ABSTRACT

Within the Zermatt-Saas Zone (ZSZ, northwestern Alps), Ti-chondrodite- and Ti-clinohumite-bearing assemblages in serpentinites indicate UHP conditions. Multiscale structural analysis (1:20 scale mapping) and petrological investigation of serpentinites at Créton (upper Valtournanche) evidenced a polyphasic deformation and metamorphic history. In this locality and at regional scale, S2 is the dominant foliation that developed under HP-UHP conditions. Pre-D2 mineral and textural relicts are preserved despite the pervasiveness of S2. Pre-D2 olivine + Ti-chondrodite + spinel assemblage implies re-equilibration at 2.8–3.3 GPa and 600–630°C, in agreement with conditions recorded by coesite- and microdiamond-bearing rocks in the Cignana Lake Unit. The PT conditions inferred for syn-D2 assemblages at Créton are similar to those estimated for D2 in the surrounding serpentinites, which were dated at 65 ± 5.6 Ma. These results suggest that portions of ZSZ were subducted at high depth before 70 Ma and widen the time span during which ZSZ recorded PT peak conditions. The comparison of these data with results of a numerical model of an ocean-continent subduction system gives insights on coupling stages of this UHP unit with the surrounding ZSZ rocks during the Alpine convergence and vertical palaeogeography over subsequent time steps.

摘要

在采尔马特-萨斯区（ZSZ，阿尔卑斯山西北部），蛇纹岩中的含钛球粒陨石和含钛斜辉石组合表明超高压条件。对克雷顿（Valtournanche 上）蛇纹岩的多尺度结构分析（1:20 比例映射）和岩石学研究证明了多相变形和变质历史。在该地区和区域尺度上，S2 是在 HP-UHP 条件下发育的主要叶理。尽管 S2 普遍存在，但前 D2 矿物和纹理遗迹仍被保留。Pre-D2 橄榄石 + 钛球粒陨石 + 尖晶石组合意味着在 2.8-3.3 GPa 和 600-630°C 下重新平衡，与 Cignana 湖单元中含有柯石英和微金刚石的岩石记录的条件一致。为 Créton 的 syn-D2 组合推断的 PT 条件与周围蛇纹岩中 D2 的估计条件相似，其日期为 65 ± 5.6 Ma。这些结果表明，ZSZ 的部分在 70 Ma 之前在高深度俯冲，并扩大了 ZSZ 记录 PT 峰值条件的时间跨度。将这些数据与大洋-大陆俯冲系统数值模型的结果进行比较，可以深入了解该超高压单元与周围 ZSZ 岩石在随后时间步长的高山辐合和垂直古地理过程中的耦合阶段。