INTERPRETING MAGMATIC CONDITIONS USING APATITE TEXTURES AND COMPOSITION

Several recent studies have shown that apatite composition and textures are underutilized yet powerful techniques for characterizing granites and understanding their petrogenesis. Samples from two peraluminous granite (Sebago Batholith and North Jay pluton) and two metaluminous granites (Mt. Waldo and Deer Isle plutons) in Maine have been studied to evaluate these techniques. Apatite grains from the peraluminous granites are anhedral, homogeneous, and relatively large (typically, >200 μm). Individual thin sections have relatively few apatite grains and these are generally not included in other minerals. Apatite grains from the metaluminous granites are euhedral, relatively small (generally < 200 μm) and occur as isolated grains or as small inclusions in hornblende, biotite and titanite. Backscatter electron imaging reveals that many crystals in the metalumious granites exhibit zoning, either as distinct cores and rims or as multiple concentric zones. Traverses across zoned grains show correlations between REE and Si suggesting the coupled substitution of trivalent cations (REE & Y) + Si⁴⁺ = divalent cations (Ca, Mn & Fe) + P⁵⁺. The biggest chemical differences in apatite from the two populations are the relatively MnO and Ce₂O₃ concentrations. In apatite from peraluminous granites MnO ranges between 0.15-0.87 wt.%, relative to 0.08-0.14 wt.% in apatite from metaluminous granites. Ce₂O₃ ranges between 0.05 – 0.12 wt.% in apatite from peraluminous granites and 0.16 and 1.02 in apatite from the metaluminous granites. The presence monazite (REE phosphate) in the peraluminous granites may account for the relatively low REE concentrations in coexisting apatite. The absence of magnetite in the peraluminous granites indicates a relatively low oxygen fugacity and suggests that most of the Mn was divalent allowing it to substitute in apatite for Ca. Concentric zoning in apatite formed during episodes of partial resorption and growth. The variable REE content suggests that apatite moved between magma batches with different REE concentrations. The metaluminous granites formed in dynamic magma chambers and diffusion was sluggish.