PLUTONS AIN'T WHAT THEY USED TO BE

Field study is central to geologic research, but plutons are challenging to interpret because processes by which they form can only be observed indirectly, and the physical conditions and time scales over which those processes operate are far from human experience. In our careers we have seen a revolution in how plutons are interpreted and have had to adjust our views accordingly. For decades, field-based conventional wisdom was that a pluton represented a large blob of magma that ascended buoyantly through the crust to a given level and then solidified. This "big tank" paradigm was accepted wisdom when we were students, persisted well into our forays into the plutonic realm, and is still presented as fact in textbooks. By the early 2000’s, however, the big tank had sprung several leaks. Especially troubling were thermal arguments against diapiric rise; evidence that plutons are generally tabular rather than globular; gradational contacts between ostensibly distinct magma bodies; lack of chemical patterns that should form in big tanks; field interpretations that are contradicted by phase equilibria, fluid dynamics, or geochemistry; and, critically, precise U-Pb dates in single plutons that span millions of years. Clearly, something was wrong.

The eastern Sierra Nevada and White-Inyo Ranges were ideal places to formulate a new framework for pluton formation, thanks to exceptional field exposures, a bewilderingly large number of diverse plutons, and critical information provided by a generation of talented field geologists led by Paul Bateman, Jim Moore, and Clem Nelson. Together they produced 27 15' geologic quadrangle maps illustrating the diversity of pluton geology. The Tuolumne Meadows quadrangle (Bateman et al., 1983) and accompanying data have proven invaluable resources in our work. Multiple waves of geochronologic study of the Tuolumne Intrusive Suite, an archetype of the big-tank model, show that it formed over 10 Myr, compelling evidence for incremental assembly. The inward gradation of U-Pb dates both within TIS plutons and across its mapped contacts leads to the inference that the gradational contacts reflect differences in thermal history rather than different magma bodies.