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Journal of Petrology Volume 42 Number 5 Pages 931-961 2001
© Oxford University Press 2001
An Exposed Hercynian Deep Crustal Section in the Sila Massif of Northern Calabria: Mineral Chemistry, Petrology and a P–T Path of Granulite-facies Metapelitic Migmatites and Metabasites
INSTITUT FÜR GEOWISSENSCHAFTEN DER CHRISTIAN-ALBRECHTS-UNIVERSITÄT ZU KIEL, 24098 KIEL, GERMANY
Received August 20, 1999; Revised typescript accepted August 15, 2000
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONIn the Sila massif of northern Calabria a continuous section is exposed through a segment of a Hercynian deep continental crust, which has been interpreted by previous workers as a stack of basement nappes (‘Monte Gariglione Complex’). The section consists essentially of metapelitic migmatites and subordinate metabasites and marbles, which were metamorphosed at medium-pressure–high-temperature granulite-facies conditions. A continuous metamorphic gradient through the exposed segment can be deduced from the systematic change in the compositions of ferromagnesian minerals in divariant metapelitic assemblages. This gradient is partly supported by conventional geothermobarometry and by applying the TWEEQU method. However, peak-metamorphic conditions are better defined by dehydration melting reactions, which reveal
4 kbar and 740°C at the top and 6 kbar and 770°C at the base of the section. Therefore the exposed rocks represent a former crustal depth range of 14–21 km at the thermal peak of metamorphism. The metamorphic evolution of the former deep crustal rocks has been reconstructed from reaction textures. The prograde P–T path took place in the stability field of sillimanite. The retrograde path is characterized by a stage of isothermal uplift after peak metamorphism to mid-crustal levels (10–15 km) followed by near-isobaric cooling to greenschist-facies conditions. The deep crustal section in northern Calabria resembles that of the Serre massif in southern Calabria, which represents the lowermost part of an exposed tilted cross-section through the Hercynian continental crust. Postkinematic mineral growth, reaction textures and isotopic age constraints indicate that the thermal conditions in the Calabrian crust during the Hercynian orogeny were mainly controlled by advective heat input through magmatic intrusions. As in southern Calabria, large granitic bodies were emplaced between the granulite-facies lower crust (‘Monte Gariglione Complex’) and the amphibolite- to greenschist-facies upper crust during regional metamorphism. KEY WORDS: Northern Calabria; lower-crustal cross-section; Hercynian medium-pressure–high-temperature metamorphism; migmatitic granulites
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INTRODUCTION |
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Exposed cross-sections through the continental crust offer the possibility to study the effect of thermal and tectonic events during orogenic cycles on a variety of lithologies at different crustal levels. A deep crustal level is exposed in the Sila massif of northern Calabria. There it forms a large unit (
900 km2) of granulite-facies, dominantly metapelitic migmatites, which was metamorphosed during the Hercynian orogeny. This unit (the ‘Monte Gariglione Complex’) represents the lower part of the Alpine ‘Sila nappe’, the core of which consists of a suite of late Hercynian granitoids overlain by upper-crustal rocks (Dubois, 1970
, 1976). The simple lithostratigraphic model has been modified by Zanettin Lorenzoni (1980) and Lorenzoni & Zanettin Lorenzoni (1983), who proposed a Hercynian nappe boundary within the Monte Gariglione Complex. The granulite-facies metapelites of the Sila massif resemble those of the southern Calabrian (Serre) massif, where they form the lowermost part of an exposed continuous profile through the Hercynian continental crust (Schenk, 1984, 1990). Until now, modern petrological methods have not been applied to the high-grade gneisses of northern Calabria. Therefore, the principal aim of this study is to check whether mineral chemistry and phase relations of metapelites point to a continuous metamorphic gradient through the Monte Gariglione Complex or to the existence of nappe slices of different metamorphic grade.
The second aim of this paper is to evaluate the thermal and baric conditions of the granulite-facies segment so as to reconstruct its former structural level within the continental crust. Furthermore, the P–T paths of these rocks are deduced to gain information on the geodynamic causes of metamorphism and to compare it with that of the lower crust exposed in southern Calabria.
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GEOLOGICAL SETTING OF NORTHERN CALABRIA |
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The Calabrian massif is situated in the southern part of the Appennine mountain system and consists of pre-Alpine crust that was involved in the Alpine orogeny. The Alpine mountain system became dismembered as a result of later movements of microplates in the western Mediterranean (e.g. Alvarez, 1976
). Some workers (Amodio-Morelli et al., 1976; Scandone, 1979; Bonardi et al., 1982) have regarded the Calabrian massif as a piece of Adriatic crust that was thrust first (Alpidic stage) onto ophiolitic units in the west and later (Appenninic stage), together with its underlying base, back onto the Appenninic carbonate platform in the east. In contrast, Dietrich (1988) regarded the northern Calabrian massif as derived from the European continent, thrust eastwards over the ophiolitic units and resting now with its ophiolitic base upon the Adriatic platform units. In contrast to the situation in southern Calabria, the supposition of Hercynian basement rocks on ophiolites is undisputed in northern Calabria.
