DIFFERENTIATION OF STRONGLY PERALUMINOUS, PERPHOSPHORUS GRANITES - THE PEDROBERNARDO PLUTON, CENTRAL SPAIN

The Pedrobemardo pluton (295 +/- 2 Ma; initial Sr-87/Sr-86 = 0.7122) i s a layered, sheet-like granite body with distinct vertical zoning: bi otite +/- muscovite-bearing granites in the lower zone, muscovite +/- biotite leucogranites in the middle zone, and muscovite-bearing aplite s and pegmatites in the upper zone. The magma was anatectically genera ted, intruded almost free of restite, and evolved as a closed system b y crystallization in situ. Geochemical modelling indicates that the lo wer and middle zones followed an equilibrium crystallization path, wit h no separation between solids and melt, and the upper zone followed a fractional crystallization path. Magmatic fractionation produced leuc ocratic differentiates with high P concentrations and a P/Ca ratio hig her than in apatite. The excess P is located in plagioclase and K-feld spar. Monazite (+apatite) fractionation in the lower and middle zones produced extreme depletions in Th, U, Y, and the REEs, and chondrite-n ormalized REE patterns with low LREEs/HREEs and a slight discontinuity between Nd and Sm. About 90% of the Th, U, Y, and REEs reside in acce ssory minerals, especially in monazite. Biotite and muscovite have ver y low REE contents with small positive or no Eu anomalies. K-feldspar and plagioclase in both the lower and upper zones have moderate REE co ntents with positive Eu anomalies. Feldspars from the upper zone, in c ontrast, have negative Eu anomalies and have very low REE contents. Co mpared to available data from literature, D(Sr,Ba,Th,U,Y,REFs)bio/mine ral are lower by one to three orders of magnitude, and D(REEs)plag/Kfs p are also lower by one order of magnitude. These huge discrepancies a re attributed not only to contamination with REE-rich inclusions in mi neral concentrates sed in previous studies, but to real differences in geochemical behavior of trace elements in peraluminous systems, with respect to subaluminous systems. Biotite/melt and, probably, K-feldspa r/melt distribution coefficients for Th, U, Y, and the REEs in peralum inous magmas must be much lower than previously considered.