Chemical and minero-petrographical changes on granulite rocks affected by weathering processes

Abstract

The purpose of this work is to study the weathering processes of the granulite rocks of the Serre Massif (southern Calabria, Italy) using a multidisciplinary approach based on field studies, geochemical modeling, and minero-petrographical analyses. The granulite rocks are plagioclase-rich with minor amphibole, clinopyroxene, orthopyroxene, biotite, and garnet and their texture are coarse-grained. The reaction path modeling was performed to simulate the evolution of groundwaters upon interaction with local granulite by means of the software package EQ3/6, version 8.0a. Simulations were performed in kinetic (time) mode under a closed system at a constant temperature of 11.5°C, (which reproduces the average temperature of local area) and fixing the fugacity of CO₂ at 10^−2.34 bar (mean value). During the most advanced stage of weathering the main mineralogical changes are: partial destruction and transformation of biotite and plagioclase associated with neoformation of ferruginous products and secondary clay minerals producing a change in the origin rock fabric. The secondary solid phases observed during the geochemical modeling (kaolinite, vermiculite and ferrihydrite) are similar to those found in this natural system. Thus, the soil-like material mainly characterized by mostly sand to gravel grain-size fractions is the final result of the weathering processes.

Keywords

Serre Massif / granulitic rocks / mineralogy / petrography / weathering profile / reaction path-modeling

Introduction

Granulite facies rocks of the lower crust are well exposed in the southern sector of the Calabrian Terranes and have been referred to the Sila Unit (Monte Gariglione-Polia-Copanello Complex; ^{Bonardi et al., 2001}). These rocks were affected by important weathering processes during their evolution. Chemical weathering is one of the main earth surface processes that through rock weathering consumes CO₂, mainly from atmospheric/soil origin and produces aqueous HCO₃⁻ and CO₃²⁻, then transported into the sea by rivers (^{Sun et al., 2010}). This process shows the links between geologic cycling of solid earth, atmosphere, and the ocean.

Several chemical and minero-petrographical changes affect both rock materials and masses due to physical disintegration and chemical and mineralogical alteration which determine a decay of the physical-mechanical properties (e.g., ^{Le Pera and Sorriso-Valvo, 2000a}, ^b; ^{Le Pera et al., 2001a}, ^b; ^{Apollaro et al., 2007a}, ^b, ²⁰¹², ^2013a, ^b, ²⁰¹⁵, ²⁰¹⁶; ^{Bloise et al., 2009}; ^{Buccianti et al., 2009}; ^{Perri et al., 2012}, ²⁰¹³, ²⁰¹⁴; ^{Guagliardi et al., 2013a}, ^b; ^{Perri and Ohta, 2014}; ^{Vespasiano et al., 2014}, ²⁰¹⁵; ^{Critelli et al., 2015}; and many others). The incongruent dissolution is a process affecting several minerals (e.g., Al-silicates) and it is associated with the precipitation of secondary sparingly soluble solid phases, such as clay minerals and oxy-hydroxides (^{Apollaro et al., 2007b}, ^2013a; ^{Scarciglia et al., 2008}; ^{Perri et al., 2015}).

The study area (Serre Massif) and the Sila Massif (Calabria, southern Italy) are characterized by instability phenomena involving weathered crystalline rocks partly due to the recent strong regional uplift started in the Lower-Middle Pleistocene in an environment characterized by a Mediterranean climate (e.g., ^{Ghisetti, 1980}, ¹⁹⁸¹; ^{Westaway, 1993}; ^{Le Pera and Sorriso Valvo, 2000b}; ^{Ietto and Bernasconi, 2005}; ^{Critelli et al., 2013}, ²⁰¹⁷). In particular, the tectonic evolution of the Serre Massif mainly consists of normal fault systems that are NE-SW and NW-SE directed (^{Ietto et al., 2016}). Thus, the tectonic uplift combined with intense weathering processes produces in the study area a wide variety of mass movement mostly during intense rainfall events (e.g., ^{Calcaterra and Parise, 2005}; ^{Ietto et al., 2007}, ²⁰¹⁵, ²⁰¹⁶).

