Strain induced volume loss during contact metamorphism: An example from southwest Colorado

The Vallecito Conglomerate of the Needle Mountains, Colorado, is a Paleoproterozoic quartz pebble conglomerate with interbedded layers of aluminous schist that both experienced deformation and metamorphism associated with emplacement of the ~1.435 Ga Eolus batholith. This contact metamorphism produced a well-defined contact aureole, defined by sillimanite near the pluton, andalusite and sillimanite together in a zone ~2 km from the pluton, and andalusite alone at greater distance from the pluton. Samples containing andalusite and sillimanite show a high degree of strain partitioning along discrete shear zones. Two samples, one recording high strain, the other recording low strain, contain identical mineral assemblages of muscovite, paragonite, chloritoid, quartz, andalusite, sillimanite, rutile and ilmenite, and were collected from the same outcrop. Based on the identical mineral assemblages and relative proximity of the two samples, it was assumed that high and low strain samples had a chemically similar protolith. An isoconcentration diagram was created using whole rock chemical analyses. This diagram shows that the highly strained sample is enriched in high field strength elements (HFSEs). Assuming that HFSEs are immobile, we interpret this result as evidence for depletion of the major elements from the highly strained sample. From the isoconcentration diagram, 50% of the major element mass was removed from the high strain sample. This observation combined with microstrustural analysis shows that solution mass transfer during prograde contact metamorphism was facilitated by localized deformation and resulted in mass loss, demonstrating a fundamental link between deformation, metamorphism, plutonism and mass balance within the middle crust.