White Mountain Magmatism of New England
The White Mountain batholith, central New Hampshire, is a member of the older White Mountain igneous province. The batholith consists of a number of overlapping centers of felsic magmatism comprised of quartz syenite porphyry ring dikes, alkali granite, metaluminous biotite granite, and felsic volcanics. Igneous activity occurred over an extended period of time from ca. 200 Ma to 155 Ma.
Both chemically and mineralogically the various suites of the batholith are A-type granitoids. Absolute abundances of alkalis, total Fe, REE (except Eu), Y, Nb, Ta, Hf, Zr, Th, and U are high relative to other granitoid types. The mafic mineralogy is characteristic of A-type granitoids; ferrohedenbergite, fayalite, ferrohastingsite, and riebeckite.
Isotopic and trace-element data indicate that a number of the suites evolved by closed system crystal fractionation. The evolution of these suites was largely controlled by feldspar (perthite, K-feldspar, plagioclase), fayalite, ferrohedenbergite, and ilmenite fractionation. Locally contaminated phases attest to limited open system fractionation at high crustal levels.
While Sr isotopic ratios are generally significantly greater than mantle values, trace-element ratios are typical of magmas derived from oceanic-island-basalt-like sources. Modeling shows that the high 87Sr/86Sr ratios can be explained by limited contamination of highly evolved magmas by radiogenic crust.
The preferred model for the origin of the batholith involves the emplacement of mantle-derived melts into the base of the crust. These magmas evolve, with some crustal contamination, at this level. The resulting homogeneous magmas are then emplaced at higher levels in the crust where further crystal fractionation and contamination can occur. This is essentially the MASH model advanced by Hildreth and Moorbath (1988).