# Eocene ultra-high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and monazite dating in petrographic context

Christian Nicollet, Valérie Bosse, Maria Iole Spalla, Federica Schiavi

► To cite this version:

Christian Nicollet, Valérie Bosse, Maria Iole Spalla, Federica Schiavi. Eocene ultra-high temperature (UHT) metamorphism in the Gruf complex (Central Alps): constraints by LA-ICPMS zircon and monazite dating in petrographic context. Journal of the Geological Society, 2018, 175 (5), pp.774 - 787. 10.1144/jgs2018-017. hal-01879915

HAL Id: hal-01879915

https://uca.hal.science/hal-01879915v1

Submitted on 22 Jul 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Eocene ultra high temperature (UHT) metamorphism in the Gruf complex (Central Alps):
constraints by LA-ICPMS zircon and monazite dating in petrographic context.

Christian Nicollet¹*, Valérie Bosse¹, Maria Iole Spalla² & Federica Schiavi¹

¹Laboratoire Magmas et Volcans, Université Clermont Auvergne, CNRS, IRD, OPGC, F-63000
Clermont-Ferrand, France

²Dipartimento di Scienze della Terra "A. Desio", Università degli Studi di Milano, Via Mangiagalli
34, 20133 Milano, Italia.

*Corresponding author (e-mail:Christian.nicollet@uca.fr)

The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic UHT metamorphism in the world but its age is still a matter of debate. Here we present LA-ICPMS dating in petrographic context of zircon and monazite from an UHT restitic granulite. Zircons and monazites are both included in large crystals and in retrograde symplectites. In such restitic rocks, partial melting or fluid interactions are unlikely precluding resetting of the monazite chronometers. Zircon cores yield Permian ages interpreted as age of charnockitisation. They are sometimes surrounded by a narrow rim at 32 Ma. Monazites are strongly zoned, but all yield a 31.8 ± 0.3 Ma age interpreted as the time of complete (re-)crystallisation during the UHT paragenesis. We propose that the zircons dated a post Hercynian metamorphism which is responsible of the widespread formation of granulites in the Southern Alps and the crust differentiation. This fluid-absent melting event produced refractory lithologies such as restites in charnockites. We suggest that Gruf UHT paragenesis is alpine in age and cristallised from these refractory lithologies. We conclude that the lower restitic crust produced at the Permian time had the ability to achieve UHT conditions during the fast exhumation and heating related to lithospheric thinning in Alpine time.

**Supplementary material:** Analytical procedures for monazite analysis and dating; table of major elements of the minerals; table of the isotope data; table of trace element measurements in zircon

**Introduction**

The Gruf complex in the Lepontine Alps is one of the rare occurrences of Phanerozoic UHT metamorphism in the world, discovered in the Val Codera by Cornelius (1916), Cornelius & Dittler (1929), and described by Barker (1964), Wenk et al. (1974). Because occurrences of UHT metamorphism are mainly of Precambrian age (eg Harley, 1998; Brown, 2007; Kelsey and Hand, 2015), this area is of major interest to understand the geodynamic signification of such extreme metamorphic conditions. Indeed, the main difficulty in understanding the geodynamic significance of UHT metamorphism is that Precambrian UHT granulites are often preserved in small-scale
33

34

35

36

37

lenses: they usually represent structural and metamorphic relics in polymetamorphic rocks belonging to polycyclic terrains. The lack of large-scale tectonic structures associated to their emplacement precludes a clear understanding of their geodynamic history. Thereby the few Phanerozoic UHT occurrences for which the geological context is well constrained are precious records that help to understand and interpret this type of metamorphism.

38

39

40

41

42

43

44

45

46

47

48

The age of the UHT metamorphism in the Gruf complex is currently a matter of debate. Based on zircon U/Pb dating, Galli et al. (2012) have proposed a Permian age (282 – 260 Ma) for the granulite facies fluid-absent biotite melting event. For these authors, the presence of orthopyroxene inclusions in zircons confirms the Permian age of the charnockites and associated sapphirine-bearing granulites. Zircon rims from the same samples yield 34-29 Ma ages interpreted as dating the Alpine amphibolite facies migmatisation. A different interpretation was suggested by Droop and Bucher (1984) and proposed by Liati and Gebauer (2003). The latter authors considered that the zircon Alpine rims (yielding a weighted mean age at 32.7 ± 0.5 Ma in their samples) grew during the UHT metamorphic event, and that the sapphirine-bearing granulites were restites formed during partial melting of the Permian granitoids. Moreover, Schmitz et al. (2009) measuring an age of 33.0 ± 4.4 by monazite chemical dating also agree with this interpretation.

49

50

51

52

53

54

55

56

57

58

59

60

61

62

These contrasting results imply two different models for the origin of the ultra-high temperature (UHT) metamorphism: 1) a Permian UHT metamorphism can be linked to the post Hercynian high-thermal regime, associated with lithospheric Permian-Triassic thinning and responsible for the widespread formation of granulites in the Austroalpine and South -Alpine continental crust (e.g.: Broadie et al., 1989; Lardeaux and Spalla, 1991; Diella et al., 1992; Barboza and Bergantz, 2000; Muntener et al. 2000; Schuster et al., 2001; Spalla and Marotta, 2007; Schuster and Stuewe 2008; Galli et al., 2013; Spalla et al., 2014). This anomalously high thermal regime would be responsible for a pervasive melting event, associated with melt loss leading to the genesis of a residual, refractory lower crust. Such processes are interpreted to be responsible of the lower continental crust differentiation (e.g. Vielzeuf and Holloway, 1988; Brown, 2008; Redler et al., 2013); 2) An Alpine UHT metamorphism would have been driven by lithospheric thinning associated with slab breakoff and asthenospheric upwelling ((e.g. Davies and von Blanckenburg, 1995; von Blanckenburg and Davies, 1995; Oalman et al., 2016) which would provide the considerable amount of heat necessary to reach the UHT conditions.

63

64

65

In this study, we combine for the first time zircon and monazite in situ dating in petrographic context for the Gruf sapphirine bearing granulite. The results provide a new opportunity to clarify the age of the UHT event and its geodynamic context.

66
67 Geological setting

68 Penninic nappes in the Central Alps consist of variegated rocks of continental and oceanic origins.
69 This nappe stack is separated from the Southern Alps by the Periadriatic Lineament and by a thin
70 ribbon of Austroalpine crust verticalised along the "southern steep belt" (e.g. Schmid et al., 1996).
71 The axial portion of Central Alps has recorded a polycyclic metamorphic evolution: structural and
72 petrologic relics of Hercynian and Caledonian imprints have been described (Schaltegger, 1994;
73 Spalla et al., 2014 and refs therein). The Alpine overprint occurs with heterogeneous intensity, and
74 the separation of Alpine from pre-Alpine metamorphic imprints has remained for a long time
75 difficult due to the similar metamorphic conditions associated with these successive orogenic cycles
76 (e.g.: Niggli, 1974; Engi et al., 2004). Alpine metamorphism in the Central Alps is characterized by
77 a polyphasic metamorphic evolution characterised by an early high pressure- low- to intermediate-
78 temperature imprint, preserved as relic blueschist- and eclogite-facies assemblages, recorded during
79 the south-verging subduction. The second metamorphic imprint is characterised by assemblages
80 indicating a Barrow-type event interpreted as consequent to the continental collision. Isograds of the
81 Barrovian metamorphism define the "Lepontine metamorphic dome" (Trommsdorff, 1966; Todd
82 and Engi, 1997) and their concentric distribution indicates that metamorphic conditions increase
83 southwards from greenschists- to upper amphibolite-facies. In this southern part partial melting
84 conditions have been attained at about 700°C and 0.6-0.8 GPa between 32 and 22 Ma (Engi et al.,
85 1995; Burri et al, 2005; Berger et al, 2009; Rubatto et al., 2009). The Gruf complex (Fig. 1) is a
86 small tectonic unit of about 200 km², located in the southeastern part of the Lepontine dome, north
87 of the Insubric Line, and limited to the east by the calc-alkaline Tertiary intrusive stock of Bergell.
88 This intrusive massif is considered synchronous with crustal anatexis at 33 - 28 Ma (Berger et al.
89 1996). The Gruf complex is composed mainly of biotite-garnet-sillimanite-cordierite metapelitic
90 rocks and migmatitic orthogneisses and paragneisses, and has been recently considered as part of
91 the tectonic mélange of continental and oceanic units accreted together in the Alpine tectonic
92 accretion channel (Engi et al., 2001). During a remarkable field work in a very difficult terrain Galli
93 et al. (2012) described structural and petrologic characters of the complex in situ for the first time,
94 focusing especially the Mg-Al-rich sapphirine granulites. The latter form schlieren and residual
95 enclaves within sheet like bodies of charnockites and migmatitic orthogneisses (Galli et al., 2013).
96 The chemical composition of the Mg-Al-rich granulites is comparable to that of restitic
97 surmicaceous enclaves in granites (e.g. Montel et al., 1991) except for magnesium. Kelsey et al.
98 (2003) suggested that the production of Mg-Al-rich compositions through melt loss is improbable
99 because of the low Mg partition into melt. For these authors Mg-rich assemblages may source from
100 Mg-enriched protoliths. Consequently, the Gruf Mg-Al-rich sapphirine granulites may represent
101 resister lenses within charnockites/orthogneisses or restites from Mg-rich protolith included in the
102 charnockites.

