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.
Acknowledgments
T.C. was supported by an Australian Postgraduate Award from the University of Sydney. Logistical and analytical funding was provided by the School of Geosciences, the University of Sydney (G.L.C.) and through the ARC Discovery Project and Future Fellowship (DP120102060 to S.P. and N.R.D.; FT1101100070 to S.P.). D. Cyprych and P. Trimby are thanked for help with data collection, and the Department of Conservation in Te Anau for permission to visit and sample localities at Breaksea Sound, Fiordland National Park. The authors acknowledge the facilities, scientific, and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Australian Centre for Microscopy & Microanalysis at the University of Sydney. The manuscript was improved after reviews from A. Indares and editorial handling of P. Cordier. This is contribution 1227 from the ARC Centre of Excellence for Core to Crust Fluid Systems (http://www.CCFS.mq.edu.au) and 1267 from GEMOC (http://www.GEMOC.mq.edu.au) The analytical data were obtained using instrumentation funded by DEST Systemic Infrastructure Grants, ARC LIEF, NCRIS, industry partners and Macquarie University.
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