Pre‐kinematic greenschist facies metamorphism is often observed in granites and basement units of mountain belts, but rarely dated and accounted for in orogenic cycle reconstructions. Studying pre‐kinematic alteration is challenging because of its usual obliteration by subsequent syn‐kinematic metamorphism often occurring at conditions typical of the brittle–ductile transition. It is, however, to be expected that pre‐kinematic alteration has major implications for the rheology of the upper crust. In the 305 Ma‐old Variscan basement of the Bielsa massif (located in the Axial Zone of the Pyrenees), successive fluid–rock interaction events are recorded in granites below 350°C. Combined microstructural and petrographic analysis, low‐T thermobarometry and in situ U–Th/Pb dating of anatase, titanite and monazite show extensive pre‐orogenic (pre‐Alpine) and pre‐kinematic alteration related to feldspar sericitization and chloritization of biotite and amphibole at temperatures of 270–350°C at 230–300 Ma. This event is followed by a second fluid–rock interaction stage marked by new crystallization of phyllosilicates at 200–280°C and is associated with the formation of mylonitic shear zones and fractures parallel to the shear planes. U–Pb anatase and monazite ages as well as the microtextural relationships of accessory minerals suggest an age for this event at 40–70 Ma, consistent with independent regional geology constraints. The Variscan basement was therefore softened at late to post‐Variscan time, at least 150–200 Ma before the main Alpine shortening while Alpine‐age compression (c. 35–50 Ma) leads to the formation of a dense net of mylonites. The associated deformation, both distributed at the scale of the Bielsa massif and localized at decametric scale in mylonitic corridors, precedes the strain localization along the major thrusts of the Axial Zone. The Bielsa massif is a good example where inherited, pre‐orogenic upper crustal softening controls the deformation patterns in granitic basement units through low‐grade metamorphic reactions.

中文翻译：

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比利牛斯山脉中部花岗岩下绿片岩相变质反应导致造山前上地壳软化

运动学前绿片岩相变质作用经常在花岗岩和山地带基底单元中观察到，但很少在造山旋回重建中确定年代和解释。研究运动学前的改变是具有挑战性的，因为它通常会被随后的同运动变质作用消除，这通常发生在典型的脆-韧转变条件下。然而，可以预料，运动学前的蚀变对上地壳的流变学具有重要意义。在 Bielsa 地块（位于比利牛斯山脉的轴带）的 305 Ma-old Variscan 基底中，在 350°C 以下的花岗岩中记录了连续的流体-岩石相互作用事件。结合显微结构和岩相分析、低 T 热气压测定法和锐钛矿的原位 U-Th/Pb 测年，钛钛矿和独居石在 270-350°C 的温度和 230-300 Ma 下显示出与黑云母和角闪石的长石绢云母化和绿泥石化相关的广泛的造山前（前阿尔卑斯山）和前运动学蚀变。此事件之后是第二个流体 - 岩石相互作用阶段，其特征是在 200-280°C 时出现新的页硅酸盐结晶，并且与糜棱质剪切带和平行于剪切面的裂缝的形成有关。U-Pb 锐钛矿和独居石年龄以及副矿物的微观结构关系表明该事件的年龄为 40-70 Ma，与独立的区域地质约束一致。因此，Variscan 基底在后期至后期 Variscan 时期软化，在阿尔卑斯山主要缩短之前至少 150-200 Ma，而阿尔卑斯时代压缩（c. 35-50 Ma）导致形成致密的糜棱岩网。相关的变形，既分布在 Bielsa 地块的尺度上，又在糜棱岩走廊中以十米尺度局部化，先于轴向带主要逆冲断层的应变局部化。Bielsa 地块是一个很好的例子，在那里继承的、造山前上地壳软化通过低级变质反应控制了花岗岩基底单元的变形模式。