Evidence for protracted prograde metamorphism followed by rapid exhumation of the Zermatt-Saas Fee ophiolite

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1 J. metamorphic Geol., 2015, 33, doi: /jmg Evidence for protracted prograde metamorphism followed by rapid exhumation of the Zermatt-Saas Fee ophiolite S. SKORA, 1,2 N. J. MAHLEN, 3 C. M. JOHNSON, 3 L. P. BAUMGARTNER, 1 T. J. LAPEN, 4 B. L. BEARD 3 AND E. T. SZILVAGYI 3 1 Institute of Earth Sciences, University of Lausanne, Geopolis, 1015 Lausanne, Switzerland 2 Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse, 25NW 8092 Zurich, Switzerland (susanne.skora@erdw.ethz.ch) 3 Department of Geoscience, University of Wisconsin-Madison, 1215 W Dayton St, Madison, WI 53706, USA 4 Department of Earth and Atmospheric Sciences, University of Houston, 4800 Calhoun Road, Houston, TX 77004, USA ABSTRACT Major and trace-element zoning in garnet, in combination with Rb Sr, Sm Nd and Lu Hf geochronology, provide evidence for a protracted garnet growth history for the Zermatt-Saas Fee (ZSF) ophiolite, western Alps. Four new Lu Hf ages from Pfulwe (c Ma) and one from Chamois (c. 52 Ma) are very similar to a previously published Lu Hf age from Lago di Cignana. Overall, the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history, commensurate with their similar structural position near the top of the ZSF. Samples from the lower part of the ZSF at Saas Fee and St. Jacques, however, produced much younger Lu Hf ages (c Ma). Neither differences in whole-rock geochemistry, which might produce distinct garnet growth histories, nor rare-earth-element zoning in garnet, can account for the age differences in the two suites. This suggests a much later prograde history for the lower part of the ZSF, supporting the idea that it was subducted diachronously. Such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions, where early oblique subduction, which produced long prograde garnet growth, changed to more orthogonal subduction, which corresponds to shorter prograde garnet growth. Six new Rb Sr phengite ages range from c. 42 to 39 Ma and, in combination with previously published Rb Sr ages, constrain the timing of the transition from eclogite to upper greenschist facies P T conditions. The proximity of the ZSF in the Saas Fee region to the underlying continental Monte Rosa unit and the similarity of peak-metamorphic ages suggest these two units were linked for part of their tectonic history. This in turn indicates that the Monte Rosa may have been partly responsible for rapid exhumation of the ZSF unit. Key words: geochronology; Lu Hf; metamorphism; Rb Sr; Zermatt-Saas Fee. INTRODUCTION High-pressure (HP) and ultrahigh-pressure (UHP) metamorphic terranes that are associated with continent continent collision record subduction of crust to deep levels, followed by rapid exhumation, as required to preserve evidence for (U)HP conditions (e.g. Coleman & Wang, 1995; Hacker & Liou, 1998; Chopin, 2003). Mechanisms for rapid exhumation of high-density mafic rocks are debated, and possibilities include exhumation through attachment to lowdensity serpentinites (e.g. Hermann et al., 2000; Schwartz et al., 2001; Pilchin, 2005) or attachment to continental fragments (e.g. Cloos, 1993; Lapen et al., 2007). Indeed, many workers argue that buoyancy mechanisms are the most likely means by which rapid (>20 mm yr 1 ) exhumation of (U)HP terranes occurs (e.g. Platt, 1987; Wheeler, 1991; Duch^ene et al., 1997b). The Alpine chain is a classical continent continent collisional orogen, where such theories might be tested. Despite intensive petrological and geochronological studies of the (U)HP terranes of the western Alps, there still exists great uncertainty in the timing of Alpine metamorphism. Part of this uncertainty lies in the relations between ages and metamorphism some ages may indeed reflect peak metamorphism, but mineral growth (of garnet and zircon, for example) along the prograde path may produce a range of ages (e.g. Lapen et al., 2003; Anczkiewicz et al., 2007; Kylander-Clark et al., 2007; Schmidt et al., 2008; Smit et al., 2010; Zirakparvar et al., 2011; Kirchenbaur et al., 2012). The aim of this study is to place better constraints on the metamorphic history across the Zermatt-Saas Fee (ZSF) unit (Fig. 1) by applying the Lu Hf garnet geochronometer to a relatively large number of samples (n = 10) from different structural positions in the alpine stack. The new 711

2 712 S. SKORA ET AL. Matterhorn Lago di Cignana: 96JA-32, 01NM-45 Chamois: CH-48 study area Pfulwe path: P-02,80b P-80c,96 Breuil Lago di Cignana: 08ES-03 ages are linked to trace-element zonation in garnet, which together place tight constraints on the timing of prograde and peak metamorphism. New Rb Sr isochron ages for phengite provide additional constraints on the transition between eclogite to upper greenschist facies P T conditions (see also de Meyer et al., 2014). Last, in the light of the new data set, as well as previously published geochronological data on the underlying continental Monte Rosa unit, we examine the timing of metamorphism across the ZSF sheet to better understand the relations between continental and oceanic units during subduction-related metamorphism (e.g. Lapen et al., 2007). GEOLOGICAL SETTING Saas Fee: 05NM-214,215 Saas Fee: 05NM-212 Zermatt St. Jacques: SJ-87 St. Jacques Allalinhorn Pfulwe pass: P-98,100 Saas Fee Saas Fee: SF-25b,26 05NM km Sesia zone & Dent Blanche nappe Combin zone (Tsaté & Mt. Fort nappe) Zermatt-Saas Fee zone Gornergrat zone Monte Rosa nappe Grand St. Bernard nappe Sample location Fig. 1. Geological map showing the Liguro-Piemont oceanic remnants at the western Swiss/Italian border (simplified after Steck et al., 1999). The studied samples from Zermatt-Saas Fee ophiolite are from the Pfulwe, Chamois, Lago di Cignana, St. Jacques and Saas