Experimental silicate-phosphate equilibria in peraluminous granitic magmas, with a case study of the Alburquerque batholith at Tres Arroyos, Badajoz,Spain

Experimental results presented here establish the P2O5 content of peralumin ous silicic melt for three potential buffering reactions at 200 MPa H2O and T from 525 to 850 degrees C: (1) biotite-sarcopside; (2) spessartine-sarco pside; (3) petalite-(amblygonite-monte-brasite)-quartz. The first two equil ibria are moderately insensitive to T and buffer the P content of melt betw een 1.30-1.57 wt % P2O5 (biotite-sarcopside) and 0.96-2.4 wt % P2O5 (spessa rtine-sarcopside). The equilibrium among petalite-(amblygonite-monte-brasit e)-quartz is strongly T dependent, and P contents of coexisting melt vary f rom 1.4 to 7.2 wt % P2O5 between 525 and 700 degrees C. with these data and the calibration of P substitution in the alkali feldspars, the phosphorus budget of granitic magmas cart be assessed. Peraluminous partial melts deri ved from metasedimentary rocks are likely to contain <0.5 wt % P2O5 at the source and to be undersaturated in apatite. IJ the incipient crystallizatio n of plagioclase lowers the Ca/P ratio of melt, these magmas can fractionat e to P-rich compositions that give rise to the diverse phosphate mineral as semblages in evolved S-type leucogranites and pegmatites. Where these silic ate-phosphate mineral equilibria apply, the P content of melt should not no rmally exceed similar to 2.5 wt % P2O5. By applying these results to a shar ply zoned, P-B-F-rich granite-pegmatite suite at Tres Arroyos, along the SE margin of the Alburquerque batholith ill Badajoz province, Spain, we can c onstrain: (1) the budget of phosphorus in the various facies of this intrus ive suite; (2) the dynamics of crystallization and the segregation of melt Slam 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.