285-6 The Nature of Jurassic Magmatism in the Sierra Nevada: A Comparison of Adjacent Plutons and Dike Swarms

Abstract:

The magmatic and tectonic history of the Jurassic arc, Sierra Nevada batholith (SNB), California, is poorly understood due to overprinting Cretaceous magmatism and deformation. This study presents age and petrologic data from Jurassic plutons and nearby dike swarms to examine timing and petrogenetic relationships and implications for the history of the Jurassic arc. In the eastern SNB, the dioritic-granodioritic King Creek and quartz-monzonitic Fish Creek plutons yielded U-Pb zircon ages of 167.7 ± 1.8 Ma and 158.8 ± 1.8 Ma, respectively. The NW-SE striking King Creek basalt-rhyolite dike swarm intruding the King Creek pluton yielded a U-Pb zircon age of 152.8 ± 2.8 Ma, indicating they all formed at slightly different times. In the Western Metamorphic belt (WMB), U-Pb zircon ages for the gabbroic-dioritic Standard (162.6 ± 2.3 Ma), dioritic-granodioritic Granite Creek (164 ± 2.5 Ma), and gabbroic diorite-monzodioritic Cobb Creek (163.72 ± 2.2 Ma) plutons overlap within uncertainty, indicating that they are coeval and may represent a ca. 700 km² magma system. The andesite-basalt, E-W striking Sonora dike swarm intrudes and mingles with a unit of the Standard pluton with an age of 159.44 ± 1.6 Ma and thus is interpreted to overlap in age.  

The WMB plutons and dikes show a Sr/Nd array ranging from depleted mantle to slightly more evolved compositions consistent with intrusion through accreted materials. In the King Creek area, the plutons and dikes have whole rock Sr/Nd isotopes indicating depleted mantle compositions consistent with intrusion through marine sediments and underlying oceanic basement. Geochemical data suggest that the rhyolite dikes formed from melts from remelting of the pluton.The plutons in both regions represent crystal cumulates inferred from the presence of plagioclase cumulates and the discrepancy between more mafic whole rock compositions and dacitic-rhyolitic melt compositions calculated from hornblende compositions.

 A broader comparison of published data on dikes in the eastern Sierra and White-Inyo Mountains (Independence dike swarm), the Snow Lake pendant, and in the Foothills terrane indicate the following: (1) dike ages vary across the arc, (2) dike chemistries match inferred basement types, (3) rhyolite dikes in swarms may be derived from melts from nearby plutons, and (4) dike swarm orientations vary from arc parallel to arc oblique, to arc perpendicular.

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