Stealth Metasomatism in Granulites from Ivrea (NW Italy): Hydration of the (Variscan) Lower Crust by Melt Flow

1. Introduction

The role of the lower continental crust in the processes that govern the transition from the late orogenic stages, such as the extensional regime linked to the orogenic collapse, to a new Wilson’s cycle, such as the continental drifting, is still poorly investigated. In this geodynamic context, underplating of large volumes of mafic magma induces relevant chemical and rheological variations in the lower crust and in the overlying middle crust, with consequences in the distribution of the stress fields, but also in the genesis of some ore deposits. Nevertheless, the evidence of melt-rock interactions and the characterization of the involved processes (porous flow vs channeled flow) have not received a lot of attention until recently (e.g., [1,2,3,4]). Moreover, the interaction between melts and deep mafic plutons straddles the boundary between igneous and high-temperature metamorphic processes, making its study a frontier topic.

The Ivrea-Verbano Zone (IVZ), in northern Italy (Figure 1), is a promising site to study the melt-rock interaction in the lower crust and its potential geodynamic consequences. It is one of the best preserved and almost integrated continental crustal sections in the world, in which distinct mafic magmatic pulses infiltrate a Variscan lower crust during the Permian orogenic collapse (e.g., [5,6,7]) and in which different geological evidences of the Mesozoic Alpine rifting are preserved [e.g., 8-11, and references therein]. The Unit is subdivided into northern (Finero Complex), central (Strona, Sesia and Sessera valleys), and southern (Ivrea and Baldissero) IVZ. Previous studies concentrate on the northern IVZ (e.g., [12,13,14,15,16,17,18,19,20,21]) and central IVZ, Valle Strona: [22,23,24,25,26,27,28,29,30], Val Sesia: [31,32,33,34] and Val Sessera: [35-37, and references therein]. To our knowledge, the only recent paper on the southern IVZ is by Mazzucchelli et al. [38] on the Baldissero peridotites, whereas the mafic rocks outcropping in and around the Ivrea town have never been studied in detail.

In the present paper, we studied the granulite rocks of the southern IVZ. We report field, macro, micro and chemical (major and trace-element mineral composition) evidence of “stealth” metasomatic hydration of a former dry mafic lower crust induced by combination of channelized flux and porous flow of orogenic hydrous basalts, and we discuss its geodynamic implications.

2. Geological Setting

The Ivrea-Verbano Zone (IVZ), located in NW Italy, is a 130 km long, and 10-50 km wide stripe that extends from Ivrea up to Locarno (Figure 1). The IVZ belongs to the Southalpine domain of the Alps, i.e., the domain consisting of Variscan continental crust constituting the Adria plate during the Triassic-Jurassic extensional tectonics responsible for the opening of the Tethys Ocean [9,39,40]. This domain, also known as Southern Alps, was only marginally involved in the compressional tectonics responsible for the Tethys closure and continental collision generating the Alpine chain and, so, it records a low-grade Alpine metamorphism that is traced only along tectonic structures [8, and references therein]. To the SE, the IVZ is separated from the Southalpine Serie dei Laghi sequence (Variscan mid-to upper crust section) by the Permian transpressional shear zone of Cossato-Mergozzo-Brissago and by the subsequent Pogallo and Cremosina Lines. To the NW, the IVZ is separated from the Sesia Zone (Austroalpine domain strongly involved in the Alpine tectonics) by a thick (1 km wide) mylonite zone called Insubric Line (e.g., [41,42,43]). To the SW, near Ivrea town, the IVZ is separated from the Canavese Zone, recording a very low-grade Alpine metamorphism, by a ductile deep fault named Internal Canavese Line (e.g., [44]). The IVZ is subdivided in three units: a) the Kinzigite Formation, b) the Mafic Complex and c) the Ultramafic Massifs (Figure 1).

2.1. Kinzigite Formation

The Kinzigite Formation, well characterized and exposed in the Strona Valley, is mainly composed of alternating metapelites, metabasites, marbles, calc-silicates and quartzites. This rock sequence has been proven to record the Variscan metamorphic evolution of the IVZ, where the metamorphic gradient increases perpendicular to the strike of the IVZ from SE to NW from amphibolite-to granulite-facies metamorphism, respectively (e.g., [46,47,48,49,50,51]). The terms ‘kinzigites’ and ‘stronalites’ that will be used here were introduced by Schnetger [52] to describe, respectively, amphibolite-and granulite-facies metapelites within the Kinzigite formation. The protolith composition is considered similar for both the granulite-facies and the amphibolite-facies metapelites in Strona Valley [53]. The metabasic rocks of the Kinzigite Formation have been presumed to be a part of the Mafic Complex (see below) by many researchers (e.g., [24,32,35,54]) that based on field relationships and petrography, stated that these rocks intruded the metasedimentary succession of the Kinzigite Formation. In fact, Kunz et al. [27] hypothesized that these rocks are metabasalts [50,55] belonging to the Kinzigite Formation, and experienced the same high temperature metamorphic event of the associated metapelites, because of their progressive granulitization and migmatization towards the Insubric Line. Additional evidence to this hypothesis suggests changes in the metamorphic texture from nematoblastic to granoblastic, and the progressive amphibole consumption to form pyroxenes [51].

The Variscan (316 ± 3 Ma; [30,56]) thermal peak in the Kinzigite Formation has been estimated at around 650–730°C and ~3.5–6.5 kbar for the amphibolite-facies and up to 900–950°C and ~10–12 kbar for the granulite-facies in both metapelites [47] and metabasites [46]. Initially, the emplacement of the Mafic Complex was considered responsible for the granulitization of the overlying Kinzigite Formation (e.g., [26,49,57,58]). More recent geochronological and petrological data indicate that the early Permian emplacement of the Mafic Complex (ca. 285 Ma, [59] more details below) was not synchronous to the Variscan thermal peak and that a 2-km wide contact mineral assemblage has been observed to overprint the Variscan high-grade assemblages in the metapelites during the Permian (ca. 275 Ma; e.g., [31,47,57,60]).