Fee areas, and sample names are listed for each locality. Alpine rocks (before collision) can be generally subdivided into: (i) pre-triassic basement rocks and (ii) Triassic to early Cenozoic cover sedimentary rocks that were deposited in a predominantly shallow water shelf environment (e.g. Tr umpy, 1980). During the Early Jurassic, this carbonate platform broke up in conjunction with establishment of the Tethyan- Atlantic junction, which gave rise to the development of the Liguro-Piemont Ocean basin (c Ma, Rubatto et al., 1998; Schaltegger et al., 2002), and separation of Europe from Apulia/Africa. The opening of the Atlantic Ocean towards the north in the Late Jurassic to Early Cretaceous led to shortening between Europe and Apulia/Africa (e.g. Stampfli et al., 1998). Southeast-directed subduction and northwest-directed thrusting during the Late Cretaceous through early Cenozoic eventually resulted in closure of the Liguro-Piemont Ocean basin and creation of the Alpine tectonic nappe system (e.g. Hunziker, 1974; Dal Piaz & Ernst, 1978). The ZSF ophiolite represents the (U)HP metamorphic remnant of that oceanic material. It is crised of peridotites, serpentinites, eclogitized metagabbros and metabasalts that contain local examples of deformed sheeted dyke systems and well preserved pillow structures, and a cover series of calcareous and siliceous metasedimentary rocks (e.g. Bearth, 1967; Dal Piaz & Ernst, 1978; Barnicoat & Fry, 1986). The paleogeographic organization used throughout this paper is from north to south (Fig. 2a): (i) European continental margin (Helvetic: Late Paleozoic crystalline basement rocks, covered by various sedimentary Mesozoic/Cenozoic nappes), (ii) Valais Basin (Lower Cretaceous B undnerschiefer/schiste Lustre ), (iii) Brianconnais domain (Late Paleozoic crystalline basement rocks, covered by various Mesozoic sedimentary rocks), (iv) remnants of the Jurassic Liguro- Piemont oceanic crust (ophiolitic HP zone, overlain by the sediment-dominated, low-pressure Combin zone), (v) northernmost Apulian/African continental margin or a distal micro-continent (Austroalpine: pre-triassic crystalline rocks), (vi) Apulian/African continental margin (non-metamorphosed Southalpine). Note that the low-p (upper greenschist/lower blueschist facies) Combin zone has been further subdivided into the (i) Mont Fort nappe (Triassic/Mesozoic cover sedimentary rocks of the Saint Bernard nappe, which is part of the Brianconnais) and (ii) Tsate nappe (ophiolitic melange zone with overlying Upper Cretaceous calcschists, Sartori, 1987). The Tsate nappe represents the accretionary wedge below which the ZSF was subducted (Sartori, 1987; Marthaler & Stampfli, 1989). Because subduction was southeast directed (Fig. 2b), the oldest peak-metamorphic ages are recorded in felsic eclogites from the Sesia-Lanzo zone (Austroalpine) at c Ma (Inger et al., 1996; Duch^ene et al., 1997a; Rubatto et al., 1999, 2011). Subsequent subduction of the ZSF unit to eclogite facies conditions produced a suite of metamorphic ages ranging between c. 50 and 40 Ma (Duch^ene et al., 1997a; Rubatto et al., 1998; Amato et al., 1999; Dal Piaz et al., 2001; Lapen et al., 2003; Rubatto & Hermann, 2003; Gouzu et al., 2006; Herwartz et al., 2008). Focusing on ages from north of the Aosta fault, the Sm Nd garnet isochron age of Ma most closely dates the peak of metamorphism (Amato et al.,

3 GEOCHRONOLOGY OF THE ZERMATT-SAAS OPHIOLITE 713 NW(a) Paleogeographic setting in Jurassic times SE Europe Valais basin Brianconnais domain Liguro-Piemont ocean Apulia/ Africa sediments Fig. 2. (a) Schematic diagram showing the paleogeographic setting of the Zermatt-Saas Fee and related units in a profile from NW to SW during Jurassic oceanic rifting (modified after Labhart, 1992). (b) Geological profile approximating the current tectonic situation of the Western Alps (modified after Labhart, 1992). sub-continental mantle? (b) Today Matterhorn asthenosphere 1: Zone Houillere/Pontis nappe 2: Grand Saint Bernard nappe 3: Mont Fort nappe 4. Monte Rosa nappe 5: Zermatt-Saas Fee zone 6: Tsaté nappe 7: Dent Blanche nappe 1999), because of strong enrichment of Sm at the garnet rim (Skora et al., 2009). Rb Sr metamorphic ages of the Tsate nappe range from c. 44 to 37 Ma (Reddy et al., 1999; Cartwright & Barnicoat, 2002). Peak-metamorphic ages from subducted continental slices of the southernmost Brianconnais continental margin such as Monte Rosa, the Gran Paradiso and the Dora Maira nappes are c Ma (Tilton et al., 1991; Duch^ene et al., 1997a; Gebauer et al., 1997; Rubatto & Gebauer, 1999; Engi et al., 2001; Meffan- Main et al., 2004; Lapen et al., 2007). Subsequent exhumation through upper greenschist facies is dated at c Ma in the Sesia-Lanzo zone (Inger et al., 1996), c Ma in the ZSF and adjacent Tsate nappe (M uller, 1989; Barnicoat et al., 1995; Reddy et al., 1999; Cartwright & Barnicoat, 2002) and c Ma in the units from the southernmost Brianconnais domain (Freeman et al., 1997; Engi et al., 2001; Meffan-Main et al., 2004). Eclogites All eclogites contain abundant hacitic clinopyroxene and garnet porphyroblasts. Other minerals, such as white mica (mostly paragonite, some phengite), glaucophane, epidote clinozoisite solid solutions (collectively called epidote hereafter in eclogites), lawsonite (pseudomorphs), carbonate, quartz and rutile also occur in the ZSF eclogites. These minerals may or may not be part of the peakmetamorphic assemblage, co-existing with garnet. Minor retrogression is present in all samples. It is characterized by transformation of hacite into Na Ca hornblende albite; garnet rims into Na Ca hornblende albite chlorite epidote; transformation of glaucophane into Na Ca hornblende (Na Ca hornblende is either barrosite, taramite or katophorite); as well as rutile that is retrograded into titanite and occasionally ilmenite at