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The evolution of fluorine‐rich felsic magmas: source dichotomy, magmatic convergence and the origins of topaz granite

Topaz granite is alkali‐feldspar granite that contains essential albite, quartz, K‐feldspar, lithium‐mica, and topaz. As a group topaz granites are characterized by their extreme enrichment in F (up to 3 wt%) and a wide variety of lithophile elements. They can be subdivided into a ‘low‐P2O5 subtype’ (P2O5 < 0.1 wt%, Al2O3 < 14.5 wt%, SiO2 > 73 wt%) and a ‘high‐P2O5 subtype’ (P2O5 > 0.4 wt%, Al2O3 > 14.5 wt%, SiO2 < 73 wt%), the δ18O values of which indicate a dichotomy of source rock: the low‐P2O5 subtype (δ18O < 10‰) having a meta‐igneous protolith and the high‐P2O5 subtype (δ18O > 10 ‰) a source with a significant component of pelitic material. The unusually high F contents enhance the efficacy of melt segregation and crystal‐melt fractionation and so facilitate extreme differentiation in topaz granite magmas. Very low melt volumes restrict the bulk composition of the partial melts regardless of the nature of the source; and extreme fractionation forces them along a path of magmatic convergence, to produce a group of granitic rocks with near‐minimum compositions so enriched in a variety of lithophile elements (Li, Nb, Ta, Sn) that economic mineralization often results.