2.2. Mafic Complex

The Mafic Complex is a large igneous body that intruded the middle-lower crust of the Kinzigite Formation, at approximately 25 km depth, in a post-Variscan transtensional tectonic setting (e.g., [5,54,57,61,62]). The Mafic Complex is a rather inhomogeneous intrusion, interpreted to have been formed by separate magmatic pulses of mantle magma progressively contaminated by hosting Variscan restitic metapelites (stronalites) and by the anatectic melt generated in the more fertile host rocks due to the crustal heating (e.g., [33] and references therein). It has been subdivided in two main units [35,36]: a) the Lower Mafic Complex (LMC), mainly consisting of granoblastic, foliated Amp-gabbros, gabbronorites and minor anhydrous charnockites; b) the Upper Mafic Complex (UMC), which contains gabbronorites, diorites and hydrous charnockites (in higher proportions compared to the LMC) with well-preserved magmatic structures. The Paragneiss-Bearing Belt (PBB) is the natural border between these two complexes and represents an arcuate-shaped, 1-2 km wide zone rich in charnockites, norites and ultramafic rocks alternating with metasedimentary septa considered as relics of the Kinzigite Formation [32,36,37,63]. The P-T conditions of crystallization are estimated at 8-9 kbar and 1000-1200°C, whereas the re-equilibration temperatures were estimated at 850-950°C ([57] and references therein). Very recent U-Pb zircon geochronological data place the timing of the mafic intrusion during early Permian (286–282 Ma), approximately 30 My after the peak metamorphic event in the Kinzigite Formation [30,64]. Ιn detail, SHRIMP U-Th-Pb isotopic data indicate that the intrusion of the LMC started in the early Permian, at ca. 295 Ma, and experienced thermal recrystallization at around 285 Ma. The crystallization of the UMC occurred between 292 and 286 Ma, and that of charnockites occurred at 277 Ma [33,59].

2.3. Ultramafic Massifs

Large bodies and smaller lenses and veins of ultramafic composition occur throughout the IVZ. The predominant lithologies are dunites, harzburgites, lherzolites and, locally, websterites with minor pyroxenites [65,66,67]. Three large ultramafic massifs are distinguished from NE towards SW: a) Finero, b) Balmuccia and c) Baldisero Canavese (Figure 1). The incorporation of these ultramafic massifs in the Mafic Complex is quite enigmatic and several opinions exist in the literature [42,43,67]. Detailed field evidence, show an igneous contact of the peridotites with the igneous rocks of the Mafic Complex suggesting that the mantle peridotites are older than the intrusion of the Mafic Complex ([68] and references therein).

3. Methods

Both EDS and WDS data have been acquired on the studied samples. SEM-EDS data were acquired using a scanning electron microscope (SEM-JEOL JSMIT300LV), combined with an energy dispersive spectrometry (EDS) Energy 200 system and an SDD X-Act3 detector (Oxford Inca Energy), installed at the Department of Earth Sciences (University of Torino, Italy). The system is equipped with the Microanalysis Suite Oxford AZtecMineral and with the following analytical set up: an accelerating voltage of 15 kV, a counting time of 50 s, a processing time of 3 µs, a working distance of 10 mm, and natural mineral standards were used. WDS data were performed with a JEOL JXA 8200 Superprobe, equipped with five wavelength dispersive spectrometers, energy dispersive X-ray spectroscopy (EDS), and cathodoluminescence detectors at the University of Milan. The operating conditions consisted of an acceleration voltage of 15 kV, at a beam current of 5 nA at 30 s counting time, with a spot size of 1 μm. Natural and synthetic minerals were used as standards. Structural formulae of minerals were processed using the NORMPLUS software of Ulmer [69] upgraded by S. Poli. Representative analyses are reported in Table S1.

Trace element data on the petrogenetic minerals (amphibole, plagioclase, orthopyroxene, clinopyroxene) from polished thin sections were determined with LA-ICP-MS at the Open University (OU), UK. The instrument at the OU is a Photon Machines Analyte G2 with a 193nm excimer laser, equipped with a HelEx II 2-volume cell, coupled with an Agilent 8800 ICP-QQQ-MS. The laser repetition rate of the analyses was at 10Hz, 3.63 J/cm2 fluence and a spot diameter of 50 μm which was reduced to 40 μm only for analysing certain sites with interstitial plagioclase films. Representative analyses for each mineral can be found in the Table S2.

Each analysis included 30 s of background measurement before ablation, 30 s of sample ablation and a 40 s washout after the ablation. Detailed instrument and analytical conditions can be found in Table S3. The calibration strategy included SRM-NIST 612 as primary standard and BCR-2G as secondary standard which were analysed every ~20 analyses. Data reduction was done with the Iolite software, v3.71 [70]. Data quality of BCR-2G analyses is in good agreement with published values [71] and with relative standard deviations of 0.47-22.88% (Table S4). Furthermore, probe data for major element concentrations were compared with major elements measured by LA-ICP-MS to ensure consistency between different methods. Element concentrations not plotted in the REE and trace element diagrams, have values below the detection limit. For a full list of detection limits for each element see Table S5.

