THE NEW HAMPSHIRE PLUTONIC SUITE: WHAT DO WE KNOW, WHAT PROBLEMS REMAIN?

The New Hampshire Plutonic Suite (NHPS) contains four members, the Bethlehem Granodiorite (BG), the Kinsman Granodiorite (KG), the Spaulding Tonalite (ST), and the Concord Granite (CG) with ages of ~ 413, 413, 400, and 365 Ma respectively. Problems pertaining to the petrogenesis of the NHPS include the tectonic setting, the nature of the source rocks, and the heat sources. The KG contains garnetites produced by biotite-dehydration melting reactions, requiring temperatures of ~ 800-850^oC. Mafic rocks from KG mingled zones have Ɛ_Nd and Sr_i as primitive as 5.2 and 0.7034 respectively, suggesting mafic magmas contributed to the high temperatures of the KG system. The most mafic ST rocks (46% SiO₂) have positive Eu anomalies, relatively sodic plagioclase (<An₄₄), and have amphibole compositions suggestive of crystallization from magmas with Mg#s of ~ 35. These rocks are cumulates from magmas of intermediate compositions that were probably produced by partial melting of amphibolites. The two mica-bearing Concord Granites are similar in composition to leucosomes in Central Maine Trough (CMT) migmatites. However, most leucosomes in the CMT crystallized at ~ 400 Ma, too old to have accumulated to produce the Concord plutons. Crustal thickening models predict post-tectonic magmatism up to ~50 m.y. after thrusting, a model that may apply to the Concord Granites. However, some 380-360 Ma plutons across coastal New England contain minor amounts of mafic rocks, suggesting a mafic magma heat source may have been influential in generating some of the 365 Ma crustal melts as well. Lithospheric delamination and upwelling of the asthenosphere and basaltic magma underplating are methods of focusing mantle heat to the base of the crust to produce the NHPS, but a comprehensive NHPS model must account for the presence of mafic magmas at 413 and 395 Ma, and younger mafic magmas at 365 Ma. The KG and BG may have been produced by biotite dehydration melting caused by the last subduction zone magmas intruding into the deforming CMT metasediments, the ST from asthenospheric upwelling following lithospheric delamination causing amphibole dehydration melting, and the CG from either crustal thickening and/or the transpressive effects of the approaching Meguma terrane, producing within-plate basaltic magmas that contributed to crustal melting.