Regional geochemical and isotopic variations of northern New England plutons: Implications for magma sources and for Grenville and Avalon basement-terrane boundaries

Abstract

Examination of 36 Devonian to Pennsylvanian plutons within and bordering the Central Maine terrane of north-central New England shows that the plutons display regional variations in mineral and bulk-rock compositions and in oxygen and sulfur isotopic values. These variations principally correspond to the regional structural trends of the Central Maine terrane. Plutons located on that terrane9s southeast flank adjacent to the Massabesic Gneiss complex, on the terrane9s northwest flank adjacent to the Bronson Hill anticlinorium, and in the northeastern part of the Northeast Kingdom batholith of Vermont have biotites with relatively low Al concentrations (≤3.0 cations p.f.u. [per formula unit]), feldspars with higher An contents (up to An 55 ), and lower δ 18 O (7‰–9‰) and higher δ 34 S (average of 0‰) bulk-rock values compared to plutons within the central parts of the Central Maine terrane. These characteristics are most compatible with metaigneous or metavolcaniclastic sources. As shown by others, the plutons of Vermont and northwestern Maine preserve Grenvillian signatures whereas those of southeastern New Hampshire (specifically, the granite at Milford, New Hampshire) and Maine (Sebago, Phillips) have Avalonian signatures. Within the interior of the Central Maine terrane of New Hampshire and western Maine and in the Connecticut Valley trough of Vermont are distinct regions characterized by peraluminous plutons with high δ 18 O values (>11‰), low δ 34 S values (−25‰), Al-rich biotites (>3.3 cations p.f.u.), and An-poor plagioclases ( 30 ). These data are most consistent with metasedimentary sources, or, at a minimum, the plutons were heavily contaminated by Central Maine terrane or Connecticut Valley trough metasedimentary rocks. These peraluminous plutons do not have the unique signatures of the underlying Precambrian basements. Several metaluminous plutons in the Central Maine terrane along the New Hampshire–Maine border separate two regions of peraluminous plutons. These metaluminous plutons have biotite and isotopic compositions that are similar to those of the plutons along the flanks of the Central Maine terrane, indicative of less Paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the Central Maine terrane. The metaluminous plutons also have trace element characteristics identical to those of plutons in Vermont and northwestern Maine that have a known Grenville basement-terrane source. We suggest that the metaluminous plutons also had sources in the underlying Grenville basement and that the Grenville-Avalon boundary cuts diagonally across northern New England. Because the plutons with Grenvillian signatures are adjacent to plutons with Avalonian signatures—the Sebago batholith and Phillips pluton—we suggest that there is no Medial New England basement in this region of New England. Some peraluminous, two-mica plutons have trace element and stable isotopic characteristics that are compatible with partial melting of metasedimentary rocks, the components of which were deposited in an anoxic basin such as the Smalls Falls Formation of the Central Maine terrane, but many other plutons have compositions that are inconsistent with this model. We suggest that the compositions of many New Hampshire Plutonic Suite plutons indicate greater heat input than that available from U-enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower crust.