Paleozoic gneissic granitoids in the Shoo Fly Complex, central Sierra Nevada, California

Abstract

Paleozoic gneissic granitoids are an important lithologic component of the Shoo Fly Complex of the central Sierra Nevada, California. Field relations and petrographic and geochemical studies indicate that the gneisses were intruded as a series of plutons ranging in composition from gabbro (oldest) to granite (median age) to syenite (youngest), with the granitoid types predominating. Compositionally they represent a calc-alkalic suite and a younger alkalic rock suite. The plutons and related smaller injections truncate an early S1 foliation in the Shoo Fly. During regional D2 and D3 deformational events, the granitoids were isoclinally folded and foliated under peak epidote-amphibolite grade conditions, and they were deformed into augen gneiss, blastomylonite, and ultramylonite. The gneisses contain porphyroclasts of feldspar, quartz, mica, and amphibole, and granitic aggregates, surrounded by an anastomosing foliation of quartzose ribbons and recrystallized quartz, feldspar, amphibole, mica, and stilpnomelane. Later deformational phases (D4–D7) typically involved open folding with the development of a spaced biotite schistosity (S4) and nonpenetrative cleavage (S5, S6, S7), and recrystallization of older fabric elements. Available geochronologic data of Sharp and others (1982) and H. K. Brueckner and W. D. Sharp (1985, personal communs.) suggest that the protoliths of the gneissic granitoids were intruded in Late Devonian time, thus establishing a pre–Late Devonian depositional age for the Shoo Fly Complex. D1 and intrusion of the orthogneiss protoliths may have been precursors of the Late Devonian to Early Mississippian Antler orogeny. Collectively, the gneisses may represent scattered remnants of a Paleozoic plutonic belt.