Oxygen isotope evidence for the evolution of Adirondack granitoids related to anorthosite

Abstract

The metamorphosed monzonitic to granitic gneiss suite of the Adirondack highlands is spatially and temporally associated with massif anorthosite bodies. Taylor (1969) recognized ^(18)O enrichment above mantle δ^(18)O values in both suites. The internal variation and cause of this enrichment is being examined through oxygen isotope and petrographic analysis of the monzonitic to granitic gneiss suite associated with the Marcy anorthosite. Corroborating evidence is being obtained from the monzonitic to granitic gniess suite associated with the Snowy Mountain Dome anorthosite. Preliminary results of 40 whole rock and mineral separate analyses show that δ^(18)O for the monzonitic to granitic gneiss suite of the Tupper-Saranac sheet (n = 11) and Snowy Mountain Dome area (n = 3) increases with differentiation, as defined by petrography and fractionation trends modeled from samples from this same suite (McLelland and Whitney, 1990). Variation is from +8.2‰ in monzonitic gneiss to +11.6‰ in evolved granitic gneiss. Variation is continuous with ranges for individual gneiss types being monzonite: +8.2-8.9‰, monzosyenite: +8.9‰, syenite: +9.1-9.6‰, Qtz syenite: +9.5-10.2‰, and granite: +10.1-11.6‰. Data for some samples in which mineral pairs (Qtz-Fsp, Fsp-Hb, Fsp-Pxn) have been analyzed are consistent with equilibration at high temperature. Samples of younger, unrelated hornblende granites have whole rock δ^(18)O values comparable to evolved members of the Tupper-Saranac sheet and Snowy Mountain Dome suites. Fayalite granites, also thought to be unrelated to the monzonitic to granitic gneiss suite, have whole rock δ^(18)O values of +6.5 and +8.8‰. Calculations show that crystal fractionation alone cannot explain the trend in ^(18)O enrichment for the monzonitic to granitic gneiss suite. It is concluded that either: 1) assimilation of, or magmatic exchange with Grenville metasediments may have been a significant process during the crystallization of monzonite to granite parental magmas, or 2) the monzonitic to granitic gneiss suite may have been derived from a heterogeneous source, as has been suggested for other Grenville granitoids (Shieh, 1985).

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