Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics

Ribeiro, B.V., Lagoeiro, L., Faleiros, F.M., Hunter, N.J.R., Queiroga, G., Raveggi, M., Cawood, P.A., Finch, M., and Campanha, G.A.C. (2020) Strain localization and fluid-assisted deformation in apatite and its influence on trace elements and U–Pb systematics. Earth and Planetary Science Letters, 545. 116421.

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Abstract

This paper presents electron backscatter diffraction (EBSD), trace element and U–Pb data of apatite grains from a granitic mylonite from the Taxaquara Shear Zone (SE Brazil). The mylonite recrystallized under upper-greenschist facies and presents two types of apatite with distinct microstructures. Type-1 apatite appears in quartz-rich layers and does not exhibit any microstructural, crystallographic, or chemical evidence of deformation/recrystallization, and resembles the original igneous apatite. Type-2 apatite appears in mica-rich layers and exhibits core-and-mantle microstructures, and intragranular subgrain development, suggesting that they have undergone dynamic recrystallization. Recrystallized tails of type-2 apatite grains exhibit a strong c-axis crystallographic preferred orientation parallel to the X-direction (stretching lineation), and lack evidence of dislocation density. This evidence from type-2 apatite grains, combined with REE depletion, high La and a negative Ce anomaly compared to type-1 grains, suggests that type-2 apatite tails underwent recrystallization via dissolution-precipitation creep, whereas parental grains underwent crystal-plastic deformation and subgrain formation through dynamic recrystallization. Phase-equilibrium modelling and quartz CPO opening-angle thermometry are consistent with recrystallization at ∼480 – 530°C and 2.2 – 5.0 kbar. We were not able to determine precise deformation ages from type-2 apatite because fluid-assisted recrystallization appears to have substantially decreased the U/Pb ratio. We find that preferential fluid flow along high-strain, biotite-rich layers in the mylonite caused type-2 apatite to recrystallise, whereas type-1 apatite in low strain layers was unaffected and retained the characteristics of the protolith.

Item ID: 73531
Item Type: Article (Research - C1)
ISSN: 1385-013X
Copyright Information: © 2020 Elsevier B.V. All rights reserved.
Date Deposited: 04 May 2022 03:54
FoR Codes: 37 EARTH SCIENCES > 3705 Geology > 370511 Structural geology and tectonics @ 50%
37 EARTH SCIENCES > 3703 Geochemistry > 370302 Inorganic geochemistry @ 50%
SEO Codes: 28 EXPANDING KNOWLEDGE > 2801 Expanding knowledge > 280107 Expanding knowledge in the earth sciences @ 100%
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