Origin of rhythmic layering in the Calamity Peak satellite pluton of the Harney Peak Granite, South Dakota: The role of boron
The Calamity Peak satellite granite pluton on the flanks of the Harney Peak Granite, South Dakota, displays prominent mineral and textural layering, the latter defined by alternating fine-grained and pegmatitic granite. Superimposed on the fine-grained granite layers is a fine-scale, 0.1-5 cm, rhythmic layering defined by modal differences in tourmaline, microcline, albite and muscovite. There is very little variation in the major element mineral compositions in the whole complex. Analyses by Prompt-Gamma Neutron Activation gave a B 2O 3 content of 0.07% in the light layers (albite, quartz, and muscovite) and 0.65% in the dark layers (tourmaline, microcline, quartz, and albite); these contents are essentially proportional to tourmaline abundance in each layer. The textural and mineralogic layering is the result of the interplay of tourmaline crystallization and shifts in phase equilibria due to changes in B concentration in the melt. Initial cotectic crystallization of albite, quartz and muscovite increased the activity of Fe and Mg in the adjacent melt to levels sufficient for tourmaline to form. Tourmaline crystallization depleted B and decreased the solubility of H 2O in the melt, causing exsolution of H 2O from the melt and further removal of B due to high fluid/melt partition coefficient. This process also resulted in the precipitation of K-feldspar, partly at the expense of muscovite and partly due to a slight increase in the solidus temperature. Once the crystallization front moved to an Fe and Mg-poor region of the melt, tourmaline precipitation ceased and the equilibrium shifted back toward quartz, muscovite, and albite. The pegmatitic layers, some of which cut across previously formed rhythmic layers and commonly have tourmaline along their borders, appear to have crystallized from melt enriched in the exsolved fluid phase.