APATITE GEOCHEMISTRY OF WESTERN MAINE PLUTONS

Apatite is a useful accessory mineral because it concentrates many trace elements that are incompatible in major minerals. It is therefore useful as a record for trace element levels, it geochemically distinguishes differing granite types, and zoning provides a record of magmatic processes.

Apatite from 14 samples from granites western Maine and southern New Hampshire were analyzed for minor constituents (Mn, Fe, S, Sr, Y, Ce, La, Si) using the electron microprobe. Backscatter electron images (BSE) and compositional maps were also acquired to document zoning in individual grains. Metaluminous samples show a positive relationship between Si and REE. Apatite from peraluminous granites show higher levels of Mn and Y. Inclusions of quartz, potassium feldspar and plagioclase are found only in apatite from metaluminous granites. Metaluminous samples show lower amounts of Mn and higher levels of Si. Two samples from metaluminous granites display complex zoning. Bright zones in BSE images denote higher levels of REE and Si, suggesting the following substitution: Si⁺⁴ + REE⁺³ = Ca⁺² + P⁺⁵.

The compositional difference between metaluminous and peraluminous granites are reflected in the chemical compositions of apatite. Complex zoning, including dissolution surfaces that truncate earlier zones, are preserved in apatite from metaluminous granites. To produce this zoning, the apatite grains were exposed to different chemical surroundings within the magma chamber, and therefore must have formed within an active, dynamic magma chamber. The age of the Songo pluton ~384 Ma and argon ages for hornblende are ~ 310 Ma. Despite remaining at elevated temperatures for >70Ma, complex zoning on the order of 2 microns is still preserved, therefore diffusion of Si in apatite must be very sluggish. The lack of zoning in peraluminous samples may point to crystallization in a less dynamic chamber.