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

Major, minor, and trace element abundances in apatites from various I- and S-type (igneous and sedimentary) granites of the Lachlan Fold Belt have bee n determined using electron microprobe and laser ablation inductively coupl ed plasma mass spectrometer. The results show that apatite can accommodate many minor and trace elements, whose concentrations and ratios are relative ly sensitive to factors controlling many of the fundamental differences bet ween I- and S-type granites. Apatites from S-type granites generally have h igher F but lower Cl contents than those from I-type granites, which is asc ribed mainly to the loss of Cl during the weathering processes forming the source rocks of S-type granites, although fractional crystallisation can ca use significant enrichment in F as well. High Mn and Fe contents in apatite s 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 p eraluminous S-type and felsic I-type magmas. There are systematic and disti nctive differences in absolute rare-earth element (REE) abundances, REE dis tribution patterns, and element ratios (e.g., La/Y, Sm/Nd, etc.) between ap atites from different types of granite. The strong Eu depletion that charac terises 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 EU2/EU3+ 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 Tn enrich ment in apatites from mafic I-type granites and marked Nd depletion in apat ites from most S-type and felsic I-type granites are caused by the precipit ation and fractionation of monazite in the parental magmas of the latter ro cks. 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 betw een apatite and melt. Thus, apatite chemistry can be used as an excellent i ndicator of granite petrogenesis. The results have important implications f or identifying different types of granite and are potentially significant f or determining the provenance of sedimentary rocks. Copyright (C) 1999 Else vier Science Ltd.