New Mexico Geological Society Annual Spring Meeting — Abstracts

The Monte Largo shear zone (MLSZ) located in the Manzano Mountains, central New Mexico, has been recognized as a northeast trending ductile shear zone separating the greenschist grade Bluesprings schist from the overlying amphibolite grade Sevilleta metarhyolite. Detailed mapping in the Capilla Peak area shows that the simple boundary recognized in Monte Largo canyon is complicated by repetition of units, interlayered quartzites, and extensive strain partitioning. The MLSZ is expressed as a series of discrete shear zones occurring in the Bluesprings schist and along lithologic contacts. Kinematic indicators (e.g. asymmetric porphyroclasts, shear bands) within the MLSZ record an east-side up sense of shear, whereas kinematic indicators in the surrounding rock have a consistent west-side-up sense of shear suggesting that the MLSZ formed in a different deformational regime. ⁴⁰Ar/³⁹Ar analysis of nine amphiboles from amphibolites within the Sevilleta metarhyolite show complex age spectra. The complexities in the age spectra may reflect retrograde actinolite growth of the hornblendes. The age spectra from these separates yield both 1.4 and 1.6-1.7 Ga apparent ages; possibly dating the original amphibolite grade metamorphic event (ca. 1.6-1.7 Ga) and the retrograde (ca. 1.4 Ga) event. A biotite and muscovite from the area yield plateauages ~1400 Ma; suggesting that the region I cooled below ~300 °C at this time.

Taken together, the observed field relations, kinematic indicators, and ⁴⁰Ar/³⁹Ar age spectra may indicate amphibolite grade metamorphism and associated west-side-up shear occurred at ~1.6-1.7 Ga. The actinolite growth may be associated with the documented 1.4 Ga metamorphism in this area. The timing of east-side-up deformation within the MLSZ remains uncertain.