JOURNAL OF PETROLOGY VOLUME 55 NUMBER 7 PAGES 1429^1456 2014 doi:10.1093/petrology/egu029 Multiple Metamorphic Stages within an Eclogite-facies Terrane (Sesia Zone, Western Alps) Revealed byTh^U^Pb Petrochronology D. REGIS1*, D. RUBATTO2,J.DARLING3,B.CENKI-TOK4,M.ZUCALI5 AND M. ENGI1 1INSTITUT FU« R GEOLOGIE, UNIVERSITY OF BERN, BALTZERSTRASSE 1þ3, 3012 BERN, SWITZERLAND 2RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, MILLS ROAD, CANBERRA, ACT 0200, AUSTRALIA 3SCHOOL OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF PORTSMOUTH, BURNABY BUILDING, PORTSMOUTH PO1 3QL, UK 4GE¤ OSCIENCES MONTPELLIER, UNIVERSITY OF MONTPELLIER 2, UMR 5243, PLACE E. BATAILLON, 34095 MONTPELLIER, FRANCE 5DIPARTIMENTO DI SCIENZE DELLA TERRA ‘ARDITO DESIO’, UNIVERSITA' DEGLI STUDI DI MILANO, VIA MANGIAGALLI 34, I-20133 MILANO, ITALY RECEIVED MARCH 2, 2013; ACCEPTED MAY 14, 2014 Convergent plate margins typically experience a transition from sub- Bonze rocks) and the Druer slice (belonging to the Eclogitic duction to collision dynamics as massive continental blocks enter the Micaschist Complex).The new data indicate separate stages of de- subduction channel. Studies of high-pressure rocks indicate that formation at eclogite-facies conditions for each recognized independ- tectonic fragments are rapidly exhumed from eclogite facies to mid- ent kilometer-sized tectono-metamorphic slice, between 85 and 60 crustal levels, but the details of such dynamics are controversial. To Ma, with evidence of intermittent decompression (ÁP 0·5GPa) understand the dynamics of a subduction channel we report the re- within only the Fondo slice. The evolution path of the Druer slice | downloaded: 6.10.2021 sults of a petrochronological study from the central Sesia Zone, a key indicates a different P^T^time evolution with prolonged eclogite- element of the internal Western Alps.This comprises two polymeta- facies metamorphism between 85 and 75 Ma. Our approach, com- morphic basement complexes (Eclogitic Micaschist Complex and bining structural, petrological and geochronological techniques, Gneiss Minuti Complex) and a thin, dismembered cover sequence yields field-based constraints on the duration and rates of dynamics (Scalaro Unit) associated with pre-Alpine metagabbros and metase- within a subduction channel. diments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro^Bonze Units) yield unequivocal petrological and geochronological evidence KEY WORDS: pressure cycles; Sesia Zone; subduction channel; tectono- of two distinct high-pressure stages. Ages (U^Th^Pb) of growth metamorphic slice; yo-yo subduction zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic ‘slices’show- INTRODUCTION ing a coherent metamorphic evolution during a given time interval Exposed high-pressure (HP) crustal rocks in Phanerozoic have been recognized: the Fondo slice (which includes Scalaro and orogens represent the result of a sequence of processes that https://doi.org/10.7892/boris.70831 *Corresponding author. Present address: Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton ß The Author 2014. Published by Oxford University Press. All Keynes MK7 6AA, UK. E-mail: [email protected], rights reserved. For Permissions, please e-mail: source: [email protected] [email protected] JOURNAL OF PETROLOGY VOLUME 55 NUMBER 7 JULY 2014 operate at lithosphere scale (i.e. rifting, subduction^ exhumation. Careful structural control in sampling and accretion, tectonic mixing, return flow^exhumation). The detailed petrography were used to select samples for petro- dynamics of assembly of HP and UHP (ultrahigh- chronology to arrive at a coherent interpretation of the pressure) terranes is central to the tectonic study of colli- P^T^t data in the context of subduction dynamics. sion zones. Despite much interest, the mechanisms by Accessory phases proved most useful in deducing parts of which HP rocks descend to mantle depths and then return the metamorphic reaction history and to relate the specific to the surface remain controversial. stages of the evolution. Microbeam Th^U^Pb geochron- For any given HP^UHP terrane (e.g. Western Alps, ology by sensitive high-resolution ion microprobe Himalaya and Dabie-Shan), exhumation mechanisms are (SHRIMP) and laser ablation inductively coupled plasma inferred from concepts derived from geological data, mass spectrometry (LA-ICP-MS) allowed dating of these notably structural data and pressure^temperature^time stages. Single grain (or subgrain) dating turned out to be (P^T^t) paths, combined with insights derived from numer- essential to utilize the preserved fine detail of (over)- ical and analogue models. Efforts to