AUREOLE STRUCTURE OF 1.4 GA PLUTONS AND THEIR TECTONIC SIGNIFICANCE

Understanding the effects of tectonism associated with 1.4 Ga granites is critical to correctly deciphering the Proterozoic history of southwestern North America. Both the Eolus Granite of the Needle Mountains of southwestern Colorado and the Oak Creek Pluton of the Wet Mountains of south-central Colorado are 1.44 Ga granitic plutons intruded along the Mazatzal Front.

In the Needle Mountains, earlier fabrics are overprinted by an E-W striking, subvertical foliation defined primarily by contact metamorphic minerals which occurs at a high angle to the pluton’s eastern and western contacts. To the west, late F3 folds in quartzite have subvertical axial surfaces and fold axes with dominantly E-W oriented transcurrent shear zones. Late L3 lineations within the gneisses to the west are E-W and shallowly plunging. Mineral textures and quantitative pressure-temperature estimates record a metamorphic field gradient adjacent to the Eolus Granite from ~450°C at distance from the pluton, to ~675°C at the pluton’s contact at 300 MPa confining pressure.

In the Wet Mountains, lineations are variable with trends ranging over about 100°. Two main populations of lineations occur, one down dip and the other oblique and trending to the E-ESE. Kinematic indicators along the margins of the pluton consistently record pluton side down normal displacements, while those at distance record reverse motion presumably from earlier orogenesis. Quantitative metamorphic petrology and mineral assemblages indicate the Oak Creep Pluton intruded at ~900 MPa and 800°C during regional metamorphism.

The type of structures differ between the two terranes as a function of depth of burial however, both exhibit features consistent with NW-SE shortening and NE-SW extension. From a tectonic standpoint, a transform or transpressional boundary for the southern margin of Laurentia is consistent with the strain field. This structural style stands to be a potential analog for similar Low Pressure-High Temperature terranes where deformation is focused into a thermally softened plutonic aureole.