Abstract

Oxygen isotopes are a valuable tool for determining the role of supracrustal rocks in the genesis of magmas and help constrain the source regions of plutonism in the northern Appalachian mountains of North America. Zircons from 35 igneous or metaigneous rocks mainly from southern New England, USA, were analyzed for 18O/16O to locate and investigate the origin of anomalous δ18O magmas and to evaluate the regional meteoric water–rock interaction. δ18O values of the zircons obtained using the laser fluorination technique vary between 2.3‰ and 11.0‰, relative to Vienna Standard Mean Ocean Water (VSMOW). Zircons from two plutons in the Avalon terrane reveal the presence of anomalously low-δ18O (<4‰) zircons, lower than typical value for mantle-derived material, viz. 5.3‰ ± 0.6‰ (2σ). These rocks are the Ediacaran Hope Valley Plagioclase Gneiss of central Connecticut, and the Early Devonian Quincy Granite of eastern Massachusetts. The data provide evidence for meteoric water alteration of the Avalonian crustal magma sources. Our search for relatively high δ18O zircons identified several plutons containing zircons with values exceeding 8‰ located across the orogen. Oxygen and Hf isotopic compositions in zircons from three high δ18O plutons, including an Early Devonian muscovite granite from Buckport, Maine, the Ediacaran Dedham Granite from eastern Massachusetts, and the Mesoproterozoic Tyringham Gneiss of western Massachusetts, as well as the Late Ordovician to Early Silurian “Yale Farm” Granite from northern Connecticut (bulk δ18O zircon = 7.56‰), indicate the incorporation of significant amounts of high-δ18O sedimentary components from older source regions into these magmas. The Avalon and Gander terranes of southeastern New England, together with the eastern margin of Laurentia, have experienced prolonged histories of burial or subduction, melting, and assimilation of surface-derived rocks during the Neoproterozoic and the middle Palaeozoic.

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