Stratiform tourmalinites in metamorphic terranes and their geologic significance

Stratiform tourmalinites are significant minor rock types in many regional metamorphic terranes of the world. Tourmalinites are more widespread than previously recognized and are especially common in Proterozoic and early Paleozoic sequences dominated by clastic metasedimentary rocks. They consist of conformable layers made up primarily of quartz and abundant tourmaline, the latter typically exceeding 15% to 20% of rock volumes. A few tourmalinites display striking sedimentary structures such as graded bedding, cross-laminations, slump and flame structures, and rip-up clasts. These and other geologic features provide important constraints for assessing the origin of these rocks, and they suggest that tourmalinites form by the early diagenetic modification of a primary boron-rich chemical precipitate. Tourmalinites are significant in preserving a valuable record of unusual chemical and paleogeographic conditions in clastic sedimentary basins, and in their close association with a variety of stratabound mineral deposits.