Some Reflections On The Origin of the Sierra Nevada Batholith

Due to excellent exposures the Sierra Nevada Batholith has become a type-example of tectonic emplacement and lithosphere delamination: granitic plutons, generated from amphibole-saturated and modestly evolved basalt magmas, are guided by dextral shear zones and releasing bends; their mafic cumulates are plausibly lost to the underlying mantle. Elastic strains from InSAR also reveal modern volume creation rates of 6,000-187,000 km 3 /Ma—the room problem appears solved. But uncertainties outweigh the advances. Some plutons are cumulates but many resemble Cascade volcanics, and some granites approximate liquids. Plutons perhaps result when stress states are unfavorable to eruption. Even less understood is what happens between and below the guiding shear zones. Magma batches could be as large as 2-4 km at the Tuolumne Intrusive Suite, but scant attention has been given to the more likely paleo-examples of the large magmatic bodies imaged beneath modern arcs. Melt aggregation, and pre-100 Ma transport also remain unclear. And crustal components are ill-defined despite there being many tools and targets. Studies could profit from high-density sampling, more complete geochemical data, and the cooling unit concept from ash-flow tuffs. Meanwhile, seismology and InSAR at active arcs should yield additional insights, if we remain alert to the potential.