The Ascutney Mountain igneous complex in southeastern Vermont is a Cretaceous member of the White Mountain plutonic-volcanic series. A subvolcanic complex, it consists of three stocks: gabbro-diorite, qvartz syenite, and granite. A syenite porphyry ring dike rims a portion of the complex and contains large xenoliths of a unique breccia.

The gabbro-diorite consists of zoned plagioslas, augite, ferroan to magnesian hornblende, phlogopite, orthoclase, and quartz. A variety of syenites occur and consist of perthite (± atbite ± orthoclase), ferro-edenite, annite, quartz, Fe-rich augite, and fayalite. The granite consists of microperthite, orthoclase, albite, phlogopite, and edenite. Magnetite, ilmenite, apatite, titanite, and zircon are common accessory minerals.

The temperature of the syenite magma was 890−1000 °C, and fo2 ranged from 10−13.7 to 10−12.9 bars, as determined from the assemblage quarlz + magnetite + ilmenite + fayalite. Pressure is constrained to have been approximately 2 kbar. Biotite equilibria indicate that fH2O of the syenite magma was 3300–4800 bars (aH2O = 1.8–2.8).

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First page of Petrology and mineral chemistry of the Ascutney Mountain igneous complex
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