The weathering processes related to the granitoid rocks in high and inland areas of the Calabrian Terranes (e.g., the Sila Massif at north and the Serre Massif at south) was investigated in previous works (e.g., ^{Le Pera and Sorriso-Valvo, 2000a}, ^b; ^{Borrelli et al., 2012}, ²⁰¹⁴, ²⁰¹⁵, ²⁰¹⁶ and many others), but this is the first study on the granulite rocks of the Serra Massif and one of the few cases studied in the world (^{Rajamani et al., 2009}).

The purpose of this work is to study and characterize the mechanisms or processes that control the weathering of these rocks. In this paper we discuss for the first time the weathering processes occurring on the granulite rocks of the Serra Massif through a multidisciplinary approach.

Field studies, geochemical modeling, minero-petrographical and chemical analyses were used in order to describe the weathering profiles of the granulite rocks.

The weathering grades of the granulite rocks were identified through visual descriptions based on different index tests (e.g., change of original colours, samples broken by hands and hammer, Schmidt Hammer tests). Data obtained from mineralogical, petrographical, and chemical analyses were cross-referenced with field descriptions and weathering characteristics and used to determine the changes in each grade of weathering on the granulite rocks. The results obtained in this study can be used to investigate other sites characterized by granulite rocks.

Geological setting

The Calabrian Terranes belong to the Internal Domain of the Alpine circum-Mediterranean chains and it is the results of post-30 Ma geodynamic processes occurring in the Mediterranean area (e.g., ^{Critelli et al., 2008}, ²⁰¹⁷). The Calabrian Terranes represents the maximum deformation of the Apennine–Maghrebian chain due to the convergence of the European and the African plates and are composed of crystalline basement nappes, some of them covered by a Meso-Cenozoic sedimentary cover (e.g., ^{Bonardi et al., 2001}; ^{Critelli et al., 2017}). The tectonic and geodynamic evolution of the Calabrian Terranes is mainly characterized by a trend with prevalently compressional tectonic movements (e.g., ^{Critelli et al., 2013}, ²⁰¹⁷; ^{Tripodi et al., 2013}, ²⁰¹⁸). The northern sector of the Calabrian Terranes is composed of Coastal Chain and Sila Massif whereas the southern sector is composed of Serre and Aspromonte Massifs. In particular, the Serre Massif lies among the Catanzaro Trough to the north, the Mesima Valley to the west, and the Ionian coast to the east (Fig. 1), and it constitutes the linkage between the southern (Aspromonte Massif and Peloritani Mountains) and the northern (Sila Massif and Coastal Chain) sectors of the Calabrian Terranes (e.g., ^{Ietto et al., 2016}). The Serre Massif is mainly constituted of Late Hercynian granitoids and granodiorites that are discontinuously surrounded by high-grade metamorphic rocks and locally covered by unmetamorphosed Cenozoic sedimentary deposits (e.g., ^{Ietto et al., 2016}).

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The Serre Massif (Fig. 1) is composed by a nearly tilted complete continental crustal section, which is made up of lower and upper crustal metamorphic rocks, sutured by late-Hercynian plutonic bodies (^{Schenk, 1980}, ¹⁹⁹⁰; ^{Langone et al., 2006}; ^{Critelli et al., 2017}). A continuous section of the lower continental crust about 7 km thick outcrops in the northern part of the Serre Massif. It consists of granulite-facies rocks (400 km²) belonging to the Monte Gariglione-Polia-Copanello Complex of the Sila Unit (e.g., ^{Bonardi et al., 2001}). ^{Schenk (1984}, ¹⁹⁹⁰) considered the granulitic unit to represent a pre-Hercynian lower crust portion exhumed, during the Hercynian orogeny, to middle crust level, where it cooled down more or less isobarically during the Mesozoic, and was finally uplifted to the surface during the Cenozoic. Alpine and pre-Alpine metamorphic effects characterize the Monte Gariglione-Polia-Copanello Complex. In particular, mineral dates (U-Pb and Rb-Sr) and petrological research (^{Schenk, 1980}, ¹⁹⁸⁴) suggest a model with two granulite-facies events during the pre-Alpine evolution (e.g., ^{Maccarrone et al., 1983}). The lowermost rocks are characterized by T ~ 800°C and P ~ 8 kbar estimated for the peak of the granulite facies metamorphism (e.g., ^{Schenk, 1980}). The granulite-facies rocks are subdivided into two lithostratigraphic units: a lower (granulite-pyriclasite) unit and an upper (metapelite) unit (^{Schenk, 1984}). The lower unit mainly consists of metabasic rocks, predominantly layered metagabbro and meta-anorthosite with subordinate and locally distributed small ultramafic bodies. The upper unit consists of migmatitic aluminous paragneiss, orthopyroxene-bearing felsic granulite, and metacarbonate rocks that sometimes include quartz monzo-gabbronorite sills and/or dikes. An Alpine fault, the Curinga-Girifalco line, separates the granulite-facies rocks from phyllonitic rocks (^{Schenk, 1984}).