the rims. SAMPLES AND PETROLOGY Samples were taken to reflect a broad geographic distribution as well as different structural levels (Fig. 1). The Lago di Cignana unit is from the structurally highest position, just beneath the Tsate nappe. The Chamois locality is only 5 km from Lago di Cignana, and likely at a similar structural position within the ZSF unit. The Pfulwe samples are also from a structurally high position, although field relations suggest that it is slightly lower than Lago di Cignana. The St. Jacques locality lies in the interior of the ZSF, but its structural position is unclear. In contrast, the Sass-Fee samples clearly reflect the structurally lowest position in the ZSF package, directly lying above the Monte Rosa nappe. The petrology of the eclogite samples is discussed first, followed by the metasedimentary samples collected at Lago di Cignana and near Saas Fee. Thin section images for representative samples are shown in Figs 3a f, 4a f and 5a d. Sample locations are given in Table S1 (eclogites only) and Fig. 1. Pfulwe The samples P-80b, P-80c, P-02, P-96 come from a small outcrop just below Pfulwe pass (~0.5 km ENE), whereas samples P-98 and P-100 come from the actual Pfulwe pass, which is located ~7.5 km east of Zermatt. The geology and petrology of this area is outlined in, for example, Bearth (1967), Ernst & Dal Piaz (1978), Oberh ansli (1982), Barnicoat & Fry (1986) and Barnicoat (1988). Both outcrops are discussed together as the Pfulwe area hereafter. Omphacite often occurs as small grains that have undulatory extinction in these samples, preserving a radial, flower-like growth structure (Fig. 3a,b). This appearance suggests that these rocks record little or no deformation. Nearly idiomorphic garnet is coarse (Fig. 3c,d) reaching up to 1 cm. Omphacite, rutile, glaucophane and quartz inclusions occur in garnet, whereas epidote and ilmenite inclusions are more abundant in the cores. Matrix paragonite and glaucophane likely belong to the prograde assemblage for

4 714 S. SK ORA ET AL. (a) P-80b (b) P-80b Na-Ca hbl Na-Ca hbl rt (c) rt gln Radial gln Radial P-96 (d) P-96 rt rt (e) 96JA-32 (f) 96JA-32 lws pseudom. lws pseudom. Fig. 3. Photomicrographs illustrating the metamorphic textures of dated samples (left columns are in PPL, and right columns are same views in XP). All ZSF eclogites contain hacitic clinopyroxene and garnet porphyroblasts that vary considerably in size. Other minerals, such as white mica (phengite or paragonite), glaucophane, epidote, lawsonite, carbonate, quartz and rutile also occur. (a, b) Eclogites from the Pfulwe area are relatively undeformed. In places, hacite even preserve a radial, flower-like growth structure. (c, d) Garnet can grow exceptionally large at Pfulwe, occasionally reaching up to 1 cm in diameter. (e, f) Lawsonite pseudomorphs are commonly observed at Lago di Cignana, as both inclusions in garnet and in the matrix., garnet;, hacite; Na Ca hbl, Na Ca hornblende; gln, glaucophane; rt, rutile; lws pseudom., lawsonite pseudomorphs.

5 GEO CHR ONOLOGY O F THE Z ERMATT-SAAS OPHIOLI TE 715 (a) CH-48 (b) pg rt CH-48 pg rt (c) SF-25b (d) SF-25b lws pseudom. lws pseudom. gln gln rt->ttn (e) rt->ttn SJ-87 (f) SJ-87 gln rt gln rt ttn incl. ttn incl. Fig. 4. Photomicrographs illustrating the metamorphic textures of dated samples (left columns are in PPL, and right columns are same views in XP). The degree of deformation in these samples is strongly increased when cared with textures of samples from Pfulwe and Lago di Cignana (Fig. 3a d), although the mineral paragenesis is carable. (a, b) The sample from Chamois exhibits a strong metamorphic foliation defined by oriented hacite and rutile. (c, d) Garnet at Saas Fee is sometimes ruptured, suggesting that deformation was very strong. (e, f) The sample from St. Jacques differs from all other samples in that garnet is much smaller on average. Abbreviations as in Fig. 3; pg, paragonite; ttn, titanite. reasons discussed in Skora et al. (2008). Epidote, Fe-rich dolomite, rutile, apatite and quartz often occur in various but small amounts. The peak-metamorphic assemblage is: hacite + garnet + paragonite + glaucophane + epidote + rutile (+ carbonate + quartz).

6 716 S. SK ORA ET AL. (a) 08ES-03 (b) pmt 08ES-03 pmt ph ph qtz qtz (c) 05NM-219 (d) ph 05NM-219 ph cb qtz cb qtz bt bt 0.5 mm 0.5 mm Fig. 5. Photomicrographs illustrating the metamorphic textures of dated samples (left columns are in PPL, and right columns are same views in XP). The appearance of metasedimentary rocks varies from quartz-rich schists to calcsilicates. All samples contain relatively coarse-grained phengite that was used for Rb Sr dating. (a, b) Sample 08ES-03 from Lago di Cignana is unusually manganiferous, as suggested by the presence of piemontite (=manganiferous epidote). (c, d) All samples from the Saas Fee area are relatively rich in carbonate in addition to phengite and quartz. ph, phengite; cb, carbonate; qtz, quartz; pmt, piemontite; bt, biotite. Lago di Cignana The petrology of this sample is described in detail in Amato et al. (1999), Lapen et al. (2003) and Skora et al. (2009). Briefly, the matrix is cosed of relatively coarse-grained hacite, glaucophane, epidote, paragonite and phengite, which define a weak metamorphic foliation (data not shown). Garnet porphyroblasts are ~0.4 4 mm in diameter. Lawsonite pseudomorphs are relatively common in this sample (Fig. 3e,f). The peak-metamorphic assemblage of 96JA-32 is garnet + hacite + glaucophane + epidote + rutile + carbonate (+ phengite). Chamois One sample (CH-48) taken from the Nuarsax/Chamois area in the Valtournenche is layered and well foliated. The layering is marked by the presence or absence of lawsonite pseudomorphs (data not shown), and foliation is defined by oriented hacite (Fig. 4a,b). In lawsonite-free layers, the foliation wraps around ~1 3 mm sized garnet. Garnet inclusion patterns differ from Pfulwe samples in that titanite is a very frequent inclusion (besides hacite), especially in garnet cores. Rutile supplants titanite as the Ti-rich mineral inclusion in garnet rims. The few paragonite grains that occur in the matrix often crosscut the foliation (Fig. 4a,b), hence, they are likely to have grown post-deformational. Given that paragonite is stable at blueschist facies conditions, it is likely that it would have been deformed alongside hacite if paragonite belonged to the prograde assemblage. Rutile grains are also present in the matrix. The peak-metamorphic assemblage of the hacite-rich layers is hacite + garnet + rutile. The other layers contain additional