103 Previous published geochronological studies are at the origin of the contrasted interpretations for
104 the geodynamic signification of the UHT conditions recorded in the Gruf complex. Liati and
105 Gebauer (2003) report SHRIMP weighted mean ages at 272.0 ± 4.1 Ma in zircon cores and at 32.7
106 ± 0.5 Ma in zircon rims from a sapphirine bearing granulite sample. The Permian ages are
107 interpreted as reflecting the age of the magmatic protolith whereas the Alpine ages are considered to
108 represent the age of the granulite facies metamorphism. For these authors, sapphirine-bearing
109 granulites represent restites formed during Alpine partial melting of Permian granitoids. Galli et al.
110 (2011; 2012) proposed different interpretation based on similar geochronological data obtained with
111 the same method (Zircon SHRIMP analyses). For these authors 282-260 Ma ages obtained in
112 oscillatory zoned zircon cores represent melts generated through granulite facies fluid-absent biotite
113 melting at 920-940°C in metapelitic rocks, whereas 34-29 Ma ages in zircon rims date the Alpine
114 amphibolite facies migmatisation. For these authors the charnockites associated with the sapphirine-
115 bearing granulites belong to the post-Hercynian European lower crust. Schmitz et al. (2009) applied
116 the method of 3D-Micro X-ray fluorescence analysis on monazite in thin section from a sapphirine-
117 bearing granulite. They obtained an age at 33.0 ± 4.4 Ma in monazites included in and intergrown
118 with HT minerals which they interpreted as the age of the high-temperature event. Finally, Oalmann
119 et al. (2013, 2016) also suggest that UHT conditions were reached slightly before 32.5 Ma followed
120 by cooling from 30 to 19 Ma recorded by rutile $^{206}$Pb/$^{238}$U ages.

121

122 Petrography of the Mg-Al granulites

123 Mg-Al-rich sapphirine granulites have been already carefully described by Barker (1964), Droop
124 and Bucher (1984), Galli et al., (2011) and Guevara and Caddick (2016). We will concentrate here
125 on the main petrological characters and mineral reactions, which appear to be of major interest to
126 link petrology and geochronology. The studied sample is a pebble which has been collected in upper
127 Val Codera (Fig. 1). Two domains have been recognized at the sample scale: one is a sapphirine-
128 bearing granulite (Fig. 2a; 3/4 of the sample)) while the other has the typical mineralogy of a
129 charnockite (Figs. 2d and e; 1/4 of the sample). The boundary between these two domains is
130 progressive. Composition of the granulitic domain is enigmatic. Galli et al. (2011) propose that the
131 granulitic domain could be a restite/schlieren of Mg-rich métapélite. It could be a resister of an
132 unknown (magnesian ?) protolith within the charnockitic domain. The charnockitic paragenesis is
133 composed of millimetric to pluri-millimetric crystals of Opx - Bt - Kfs ± Pl - Qz - Mnz - Zrn and Ap
134 (Figs. 2d and e; abbreviations in text and figures are from Whitney and Evans (2010)). Rare
135 inclusions of zircon, monazite, plagioclase and quartz are observed in the phenocrysts. In some
136 places, clusters of K-feldspar - quartz-apatite microcrysts are following the grain boundaries in the
137 charnockitic assemblage (Fig. 2e). They seem to represent incipient melting or residual melt after
138 extraction. In the charnockite domain, K-feldspar is orthose (XOr: 80%; XAb: 20%), plagioclase
139 has an intermediate composition with XAn: 40%. The compositions of orthopyroxene and biotite are
140 substantially the same as in the granulitic domain of the rock.

141 The primary crystals of the residual/resister granulite are millimetric to plurimillimetric. The peak
142 UHT paragenesis was: Al rich-Opx - Sil - Spr - Bt - Grt - Crd - Rt - Ap - Zrn - Mnz (Fig. 2a). Here
143 again some tiny inclusions are rarely present. Detailed observations of the mineral associations
144 reveal complex relationships with (at least) 2 generations of Al-rich orthopyroxene, sillimanite,
145 cordierite and sapphirine ± spinel ± biotite, garnet and rare inclusions of quartz, rutile and apatite.
146 Secondary minerals are abundant. Except for orthopyroxene, the chemical composition of the
147 minerals does not change much and the crystals are weakly zoned (Table A supplementary
148 material). Garnet is almost pure almandine – pyrope solid solution with a slight decrease of pyrope
149 relative to almandine at the rim of the crystals (from the core to the rim: XPy: 48-46 %; XAlm: 42-
150 45 %). Grossular and spessartine contents are low: XGrs: 2.2-3.1% and XSp: ≤1%. Opx is Al₂O₃-
151 rich and zoned (core: 7.5-8.6% and rim: 5.5 to 7 % Fig. 3). Biotite is Mg- (XMg: 75-80%) and Ti-
152 rich (TiO₂: 2.6-3.6%). Cordierite composition is homogeneous both in primary minerals and in
153 symplectites. Galli et al. (2011), Oalmann et al. (2013), Guevara and Caddick (2016) estimated the
154 conditions of the primary paragenesis in these granulites at T = 920-940 °C and P= 0.85-0.95 GPa.

155 The secondary symplectites in the granulites are varied and complex, with Spr, Sil, Crd, Opx ± Sp.
156 Spinel is hercynite - spinel solution, homogeneous in composition. Two main reactions dominate
157 (Fig. 2b-c): Sil + 1 phenocrysts are surrounded by Spr + Crd + 2 in contact with Opx ± Bt; the garnet
158 is destabilized into symplectites of Opx + 2 + Crd + 2. These two reactions indicate a pressure decrease.
159 Al₂O₃ content in Opx + 2 is between 7.4 to 5.9 % similar to the rims of the primary phenocrysts (Fig.
160 3) whereas the XMg is slightly lower than in the primary crystals. Primary Opx probably continues
161 to grow at the beginning of the retrograde evolution during decompression, while garnet is
developed. This suggests that primary paragenesis and retrograde symplectites are the product of a
163 single metamorphic event. These retrograde textures demonstrate a re-equilibration of the UHT
164 peak assemblages at lower metamorphic conditions: 720-740 °C at 6.5-7.5 kbar which starts with a
165 decompression and continue with a temperature decrease (Galli et al. 2011; Oalmann et al. 2013;
166 Guevara and Caddick, 2016). These conditions are similar to those inferred for the migmatisation
167 occurring during the Tertiary Barrovian regional metamorphism (Burri et al, 2005; Engi et al.,
168 1995).

169 Both zircons and monazites are included in the large crystals (namely primary phenocrysts) :
orthopyroxene, orthoclase from the charnockitic assemblage ; sapphirine, orthopyroxene,
sillimanite, cordierite from the UHT assemblage as well as in the cordierite – biotite matrix and in
the late symplectites. Mineral inclusions are rare in garnet and zircon and monazite have not been
detected in it.

174 Zircon and monazites textures

175 Zircons are subhedral elongated and/or resorbed rounded crystals 30-100 µm long. They contain
176 rare and very tiny inclusions among which biotite, white mica, quartz and apatite were only
177 unequivocally identified by Raman spectrometry. Sillimanite has not been observed. Zircons
178 included in the primary phenocrysts of the two domains are resorbed grains and seem to be relictual
179 (Fig. 4a). Cathodoluminescence images (CLI) show that most of the grains display a large inner
180 domain with usually oscillatory or rarely complex zoning (Fig. 4b - c). In zircon included in
181 cordierite – biotite matrix and in late symplectites, this inner domain is sometimes surrounded by a
182 thin rim (<5 µm to 15 µm) with euhedral faces and bright in CDI (Fig. 4c). U and Th contents are
183 variable (150<U< ~4860 ppm and 4 <Th<1180 ppm; Table B supplementary material) with no
184 correlation with the internal structure nor ages (see the next section). As a result, Th/U ratios are
185 also variable (0<Th/U<0.50) and not related to the different zircon domains (i.e. oscillatory zoned
186 inner domain or external rim). Trace elements have been analysed in the core and in the rims of the
187 zircon grains. Because the rims are very thin compared to the spot size (15µm), we were not able to
188 analyse properly the rims. Rare earth elements (REE) pattern of the zircons show an enriched
189 distribution of the heavy REE, negative Eu anomalies (Eu/Eu*= 0.05 – 0.09) and positive Ce
190 anomalies (see Supplementary material – Table C) which are similar to those described in zircon
191 from Grt-bearing granulite facies rocks (Rubatto et al. 2002). Zircons from the charnockite domain
192 display same REE pattern as those of granulite.

193 Monazites are 50 to 150 µm in size and are present in the core of large Spr, Opx, Crd crystals or
194 form clusters of small grains (10 to 40 µm) in the late symplectites. The shape of the grains varies
195 from large rounded grains to small elongated ones with rarely indented grain boundaries (Fig 5b
196 and 6a), without any correlation with the textural position. Rare inclusions are needles of sillimanite
197 (Fig. 6a). All the grains are strongly zoned (Table 1). Chemical variations of the LREE-, Th- and
198 Ca-contents broadly follow the brabantite substitution (2REE³⁺ = Th⁴⁺ + Ca²⁺). ThO₂ content varies
199 from 1.3 to 10.8 wt%. Y₂O₃ and UO₂ contents are highly variable (from 0.1 to 5.5 wt% and from
200 0.2 to 3.5 wt%, respectively) and inversely correlated. These variations are clearly related to
201 mineral position within the sample (Fig. 7a): monazites in the charnockite domain are Y-rich and U-
202 poor whereas the opposite is true in the granulite domain. Variations in LREE, MREE and Y₂O₃
203 contents are also correlated to the sample zonation: monazites from the charnockite domain are
enriched in Gd₂O₃ and Y₂O₃ whereas monazites from the granulite domain are enriched in La₂O₃ and poor in Gd₂O₃ and Y₂O₃ (Fig. 7 b - c). Chemical variations are observed from grain to another but also within single crystal: for example T50M1 crystal on Fig. 6 a and 7 in the Al-granulite domain; T14M1 crystal on Fig. 7 and T12M1 crystal on Fig. 6 b in the charnockite domain. Some of the grains display very tiny Y-rich overgrowths (Fig. 6a).

