FABRIC DEVELOPMENT AND INDEX MINERAL GROWTH IN THE PEQUOP MTS., NEVADA

Biotite and garnet zone metapelites in the Pequop Mountains, Nevada, provide an opportunity to study details of fabric development and index mineral growth not commonly seen in higher-grade rocks.  This study focuses on the Cambrian Dunderberg Shale, metamorphosed by thrust burial during the Sevier orogeny.  The biotite-in isograd crosses this north-south trending unit, allowing observation of the first appearance of biotite and its textural relationship to the dominant foliation (S₂), which becomes more pronounced with increasing metamorphic grade.  In the highest grade rocks S₂ is the only foliation visible in outcrop and parallels compositional layering.  In thin section this foliation is a homogeneous schistosity defined by parallel alignment of muscovite and chlorite.  No evidence of any prior foliation (S₁) exists save as inclusion trails within biotite porphyroblasts.  The biotite porphyroblasts are over 0.5mm across and are mostly prekinematic to S₂; however, some biotite rim and tail growth was syn- to post-kinematic to S₂.  In our interpretation S₁ formed during thrust burial, biotite porphyrblasts grew during thermal relaxation, and S₂ formed during crustal extension.  In biotite porphyroblast-bearing rocks near the biotite-in isograd S₂ is a more poorly developed spaced cleavage, and S₁ is discernable in quartz domains within the matrix.  Biotite porphyroblasts contain inclusion trails of S₁ that is at a higher angle to S₂ than it is in the matrix, indicating that progressive development of S₂ after biotite growth rotated matrix S₁ in quartz domains.  In the lowest grade rocks (biotite absent) both S₁ and S₂ are measurable in outcrop, and S₂ in thin section is a closely spaced crenulation cleavage crosscutting S₁ at a high angle.  With increasing metamorphic grade rock texture coarsens, the angle between S₁ and S₂ decreases, the spacing of S₂ increases (until it becomes homogeneous), and S₁ becomes progressively obliterated in the matrix.