Geochemical behavior of sulfur in granitoids during intrusion of the South Mountain batholith, Nova Scotia, Canada

Abstract A detailed stable isotope (S, O) and elemental (major, trace, S content) study of peraluminous S-type granites and granodiorites of the South Mountain Batholith and the Meguma Group metasediments into which the batholith was intruded has been conducted on a regional and local scale near Mt. Uniacke. Sulfur contents of the metasediments range from 5 to 2440 ppm, and δ 34 S ranges from −3.7 to +26.3%. Sulfur contents of the igneous rocks range from 6 to 570 ppm, with the granites ( ≈20 ppm) having lower sulfur contents than the granodiorites ( ≈300 ppm). Approximately 30 to 60% of the sulfur in the granodiorites is present as pyrrhotite (a late phase), with most of the remainder being present as sulfur substituted into biotite (an early phase). δ 18 OQ Quartz values of the Mt. Uniacke granodiorites are heavy (≈a + 12.2%.) indicating formation of the magmas by partial melting of sedimentary rocks. Major element variations suggest that the granites are related to the granodiorites by fractional crystallization of plagioclase, quartz, and biotite. Comparison of the sulfur contents of the granodiorites with experimental determinations of sulfur solubility suggest that the distribution coefficient of sulfur ( D s ) between the cumulate and the magma during fractional crystallization ≈4. The regional granodiorites have a relatively restricted range of δ 34 S values (+5.4 to + 8.4%.), while the regional granites have a wide range of δ 34 S values ( + 1.6 to + 15.0%.). Granodiorites at Mt. Uniacke with normal magmatic values (+0.76 to +1.42%.) for Δ 18 O (Quartz-Feldspar) have δ 34 S values between +6.6 and +12.3%., and show good negative correlations between δ 34 S and S, FeO, and MgO contents. Some granodiorites at Mt. Uniacke have anomalously low sulfur contents for their major element composition, and Δ 18 O (Quartz-Feldspar) values (−0.45 to +0.67%. ) indicate that these samples have undergone hydrothermal alteration, and this alteration event is probably responsible for the low sulfur contents. A number of processes have been considered in order to understand the variations of δ 34 S. Partial melting of inhomogeneous source rocks may be responsible for some of the regional δ 34 S variations, but this implies that the granites are not genetically related to the granodiorites. Inhomogeneous source rocks are an unlikely explanation for the systematic variations of δ 34 S observed over only a few kilometres at Mt. Uniacke. Rayleigh fractionation accompanying the fractional crystallization of biotite [ Δ 34 S(pyrrhotiteS — silicate-S) ≈ +0.5 to +1.0%. ] can account for only a small component of the observed δ 34 S variations. The assimilation of isotopically variable sulfur derived from the Meguma Group metasediments during fractional crystallization (combined assimilation-fractional crystallization, or AFC) provides the best explanation for the δ 34 S variations. Transfer of sulfur from the country rock may have been possible by either bulk AFC (where all of the country rock material is incorporated into the magma) or by selective AFC (where sulfur and other volatile elements only are transferred from the country rock into the magma). Selective AFC could have been effected by a sloping mechanism of intrusion, followed by the heating of country rock xenoliths to the point where extensive desulfidation reactions took place, with sulfur being transferred from the country rock to the magma by a fluid phase.