THE AGE OF PROTEROZOIC OROGENESIS IN NEW-MEXICO, USA

The Magdalena granite (1654+/-1 Ma) cross-cuts strongly deformed supra crustal rocks (1664+/-3 Ma) that display a NE-striking foliation and w ell developed mineral lineation. The foliation is truncated, not defle cted, by the pluton, and no spatial relationship exists between pluton margin and fabric intensity. Moderately annealed country rocks show e vidence of dynamic recrystallization. Quartz displays dimensional and crystallographic preferred orientation and evidence for grain size red uction. The Magdalena granite is a coarse-grained, two feldspar biotit e granite with a 15-m-wide, fine-grained border phase that is unfoliat ed and lacks deformational microstructures. Quartz crystals show some undulatory extinction, but no evidence of grain size reduction. Feldsp ars are euhedral and zoned. Although strain partitioning may explain t he lack of deformational fabric in the granite, it can not explain the lack of fabric in the border phase, which is similar in texture and m ineralogy to the deformed supracrustal rocks. Instead, the pluton is i nterpreted to have been passively emplaced subsequent to or during the final phases of regional deformation, and thus, deformation occurred between 1664 and 1654 Ma. Deformational features in the Magdalena Moun tains are comparable to NE-striking structures throughout central New Mexico. These structures are attributed to N-directed crustal shorteni ng during Proterozoic accretion. This indicates that a major orogenic event occurred in central New Mexico at approximately 1.66 Ga, similar in time and style with the approximately 1.66-1.65 Ga Mazatzal orogen y of southeastem Arizona.