7 GEOCHRONOLOGY OF THE ZERMATT-SAAS OPHIOLITE 717 coarse-grained paragonite + epidote albite assemblages, in which minerals are not oriented. These assemblages are similar to the lawsonite pseudomorphs described by Skora et al. (2009) for Lago di Cignana (Fig. 3e,f), except that the lawsonite habit is often not clearly preserved. The matrix lawsonite is nevertheless interpreted to have formed during early prograde metamorphism followed by transformation on the retrograde path, given the undeformed and unoriented appearance of mica. Saas Fee Two samples were collected close to the Britannia Hut (Saas Fee). The matrix of the eclogites consists of fine-grained hacite, epidote and rare glaucophane, all of which define a very strong foliation. Garnet in these samples is up to 3 mm in diameter, and extremely inclusion rich. Grains are partly broken, suggesting that some deformation occurred after garnet growth ceased. Titanite is more abundant than rutile in the inclusion population, especially in garnet cores. In addition, titanite (ilmenite) surrounds or replaces rutile in the matrix. Lawsonite pseudomorphs, marked by epidote + paragonite albite, occur in both samples as garnet inclusions (Fig. 4c,d). Matrix lawsonite pseudomorphs appear to be restricted to sample SF-26. Saas Fee samples are characterized by the peak-metamorphic assemblage hacite + garnet + rutile + glaucophane + epidote. St. Jacques One sample was taken northeast of St. Jacques in the Val d 0 Ayas (SJ-87). Matrix hacite is aligned and defines a foliation (Fig. 4e,f). Garnet in this sample differs from all other samples in that it is much smaller (, Fig. 4e,f) but very abundant. In terms of inclusion patterns, it has abundant titanite in the cores, and rutile in the rims. Other minerals that are aligned in the matrix include epidote, hacite, glaucophane and paragonite. Glaucophane commonly has rutile inclusions, in contrast to garnet cores, suggesting that it started to grow close to peak conditions. This sample also contains lawsonite pseudomorph textures (paragonite + epidote albite), very similar to the Saas Fee samples. The peak-metamorphic assemblage is hacite + garnet + glaucophane + epidote + rutile + paragonite. Metasedimentary rocks Metasedimentary rocks in the ZSF were sampled for Rb Sr geochronology at Lago di Cignana, and near Saas Fee, representing the lower and upper parts of the ZSF respectively. The mineralogy and cositions of metasedimentary rocks in the ZSF vary greatly, as primarily reflected in variable proportions of quartz, garnet, carbonate and mica. Lago di Cignana Samples are garnet-bearing, quartz-rich schists (quartz: 40 65%, phengite: 15 25%, carbonate: <5%, garnet: 5 10%, epidote-group mineral: 5 15%). Sample 01NM-45 was collected from the eastern side of Lago di Cignana, and contains garnet, quartz, phengite and chlorite, with some clinozoisite, rutile and traces of biotite and carbonate. Sample 08ES-03 comes from a blocky outcrop on the south side of Lago di Cignana and is a phengite-bearing, manganiferous, quartz-rich schist (Fig. 5a,b). At outcrop scale, centimetre-sized, piemontite (=manganiferous epidote)-rich lenses readily indicate high Mn contents. This sample contains phengite, quartz, garnet, piemontite, manganiferous biotite, plagioclase, oxides and traces of carbonate and rutile. Saas Fee Samples are garnet-bearing calcschists [quartz: 20 30%, white mica (phengite & paragonite): 10 15%, carbonate: 30 50%, garnet: 5% and (clino)zoisite (=clinozoisitezoisite: 5 10%)]. Sample 05NM-212 was collected along the Britannia Hut trail. Samples 05NM-214 and -215 were taken from an outcrop located ~0.5 km northwest of Britannia Hut. Sample 05NM-212 has quartz, carbonate, phengite, with minor paragonite, chlorite, rutile with titanite rims and apatite, as well as poikilitic garnet porphyroblasts. Strong foliation and relict micro-folds are defined by mica. Samples 05NM-214 and 05NM-215 display the same assemblage apart from containing additional zoisite, biotite is absent, and the sample has a weaker schistosity. Sample 05NM-219 (from Britannia Hut) is rich in carbonate, quartz and phengite (Fig. 5c,d), and contains garnet, clinozoisite, titanite and traces of zoisite and biotite. Aggregates of zoisite, clinozoisite, quartz and white mica form rhombohedral shapes that are reminiscent of lawsonite pseudomorphs. Metamorphic conditions Peak-metamorphic conditions in eclogites of the Pfulwe area were estimated to be ~18 24 kbar and C (Oberh ansli, 1982; Barnicoat & Fry, 1986). Studies of the nearby Allalin metagabbro suggest broadly similar peak P T conditions (Chinner & Dixon, 1973; Bucher & Grapes, 2009), which are all roughly in agreement with the study of Angiboust et al. (2009) on a large number of metabasites collected over the entire ZSF zone (23 1 kbar; C). Conditions of kbar and C are suggested for the Lago di Cignana area due to the local occurrence of coesite inclusions (Reinecke, 1998; Groppo et al., 2009). It has been suggested that similar conditions have prevailed in other parts of the ZSF (Bucher et al., 2005), but this has not been confirmed by other workers.

