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A rare garnet-tourmaline-sillimanite-biotite-ilmenite-quartz assemblage from the granulite-facies region of south-central Massachusetts

Department of Geology, Eastern Washington University, 130 Science Building, Cheney, Washington 99004, U.S.A.

Correspondence: ^* E-mail: jthomson{at}mail.ewu.edu

A rare lithology consisting of garnet-tourmaline-sillimanite-biotite-ilmenite-quartz has been found within the granulite-facies region of south-central Massachusetts. The homogeneous, Ti-rich oxy-dravitic tourmaline, X_Mg = Mg/(Mg + Fe) = 0.72–0.77, falls into the alkali group, and is similar in composition to lower grade tourmaline found in corresponding metapelitic rocks in Maine. Charge-balancing calculations and binary diagrams suggest that, like biotite in the region, tourmaline has undergone deprotonation by means of the exchange vectors AlOR_–1(OH)_–1 and TiO₂R_–1(OH)_–2, where R represents Fe + Mg. The restriction of a concordant tourmaline-rich horizon in otherwise tourmaline-free rocks of this granulite-facies region suggests that either: (1) B, released during prograde fluid-absent dehydration reactions of muscovite and biotite, was locally available in a fluid phase or melt for later crystallization near the peak of granulite-facies metamorphism along pathways that provided a conduit for fluid migration; or (2) that this is simply a B-rich compositional horizon (tourmalinite) that survived anatexis and granulite-facies metamorphism and that records the incipient conditions of tourmaline breakdown and subsequent recrystallization near or post-peak metamorphism.

Key Words: Massachusetts • tourmaline • granulite-facies • partial melting • deprotonation

This article has been cited by other articles:

				S. H. Buttner and S. A. Kasemann Deformation-controlled cation diffusion in tourmaline: A microanalytical study on trace elements and boron isotopes American Mineralogist, November 1, 2007; 92(11-12): 1862 - 1874. [Abstract] [Full Text] [PDF]