COLLISIONAL METAMORPHIC SIGNATURE IN THE SIERRA NEVADA, CALIFORNIA? HIGH-GRADE METAMORPHISM OF THE SHOO FLY COMPLEX

The Shoo Fly Complex, California is a subduction complex metamorphosed at lower green schist facies in the northern Sierra Nevada. Central Sierra Nevada exposures include higher grade assemblages, and published metamorphic ages are much older than the Sierra Nevada batholith. In the North Fork Mokelumne River drainage The Shoo Fly consists of mostly phyllites, mica schists (metastones) and cherts, with some metabasites. Metabasite assemblages range from fine-grained (to 0.2 mm) calcic amphibole with plagioclase (An29-34) and ilmenite (sample TCR2) to coarse-grained (1 mm or larger) calcic amphibole with clinopyroxene, plagioclase (An92-96 with calcic amphibole-rich layers, An34-39 in cpx-rich layers), and ilmenite, locally rimmed by titanite. Hornblende-plagioclase thermometry on TCR2 yields temperatures in the 600-680°C range at a nominal pressure of 0.5 GPa, whereas for TCR12 pairing with the plagioclase intergrown with amphibole yields unrealistically high temperatures (most pairs are >1000°C), whereas pairing of the amphibole with the more albitic feldspar in cpx-rich layers yields somewhat lower temperatures (680-785°C) but is difficult to justify on textural grounds. Amphibole thermobarometry from inclusion-free amphiboles in TCR12 yields T=790-825°C at P=0.75 to 1.1 Gpa. Such high pressures are consistent with occurrence of kyanite in nearby quartz-rich mica schists. TCR 2 amphibole compositions suggest temperatures ranging from 500-600°C with pressures ranging from 0.7 to a clearly unreaslistic 2.5 GPa. Overall the area appears to reflect a west-to-east increase in metamorphic grade over a distance of about 8 km. The metamorphism of accretionary wedge lithologies under high-temperature medium-to-high pressure conditions may reflect burial and subsequent heating of the subduction complex following subduction termination by collision or other transition. Collisions have been proposed as a part of the evolution of the Sierran basement, but subduction termination thermal signatures have not been documented.