## U-Th-Pb geochronology

Monazite and zircon have been analysed in thin section by LA-ICPMS in order to control the position of the grain (and thus the measured ages) relative to the textures observed in the sample. Zircons and monazites were ablated using a Resonetics Resolution M-50 equipped with a 193 nm Excimer laser system coupled to an Agilent 7500 cs ICP-MS with a 1 Hz repetition rate, a fluence of 7 J/cm² and a spot size of 7 µm for monazite dating and 3 Hz repetition rate, a fluence of 2.6 J/cm² and a spot size of 20 µm for zircon dating. Helium carrier gas was supplemented with N₂ prior to mixing with Ar for sensitivity enhancement (Paquette et al., 2014). Detailed analytical procedures are reported in Paquette and Tiepolo (2007) and Didier et al. (2015) and in Supplementary – Analytical procedures. Monazite can be dated using both U-Pb and Th-Pb decay schemes. Here we will only consider ²⁰⁸Pb/²³²Th ages for the following reasons. Firstly, ²³²Th is largely predominant in monazite, allowing small spots (7 µm) to be performed during laser ablation. Secondly, U decay series could be in disequilibrium in young monazites (Schärer, 1984), resulting in overestimated ²⁰⁶Pb/²³⁸U ages. Because secular equilibrium among the intermediate daughters of ²³²Th occurs after about 30 years, it seems reasonable to assume that initial ²⁰⁸Pb is absent. Thirdly, ²³²Th is so abundant that ²⁰⁸Pb originating from common Pb is usually negligible compared to radiogenic ²⁰⁸Pb. To reinforce this hypothesis, we only consider here the concordant ²⁰⁸Pb/²³²Th – ²⁰⁶Pb/²³⁸U ages (Fig. 8a). Whatever the textural position of the monazite grains or the chemical composition of the monazite domain, the ²⁰⁸Pb/²³²Th ages are between 29.6 ± 1.0 and 34.1 ± 1.1 Ma, allowing to calculate a weighted average age of 31.8 ± 0.3 Ma (MSWD = 3,2, N = 51; Fig. 8b; Supplementary material: Table B). We notice the important chemical variations in the same crystal, but identical ages (Figs. 6 and 7). This suggests significant chemical transfer and/or variation of the redox state through the rock during this brief metamorphic episode of UHT.

²⁰⁶Pb/²³⁸U ages measured in zircon from our sample are scattered from 30 ± 1 to 304 ± 12 Ma (only U/Pb concordant ages; Fig. 8c). Ages are similar in the charnockite and granulite domains. The youngest ages have been obtained in the bright rims. Plotted in a Tera Wasserburg diagram, we see that these ages are in total agreement with the ages measured in the monazite grains (Fig. 8c). The oldest ages are upper Carboniferous-lower Permian and have been obtained in the inner oscillatory zoned domains. Two reasons can be provided to explain intermediate ages between the oldest and
the youngest ones: i) they reflect mixing between the Carboniferous core and the narrow Cenozoic rim (<5 µm to 15 µm) due to analysis of distinct adjacent domains; ii) they result from Pb loss. Indeed, in some of the grains the U content is high (U> 1000 ppm). Such a high U content could be responsible for radiation damage of the crystal lattice especially in Paleozoic zircons and induce Pb loss. This results in younger core compared to the rim as observed in Figure 4b. With the aim of limiting these two effects and to better restrict the range of the oldest ages, we decide to only consider the ages which were measured i) in the zircon crystals which do not present the luminescent rim on the CLI, ii) measured in domains with U content ≤1000 ppm. By doing this the scattering of the ²⁰⁶Pb/²³⁸U ages remains between 247 ± 6 and 304 ± 12 Ma (Fig. 8d).

247

## Discussion

### 249 ■ Ages recorded in zircon

Two groups of ages have been recorded in the zircons: between 247 ± 6 and 304 ± 12 Ma in the core and ca 30 Ma in the rim. These ages are in total agreement with the results obtained by Liati and Gebauer (2003) and Galli et al. (2012). Following these last authors, who found the charnockitic paragenesis included within the zircon cores, the ages measured in the oscillatory zoned inner domain could be interpreted as the age of the charnockitisation. As no older inherited domains have been observed in our zircons, we favour this interpretation. This charnockitisation can be developed under the Permian-Triassic high-thermal regime (Galli et al., 2013) responsible of the widespread formation of granulites in the Southern Alps and Austroalpine (Bertotti et al., 1993; Spalla and Marotta, 2007; Spalla et al., 2014 and references herein). The scattering of the ages between 247 to 304 Ma in a single sample is rather surprising and, as previously suggested, can not be only attributed to mixing ages. Such a large spread of zircon ages is known in other Austroalpine and Southalpine units (Marotta et al., 2016 and refs therein) representing lower continental crust: Ivrea zone (Vavra and Schaltegger, 1999; Peressini et al., 2007), Malenco (Müntener et al., 2000), Sondalo (Tribuzio et al., 1999), Valtournanche (Manzotti et al., 2012). Vavra and Schaltegger (1999) suggest that the U-Pb system in zircons from the Ivrea Zone could have been disturbed by different types of alteration and Pb-loss processes during the Permian, Triassic and possibly early Jurassic times. These perturbations were differently interpreted as consequent to the early stages of Mesozoic rifting related to thermal and/or decompression pulses during extensional unroofing in the Permian (Marotta et al., 2009), as for example in Valtournanche, where the pre-Alpine evolution is associated with LP-HT metamorphism related to Permian-Triassic lithospheric thinning (Manzotti et al., 2012; Manzotti & Zucali, 2013). Our results, although slightly older than the previously published ages (but within 2 σ error bar), are in agreement with these interpretations.
The second group of ages between 30 ± 1 Ma and 34 ± 2 Ma has been recorded in euhedral rims from some crystals included in the Bt-Crd matrix or Spr-Crd symplectites. Most of the zircons included in the primary phenocrysts do not present such a younger rim (Fig 4 b). Furthermore, these results are similar to the $^{208}$Pb/$^{232}$Th ages measured in the adjacent monazites, whatever the textural position of the latter. Thus, these results argue for a zircon recrystallisation event at ca. 32 Ma simultaneous with the crystallisation (or re-crystallisation, see the discussion below) of the monazite grains.

## Monazite ages

Contrary to the zircon results, monazites from the studied sample provide narrow age estimation at 31.8 ± 0.3 Ma (between 29.6 ± 1.0 and 34.1 ± 1.1 Ma). The ages are the same in all the different textural positions of the monazite (included in large Spr, Opx or Crd crystals or in the Crd-Bt matrix or as cluster of small grains within secondary symplectites), or the chemical zoning of the granulitic (Fig. 6a) or charnockitic (Fig. 6b) paragenesis. The Y-rich overgrowths observed in some monazite grains which could be related to the garnet destabilisation (Didier et al. 2014) do not yield younger ages (Fig. 6a). Similarly, no older ages (i.e. Paleozoic) have been recorded by the monazites. Usually the shape of the monazite grains suggests equilibrium with, on one hand, the large primary crystals recording the peak of the UHT conditions and on the other hand, with the secondary symplectites during the beginning of the retrograde evolution. This demonstrates that the Spr-Opx-Sil UHT paragenesis as well as the retrograde symplectites in the restitic granulites equilibrated at ca. 32 Ma, in agreement with earlier works (Liati and Gebauer, 2003, Schmitz et al., 2009, Droop and Bucher, 1984). The Alpine UHT event is recorded by the monazite and by the zircon rims.

## Age of UHT metamorphism in the Gruf complex

Our zircon and monazite age results point to an age of 32 Ma for UHT. Galli et al. (2011, 2012, 2013) disagree with this interpretation. In their model, the Alpine age recorded in the zircon rims represents the age of the migmatisation of the Gruf lithologies in the upper amphibolite facies conditions. Moreover, they interpreted that the ages recorded at 33.0 ± 4.4 Ma by monazite (Schmitz et al., 2009) are supposedly the result of the monazite resetting during fluid-assisted migmatisation (Galli et al. 2012). We cannot support this interpretation for two reasons. First, many examples exist in the literature that show that monazite is a robust geochronometer able to retain age records during anatectic processes (Paquette et al. 2004; Rubatto et al. 2006; Stepanov et al. 2012; Rubatto et al. 2013; Didier et al. 2014; Didier et al. 2015) or even UHT conditions (Korhonen et al. 2013; Rocha et al. 2017). The main causes of possible disturbance of the Th-U-Pb systems in monazite are dissolution/recrystallisation processes during melt or hydrous fluid interaction (eg: Cherniak et al., 2004; Kelsey et al. 2012). Fluid interaction in the present context is
difficult to argue and in any case supposes an external input (see the next point below).
Dissolution/recrystallisation processes during melt interaction strongly depend upon the melt
composition and its H2O content (Montel, 1986; Rapp and Watson 1986; Spear and Pyle 2002).
Monazite solubility is low in meta- or peraluminous melts and thus monazite is able to resist to
crustal anatexis. To conclude, if fluid-assisted migmatisation in the amphibolite conditions occurred
during the Alpine times following a Permian UHT event, we would expect to measure Permian ages
in part of the monazite domains or grains, which is not the case. "Resetting" cannot be an
explanation for the absence of Permian age records in monazite in the present conditions.

