Anatomy and assembly of the McDoogle pluton near Sawmill Lake, Central Sierra Nevada, California

This study examined the internal anatomy and assembly processes of the xenolithrich (-30 vol %) McDoogle pluton of the John Muir Intrusive Suite (JMIS), part of the Sierra Nevada batholith. The McDoogle pluton is interpreted to have grown incrementally as a plexus of dikes emplaced synkinematically into the steep Sawmill Lake shear zone. The best exposed portion of the pluton near Sawmill Lake was investigated using detailed geologic and bulk magnetic susceptibility (BMS) mapping (1:3000 scale), structural analysis, and high-precision U-Pb geochronology. The pluton is composed of five composite sheets of three different lithologic types arranged in mirror symmetry. The oldest, most heterogeneous, central phase grew antitaxially, meaning increments were added at the intrusive margins. Antitaxial growth preserves abundant in-situ wall-rock screens which are interpreted to mark contacts between increments. Folding patterns of the wall-rock screens suggest that the central phase grew from east to west. The younger intermediate and border phases were emplaced on both sides of the central phase, thus creating the mirror symmetry. The outer increments grew syntaxially, meaning increments were added at the centers of previous increments. The central phase terminates bluntly in the map area whereas the border phases continue southward and terminate as dike tips. The McDoogle pluton rapidly intruded the Sawmill Lake shear zone at 95 Ma. Three samples were processed for high-precision U-Pb zircon dating and produced ages that overlap within error (±0.1 Ma). The minimum permissible wall-rock displacement rate is 4 mm/yr, which exceeds normal local tectonic dilation rates of ~1 mm/yr. Data from the Bishop Pass phase of the Lamarck granodiorite, located further north in the JMIS, define a similar duration and minimum rate of growth.. Magmatic overpressure may account for the rapid dilation rate. BMS mapping can distinguish and communicate subtle petrologic variations that traditional geologic mapping cannot. However, it is necessary to investigate what controls the BMS signal before large BMS surveys. The Sawmill Lake BMS map reflects the geologic map, but contains areas of high and low values that define tabular bodies which follow folded wall-rock screens, supporting structural interpretations. The correlation between BMS and color index (CI) was tested in nine samples. Ilmenite has much lower bulk susceptibility than magnetite, and therefore a rock that contains ilmenite in place of magnetite has a lower BMS for a given CI. Though, removing samples with significant ilmenite proportions did not improve the correlation between CI and BMS.