Origin of Comb Layering and Orbicular Structure, Sierra Nevada Batholith, California

A new descriptive term, comb layering, is proposed to replace the informal term Willow Lake-type layering, first introduced by Poldervaart and Taubeneck (1959) to describe layering in granitoid rocks in which constituent crystals are oriented approximately perpendicular to individual layers. The term schlieren layering is proposed to describe the “normal” layering of granitic rocks defined by alternating layers enriched or depleted in the normal mafic minerals. In such layers, elongate minerals commonly lie in the plane of the layering. Comb layering is widespread in plutonic rocks of California and is commonly associated with orbicular diorites. Evidence from a detailed study of three localities in the Sierra Nevada indicates that comb layering forms chiefly in overturned troughs along overhanging walls of plutons or along walls of dikes or pipes that cut country rocks adjoining plutons. Orbicular rocks associated with the comb layering are formed by a nucleus surrounded by multiple comb layers.

The growth direction in comb layers can be determined by the branching and widening of plagioclase and hornblende crystals and is invariably toward the parent pluton. Field data indicate that comb layering cannot have formed from silicate magma, and further suggest that the layers have been deposited by large volumes of aqueous fluids that migrated upward along contacts between magma and wallrock or along the interface between magma and previously solidified melt. Comb layering and orbicules are largely restricted to structural traps into which upwardly migrating, solute-rich water was channeled owing to its low density. The comb layers grew on the solid walls of fluid-filled channels, whereas orbicules formed by precipitation of comb layers on hobbling inclusions suspended within the upward-flowing fluid.