The second point questions the migmatisation processes in such refractory lithologies. As reported
by Galli et al. (2011), Mg-Al granulites occur as restites or schlieren in the charnockites and thus
represent highly refractory rocks formed during partial melting processes in the Permian times.
Partial melting of such refractory lithologies is improbable as also suggested by McDade and
Harley (2001) unless a high amount of fluid is added. It is very unlikely that this fluid-assisted
migmatisation event in the amphibolite facies conditions could be responsible for all the textures
observed in the UHT granulites. There is no evidence in the retrograde reactions of significative
fluid input as argued by Galli et al. (2011). In contrast we propose that all the metamorphic textures
in the Mg-Al granulites correspond to the UHT conditions during the Alpine time, as demonstrated
by the age recorded in the monazites equilibrated with the UHT mineral assemblages.

324 ■ A model for the evolution of the Gruf granulites and charnockites

Based on the petrological observations and geochronological results obtained in our sample we
propose the following model for the time evolution of the Gruf granulites and charnockites. The
oldest ages we obtained in the zircons from our UHT granulite sample are between 304 ± 12 Ma
and 247 ± 6 Ma. These ages, despite slightly older, are similar to those measured by Galli et al.
(2012) and Liati and Gebauer (2003) in the Gruf UHT granulites. Older ages at 513 ± 8 Ma and 453
± 8 Ma (weighted average ages in magmatic zircon cores) have only been reported in an enclave-
rich biotite orthogneiss in the Gruf complex (Galli et al. 2012). Our results suggest that oscillatory
zoned zircons from the UHT granulites crystallized in the Permian-Triassic times during a partial
melting event. This Paleozoic anatectic event occured under the granulite facies conditions, as
shown by the presence of the charnockitic paragenesis included in the zircons cores (Galli et
al.2013), inducing the formation of both charnockitic melt and restitic/resister rocks, the precursor
of the sapphirine-bearing granulites. Unfortunately, the few tiny inclusions observed in the zircon
grains do not allow to precise what was the mineral assemblage in this precursor during the
Paleozoic event. We suggest that the precursor of the sapphirine-bearing granulites was a biotite –
cordierite rich selvedge probably containing also orthopyroxene and garnet could have been added.
Considering the abundance of the monazites in our sample, it is likely that monazites were present during the Permian times. During the Alpine times, UHT metamorphism occurred during a short-lived event (less than 5 Ma) between 30 to 34 Ma recorded by both zircon rims and monazites. UHT metamorphic conditions at 920-940 °C/ 0.85-0.95 GPa (Galli et al., 2011) are responsible for the complete re-crystallization of both the charnockites and the Bt-Cd-Grt(?)-Opx(?)-bearing rocks and enable the development of UHT parageneses appearance: Al rich-Opx - Sil - Spr - Bt - Grt - Crd - Rt - Ap- Zrn - Mnz in the Mg-Al granulite and Opx, Bt, Kfs, ± Pl, Qz, Zrn, Mnz and Ap in the charnockite. In both the charnockites and the Mg-Al granulites, this UHT event is responsible for the partial dissolution of the Paleozoic zircons and the growth of the Alpine rims. Because no Paleozoic ages were found in monazites from our samples (as well as in the previous monazite study, Schmitz et al. 2009), crystallisation processes for the Alpine monazites are more difficult to assess. Two processes can be proposed: crystallisation from a mineral precursor such as allanite or apatite, or complete recrystallisation of an earlier monazite generation, possibly late Carboniferous - Permian in age. In our sample petrographic evidences of a monazite precursor are missing and it is not possible to solve the question in the present context.

* Geodynamic implications and the origin of the UHT metamorphism in the central Alps

The oldest ages we obtained in the zircon cores between 304 ± 12 Ma and 247 ± 6 Ma interpreted as the age of the charnockitisation, unknown elsewhere in the Penninic domain of the Central Alps (Galli et al, 2011), developed during Permian-Triassic high thermal regime responsible for HT-LP metamorphism widespread in the Austroalpine and Southalpine domains (Galli et al., 2013, Barboza and Bergantz, 2000; Lardeaux and Spalla, 1991; Diella et al., 1992; Bertotti et al., 1993; Muntener et al. 2000; Marotta et al., 2016). These granulites are interpreted as residual and refractory rocks following partial melting events and extensive melt loss. This process is responsible for the differentiation of the continental crust via fluid-absent melting reactions involving muscovite and biotite (and amphibole in metabasic rocks) destabilisation. As a consequence, the precursor of the Al-Mg granulites of the Gruf complex acquired its restitic character during this post Hercynian event.

UHT metamorphism occurs at temperatures above the fluid-absent melting in most crustal rocks which is an endothermic process that consumes heat and buffers the temperature (Vielzeuf and Holloway, 1988; Stüwe 1995). Thus partial melting limits heat production avoiding a fertile crust to attain UHT. Nevertheless, UHT conditions are more easily reached in refractory/restitic rocks and develop preferentially in terrains that previously underwent metamorphism and melt loss (Clark et al., 2011; Kelsey and Hand, 2015). The Gruf complex is the sole unit in Penninic Domain of Central
374 Alps recording the Permian-Triassic granulitic metamorphism, which permitted its rocks to reach
375 and record UHT conditions during the Alpine cycle.

376 Zircon and monazite ages, together with their microstructural relationships with granulite facies
377 minerals, show that the typical UHT paragenesis crystallised at Alpine time during a short-lived
378 event (less than 5 Ma) between 34 to 30 Ma. These results are in agreement with the garnet
379 diffusion modelling proposed by Galli et al. (2011) also suggesting that UHT metamorphism was
380 brief. This UHT metamorphism is contemporaneous with the Bergell tonalite-granodiorite intrusion
381 dated between 33 and 28 Ma (Berger et al. 1996) and the beginning of the anatectic event in the
382 Southern part of the Central Alps at 32 Ma (Köppel et al, 1981; Berger et al, 2009; Rubatto et al.,
383 2009). Secondary symplectites in the Mg-Al granulites just following the UHT peak conditions are
384 equilibrated in the same conditions as the Lepontine migmatisation (e.g. Engi et al. 1995; Burri et
385 al. 2005). However, the 38-34 Ma UHP metamorphism in the Lepontine domain, is quickly
386 followed by a fast exhumation, then the UHT event and the final amphibolite faciès migmatisation
387 (Brouwer et al. 2004). This fast exhumation is related to a short-lived (less than 4 Ma) episode of
388 lithospheric thinning associated with a rise of hot asthenospheric material (Brouwer et al., 2004)
389 from 34 Ma to 32-30 Ma (Beltrando et al., 2010). Oalman et al. (2016) proposed that similar
390 lithospheric thinning associated with slab breakoff or roll back and asthenospheric upwelling could
391 also be responsible for the short-lived Alpine UHT metamorphism in the Gruf complex. A similar
392 geodynamic context is proposed for another example of Phanerozoic UHT metamorphism (16Ma)
393 of the island of Seram (Indonesia) where the UHT conditions were produced by slab rollback-
394 driven lithospheric extension and the exhumation of hot subcontinental lithospheric mantle
395 (Pownall et al, 2014; Pownall, 2015). Harley (2016) proposed that shorter duration UHT granulite
396 event can be formed as a consequence of severe lithospheric thinning and crustal extension
397 accompanied by voluminous magmatism in arc settings affected by subduction roll-back. Brouwer
398 et al. (2004) notice that heating by slab detachment is fast and transient (more than 100°C in up to
399 10 million years,), whereas radiogenic heating requires time spans of the order of tens of millions of
400 years and cessation of the subduction process. Finally, Perchuk et al. (2017) used numerical models
401 to explain ultra-hot orogeny and showed that UHT conditions at the bottom of the crust might be
402 produced by lateral propagation of the hot asthenospheric front during plate convergence associated
403 with lithospheric delamination. In this model, a close relationship of HT-UHT metamorphism with
404 tonalitic magmatism is proposed which could explain the relation between the Gruf UHT
405 metamorphism and the Bergell intrusion, which is one of the main Periadriatic igneous bodies
406 emplaced during Late Alpine times, characterised by an enriched mantle source (von Blanckenburg
407 et al., 1998), and interpreted as triggered by post-collisional slab break off.
408 **Conclusions**

409 In situ and in context zircon dating on zircon from a restitic granulite within charnockites gives both
410 Permian-Triassic (304-247 Ma) and Alpine ages (ca 34-30 Ma). $^{208}$Pb/$^{232}$Th ages measured in
411 monazite from the same sample yield a weighted average age of 31.8 ± 0.3 Ma (MSWD = 3.2; N =
412 51) interpreted as the time of complete (re-)crystallisation of the monazite in equilibrium with the
413 UHT paragenesis. The Alpine UHT event was thus recorded by zircon rims and monazites. The
414 oldest ages we obtained in the zircons between 304 ± 12 Ma and 247 ± 6 Ma are interpreted as the
415 age of the charnockitisation during which the precursor of the Al-Mg granulites of the Gruf
416 complex acquired its restitic character. Our results show that the Gruf complex is the sole Penninic
417 unit in the Central Alps recording the post Hercynian granulitic metamorphism which permits to
418 reach and record UHT conditions during the Alpine cycle. The typical UHT paragenesis crystallised
419 at Alpine time during a short-lived event (less than 5 Ma) related to lithospheric thinning associated
420 with a rise of hot asthenospheric material.