Materials and methods

Field observations

Weathered bedrock of granulite of the study area was divided into different and progressive weathering stages (^{Irfan and Dearman, 1978}; ^{Gullà and Matano, 1997}), based on field observation. The evaluation of the weathering stages, its characteristics and thickness of the altered rocks along several cut slopes (Figs. 2 and 3) were observed during field survey.

In the studied area, five classes of weathering (from the class II–III to the class VI) have been recognized. Corestones of slightly and moderately weathered rocks (classes II–III) and corestones of moderately and highly weathered rocks (classes III–IV) outcrop along limited portions of the studied cut slopes. Highly weathered and completely weathered granulite rocks, located on the medium-high portions of the slopes (classes IV–V), are covered by colluvial and detritical–colluvial soils with variable volumes of completely weathered rocks (classes V–VI) (Figs. 2 and 3). The total weathered rocks and the thin layers of colluvial and detritical detritical–colluvial (class V–VI) characterizing the highest reliefs of the studied cut slopes are mainly related to very active morphodynamic processes (e.g., ^{Borrelli et al., 2014}). Three main weathering classes that characterize the studied area and selected for this study are: slightly and moderately weathered rocks (classes II–III); highly weathered and completely weathered granulite rocks (classes IV–V); completely weathered rocks with colluvial and detritical–colluvial soils (classes V–VI).

The typical weathering profiles of the sampled granulite rocks show complex and irregular transition among the weathering classes. The weathered horizons of weathering profiles studied are characterized by irregularities in the spatial distribution and by faults and fractured zones with sharp contacts between the different weathering classes, thus resulting in a complex profile (sensu ^{Baynes et al., 1978}). The development of the weathering profiles (Figs. 2 and 3), characterized by important fractures and fault zones, has occurred between the Late Miocene and Pleistocene (e.g., ^{Borrelli et al., 2014}). The current temperate and Pleistocene glacial climatic conditions combined with the tectonic uplift of the Serre Massif, induced erosional processes that partially removed the exposed deeply weathered horizons.

Rocks: sampling and analytical methods

The weathering profiles have been studied following the methodology proposed by ^{Gullà and Matano (1997)}, also used to classify the observed weathering grades of the weathered samples collected on each cut slope. Eleven representative samples (3 samples of the classes II–III, 3 samples of the classes IV–V, and 5 samples of the classes V–VI) were selected and analysed using an optical microscopy of thin sections and Electron Probe Micro Analyzer (EPMA – JEOL- JXA 8230 equipped with Spectrometer EDS – JEOL EX-94310FaL1Q- Silicon drift type) for the petrographic evaluations and a Bruker D8 Advance diffractometer (step size of 0.02° 2q with 1 s for step) for the mineralogical evaluations. In particular, the X-ray Diffraction (XRD) analyses have been carried out on untreated specimens for the bulk mineralogical composition and on clay suspensions, separated from the bulk sample by centrifugation, exposed to thermo-chemical treatments for the clay-mineral identification and features. The differences among the<2 µm XRD patterns of the air dried samples, and samples exposed to thermal treatment by heating at 375°C and chemical treatment by glycolation with ethylene glycol at 60°C overnight, allow us to calculate the relative percentages of clay minerals in the<2 µm fraction and their main features. The methodology used for these X-ray diffraction analyses is according to ^{Moore and Reynolds (1997)}.