4. Results

4.1. Field Evidence

In the area nearby Ivrea town, the IVZ mainly consists of mafic granulites and minor enderbites, with very rare metapelitic granulites (stronalites; Figure 2). These rocks are ubiquitous in the study area from Pavone Canavese and Lessolo, up to the contact of the Lower Mafic Complex with the Insubric Line, approximately 5-7 kilometers north of Ivrea town. Particularly, they homogenously cover the NE part of the studied area, as well as the area around the villages of Fiorano and Pavone. The enderbites largely outcrop in the south-eastern part of the studied area. The size and shape of the enderbites can vary from the large and irregular bodies of several hundred meters long and wide, outcropping at Monte Marino and Bric Appareglio, up to the very thin (<10 meters) dykes which can be traced for several hundred meters and are sporadically outcropping in the area between Ivrea and Lago Sirio. Where visible, the contact between mafic granulites and enderbites is usually transitional intrusive (see below). The stronalites outcrop only in the area between Ivrea and Lago Sirio and constitute decametric-thick septa, apparently aligned in direction SW-NE, within the mafic granulites. Moving towards the contact with the Insubric Line (i.e., the Internal Canavese Line) the lithologies are progressively fine-grained and mylonitized.

Below, the field appearance and relationships among mafic granulites, enderbites and stronalites are described in detail. More than 60 thin sections from collected samples and samples already present in the collection in the University of Turin, have been characterized under an optical microscope. Five samples were selected for more detailed analyses (Table 1). Mineral abbreviations used in text and figures are from Whitney and Evans [72].

4.1.1. Mafic Granulites

The mafic granulites macroscopically exhibit an apparent homogenous texture and composition ranging from gabbronorite to Amp-gabbronorite. However, a detailed examination revealed local variations in grain-size (from medium to coarse-grained; Figure 3a-b), in texture (from granoblastic to faintly foliated; Figure 3a-b), and in modal composition (variation in Pl and Amp contents, Figure 3c). Locally, they exhibit 0.5–5-centimeter-wide pyroxene-rich layers showing infiltrating Pl from the selvages, and fractures filled by infiltrated coarse-grained Pl + Amp (Figure 3c-d). The mafic granulites are crosscut by enderbites and Amp-rich dykes (see below).

4.1.2. Enderbites

The enderbites are predominantly medium- to coarse-grained and granoblastic (Figure 4a), but locally, a faint foliation marked by orthopyroxene (crystal size up to ~0.5 millimeter long) is also common, regardless of the rock grain size (Figure 4b). Fine-grained enderbites appear as sporadic dykes with a width of less than 10 centimeters that crosscut the coarser-grained ones (Figure 4c). The selvages are straight and do not show any reaction with the host enderbite. Enderbitic, fine-grained dykes are also found to crosscut the mafic granulites (Figure 4d). The observed selvages are usually overprinted by the Alpine orogeny, but they seem to lack evidence for reactions with the host mafic granulites.

Enderbites outcropping in Ivrea town, particularly those located north of the river Dora Baltea, are fine-grained, richer in quartz (>20 vol%), and poorer in Opx (<10%) compared to those outcropping in the southern part of the studied area. According to the QAP diagram for Opx-bearing granitic rocks [73], the threshold for quartz to classify a rock as a true enderbite is 20 vol%. The enderbites from the southern area do not contain as much quartz (~5-6 vol%) and therefore, should be classified as “Opx-diorite or Opx-granitoids” [73]. However, because of similar field relationships, microstructures and mineral assemblage, we consider these rocks as a more mafic variety of the Ivrea enderbites.

4.1.3. Amp-Rich Dykes, Pods, and Lenses

Both mafic granulites and enderbites are infiltrated by Amp-rich elongated dykes, pods, or lenses (Figure 5) that show irregular (Figure 5a,c) or gradual (Figure 5b,d) contacts. Usually, they show a hypidiomorphic granular structure from coarse-grained (Figure 5c-d) to pegmatitic (Figure 5a-b). The Pl content is variable, but always significantly lower than in the mafic granulites (Fig. 5c-d) Their composition ranges from Amp-gabbronorite to hornblendite.

4.1.4. Quartz-Feldspathic Dykes

Locally, both mafic granulites and enderbites are crosscut by coarse-grained quartz-felspathic dykes that show straight contacts (Figure 6a). Crosscutting relationships with the Amp-rich dykes are not observed.

4.1.5. Stronalites

The stronalites are rare and outcrop in the central part of the studied area (Figure 2). They constitute a few ‘septa’ (i.e., thin, lensoidal bodies that can be traced for a rather long distance) with dimensions that reach up to 100 meters in length and 20-50 meters in width. The septa appear oriented ca. NE-SW (Figure 2), i.e., parallel to the main strike observed in other localities of the IVZ (Figure 1). The stronalites are usually intercalated between enderbites and mafic granulites, but their contact is not visible. They exhibit partial melting structures such as leucosome formation, melt segregation and formation of coarse-grained quartz veins (Figure 6b). The quartz veins commonly have an amethyst-like color, probably due to numerous inclusions of rutile needles. The coarse-grained stronalites contain garnet porphyroblasts up to 1 centimeter in diameter (Figure 6b), while the fine-grained varieties contain plagioclase and garnet with a maximum diameter of <0.5 millimeter. Plagioclase and sillimanite, both fibrolitic and prismatic, are abundant, but very minor K-feldspar and biotite are observed. The majority of the stronalites are not well preserved and show extended retrogression reflected in retrograde veins that usually contain quartz + scapolite or quartz +(clino)zoisite and in plagioclase alterations. Further characterization of stronalitic septa is beyond the scope of this work.

4.1.6. Alpine Overprint

The field evidence of the Alpine overprint increases moving towards the contact with the Insubric Line near Montalto Dora (Figure 2). It is represented by ductile (mylonites) and fragile (centimeter-thick Chl-Pl-Ep veins) structures and by replacement of the mineral by very-low grade mineral assemblage (Figure 6c).