421 **Acknowledgments**

422 The subject editor Igor Villa and the two reviewers Simon Harley and Jean-Marc Lardeaux are
423 thanked for their constructive comments. We also thank Dr. E. Bruand for reading the English.
424 Thanks are due to Jean Luc Devidal and Jean-Marc Hénot for their help in the use of EMP and
425 SEM.

426 **References**

427 Barker, F., 1964. SAPPHIRINE-BEARING ROCK, VAL CODERA, ITALY. AMERICAN MINERALOGIST 49, 146-152.

428 Barboza S.A. and Bergantz G.W. (2000) - Metamorphism and anatexis in the mafic complex contact
429 aureole, Ivrea Zone, northern Italy. Journal of Petrology 41:1307-1327.

430 https://doi.org/10.1093/petrology/41.8.1307

431 Beltrando M., Lister G.S., Rosenbaum G., Richards S. and Forster M.A. (2010) - Recognizing
432 episodic lithospheric thinning along a convergent plate margin: The example of the Early Oligocene

433 Alps. Earth-Science Reviews 103; 81-98. https://doi.org/10.1016/j.earscirev.2010.09.001

434 Berger A., Rosenberg C., Schmid S.M. (1996) Ascent, emplacement and exhumation of the Bergell
435 pluton within the Southern Steep Belt of the Central Alps. Schweizerische Mineralogische und
436 Petrographische Mitteilungen 76:357-382.

437 Berger A., Rosenberg C., Schaltegger U. (2009) Stability and isotopic dating of monazite and
438 allanite in partially molten rocks: examples from the Central Alps. Swiss Journal of Geoscience

439 102:15-29, https://doi.org/10.1007/s00015-009-1310-8

440 Bertotti G., Siletto G. B. & Spalla M. I. (1993) Deformation and metamorphism associated with
441 crustal rifting: Permian to Liassic evolution of the Lake Lugano-Lake Como area (Southern Alps).

442 Tectonophysics, 226, 271-284. DOI: 10.1016/0040-1951(93)90122-Z

443 Brodie K. H., Rex D. & Rutter E. H. (1989) On the age of deep crustal extensional faulting in the
444 Ivrea zone, Northern Italy. Geological Society, London, Special Publications, 45, 203-210.

445 DOI: 10.1144/GSL.SP.1989.045.01.11
446 Brouwer F.M., van de Zedde D.M., Wortel M.J. and Vissers R.L. (2004) Late-orogenic heating during exhumation: Alpine PTt trajectories and thermomechanical models. Earth and Planetary Science Letters 220 (2004) 185-199. https://doi.org/10.1016/S0012-821X(04)00050-0

449 Brown M. 2007. Metamorphic conditions in orogenic belts: a record of secular change. International Geological Review, 49:193–234, https://doi.org/10.2747/0020-6814.49.3.193

451 Brown, M., (2008) - Granites, migmatites and residual granulites: relationships and processes. In: Sawyer, E.W., Brown, M. (Eds.), Working With Migmatites: Mineralogical Association of Canada Short Course, 38, pp. 97–144.

454 Burri T., Berger A., Engi M. (2005) Tertiary migmatites in the Central Alps: regional distribution, field relations, conditions of formation, and tectonic implications. Schweizerische Mineralogische und Petrographische Mitteilungen, 85:215–232

457 Clark C., Fitzsimons I.C.W., Healy D., Harley S.L. (2011) - How does the continental crust get really hot? Elements 7: 235-240. https://doi.org/10.2113/gselements.7.4.235

459 Cornelius, H.P. (1916) Ein alpines Vorkommen von Sapphirine. Cent. Mineral., 265-269.

460 Cornelius, H.P. & Dittler E. (1929) Zur Kenntnis des Sapphirinvorkommens von Alpe Bresciadega in Val Codera (Italien, Prov. Sondrio). Neues Jahrbuch Fur Mineralogie-Abhandlungen, 59, 27-64.

462 Cherniak D.J., Watson B.E., Grove M., Harrison T.M. (2004) Pb diffusion in monazite: a combined RBS/SIMS study. Geochimica et Cosmochimica Acta 68:829–840.

464 https://doi.org/10.1016/j.gca.2003.07.012

465 Didier A., Bosse V., Cherneva Z., Gautier P., Georgieva M., Paquette J.L. and Gerdjikov I. (2014).

466 Syn-deformation fluid-assisted growth of monazite during renewed high-grade metamorphism in metapelites of the Central Rhodope (Bulgaria, Greece) Chemical Geology 381, 206–222.

468 https://doi.org/10.1016/j.chemgeo.2014.05.020

469 Davies J.H. & Von Blanckenburg F. (1995) Slab breakoff: A model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens. Earth and Planetary Science Letters, 129, 85–102. Doi: 10.1016/0012-821X(94)00237-S

472 Didier A., Bosse V., Bouloton J., Mostefaoui S., Viala M., Paquette J.L., Devidal J.L., Duhamel R. (2015). NanoSIMS mapping and LA-ICP-MS chemical and U–Th–Pb data in monazite from a xenolith enclosed in ndesite (Central Slovakia Volcanic Field). Contribution to Mineralogy & Petrology vol.170, p.45, doi:10.1007/s00410-015-1200-1.

476 Diella V., Spalla M. I. & Tunesi, A. (1992) Contrasted thermo-mechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy). Journal of Metamorphic Geology, 10, 203–219. Doi: 10.1111/j.1525-1314.1992.tb00079.x

479 Droop G.T.R. & Bucher-Nurminen K. (1984) Reaction textures and metamorphic evolution of sapphirine-bearing granulites from the Gruf Complex, Italian Central Alps. Journal of Petrology, 25, 766-803.

482 Engi, M., Todd, C.S., Schmatz, D.R., 1995. Tertiary metamorphic conditions in the eastern Lepontine Alps. Schweizerische Mineralogische und Petrographische Mitteilungen 75, 347–369.

484 Engi M., Bousquet R. & Berger A. (2004) Explanatory notes to the map: Metamorphic structure of the Alps - Central Alps. Mitteilungen der Österreichischen Mineralogischen Gesellschaft, 149, 157-173.

487 Galli A., Le Bayon B., Schmidt M.W., Burg J.P., Caddick M.J., Reusser E. (2011) Granulites and charnockites of the Gruf Complex: evidence for Permian ultra-high temperature metamorphism in the Central Alps. Lithos 124:17–45. doi:10.1016/j.lithos.2010.08.003

490 Galli, A., Le Bayon, B., Schmidt, M.W., Burg, J.P., Reusser, E., Sergeev, S.A., & Larionov, A. (2012). U-Pb zircon dating of the Gruf Complex: disclosing the late Variscan granulitic lower crust
492 of Europe stranded in the Central Alps. Contributions to Mineralogy and Petrology, 163(2), 353–

493 378. DOI 10.1007/s00410-011-0676-6

494 Galli A., Le Bayon B., Schmidt M.W., Burg J.P. & Reusser E. (2013) Tectonometamorphic history

495 of the Gruf complex (Central Alps): exhumation of a granulite–migmatite complex with the Bergell

496 pluton. Swiss Journal of Geoscience, 106:33–62. DOI 10.1007/s00015-013-0120-1

497 Guevara V.E. & Caddick M.J. (2016) Shooting at a moving target: phase equilibria modelling of

498 high temperature metamorphism. Journal of metamorphic Geology, 34, 209–235.

499 doi:10.1111/jmg.12179

500 Harley S.L. (1998). – On the occurrence and characterization of ultrahightemperature crustal

501 metamorphism. *In:* Treloar, P.J., O’Brien, P.J. (Eds.), What Drives Metamorphism and Metamorphic

502 Relations? *Geological Society, London, Special Publication*, **138**, p. 81-107.

503 Harley S.L. (2016) – A matter of time: the importance of the duration of UHT metamorphism.

504 Journal of Mineralogical and Petrological Sciences, v. 111, p. 50-72. doi:10.2465/jmps.160128

505 Kelly N. M., Harley S. L., & Möller A. (2012). Complexity in the behavior and recrystallization of

506 monazite during high-T metamorphism and fluid infiltration. Chemical Geology, 322–323, 192–

507 208. https://doi.org/10.1016/j.chemgeo.2012.07.00

508 Kelsey D.E., and Hand M. (2015) - On ultrahigh temperature crustal metamorphism: Phase equilibri

509 ia, trace element thermometry, bulk composition, heat sources, timescales and tectonic settings.

510 Geoscience Frontiers 6, 311 – 356. http://dx.doi.org/10.1016/j.gsf.2014.09.006

511 Kelsey D.E., White R.W., Powell R. (2003) – Orthopyroxene-sillimanite-quartz

512 assemblages: distribution, petrology, quantitative P-T-X constraints and P-T paths. Journal of

513 Metamorphic Geology 21, 439-453. DOI: 10.1046/j.1525-1314.2003.00456.x

514 Köppel V., Günthert A., Grünenfelder M. (1981). Patterns of U–Pb zircon and monazite ages in

515 polymetamorphic units of the Swiss Alps. Schweizerische Mineralogische und Petrographische

516 Mitteilungen 61, 97–119.

517 Korhonena F.J., Clarka C., Brown M., Bhattacharyac S., Taylora R. (2013) How long-lived is

518 ultrahigh temperature (UHT) metamorphism? Constraints from zircon and monazite geochronology

519 in the Eastern Ghats orogenic belt, India. Precambrian Research. Vol. 234, 322-350.

