White mica K–Ar geochronology of HP–UHP units in the Lago di Cignana area, western Alps, Italy: Tectonic implications for exhumation

Abstract

High-pressure and ultra-high pressure (HP–UHP) blueschist- and eclogite-facies metabasaltic and metasedimentary rocks occur in four different tectonic units near Lago di Cignana, western Alps. We have determined K–Ar ages for white micas (matrix phengite and paragonite) from the Lago di Cignana UHP unit (LCU; 39–41Ma); the lower and upper units of the Zermatt–Saas meta-ophiolite (LU and UU; 37–38Ma and 38–41Ma respectively), and the Combin unit (CU; 36–40Ma). These K–Ar ages overlap with single-grain Ar–Ar plateau ages (36–42Ma) previously determined for phengites from LCU metasediments. Matrix white micas have been severely deformed during exhumation, and their chemistries differ from those of micas included in garnet. Although individual mica grains in the matrix could have experienced different degrees of deformation which have reset their K–Ar systems, “bulk” white mica separates provide the average age of all the individual grains in the separate. The similarity of ages determined for white micas from the LCU, LU, UU and CU units, regardless of rock type and mineral species, suggests that these four units were metamorphosed together as part of a single metamorphic sequence in the Piemonte–Liguria paleosubduction zone and were subsequently exhumed together. However, present-day structural relationship among those units and the limited occurrence of UHP minerals in LCU suggests that the exhumation of LCU was more rapid than that for LU, UU and CU. The age gaps between the youngest value of white mica K–Ar ages in each unit and the inferred timing of the metamorphic peak (U–Pb age: 44Ma) is 5, 7, 6 and 8Myr for LCU, LU, UU and CU, respectively. These intervals are considerably shorter than that determined for the Sanbagawa HP metamorphic belt of Southwest Japan (>31Myr). The short interval observed for the Lago di Cignana units that we have studied is consistent with the model of rapid exhumation of the UHP-bearing metamorphic domain, suggesting the exhumation rate is higher than 18mm/y in the early stage of exhumation from the deepest level (ca. 120km) to the lower crust (ca. 30km).