New Mexico Geological Society Annual Spring Meeting — Abstracts

The Sandia Granite is one of a series of 1.4 Ga granitoid plutons exposed in ranges bounding the Rio Grande Rift. The pluton cores the Sandia Mountains and crops out over 300 km². Recent structural work suggests that the pluton was emplaced syntectonically with respect to a transtensional shear zone on its southeastern side. The shear zone is exposed just north of Tijeras Canyon; it is 1-2 km wide, strikes 030°, and dips 45° NW under the pluton. We thus infer it to represent a deformed base or lower side of the pluton.

A NW - SE transect across the zone consists of 1) undeformed Sandia plutonmegacrystic monzogranite with a locally developed magmatic foliation; 2) sheared Sandia pluton -a mylonitic, augen orthogneiss whose matrix is depleted in quartz and K-feldspar and enriched in biotite relative to the undeformed pluton; 3) Cibola granite -leucocratic, fine to coarse grained, equigranular granite that intrudes laterally continuous screens of quartzite. Contacts between the lithologies are irregular and, in many places, diffuse. The shear zone is truncated to the SE by the Tijeras fault, a Phanerozoic brittle fault which juxtaposes the Cibola granite with the unsheared amphibolites, pelitic schists, and quartzites of the Tijeras greenstone.

Kinematic indicators (asymmetric porphyroclasts, S-C fabrics) in the shear zone suggest top to the north, extensional movement with inferred slip parallel to an average stretching lineation plunging 40° to 330°. Small-scale synthetic shear bands often transpose foliation into C-C' relationships, while antithetic shear bands appear to have accommodated back-rotation of discrete blocks. Field and microstructural relationships suggest that pluton crystallization was synchronous with deformation in the shear zone. Evidence includes: 1) fine grained melt injections of leucocratic Cibola granite fill Sand C planes and shear bands in mylonitic orthogneiss; 2) a general correlation between degree of deformation and intrusion sequence -the deformed host Sandia pluton is cross-cut by progressively less deformed Cibola granite, aplites, and pegmatites; 3) undeformed pegmatite dikes which cross-cut early shear fabrics.

Shear zone movement was cleady synchronous with pluton emplacement and produced impressive deformation in the margin of the pluton. In light of this, we suggest that the Sandia pluton is not "anorogenic" in the strictest sense of the word and that other 1.4 Ga plutons should be examined for similar deformation. Further work is required, however, to constrain whether deformation was related to pluton emplacement or to regional deformation.