New Mexico Geological Society Annual Spring Meeting — Abstracts

In multiply deformed, medium-to high-grade metamorphic rocks, tectonite fabrics (foliations and lineations) are commonly overprinted or obliterated by later generations of structures. However, in many rocks these earlier fabrics tend to be preserved as inclusion trails within and adjacent to porphyroblasts such as garnet, staurolite, andalusite, kyanite, and plagioclase. Petrographic analysis of such microstructures in and around porphyroblasts and megacrysts in metamorphic rocks provides a method of reconstructing the occurrence and relative timing of fabric-forming deformational events. Evidence for these early fabrics may be indistinguishable at larger scales of observation. In many rocks these porphyroblasts reveal consistent relationships between . mineral growth and deformation.

Early Proterozoic metasedimentary rocks in the picuris Range of northern New Mexico contain a variety of well-preserved porphyroblasts containing inclusion trails. Although unambiguous interpretations of the more complex microstructures in these minerals are rare, they do indicate a metamorphic/kinematic history that is more complicated than is apparent from outcropscale and map-scale structures. They also suggest that mineral growth and deformation are somehow interdependent, and that relatively short periods of time separated the major strain events. In the picuris Range, porphyroblast microstructures J make it possible to integrate anomalous large-scale structural and stratigraphic relationships into a coherent picture of a . progressive strain history.