New Mexico Geological Society Annual Spring Meeting — Abstracts

The 1.4 Ga. Sandia Granite is exposed in the Sandia Mountains of Central New Mexico. The southern margin of the pluton is bounded by a transtensional ductile shear zone, here termed the Seven Springs Shear Zone (SSSZ). The plutons northern margin is an intrusive contact with supracrustal rocks. Both margins of the pluton show evidence of synchronous deformation, magmatism and metamorphism.

Supracrustal rocks of the pluton's northern margin record a complex structural and metamorphic history involving at least three generations of deformation. The D1 deformation is recorded by compositional layering, which strikes E-W to NW. D1 fabrics are overprinted by D2 deformation and contact metamorphism. Textures which support the interpretation that contact metamorphism is synchronous with deformation include: 1) contact metamorphic andalusite that overgrows S1 foliation and is aligned in S2 foliation; 2) sillimanite that is lineated parallel to L₂ extension lineations which are oriented 45 to 030; 3) shear bands and S-C fabrics that are filled with pegmatite and aplite dikes. Orientations of shear bands and S-C fabrics are consistent with top to NE extension.

Deformation in the SSSZ is interpreted to have promoted segregation of melt fractions during crystallization of the Sandia Granite as indicated by the distribution of diverse compositional units. Granitic rocks can be divided on the basis of mineralogy and textures in the field. Coarse grained biotite-rich granites (depleted Sandia Granite) are concentrated in the NW margin of the SSSZ, while fine-grained granites (interpreted to be segregated melts) are concentrated and SE of the SSSZ. Granites in the central portion of the SSSZ show complex intrusive relationships between fine grained leucogranites and coarse grained biotite granites. Leucogranites cross cut coarse grained biotite granites as dikes, shear bands and C planes in S-C fabrics. These field relations suggest that the Cibola Gneiss (here named the Cibola Granite) is in fact comagmatic with the Sandia Granite. This interpretation is supported tentatively by geochemical data. Major elements show a linear fractionation trend between the Sandia Granite and the Cibola Granite.