Strain localization and fluid assisted deformation in apatite and its influence on trace elements and U Pb systematics
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 530degC 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.