EVOLUTION OF COMPOSITIONAL POLARITY AND ZONING IN TOURMALINE DURING PROGRADE METAMORPHISM OF SEDIMENTARY-ROCKS IN THE SWISS CENTRAL ALPS

The compositional evolution of tourmaline from diagenetic to lower amp hibolite-facies conditions was investigated in two metasedimentary uni ts (redbed and black shale formations) along a traverse across the Cen tral Alps. With increasing metamorphism three distinct rim zones grew around detrital tourmaline cores, simultaneously with the development of prismatic neoblasts. Detrital cores and all successive rim zones ar e preserved in the amphibolite-facies rocks. As indicated by whole-roc k data, the B required for tourmaline growth was released from illite and muscovite and was not introduced from external sources. The studie d samples of tourmaline exhibit compositional polarity, i.e., their co mpositions are different at the apposite ends of the c axis. The compo sitional difference between the two poles can be expressed by the subs titutions Na + Mg reversible arrow square + Al and 2Al reversible arro w Ti + Mg, with the positive pole always richer in Al and having more X-site vacancies, Both chemical and optical polar effects are most pro nounced in the internal rim zones and become less prominent toward the external zones, The systematic prograde compositional trends are depi cted in a vector space for Li-poor tourmaline. At low-grade conditions , the tourmaline composition is clearly controlled by the host-rock co mposition, but with increasing metamorphic grade the compositions of t ourmaline from a variety of rocks converge. The data show that in the Central Alps, increasing metamorphic grade is reflected by increases i n Ca/(Ca + Na) and Mg/(Mg + Fe2+), a decrease in Fe3+, and an increase in the occupancy af the X site of tourmaline.