Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the Adirondack Mountains, New York, USA
The Adirondack Mountains, New York, expose a diverse group of Proterozoic igneous rocks that were metamorphosed to granulite facies conditions during the Ottawan phase of the Grenville orogeny. Oxygen isotope data for seventy whole rock samples of gabbroic to granitic meta-igneous rocks, primarily from the charnockite suites from the Tupper and Saranac sheets in the central Adirondacks, demonstrate a correlation between δ^(18)O_(wr) and major element composition within continuous, mappable, meta-plutonic units. No such relationship is seen among nonconsanguineous granitoids. Variations in mineral δ^(18)O values and large differences in δ^(18)O between rocks with nonconsanguineous protoliths but similar bulk composition demonstrate that these rocks were not infiltrated by, or isotopically equilibrated through, a pervasive metamorphic fluid. Values of δ^(18)O for mineral separates preserve generally high temperature fractionations indicative of dominantly closed-system retrograde exchange. Values of δ^(18)O_(wr) in Adirondack orthogneisses were not significantly shifted during granulite facies metamorphism and were dominantly controlled by processes active during premetamorphic magmatism. Values of δ^(18)O_(wr) may thus serve as a petrogenetic indicator, allow discrimination of rock units, and serve as a source constraint for meta-igneous rocks. Fayalite meta-granites with low δ^(18)O values can be discriminated from surrounding granitoids having high δ^(18)O values. A record of assimilation in the evolution of differentiated granitic units is preserved. The preservation of primary igneous δ^(18)O values in granulite facies orthogneiss imposes constraints on the synmetamorphic and postmetamorphic fluid history of the Adirondack Highlands. Oxygen isotopic compositions at the hand-sample scale have been preserved through granulite-facies metamorphism. Fluid absence or low fluid/rock ratios on a regional scale are indicated.
© 1994 Elsevier Science Ltd. Received March 21, 1994; accepted in revised form July 18, 1994. We gratefully acknowledge Shelby Boardman and Cathy Manduca for use and assistance in the X-ray fluorescence laboratory at Carlton College. We also wish to thank Jim McLelland and Lukas Baumgartner for stimulating and helpful discussion; Kevin Baker and Mike Spicuzza for assistance in the stable isotope laboratory, and Clark Johnson for use of specimen preparation facilities and a helpful review of an earlier version of this work. We thank S. Hoernes, U. Lichtenstein, J. Morrison, and Z. Sharp for thorough and helpful reviews. This work was supported by the National Science Foundation (grant EAR89-05101 and a graduate research fellowship), and a Geological Society of America Penrose research grant. Editorial handling· K. Mezger.