The studied samples were also chemically analysed by X-ray fluorescence spectrometry (XRF) using a Bruker S8 Tiger equipment. Total loss on ignition (LOI) was determined after heating the samples for 3 h at 900°C. The certified international reference materials AGV-1, BCR-1, BR, DR-N, GA, GSP-1, and NIM-G were used as monitors of data quality. The chemical compositions of the studied samples were used to calculate the chemical index of alteration (CIA; Nesbitt and Young, 1982), the chemical index of weathering (CIW; ^{Harnois, 1988}) and the Plagioclase Index of Alteration (PIA; ^{Fedo et al., 1995}) and, thus, to evaluate the increase of the weathering processes in the studied samples.

Waters: sampling and analytical methods

Five samples of groundwater, taken near the alteration profiles, and some samples of rain water were collected in the study area. Temperature, pH, Eh, and electrical conductivity (EC) were measured in the field by means of portable instruments. Two pH buffers, with nominal pH values of 4.01 and 7.01, at 25°C, were used for pH calibration at each sampling site. Total alkalinity was also determined in the field by acidimetric titration using HCl 0.01 N as titrating agent and methyl orange as indicator. Waters were filtered in situ through a 0.4 µm poresize polycarbonate membrane filter (Nuclepore). Samples for the determination of anions were stored with- out further treatment. Samples for the determination of cations were acidified by addition of suprapure acid (1% HNO₃) after filtration, and stored. New polyethylene bottles were used for all the samples. In the laboratory, the concentrations of Na⁺, K⁺, Mg²⁺, Ca²⁺, F⁻, Cl⁻, SO₄²⁻, and NO₃⁻ were determined by high performance liquid chromatography (HPLC, Dionex DX 1100). All the chemical data were determined in the laboratory of the Department of Biology, Ecology and Earth Sciences of the University of Calabria and are reported in Table 1.

Geochemical modeling

The fate of the chemical constituents during weathering is a complex topic that can be elucidated by means of the geochemical modeling, or more precisely, by the reaction path modeling of water-rock interaction. Reaction path modeling is a powerful geochemical tool, formulated in the late 1960s by ^{Helgeson (1968}, ¹⁹⁷⁹), who developed the equations required to describe the irreversible mass exchanges between the aqueous solution, a gas phase (if any), and one or more solid phases. Reaction path modeling can be applied to reproduce the progressive dissolution of one or more mineral phases in an aqueous solution, the mixing of two solutions and the heating or cooling of an aqueous solution. In this study, the most recent release of the software package EQ3/6, version 8.0a (^{Wolery and Jarek, 2003}; ^{Wolery and Jove-Colon, 2007}), was used to simulate the progressive dissolution of a granulite rock. In order to constrain the modeling exercise, it was necessary to provide some input parameters such as the composition and abundance of each mineral of interest, the initial composition of the aqueous solution (before the beginning water-rock interaction), relevant thermodynamic and kinetic data, etc.

Simulations were performed in kinetic (time) mode under closed system (^{Apollaro et al., 2009}, ²⁰¹¹). The solid reactants taken into account (starting granulite) are: plagioclase rich (60%) with minor amphibole (10%), clinopyroxene (8%), orthopyroxene (4%), biotite (10%) and garnet (8%). The average concentration of some rainwater sampling in the study area was used as a starting solution for modeling (Table 2).

Thermodynamic data of some minerals such as anorthite, K-feldspar, albite, annite, phlogopite, muscovite, 1.4 nm clinochlore, magnesite, calcite, rhodochrosite, siderite, witherite, strontianite, and aragonite were evaluated by review work of ^{Helgeson et al. (1978)}. Thermodynamic data of clay minerals (Mg, Na, K, and Ca endmembers of beidellite, saponite and montmorillonite), 1.4 nm chamosite and celadonites were calculated by ^{Wolery and Jove-Colon (2007)} and references therein. Those of vermiculites such as Me–Al vermiculites, Me–Fe vermiculites, Me–Mg–Al vermiculites, Me–Mg–Fe vermiculites with Me= Na, K, Mg and Ca were evaluated by ^{Apollaro et al. (2013a}, ^b). From Perri et al. (2015) were obtained the thermodynamic data of illite and those of ferrihydrites are from the work of ^{Majzlan et al. (2004)}.

