CHARACTERISTICS OF MODAL LAYERING IN THE ROUND VALLEY PEAK GRANODIORITE, EASTERN SIERRA NEVADA, CALIFORNIA

Many granitic plutons around the world exhibit alternating mafic and felsic layers. These structures, known as modal layering, have been classically interpreted as sedimentary features that formed by crystal sedimentation, slumping, and turbidity currents. However, field observations and geochemical data from a modally layered domain of the Round Valley Peak Granodiorite and surrounding un-layered granodiorite, eastern Sierra Nevada, California, contradict the standard interpretation. The layered domain forms a glacially sculpted rock mullion about 70 m by 40 m in extent, with about 20 m of vertical relief. A typical layer consists of a fine-grained mafic base containing abundant hornblende, grading upwards to a coarse-grained felsic top containing quartz, feldspar, and large biotite crystals. Contradictory features include: (1) sedimentary-like features that resemble cross bedding, with paleo-up directions within adjacent layers that are contradictory; (2) large planar layering sets that intersect each other at right angles; (3) poles to layering that scatter unsystematically over about one quarter of a stereonet; and (4) presence of large, dense biotite crystals among smaller quartz and feldspar crystals within the layered sequence. These features are all inconsistent with formation of layering by crystal sedimentation. An alternative hypothesis is that layering results from self-organization owing to crystal aging. Boudreau et al. (1997) showed that crystal aging and chemical diffusion could produce the layering seen in classic layered mafic complexes such as the Skaergaard intrusion, and these processes may apply in granodioritic systems. Ongoing investigations of the Round Valley Peak Granodiorite, such as XRF core scanning, SEM imaging, electron microprobe analyses and whole rock geochemical analyses will be used to further test the two opposing hypotheses.