PETROLOGIC MINERALOGY IN METAPELITIC ROCKS: EXAMPLES FROM BIOTITE AND TOURMALINE GROUP MINERALS

The treatment of crystal chemistry and crystallography of minerals in the context of the rocks in which they form is a fundamental approach long advocated by Charles Guidotti. Metapelitic rocks are particularly effective rock types to practice this approach because of the facile manner in which mineral chemistry and crystallography respond to changing temperature (T), pressure (P) and fluid compositions. However, to study complex metapelitic minerals in a systematic way it is necessary to demonstrate chemical equilibrium and to reduce the number of compositional variables that influence the minerals e.g. through the presence of minerals that can act as saturating phases for certain elements such as ilmenite for Ti. Such precepts have been central to several studies of biotite and tourmaline from metapelitic rocks. (1) Biotite. The change of Ti concentrations in biotite as a function of T and biotite chemistry is an exceptional application of a petrologic mineralogy approach. Based on a large natural biotite data set from metapelites of W Maine and SC Massachusetts, a Ti-saturation surface (TºC - Mg/(Mg+Fe) – Ti surface) was generated for biotites from graphitic, peraluminous metapelites that contain ilmenite or rutile and have equilibrated at roughly 4-6 kbar. The surface fit equation was reformulated to produce an empirical Ti-in-biotite geothermometer for peraluminous metamorphic rocks. In addition, the systematics of Ti found in these biotites provide a means to evaluate the approach to chemical equilibrium as well as Ti-substitution mechanisms. (2) Tourmaline. Tourmaline has long been a difficult mineral to understand, but some of the greatest progress has been made by considering the development of tourmaline in metapelites during progressive metamorphism. Despite its refractory nature, tourmaline does approach equilibrium, tends to grow with progressive metamorphism as B is released or added to the rocks, and can locally react depending on the fluids. It preserves a chemical record (zoning) that can be related to the reactions and T changes in the metapelitic rocks. Based on the refractory mechanical nature of the mineral and its wide-ranging chemistry that responds to the rock in which it formed, it can be a superb provenance indicator in both sedimentary and metasedimentary rocks.