Abstract

New mapping of the southern McDoogle pluton, an early intrusion in the John Muir suite in the central Sierra Nevada, California, has resolved important details of incremental assembly by magmatic crack-seal. The tabular McDoogle pluton concordantly intruded the steeply dipping mylonitic Sawmill Lake shear zone around 95 Ma. The pluton was previously divided into three phases based on wall-rock screen abundance and lithology. The earliest central phase consists of steep dikes separated by abundant wall-rock screens. These screens are largely absent from border phases located on either side of the central phase. The inferred relative ages of intrusive phases and the wall-rock screen distribution signify a change from antitaxial (addition at the margins of the central phase) to syntaxial (addition at the center of each border phase) assembly during pluton growth. Interaction among the effects of preexisting foliation, older nondeformed intrusions, and synintrusive static recovery of shear zone rocks likely caused the transition from antitaxial to syntaxial growth. The trend from antitaxial to syntaxial growth during emplacement has been observed in other intrusions and intrusive suites and may be a general pattern of incremental intrusion associated with the feedback between preexisting deformation and thermal addition in magmatic arcs.

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