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.

中文翻译：

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磷灰石的应变定位和流体辅助变形及其对微量元素和 U-Pb 系统学的影响

摘要 本文介绍了来自 Taxaquara 剪切带（巴西东南部）花岗岩糜棱岩的磷灰石颗粒的电子背散射衍射 (EBSD)、微量元素和 U-Pb 数据。糜棱岩在上绿片岩相下重结晶并呈现两种具有不同微观结构的磷灰石。1 型磷灰石出现在富含石英的层中，不表现出任何变形/再结晶的微观结构、晶体学或化学证据，类似于原始火成磷灰石。2 型磷灰石出现在富含云母的地层中，显示出核幔微观结构和粒内亚晶发育，表明它们经历了动态再结晶。2 型磷灰石晶粒的再结晶尾部表现出与 X 方向平行的强 c 轴晶体择优取向（拉伸线），并且缺乏位错密度的证据。来自 2 型磷灰石晶粒的这一证据，结合 REE 耗尽、高 La 和与 1 型晶粒相比的负 Ce 异常，表明 2 型磷灰石尾部通过溶解-沉淀蠕变进行了再结晶，而母体晶粒经历了晶体-塑性通过动态再结晶变形和亚晶形成。相平衡模型和石英 CPO 开角温度测量与在 ~480 – 530°C 和 2.2 – 5.0 kbar 下的再结晶一致。我们无法确定 2 型磷灰石的精确变形年龄，因为流体辅助再结晶似乎大大降低了 U/Pb 比。我们发现沿着糜棱岩中高应变、富含黑云母的层的优先流体流动导致 2 型磷灰石重结晶，而低应变层中的 1 型磷灰石不受影响并保留了原岩的特征。