A NEW OCCURRENCE OF THE BOROSILICATE SERENDIBITE IN TOURMALINE-BEARING CALC-SILICATE ROCKS, PORTAGE-DU-FORT MARBLE, GRENVILLE PROVINCE, QUEBEC: EVOLUTION OF BORON ISOTOPE AND TOURMALINE COMPOSITIONS IN A METAMORPHIC CONTEXT

Serendibite, Ca1.93Na0.08Mg2.58Al5.03B1.53Si2.71O20, was recently discovered in a lens of B-rich calc-silicate rock metamorphosed at 6-7 kbar, 650-700 degrees C in the Central Metasedimentary Belt, Grenville Province. Based on paragenesis and tourmaline composition, which were used to monitor changes in fluid composition, the following stages of mineralization are recognized: (1) a prograde assemblage consisting of K-feldspar, tourmaline [T1 with X-Ca = Ca/(Ca + Na) = 0.54, T2 with X-Ca = 0.71], and calcite, inferred from relicts in scapolite; (2A) a peak metamorphic assemblage of aluminous diopside, serendibite, lesser phlogopite, and local scapolite (Me-62); (2B) continued formation of phlogopite around serendibite in calcite pockets although serendibite was stable; and (3) high-temperature breakdown of serendibite to uvite (T3; X-Ca = 0.82) + spinel + calcite, and of aluminous diopside to pargasite. Three generations of idiomorphic magnesian tourmaline that crystallized in calcite pockets (T4 to T6) recorded fluid evolution as follows: (A) X-Ca approximate to 0.5 and relatively moderate Ti concentrations (generation T4); (B) increase in Na relative to Ca, X-Ca approximate to 0.3 (generation T5), with the rare occurrence of oligoclase (X-Ca = 0.17) and relatively low Ti concentrations; and (C) X-Ca approximate to 0.5, relatively high Ti concentrations (generation T6). The final stage is localized, low-temperature alteration to fine-grained phyllosilicates. The boron isotopic composition of peak metamorphic serendibite (delta B-11 = +4.3 +/- 1.7 parts per thousand) is lower than that of prograde tourmaline (delta B-11 = +10.6 +/- 2.3 parts per thousand) from which it formed. The fractionation difference between serendibite and metamorphic fluid (with B derived from prograde tourmaline), Delta B-11(Srd-Tur) = delta B-11(Srd) - delta B-11(Tur) = -6.3 parts per thousand, is similar to the ab initio calculated fractionation factor between serendibite and fluid (Delta B-11(Srd-fluid) = -7.6 +/- 1.4 parts per thousand at 727 degrees C). Tetrahedral boron coordination in serendibite is largely responsible for this fractionation. High-temperature uvite replacement of serendibite is significantly higher in delta B-11 relative to serendibite (+10.6 parts per thousand versus +4.3 parts per thousand), which is attributed to buffering of boron isotopes by serendibite in local equilibrium with uvite. Tourmalines formed in calcite pockets range from +8 parts per thousand to almost +12 parts per thousand on average, while the core of a T4 tourmaline has delta B-11 = +3.4 parts per thousand. This low delta B-11 value is attributed to a temporary departure from equilibrium due to fluid influx. The boron isotope signature of prograde tourmaline suggests a marine evaporite source.