4.2. Petrography, Mineral Chemistry and Thermobarometry

4.2.1. Mafic Lithologies

The sampled mafic granulites show medium- to coarse-grained (~1 millimeter average grain size) granoblastic texture, locally polygonal, although faintly foliated samples are also present. Locally, relic magmatic structures are still well recognizable. They consist of plagioclase, orthopyroxene, clinopyroxene and variable modal amounts of amphibole. The accessory minerals include opaques (magnetite and ilmenite), spinel (hercynite, hereafter Hc) and apatite. Sample IZ23-02b has low Cpx content and the mineral composition is intermediate between the more mafic samples and the enderbitic ones (see below). Millimeter-to centimeter-thick pyroxene-layers parallel to the main foliation locally occur (Figure 7a).

Two generations of Pl and Px can be easily distinguished. The first one consists of coarse-grained (2-9 millimeters) porphyroclasts of Pl, Opx and Cpx with lobate grain boundaries, while the second one consists of medium- to fine-grained neoblasts with granoblastic polygonal structure (Figure 7b). The Pl porphyroclasts include Ap and Px in their core whose composition, although varying depending on the sample, is the richest in Na (An content ranging from 0.59 to 0.65; Figure 8a). The Pl neoblasts and the rim of the porphyroclasts are almost devoid of mineral inclusions and their composition is variable, but always slightly higher in Ca with respect to the corresponding core of porphyroclasts (An content ranging from 0.64 to 0.67; Figure 8a). Another Pl generation constitutes a micrometer-wide interstitial phase between Cpx and Mag ± Hc (Figure 7c-e) and, very rarely, between Opx and Mag ± Hc, but not between Amp and Mag ± Hc (Figure 7e). Its composition is the highest in Ca and shows a large spread (An content from 0.76 to 0.88; Figure 8a). In sample IZ21-20a (foliated mafic granulite), the Pl neoblasts show a chemical re-equilibration towards the more calcic compositions of the interstitial Pl (Figure 8a). Both Opx generations are characterized by evident pleochroism from light-green to light-pink (Figure 7b). The Opx porphyroclasts show dark brown exsolution lamellae (of a probable opaque phase) obliquely to its cleavage (Figure 7f), whereas the neoblasts are devoid of exsolutions (Figure 7b). The Opx porphyroclastic cores show higher Ca content (> 0.028 a.p.f.u.) than porphyroclastic rims and neoblasts (< 0.024 a.p.f.u.) and constant Mg# (from 0.69 to 0.70; Figure 9a). This Mg# is also similar to that of the porphyroclastic rims and neoblasts (from 0.68 up to 0.72; Figure 9a). The Al content is more scattered, but it is usually higher in porphyroclasts (> 0.217 a.p.f.u) than neoblasts (Figure 9b). In the low-Cpx sample, Opx has lower, but overlapping Mg# in the porphyroclastic cores and rims (from 0.59 to 0.61) and the neoblastic cores and rims (from 0.60 to 0.61). The Ca content is similar to the porphyroblastic rims and neoblasts from the mafic granulites (< 0.026 a.p.f.u.), while the Al content is significantly lower compared to analyses from the mafic granulites (< 0.109 a.p.f.u.; Fig 9a-b). The Cpx is pale green and not pleochroic (Figure 7b). The Cpx porphyroclasts usually contain opaque minerals and show exsolution lamellae of Opx (Figure 7b, d-e), but the neoblasts lack exsolution lamellae (Figure 7b). The Cpx is diopside with a varying composition which is not related to the different Cpx generations. The Mg# ranges from 0.75 to 0.81, the Ca content ranges from 0.83 to 0.87 a.p.f.u., the Al content from 0.26 to 0.32 a.p.f.u., the Ti content from 0.015 to 0.026 a.p.f.u., and the Na content from 0.050 to 0.067 a.p.f.u. (Figure 10). In the low-Cpx sample, clinopyroxene has lower Mg# (around 0.72), the Ca content ranges from 0.87 to 0.88 a.p.f.u., the Al content is around 0.14 a.p.f.u., the Ti content is 0.01 a.p.f.u., and the Na content ranges from 0.03 to 0.04 a.p.f.u. (Figure 10). Brown Amp is medium-to-coarse grained and is recognized by two families that show the same pleochroism from pale brown to dark brown. The first one constitutes large interstitial crystals, usually with shape preferred orientation, between Pl-Pl or Px-Px grain-boundaries (Figure 7g) and partly replaces rounded Cpx (Figure 7h). The second one is represented by granoblastic Amp with Ilm ± Mag “beads” at the Amp-Amp grain-boundaries in the triple joints (Figure 7i). The chemical composition of these two families overlaps and corresponds to a Ti-rich pargasite characterized by Mg# = 0.65-0.73, and (in a.p.f.u.) Ca = 1.70-1.84, Mn = 0.02-0.03, Al = 2.48-2.67, K = 0.12-0.24, Na = 0.74-0.90, Ti = 0.31-0.41, Cl ≤ 0.02, F ≤ 0.12 (Figure 11). In the low-Cpx sample, Amp has lower Mg# (0.60-0.67), and (in a.p.f.u.) Ca = 1.83-1.87, Mn = 0.003-0.017, Al = 2.25-2.34, K = 0.32-0.42, Na = 0.31-0.49, Ti = 0.19-0.28, Cl ≤ 0.07, F = 0.19-0.31 (Figure 11). The Mag is usually associated with pyroxenes and constitutes allotriomorphic or polygonal crystals (Figure 7k), depending on the structure of the rocks. It appears as blebs inside composite crystals with deep green Hc that are partially surrounded by interstitial Pl (Figure 7k).