520 doi.org/10.1016/j.precamres.2012.12.001

521 Lardeaux J.M. and Spalla I. (1991) - From granulites to eclogites in the Sesia zone (Italian Western

522 Alps): a record of opening and closure of the Piedmont ocean. Journal of Metamorphic Geology,

523 9:35–59

524 Liati A., Gebauer D. (2003) Geochronological constraints for the time of metamorphism in the Gruf

525 Complex (Central Alps) and implications for the Adula-Cima Lunga nappe system. Schweizerische

526 Mineralogische und Petrographische Mitteilungen, 83:159–172

527 McDade P., Harley S.L. (2001) - A petrogenetic grid for aluminous granulite facies metapelites in

528 the KFMASH system. Journal of Metamorphic Geology, p. 19, p. 45 – 59 DOI: 10.1046/j.0263-

529 4929.2000.00296.x

530 Manzotti P., Rubatto D., Darling J., Zucali M., Cenki-Tok B., Engi M. (2012) - From Permo-

531 Triassic lithospheric thinning to Jurassic rifting at the Adriatic margin: Petrological and

532 geochronological record in Valtournenche (Western Italian Alps). Lithos 146–147; 276–292.

533 doi:10.1016/j.lithos.2012.05.007

534 Manzotti, P. & Zucali, M. 2013. The pre-Alpine tectonic history of the Austroalpine continental

535 basement in the Valpelline unit (Western Italian Alps). Geological Magazine, 150, 153–172. Doi:

536 10.1017/S0016756812000441.

537 Marotta A.M., Roda M., Conte K., Spalla M.I. (2016) Thermo-mechanical numerical model of the

538 transition from continental rifting to oceanic spreading: the case study of the Alpine Tethys.
539 Geological Magazine, in press, 1-30. Doi: 10.1017/S0016756816000856.

540 Marotta A.M., Gosso G., Spalla M.I. (2009) Upper and lower crustal evolution during lithospheric extension: Numerical modelling and natural footprints from the European Alps. Geological Society
541
542 Special Publication, 321, 33-72. Doi: 10.1144/SP321.3.

543 Montel J.M.(1986) Experimental determination of the solubility of Ce-monazite in Si02-Al203-K20-Na20 melts at 800 °C, 2 kbar,under h2O-saturated conditions Geology v. 14 no. 8 p. 659-662

545 Montel J.M., Didier J. & Pichavant M. (1991) Origin of surmicaceous enclaves in intrusive granites. Developments in Petrology : Enclaves and Granite Petrology, 509-528. Elsevier.

547 Müntener O., Hermann J. & Trommsdorff V. (2000). Cooling history and exhumation of lower crustal granulite and upper Mantle (Malenco, eastern Central Alps). Journal of Petrology, 41, 175–200.

550 Niggli E. (1974) Metamorphism and tectonics of the Alps. Memorie della Società geologica Italiana, 13, 285-289.

552 Oalmann J., Möller A. & Bousquet R., (2013). P-T modelling reveals juxtaposition of units within the Gruf Complex (Central Alps) during orogenesis. Mineralogical Magazine, 77, 1873.
553
554 DOI:10.1180/minmag.2013.077.5.15

555 Oalmann J. Möller A. & Bousquet R., (2016) – Unravelling the P-T-t evolution of the crust of the Gruf complex by in situ accessory mineral dating and thermometry combined with P-T modelling of microdomains. European Mineralogical Conference, Rimini.

558 Paquette J.L., Goncalves P., Devouard B. and Nicollet C. (2004). Micro-drilling ID-TIMS U-Pb dating of single monazites: A new method to unravel complex poly-metamorphic evolutions.

560 Application to the UHT granulites of Andriamena (North-Central Madagascar). Contributions to
561 Mineralogy and Petrology, 147: 110–122 DOI: 10.1007/s00410-003-0549-8

562 Paquette, J.-L., Tiepolo, M., 2007. High resolution (5 µm) U-Th-Pb isotopes dating of monazite with excimer laser ablation (ELA)-ICPMS. Chemical Geology 240, 222–237.

564 DOI:10.1016/j.chemgeo.2007.02.014

565 Paquette, J.L., Piro, J.L., Devidal, J.L., Bosse, V., Didier, A., 2014. Sensitivity enhancement in LA-ICP-MS by N2 addition to carrier gas: application to radiometric dating of U-Th bearing minerals.

567 Agilent ICP-MS J. 58, 4–5

568 Perchuk A.L., Safonov O.G., Smit C.A., van Reenen D.D., Zakharov V.S, Gerya T.V. (2017)

569 Precambrian ultra-hot orogenic factory: Making and reworking of continental crust;

570 Tectonophysics, in press. https://doi.org/10.1016/j.tecto.2016.11.041

571 Peressini G., Quick J. E., Sinigoi S., Hofmann A. W. & Fanning M. (2007) Duration of a large mafic intrusion and heat transfer in the lower crust: a SHRIMPU-Pb zircon study in the Ivrea-Verbano Zone (Western Alps, Italy). Journal of Petrology, 48, 1185–1218. Doi: 10.1093/petrology/egm014

574 Pownall J.M. (2015) - UHT metamorphism on Seram, eastern Indonesia: reaction microstructures and P-T evolution of spinel-bearing garnet-sillimanite granulites from the Kobipoto Complex. Journal of metamorphic Geology, 33, 909-935 doi:10.1111/jmg.12153

577 Pownall J.M., Hall R., Armstrong R.A. & Forster M.A. (2014) - Earth's youngest known ultrahigh-
578 temperature granulites discovered on Seram, eastern Indonesia. Geology, 42, 279-282. DOI:

579 https://doi.org/10.1130/G35230.1

580 Rapp R.P. & Watson E.B. (1986). Monazite solubility and dissolution kinetics: implications for the thorium and light rare earth chemistry of felsic magmas. Contributions to Mineralogy and Petrology. 94 : 304-316.

583 Redler C., White R.W., Johnson. T.E., (2013) Migmatites in the Ivrea Zone (NW Italy): Constraints on partial melting and melt loss in metasedimentary rocks from Val Strona di Omegna Lithos Vol.
585 190–191, March 2014, Pages 1-12. doi.org/10.1016/j.lithos.2013.04.019

586 Rocha B.C., Moraes R., Möller A., Cioffi C.R., Jercinovic M.J. (2017) Timing of anatexis and melt

587 crystallization in the Socorro–Guaxupé Nappe, SE Brazil: Insights from trace element composition
588 of zircon, monazite and garnet coupled to UPb geochronology. Lithos 277, 337-355.

589 https://doi.org/10.1016/j.lithos.2016.05.020

590 Rubatto D. (2002) Zircon trace element geochemistry: partitioning with garnet and the link between

591 U–Pb ages and metamorphism. Chemical Geology 184, 123–138 doi.org/10.1016/S0009-

592 2541(01)00355-2

593 Rubatto D., Hermann J., Buick I.S., (2006). Temperature and bulk composition control on the

594 growth of monazite and zircon during low-pressure anatexis (Mount Stafford, central Australia).

595 Journal of Petrology, 47, 1973–1996. https://doi.org/10.1093/petrology/egl033

596 Rubatto D., Hermann J., Berger A., Engi M., (2009). Protracted fluid-present melting during

597 Barrovian metamorphism in the Central Alps. Contributions to Mineralogy and Petrology.

598 doi:10.1007/s00410-009-0406-5.

599 Rubatto D., Chakraborty S. and Dasgupta S. (2013) Timescales of crustal melting in the Higher

600 Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained

601 monazite and zircon chronology. Contributions to Mineralogy and Petrology 165:349–372.

602 doi: 10.1007/s00410-012-0812-y

603 Schaltegger U. (1994) Unravelling the pre-Mesozoic history of Aar and Gotthard massifs (Central

604 Alps) by isotopic dating - a review. Schweizerische Mineralogische und Petrographische

605 Mitteilungen, 74, 41–51.

606 Schärer U. (1984) The effect of initial 230Th disequilibrium on young U-Pb ages: the Makalu case,

607 Himalaya. Earth Planetary Science Letters 67 : 191-0204.

608 Schmid S.M., Berger A., Davidson C., Gieré R., Hermann J., Nievergelt P., Pusching A.R. &

609 Rosenberg C. (1996) The Bergell Pluton (Southern Switzerland-Northern Italy): overview

610 accompanying a geological-tectonic map of the intrusion and surrounding country rocks.

611 Schweizerische Mineralogische und Petrographische Mitteilungen, 76, 329–355.

612 Schmitz S., Möller A., Wilke M., Malzer W., Kannigiesser B., Bousquet R., Berger A., Schefer S.

613 (2009) Chemical U-Th-Pb dating of monazite by 3D-Micro X-ray fluorescence analysis with

614 synchrotron radiation. European Journal of Mineralogy 21:927–945. https://doi.org/10.1127/0935-

615 1221/2009/0021-1964

616 Schuster R., Scharbert S., Abart R. & Frank W. (2001) Permo-Triassic extension and related HT/LP

617 metamorphism in the Austroalpine-Southalpine realm. Mitteilungen der Gesellschaft der Geologie

618 und Bergbaustudenten in Österreich, 45, 111–141.

619 Schuster R. & Stuewe K. (2008) Permian metamorphic event in the Alps. Geology, 36, 603–606.

620 Doi: 10.1130/G24703A.1.

621 Spalla M. I. & Marotta A. M. (2007) P–T evolutions vs. numerical modelling: a key to unravel the

622 Paleozoic to early-Mesozoic tectonic evolution of the Alpine area. Periodico di Mineralogia, 76,

623 267–308. Doi: 10.2451/2007PM0029.

