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.

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