The Px-rich layers are medium- to very coarse-grained and consist of Opx, Cpx, and variable amounts of Mag, Ilm, and Pl (Figure 12a). The layers exhibit a granoblastic structure, but the magmatic one is still commonly preserved. Both Cpx and Opx show exsolution lamellae, and opaque minerals show interstitial or polygonal habit. Opx from the pyroxenitic layer of sample IZ23-02b has a relatively homogeneous composition with Mg# = 0.62-0.63, Ca content ranging from 0.015-0.026 a.p.f.u. and Al content less than 0.1 a.p.f.u. (Figure 9a-b). The associated Pl in the pyroxenitic layers has an anorthite content ranging from 0.62-0.66, while interstitial Pl can reach up to 0.75. Locally, the layers are infiltrated by coarse-grained brown Amp + Pl (Figure 12b) that exhibit the interstitial structures already described for the mafic granulites (Figure 12c-d). These structures are well preserved in the samples that correspond to the coarse-grained Amp-rich elongated bodies, pods, or lenses, locally pegmatitic infiltrating both mafic granulites and Px-rich layers. They show increased proportions in brown Amp and/or Pl (Figure 12e), and they usually exhibit interstitial or granoblastic structure. In particular, the Amp-Amp grain-boundaries are marked by the Ilm ± Mag “beads” already described for the mafic granulites (Figure 12f). The composition of the interstitial Pl is Ca-rich (An content from 0.72 to 0.79) and similar to the interstitial Pl present in the mafic granulites (Figure 8a). Interstitial Amp and neoblasts have overlapping compositions of Ti-rich pargasite characterized by Mg# = 0.67-0.72, and (in a.p.f.u.) Ca = 1.73-1.82, Mn = 0.02-0.03, Al = 2.49-2.67, K = 0.17-0.22, Na = 0.74-0.84, Ti = 0.30-0.36, Cl = 0.01-0.02; F ≤ 0.10 (Figure 11). These compositions are similar to those of the Amp from mafic granulites but characterized by slightly higher K and lower Ti (Figure 11).

4.2.2. Enderbites

The enderbites from Ivrea range from fine-grained (dykes) to very coarse-grained (~1 millimeter average grain size) with local faintly foliation defined by Opx, either as elongated porphyroclasts (Figure 13a) or as neoblastic aggregates (Figure 13b). The mineral assemblage mainly consists of Pl, Opx, Opq ± Qz ± Amp. Although relict porphyroclastic Pl and Opx exhibit lobate grain boundaries (Figure 13c), the more abundant neoblasts show granoblastic texture with triple joints (Figure 13d). The composition of the core of porphyroclastic Pl is the Na-richest (An content from 0.45 to 0.47; Figure 8b), whereas the porphyroclastic rims and the neoblasts are slightly Ca-richer (An content from 0.47-0.52; Figure 8b). Locally, the Pl contain, in their interior or at their grain-boundaries, irregularly shaped Qz blebs that exhibit uniform extinction (Figure 13e) and are closely associated with replacement antiperthitic Kfs (Figure 13f). The Pl composition around the blebs and replacement antiperthites is enriched in anorthitic components (An from 0.61 to 0.69; Figure 8b). The Opx constitutes two generations: the porphyroclastic one contains Mag exsolution lamellae (Figure 13a) that lack in the neoblastic one. Porphyroclastic Opx has slightly higher Ca content (ranging from 0.023–0.026 a.p.f.u.) than neoblasts (ranging from 0.018–0.022 a.p.f.u.) and similar Mg# (0.46 to 0.52; Figure 9c). The Al content ranges from 0.051 to 0.077 in a less coherent trend, but the highest values are preserved in the porphyroclastic cores (up to 0.077 a.p.f.u.; Figure 9d). The Amp is very rare, has grey-greenish color and forms around opaque minerals. Its composition, different to that of the mafic lithologies, is tschemarkitic and characterized by Mg# = 0.50-0.57, and (in a.p.f.u.) Ca = 1.82-1.89, Mn = 0.01-0.02, Al = 2.16-2.49, K = 0.31-0.39, Na = 0.31-0.42, Ti = 0.12-0.23, Cl = 0.01-0.02; F ≤ 0.09 (Figure 11). Most of the opaque minerals in enderbites are Ilm, in contrast with the mafic granulites.

4.2.3. Mineral Trace Element Composition in Mafic Lithologies

LA-ICP-MS trace element data were collected on clinopyroxene, amphibole, orthopyroxene and plagioclase from the most promising mafic lithologies, i.e., a foliated variety of the mafic granulite in the Ivrea town area (sample IZ21-20a), a typical, homogenous mafic granulite lacking foliation with universal distribution of the minerals (sample IZ21-18a) and a mafic granulite whose Amp-content is much higher than in the samples mentioned above, located in the proximity of a 4 centimeter-thick hornblenditic layer (sample IZ21-17). The trace element composition of each mineral within a sample and across different samples is strikingly homogenous (Figure 14).

Clinopyroxenes have a chondrite-normalized REE pattern slightly enriched and characterized by a convex-upward LREE distribution, a small negative Eu anomaly and an almost flat M-to-HREE distribution (Figure 14a). With respect to the primitive mantle, they are poor in HFSE and variably depleted in Rb, Ba, Pb and Sr, with marked negative anomalies in Pb, Sr and Ti (Figure 14b). Amphiboles have trace-element compositions pointing to chemical equilibrium with clinopyroxenes. They display REE patterns parallel to those of clinopyroxenes, although slightly enriched (Figure 14c) and enrichments LILE and HFSE, with slightly negative anomalies in Pb, Sr, Zr, Hf and without Ti anomaly (Figure 14d). The orthopyroxene trace-element contents testifying chemical equilibrium with clinopyroxenes and are characterized by slightly depletion in LREE and slightly enrichments in HREE (Figure 14e), and by depletion in whole trace elements with negative anomalies in Sr (Figure 14f). Also, the plagioclase trace-element contents point to chemical equilibrium with the other minerals, i.e., enrichments in LREE, depletion in HREE and HFSE, and positive Ba, Sr and Eu anomalies (Figure 14g-h).

