Description
In the northern Sierra Nevada, California, at about the latitude of Lake Tahoe, rocks that predate the emplacement of the Cretaceous Sierra Nevada batholith are commonly subdivided into four belts. From west to east the belts are: the Smartville, central, Feather River peridotite, and eastern belts. Cretaceous and younger sediments form the western boundary of the Smartville belt, while the other belts are separated by the Foothills fault system. The eastern belt includes the quartz-rich largely sedimentary rocks of the pre-Late Devonian Shoo Fly Complex, and the remnants of Late Devonian through Permian, and Early to Middle Jurassic arc rocks. The north-to-northwest trending Foothills fault system and related folds and cleavages have been attributed to the Late Jurassic Nevadan orogeny. However, recent work suggests that some of the north to northwest trending structural fabric within the pre-Cretaceous rocks of the four belts may not have formed during the Late Jurassic Nevadan orogeny as previously thought, but instead may have formed between Early and Middle Jurassic time. The Middle Jurassic Emigrant Gap composite pluton, which crops out in the eastern belt, is composed of four major units: hornblende-biotite granodiorite, two-pyroxene diorite, two-pyroxene gabbro, olivine websterite. Geochemical data suggests that the Emigrant Gap composite pluton is: (1) composed of four major units with distinct chemistries (2) calcic according to Peacock (1931); (3) cafemic according to Debon and Lefort (1983) which implies a mantle source or a mixed sialic-mantle hybrid source; and (4) a granitoid emplaced within a volcanic arc. To quantitatively estimate the physical conditions during emplacement of the pluton, microprobe work was completed on eleven samples from the hornblende-biotite granodiorite unit. Pressures across the pluton range from a low of 1.3 kbar to a high of 2.7 kbar with an average pressure of 2.1 ± 0.3 kbar. Temperature varied across the pluton from a low of 602°C to a high of 686°C, with an average of 646 ± 19°C. The Al-in-hornblende barometry and amphibole-plagioclase thermometry is supported by data collected from the contact metamorphic aureole of the Emigrant Gap composite pluton which suggest pressures of ~2.2 kbar and temperatures of ~650 °C. The Middle Jurassic Emigrant Gap composite pluton intrudes a wide variety of rocks which contain a domainal, locally penetrative cleavage that trends ~N20W, and dips ~72 NE. This northwest trending cleavage is deflected into parallelism with the northwestern margin of the pluton and is truncated by the pluton in the southeast. These relationships along with the fact that the Emigrant Gap composite pluton is unstrained suggest that the northwest trending structural fabric in the eastern belt was not produced by the Late Jurassic Nevadan orogeny, but must have formed prior to the emplacement of the Middle Jurassic Emigrant Gap composite pluton.
