Preservation of Acadian deformation and metamorphism through intense Alleghanian shearing Tim H. Bell * , Hyeong Soo Kim School of Earth Sciences, James Cook University, Townsville, Qld 4811, Australia Received 5 May 2003; received in revised form 28 December 2003; accepted 5 January 2004 Available online 12 April 2004 Abstract The Northfield syncline in Massachusetts, USA, preserves the same Acadian succession of FIA trends (foliation intersection axes preserved in porphyroblasts) as that in Southeast Vermont, in spite of the overprinting effects of intense Alleghanian deformation and metamorphism resulting from this syncline being thrust southwards over the Pelham gneiss dome. Therefore, both regions were multiply tectonized about the same succession of directions of shortening prior to the Alleghanian, but all relics of Acadian metamorphism were obliterated in the matrix of the Northfield syncline rocks during southwards thrusting. Within the most intensely foliated rocks at the contact between the Northfield syncline and the Pelham gneiss dome, the compositional zoning within the garnet porphyroblasts was homogenized. The 55 8C increase in the rims of these garnet porphyroblasts against the matrix appears to be a product of shear heating that occurred when these rocks were thrust over the Avalonian rocks of the Pelham dome. The lack of equivalent intense Alleghanian shearing in Vermont suggests north central Massachusetts marks the upper contact of the northwest extremity of Avalon. This extremity was less than 10 km thick if it reached SE Vermont suggesting that these rocks were tectonically wedged into North America rather than simply underthrust, and that some delamination of the upper part of this portion of Avalon occurred in Alleghanian times. q 2004 Elsevier Ltd. All rights reserved. Keywords: Shear heating; Homogenization of compositional zoning; Effect of homogenization on inclusion trail geometry; Porphyroblasts; FIAs; Correlation of metamorphism 1. Introduction It has long been apparent that porphyroblasts preserve evidence for foliations and other microstructures destroyed in the matrix, and, potentially provide windows into the geologic history. It has recently been suggested that the extent to which this occurs in polydeformed and poly- metamorphosed rocks is far greater than previously conceptualized (Bell et al., 2003). Protracted histories of deformation and metamorphism, for which all evidence has been destroyed in the matrix, have been revealed by the measurement of foliation intersection/inflection axes pre- served within porphyroblasts (FIAs) and the dating of monazite inclusions that help define them (e.g. Bell and Welch, 2002). The amount of metamorphic information available from porphyroblasts is limited by the modification or homogenization of compositional zoning patterns, such as those in garnet porphyroblasts. Such modifications occur with time at metamorphic temperatures around 700 8C (Tracy, 1982). However, this should not affect the structural information preserved by inclusion trails, where there has been no internal deformation of the porphyroblast and homogenization of compositional zoning occurs by internal diffusion. Consequently, the history of phases of porphyro- blast growth may be preserved by successions of FIAs in rocks where there has been very intense overprinting by deformation and metamorphism associated with subsequent periods of orogenesis, especially where it can be correlated with rocks that have been left unaffected. Where the temperatures have not reached 700 8C, the history of metamorphism may also be preserved within the compo- sitional zoning and be able to be correlated sample to sample through the FIAs. Porphyroblasts preserve long histories probably because they appear to grow during successive deformation and metamorphic events that are partitioned through their immediate vicinity (Bell and Hayward, 1991; Spiess and 0191-8141/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2004.01.006 Journal of Structural Geology 26 (2004) 1591–1613 www.elsevier.com/locate/jsg * Corresponding author. Tel.: þ 61-77-81-4766; fax: þ61-77-25-1501. E-mail addresses: tim.bell@jcu.edu.au (T.H. Bell), hyeong.kim@jcu. edu.au (H.S. Kim).