4.2.4. Thermobarometry

We used Amp as geothermobarometer because it is a sensitive mineral to T and P changes. The recent Ti-in-Amp thermometer ([75]; standard error of the calculation,1σ, ±35°C and 2σ ±70°C [76]) was applied to the mafic granulites as well as pegmatitic mafic rocks and enderbites (Table S1). The calculated data indicate T > 900°C with a peak at 965°C for the mafic granulites, T > 900°C with a peak at 933°C for the pegmatitic mafic rocks, and T < 840°C with a peak at 810°C for the enderbites (Figure 15a). Note that the T variation between mafic granulites and pegmatitic rocks is within the standard error of the calculation. The Si vs Ti distribution for mafic granulites and pegmatites corresponds to that of UHT granulites, whereas that for enderbites is within the HT field (Figure 15b). Regarding the pressure, the Al^IV vs Al^VI distribution is compatible with medium-P granulites (Figure 15c) for all the studied lithologies.

5. Discussion

5.1. Comparison between the Ivrea Granulites and the Other Granulites of the Ivrea-Verbano Zone

Our study indicates that the area of Ivrea town consists of faintly foliated granulites still preserving intrusive field relationships (Figure 1). Most of the area is composed of mafic granulites, with a composition corresponding to Amp-gabbronorite, with local mineral banding parallel to the magmatic foliation and consisting of pyroxenitic layers. Field evidence between mafic granulites and enderbites reveals that the mafic rocks were intruded by enderbitic bodies and, subsequently, by Amp-gabbronoritic to hornblenditic dykes and, finally, by quartz-feldspathic dykes. All the lithologies (excluding quartz-feldspathic dykes) preserve two generations of plagioclase and pyroxenes. The first porphyroclastic generation represents igneous relics, while the second granoblastic generation likely represent the recrystallization occurred during the ‘slow cooling’ of the intrusion, due to a prolonged, stationary position at the base of the crust [79].

In the Ivrea-Verbano zone, mafic granulites occur in the Strona and Sessera Valleys. In the Strona Valley (Figure 1), the metabasites of the Kinzigite formation are interlayered with migmatitic metapelites, quartzites and marbles (Figure 1). They have been distinguished, based on their metamorphic grade which increases towards the Insubric Line, into a) amphibolites with hornblende, plagioclase and minor quartz and biotite; b) garnet-bearing metagabbros with plagioclase, garnet, amphibole and pyroxene and c) ‘pyriclastites’ (i.e., mafic granulites) with plagioclase, clinopyroxene, orthopyroxene, quartz, and biotite [55]. Middle-to-high grade metamorphic rocks exhibit migmatitic structures consisting of patchy leucosomes (2–5 centimeter in size) with quartz-feldspathic composition. In the highest-grade rocks close to the Insubric Line, the degree of partial melting is even higher, making any recognition of field relationships difficult. Partial melting structures in the granulite-facies migmatites of the Kinzigite formation are characterized by interconnected networks of veins or patches of leucosomes. They consist of plagioclase, quartz but also peritectic clinopyroxene [27].

The mafic granulites of the Sessera and Sesia Valleys belong to the Mafic Complex (Figure 1). Those belonging to the LMC range from Amp-gabbro to gabbronorite in composition and constitute an intermingling structure with leucocratic charnockites, enderbites, ultramafic rocks (pyroxenites and peridotites sequences similar to those of Rocca d’Argimonia; [35,37]) and to Amp-rich pods, and lenses (Figure 1). The lithologies of the LMC are characterized by the granoblastic recrystallization of the former magmatic mineral assemblage given to the slow cooling of the batholith (i.e., Mafic Complex; [57]). On the contrary, the UMC (Figure 1) consists of homogenous diorites, monzodiorites and gabbros with well-preserved magmatic structure, and occurs in the Sesia Valley [35].

If compared with the rocks from Strona, Sesia and Sessera valleys, the granulites of Ivrea town studied in the present work exhibit structures and compositions compatible to those of the LMC. However, they are lacking charnockites and cumulitic peridotites. The temperatures around 950°C are compatible with the temperatures calculated for the lower crust in Sesia and Strona valleys [57,76]. The LMC is presumed to have been generated by mantle-derived melts with MORB affinity, contaminated with melts generated by the partial melting of the hosting metapelites [33]. Enderbitic magma could represent a more evolved mantle-derived magma [33,35,36], possibly by reactive melt flow [3] or a higher degree of magma mixing or crustal assimilation [33,35,36]. The lack of K-feldspar and biotite in the studied enderbitic samples point to a lack, or a very low degree, of magma mixing or crustal assimilation. The large enderbitic body to the south of Ivrea town could be generated by accumulation of melt in low-strain sectors during the activity of high-temperature shear zones, as proposed for the Sesia Valley charnockites [34].

The NE-SW oriented stronalitic septa show both dimensions (tens or hundreds of meters) and mineral composition (presence of rare K-feldspar and biotite) compatible with the metapelitic bodies belonging to the PBB [33]. In the literature, they are interpreted as restitic roof rocks, showing variable degree of partial melting, incorporated into the MC during the multiple intrusion process coupled with roof rock melting (e.g., [63]). Their presence in the south-eastern part of the studied area could indicate the proximity of the not-outcropping UMC.