understand the mech- growths within each accessory mineral. Where possible, anical behaviour of the lithosphere have singled out several mineral chronometers were combined to exploit their cap- mechanisms in the transition from a subduction to an ex- acity to record different stages of the metamorphic evolu- humation stage (e.g. Warren et al., 2008).These mechanisms tion. Particular attention was paid to the distribution of can be grouped into those driven by buoyancy forces (e.g. rare earth elements (REE) between major metamorphic Ernst, 2001) and those driven by a range of different tectonic minerals and the accessory phases used for geochronology processes (e.g. Platt, 1993; Kurz & Froitzheim, 2002). (zircon and allanite) to gain additional insight into the Structural and P^T^t data from several HP terranes relative timing of accessory and major mineral growth suggest two separate stages of exhumation: a first rapid (e.g. Hermann & Rubatto, 2003; Gabudianu Radulescu stage from mantle to lower crustal levels and a slower ex- et al., 2009). Two independent tectono-metamorphic ‘slices’ humation to the surface (Terry et al., 2000; Rubatto & (termed the Druer and Fondo slices, respectively) were Hermann, 2001; Carswell et al., 2003; Mukherjee et al., recognized within the Sesia Zone because they followed 2003; Parrish et al., 2006). Although various mechanisms different P^T^t paths within the subduction channel. may operate at different depths, buoyancy forces related Independent mobility of these slices has been unequivo- to the density contrast are commonly inferred to be par- cally demonstrated because they underwent different P^T ticularly significant for the first stage of exhumation, conditions at the same time. The results obtained on the whereas the second stage can be accomplished by a variety P^T^t evolution of each tectono-metamorphic slice are of syn- to post-convergent processes (e.g. Ernst, 2001). used to estimate the rates of subduction and exhumation. Moreover, some numerical models invoke extensive tec- tonic mixing during the subduction and exhumation stages (Gerya et al., 2002; Stoeckhert & Gerya, 2005; GEOLOGICAL BACKGROUND AND Roda et al., 2012), suggesting complex P^T^time paths for SAMPLING STRATEGY lithospheric fragments within a subduction channel. The Alps are a double-vergent orogen that formed as a In comparing P^T data from HP terranes with model result of the convergence between the European and predictions several questions arise, such as the following. African (Apulia) tectonic plates (e.g. Dewey et al., 1989). Under which conditions do fragments from the subducting The Sesia Zone (SZ) is the largest exposed slice of plate accrete to form an HP terrane? What temporal and Apulian continental crust in the Western Alps that under- spatial scales of tectonic mixing in the subduction channel went subduction prior to the Alpine orogeny and exten- are realistic? What triggers exhumation and at what rates sively preserves eclogite-facies assemblages. It has does it occur? traditionally been divided into three subunits (from west The dynamics in a subduction channel leading to a spe- to east, Fig. 1a, Table 1): the Gneiss Minuti Complex cific HP terrane are bound to be complex. For example, (GMC), the Second Dioritic^Kinzigitic Zone (IIDK), and results presented by Rubatto et al. (2011) have already es- the Eclogitic Micaschist Complex (EMC). These tectonic tablished the existence of HP cycles for the specific case of complexes consist of a metasedimentary basement com- the Sesia Zone. To corroborate this complex evolution and posed of Variscan amphibolite-facies rocks intruded by late quantify it in detail, we adopted an integrative approach Variscan granitoids that experienced polyphase tectono- that links observations and data across a range of scales, metamorphic reworking during the Alpine orogeny (Dal from structural patterns at kilometer scale in the field Piaz et al. 1972; Compagnoni, 1977; Compagnoni et al., 1977; down to petrological and chronological data obtained at Gosso, 1977; Lardeaux & Spalla, 1991). The three units pre- micrometer scale. This field-based study of a significant serve to different degrees an early HP imprint (peak pres- portion of the Sesia Zone reveals a multi-stage evolution sures of 1·0^1·5, 1·0^1·2, and 1·6^2·4 GPa respectively) that of this HP continental terrane, from subduction to partially re-equilibrated under greenschist-facies conditions 1430 REGIS et al. SESIA ZONE EVOLUTION Fig. 1. (a) Geological map of the central Sesia Zone (SZ), Western Italian Alps. Greenschist-facies overprinted portion of the Eclogitic Micaschist Complex