8 718 S. SKORA ET AL. Petrological studies conducted on eclogites from the Western Alps all indicate that T peak and P peak were reached at roughly the same point in time (e.g. Reinecke, 1998). This is in contrast to studies on eclogites from the Central Alps (e.g. Brouwer et al., 2005) that appear to have reached their T peak during the collisional stage. Eclogites from the Central Alps are heavily overprinted. Their overprint assemblage is consistent with exhumation through amphibolite facies, in contrast to eclogites from the Western Alps that only show a greenschist facies overprint. The latter is indicative of cooling during exhumation. Because of the shape of garnet isopleths in P T space in eclogites (e.g. Hoschek, 2001), garnet cannot have grown during exhumation unless it was subjected to heating upon exhumation. The term peak metamorphism is thus used to describe the point of deepest burial, which equates to T peak and P peak in the Western Alps, and it is assumed that garnet growth must have ceased upon reaching this point. ANALYTICAL METHODS Whole-rock () powders were prepared from slabs of rock, which were trimmed to remove weathered portions, crushed in a steel jaw crusher and reduced to sand-sized particles using an alumina-lined disc mill (University of Wisconsin). The samples were then split, where a portion was saved for mineral separates, a portion was powdered using an alumina-lined shatterbox and a portion was powdered using a tungstencarbide shatterbox. data given in Table 1 are XRF data (University of Lausanne), obtained on the same powder that was used for geochronology. Mineral chemistry Garnet Central cuts of garnet in thick sections of eclogites were prepared for major and trace-element analyses, using X-ray tomography at the University of Lausanne. Major-element X-ray maps of all measured grains were obtained prior to acquiring wavelengthdispersive quantitative analyses using a Cameca SX- 50 (5 spectrometers) electron microprobe (Lausanne). The same thick sections and garnet profiles were then used to obtain trace-element profiles using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Trace-element data were acquired on a Perkin-Elmer ELAN 6100 DRC ICP-MS at Lausanne. Samarium data for one Pfulwe garnet were obtained by secondary ion mass spectrometry (SIMS) at the Max Planck Institute of Chemistry (Mainz). A detailed description on garnet sectioning, EMPA, LA-ICP-MS and SIMS is published in Skora et al. (2006). Garnet chemical data are plotted in Fig. 6a c. Phengite wt% P-80b P-80c P-96 P-02 P-98 P JA-32 a CH-48 SF-25b SF-26 SJ-87 Chemical cositions of phengite in the metasedimentary rocks were measured using the Cameca SX51 electron microprobe at 15 kev and 20 na at the University of Wisconsin-Madison. Data are given in Table S2. Rb Sr, Sm Nd and Lu Hf isotopes Mineral separates and samples of eclogites were processed for Sm Nd and Lu Hf geochronology following the methods in Lapen et al. (2004) at the University of Wisconsin-Madison. Analytical details are only briefly summarized here, whereas a full description is given in Appendix S1. Eclogite samples and minerals were dissolved in Parr bombs, followed by several dry-down steps. Clete dissolution was confirmed after centrifuging each sample. The Amato et al. (1999) leaching procedure was used on garnet fractions spiked only with the mixed 149 Sm 150 Nd tracer. Garnet-rich fractions spiked with both mixed 176 Lu 178 Hf and 149 Sm 150 Nd tracers were not subjected to sequential dissolution procedures as outlined in Amato et al. (1999), because Mahlen et al. (2008) found that this method could potentially fractionate Lu and Hf. There was further concern that leaching or partial dissolution may preferentially dissolve certain garnet populations, and our geochronologic modelling is intended to model total garnet evolution. Table 1. Whole-rock data for eclogites (wt%). SiO TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K2O P2O LOI Sum Major elements determined by XRF; LOI, loss of ignition. a Sample from Amato et al. (1999) and Lapen et al. (2003).

9 GEOCHRONOLOGY OF THE ZERMATT-SAAS OPHIOLITE 719 (a) Pfulwe (sample P-80b) Cations [pfu] Mn Fe Mg Ca Lu [ppm] Lu Sm (ICP) Sm (SIMS) Sm [ppm] (b) Chamois (sample CH-48) Cations [pfu] Lu [ppm] average quant. limit of Sm Sm [ppm] (c) Saas Fee (sample SF-25b) Cations [pfu] Lu [ppm] average quantification limit of Sm Sm [ppm] Rim-to-rim zoning [mm] Fig. 6. Dual plots of major elements (EMPA) and Lu and Sm (LA-ICP-MS, SIMS) zoning pattern for representative garnet samples, one from each investigated location. All data were collected on garnet central sections, as determined by X-ray tomography. Most Sm concentrations of garnet cores are below the limit of quantification for LA-ICP-MS. The data are still plotted, but this only serves to illustrate that the garnet core is depleted in Sm when cared with the rim cositions. Lutetium and Hf isotopes were measured on a Micromass Isoprobe. The 176 Lu decay constant of S oderlund et al. (2004) was used, at yr 1, which is essentially identical to that of Scherer et al. (2001). Samarium and Nd isotopes were measured on a Micromass Sector 54 thermal ionization mass spectrometer (TIMS). The decay constant for 147 Sm used was yr 1. Isotope data for both Lu Hf and Sm Nd are given in Tables 2 and 3, and are plotted in Fig. 7a s. Mineral separates and samples of metasedimentary rocks were analysed for Rb Sr isotopes at

10 720 S. SKORA ET AL. Table 2. Sm Nd isotope data for Zermatt-Saas Fee eclogites. Table 3. Lu Hf isotope data for Zermatt-Saas Fee eclogites. Mat. Sm (ppm) Nd (ppm) 147 Sm/ 144 Nd 143 Nd/ 144 Nd 2SE e Nd Mat. Lu (ppm) Hf (ppm) 176 Lu/ 177 Hf 176 Hf/ 177 Hf 2SE e Hf Pfulwe sample: P-80b l l a gln Pfulwe sample: P-80c l Pfulwe sample: P-02 l l a l a c Pfulwe sample: P a Pfulwe sample: P Chamois sample: CH Saas Fee sample: SF-25b Saas Fee sample: SF-26 l l a gln St. Jacques sample: SJ Mat., material;, garnet;, hacite; gln, glaucophane;, whole rock; l, leached; c, core. 2SE are from in-run statistics. Where replicate analyses occur, isochrons are calculated from averages. a Analyses that are repeat measurements of the same dissolution. the University of Wisconsin. A 87 Rb 84 Sr mixed isotope tracer was added to the samples; two mixed spikes were used, one for high Rb/Sr ratio phases such as mica (spike molar Rb/Sr = 191) and one suited for materials with lower Rb/Sr ratios (spike molar Rb/Sr = 3.8). Rubidium and Sr were separated using cation-exchange chromatography and analysed using a VG Instruments Sector 54 TIMS. The 87 Rb decay constant of Rotenburg et al. (2012) was used, yr 1, which is close to other recent determinations by Kossert (2003) and Nebel et al. (2011). For samples in the age range of 40 Ma, the decay constant of Rotenburg et al. (2012) produces ages c. 0.6 Ma older than those calculated using the old decay constant of Steiger & J ager (1977). Rb Sr data for the metasedimentary rocks are given in Table 4 and are plotted in Fig. 8a f. Errors used in isochron calculations reflect measured uncertainties of individual analyses (internal 2 standard error, SE) for 143 Nd/ 144 Nd, 176 Hf/ 177 Hf and 87 Sr/ 86 Sr. The uncertainties in all parent/daughter Pfulwe sample: P-80b a a a gln Pfulwe sample: P-80c a a Pfulwe sample: P a a a a Pfulwe sample: P a a a c a a Pfulwe sample: P a a Pfulwe sample: P a a Chamois sample: CH a a a a a Saas Fee sample: SF-25b a a a Saas Fee sample: SF a a a gln St. Jacques sample: SJ a Mat., material;, garnet;, hacite; gln, glaucophane;, whole rock; c, core. 2SE are from in-run statistics. Where replicate analyses occur, isochrons are calculated from averages. a Analyses that are repeat measurements of the same dissolution.

