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

The Tourem granitic complex (North Portugal) consists of quartz-and alkali-feldspar-rich felsic granites, biotite- and plagioclase-richheterogeneous granites, and cordierite-biotite granites, containingnumerous enclaves of orthogneisses and metapelitic schists.Mineralogical, chemical, and experimental data suggest thatall the granites and the orthogneiss enclaves are geneticallyrelated. The felsic granites are characterized by normally zoned plagioclase,absence of cordierite, high SiO₂ and K₂O (72–74 wt.% and5?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 thoseof restite-free melts formed by low extents of melting. Meltingexperiments show that these felsic granites are likely to bederived by melting of a source material similar to the orthogneissenclaves under low water activities (0?5), at relatively hightemperature ( 800?C) and <30% melting. The heterogeneous and cordierite-biotite granites display highcordierite 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?1ppm) contents, suggesting that the melts contained significantproportions of residual biotite, cordierite, plagioclase, andaccessories. Experimental data indicate that the melts weregenerated under water-undersaturated conditions but by higherextents of melting (30–60% melting) with probably a largeramount 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 Touremanatectic complex cannot be explained by the restite unmixingmodel but could have resulted from sequential low extents ofmelting with efficient melt segregation followed by higher extentsof melting with restite retention.