干旱区生态环境知识资源中心

The 148 Ma Independence dike swarm is a prominent feature of the Jurassic Cordilleran arc, extending >600 km from the eastern Sierra Nevada to the Mojave Desert, California. The swarm is fundamentally mafic in composition (<55 wt% SiO2), although dikes range in composition from basalt to rhyolite. Many dikes in the swarm are composite and contain multiple subparallel sheets or abundant enclaves. Whereas most Sierran composite dikes contain only mafic intrusions, some contain both mafic and felsic sheets. In more southerly portions of the swarm (the Spangler Hills and Granite and Fry Mountains), composite dikes rarely contain subparallel intrusions but instead contain abundant enclaves that locally comprise >50 vol% of a dike. Compositional variability in the Independence swarm as a whole may be correlated with physical characteristics of composite dikes. In the Sierra, where composite dikes show little evidence for interaction between mafic and felsic magmas, compositions are bimodally distributed. In contrast, in the south, where composite dikes are characteristically enclave-rich, intermediate-composition dikes are more common. Elemental and isotopic data for the Independence dikes are consistent with chemical controls on mixing processes. The source for the mafic dikes has a consistent epsilon(Nd)(t) value of similar to-2, independent of location. This probably reflects derivation from a widespread, isotopically homogeneous source rather than lateral intrusion of the dikes over a great distance from a single source. The isotopic data for the dike swarm as a whole arepart of a long-term trend of decreasing isotopic variability over a broad range of bulk composition in the Jurassic through Cretaceous Sierran batholith.

Mylonitic shear zones and limited geobarometric data suggest that Sierran dikes represent deeper levels of exposure than dikes in the Mojave Desert, where host rocks are not mylonitized. If dikes along the swarm tapped magmas emplaced at similar paleodepths, then variations in composite dike features and dike compositions along the swarm may reflect different degrees of mixing vertically within dike conduits.