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Geological Society, London, Special Publications; 1989; v. 43; p. 453-460;
DOI: 10.1144/GSL.SP.1989.043.01.41
© 1989 Geological Society of London

Acadian metamorphism in central massachusetts and south-western New Hampshire: evidence for contrasting P-T trajectories

J. C. Schumacher & Renate Schumacher

Mineralogisches Institut, Universität Kiel, Olshausenstrasse 40, D-2300 Kiel, Federal Republic of Germany

Peter Robinson

Department of Geology & Geography, University of Massachusetts, Amherst, MA 01003, USA

Early work by numerous investigators (Robinson 1963, Tracy et al. 1976, Robinson et al. 1982, 1986a, b) provides a structural and metamorphic framework for the Acadian metamorphic high in central Massachusetts within which changes in phase composition and assemblage in the rocks can be correlated. Six metamorphic zones have been distinguished (Tracy et al. 1976, Robinson et al. 1982), reflecting prograde (T) metamorphic conditions in graphite-bearing pelitic rocks (Fig. 1) that range from the amphibolite through to the lower granulite facies. The T and P estimates are based mainly on A. B. Thompson’s (1976) pioneering work in quantifying T and P using biotite-garnet Fe/Mg exchange and the garnet-cordierite-sillimanite-quartz assemblage, respectively.

Background and setting

Understanding of P-T trajectories in a region of complex deformation and metamorphism requires three types of information: (i) major and minor structural features that can be linked in a local kinematic sequence and regionally, to provide time correlation of the tectonic events; (ii) petrological features that give P-T information, and that can be correlated with the sequence of structural events; and (iii) significant geochronological information to provide an absolute time-frame for metamorphic events.

The structural development and metamorphism of the region took place mainly during the Devonian Acadian orogeny and affected stratified and intrusive rocks of late Precambrian, Ordovician, Silurian, and Lower Devonian age. In termsof present tectonic inter-pretations, the area lay east (present geography) of the early Palaeozoic ocean, Iapetus, that closed during the Taconian orogeny, and east of the pertinent subduction zone. It also lay

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M. Brown
P-T-t evolution of orogenic belts and the causes of regional metamorphism
Geological Society, London, Memoirs, 1995; 16: 67 - 81.
[Abstract] [PDF]