Contamination of the Marcy Anorthosite Massif, Adirondack Mountains, NY: petrologic and isotopic evidence

Abstract

Oxygen isotope analyses of 101 samples from the Marcy Anorthosite Massif (61 from this study, 40 from Taylor 1969), indicate that two major and distinct processes of crustal contamination have affected the massif. Ninety percent of the 93 samples with over 65% plagioclase are enriched in 18O by ∼ 2.6 ‰ relative to “normal” anorthosites or gabbros: the average δ18O for 83 enriched samples is 9.5 ‰ Depletions in 18O occur in 8% of the samples which have values ranging from 3.0 to 5.8 ‰ Only 2 of the samples fall within the normal magmatic range for anorthosites. Low δ18O values of 3.0 to 5.8 ‰ in the anorthosite occur only near contacts, and a gradient in δ18O occurs near the contact within the border zone of the massif. Low δ18O values in both the anorthosite and adjacent wollastonite skarns (with δ18O down to −1.3) were probably caused by isotopic exchange with heated meteoric water when the anorthosite intruded at shallow levels, prior to Grenvilleage (∼ 1.1 by) granulite facies metamorphism. The 18O-enrichment was ascribed to exchange between anorthosite and 18O-rich marble by means of a pervasive, H2O-CO2 fluid during the regional metamorphism by Taylor (1969). However, a number of lines of evidence argue against this hypothesis: 1) the preservation of premetamorphic low δ18O values in anorthosite from the border zone as well as preserved gradients in δ18O from a number of localities, 2) mass balance calculations of the amount of marble necessary to produce the 18O enrichment 3) metamorphic phase equilibria which buffer $$P_{{\text{CO}}_{\text{2}} } $$ and $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ to low values, and, 4) recent oxygen isotope analyses show homogeneity which indicates that magmatic oxygen isotope compositions have been preserved. We evaluated the importance of magmatic assimilation of country rock at the present level of intrusion as an alternative cause of the 18Oenrichment. Samples from 2 distinct lobes of the massif were analyzed: the NE lobe where xenoliths of metasedimentary country rock are common, and the NW lobe, where xenoliths are scarce and the country rock is dominantly granitic. The mean values of δ18O for these two lobes are 9.6 in the NE and 9.3 in the NW. Thus, magmatic assimilation at the present level of exposure probably had only localized and relatively minor effects on the oxygen isotope composition of the massif. This conclusion is supported by Rb/Sr data: variations in Rb content and (87Sr/ 86Sr)i show that such crustal contamination is localized, generally occurring only in samples near the border zone. All of the available results suggest that the 18O-enrichment is a magmatic feature, acquired prior to intrusion at the present level of exposure.