Apatite chemical composition, determined by electron microprobe and laser-ablation inductively coupled plasma mass spectrometry, as a probe into granite petrogenesis

Abstract

Major, minor, and trace element abundances in apatites from various I- and S-type (igneous and sedimentary) granites of the Lachlan Fold Belt have been determined using electron microprobe and laser ablation inductively coupled plasma mass spectrometer. The results show that apatite can accommodate many minor and trace elements, whose concentrations and ratios are relatively sensitive to factors controlling many of the fundamental differences between I- and S-type granites. Apatites from S-type granites generally have higher F but lower Cl contents than those from I-type granites, which is ascribed mainly to the loss of Cl during the weathering processes forming the source rocks of S-type granites, although fractional crystallisation can cause significant enrichment in F as well. High Mn and Fe contents in apatites from S-type granites, and high S and As abundances in apatites from mafic I-type granites, result from different oxygen fugacities and degrees of Al saturation (or aluminosity) between metaluminous mafic I-type magmas and peraluminous S-type and felsic I-type magmas. There are systematic and distinctive differences in absolute rare-earth element (REE) abundances, REE distribution patterns, and element ratios (e.g., La/Y, Sm/Nd, etc.) between apatites from different types of granite. The strong Eu depletion that characterises apatites from S- and felsic I-type granites is interpreted here to be a result of the uniqueness of crystal chemistry of apatite and high Eu 2+/Eu 3+ ratios in S-type and felsic I-type magmas, which are more reduced and peraluminous than mafic I-type magmas. Strong REE (La to Eu) and Th enrichment in apatites from mafic I-type granites and marked Nd depletion in apatites from most S-type and felsic I-type granites are caused by the precipitation and fractionation of monazite in the parental magmas of the latter rocks. Substitution mechanisms are responsible for high Na in apatites from S-type and felsic I-type granites, and for high Si in apatites from mafic I-type granites, and may also have important effects on REE partitioning between apatite and melt. Thus, apatite chemistry can be used as an excellent indicator of granite petrogenesis. The results have important implications for identifying different types of granite and are potentially significant for determining the provenance of sedimentary rocks.