SPATIALLY RESTRICTED DEFORMATION AROUND THE 1.4 GA EOLUS GRANITE, NEEDLE MOUNTAINS, COLORADO: CONSTRAINTS ON REGIONAL DEFORMATION FIELDS AT 1.4 GA

Understanding the strain field around 1.4 Ga Plutons is critical to correctly deciphering the processes responsible for the generation of this enigmatic magmatic belt. Here we present structural data from the northeast fringe of the 1440 Ma Eolus Granite in the Needle Mountains of SW Colorado which record regional deformation fields in a thermally softened aureole.

The eastern side of the Eolus granite in the Vallecito River valley is north striking and subvertical, concordant with the bulk of fabrics in country rock gneisses. However, near the plutons margin domains of migmatic gneisses occur which contain an east-west striking, steeply dipping foliation. This foliation is defined by migmatitic banding and sillimanite, both spatially restricted to the aureole of the pluton. These relationships suggest that this foliation formed during contact metamorphism at 1.4 Ga. Furthermore, since this fabric intersects the pluton contact at a high angle, it cannot have been produced by expansion of the pluton.

At Sunlight Creek, the orientation of the margin of the pluton changes from north striking to east-west striking with a steep southern dip. Quartzites at the contact of the pluton are isoclinally folded as indicated by reversals in facing direction recorded by cross-bedding along this margin. In schist layers, a crenulation cleavage is present which is east-west striking and sub-vertical similar to that seen in migmatite farther south. Thin leucosomes developed in these schists are intruded into shallowly dipping vein arrays, suggesting vertical incremental extension. Sillimanite defined stretching lineations are down dip, consistent with the observations from the veins.

Taken together, the fabrics within the contact aureole record approximately north-south shortening during pluton emplacement. The apparent restriction of these fabrics to the aureole of the pluton combined with the lack of pervasive solid state fabrics in the interior of the pluton suggests the regionally imposed deviatoric stresses may have been small. This in turn may have lead to focusing of deformation into the aureoles of 1.4 Ga plutons.