EXPLORING PALEOMAGNETISM AS A TOOL FOR RESOLVING THE TIMING OF IGNEOUS INTRUSIONS IN THE MOUNT TALLAC METAMORPHIC ROOF PENDANT; DESOLATION WILDERNESS AREA, ELDORADO NATIONAL FOREST, CA

An important chapter in the history of the subduction-related volcanic arc along western North America is recorded in the metamorphic roof pendants in the Sierra Nevada. Among these remnants, the Mt Tallac pendant is the largest and most well exposed. Outcrops near Grass Lake in the southern Mt Tallac pendant include the polymict, matrix-supported volcanic breccia of the Tuttle Lake Fm, which is interpreted as Middle Jurassic (166-163 Ma) based upon fossils in the underlying Sailor Canyon Fm. The Tuttle Lake Fm is first cut by at least five sets of intermediate dikes and finally by the Keiths Dome quartz monzonite pluton. While not directly dated, the Keiths Dome pluton is estimated to be Late Jurassic (158-148 Ma) based upon contact relationships elsewhere. Taken altogether, the relationships at Grass Lake suggest that in as little as 10-20 Ma, the area underwent an incredibly rapid geologic evolution from the deposition of debris flows in intra-arc submarine basins, to the intrusion of hypabyssal networks of dikes feeding stratovolcanoes, and ultimately the intrusions of the plutons supplying magma to the arc. In an attempt to further resolve the timing of events in the Mt Tallac pendant, we present results of a paleomagnetic analysis of the youngest of the Grass Lake dikes. Seven samples were collected at ~20 m intervals along the length of a single porphyritic andesite dike. The dike strikes perpendicular to and is cut by the contact with the Keith’s Dome pluton, which provides an opportunity to determine if the thermal pulse from plutonism altered and/or reset the primary thermal magnetization carried by the dikes. Analysis of mean coercivities and dispersions suggests the samples contain magnetite, hematite, and goethite. Analysis also suggests that six of the seven samples contain single domain magnetite (Mr/Ms ~ 0.5, Hr/Hc 1-2) while one is dominated by single domain hematite (Mr/Ms 0.5-0.7, Hr/Hc 1.45-1.62). Six of seven sites responded well to alternating field demagnetization up to 200 mT, all of which display one or two component systems. The high coercivity component for individual sites are well clustered (a₉₅= 4-13˚), with no statistically significant variation in site mean orientation as a function distance from the pluton contact. Orientations are consistent with emplacement in the Late Jurassic (158-146 Ma).