Abstract The problem of how dense high- (HP) and ultrahigh-pressure (UHP) metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep within subduction zones is crucial to understanding processes occurring at the interface between the subducting slab and the overlying plate and mantle wedge. In this study, we use the P-T-t evolution of metamorphosed Fe Ti gabbros hosted in meta-serpentinite from the Voltri Ophiolite (Ligurian Alps, Italy) to evaluate potential exhumation mechanisms and assess whether HP mafic rocks were exhumed as blocks within a complex serpentinite-hosted melange or as coherent sections of oceanic lithosphere. A number of observations provide evidence for subduction and exhumation of the central portion of Voltri Ophiolite as a coherent package of ultramafic and mafic material: (1) a similar evolution in mineral assemblages, textures and compositions suggesting peak metamorphism in the eclogite-facies, followed by a retrograde evolution through blueschist-, amphibolite- and greenschist-facies conditions; (2) uniform P-T conditions of both initial garnet growth (~475–480 °C and 2.0–2.2 GPa) and peak metamorphism (~500–525 °C and 2.3–2.5 GPa); and (3) bulk Sm Nd garnet ages that reveal a very tight grouping of ages for four well-equilibrated foliated Fe Ti meta-gabbros clustering at 39.98 ± 0.84 Ma. A massive undeformed gabbro also from the central Voltri area gave a slightly younger age of 38.15 ± 0.89 Ma, which is interpreted to reflect kinetic overstepping and delayed growth of garnet. By contrast, an Sm Nd garnet age from the NW portion of the Voltri Ophiolite indicates peak eclogite-facies metamorphism occurring ~10–12 Ma earlier at 49.68 ± 0.35 Ma. A compilation of the current geochronological data suggests that the NW portion of the Voltri Ophiolite underwent peak eclogite-facies metamorphism first at ~50 Ma, the sedimentary portion of the Voltri Unit (the Voltri-Rossiglione meta-sediments) experienced peak metamorphism around ~47–44 Ma, whilst subduction and peak metamorphism of the central part of the ophiolite occurred later at 41–38 Ma. Alongside recent studies of other Alpine ophiolites (Zermatt Saas, Monviso, Alpine Corsica), this study suggests that detachment and buoyant exhumation of large kilometric-scale coherent sections of slab material, rather than exhumation within a chaotic serpentinite melange, is the dominant mechanism of preserving and returning eclogite-facies metamorphic rocks to the surface in the Western Alps. In the case of the Voltri Ophiolite, which lacks any associated continental massif, this exhumation was likely aided by the buoyancy of the serpentinite-dominated oceanic lithosphere, rather than by coupling to continental crust.

中文翻译：

---

意大利沃尔特里蛇绿岩的俯冲和折返历史：评估高压变质地块的折返机制

摘要 俯冲带深处的高压（HP）和超高压（UHP）变质大洋基性和超基性岩的致密程度问题对于理解俯冲板片与上覆板块和地幔之间的界面处发生的过程至关重要。楔。在这项研究中，我们使用位于 Voltri蛇纹石（意大利利古里亚阿尔卑斯山）的变蛇纹岩中的变质 Fe Ti 辉长岩的 PTt 演化来评估潜在的折返机制，并评估 HP 基性岩是否在复杂的蛇纹岩内以块状开采。混杂或作为海洋岩石圈的连贯部分。许多观察结果为 Voltri蛇绿岩的中心部分作为超镁铁质和镁铁质材料的连贯组合进行俯冲和挖掘提供了证据：(1) 类似的矿物组合、结构和成分演化表明榴辉岩相出现了峰值变质作用，随后通过蓝片岩、角闪岩和绿片岩相条件逆行演化；(2) 石榴石初始生长（~475-480°C 和 2.0-2.2 GPa）和峰值变质（~500-525°C 和 2.3-2.5 GPa）的均匀 PT 条件；(3) 大块 Sm Nd 石榴石年龄，揭示了四个平衡良好的叶状 Fe Ti 变辉长岩聚集在 39.98 ± 0.84 Ma 的非常紧密的年龄分组。同样来自 Voltri 中部地区的大量未变形辉长岩的年龄略小，为 38.15 ± 0.89 Ma，这被解释为反映了石榴石的动力学超步和延迟生长。相比之下，来自 Voltri 蛇绿岩 NW 部分的 Sm Nd 石榴石年龄表明，榴辉岩相变质作用发生在 49.68 ± 0.35 Ma 早约 10-12 Ma。当前地质年代学数据的汇编表明，Voltri蛇绿岩的 NW 部分首先在~50 Ma 经历了高峰榴辉岩相变质作用，Voltri 单元的沉积部分（Voltri-Rossiglione 变质沉积物）在~47 左右经历了峰值变质作用–44 Ma，而蛇绿岩中心部分的俯冲和峰值变质作用发生在 41-38 Ma。除了最近对其他高山蛇绿岩（采尔马特萨斯、蒙维索、高山科西嘉岛）的研究外，这项研究表明，大千米尺度连贯的板坯材料部分的分离和浮力剥落，而不是混沌蛇纹石混杂岩中的剥落，是西阿尔卑斯地区榴辉岩相变质岩保存和返回地表的主要机制。在没有任何相关大陆地块的 Voltri蛇绿岩的情况下，这种挖掘可能是由以蛇纹岩为主的海洋岩石圈的浮力的帮助，而不是通过耦合到大陆地壳。