Tectonic setting of the Sandia pluton: An orogenic 1.4 Ga granite in New Mexico

Structural studies of the circa 1.42 Ga Sandia pluton and its aureole document significant deformation synchronous with pluton emplacement and call into question the “anorogenic” label associated with this and other 1.4 Ga granites in the southwestern United States. The SE margin of the pluton is a 1‐ to 2‐km‐wide NW dipping ductile shear zone. Field and microstructural observations (melt‐filled shear bands, high‐temperature dynamic recrystallization of K‐feldspar megacrysts, and crosscutting pegmatite dikes) indicate that top‐to‐the‐NW (normal) movement in the shear zone took place in the presence of melt. Subparallel magmatic fabrics north of and structurally above the shear zone contain kinematic indicators consistent with top‐to‐the‐NW shear sense, suggesting that over large regions of the pluton, magmatic flow mimicked solid‐state strain. In the northern aureole, contact metamorphic aluminosilicate porphyroblasts grew during the formation of a NE striking crenulation cleavage (S₃) and related folds of late‐stage pegmatite dikes. These features document the synchroneity of magma emplacement, shortening, and metamorphism and indicate that the Sandia pluton is syntectonic, not anorogenic. We interpret the kinematic consistency of structural elements from the base of the pluton, the interior of the pluton, and the northern aureole to reflect a regional (larger than the pluton) strain field and suggest that the “orogeny” recorded in and around the Sandia pluton involved a three dimensional strain field with subhorizontal extension (N–S) and contraction (E–W) directions. N–S extension is documented by the orientation of mineral lineations and movement directions in the basal shear zone and in high‐strain zones in the northern aureole and by the orientations of tabular pegmatite and aplite dikes in the pluton and aureole. East to SE shortening is documented in the northern aureole by orientations of folded pegmatite dikes and associated S₃ crenulation cleavage, and east to SE shortening (or least extension) directions in the pluton proper are documented by the intersections of orthogonal dikes. Thus emplacement of the Sandia pluton is interpreted to record a snapshot of regional strains inboard of an active plate margin, rather than local strains generated by emplacement.