11 GEOCHRONOLOGY OF THE ZERMATT-SAAS OPHIOLITE (a) P-80b Pfulwe leached glc 20 ± 21 Ma MSWD = 1.7 Sm-Nd (e) P-100 Pfulwe 44 ± 16 Ma MSWD = (j) P-80b Pfulwe glc 49.6 ± 2.5 Ma MSWD = 0.68 Lu-Hf (o) P-100 Pfulwe 40.3 ± 5.9 Ma MSWD = (b) P-80c Pfulwe leached 43 ± 43 Ma (f) CH-48 Chamois 58 ± 17 Ma (k) P-80c Pfulwe 51.7 ± 2.1 Ma MSWD = 1.04 (p) CH-48 Chamois 52.6 ± 1.7 Ma MSDW = Nd/ 144 Nd no Sm-Nd data available for sample P-96 (Pfulwe) (g) SF-25b Saas Fee 78 ± 25 Ma 176 Hf/ 177 Hf (l) P-96 Pfulwe 46.1 ± 2.2 Ma MSWD = 1.3 (q) SF-25b Saas Fee 38.1 ± 2.7 Ma (c) P-02 Pfulwe leached 97 ± 320 Ma MSWD = 8.2 (h) SF-26 Saas Fee glc leached 20 ± 14 Ma MSWD = (m) P-02 Pfulwe core 46.7 ± 2.1 Ma MSWD = 2.2 (r) SF-26 Saas Fee 40.7 ± 1.8 Ma (d) P-98 Pfulwe (i) SJ-87 St. Jacques (n) P-98 Pfulwe (s) SJ-87 St. Jacques ± 51 Ma 65 ± 37 Ma ± 6.4 Ma 39.2 ± 1.6 Ma MSWD = Sm/ 144 Nd 147 Sm/ 144 Nd 176 Lu/ 177 Hf 176 Lu/ 177 Hf Fig. 7. (a i) Sm Nd and (j s) Lu Hf isochrons for Alpine eclogites. Data are from Tables 2 and 3. Note that all isochrons are at the same scale for Sm-Nd and Lu-Hf respectively., garnet;, whole rock;, hacite; glc, glaucophane. ratios were set equal to 0.5%, but this has little effect on the propagated errors for the isochrones due to the relatively young age of the samples. RESULTS Results are presented first for garnet and phengite chemistries, which were used for Sm Nd and Lu Hf and Rb Sr geochronology respectively. Sm Nd and Lu Hf ages for eclogites and Rb Sr ages for metasedimentary rocks are summarized in Tables 5 and 6 respectively. Garnet chemistry Major-element profiles determined by electron microprobe exhibit concentric prograde growth zoning that is characterized by spessartine and grossular contents that are highest in the cores, and pyrope and almandine contents that are highest at the rims. The exact zoning pattern, as well as the magnitude, varies slightly by location. Rare-earth-element profiles for all garnet grains vary systematically from Lu to Sm, very similar to that described in Skora et al. (2006). Only Lu (enriched in early grown cores) and Sm (enriched in late-grown rims) data are shown; these elements are the most important for understanding the geochronological results. Selected examples of characteristic zoning patterns of each locality are given in Fig. 6a c. Note that no central garnet section of the St. Jacques samples was obtained because they are too small. Phengite chemistry Phengite from Lago di Cignana exhibits Si that varies from 3.26 to 3.48 atoms per formula unit (apfu; aver-

12 722 S. SKORA ET AL. Table 4. Rb Sr isotope data for Zermatt-Saas Fee metasedimentary rocks. Mat. Sr (ppm) Rb (ppm) 87 Rb/ 86 Sr 87 Sr/ 86 Sr 2SE Lago di Cignana sample: 01NM czo ph Lago di Cignana sample: 08ES pmt ph Saas Fee sample: 05NM-212 cb czo ph Saas Fee sample: 05NM-214 cb (c)zo ph Saas Fee sample: 05NM-215 cb (c)zo ph Saas Fee sample: 05NM-219 cb (c)zo ph Mat., material;, garnet; cb, carbonate; czo, clinozoisite; (c)zo, clinozoisite+zoisite; piemontite, pmt;, whole rock. 2SE are from in-run statistics. Where replicate analyses occur, isochrons are calculated from averages. 87 Sr/ 86 Sr (a) czo NM-45 Lago di Cigana (c) 05NM-212 Saas Fee ph 41 ± 19 Ma MSWD = ± 0.2 Ma* ph czo 40.3 ± 3.8 Ma MSWD = 376 cb/ 39.1 ± 0.3 Ma* (e) NM-215 Saas Fee ph ± 0.9 Ma MSWD = 25 cb/(c)zo ± 0.2 Ma* Rb/ 86 Sr Ma* age = phengite-(clino)zoisite only (b) 08ES-03 Lago di Cigana pmt (d) 05NM-214 Saas Fee cb/(c)zo (f) 05NM-219 Saas Fee 39.6 ± 8.4 Ma MSWD = ± 0.2 Ma* ph ph 38.5 ± 1.2 Ma MSWD = ± 0.2 Ma* ph 40.8 ± 0.7 Ma MSWD = 16 cb/(c)zo 40.6 ± 0.2 Ma* Rb/ 86 Sr Fig. 8. (a f) Rb Sr isochrons for Alpine metasedimentary rocks. Data are from Table 4. Ages are calculated for multiple-minerals and whole rock, as well as for phengite epidote-group mineral pairs only (age denoted with an asterisk). Note that the size of the symbol is larger than the individual analytical errors for 87 Rb/ 86 Sr and 87 Sr/ 86 Sr. ph, phengite;, whole rock; cb, carbonate; pmt, piemontite; czo, clinozoisite; (c)zo, (clino)zoisite. age = ; calculations are based on 11 oxygen), indicating moderately high celadonite contents (Table S2). Saas Fee samples display generally lower average phengite contents and larger variation (Si range = apfu; average = ). Paragonite contents in all samples are low, varying between ~0.04 and 0.08 apfu. Phengite in the piemontite-bearing sample 08ES-03 has measurable Mn contents ( wt%). Calcium contents are always low, often below detection. All phengite in the metasedimentary rocks at Lago di Cignana and Saas Fee is interpreted to reflect eclogite to upper greenschist facies P T conditions. Sm Nd and Lu Hf geochronology eclogites The Sm Nd and Lu Hf results obtained on the exact same samples and dissolutions are cared in Fig. 7a s, and it is immediately apparent that virtually none of the Sm Nd ages (Fig. 7a i) produced useful isochron ages due to very low measured 147 Sm/ 144 Nd ratios for garnet, which in turn produced ages of very low precision. Errors in Sm Nd ages range from 14 to 320 Ma. In contrast to the generally unsuccessful Sm Nd geochronological results from the ZSF, most samples produced Lu Hf isochrons that are geologically meaningful (Fig. 7j s). We note that although some samples produced ages with relatively high uncertainties, sequential dissolution methods were not used for Lu Hf garnet analyses for reasons detailed in the methods section. Lu Hf isochron precision is proportional to the Lu/ Hf ratios that were obtained on garnet, as expected. At Pfulwe, four samples produced Lu Hf ages