Characteristics of modal layering in the Round Valley Peak Granodiorite, eastern Sierra Nevada, California

Many granitic plutons exhibit alternating mafic and felsic layers known as modal layering. They have been classically interpreted as sedimentary features that formed by crystal sedimentation. However, field observations and geochemical data from a modally layered domain of the Round Valley Peak Granodiorite and nearby non-layered granodiorite contradict the standard interpretation. A typical layer consists of three units: a mafic, fine-grained layer, an intermediate layer, and a more coarse-grained felsic layer. Features that contradict a sedimentary model include: (1) features that resemble cross bedding but have contradictory paleo-up directions within adjacent layers (2) poles to layering that scatter over about one quarter of a stereonet (3) large, dense biotite crystals among quartz and feldspar crystals in parts of layers that are deemed stratigraphically high. An alternative hypothesis is that layering results from self-organization owing to crystal aging. Characterization of the modally layered unit has allowed for assessment of the hypotheses.