COUPLED DEFORMATION AND METAMORPHISM: DEFORMATIONAL CONTROLS on THE ALUMINOSILICATE PHASE TRANSITION

The Vallecito Conglomerate of the Needle Mountains, Colorado, is a Paleoproterozoic quartz pebble conglomerate with aluminous layers. The conglomerate was intruded by the Eolus granite at 1.42 Ga, producing a well-defined contact aureole defined by sillimanite close to the pluton, coexisting andalusite and sillimanite in a diffuse zone ~2 km from the pluton, and andalusite alone at greater distance from the pluton. Additionally, quartz microstructures vary systematically, defining a thermal gradient associated with the pluton. Andalusite-sillimanite bearing samples exhibit varying degrees of deformation. This deformation is strongly partitioned into discrete shear zones, which was sampled and studied in depth.

The sample collected contains a mineral assemblage of muscovite, chlorite, chloritoid, quartz, andalusite, sillimanite, rutile and ilmenite. Shear bands, defined by sillimanite and the absence of andalusite, separate low strain andalusite rich domains. WDS X-ray element maps of these sillimanite shear zones show that they are enriched in Al and Na and are depleted in K and Fe. Notably, micas also have distinct compositions in the shear zones and the intervening zones. Furthermore, oxide rich zones run parallel to the shear zones, but are absent from areas of high strain. The compositional and textural differences are consistent with both mass transfer and dehydration during the development of the shear zones. We conclude that the shear zones formed by two mechanisms: 1) initial pressure solution dissolving muscovite combined with slip along muscovite grain boundaries and, 2) dehydration of muscovite, producing sillimanite and localizing slip into sillimanite rich shear bands as mica was removed. These textures demonstrate localized ductile deformation, which is most likely related to emplacement of the Eolus granite as sillimanite is restricted to the contact aureole of the batholith. From these observations, we suggest that the mineral assemblages and textures observed record strain localization as the rocks were heated, indicating that the shear zones are prograde and accommodated strain during emplacement of Eolus granite.