Abstract

Located in the northern Sierra Nevada, the Middle Jurassic Pyramid Peak granite and associated dioritic rocks display superbly exposed interlayered sheets of mafic, felsic, and hybrid rocks. The mafic-hybrid layers show a consistent asymmetry (sharply chilled on one side and gradational through hybrids to granite on the other) and dip to the southwest. Load casts and silicic pipes along the chilled, mafic northeastern margins of the sheets indicate that depositional way-up is to the southwest and that the adjacent Mount Tallac “roof pendant” was the original floor of the intrusion. Orientations of the pipes indicate that the layers were tilted after deposition and solidification. We interpret the sheets to be intramagmatic flows that formed by injections of mafic magma into a silicic magma chamber followed by flow across the crystal-rich base of that chamber. The granitic and hybrid rocks interlayered with the mafic sheets represent material that accumulated on the chamber floor between episodes of mafic replenishment. Most contamination of the mafic sheets probably occurred because of flow-front instabilities generated as the mafic magma entered and spread across the chamber floor. The upward gradation from diorite to granite with decreasing abundance and size of mafic enclaves can best be explained by convective stirring at the upper boundaries of the dioritic replenishments. The Pyramid Peak granite was probably initiated as a thin, concordant sheet intruding into gently dipping Jurassic supracrustal rocks; the pluton was constructed incrementally with many replenishments of felsic and mafic magma and dominantly solidified by accumulation of crystals on the floor of the chamber. The present steep dips of the layers suggest that downward warping of the chamber floor may have been a factor in accommodating the incrementally constructed pluton.

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