APATITE GEOCHEMISTRY: A POWERFUL TOOL FOR CHARACTERIZING MAGMA SUITES AND INTERPRETING IGNEOUS PETROGENESIS

Apatite from the metaluminous (ASI > 1) Deer Isle Granite Complex (DIGC), weakly metaluminous (ASI ≈1) Mt. Waldo pluton (MW), and the peraluminous (ASI < 1) Sebago and North Jay plutons (SBG and NJ respectively) were analyzed for minor constituents (Mg, Na, Si, Fe, S, Sr, Y, Ce and La) as well as the major constituents CaO, P₂O₅, F and Cl. The three groups can be identified on the basis of Mn, Y, Si, La and Ce. Y and Mn concentrations are high and Si, La and Ce are low in apatite from the SBG and NJ plutons. The DGIC has high Si, La and Ce concentrations but low Y and Mn. The MW pluton has high Si, La, Ce and Y but low Mn. The concentration of trace elements in apatite is strongly related to the accessory mineral assemblage of the rock. La and Ce concentrations are strongly correlated in the plutons: La/Ce = 0.32 (SBG & NJ); = 0.34 (MW); = 0.77 (DIGC). Previous research indicates that other peraluminous and weakly metaluminous granites have lower La/Ce ratios (≤ 0.34), whereas metaluminous granites have higher La/Ce ratios, with this ratio fundamentally related to alumina saturation. Concentrations of Na and S are stongly correlated, as are trivalent ions and Si, suggesting the following coupled substitutions: Na¹⁺ + S⁶⁺ = Ca²⁺ + P⁵⁺ and X³⁺ + Si⁴⁺ = Ca²⁺ + P⁵⁺.

Backscatter electron imaging and compositional mapping were used to search for zoning in apatite crystals. Zoning is common in apatite from the MW and DIGC. The total number of zones observed in any crystal is ≤ 6, and can vary in width from a few to 10's of µm. Bright areas on backscatter images correlate with high concentrations of trivalent cations and silica. Darker areas have high Na and S concentrations. Virtually no zoning was observed in apatite from SBG and NJ plutons, apart from a tiny decrease in Mn near crystal edges. Zoning in apatite from the MW and DIGC is attributed to dynamic magmatic processes in which crystals move between domains with different compositions. The lack of zoning in apatite from the peraluminous granites probably indicates a very different magmatic environment, but post-crystallization annealing cannot be ruled out at this time. Therefore, apatite chemistry can be used to help characterize magma types and to identify rocks in which magma hybridization played an important petrogenetic role.