Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the adirondack mountains, New York, USA

Details Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the adirondack mountains, New York, USA Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the adirondack mountains, New York, USA

Dec 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.5525e&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 58, Issue 24, pp.5525-5535

Computer Science

Scientific paper

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 18 O wr and major element composition within continuous, mappable, meta-plutonic units. No such relationship is seen among nonconsanguineous granitoids. Variations in mineral 18 O values and large differences in 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 18 O for mineral separates preserve generally high temperature fractionations indicative of dominantly closed-system retrograde exchange. Values of 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 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 18 O wr values can be discriminated from surrounding granitoids having high 18 O wr values. A record of assimilation in the evolution of differentiated granitic units is preserved. The preservation of primary igneous 18 O wr 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.

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