Combining compositional zoning and foliation intersection axes (FIAs) in garnet to quantitatively determine early P-T-t paths in multiply deformed and metamorphosed schists: North central Massachusetts, USA

Hyeong Soo Kim, Tim H. Bell

Research output: Contribution to journalArticlepeer-review

31 Citations (Scopus)

Abstract

Quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite-staurolite-garnet grade rocks from the northeastern flank of the Pelham dome, north central Massachusetts, distinguishes the effects of Acadian deformation and metamorphism from extensive overprinting Alleghanian shearing. The P-T conditions and the metamorphic path during the Acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts (FIAs). This Acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an Alleghanian overprint. For each sample where garnet first nucleated during one of these stages in the tectonic history, the PT of core growth was determined by plotting the intersection of the Mn, Fe and Ca isopleths calculated for the core composition on a P-T pseudosection for that sample using THERMOCALC. Combining the PT data from all these samples indicates that the temperature and pressure increased throughout Acadian orogenesis, causing episodic garnet growth. During the Alleghanian, locally intense shearing, especially against the margin of the Pelham dome, formed the dominant schistosity, which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of Acadian deformation and metamorphism in the rock matrix. Shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700°C in those rocks lying directly adjacent to the Pelham dome, and resulted from the rocks of the Northfield syncline being thrust a large distance southwards over the gneisses in the dome.

Original languageEnglish
Pages (from-to)141-163
Number of pages23
JournalContributions to Mineralogy and Petrology
Volume149
Issue number2
DOIs
Publication statusPublished - 2005 Apr
Externally publishedYes

Bibliographical note

Funding Information:
Acknowledgments We acknowledge the Australian Research Council for providing the funds that enabled the research to be done. We thank Dave Moecher and Rodger Powell for careful reviews and suggestions on the first version of this manuscript. We thank Peter Welch for preparing some of the garnet compositional maps and Tom Evans for discussions re all things metamorphic, including THERMOCALC. We thank Peter Robinson for letting us use his detailed maps of the NE margin of the Pelham Dome. We thank JCU for providing superb facilities for microstructural and metamorphic research. Kim thanks to the Industry Mineral Bank (IMB) at the Kangwon National University for partly funding the compositional mapping of garnet.

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

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