Results

Petrographic analyses

Several types of crystalline basement rocks are present in the studied area. In this work we only focused on granulite-bearing plagioclase source rock lithotypes (e.g., ^{Rizzo et al., 2004}, ²⁰⁰⁵) because of their larger areal distribution within the study area and in order to understand the degree or extent to which these mafic rocks have been chemically weathered. The assessment of rock weathering efficiency has been done through petrographic analysis.

Slightly weathered granulite is plagioclase rich (60%) with minor amphibole (10%), clinopyroxene (8%), orthopyroxene (4%), biotite (10%), and garnet (8%) and its texture is coarse-grained.

Petrologic analysis of the weathered bedrock was done to ascertain weathering efficiency on primary minerals which could be replaced by neogenic products (e.g., ^{Perri et al., 2016}; ^{Barros dos Santos et al., 2017}). The sericitization of plagioclase, preferentially, along cleavage traces and twin planes, the partial chloritization of micas (biotite) along rims and lamellae and their transitions into Fe-Oxides, the neoformation of clay minerals were the main processes observed as results of the parent rocks alteration. Microcracks in quartz crystals are mainly related to the physical weathering processes.

The observed changes are consistent and predictable and have led to a mineralogical differentiation of the samples set with progressive increase (from II–III to V–VI stage) degree of weathering.

Specifically, an increase of clay minerals associated at decrease of primary mineral, such as feldspar, an increase of microcracks and voids characterize the transition from weathering stage II–III to weathering stage V–VI. Petrographic analysis was carried out with a polarizing microscope from the thin sections of the rock samples on the samples from the least altered sample to the sample most affected by the weathering process (Fig. 4).

The transition from slightly weathered protolith (Figs. 4(a)–4(b)) to moderately weathered sample (Fig. 4(c)) to highly weathered samples (Figs. 4(d)–4(f)) is marked by both chemical and granular disintegration.

Weathering class stage II–III (GR1A, GR1B, and GR2A) – The alteration consists mainly in the oxidation process of garnet, preferentially along clevage traces. Oxidation process occurs along isomineralic interfaces (e.g., ^{Heins, 1995}; ^{Caracciolo et al., 2012}) developed along crystal boundaries between garnets and along non-isomineralic interfaces (e.g., along a crystal boundary microcline/garnet or plagioclase/garnet). Besides both core and rim of garnet crystals are affected by neoformation of clay minerals unidentified in thin sections under a polarizing microscope.

Weathering class stage IV–V (GR3A, GR3B, and GR3C) – The most pronounced alterations consist in the sericitization of plagioclase mainly within the core of the crystal (Fig. 4(c)) and also along cleavage traces and twin planes of the crystals, and in the neoformation of unidentified clay minerals replacing plagioclase and in the chloritization of biotite along lamellae.

Intra-, inter- and trans-crystalline microcracks characterize the more advanced stage of rock weathering (class stage V–VI of GR4A, GRB4B, GR5A, GRA5B, and GR5C samples). These can be void, suggesting a process of leaching and dissolution (e.g., ^{Velbel, 1999}; ^{Price and Velbel, 2003}; ^{Perri et al., 2016}, ^{Barros dos Santos et al., 2017}), or filled by oxidation products, increasing crystal microfracturing. Furthermore, ferruginous weathering products appear to have lined fractures within and around plagioclase crystals (Figs. 4(d)–4(e)). In some cases, the ferruginous alteration products appear to have originated, preferentially, along cleavage planes and then grew pervasively. The green amphibole shows linings of microcracks within and around the crystal and crystal fragments, formed by ferruginous weathering products (Fig. 4(f)). This happened during the last stage of the alteration process. Side-by-side coalescence of lenticular etch pits, moreover, produced also characteristic “denticulated” or “sawtooth” terminations (Fig. 4(f)) indicative of chemical corrosion (e.g., ^{Velbel, 1989}; ^{Andò et al., 2012}). Oxidation process propagates along mineral interfaces both of iso-mineralic type (e.g., amphibole/amphibole contact) and of non-isomineralic type (e.g., plagioclase/amphibole contact; microcline/amphibole contact).

