GEOCHEMISTRY AND GEOCHRONOLOGY OF A PRECAMBRIAN PLAGIOGRANITE IN THE SIERRA CITY MELANGE, NORTHERN SIERRA NEVADA, CALIFORNIA

The easternmost belt of accreted rocks in the Sierra Nevada of Northern California is the Northern Sierra Terrane, which consists of the Shoo Fly Complex along with the overlying Paleozoic volcanic arc rocks. The structurally-highest unit of the Shoo Fly Complex is the Sierra City Mélange (SCM), which is composed of blocks of serpentinite, gabbro, basalt, felsic tuff, limestone, and chert in a sandstone-serpentinite matrix mélange. Based on fossils in the limestone blocks as well as U-Pb dating of the felsic tuff, the SCM is considered Ordovician to Silurian in age. However, a previously published age of 612 土 29 Ma on a plagiogranite using U-Pb zircon geochronology significantly predates the other rocks in the unit. While the plagiogranite has been dated, it has not previously been described. Here, we use petrography and geochemistry to characterize the plagiogranite and modern geochronology to supplement the published age.

Samples of the plagiogranite were collected for thin-section petrography, geochronology, and major- and trace-element geochemistry. Whole-rock geochemistry was performed by a commercial laboratory with ICP-ES and ICP-MS methods to determine the composition of the rock. Zircon was separated using standard heavy mineral separation techniques then analyzed via U-Pb geochronology on a LA-ICP-MS to understand possible inherited age components.

Petrography shows the rock is composed of quartz, plagioclase, mafic minerals that have been altered to chlorite, and opaques. Plagioclase is altered to unknown fine-grained minerals. The modal mineral composition makes this a tonalite. Major-element geochemistry shows that the rock is about SiO2 71%, Al203 15%, Na2O 8%, and Fe2O3 1%, with MgO, CaO, and K2O under 1%, making this a magnesian, peraluminous, alkali-calic granitoid. Trace-element geochemistry shows depletion of all elements compared to typical continental granites.