The role of Diffusion-controlled Oscillatory Nucleation in the Formation of Line Rock in Pegmatite-plite Dikes

Abstract

The George Ashley Block (GAB), located in the Pala Pegmatite INTRODUCTION District, San Diego County, California, is a composite pegmatite– Modal or phase layering is a phenomenon that is seen aplite dike of 8 m thickness displaying striking mineralogical layering in plutonic igneous rocks of all compositions from highin the aplite portion of the dike, referred to as line rock. Rhythmic temperature, low-viscosity, mafic complexes to low-temlayering is characterized by garnet-rich bands alternating with perature, high-viscosity, felsic complexes. The sizes of albite–quartz–muscovite-rich bands. Cumulus textures are notably such plutonic bodies can range from the large layered absent from the layered portion of the dike. Elongated quartz intrusive complexes such as the 3000 m thick Skaergaard megacrysts are oriented perpendicular to the garnet-rich layers and complex down to dikes a few meters wide. There is, poikilitically include garnet, albite, and muscovite. Calculated however, no consensus as to the origin of the layering. crystal-free magma viscosity with 3% H2O is 10 Pa s and the Modal layering that results from the gravitational settling calculated settling velocity for garnet is 0·51 cm/year. Conductive of crystals from a magma is a model often invoked to cooling calculations based on emplacement of a 650°C dike into explain the layering in mafic magmatic systems. This is 150°C fractured gabbroic country rock at 1·5 kbar, and accounting more problematic in granitic systems because of the for latent heat of crystallization, demonstrate that the line rock higher viscosities of the melts and the smaller density portion of the dike cools to 550°C in about 1 year. Crystal difference between crystals and melt. size distribution studies also suggest very rapid nucleation and This paper presents a temperature–crystallization crystallization. Diffusion-controlled gel crystallization experiments model for the George Ashley composite pegmatite–aplite yield textures virtually identical to those observed in the layered dike, which displays well-developed rhythmic layering aplite, including rhythmic banding, colloform layering, and band (line rock) in the aplite portion of the dike. Kleck (1996) discontinuities. Thus, observed textures and calculated magmatic proposed that the layering in the George Ashley Block parameters suggest that mineralogical layering in the GAB results (GAB), a float slide block from this dike, could be exfrom an in situ diffusion-controlled process of oscillatory nucleation plained by the gravitational settling of garnet and albite and crystallization. We propose that any event that promotes from the melt, and that many of the textural features strong undercooling has the potential to initiate rapid heterogeneous which characterize the line rock, such as offsets in the nucleation and oscillatory crystal growth, leading to the development layers, formed as a result of soft sediment deformation of a layer of excluded components in front of the crystallization of a crystal–liquid mush. The purpose of this investigation front, and the formation of line rock. was to evaluate crystal settling as a possible mechanism