HIGH-PRESSURE, HIGH-TEMPERATURE MAFIC AMPHIBOLITE OF THE CENTRAL BELT OF THE SIERRA NEVADA MOUNTAINS NEAR COLFAX, CALIFORNIA

Framework rocks of the northern Sierra Nevada consist of five north-south trending accreted terranes that range in age from Ordovician to Jurassic. The second belt from the west is the Central Arc and Mélange Belt (Central Belt), which consists of serpentinite basement overlain by Triassic and Jurassic volcanic and sedimentary rocks. While mostly low grade, the Central Belt has scattered high-pressure (HP) and high-temperature (HT) amphibolite bodies present along its length. Near the town of Colfax, the Central Belt consists of the Clipper Gap unit, comprised of mostly chert and shale, and the Colfax unit, consisting of fine-to-coarse grained siliciclastic rocks. Both units contain olistoliths of various composition. Mapped between and within these sedimentary units are 200-to-600-m long bodies of mafic amphibolite that may correlate with high-pressure amphibolite units in the Central Belt to the south. While they have been mapped, these rocks have not been previously described in the literature.

To understand the composition, age, and nature of these rocks, we located and sampled five of the mapped amphibolite bodies within the Colfax and Clipper Gap units. Thin-section petrography was used to determine the mineralogy and metamorphic history of the samples. Trace-element geochemistry collected by ICP-MS was used to determine the composition of the rock and zircon was separated for U/Pb geochronology to determine the age of crystallization.

Petrographic observations show the amphibolite is composed of a peak assemblage of hornblende, plagioclase, epidote, and garnet, overprinted by retrograde greenschist-facies minerals. Rutile and ilmenite are present and overgrown by sphene. These minerals suggest the mafic protolith underwent HP and HT metamorphism, followed by decompression and cooling. The amphibolite and surrounding low-grade sedimentary rocks are not isofacial, leading us to suggest that the amphibolite was exposed to early HP and HT conditions, exhumed, then later incorporated into sediment making up the Colfax and Clipper Gap units. The amphibolite is similar to those found elsewhere in the Central Belt and likely preserve a similar tectonic history. The geochemistry and geochronology of these rocks will further help test this idea.