GEOLOGIC MAPPING AND U-Pb ZIRCON GEOCHRONOLOGY OF THE IRON MOUNTAIN PENDANT, SIERRA NEVADA, CA TO EVALUATE REGIONAL TECTONOSTRATIGRAPHIC RELATIONSHIPS

Detailed mapping of the Iron Mountain Pendant (IMP), Sierra Nevada, CA, is crucial to establish the tectonostratigraphy of the pendant and determine its origin in relation to other pendants of the region. The IMP trends NW along strike of previously established Kings Sequence locations which represents the base of a Mesozoic continent-derived sedimentary sequence and could trace the edge of the North American continent (Saleeby and Busby, 1993).

Our new 1:10,000 scale geologic map is used to evaluate the connections between IMP and nearby pendants, building on USGS quadrangle mapping of Bateman (1989), Huber (1969) and Peck (1980; 2002). The new map provides a cohesive, more detailed representation of the IMP and surrounding plutons, and provides U-Pb zircon LA-ICP-MS geochronology constraints on the depositional history.

The IMP is composed of a metasedimentary package in fault contact with an overlying metavolcanic package. The metasedimentary units are layered quartzites and phyllites, interpreted to be of marine origin, which are complexly folded. Low zircon yields produced grain ages of 1140, 1000, 550, and 157 Ma. The foliated metasedimentary rocks are steeply dipping and are juxtaposed against shallowly bedded metavolcanic units. The metavolcanic package consists of basal metaandesite debris flow deposits and overlying lava flow unit with plagioclase and hornblende phenocrysts and an upper metarhyolite unit that is a fine-grained, layered lava flow with interbedded clastic layers. Flow-banding marks contacts within the volcanic sequence. The metaandesite yielded four zircon grains with weighted mean age of 155.6 +/- 3.58 Ma. The metarhyolite yielded zircons from two samples and produced weighted mean ages of 123.0 +/- 0.62 Ma (n= 27 zircons) and 124.6 +/- 0.83 Ma (n= 13 zircons). The pendant was intruded by 120-104 Ma plutons that were emplaced during the Cretaceous flare-up. The pendant rocks were metamorphosed to greenschist facies, based on presence of distinctive minerals including epidote and chlorite, and experienced localized pervasive fluid alteration. Based on initial results, the IMP is potentially a part of the Kings Sequence. Ongoing geochronological and geochemical studies of these rocks will aim to clarify their source and significance in the formation of the Sierra Nevada.