624 Spalla M. I., Zanoni D., Marotta A. M., Rebay G., Roda M., Zucali M. & Gosso G. (2014) The

625 transition from Variscan collision to continental break-up in the Alps: insights from the comparison

626 between natural data and numerical model predictions. Geological Society, London, Special

627 Publication, 405, 363–400. Doi: 10.1144/SP405.11

628 Spear F.S., Pyle J.M. (2002) Apatite, monazite, and xenotine in metamorphic rocks, reviews in

629 mineralogy and geochemistry, phosphates: geochemical, geobiological, and materials importance.

630 Mineralogical Society of America, 48:523–558.

631 Stepanov A.S., Hermann J., Rapp R.P., Rubatto D. (2012) Experimental study of monazite/melt
partitioning with implications for the REE, Th and U geochemistry of crustal rocks. Chemical Geology. 300-301:200-220. https://doi.org/10.1016/j.chemgeo.2012.01.007

Stüwe K. 1995 - Thermal buffering effects at the solidus. Implications for the equilibration of partially melted metamorphic rocks. Tectonophysics vol. 248 p. 39-51.

Todd, C.S. and Engi, M., 1997. Metamorphic field gradients in the Central Alps. Journal of Metamorphic Geology 15, 513-530

Tribuzio R., Thirlwall M.F., Messiga B. (1999) Petrology, mineral and isotope geochemistry of the Sondalo gabbroic complex (Central Alps, Northern Italy): implications for the origin of post-Variscan magmatism. Contributions to Mineralogy and Petrology, 136: 48-62.

Trommsdorff V., (1966). Progressive Metamorphose kieseliger Karbonatgesteine in den Zentralalpen zwischen Bernina und Simplon. Schweizerische Mineralogische und Petrographische Mitteilungen 46, 431-460.

Vavra G. and Schaltegger, U. (1999) - Post-granulite facies monazite growth and rejuvenation during Permian to Lower Jurassic thermal and fluid events in the Ivrea Zone (Southern Alps. Contributions to Mineralogy and Petrology, 134: 405-414

Vielzeuf, D. and Holloway, J.R. (1988). Experimental determination of the fluid-absent melting reactions of pelitic rocks — consequences for the crustal differentiation. Contributions to Mineralogy and Petrology, 98, 257-276.

Von Blanckenburg F. & Davies J.H. (1995) Slab breakoff. A model for syncollisional magmatism and tectonics in the Alps. Tectonics, 14, 120-131. DOI: 10.1029/94TC02051

Wenk H.R., Wenk E. and Wallace J.H. (1974) Metamorphic mineral assemblages in politic rocks of the Bergell Alps. Schweizerische Mineralogische und Petrographische Mitteilungen, 54:507-554

Whitney D.L. & Evans B.W., 2010. Abbreviations for names of rock-forming minerals. American Mineralogist, 95, 185-187. DOI: 10.2138/am.2010.3371

## 657 Figures captions

Fig. 1: Sketch map of the Gruf complex in the eastern Central Alps after Galli et al., (2011); Asterisk locates upper Val Codera. The sketch is located in the Alpine chain (inset).

Fig. 2: Microphotographs of the parageneses and textures of the granulitic rock (Plane-(PPL) and cross (XPL)-polarized light; (a): The Spr bearing granulitic paragenesis (PPL); (b): Complex retrograde symplectites in the Spr bearing portion of the rock granulite with spr2, sp, cord, sil2. Opx + Crd symplectites replace Grt (PPL); (c): Spr2+crd resulting of the retrograde reaction Opx + Sil = Spr + Crd (PPL); (d): the charnockitic paragenesis (XPL); (e): textures of incipient melting or residual melt after extraction in the charnockite portion of the rock (XPL). Abbreviations in text and figures are from Whitney and Evans (2010).

Fig. 3: Al₂O₃ profiles in the primary Opx (C: core and R: rim) and Al₂O₃ concentrations (contents values) in the Opx from the Opx + Crd symplectite (sym) around garnet.

Fig. 4 (a): Resorbed crystals of zircons included in primary phenocrysts, Spr and Sil (PPL);
(b): Cathodoluminescence images of zircons in the Crd – Bt matrix with oscillatory zoning; (c): zircon grains surrounded by highly luminescent rim (<5 µm to 15 µm) with euhedral faces with Alpine or intermediate ages ($^{206}$Pb/$^{238}$U ages; $2\sigma$)

**Fig. 5 (a):** Monazite inclusions in the phenocrysts (PPL) of SAPPHIRINE and Al-rich orthopyroxene of the Spr bearing domain of the rock; (b): Monazite inclusions in an Opx of the charnockite portion of the rock; note the tiny overgrowths (rich in Y) in the BSE image (crystal T14M1). Circles: $^{208}$Pb/$^{232}$Th ages in Ma (with ±1.1 Ma for all data).

**Fig. 6 (a):** BSE images of figure 2b and of the monazites in Spr, Crd and in clusters in late symplectites; cathodoluminescence of a zircon in Crd. ThO₂ and Y₂O₃ X-ray maps (spots; in wt%) in the crystal T50M1 and $^{208}$Pb/$^{232}$Th ages (circles) ages in Ma (with ±1.1 Ma for the monazites). Black inclusions are needles of sillimanite; arrows: diamond-shaped basal sections.

**Fig. 6 (b):** ThO₂ and Y₂O₃ X-ray maps (spots; in wt%) and BSE image and $^{208}$Pb/$^{232}$Th ages (circles) ages in Ma with ±1.1 Ma for a monazite in the charnockite portion of the rock (crystal T12M1).

**Fig. 7:** Chemical composition of the monazites; (a): Ternary diagram showing the relative Y₂O₃, ThO₂ and UO₂ (wt%) contents; (b): Y₂O₃ versus Gd₂O₃; (c): La₂O₃ versus Gd₂O₃.

**Fig. 8:** Geochronological results; (a): $^{208}$Pb/$^{232}$Th versus $^{206}$Pb/$^{238}$U diagram for the monazite; (b): Weighted average $^{208}$Pb/$^{232}$Th ages in monazite; (c): Tera Wasserburg diagram showing the zircon and monazite data (for more clarity, only concordant U/Pb data are shown); (d): Tera Wasserburg diagram showing U/Pb results for zircon cores with U content ≤ 1000 ppm and zircon grain without apparent Alpine rim in CLI.