that had uncertainties of <3 Ma at Ma (P-80b; Fig. 7j), Ma (P-80c; Fig. 7k), Ma (P-96; Fig. 7l) and Ma (P- 02; Fig. 7m). The weighted average of all samples is Ma. Two samples have much lower Lu/ Hf ratios, coupled to significantly higher age uncertainties (samples P-98 and P-100: Ma and Ma respectively; Fig. 7n,o). All of the Lu Hf ages at Pfulwe broadly overlap the Lu Hf age determined at Lago di Cignana of Ma (Lapen et al., 2003). The Lu Hf age of sample CH- 48 from Chamois in Valtournenche is Ma (Fig. 7p). A 50 Ma age group is thus defined based on these samples. In contrast, the Lu Hf ages from the Saas Fee area are significantly younger at Ma (SF-25b; Fig. 7q) and Ma (SF-26; Fig. 7r), and most weight is put in the betterconstrained age for sample SF-26. These young ages overlap with that obtained for sample SJ-87 from St. Jacques at Ma (Fig. 7s). Based on these

13 GEOCHRONOLOGY OF THE ZERMATT-SAAS OPHIOLITE 723 Table 5. Sm Nd and Lu Hf ages (Ma) for Alpine eclogites. Sample Location Sm Nd Age Initial 143 Nd/ 144 Nd 2SE e Nd Lu Hf Age Initial 176 Hf/ 177 Hf 2SE e Hf P-80b Pfulwe P-80c Pfulwe P-96 Pfulwe P-02 Pfulwe P-98 Pfulwe P-100 Pfulwe JA-32 Lago di Cignana a b CH-48 Chamois SF-25b Saas Fee SF-26 Saas Fee SJ-87 St. Jacques Sm Nd and Lu Hf isochrons are calculated with relative uncertainties of 0.5% for 147 Sm/ 144 Nd and 176 Lu/ 177 Hf, and the analytical uncertainties (internal 2SE) for 143 Nd/ 144 Nd and 176 Hf/ 177 Hf as given in Tables 2 and 3. a Amato et al. (1999). b Lapen et al. (2003). Table 6. Rb Sr ages (Ma) for Alpine metasedimentary rocks. Sample Location Rb Sr age a Rb Sr age b Initial 87 Sr/ 86 Sr c 2SE 01NM-45 Lago di Cignana ES-03 Lago di Cignana NM-212 Saas Fee NM-214 Saas Fee NM-215 Saas Fee NM-219 Saas Fee Rb Sr isochrons are calculated with relative uncertainties of 0.5% for 87 Rb/ 86 Sr, and the analytical uncertainties (internal 2SE) for 87 Sr/ 86 Sr as given in Table 4. a Age calculated using the and all mineral fractions. b Age calculated using phengite (clino)zoisite only. c Given are initials and 2SE that are calculated from phengite (clino)zoisite isochrons; they overlap within error with calculated intercepts using multiple minerals. ages, a second age group of 40 Ma is defined. A broadly similar age range (c Ma) was obtained by Herwartz et al. (2008) for the upper Valle di Gressonay. Rb Sr geochronology metasedimentary rocks Rb Sr isochrons for the metasedimentary rocks are largely controlled by phengite, the highest Rb/Sr mineral in the samples (Table 4). The intercepts are controlled by carbonate and an epidote-group mineral. analyses provide an assessment of isochron integrity and may identify open-system behaviour. Isochron ages that include the for Lago di Cignana samples have large errors. Phengite clinozoisite (01NM-45) and phengite piemontite (sample 08ES- 03) ages at Lago di Cignana, however, have relatively small errors of Ma (Fig. 8a) and Ma (Fig. 8b) respectively. Phengite (clino)zoisite ages for the Saas Fee samples are Ma (05NM-212; Fig. 8c), Ma (05NM-214; Fig. 8d), Ma (05NM-215; Fig. 8e) and Ma (05NM-219; Fig. 8f). The weighted average of these samples is Ma. All these ages are indistinguishable with isochron ages constructed using all conents, including whole rocks (Table 6). Although very small age uncertainties are reported here (0.3 Ma or less, based on analytical 87 Rb/ 86 Sr and 87 Sr/ 86 Sr errors), the uncertainty in the geological age interpretation is certainly much larger; this will be important to consider in the discussion section. Collectively, all of the Rb Sr phengite ages are close to the 40 Ma age group, as defined in the previous section for eclogites. The initial 87 Sr/ 86 Sr ratios are all high, >0.710 (Table 4), consistent with significant sourcing of the metasedimentary rocks within the ZSF from continental basement, as reflected by the local granitic nappes (Mahlen et al., 2005). DISCUSSION Below, the discussion commences with some important issues related to Rb Sr, Sm Nd and Lu Hf isochron geochronology, such as isotopic closure (Dodson, 1973) and equilibrium assemblages, and the very high errors obtained for the Sm Nd technique are addressed in the context of REE abundances in garnet. This is followed by a short explanation on how Lu Hf and Rb Sr ages need to be interpreted based on REE zoning pattern and other petrological considerations. Individual Lu Hf and Rb Sr ages are then placed in the context of previously published data, as well as their structural position in the alpine stack. Lastly, the implications of the new data for the subduction history of the ZSF unit are discussed. Assessment of Rb Sr, Sm Nd and Lu Hf geochronology Geochronological investigations aimed at determining the P T path of (U)HP terranes require rocks that did not exceed the closure temperatures of appropriate geochronometers for significant periods of time. In addition, meaningful geochronology requires lithologies that do not contain inherited conents (e.g. Scherer et al., 2000), as well as samples that exhibit minimal retrograde overprints. Retrograde garnet resorption is especially problematic for Lu Hf geochronology (Kelly et al., 2011). The eclogites of the ZSF unit, however, satisfy all of the above crite-

