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

Examination of 36 Devonian to Pennsylvanian plutons within and bordering th e Central Maine terrane of north-central New England shows that the plutons display regional variations in mineral and bulk-rock compositions and in o xygen and sulfur isotopic values. These variations principally correspond t o the regional structural trends of the Central Maine terrane, Plutons loca ted on that terrane's southeast flank adjacent to the Massabesic Gneiss com plex, on the terrane's northwest flank adjacent to the Bronson Hill anticli norium, and in the northeastern part of the Northeast Kingdom batholith of Vermont have biotites with relatively low Al concentrations (less than or e qual to 3.0 cations p.f.u. [per formula unit]), feldspars with higher An co ntents (up to An(55)), and lower delta(18)O (7 parts per thousand-9 parts p er thousand) and higher delta(34)S (average of 0 parts per thousand) bulk-r ock values compared to plutons within the central parts of the Central Main e terrane. These characteristics are most compatible with metaigneous or me tavolcaniclastic sources. As shown by others, the plutons of Vermont and no rthwestern Maine preserve Grenvillian signatures whereas those of southeast ern New. Hampshire (specifically, the granite at Milford, New Hampshire) an d Maine (Sebago, Phillips) have Avalonian signatures. Within the interior of the Central Maine terrane of New Hampshire and weste rn Maine and in the Connecticut Valley trough of Vermont are distinct regio ns characterized by peraluminous plutons with high delta(18)O values (>11 p arts per thousand), low delta(34)S values (-25 parts per thousand), Al-rich biotites (>3.3 cations p.f.u.), and An-poor plagioclases (<An(30)) These d ata 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 Ham pshire-Maine border separate two regions of peraluminous plutons. These met aluminous plutons have biotite and isotopic compositions that are similar t o those of the plutons along the flanks of the Central Maine terrane, indic ative of less Paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the Central Maine terrane, The metaluminous plut ons also have trace element characteristics identical to those of plutons i n Vermont and northwestern Maine that have a known Grenville basement-terra ne 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 Grenvillia n signatures are adjacent to plutons with Avalonian signatures-the Sebago b atholith and Phillips plutons-we suggest that there is no Medial New Englan d 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 t he Smalls Falls Formation of the Central Maine terrane, but many other plut ons have compositions that are inconsistent with this model. We suggest tha t the compositions of many New Hampshire Plutonic Suite plutons indicate gr eater heat input than that available from U-enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower cr ust.