The geochemistry of phosphorus in different granite suites of the Western Carpathians, Slovakia: the role of apatite and P-bearing feldspar

Abstract

The geochemical behaviour of phosphorus in granites, coupled with compositional variations in apatite, discriminate the different genetic suites of Variscan granitic rocks in the West-Carpathian orogenic belt. Meso-Variscan granites, mostly of S-type affinity, show a good correlation between SiO 2 and P 2O 5, with decreasing amounts of P in the poorly and moderated fractionated granites, and increasing phosphorus in the most evolved and late differentiation products. The aluminium saturation index (ASI) correlates negatively with bulk P contents in I-type granites, and positively with P in S-type granites. The phosphorus content of alkali feldspar (mainly Kfs) is usually very low (typically <0.05 wt.% P 2O 5), or not detected in the slightly differentiated granites. In the lateVariscan I/S-type granitoids, phosphorus is hosted mainly in the early-magmatic apatite, but is present also in the late K-feldspar (up to 0.15 wt.% of P 2O 5) that formed from the residual melts. In the post-orogenic S-type granites, the bulk of the phosphorus occurs in alkali feldspar (∼0.3 wt.% P 2O 5), with a minor proportion present in primary early-magmatic apatite. Clusters of small secondary apatite grains with a distinctive composition are distributed in alkali feldspar grains of the post-orogenic S-type granites. This apatite has formed post-magmatically from P released from P-rich alkali feldspar during decreasing temperature, by reaction with a fluid rich in volatiles (including F and B), and alkali and alkali earth metals. A-type granites have the lowest bulk phosphorus contents. Consequently, phosphate minerals are relatively rare in this granite type, and the alkali feldspar has negligible P 2O 5 contents. Except for Sr, the minor element contents in apatite partly correlate with the bulk composition of their host rocks and partly reflect some thermodynamic properties of melt. The Mn contents of apatite increase with the peraluminous character of the melt and with decreasing fO 2 and so apatite from the S-type granites has significantly higher Mn contents than apatite from the I-type granites. Apatite from A-type granites is enriched in Fe and HREE, reflecting the higher bulk Fe and HREE content of this granite type. Apatite from specialized S-type granites is enriched in Y and HREE. In general, the Cl and S content of apatite is higher in the basic members of the I-type affinity granitic rocks.