14 724 S. SKORA ET AL. ria for robust geochronology. For example, all samples show only minor indications of retrogression. In addition, the peak temperatures of the ZSF unit did not exceed ~600 C, which is below the minimum blocking temperatures of both Sm Nd and Lu Hf garnet geochronometers (e.g. Th oni & Miller, 1996; Duch^ene et al., 1997a; Ganguly et al., 1998; Van Orman et al., 2002; Tirone et al., 2005; Skora et al., 2008), and this is confirmed by the preservation of prograde REE zoning in garnet (Fig. 6a c). Hence, Lu Hf and Sm Nd ages should represent times within the prograde-metamorphic cycle. The precursors of the eclogites were young, oceanic material that should contain minimal or no inherited conents nor relict minerals that would clicate the isotope systematics. The high e Nd (+6 to+8) and e Hf (+10 to +13) values (Table 5) indicate that the ZSF eclogites were derived from depleted mantle and therefore should contain no inherited conents. This is important because eclogitic garnet is inclusion rich, which could potentially supply non-radiogenic Hf through, for example, sub-microscopic zircon inclusions, or inherited Nd from epidote inclusions. Although zircon was not observed in the samples, LA-ICP-MS data suggest its presence at sub-micron scale. Such zircon most likely grew during prograde metamorphism when formerly Zr-rich host minerals broke down. This is corroborated by a published average U Pb zircon age from the Lago di Cignana unit, which is relatively young (c. 44 Ma, Rubatto et al., 1998), and only slightly older than the Sm Nd age determined from the same area (c. 41 Ma; Amato et al., 1999). Baxter & Scherer (2013) show that although inclusions of the same age as the dated mineral may decrease the isochron precision, they would not alter the accuracy of the age. Hence, we conclude that the small amount of zircon dissolved alongside garnet cannot significantly alter the Lu Hf ages except to decrease precision. It is commonly assumed that failure of Sm Nd garnet geochronology in terms of spread in 147 Sm/ 144 Nd ratios reflects the impact of LREE-rich inclusions such as metamorphic epidote, which is a refractory sink for all LREE, and this has prted development of leaching techniques (e.g. Amato et al., 1999; Baxter et al., 2002; Anczkiewicz & Thirlwall, 2003; Pollington & Baxter, 2011). The method of Amato et al. (1999) was adopted here for selected samples (Pfulwe, Saas Fee), given its previous success in removing the majority of LREE-rich inclusions from the Lago di Cignana area. It is immediately apparent from Fig. 7a i, however, that this leaching procedure was unsuccessful. The measured range of 147 Sm/ 144 Nd ratios of leached garnet fractions of ~ , with absolute Nd concentrations as high as ppm (Table 2), is indicative of contamination by LREE-rich inclusions (e.g. Th oni, 2002; Baxter & Scherer, 2013). A key question is: why were the Sm Nd geochronology attempts at localities other than at Lago di Cignana so unsuccessful? One explanation is that the leaching methods are rather sensitive to garnet sizes, the sizes of inclusions, different inclusion populations, and the duration of acid leaching and leaching temperatures, raising the possibility that every sample potentially behaves differently (e.g. Pollington & Baxter, 2011). Success or failure in leaching methods applied to Sm Nd garnet geochronology may also be dependent on absolute Sm Nd concentrations in garnet. Garnet enriched in Sm and Nd is more resistant to the influence of LREE-rich inclusions before 147 Sm/ 144 Nd ratios are decreased to levels too low to provide useful isochrons. In contrast, garnet that contains very low Sm and Nd will be much more difficult to use for Sm Nd geochronology, and leaching methods may be less effective. This is illustrated for the different examples in Fig. S1. Measured REE profiles across garnet indicate that Sm concentrations in Pfulwe (Fig. 6a), Chamois (Fig. 6b) and Saas Fee (Fig. 6c) are enriched towards the rim, similar to that found in Lago di Cignana (Skora et al., 2009). Absolute concentrations of Sm, however, vary significantly among samples from specific localities, as well as between localities, reflecting differences in garnet growth histories. Rim-Sm concentrations are highest at Lago di Cignana (2.5 ppm, Skora et al., 2009) and Chamois (2 ppm, Fig. 6b), lower in Saas Fee samples (0.8 ppm, Fig. 6c) and lowest in Pfulwe samples (<0.5 ppm, Fig. 6a). Hence, we believe that the majority of the hand-picked garnet had exceptionally low Sm contents and by inference, Nd contents. Despite application of the same leaching method to each sample, the effects of survival of tiny amounts of LREE-rich inclusions will have likely dominated the Sm and Nd budgets in many of the garnet separates, leading to low 147 Sm/ 144 Nd ratios that make the samples unsuitable for Sm Nd geochronology (Fig. S1). In the Lago di Cignana sample, in turn, even 147 Sm/ 144 Nd ratios of unleached garnet fractions were significantly higher (by a factor of ~2) when cared with any of the 147 Sm/ 144 Nd ratios measured in this study on leached garnet fractions. Therefore, the high Sm and Nd contents for the Lago di Cignana garnet have the highest likelihood that leaching procedures would be sufficiently successful. Although the LREE-poor garnet proved to be unsuitable for Sm Nd geochronology in our sample suite, it is certainly possible that different samples contain different levels of LREE inclusions, and application of different leaching procedures can produce successful Sm Nd ages in garnet that has low Nd concentrations (e.g. ~0.03 ppm: Dragovic et al., 2012). The Rb Sr isotope system is affected by additional clexities, including distinct closure temperatures, which can yield variable isochrons, reflecting variable levels of equilibration of different rock-forming minerals (e.g. mica, epidote-group mineral, carbonate,