The Rocca Canavese Thrust Sheets Unit (RCTU) is a subduction-related mélange that represents the eastern-most complex of the Sesia–Lanzo Zone (SLZ), bounded by the Periadriatic (Canavese) Lineament that separates the Alpine subduction complex from the Southalpine domain. The RCTU is limited to the south by the Lanzo Massif (LM) and to the east by the Eclogitic Micaschists Complex (EMC). Particularly the tectonic contact area of the RCTU, adjacent to the neighbouring SLZ and the LM is characterised by a 100–200-m-thick mylonitic to ultra-mylonitic zone (MZ) that was active under blueschist-to greenschist-facies conditions. Despite the dominant mylonitic structure, some rocks (garnet-bearing gneiss, garnet-free gneiss and orthogneiss) still preserve pre-mylonitic parageneses in meter-sized domains. The scarcity of superposed structures and the small size of relicts impose a detailed microstructural analysis supported by chemical investigation to reconstruct the tectono-metamorphic history of the MZ. Therefore, we integrated the classical meso- and microstructural analysis approach with a novel quantitative technique based on the Quantitative X-Ray Map Analyzer (Q-XRMA), used to classify rock-forming minerals starting from an array of X-ray elemental maps, both at whole thin section and micro-domain scale, as well as to calibrate the maps for pixel-based chemical analysis and end-member component maps, relevant for a more robust conventional geothermobarometer application as well for calculating reliable PT pseudosections. Pre-Alpine relicts are garnet and white mica porphyroclasts in the garnet-bearing gneiss and biotite and K-feldspar porphyroclasts in garnet-free gneiss and orthogneiss, respectively, providing no PT constraints. The Alpine evolution of the MZ rocks, has been subdivided in three deformation and metamorphic stages. The first Alpine structural and metamorphic equilibration stage (D1 event) occurred at a pressure of ca. 1.25–1.4 GPa and at a temperature of ca. 420–510 °C, i.e. under blueschist-facies conditions. The D2 event, characterised by a mylonitic foliation that is pervasive in the MZ, occurred at ca. 0.95–1.1 GPa and ca. 380–500 °C, i.e. under epidote-blueschist-facies conditions. The D2 PT conditions in the MZ rocks are similar to those predicted for the blocks that constitute the RCTU mélange, and they overlap with the exhumation paths of the EMC and LM units. Therefore, the RCTU, EMC and LM rocks became coupled together during the D2 event. This coupling occurred during the exhumation of the different tectono-metamorphic units belonging to both continental and oceanic lithosphere and under a relatively cold thermal regime, typical for an active oceanic subduction zone, pre-dating Alpine continental collision.

中文翻译：

---

可容纳深埋大陆壳的顺冲俯冲的Blueschist lon回带：以Rocca Canavese冲断层单元为例（Sesia–Lanzo区，意大利西部阿尔卑斯山）

罗卡卡纳威斯冲断片单元（RCTU）是俯冲相关的混杂岩，代表着Sesia–Lanzo地带（SLZ）最东端的复合体，其边界是四海相间（Canavese）界线，将阿尔卑斯山的俯冲复合体与南阿尔卑斯山脉分开。RCTU由Lanzo Massif（LM）限制在南部，由Eclogitic Micaschists Complex（EMC）限制在东部。特别是RCTU的构造接触区域，与邻近的SLZ和LM相邻，其特征是在蓝片岩相到绿片岩相条件下活跃的100-200米厚的镁铁矿床至超亚冰晶岩带（MZ）。尽管具有支配性的丙二烯系结构，但一些岩石（含石榴石的片麻岩，无石榴石的片麻岩和直生的片麻岩）仍在米级区域保留了早木世的同生代。叠置结构的稀缺性和遗物的小尺寸要求进行详细的微观结构分析，并由化学研究支持以重建MZ的构造变质历史。因此，我们将经典的介观和微观结构分析方法与基于定量X射线图分析器（Q-XRMA）的新型定量技术相结合，用于从一系列X射线元素图开始对成岩矿物进行分类，无论是在整个薄片截面还是在微域尺度上，以及校准基于像素的化学分析图和最终成员组成图，都与更强大的常规地热气压计应用以及计算可靠的PT伪截面有关。阿尔卑斯山前的遗迹分别是含石榴石的片麻岩中的石榴石和白云母卟啉岩，而不含石榴石的片麻岩和直生片岩中的黑云母和钾长石卟啉岩均无PT限制。MZ岩石的高山演化分为三个变形和变质阶段。阿尔卑斯山的第一个结构和变质平衡阶段（D1事件）发生在约200摄氏度的压力下。1.25–1.4 GPa且温度约为 420–510°C，即在蓝片岩相条件下。D2事件的特征是在MZ中普遍存在的mylonitic叶状化，发生在大约2月。0.95–1.1 GPa和 380–500°C，即在显性blueschist相条件下。MZ岩石中的D2 PT条件类似于对构成RCTU混杂岩块的预测条件，它们与EMC和LM装置的挖掘路径重叠。因此，在D2事件中，RCTU，EMC和LM岩石耦合在一起。这种耦合发生在掘出属于大陆和海洋岩石圈的不同构造变质单元的过程中，并且在相对寒冷的热力条件下，这是活跃的俯冲带的典型特征，早于高山大陆碰撞。