A comparison of microstructural and chemical patterns in garnet from the Fleur de Lys Supergroup,Newfoundland |
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| Institution: | 1. School of Earth Sciences, China University of Geosciences, Wuhan 430074, China;2. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;3. Engineering Research Center of Nano-Geo Materials of Ministry of Education, China University of Geosciences, Wuhan 430074, China;1. Institute of Mineralogy, University of Münster, 48149 Münster, Germany;2. School of Earth and Space Sciences, USTC, Hefei, Anhui 230026, China |
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| Abstract: | This study uses compositional analyses of garnet porphyroblasts to test a previously published microstructure-based model of garnet growth in the Fleur de Lys Supergroup. X-ray maps reveal significant compositional anomalies within garnet, including zoning reversals and steepened compositional gradients. These anomalies occur at the margin of the proposed first stage of garnet growth (G1), and coincide with truncations of inclusion trails and changes in the inclusion assemblage. Intervals of reversed composition zoning and steepened compositional gradients across this boundary are interpreted to represent a hiatus in garnet growth, possibly accompanied by garnet consumption, during which changes in the garnet-forming reaction, P–T conditions and deformation kinematics occurred. The junction of the proposed second and third stages of garnet growth (G2 and G3) coincides with the transition between successive crenulation cleavages, without substantial microstructural truncations or changes in the inclusion assemblage. The G2–G3 boundary is generally marked by uninterrupted normal zoning, with subtle compositional anomalies in some samples. This boundary may in fact record continuous garnet growth, or alternatively mark a relatively short intra-orogenic pause in garnet growth with minimal affect on zoning patterns. Individual porphyroblasts with contrasting inclusion trail microstructures also have different zoning patterns, and this supports the previous recognition of contrasting growth histories between individual porphyroblasts. A combined structural–metamorphic model is presented that integrates the timing of garnet growth and foliation development, reaction history and the P–T–t path in the Fleur de Lys Supergroup. |
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