The petrogenetic and metallogenetic significance of topaz granite from the Southwest England orefield

Late-stage volatile-rich topaz granites occur widely but sparsely throughout the southwest England Sn-Cu-polymetallic mineralized S-type biotite granite batholith. New observations from the St. Austell area have clarified field relations, and demonstrate the importance of an aureole of tourmalinization affecting both granitic and sedimentary host rocks. Topaz granite contacts are often marked by pegmatitic zones showing undirectional solidification textures and carrying vugs of quartz-tourmaline; minor intrusive sheets have symmetrical haloes of tourmalinization within adjacent host rocks. The topaz granites are mineralogically complex, containing primary topaz, zinnwaldite, or lepidolite, amblygonite (and other phosphates), and various Nb-Ta rich accessory phases, as well as albite, orthoclase, and quartz. Fluorite is secondary. They are chemically distinct from the biotite granites, showing markedly higher concentrations of Li (sub 2) O (as much as 0.5 percent), F (as much as 1.5 percent), P (sub 2) O (sub 5) (0.5 percent), Nb (as much as 65 ppm), Ta (as much as 30 ppm), Ga (as much as 50 ppm), and Rb (as much as 2,000 ppm), in particular. Late differentiates of the biotite granites include tourmaline granites, but this differentiation trend principally involves an increase in B with little change in Li, F, or P. It is considered that the topaz granites are unlikely to be derived by fractional crystallization of the southwest England biotite granite magma (they are intruded by rhyolite porphyry dikes, which belong to the biotite granite suite), and an origin involving limited partial melting of subbatholithic fusion residues during an episode of potassic basic magmatism is preferred. Comparison with other volatile-rich granitic rocks indicates that certain lithium pegmatites (e.g., Tanco), other topaz granites (Seward Peninsula, Erzgebirge, etc.) and volcanic glasses (e.g., Macusani) share important characteristics with the southwest England topaz granites. It is suggested that these rock types may represent a fundamentally similar volatile-rich granite magma type whose formation, while debatable, may be controlled by limited partial melting of lower crustal fusion residues that had previously generated more "normal" granite magmas.