The Late Proterozoic transition from magmatic arc to a transtensional or extensional regime within the Avalon terrane of New England is recorded by granitoid magmatism in southeastern Rhode Island. Plutonism in New England was nearly continuous across this transition. Geochemical discrimination diagrams reveal the transitional nature of the Bulgarmarsh Granite in contrast with the orogenic character of the earlier Metacom Granite Gneiss. The Bulgarmarsh Granite is a mildly peraluminous monzogranite with A-type petrographic features that include clots of mafic minerals and complex feldspar textures indicative of an involved evolution under changing physical conditions. The granite shows geochemical affinities with A-type granitoids, mature arc granitoids, and post-orogenic granitoids. The Metacom Granite Gneiss plots within the I-type orogenic field consistent with its content of plagioclase > potassium feldspar, relict amphibole, and high mafic content. The ca. 595 Ma Dartmouth Pluton, southeast of the Bulgarmarsh Granite, has been interpreted to be a post-collisional alkaline granite marking the transition from compressional to transtensional regimes (Hermes and Zartman, 1992). The geochemistry of the Bulgarmarsh Granite may reflect the onset of that transition. The geochemically distinct Metacom Granite Gneiss likely records the arc magmatism that preceded the transition.
|Original language||English (US)|
|Number of pages||14|
|Journal||Special Paper of the Geological Society of America|
|State||Published - Jan 1 1996|
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