are then stoped into the main body of the Curecanti and digested. Numerous elongate stocks and dikes of Curecanti quartz monzonite contain a weak solid-state fabric generally concordant with the host rock foliations. A pervasive set of east-west vertically dipping pegmatites and aplite dikes cut the Curecanti pluton and are interpreted as recording the last stage of crystallization. Original Rb-Sr data suggest that the main body of Curecanti quartz monzonite was emplaced at 1420±15 Ma. The Curecanti pluton was previously interpreted as a laccolith which lifted its roof rocks, but new field observations and structural data indicate that the pluton's emplacement was also facilitated by other processes, including extensive brecciation and assimilation of wall rocks. Field observations suggest that the emplacement depth and geometry of the Curecanti pluton was likely a function of the magma's driving pressure, rather than being controlled by a crustal anisotropy such as a shear zone. The consistent orientation of the pluton and dikes suggests that emplacement may have been controlled by regional deformation at 1.4 Ga despite the lack of solid-state fabric development in the main pluton. Further investigation will help to determine how this fine-grained body relates to the regional suite of megacrystic 1.4 Ga granites, providing new insights into the tectonic setting of 1.4 Ga magmatism throughout southern Laurentia.