Rapid Emplacement of the Mcdoogle Pluton into the Sawmill Lake Shear Zone, Sierra Nevada, California

The steep, tabular McDoogle pluton measures ~10 km x 2 km and comprises five mapped sheets of three lithologic types arranged with mirror symmetry. The pluton was emplaced late synkinematically into the subvertical Sawmill Lake shear zone (Mahan et al., 2003, GSA Bulletin). The pluton contains abundant tabular wall-rock inclusions that are interpreted to be in situ screens, because the inclusions define a ghost stratigraphy and contain pre-intrusive solid-state fabrics that are concordant both among the screens and with external wall rocks. The pluton thus is interpreted to have grown incrementally by magmatic crack-seal (Bartley et al., 2007). Mahan et al. (2003) reported ID-TIMS U-Pb zircon dates of 95±4 Ma and 94.8±0.6 Ma from the central and border phases, respectively. New higher-precision U-Pb data, from two central-phase samples and from additional fractions from Mahan et al.'s border-phase sample, were collected to test for resolvable age differences. The central-phase samples yielded identical dates of 95.0±0.1 Ma, and the new border-phase results improve the precision of that date to 94.9±0.1 Ma. The three ages overlap within error and indicate that the pluton grew in no more than 300 ka. These results resemble those of Gracely and Coleman (2007) from the southern Lamarck pluton, which intruded the same shear zone ~0.5 Ma later and ~20 km farther north. The McDoogle data set a lower limit of 4 mm/yr for the dilation rate required to make space for the pluton. Tectonic dilation this rapid is not out of the question but local tectonic dilation rates within continents rarely exceed 1 mm/yr. Therefore, either (a) the McDoogle and Lamarck plutons were intruded during relatively rapid intra-arc extension or (b) a significant proportion of wall-rock opening was driven by magma overpressure rather than by far-field stress.