The petrogenesis and tectonic setting of the New Hampshire Plutonic Suite: Towards a more comprehensive model for the magmatism of the Acadian Orogeny

Abstract

The Wamsutta and Nineteenmile Brook Diorites, two small plutons located in the White Mountains of New Hampshire, have bearing on the tectonic setting of magmatism between 410 and 407 Ma in this portion of the northern Appalachians. The unmetamorphosed, undated Nineteenmile Brook pluton has arc basalt affinities, produced in the same arc as the mafic magmas that were mingled with the ∼410 Ma Meredeth Porphyric Granite of central New Hampshire. These volcanic arc magmas from a westerly dipping subduction zone contributed both heat and mass to the petrogenesis of the New Hampshire Plutonic Suite (NHPS), producing the high temperature melts of the Kinsman Granodiorite of the NHPS. These peraluminous NHPS magmas were emplaced during collision of Avalonia with Laurentia, forming the bases of Acadian thrust sheets. The ∼408 Ma Wamsutta Diorite has appinite-like textures and chemically is a low SiO₂ adakite, with Sr/Y ratios of ∼ 400 and (La/Yb)_N between 80 and 130. These magmas were generated after flat slab, subduction erosion mixed basaltic rocks into the mantle wedge and partially melted the mafic rocks in the garnet stability field. The melts interacted with the surrounding peridotite to attain the low SiO₂ adakite characteristics. At this same time, the ∼407 Ma Exeter Diorite and other arc plutons were emplaced in the Merrimack belt of southeastern New Hampshire. By 400 Ma, continued westerly dipping subduction provided mafic magma underplating to partially melt lower crustal amphibolites, generating the Spaulding Tonalite. Subsequently, lower crustal delamination and asthenospheric upwelling provided the heat source that produced a younger, post-tectonic suite of magmas between 390 and 370 Ma that, while having arc signatures because of the heritage of their crustal source rocks, are not arc magmas because subduction is thought to have ceased by this time. These plutons include the mafic rocks of the Northeast Kingdom of Vermont and the Mooselookmeguntic Igneous Complex of NH and ME. This same heat source may have contributed to melting lower to midcrustal metasediments to produce the widespread peraluminous Concord Granite of Vermont, New Hampshire, and western Maine.