THE GARNET - PHOSPHATE BUFFER IN PERALUMINOUS GRANITIC MAGMAS: A CASE STUDY FROM PEGMATITES IN THE POCHO DISTRICT, CORDOBA, ARGENTINA

Abstract

Members of the beusite–graftonite series [Mn/(Mn+Fe) between 0.47 and 0.65] appear associated to P-bearing garnet (up to 1.10 wt.% P 2 O 5 in the crystal cores) in two primitive pegmatites of the Pocho district, Cordoba, central Argentina. Elemental maps and electron-microprobe analyses show that garnet has an irregular P distribution, with depleted rims. Textural features of garnet (resorbed margins, isolated relics in beusite, truncated zoning) and similar Mn/Fe values in garnet and beusite indicate that the phosphate formed by destabilization of garnet. The crystallization temperatures of the zones where garnet + beusite occur are 420°C (Tablada I) and 522°C (Cerro Moreno), and the P 2 O 5 content in the melt of the middle intermediate zone at Tablada I is 0.82–0.91 wt.% (estimated using P in feldspars). One possible explanation involves the onset of garnet crystallization before the nucleation of beusite, while the P content of the magma was not buffered. As magmatic evolution progressed, P content increased and eventually reached levels high enough to stabilize mafic phosphates. For some time garnet co-crystallized with beusite; this is reflected by P-depleted zones in garnet. With P build-up, eventually mafic silicates became unstable and were replaced by phosphates. The low activities of F, Li and Na contributed to stabilize beusite. The modest degree of evolution of the pegmatites is indicated by very high Mg content of the phosphates. Beusite has primary inclusions of Mg-rich phosphates, one belonging to the alluaudite group and the other one possibly belonging to the triplite or wolfeite groups. Breakdown of beusite under oxidizing hydrothermal conditions produced, among others, mitridatite and jahnsite-(CaMnMg).