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Inefficient high-temperature metamorphism in orthogneiss

Authors
  • Chapman, Timothy
  • Clarke, Geoffrey L.
  • Piazolo, Sandra
  • Daczko, Nathan R.
Type
Published Article
Journal
American Mineralogist
Publisher
Mineralogical Society of America
Publication Date
Jan 02, 2019
Volume
104
Issue
1
Pages
17–30
Identifiers
DOI: 10.2138/am-2019-6503
Source
De Gruyter
Keywords
License
Yellow

Abstract

A novel method utilizing crystallographic orientation and mineral chemistry data, based on large-scale electron backscatter diffraction (EBSD) and microbeam analysis, quantifies the proportion of relict igneous and neoblastic minerals forming variably deformed high-grade orthogneiss. The Cretaceous orthogneiss from Fiordland, New Zealand, comprises intermediate omphacite granulite interlayered with basic eclogite, which was metamorphosed and deformed at T ≈ 850 °C and P ≈ 1.8 GPa after protolith cooling. Detailed mapping of microstructural and physiochemical relations in two strain profiles through subtly distinct intermediate protoliths indicates that up to 32% of the orthogneiss mineralogy is igneous, with the remainder being metamorphic. Domains dominated by igneous minerals occur preferentially in strain shadows to eclogite pods. Distinct metamorphic stages can be identified by texture and chemistry and were at least partially controlled by strain magnitude. At the grain-scale, the coupling of metamorphism and crystal plastic deformation appears to have permitted efficient transformation of an originally igneous assemblage. The effective distinction between igneous and metamorphic paragenesis and their links to deformation history enables greater clarity in interpretations of the makeup of the crust and their causal influence on lithospheric scale processes.

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