Genesis of Peraluminous Granites II. Mineralogy and Chemistry of the Tourem Complex (North Portugal). Sequential Melting vs. Restite Unmixing

Abstract

The Tourem granitic complex (North Portugal) consists of quartz- and alkali-feldspar-rich felsic granites, biotite- and plagioclase-rich heterogeneous granites, and cordierite-biotite granites, containing numerous enclaves of orthogneisses and metapelitic schists. Mineralogical, chemical, and experimental data suggest that all the granites and the orthogneiss enclaves are genetically related. The felsic granites are characterized by normally zoned plagioclase, absence of cordierite, high SiO2 and K2O (72–74 wt.% and 5·4–6·4 wt.%, respectively), moderate P2O5 and REE (0·22–0·24% and 85·0–95·7 ppm), and low Fe2O3* and Zr contents (1·3–1·5% and 80–90 ppm). These features are consistent with those of restite-free melts formed by low extents of melting. Melting experiments show that these felsic granites are likely to be derived by melting of a source material similar to the orthogneiss enclaves under low water activities (∼0·5), at relatively high temperature (∼ 800°C) and <30% melting. The heterogeneous and cordierite-biotite granites display high cordierite contents (up to 30%) in addition to biotite (5–25%), complexly zoned plagioclase, and high Fe2O3 (2·72–6·99%), CaO (0·56–1·95%), Zr (101–213 ppm), and Ce (39·8–98·1 ppm) contents, suggesting that the melts contained significant proportions of residual biotite, cordierite, plagioclase, and accessories. Experimental data indicate that the melts were generated under water-undersaturated conditions but by higher extents of melting (30–60% melting) with probably a larger amount of available water compared with the felsic granites. The major and trace element chemical trends of the granites, which do not define single arrays on two-element variation diagrams, and experimental data show that the generation of the Tourem anatectic complex cannot be explained by the restite unmixing model but could have resulted from sequential low extents of melting with efficient melt segregation followed by higher extents of melting with restite retention.