The Rattlesnake pluton of Sharon, Massachusetts, is an elliptical alkalic granite stock composed principally of coarse biotite granite and fine riebeckite granite; minor amounts of granite porphyry, riebeckite-biotite granite, and coarse riebeckite granite; and traces of trachyte and ferrohastingsite granite. Major-element chemical analyses are reported for the coarse biotite granite, fine riebeckite granite, trachyte, biotite from the coarse biotite granite, and four riebeckite samples (including one from Rhode Island). Sodium and potassium analyses are reported for 11 samples of alkali feldspar, including three from the Quincy pluton. Feldspar from the Rattlesnake pluton shows a trend in Or enrichment, reflecting a decrease in temperature of crystallization that roughly follows the evolutionary order of the various rock types. Feldspar phenocrysts of the granite porphyry are chemically related to feldspar of the associated coarse biotite granite; however, thegroundmass feldspar is more sodic, indicating a markedly different crystallization history, most likely related to a water-vapor pressure-quench episode. In contrast to the fine riebeckite granite, the quartz veins and pegmatite have higher concentrations of Na, Li, Fe, and F, which indicate enrichment in those elements in the late-stage fluids. Modal and chemical analyses of the principal granites fall close to the ternary minimum in the system Ab-Or-SiO 2 -H 2 O and indicate maximum crystallization temperatures of 790° to 800° C. The trachyte probably crystallized at a temperature around 925° C and at a water-vapor pressure of about 500 kg/cm 2 . Most granite bodies of the Rattlesnake pluton crystallized at temperatures between 660° and 800° C and at water-vapor pressures probably close to 1,000 kg/cm 2 . Minor oxide content in the rocks of the pluton suggests their consanguinity with other alkalic granite plutons of southern New England. Granite magma forming the pluton presumably underwent differentiation associated with transfer of Na, Li, Fe, and F toward the roof of the magma chamber. Two stages of magma upwelling associated with blister collapse and cauldron subsidence are hypothesized. The first stage resulted in the upwelling of a peralkaline magma that formed the fine riebeckite granite. This was followed by vapor-state transfer of Na, Li, Fe, and Si (probably as fluorides) and of H 2 O, which produced a pegmatite zone in the fine riebeckite granite and in the country rocks. The residual magma thus became peraluminous by loss of Na and later moved upward during another stage of collapse, crystallizing as coarse biotite granite. K-Ar dating of the granite at Rattlesnake Hill indicates a Middle Devonian age (about 370 m.y. B.P.), which is in agreement with radiometric dates for the Peabody Granite and with widespread igneous activity thoughout New England during Acadian time. This date, however, is lower than radiometric dates for the Cape Ann and Quincy Granites, which fall around 400 to 450 m.y. B.P.; this variance suggests two or more periods of alkalic magma intrusion in eastern Massachusetts.