**Table 1:** Electron microprobe analyses of the monazites. G = granulite domain; Ch = charnockite domain.
<table><caption>Table 1: analyses of monazites.</caption><thead><tr><th rowspan="2">Analyses</th><th colspan="3">G</th><th colspan="3">G</th><th colspan="3">Ch</th><th colspan="2">G</th><th colspan="2">Ch</th><th colspan="2">G</th><th colspan="2">G</th><th colspan="2">G</th></tr><tr><th>2</th><th>7</th><th>14</th><th>17</th><th>18</th><th>23</th><th>25</th><th>32</th><th>33</th><th>34</th><th>42</th><th>45</th><th>49</th><th>53</th><th>56</th><th>57</th><th>58</th><th>59</th><th>61</th><th>64</th></tr><tr><th></th><th colspan="3">T50 - M2</th><th colspan="3">T50-M1</th><th colspan="3">i saphirine</th><th colspan="2">T50 - cluster</th><th colspan="2">T14 M1</th><th colspan="2">T13 M1</th><th colspan="2">T12-M1</th><th colspan="2">T10bis-M2</th><th colspan="2">T10 bis</th><th colspan="2">T10bisM1</th><th colspan="2">T18-i OPX</th><th colspan="2">T33-i OPX</th></tr></thead><tbody><tr><td>SiO2</td><td>0.17</td><td>0.27</td><td>0.12</td><td>0.19</td><td>0.21</td><td>0.10</td><td>0.16</td><td>0.25</td><td>0.31</td><td>0.47</td><td>0.22</td><td>0.38</td><td>0.39</td><td>0.59</td><td>0.41</td><td>0.32</td><td>0.19</td><td>1.34</td><td>0.19</td><td>0.32</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>P2O5</td><td>30.15</td><td>29.51</td><td>29.77</td><td>29.54</td><td>29.93</td><td>29.99</td><td>29.67</td><td>30.20</td><td>29.96</td><td>29.65</td><td>30.29</td><td>29.57</td><td>30.02</td><td>29.09</td><td>29.60</td><td>29.57</td><td>29.80</td><td>27.70</td><td>29.65</td><td>30.12</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ce2O3</td><td>29.92</td><td>28.92</td><td>30.59</td><td>29.74</td><td>29.23</td><td>30.28</td><td>29.79</td><td>25.18</td><td>25.44</td><td>25.61</td><td>28.36</td><td>25.20</td><td>24.73</td><td>25.71</td><td>29.57</td><td>29.48</td><td>29.22</td><td>30.89</td><td>29.94</td><td>27.14</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>La2O3</td><td>13.58</td><td>13.48</td><td>14.31</td><td>13.26</td><td>13.14</td><td>13.30</td><td>13.59</td><td>10.70</td><td>11.09</td><td>11.10</td><td>12.51</td><td>11.39</td><td>10.70</td><td>11.22</td><td>13.47</td><td>13.39</td><td>13.20</td><td>14.78</td><td>13.59</td><td>12.29</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pr2O3</td><td>3.30</td><td>3.18</td><td>3.10</td><td>3.39</td><td>3.19</td><td>3.45</td><td>3.21</td><td>2.78</td><td>2.98</td><td>2.87</td><td>2.93</td><td>3.01</td><td>2.77</td><td>2.89</td><td>3.08</td><td>3.09</td><td>3.23</td><td>3.65</td><td>3.05</td><td>3.02</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Nd2O3</td><td>11.49</td><td>10.97</td><td>11.95</td><td>11.58</td><td>11.36</td><td>11.32</td><td>11.06</td><td>9.88</td><td>10.44</td><td>10.49</td><td>10.75</td><td>8.71</td><td>10.32</td><td>11.27</td><td>11.65</td><td>11.66</td><td>10.92</td><td>11.99</td><td>10.52</td><td>11.02</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sm2O3</td><td>1.95</td><td>1.46</td><td>1.54</td><td>1.66</td><td>1.74</td><td>2.17</td><td>1.66</td><td>2.14</td><td>1.90</td><td>2.03</td><td>1.92</td><td>1.64</td><td>1.85</td><td>1.82</td><td>1.73</td><td>1.52</td><td>1.75</td><td>1.19</td><td>1.77</td><td>1.74</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Gd2O3</td><td>1.33</td><td>0.44</td><td>0.69</td><td>1.09</td><td>1.54</td><td>1.58</td><td>1.06</td><td>2.44</td><td>1.91</td><td>2.05</td><td>1.39</td><td>1.01</td><td>2.04</td><td>1.60</td><td>1.07</td><td>1.11</td><td>1.01</td><td>0.64</td><td>0.91</td><td>1.82</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Y2O3</td><td>0.39</td><td>0.13</td><td>0.35</td><td>0.31</td><td>0.50</td><td>0.69</td><td>0.32</td><td>4.78</td><td>3.78</td><td>3.24</td><td>0.98</td><td>0.47</td><td>3.89</td><td>2.63</td><td>0.27</td><td>0.58</td><td>0.91</td><td>0.14</td><td>0.65</td><td>2.14</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>CaO</td><td>1.34</td><td>1.75</td><td>1.16</td><td>1.39</td><td>1.45</td><td>1.15</td><td>1.46</td><td>1.61</td><td>1.76</td><td>1.77</td><td>1.83</td><td>2.90</td><td>1.78</td><td>1.67</td><td>1.31</td><td>1.45</td><td>1.53</td><td>0.44</td><td>1.53</td><td>1.49</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ThO2</td><td>5.79</td><td>8.43</td><td>5.15</td><td>6.42</td><td>6.07</td><td>3.92</td><td>6.73</td><td>7.84</td><td>8.75</td><td>9.32</td><td>6.78</td><td>10.82</td><td>9.59</td><td>10.07</td><td>7.17</td><td>5.74</td><td>5.19</td><td>6.96</td><td>5.48</td><td>8.00</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>UO2</td><td>0.98</td><td>0.33</td><td>0.50</td><td>0.78</td><td>0.83</td><td>1.35</td><td>0.75</td><td>0.74</td><td>0.41</td><td>0.32</td><td>2.05</td><td>3.50</td><td>0.40</td><td>0.26</td><td>0.45</td><td>1.29</td><td>2.01</td><td>0.20</td><td>1.78</td><td>0.37</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>PbO</td><td>0.03</td><td>0.00</td><td>0.00</td><td>0.04</td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.03</td><td>0.00</td><td>0.01</td><td>0.02</td><td>0.00</td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.00</td><td>0.03</td><td>0.01</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td><strong>Total</strong></td><td><strong>100.41</strong></td><td><strong>98.87</strong></td><td><strong>99.23</strong></td><td><strong>99.38</strong></td><td><strong>99.20</strong></td><td><strong>99.30</strong></td><td><strong>99.48</strong></td><td><strong>98.54</strong></td><td><strong>98.76</strong></td><td><strong>98.93</strong></td><td><strong>100.00</strong></td><td><strong>98.63</strong></td><td><strong>98.49</strong></td><td><strong>98.83</strong></td><td><strong>99.76</strong></td><td><strong>99.21</strong></td><td><strong>98.96</strong></td><td><strong>99.90</strong></td><td><strong>99.08</strong></td><td><strong>99.47</strong></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Si</td><td>0.007</td><td>0.011</td><td>0.005</td><td>0.007</td><td>0.008</td><td>0.004</td><td>0.007</td><td>0.010</td><td>0.012</td><td>0.018</td><td>0.009</td><td>0.015</td><td>0.015</td><td>0.023</td><td>0.016</td><td>0.013</td><td>0.007</td><td>0.054</td><td>0.008</td><td>0.012</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>P</td><td>0.996</td><td>0.991</td><td>0.995</td><td>0.990</td><td>0.997</td><td>0.999</td><td>0.992</td><td>0.995</td><td>0.991</td><td>0.984</td><td>0.998</td><td>0.990</td><td>0.993</td><td>0.975</td><td>0.987</td><td>0.988</td><td>0.995</td><td>0.941</td><td>0.992</td><td>0.995</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ce</td><td>0.427</td><td>0.420</td><td>0.442</td><td>0.431</td><td>0.421</td><td>0.436</td><td>0.431</td><td>0.359</td><td>0.364</td><td>0.368</td><td>0.404</td><td>0.365</td><td>0.354</td><td>0.373</td><td>0.426</td><td>0.426</td><td>0.422</td><td>0.454</td><td>0.433</td><td>0.388</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>La</td><td>0.195</td><td>0.197</td><td>0.208</td><td>0.194</td><td>0.191</td><td>0.193</td><td>0.198</td><td>0.154</td><td>0.160</td><td>0.161</td><td>0.179</td><td>0.166</td><td>0.154</td><td>0.164</td><td>0.196</td><td>0.195</td><td>0.192</td><td>0.219</td><td>0.198</td><td>0.177</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pr</td><td>0.047</td><td>0.046</td><td>0.045</td><td>0.049</td><td>0.046</td><td>0.049</td><td>0.046</td><td>0.039</td><td>0.042</td><td>0.041</td><td>0.042</td><td>0.043</td><td>0.039</td><td>0.042</td><td>0.044</td><td>0.044</td><td>0.046</td><td>0.053</td><td>0.044</td><td>0.043</td><td></td><td></td><td></td><td></td></tr><tr><td>Nd</td><td>0.160</td><td>0.155</td><td>0.169</td><td>0.164</td><td>0.160</td><td>0.159</td><td>0.156</td><td>0.137</td><td>0.146</td><td>0.147</td><td>0.149</td><td>0.123</td><td>0.144</td><td>0.159</td><td>0.164</td><td>0.164</td><td>0.154</td><td>0.172</td><td>0.149</td><td>0.153</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Sm</td><td>0.026</td><td>0.020</td><td>0.021</td><td>0.023</td><td>0.024</td><td>0.029</td><td>0.023</td><td>0.029</td><td>0.026</td><td>0.027</td><td>0.026</td><td>0.022</td><td>0.025</td><td>0.025</td><td>0.023</td><td>0.021</td><td>0.024</td><td>0.016</td><td>0.024</td><td>0.023</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Eu</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Gd</td><td>0.017</td><td>0.006</td><td>0.009</td><td>0.014</td><td>0.020</td><td>0.021</td><td>0.014</td><td>0.031</td><td>0.025</td><td>0.027</td><td>0.018</td><td>0.013</td><td>0.026</td><td>0.021</td><td>0.014</td><td>0.015</td><td>0.013</td><td>0.009</td><td>0.012</td><td>0.023</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Y</td><td>0.008</td><td>0.003</td><td>0.007</td><td>0.006</td><td>0.010</td><td>0.015</td><td>0.007</td><td>0.099</td><td>0.079</td><td>0.068</td><td>0.020</td><td>0.010</td><td>0.081</td><td>0.055</td><td>0.006</td><td>0.012</td><td>0.019</td><td>0.003</td><td>0.014</td><td>0.044</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ca</td><td>0.056</td><td>0.074</td><td>0.049</td><td>0.059</td><td>0.061</td><td>0.048</td><td>0.062</td><td>0.067</td><td>0.074</td><td>0.075</td><td>0.076</td><td>0.123</td><td>0.074</td><td>0.071</td><td>0.055</td><td>0.062</td><td>0.064</td><td>0.019</td><td>0.065</td><td>0.062</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Th</td><td>0.051</td><td>0.076</td><td>0.046</td><td>0.058</td><td>0.054</td><td>0.035</td><td>0.060</td><td>0.069</td><td>0.078</td><td>0.083</td><td>0.060</td><td>0.097</td><td>0.085</td><td>0.091</td><td>0.064</td><td>0.052</td><td>0.047</td><td>0.064</td><td>0.049</td><td>0.071</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>U</td><td>0.008</td><td>0.003</td><td>0.004</td><td>0.007</td><td>0.007</td><td>0.012</td><td>0.007</td><td>0.006</td><td>0.004</td><td>0.003</td><td>0.018</td><td>0.031</td><td>0.004</td><td>0.002</td><td>0.004</td><td>0.011</td><td>0.018</td><td>0.002</td><td>0.016</td><td>0.003</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Pb</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td><strong>Summe</strong></td><td><strong>2.000</strong></td><td><strong>2.001</strong></td><td><strong>2.001</strong></td><td><strong>2.002</strong></td><td><strong>1.999</strong></td><td><strong>2.000</strong></td><td><strong>2.001</strong></td><td><strong>1.997</strong></td><td><strong>1.999</strong></td><td><strong>2.001</strong></td><td><strong>1.998</strong></td><td><strong>2.000</strong></td><td><strong>1.995</strong></td><td><strong>2.001</strong></td><td><strong>1.999</strong></td><td><strong>2.003</strong></td><td><strong>2.001</strong></td><td><strong>2.006</strong></td><td><strong>2.003</strong></td><td><strong>1.995</strong></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Figure 4</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Figure 5</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Figure 6a</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6b</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Figure 7</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 8 a</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 8 b</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Figure 8c</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Figure 8 d</figcaption></figure>