ASSEMBLY OF THE INCREMENTALLY EMPLACED MCDOOGLE PLUTON, CENTRAL SIERRA NEVADA, CA

This study examines the internal structure of the dike-like ~95 Ma McDoogle pluton, which is xenolith-rich (~30 vol. %) and interpreted to have grown incrementally by crack-seal (Ramsay, 1980; Mahan et al., 2003; Bartley et al., in press). In antitaxial crack-seal, increments are added at the margins of a pluton, whereas during syntaxial growth increments are added at the center. A hallmark of antitaxial crack-seal that is exemplified by the McDoogle is in-situ inclusion of wall-rock bodies where dike paths deviate into the wall rock at the margins of a growing pluton. Syntaxial growth does not isolate wall-rock bodies and is more difficult to recognize. A well-exposed ~2 km2 portion of the McDoogle pluton was mapped at 1:3000. Also, 143 bulk magnetic-susceptibility measurements were made and contoured by inverse-distance weighting; Quaternary cover caused irregular areal coverage but this does not appear to cause artifacts in the magnetic map pattern. Samples that span the range of magnetic susceptibility were examined using optical petrography. Mahan et al. (2003) distinguished three phases at 1:10,000 scale, whereas our more detailed map resolves five lithologic phases with mirror-image across-strike symmetry. The lithologic phases are internally composite based on in-situ inclusion of wall rocks and on internal contacts that truncate the magmatic fabric of the pluton. Each phase has a distinct bulk magnetic susceptibility and screen abundance; in the center of the pluton, magnetic susceptibility and screen abundance are highest and the susceptibility and rock type vary the most. Dikes (0.5 - 10 m) intruded into screens differ in magnetic susceptibility, even where only 1 to 10 m apart. The tabular shape of the pluton, sheeted patterns of lithologic variation and magnetic susceptibility, and abundant in-situ wall-rock inclusions indicate that the McDoogle was constructed by amalgamation of dikes. The sizes and distribution of dikes and screens suggest that increments may be 0.5 to 10 m thick. The distinct abundances of wall-rock screens in each phase suggest that assembly varied from antitaxial to syntaxial during growth of the pluton: the abundant screens in the central phase record antitaxial crack-seal, whereas less abundant screens in the other phases suggest a shift toward syntaxial growth.