Journal of Petrology, Volume 40, Issue 1: January 1999.

Experimental Silicate-Phosphate Equilibria in Peraluminous Granitic Magmas, with a Case Study of the Alburquerque Batholith at Tres Arroyos, Badajoz, Spain

DAVID LONDON¹, MICHAEL B. WOLF¹, GEORGE B. MORGAN VI¹ AND MARCOS GALLEGO GARRIDO²

¹SCHOOL OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF OKLAHOMA, 100 EAST BOYD STREET, 810 SARKEYS ENERGY CENTER, NORMAN, OK 73019, USA
²AVENIDA DE AJALVIR, NO. 8, 7D, 28806 ALCALA DE HENARES, MADRID, SPAIN

Experimental results presented here establish the P₂O₅ content of peraluminous silicic melt for three potential buffering reactions at 200 MPa H₂O and T from 525 to 850°C: (1) biotite-sarcopside; (2) spessartine-sarcopside; (3) petalite-(amblygonite-montebrasite)-quartz. The first two equilibria are moderately insensitive to T and buffer the P content of melt between 1·30-1·57 wt % P₂O₅ (biotite-sarcopside) and 0·96-2·4 wt % P₂O₅ (spessartine-sarcopside). The equilibrium among petalite-(amblygonite-montebrasite)-quartz is strongly T dependent, and P contents of coexisting melt vary from 1·4 to 7·2 wt % P₂O₅ between 525 and 700°C. With these data and the calibration of P substitution in the alkali feldspars, the phosphorus budget of granitic magmas can be assessed. Peraluminous partial melts derived from metasedimentary rocks are likely to contain <0·5 wt % P₂O₅ at the source and to be undersaturated in apatite. If the incipient crystallization of plagioclase lowers the Ca/P ratio of melt, these magmas can fractionate to P-rich compositions that give rise to the diverse phosphate mineral assemblages in evolved S-type leucogranites and pegmatites. Where these silicate-phosphate mineral equilibria apply, the P content of melt should not normally exceed ~2·5 wt % P₂O₅. By applying these results to a sharply zoned, P-B-F-rich granite-pegmatite suite at Tres Arroyos, along the SE margin of the Alburquerque batholith in Badajoz province, Spain, we can constrain: (1) the budget of phosphorus in the various facies of this intrusive suite; (2) the dynamics of crystallization and the segregation of melt from crystals; and (3) the magmatic to subsolidus history recorded by the alkali feldspars. The various equilibria presented here agree well with the actual mineralogy and whole-rock P in the granite-aplite-pegmatite facies at Tres Arroyos.

Keywords: phosphorus;phosphates;alkali feldspar;peraluminous granites;experimental petrology

Pages 215-240

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