THE TIMING AND MAGMA SOURCE OF THE SONORA DIKE SWARM AND STANDARD PLUTON, SONORA, CALIFORNIA, AND COMPARISON TO OTHER JURASSIC DIKE SWARMS IN THE SIERRA NEVADA BATHOLITH

The Sierra Nevada Batholith (SNB) contains several swarms of late-Jurassic dikes and small mafic intrusions. Among these is the Independence dike swarm (IDS), a ca. 148 Ma dike swarm that is found in the eastern SNB and to the east in the Inyo Mountains and the Mojave Desert with a wide range of lithologies and isotopic values ranging Sr_i= 0.705324-0.710445 and εNd= -9.74 to -1.18 (Glazner et al., 2008). Dikes with bimodal compositions intruding the central SNB King Creek pluton are slightly older and much more primitive than the IDS. They have a LA-ICPMS U/Pb zircon age of 152.5±2.7 Ma and isotopic values of Sr_i= 0.70465-0.70470 and εNd= 5.40 to 7.63 (Wesley et al., 2019).

In the western SNB, the basaltic-andesitic Sonora dike swarm is less well-known and understood. It intrudes metasedimentary host rocks and the granodioritic to dioritic Standard pluton. Field observations and samples were collected to determine the timing and source of the Sonora dike swarm and its relation to other dike swarms in the SNB. Mafic dikes intruding the Standard pluton show a mingling relationship with the host magma. The Standard pluton revealed a LA-ICPMS U/Pb zircon age of 162.27±0.36 Ma. Isotopic analysis of three Sonora dikes and the Standard pluton range from Sr_i= 0.703701-0.705657 and εNd= -1.49 to 6.83. Based on the dike and pluton mingling relationships in the field, we conclude that the Sonora dikes and Standard pluton are coeval and thus that the Sonora dike swarm was emplaced before the central SNB King Creek dikes and the IDS. Additionally, isotopic analysis revealed that the magma source for the Sonora dikes and King Creek dikes as well as the associated Standard and King Creek plutons, respectively, is depleted mantle with minor crustal or enriched mantle input, while the source for the IDS is more evolved. The geographic location of these three Jurassic dike swarms and associated intrusions and their isotopic affinities indicate that the Sonora dikes in the northwestern SNB and King Creek dikes in the central SNB were sourced from asthenospheric mantle, while the IDS in the southeastern SNB and to the east and southeast thereof likely tapped a lithospheric mantle and/or a crustal source. Finally, the three dike swarms expand the Jurassic extensional period in the SNB to at least episodic (spatially and temporally) events ranging from 162-148 Ma.