The Role of Fluids During Metamorphism of Marbles and Associated Rocks in the Adirondack Mountains, New York.

Abstract

Mineral equilibria and stable C-O isotopic compositions have been studied in amphibolite and granulite facies marbles of the Adirondack Mountains, New York. In combination, these results suggest that complex fluid conditions prevailed at the peak of metamorphism. No mappable isograds were located for five volatilization and one solid-solid reaction within the system CaO-MgO-KAlO(,2)-SiO(,2)-H(,2)O-CO(,2) due to local variations in fluid composition and mineral solid solutions. Common assemblages of tremolite + calcite + quartz and phlogopite + calcite + quartz would not be stable at inferred conditions of 6 to 8 kb, 600-800(DEGREES)C unless P(,total) (TURNEQ) P(,H(,2)O) + P(,CO(,2)). Electron microprobe analysis for 12 major and minor elements indicates that fluorine is an important diluent in some hydrous phases, but that otherwise mineral compositions are close to endmember. The presence of such large amounts of fluorine can significantly extend the stability of tremolite or phlogopite in quartz-saturated marbles. The influence of solid solutions on mineral stabilities has been estimated by thermodynamic calculations using an ideal ionic model for activities. Changes in mineralogy within single outcrops, such as calcite + quartz vs. wollastonite or tremolite + calcite + quartz vs. diopside, cannot be accounted for by solid solution, indicating that fluid compositions have varied at the scale of a few meters. Akermanite (Ak(,90)) coexists with monticellite (Mo(,92)) and wollastonite (Wo(,99)) in an unusual calc-silicate xenolith in anorthosite at Cascade Slide in the Adirondack Highl and s. When the effects of solid solution are modeled, this assemblage, in conjunction with experimental data (Yoder, 1975), indicates that the conditions of metamorphism were 7.4 (+OR-) 1kb and 750 (+OR-) 30(DEGREES)C. Thus, the exotic mineralogy in this xenolith (akermanite, monticellite, cuspidine and wilkeite), which might otherwise be interpreted as indicative of the sanidinite facies, was stabilized during granulite facies metamorphism by low CO(,2) fugacity. Analysis of stable C-O isotopic compositions for assemblages of up to 5 minerals shows concordance of isotopic fractionations, which is consistent with equilibrium at peak metamorphic temperatures. The fractionation of ('13)C between calcite and graphite is consistently small (2.6 to 4.8) in 34 assemblages of the Grenville Province and for 25 Adirondacks samples fractionation decreases regularly with increasing metamorphic temperature. For T = 600(DEGREES) to 800(DEGREES)C, the Adirondack data are described by (DELTA)(Cc-Gr) = 0.00784T((DEGREES)C) + 8.68. This good correlation between (DELTA) and T and the independence of (DELTA) and (delta)('13)C suggests that a theoretical calculation of this fractionation (Bottinga, 1969) is approximately 2('0)/00(, )too large in this temperature range and that (DELTA)(Cc-Gr) should provide an excellent thermometer for high grade marbles. Calcites from granulite facies marbles are similar in (delta)('18)O to those from lower grades and from unmetamorphosed limestones of the same age. Isotopic exchange is seen across marble-anorthosite contacts at the scale of only a few meters. Sedimentary isotopic compositions have been preserved in the core of the Cascade Slide xenolith and in interlayered non-calcareous metasediments. Mass balance calculations of the isotopic effects of pervasive fluid migration and volatilization suggest that if fluids percolated into this xenolith their molecular (H(,2)O + CO(,2))/rock ratio was from 0.0 to 0.1. No isotopic evidence is seen that indicates massive amounts of pervasive oxygen-bearing fluid movements during granulite facies metamorphism. These isotopic results are consistent with amphibole, biotite and melting equilibria which indicate low H(,2)O fugacities in anorthosites and charnockites, but high H(,2)O + CO(,2) fugacities in marble.