Electron Probe Micro Analyzer images (Fig. 5) show the main chemical weathering processes and neoformed phases of the highly and completely weathered/residual and colluvial soil samples (classes IV–V and V–VI). Weathering processes act mainly along non-isomineralic interfaces such as between green amphibole and plagioclase with dissolution features and neoformation of clay minerals along cleavage planes of green amphibole and plagioclase (Fig. 5(a)). Neoformation of clay minerals, interpreted as kaolinite, and expandable clays (vermiculite) fill inter- intra- and trans-granular microcracks (Figs. 5(b)–5(c)). Coatings of Fe-oxides/hydroxides and clay minerals replace unidentified precursor minerals (Fig. 5(d)).

Mineralogical and chemical analyses

Differences among the weathering classes can be evaluated by mineralogical analyses on the clay fraction (e.g., ^{Perri et al., 2012}, ²⁰¹⁵, ²⁰¹⁶; ^{Borrelli et al., 2014}; ^{Scarciglia et al., 2016}; ^{Barros dos Santos et al., 2017}). In particular, the clay fraction of the highly and completely weathered samples analyses by X-ray Diffraction (XRD) shows important information about the neoformed clay related to the weathering processes. The mineralogical composition of the bulk fraction of the studied samples is shown in Table 3. The XRD pattern of the bulk fraction of the studied weathered granulite rocks are main composed of plagioclase and amphibole (Fig. 6). Micas, chlorite, pyroxenes and Fe-oxides/hydroxides are present in minor amounts, whereas garnet and quartz are present in traces (Fig. 6). In particular, the XRD patterns of the bulk fraction (Fig. 6) show an increase of clay minerals and Fe-oxides/hydroxides and a decrease of plagioclases from the slightly and moderately weathered sample (class II–III; GR1B) to the completely weathered/residual and colluvial soil samples (class V–VI; GR5B). The difference of clay mineral amounts is better identified in the XRD pattern of the clay fraction. The XRD patterns of the<2 mm fraction of the completely weathered/residual and colluvial soil (class V–VI) samples (GR4A and GR5A) show high value of expandable clay minerals 2:1 (smectite and/or vermiculite types); 1:1 and 2:1:1 phases (such as kaolinite and/or chlorite) cannot be excluded (Fig. 7).

The chemical variations among the studied samples show an increase (e.g., for Al₂O₃ from 13.13 of the GR1A to 21.48 of the GR5B) or a decrease (e.g., SiO₂ from 58.62 of the GR1A to 39.12 of the GR4A) of certain chemical species due to the weathering processes through the different weathering classes (Table 4). A similar increase is further shown by the CIA (from 49.96 of the GR1A to 60.81 of the GR5C), CIW (from 53.78 of the GR1A to 62.58 of the GR5C) and PIA (from 49.95 of the GR1A to 61.46 of the GR5C) indices (Table 4).

Water chemistry

Results of field measures and chemical analyses reported in Table 1 show that the temperature of collected water samples varies from 13.1°C to 15.2°C around an average of 14.2°C. Their pH ranges from 6.3 to 7.3 with a mean of 6.8 pH-units. Eh values are typical of shallow, oxidizing or relatively oxidizing environments close to the earth surface. Based on the results of Apollaro et al. (2013), the local groundwater samples are plotted in the activity diagram for the system K₂O–MgO–Al₂O₃–SiO₂–H₂O (Fig. 8(a)). Water samples distribute partly in the stability field of kaolinite and partly in that of Mg–Al–vermiculite. Measured pH and Eh values of sampled groundwaters distribute in the field of ferrihydrite (Fig. 8(b)).

Reaction path modeling

The simulations were performed at the average temperature of the local area (11.5°C; data from Arpacal-CFM, fixing the fugacity of CO₂ at 10^−2.34 bar (mean value). The simulations show a progressive dissolution dominated by plagioclase followed by a minor amounts of amphibole, clinopyroxene, and biotite and negligible amounts of orthopyroxene and garnet (Fig. 9).

The differences in the type and amount, along the reaction path of the secondary minerals (Fig. 10), mainly reflect the different dissolution of primary minerals and, therefore, a different contribution during the reaction of chemical elements.

