GROWTH OF BIOTITE PORPHYROBLASTS DURING DOMINANTLY COAXIAL DEFORMATION AT LOW METAMORPHIC GRADES IN THE PEQUOP MOUNTAINS, NEVADA: CONTRADICTORY MICROSTRUCTURE AND THE EFFECTS OF SPATIAL AND TEMPORAL PARTITIONING OF STRAIN

Greenschist and amphibolite facies rocks in the Pequop Mountains contain a single prograde metamorphic foliation wherein biotite porphyroblasts in metapelites form a uniform lineation and overall have a shape and crystallographic preferred orientation suggesting they grew during a single deformational event. On the scale of the range, foliation varies in intensity and is spatially partitioned wrapping around low to no strain domains. Microscopically, microstructures from various rock types suggest deformation was dominantly coaxial. Comparison of biotite porphyroblasts from various samples taken throughout the range indicates they exhibit contradictory microstructure suggesting pre-, syn-, or post-tectonic growth. The following microstructures are observed in different areas: 1) strain caps mantling biotite porphyroblasts and associated quartz-filled strain shadows suggesting pre-tectonic growth; 2) growth of biotite in cleavage splits, strain shadows, and growth over arcuate strain caps suggesting syntectonic growth; and 3) growth of biotite over the foliation suggesting post-tectonic growth.

The presence of biotite porphyroblasts that exhibit pre-tectonic, syn-tectonic, and post-tectonic microstructure (or a combination thereof) in a terrain where strain is spatially partitioned implies that strain did not evolve synchronously everywhere but rather was focused on certain areas at a given time and then shifted focus to other areas perhaps in response to fluxing rheologic conditions. Such shifting of the locus of strain over time (temporal strain partitioning) during porphyroblast growth would explain the presence of pre-tectonic, syn-tectonic, and post-tectonic microstructure. These data imply 1) that evaluation of a small area may lead to an erroneous conclusion about the timing of growth of porphyroblasts relative to tectonics on a regional scale, and 2) temporal and spatial partitioning of strain during a progressive, dominantly coaxial, deformational event may result in the production, and diagnosis, of pre-, syn-, and post-tectonic porphyroblasts.