AMS REVEALS INCREMENTAL EMPLACEMENT AND FINAL CONSTRUCTION OF THE EJB PLUTON, EASTERN CALIFORNIA

The Jurassic Eureka Valley-Joshua Flat-Beer Creek (EJB) composite pluton in eastern California sits in a structural basin of early Cambrian sedimentary units which rotate downward abruptly within 2 km of the contact. New modeling and additional Anisotropy of Magnetic Susceptibility (AMS) data, combined with recent U-Pb zircon ages and new wall rock data has led us to model the emplacement of the pluton as incremental and which grew outward horizontally. Over 300 AMS sample locations (1,678 individual measurements) over the three magmatic units reveals remarkably consistent foliation and lineation orientations. The lineation plunges steeply to the NW in all three igneous units (Eureka Valley monzonite (EV), Joshua Flat quartz monzonite (JF), and Beer Creek granite (BC)) and in the wall rocks. The AMS foliation is subparallel to the wall-rock contact and cuts across the internal contact between the JF and BC. Susceptibility (Km) values decrease abruptly across compositional boundaries and are consistent with discrete pulses of separate magma batches defined by the present day internal contacts, although, anisotropy (P%) is highest on the margins and lowest in the interior. The JF records overall higher P% than the BC. The EV has a concordant U-Pb age of 180+2 Ma. The JF has five U-Pb ages between 172 and 175 Ma and the BC has five U-Pb ages between 168 and 172 Ma. U-Pb ages were determined from individual zircons by LA-ICP-MS. Three traverses across the aureole in three locations reveal shortening strains between 50 & 71% and regional structures are deflected by the aureole. The three igneous units vary in age over 10 million years, yet, the lineation is same for all three and is magmatic. We infer the lineation to be the transport direction for all three pulses of magma. The foliation is later than the internal contacts and therefore must record later deformation. We infer that the deformation is related to pulses of BC magma entering the chamber from below and forcing the older JF magma outward, which is consistent with the higher P% in the JF and higher P% values towards the margins. Therefore, the fabrics record the emplacement of individual parts and also the final construction of the entire pluton by outward growth.