5.2. The Stealth Metasomatism of the Mafic Complex in Ivrea

Microstructures reported in mafic granulites reveal the presence of melt in the late stages of their evolution. In fact, the interstitial amphibole with cuspate grain-boundaries (Figure 7g: Figure 8c) and the film of interstitial plagioclase around opaques and pyroxenes (Figure 7c-e; Figure 8d) are microstructures indicative of a former presence of melt (i.e., melt pseudomorphs; [80,81,82]). However, instead to be evidence of in situ partial melting by amphibole breakdown, these melt pseudomorphs are indicative of infiltration of a melt in chemical disequilibrium with the host rock, as testified by the lack of amphibole relics and by the presence of relics of rounded pyroxenes and opaques partly replaced by amphibole with shape preferred orientation (Figure 7g,h and Figure 8c). The melt infiltration is also testified by local preservation of a modal increase of amphibole towards the Amp-rich elongated dykes, pods, or lenses, locally pegmatitic, (Figure 5), that reveals a genetic relationship between them and the melt pseudomorphs. The chemical disequilibrium between the infiltrating melt and the hosting rock is still preserved, as major element composition, in the higher An content in the interstitial plagioclase than in the older generations (Figure 8). Nevertheless, the scattered major element composition of clinopyroxene (Figure 10) and its strongly homogenous trace element content, that perfectly mimic the one of amphibole (Figure 14), indicate a strong chemical re-equilibration between the infiltrating melt and the host rock. All this evidence point to: i) a metasomatic origin of both the pargasitic amphibole and the An-rich plagioclase induced by chemical disequilibrium between a former “dry” gabbronorite, still rarely preserved (Figure 7a), and a more mafic hydrous-silicate melt; Figure 5); ii) a syn-tectonic metasomatism occurred at suprasolidus conditions; iii) a flux of the metasomatic melt along veins/dykes combined with percolation along mineral grain boundaries.

The high volume of Hbl-mafic granulites in the studied area (ca. 70 vol% of the mafic granulites) and their high modal amounts of Hbl (up to 45 vol%) indicate that this metasomatic process was pervasive and generated by impregnating large rock volumes. Because the magmatic hornblende is a common constituent of mafic plutons and its metasomatic origin can be recognized only by microstructural observations and trace element content, the related metasomatism can be named “stealth metasomatism”, in agreement with the proposed nomenclature used to characterize the types of mantle metasomatism [83].

The evidence of metasomatic increase of modal amount of hornblende in the lower crust up to the generation of hornblendites was already reported by Daczko et al. [1] and Meek et al. [4] and defined “reactive infiltration followed by melt flux through an armored zone” by Stuart et al. [2]. Following their proposed process, the mafic granulites from Ivrea should have experienced a syn-tectonic channeled high-melt flux, enhanced by the former “gabbro glacier”-derived anisotropy [54], that generates hornblendite in the conduits. Starting from the conduits, the melt was transported from the grain-scale to the meso-scale by porous flow in an interconnected network promoting melt migration. Because porous flow can occur in the lower crust only in metaigneous rocks and only if the lower crust is particularly hot [84], this process could have been active in the MC. Therefore, the porous flow of rising melt promoted the suprasolidus replacement, through grain-scale magmatic assimilation of Cpx, of the former dry gabbro-norite into hydrous Amp-gabbronorite. In this reconstruction, the pegmatitic bodies should represent the final crystallization products of the infiltrating metasomatic melt.

The metasomatic melt was likely hydrous basaltic in composition, as testified by the presence of interstitial Amp with Mg# 0.70 and plagioclase with labradoritic-bytownitic composition. The increased water content in the melt could derive from crystal fractionation (e.g., [85,86,87]) or from a primary feature (e.g., [88] and references therein). In fact, it is known that the solubility of water increases at depth and, at 1 GPa, the water concentration in a MORB melt could even reach ca. 15 wt%, depending on temperature (e.g., [89]). Whatever is the process, the presence of water in the basaltic metasomatic agent could have been the cause of the channeled flux. The decrease of melt density and viscosity and the increase of melt compressibility (e.g., [89]) could have triggered its rapid rising from the mantle source into the lower crust by buoyancy along high-permeability channels, likely former shear zones. This channeled melt flow allows the migration of large melt volume in small rock volume whose composition is completely modified, but without significant compositional effect outside the channel. The prolonged melt migration in the armored channel [1] allows porous flow percolation likely promoted by the high water content in the infiltrating melt [90] and by the increased connectivity induced by the magmatic assimilation of the minerals, e.g., Cpx, with consequent decrease of their dihedral angles [83]. This process, allowing percolation of a small melt volume in a large rock volume with pervasive melt-rock interaction, explains the widespread stealth metasomatism in the studied area.

