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BAUER PW , WILLIAMS ML

PRECAMBRIAN RESEARCH

v. 67(#3-4) pp. 349-356 MAY 1994

Abstract:

The Magdalena granite (1654+/-1 Ma) cross-cuts strongly deformed supracrustal rocks (1664+/-3 Ma) that display a NE-striking foliation and well developed mineral lineation. The foliation is truncated, not deflected, by the pluton, and no spatial relationship exists between pluton margin and fabric intensity. Moderately annealed country rocks show evidence of dynamic recrystallization. Quartz displays dimensional and crystallographic preferred orientation and evidence for grain size reduction. The Magdalena granite is a coarse-grained, two feldspar biotite granite with a 15-m-wide, fine-grained border phase that is unfoliated and lacks deformational microstructures. Quartz crystals show some undulatory extinction, but no evidence of grain size reduction. Feldspars are euhedral and zoned. Although strain partitioning may explain the 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 mineralogy to the deformed supracrustal rocks. Instead, the pluton is interpreted 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 Mountains are comparable to NE-striking structures throughout central New Mexico. These structures are attributed to N-directed crustal shortening 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 orogeny of southeastem Arizona.

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