Oxygen isotope studies of anorthosites, with particular reference to the origin of bodies in the Adirondack Mountains, New York

Abstract

Oxygen isotope analyses have been obtained on samples from many anorthosite bodies throughout the world, including both the large Precambrian domical massifs (Egersund, San Gabriel Mts., Laramie, Lac St. Jean, etc.) and the layered gabbroic complexes (Bushveld, Stillwater, Duluth, etc.). The O¹⁸/O¹⁶ ratios of plagioclase from all these anorthosites are very similar, with δ-values of 5.8 to 7.6 per mil relative to standard mean ocean water; this is identical to the range of O¹⁸/O¹⁶ in plagioclase from basalts and gabbros. These data are compatible with the hypothesis that both types of anorthosites are related to gabbroic magmas. The anorthosite bodies in the Adirondack Mountains, New York, have been intensely metamorphosed, and they also have considerably higher O¹⁸/O¹⁶ ratios than the anorthosites mentioned above (δ = 8.9 to 11.2 per mil). It is logical to attribute this O¹⁸-enrichment to the metamorphic episode, in as much as similar isotopic relationships are observed in other regional metamorphic terranes, though on a much smaller scale. Igneous rocks are generally lower in O¹⁸ than sedimentary rocks, but regional metamorphism tends to smooth out any oxygen isotope gradients that may exist in a series of interbedded rock types. In general, the metaigneous rocks become richer in O¹⁸, and the metasedimentary rocks are depleted in O¹⁸; it may be that the limestone and dolomite-rich metasedimentary section was the source of the O¹⁸-rich fluids (H₂O and/or CO₂) that interacted with the anorthosite bodies. This is particularly feasible if carbonate-rich metasediments are assumed to underlie the slab-like anorthosite body. Most of the meta-igneous rock types of the Adirondack area are enriched in O¹⁸ relative to their unmetamorphosed counterparts in other localities, so a large fraction of the present Adirondack terrane cannot have been a closed system during metamorphism. Only a few relatively unfoliated metagabbros and some granitic plutons (synchronous with the metamorphism?) have "normal" igneous O¹⁸/O¹⁶ ratios. In two other anorthosite occurrences, Boehls Butte, Idaho, and Morin, Quebec, limited sampling has shown some O¹⁸-rich plagioclase. In both cases there is field and petrologic evidence of metamorphism of the anorthosite bodies and there are juxtaposed metasediments containing abundant limestones or calcsilicate rocks. Some unusually low O¹⁸/O¹⁶ ratios have been observed in skarn zones peripheral to the Adirondack anorthosite massif, particularly in the Willsboro wollastonite deposits (δ = -0.9). Some garnet-rich anorthosites at the margins of the massif are appreciably lower in O¹⁸ than "normal" unmetamorphosed anorthosites, and another specimen adjacent to a skarn zone in the Boehls Butte area has the lowest O¹⁸/O¹⁶ ratio of any plagioclase yet analyzed (δ = -1.6). It is tentatively concluded that these abnormally low O¹⁸/O¹⁶ ratios are the result of strong decarbonation and expulsion of O¹⁸-rich CO₂, because at equilibrium, CO₂ is known to concentrate O¹⁸-relative to the minerals in question.