TOURMALINE TEXTURES IN THE LAND’S END GRANITE, SW ENGLAND – A STROLL THROUGH THE MAGMATIC-HYDROTHERMAL TRANSITION

Tourmaline is a common mineral throughout the granites of SW England. It occurs in multiple textural varieties, including veins, quartz-tourmaline orbicules, massive quartz-tourmaline rocks (MQT), pegmatitic pockets, and disseminated in the granite. Field relations indicate both magmatic tourmaline and post-magmatic, hydrothermal tourmaline, occuring as disseminated grains and veins, respectively. In addition, more genetically complex textures are observed as gradual exsolutions of quartz-tourmaline rocks within the granite, with or without associated veining. These appear on the cm scale and up to several meter large bodies. Such textures are observed in localities several km apart, and are a regional feature. Occasionally, the gradual transition from granite to MQT to miarolitic tourmaline or a pure tourmaline zone, can be observed over tens of cm. The microtextures are strongly linked to the macroscopic textures, reflecting the diverse processes that formed the different tourmaline occurrences. Strongly birefringent, green, acicular tourmaline dominates in vein-style tourmaline; disseminated tourmaline in the granite is weakly zoned and brown; tourmaline from MQT is brown with blue rims. Later tourmaline generations overgrow earlier generations, adding to the complexity. Cassiterite is only associated with the hydrothermal green acicular tourmaline. Tin concentrations in other tourmaline-rich rocks are less than 100 ppm, and these rocks are not a part of the mineralizing events of the Land’s End granite.