TOURMALINE – A KEY MINERAL FOR UNDERSTANDING PROCESSES IN COLLISIONAL OROGENS (Invited Presentation)

An important feature of most collisional orogens is the association of leucogranites and pegmatites with thick, metapelitic sequences. Isotopic systems demonstrate that the granites are derived from the metapelites. Tourmaline, along with biotite, is the most important ferromagnesian mineral in the granite-metapelite systems, and records several processes, including partial melting of muscovite-bearing schists, rapid fractional crystallization of melts, and metasomatism. A typical tourmaline in leucogranites ranges from a few millimeters in length in aplitic layers to several decimeters in pegmatitic layers, with the difference in morphology related to competition between rates of nucleation and growth. In pegmatitic layers, tourmaline’s c-axis usually points away from wall-rocks. Crystals have skeletal, patchily-zoned cores that are wrapped by more homogeneous mantles. The tourmaline is schorl zoned from Mg# [Mg/(Mg+Fe)] 0.5 to 0.15 and has excess of c. 0.5 Al atoms on the three Y sites. The middle of the Mg# range reflects equilibrium between residual biotite and melt in metapelitic source rocks. Boron is mostly derived from muscovite in the source rocks. Muscovite together with feldspar and quartz is the first mineral to participate in melting of metapelites, thus tourmaline leucogranites are probably the first melts produced during partial melting. Biotite and tourmaline usually coexist only on the pluton-scale where one portion of a pluton is biotite-bearing and another tourmaline-bearing. In collisional terranes, Li-bearing tourmaline (elbaite) is typically confined to spatially separate pegmatite intrusions. The proportion of elbaite in the tourmaline typically increases from wall zones to cores of pegmatites, reflecting the incompatibility of Li with early-crystallizing schorl tourmaline and other minerals.

Metasomatic replacements of micas by tourmaline are common in aureoles of leucogranites. They underscore the high mobility of B in fluids that emanate from the intrusions. Zoning in the tourmaline suggests a preferential replacement of muscovite over biotite in schists during early stages of metasomatism. The tourmaline often coexists with graphite and has methane-bearing fluid inclusions, demonstrating that tourmaline can be stable in low-fO₂environments.