APATITE AND GRANITE PETROGENESIS

Virtually all igneous rocks contain apatite. A study of the petrography, trace element composition and zoning characteristics of apatite from six peraluminous and six metaluminous granites bodies in Maine and New Hampshire suggests that it preserves information that may help constrain their petrogenesis.

Apatite from the metaluminous granites consists of abundant, small, inclusion-free, prismatic grains that are enclosed in hornblende, biotite, titanite, magnetite and plagioclase. These are enriched in Si, S, La and Ce relative to their counterparts in the peraluminous granites. Compositional zoning is common. In the peraluminous granites, the apatite grains are larger, rare, equant, grains that are rarely included in other minerals and often contain inclusions or zircon and monazite. These grains lack chemical zoning and are enriched in Mn and Y.

These characteristics are attributed to differences in the chemical composition of the magmas, oxygen fugacity, and magma chamber processes. The low solubility of monazite in peraluminous magma leads to its early crystallization and consequent removal of REE. Apatite crystallization is delayed. The higher CaO content and greater solubility of monazite in metaluminous magmas leads to early crystallization of apatite. The more oxidizing nature of metaluminous magmas allows for the substitution of S⁶⁺ in apatite. The lack of chemical zoning in apatite from peraluminous granites, suggests a relatively simple petrogenetic history after apatite joined the liquidus. Zoned apatite from metaluminous samples indicates crystallization in an active, dynamic chamber with gradients in temperature and/or composition. Preservation of this zoning in samples from the high-grade metamorphic terrain of western Maine suggests that zoning is robust.