Stable isotopic and petrological constraints on scapolitization of the Whitestone meta-anorthosite,Grenville Province,Ontario*

The Whitestone Anorthosite (WSA), located in the Central Gneiss Belt of the south-western Grenville Province, Ontario, exhibits a nearly concentric metamorphic envelope characterized by an increase in modal scapolite, hornblende, epidote and garnet, developed around a core of granulite facies clinopyroxene ± orthopyroxene ± garnet meta-anorthosite. Scapolite- and hornblende-bearing assemblages develop mainly at the expense of plagioclase and pyroxene within the envelope. Stable isotopic and petrological data for scapolite-bearing mineral assemblages within meta-anorthosite constrain the source of carbon responsible for CO₃-scapolite formation and the extent of fluid/rock interaction between the anorthosite and adjacent lithologies. Stable isotopic data indicate increasing δ¹⁸O and δ¹³C from core to margin of the meta-anorthosite and for samples from the southern extension of the WSA, where it is ductilely deformed within the Parry Sound Shear Zone (PSSZ). The average δ¹⁸O_SMOW value (whole rock) for the WSA core is 6.9‰, increasing to 11.5‰ where the WSA is in tectonic contact with marble breccia. The average δ¹³C_PBD value of scapolite in meta-anorthosite from the centre of the WSA is -3.4‰, increasing to -0.5‰ at the eastern (marble) contact. Average values of δ¹³C for scapolite and whole-rock δ¹⁸O for samples from the shear zone are -1.0 and 8.0‰, respectively. Marbles have average δ¹⁸O and δ¹³C values of 19.2 and -0.4‰, respectively. The sulphate content of texturally primary scapolite decreases from the core of the WSA (X_SO4= 0.48) to the eastern contact (≤0.05). Texturally late scapolite after plagioclase and garnet tends to be CO₃-rich relative to texturally primary scapolite, and some scapolite grains show zoning in the anion site with CO₃-enriched rims. Scapolite composition may vary at any scale from a single grain to outcrop. The pattern of isotopic enrichment in ¹³C and ¹⁸O preserved in the eastern margin of the WSA is consistent with marble as the major source of fluid contributing to the formation of the metamorphic envelope. The decrease in X_SO4 and increase in X_CO3 in scapolite toward the margin of the WSA indicate that the volatile content was reset by, or developed from, a CO₂-bearing fluid. Assuming derivation of fluid from marble, minimum fluid/rock values at the margin of the WSA range from 0.03 for the least enriched, to 0.30 for the most isotopically enriched samples. Although marble is not found in immediate contact with samples of sheared meta-anorthosite from the PSSZ, a marble source is also consistent with the C and O isotope composition and anion chemistry of scapolite within these samples.