Compositions of amphibole and pyroxene in the various units of the Belknap Mountain complex of central New Hampshire indicate a diversity of parental magmas. Amphibole and pyroxene in camptonites and Moat Volcanic trachyandesite, and amphibole in the Vent Agglomerate dike, indicate crystallization from silica-undersaturated magmas similar to those of the Monteregian Hill intrusions. The Belknap Syenite, Cobble Hill Syenite, and Conway Granite are silica-saturated; amphibole and pyroxene from these rocks are typical of the White Mountain Magma Series.

Pyroxene in the camptonites, Gilford Gabbro, Vent Agglomerate dike, and the Endicott Diorite has similar Mg/(Mg + Fetot) values, but displays different [4]Al, Ti, Na, and Ca contents and zoning. These differences imply that even the units that may have had a common parent evolved independently from each other and are probably not related by shallow level processes.

Amphibole samples from the Conway Granite have higher Mg/(Mg + Fetot) values and are richer in Si and poorer in Al, Ti, and Na + K than those from the Vent Agglomerate dike. Amphibole from the Belknap Syenite forms a compositional continuum between amphibole in the Vent Agglomerate dike and Conway Granite. The nature of Belknap Syenite amphibole evolution, coupled with the presence of nepheline inclusions in oligo-clase and interstitial quartz, suggests that the Belknap Syenite may have originated through mixing of a silica-poor magma similar to the Vent Agglomerate dike with a magma similar to the Conway Granite.

Underplating of the crust by strongly silica-undersaturated magmas similar to those in the Monteregian Hills may have partially melted lower crustal rocks to form the Conway Granite. Mixing of these two main magma types can account for the origin of the Belknap Syenite and may serve as a model for many of the alkaline, silica-saturated rocks of the White Mountain Magma Series.

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