5.3. Nature of the Metasomatic Melt

Evidence for multiple infiltration of mafic magmas occurs also in other sectors of the IVZ. In the central IVZ, hornblendite and Amp-rich (45-90 vol%) gabbro and gabbronorite veins or dykes are observed to crosscut peridotite-pyroxenite associations [28]. The amphibole shows Mg# from 0.66 (in Amp-gabbronorite) to 0.82 (in hornblendites), Al₂O₃ from 13.7 to 15.6 wt%, TiO₂ from 1.3 to 3.1 wt%, Na₂O from 1.9 to 2.8 wt%, low HFSE content and a convex-upward LREE distribution characterized by slightly enrichments and lack of negative Eu anomalies. In the northern IVZ, Amp metasomatism of gabbroic rocks by infiltration of external hydrous mafic melt was recently reported by Munnikhuis et al. [13]. The pargasitic amphibole has Mg# from 0.62 to 0.66, Al₂O₃ from 12.3 to 13.2 wt%, TiO₂ from 2.06 to 3.1 wt%, Na₂O from 1.9 to 2.2 wt%, low HFSE content and a usually convex-upward LREE distribution characterized LREE enrichments and small negative Eu anomalies. A detailed study on the dykes crosscutting the Finero peridotites ([14]; Figure 16) revealed the presence of three kinds of hornblenditic dykes, and associated gabbroic, dioritic, and anorthositic dykes: i) HFSE-poor hornblendites with pargasitic amphibole characterized by low #Mg (ca. 0.65) and SiO₂ (< ca. 43 wt%), high Al₂O₃ (> ca. 13 wt%) and TiO₂ (< ca. 2.0 wt%) and K₂O at ca. 0.5 wt%; ii) HFSE-rich hornblendites with pargasitic amphibole characterized by higher #Mg (ca. 0.80) and SiO₂ (up to ca. 46 wt%), lower Al₂O₃ (< ca. 13 wt%) and TiO₂ (ca. 2.5 wt%) and K₂O from ca. 0.4 to ca. 0.7 wt%; iii) hornblendites with intermediate composition. The HFSE-poor dykes are considered to have calc-alkaline orogenic affinity and, being derived from partial melting of metasomatized lithospheric mantle, to belong to the late events (Early Mesozoic) of the post-collisional Variscan magmatism [14]. On the contrary, the subsequent HFSE-rich dykes show anorogenic affinity and derived from asthenospheric or deep lithospheric mantle, representing the intraplate magmatism in the extensional setting at the beginning of the Alpine cycle. Overall, the major and trace element composition of the pargasitic amphiboles from Ivrea town area, both those from Amp-rich dykes/pods/lenses and those from hosting mafic granulites, are compatible with the amphiboles reported by Berno et al. [28] and by Munnikhuis et al. [13], and also, correspond to those of the HFSE-poor dykes described by Ogunyele et al. ([14]; Figure 17). Possible minor differences in major and trace elements contents are likely due to variations in the magma evolution and/or in the mixed crustal component. Thus, the comparison between the data from the Ivrea area and the other sectors of the IVZ indicates that the MC in the southern IVZ experienced a stealth metasomatic event induced by infiltration of post-collisional orogenic basaltic magmas.

5.4. Permian Hydration of the Southalpine Lower Crust and Implications for Alpine Magma-Poor Rifting

The field and chemical similarities regarding Amp-rich dykes and hosting Amp-rich mafic granulites among southern (this study), central (LMC; [28,32,36,37,63]) and northern IVZ (the Finero Complex, e.g., [12,13,14] and references therein), such as their large volumes deduced by geophysical data [7] point to a widespread Middle Triassic (?) rising of orogenic basaltic magmas. The rising magmas were responsible for the hydration of the whole lowermost part of the IVZ and likely composed of more depleted gabbronorite (Figure 41 of [68] and references therein), at least in the southern and central IVZ. The transfer of magmatic heat allowed the infiltrated crust to reach suprasolidus conditions (from supersolidus rocks to melt-poor mush, as described by Sparks et al. [91], and to exhibit plastic behavior, also testified by the irregular or gradual contacts of the studied mafic dykes with the hosting rock (Figure 5). On the contrary, the older enderbitic dykes, displaying sharp contacts (Figure 4c,d), indicate that the related colder melt (Figure 14), likely originated by chemical differentiation [3], was injected in a lower crust still characterized by a rigid behavior. Thus, our study suggests a rapid change in the rheological behavior of the lower crust, from rigid to plastic, due to its metasomatic hydration.

Considering that the infiltration of the metasomatic hydrous basaltic melts was the last event occurring during the Permian collapse of the Variscan orogen [10,14], the abrupt variation of the lower crust mechanical properties could have had a role that needs extensive investigation. This could have favored the Triassic thinning of the Variscan crust, the location of the necking domains (e.g., [9] and references therein), and ultimately, the beginning of the Alpine cycle.

At present, the numerical modellization of a magma-poor rifting, like the one responsible for the aperture of the Alpine Tethys (e.g., [9] and references therein), consider a Permian lithospheric section characterized by a strong lower crust made by dry mafic and felsic granulites (e.g., [11,92,93]). Nevertheless, some seismic profiles along rifted margins reveal indication of ductile deformation of the lower continental crust along shear zones (e.g., [94]).

In the present study, we demonstrate that, at the end of the Permian, the continuous infiltration of metasomatic melts within dry mafic plutons at ca. 25-30 km depth, coupled with its suprasolidus migration by porous flow and its crystallization [95] at ca. 950°C, generates large volumes of hydrous lower crust at the crust-mantle boundary and promotes the rising of the isotherms of the middle crust, at least above the largest mafic bodies (Figure 18). Both of them could represent the necessary conditions for a more intense deformation during the stretching and necking phases of the rifting.

6. Conclusions

The present work describes the granulites occurring in the southern IVZ (Ivrea town area) and the evidence of melt-rock interactions occurring in the mafic lithologies. The main results are:

• The granulites of the southern IVZ show many similarities with those belonging to the LMC of the central IVZ (Sessera Valley), and are supposed to be genetically similar to them, although with a possible lower degree of crustal contamination. The presence of stronalitic septa could indicate the proximity of a not-outcropping UMC to the south-east.
• The interstitial pargasitic amphibole and An-rich plagioclase in the mafic granulites have a metasomatic origin (stealth metasomatism) and were derived by the suprasolidus interactions between the former dry gabbro-norite with a more mafic hydrous-silicate melt infiltrated along channel (now hornblenditic dykes) and then percolated along grain boundaries.
• The metasomatic melt likely corresponds to the Early-Mesozoic post-collisional orogenic basaltic magmas, whose infiltration is well documented in the northern IVZ (Finero area).
• The metasomatic event is responsible for the hydration of large volumes of the thick hot lower crust, and for the changes in rheology of this crustal level.