PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF IGNEOUS DIKES AND KEITHS DOME PLUTON IN THE GRASS LAKE AREA IN THE MOUNT TALLAC ROOF PENDANT; ELDORADO COUNTY, CALIFORNIA

Preliminary geochemical and petrological analysis of the Keiths Dome Pluton and intrusive dikes in the Tuttle Lake Formation shows evidence for low pressure fractionation of magmas from multiple magmatic systems. A previous study (Severson et. al 2014) of this area details the field relationships between the Keiths Dome pluton, igneous dikes, and the host metamorphic volcaniclastic sequence. The dikes were grouped by their field orientation into four sets (D1-D4, Severson et. al 2014). Our petrographic analysis shows that each of these groups contain a variety of rocks with distinct mineral assemblages suggesting they are not from the same magma. This is reflected in the chemical classification of the dikes that range from granodiorite, quartz monzodiorite, quartz monzonite, monzonite to syenite. Major element models show that the igneous rocks can be related through low pressure (1kilobar) fractionation over reasonable ranges of oxidation potential (NNO and QFM) and water concentrations (0-1%). Contrary to this, trace element systematics suggests fractionation of a single magma system is unlikely. Trace element concentrations show the dikes and pluton are rare earth element enriched and have evidence of enrichment of fluid mobile elements (Ba, K , and Th). This complex petrogenetic history suggested by the trace element systematics will be investigated further.