Precipitation of kaolinite occurs at the beginning of the simulation and it has ephemeral existence, since it is quickly substituted by the solid solution of vermiculite. Finally, the solid solution of hydroxides precipitates and persists throughout the run.

Discussion

The weathering process developing from poorly to intensely weathered rock samples is mainly shown by progressive destruction of plagioclase and exfoliation and oxidation of micas (especially biotite) coupled with an increase of clay neoformed minerals, and a consequent generation of complete or partial filling along cleavage planes. The process generates the formation of randomly distributed microvoids and microcraks that propagate into surrounding less weathered mineral grains (e.g., ^{Scarciglia et al., 2007}; ^{Barros dos Santos et al., 2017}); thus, the migration of the Fe-oxyhydroxide (and neoformed clays) is promoted at the beginning weathering process by circulating waters (e.g., ^{Scarciglia et al., 2016}). The neoformed clay minerals identified on weathered primary minerals of the intensely weathered rock samples are similar to those observated in Sila Massif and Capo Vaticano area (e.g., ^{Scarciglia et al., 2005a}, ^b, ²⁰⁰⁷; ^{Perri et al., 2015}, ²⁰¹⁶) and consist mainly in vermiculite and kaolinite components (2:1 and 1:1 clays).

The chemical indices of alteration (CIA, CIW, and PIA) record an incipient to moderate chemical weathering stage. The overall decrease in SiO₂ with the increases of the weathering grade is associated to an increase of Al₂O₃. The progressive dissolution of plagioclase, amphibole, and micas promote desilication, whereas the presence of neoformed clay minerals promote the increase of Al₂O₃ (e.g., ^{Scarciglia et al., 2016}). The positive relation among the chemical indices of alteration (CIA, CIW, and PIA) and the Al₂O₃/SiO₂ ratio support this interpretation (Fig. 11). The increase of the weathering process among the studied samples is also shown by the analogous distribution of the Fe₂O₃/TiO₂ ratios and LOI values (Fig. 12). The increases of Fe₂O₃/TiO₂ ratios and LOI values according to the weathering grade classes are mainly related to the precipitation of Fe-oxy-hydroxides and neoformed clay minerals, respectively.

Geochemical modeling showed that the progressive dissolution of the granulite rock is characterized mainly by plagioclase dissolution, followed by a minor amounts of amphibole, clinopyroxene and biotite, and negligible amounts of orthopyroxene and garnet (Fig. 9).

Result of geochemical modeling, furthermore, provide a good indications on the sequence of precipitating clay minerals during weathering of these rocks, namely kaolinite, vermiculite, and ferrihydrite (Fig. 10).

Concluding remarks

The studied weathering profiles of the granulite rocks show complex and irregular transition among the weathering classes. Chemical analyses coupled to the minero-petrographic studies are used to highlight the variations and the principal features of each weathering class. The transitions among the various weathering stages are thus well documented by thin-section investigations combined to the XRD profile evaluations. The crystallographic and chemical affinities are directly linked to the durability of mineral interfaces and associated textural relationships, and the greatest effects of weathering occur along non-isomineralic interfaces. Weathering efficiency acts and propagates along both iso- and non-isomineralic interfaces creating favorable conditions for disintegration and alteration of this bedrock lithotype.

Chemical weathering of the studied granulite rocks seems to be relatively close to an iso-chemical transformation, as showed by the variations from the initial to the final state in the concentrations of solutes within the aqueous solution involved in geochemical modeling. The CO₂ driven reactions of interest are dissolution of plagioclase accompanied by minor amounts of amphibole, clinopyroxene and biotite, and negligible amounts of orthopyroxene and garnet. The secondary product minerals are kaolinite, vermiculite, and hydroxides. In particular, the secondary clay minerals expected by the reaction path modeling approach correspond well to those identified by XRD and Electron Probe Micro Analyzer studies.

The multidisciplinary approach used in this work is positively applied for the first time on mafic rocks and thus plays a key role for the evaluation of the weathering processes and the derivative products occurring along weathering profiles. This type of investigation and the obtained results can be transferred to other sites where granulite rocks occur.

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