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

Abstract

Abstract Serendibite, Ca 1.93 Na 0.08 Mg 2.58 Al 5.03 B 1.53 Si 2.71 O 20 , was recently discovered in a lens of B-rich calc-silicate rock metamorphosed at 6–7 kbar, 650–700 °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 ≈ 0.5 and relatively moderate Ti concentrations (generation T4); (B) increase in Na relative to Ca, X Ca ≈ 0.3 (generation T5), with the rare occurrence of oligoclase ( X Ca = 0.17) and relatively low Ti concentrations; and (C) X Ca ≈ 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 (δ 11 B = +4.3 ± 1.7‰) is lower than that of prograde tourmaline (δ 11 B = +10.6 ± 2.3‰) from which it formed. The fractionation difference between serendibite and metamorphic fluid (with B derived from prograde tourmaline), Δ 11 B Srd−Tur = δ 11 B Srd − δ 11 B Tur = −6.3‰, is similar to the ab initio calculated fractionation factor between serendibite and fluid (Δ 11 B Srd−fluid = −7.6 ± 1.4‰ at 727 °C). Tetrahedral boron coordination in serendibite is largely responsible for this fractionation. High-temperature uvite replacement of serendibite is significantly higher in δ 11 B relative to serendibite (+10.6‰ versus +4.3‰), which is attributed to buffering of boron isotopes by serendibite in local equilibrium with uvite. Tourmalines formed in calcite pockets range from +8‰ to almost +12‰ on average, while the core of a T4 tourmaline has δ 11 B = +3.4‰. This low δ 11 B 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.