THE DRY RIVER DIORITE: INSIGHTS TOWARDS THE MANTLE CONTRIBUTION TO THE WHITE MOUNTAIN BATHOLITH, NEW HAMPSHIRE AND THE TRANSITION FROM THE CENTRAL ATLANTIC MAGMATIC PROVINCE

The Dry River Diorite of the White Mountain Batholith is one of the few mafic bodies associated with the batholith. It offers a rare glimpse at the mafic endmember that contributed to the White Mountain Magma Series. The diorite contains aluminous and titaniferous diopside (average Mg# is 72), olivine (Fo_40-46), titaniferous biotite (up to 7.4 wt. % TiO₂), anorthoclase, and secondary amphibole (dominantly actinolite). The Dry River Diorite is mildly silica undersaturated, with chondrite-normalized REE and spider diagram patterns that indicate ocean island basalt compositions. Ce/Yb, (Dy/Yb)_N, (Sm/Yb)_N and (Ce/Sm)_N ratios indicate the magmas are <5% partial melting of garnet peridotites. Modeling of the Dry River Diorite as the mafic endmember of the White Mountain Batholith indicates that the granites of the batholith contain up to 50% crustal endmember.

The western exposures of Conway Granite of the batholith were emplaced as soon as 2 million years after the tholeiitic flood basalts of the Central Atlantic Magmatic Province. The close association in time and space between the batholith and the flood basalts prompts questions regarding their relationship. Ce/Yb, (Dy/Yb)_N, (Sm/Yb)_N and (Ce/Sm)_N ratios indicate that CAMP was partial melts of spinel peridotites compared to the garnet peridotite source of the Dry River Diorite, the inferred mafic endmember to the Conway Granite. Additionally, CAMP has higher initial ⁸⁷Sr/⁸⁶Sr ratios and lower Ɛ_Nd values than the Dry River Diorite. Hence, both trace element and isotopic compositions indicate the Dry River Diorite had a distinctly different source and has different petrogenetic history than CAMP. We propose a propagating fracture model where tensional fractures produced during lithospheric migration caused partial melting of garnet peridotite with the magmas produced in the asthenosphere migrated and penetrated the lithosphere along these fault systems.