The Voltri Unit and adjacent Tertiary Piedmont Basin, Western Alps, preserve complementary bedrock and sedimentary archives of Alpine subduction and exhumation. Combined PT modeling and phengite Rb–Sr geochronology of bedrock and clast samples shows that Fe–Ti metagabbros and metasediments preserve a record of protracted high-pressure metamorphism, between ~ 50 and ~ 40 Ma. Bedrock and clast Fe–Ti metagabbros yield similar eclogite-facies peak conditions, between 23 and 25 kbar, 510–530 °C; phengite, zoned in celadonite content, with cores > 3.5 Si c.p.f.u. and rims < 3.3 Si c.p.f.u., constrains the timing of exhumation between 24 and 20 kbar to 45–49 Ma. A single impure quartzite bedrock sample records peak-P conditions of 18–19 kbar, 450–470 °C; partial phengite equilibration in this sample occurred continuously between 19 and 12 kbar, between ~ 45 and ~ 40 Ma. Exhumation-related recrystallization of high-pressure phengite to low-Si phengite in two metasedimentary samples occurred between ~ 33 and ~ 30 Ma, contemporaneous with the onset of deposition in the Tertiary Piedmont Basin and consistent with previous ⁴⁰Ar–³⁹Ar constraints on the timing of greenschist metamorphism. Combined with existing ages from the ophiolite, these data show that peak subduction-related high-pressure conditions, between ~ 18 and ~ 25 kbar, were attained at different times across the Voltri Unit, between ~ 50 and ~ 40 Ma, implying that the Voltri Unit comprises an assembly of discrete lithotectonic units that were juxtaposed prior to erosion and deposition in the Tertiary Piedmont molasse basin. The PTt data reported here support a model in which individual sheets of high-pressure material were detached from the downgoing plate, partially exhumed from peak pressures to blueschist facies conditions, while subduction continued, and were stored for > 10 Myr until subduction ceased on arrival of the European continent into the orogenic wedge. As shear tractions on the plate interface are considered to exceed available buoyancy forces for exhumation, we suggest that syn-subduction exhumation occurred along the wedge-plate interface.

西阿尔卑斯山的沃尔特里单元和邻近的第三纪山麓盆地，保留了俯冲和掘尸的互补基岩和沉积档案。合并的PT的基岩和碎屑样品节目建模和多硅白云母铷锶年代学的Fe-Ti系metagabbros和沉积岩保持长期高压变质的记录，〜50和〜40马之间。510-530°C，基岩和碎屑Fe-Ti变质岩产生相似的榴辉岩相峰条件，温度在23至25 kbar之间。以青瓷石含量划分的亚硫酸盐，岩心> 3.5 Si cpfu，轮辋<3.3 Si cpfu，限制了在24至20 kbar到45-49 Ma的掘尸时间。单个不纯石英岩基岩样品记录峰P18–19 kbar，450–470°C的条件；该样品中的部分亚硫酸盐平衡在19至12 kbar之间连续发生，在〜45至40 Ma之间。在两个沉积沉积样品中，高压变质-辉石重结晶为低硅变质辉石发生在〜33 Ma〜〜30 Ma之间，与第三山麓盆地沉积的开始同时发生，并且与先前的⁴⁰ Ar– ³⁹相一致。Ar限制了绿色分裂变质的时间。结合蛇绿岩的现存年龄，这些数据表明，Voltri装置在不同的时间（约50至40 Ma）达到了与俯冲相关的高压峰，压力在〜18 kbar〜〜25 kbar之间。沃尔特里单元（Voltri Unit）由不连续的岩性构造单元组成，这些单元在侵蚀和沉积之前并列于第三纪山麓糖蜜盆地中。在加温加压此处报道的数据支持一种模型，其中将高压材料的各个片层从下降的板块上取下，从峰值压力中发掘出一部分至蓝胶岩相条件，同时继续俯冲，并存储> 10 Myr，直到俯冲停止时俯冲停止。欧洲大陆进入造山楔。由于板界面上的剪切力被认为超过了挖掘时可用的浮力，因此我们建议沿楔-板界面发生同生俯冲法。