The tectonically higher units of the Sila massif in northern Calabria (Monte Gariglione, Bocchigliero and Mandatoriccio Complexes) form the so-called ‘Sila nappe’ of Dubois (1970, 1976), which consists in its deeper part of granulite-facies gneisses, the Monte Gariglione Complex. Late Hercynian granitoids intruded these underlying high-grade gneisses as well as the overlying amphibolite-facies upper-crustal gneisses and low-grade Palaeozoic rocks (Bocchigliero and Mandatoriccio Complexes) (Fig. 1). The ‘Sila nappe’ thus consists of rocks from very different crustal levels and—if they were part of the same Hercynian crustal segment—may store valuable information about the former thermal conditions at different levels of that Hercynian crust.
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The relatively simple tectonic model for the Sila nappe described above, which assigns the granulite-facies rocks to a single Hercynian unit (Monte Gariglione Complex), is, however, debated (e.g. Dubois, 1970
, 1976; Messina et al.; 1991; Borghi et al., 1992; and references therein). According to, for example, Amodio-Morelli et al. (1976), Zanettin Lorenzoni (1980) and Lorenzoni & Zanettin Lorenzoni (1983), the high-grade metamorphic rocks are subdivided into two Alpine nappes of which only the upper one is characterized by a considerable amount of migmatite. These workers proposed a Hercynian nappe boundary within the migmatitic unit separating high-grade metamorphic rocks from overlying intermediate-grade rocks. As will be discussed below, on the basis of metamorphic phase equilibria and mineral chemistry systematics of the granulite-facies rocks, we favour the simpler of the two tectonic models for the Monte Gariglione Complex. In addition, we will deduce that the high-grade rocks of this unit represent an intermediate crustal level in relation to the upper- and the lower-crustal rocks exposed in southern Calabria (Schenk, 1984; Graessner & Schenk, 1999). |
FIELD RELATIONS AND MINERAL ASSEMBLAGES |
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To evaluate the metamorphic field gradient in the high-grade rocks of the Sila nappe, metapelitic and metabasic rocks were collected along two SW–NE-trending traverses through the southern part of the Monte Gariglione Complex (Fig. 2). A few additional thin sections were provided by N. Le Breton (see Le Breton, 1983
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The Monte Gariglione Complex (Fig. 2) consists mainly of metapelites as well as subordinate garnet–biotite gneisses. Both rock types contain numerous, mainly concordant leucosomes. The amount of these is roughly estimated at 10 vol. % but reaches locally 20 vol. % in the upper part of the section. These leucosomes form lenses and bands mostly parallel to the foliation (stromatic leucosomes) and are interpreted as crystallized partial melts of the host metapelites. Silica-undersaturated aluminous restitic rocks are found as small layers on a thin-section scale but only rarely on a hand-specimen scale. In most of the coarse-grained gneisses a well-developed foliation affected by isoclinal folding is recognizable. It dips 20–30° towards the east.
The metapelites have been metamorphosed at sillimanite + K-feldspar grade, so that prograde muscovite + quartz was not stable during peak metamorphism. Fourteen mineral assemblages have been observed in the metapelites, among which the most common are Grt–Sil–Crd [mineral abbreviations are from Kretz (1983)] and Grt–Sil–Kfs, both in equilibrium with Qtz–Bt ± Pl. The ‘univariant’ KFMASH assemblage Grt–Bt–Crd–Kfs–Sil–Qtz, forming an isograd in the Serre massif further to the south (Schenk, 1990), has been found at 24 localities and at very different structural levels of the Sila massif. A discontinuity in metamorphic grade from high-grade granulite-facies to intermediate-grade rocks, which would support the model of tectonic slices in the Monte Gariglione Complex as proposed by Zanettin Lorenzoni (1980) and Lorenzoni & Zanettin Lorenzoni (1983), is not indicated by the regional distribution of assemblages. Late-stage myrmekite, muscovite, kyanite (rare), andalusite and staurolite are interpreted as retrograde formations on textural grounds (Fig. 2).
Metabasites and some intermediate rock types of enderbitic composition are mainly restricted to two large bodies near the village of Villaggio Mancuso and to a few small lenses and layers within the metapelites (Fig. 2). Leucosomes in metabasites have not been observed in the field or on a thin-section scale but cannot strictly be ruled out, because of the scarcity of outcrops. In metabasic rocks seven different mineral assemblages have been distinguished, not considering amphibole as a critical mineral for subdivision (Fig. 2). The granulite facies assemblage Opx ± Grt + Pl + Qtz is found next to retrogressed Amp–Pl ± Grt-bearing rocks, which occur in all parts of the study area. As the assemblage Grt–Cpx–Qtz is missing, the granulites are assigned to the medium-pressure type of Green & Ringwood (1967).
Olivine metagabbros are common as river pebbles in the southwestern part of the study area (Fig. 2). They comprise the primary magmatic assemblage Ol + Pl + Opx ± Cpx and late-stage reaction rims around olivine of Opx + Cpx + Spl and Prg–Hbl + Spl. This rock type is similar to the Rovale Gabbro (Acquafredda et al., 1992; Caggianelli et al., 1994) in the northern part of the study area and resembles metagabbroic rocks described by Dubois (1976) in the west of the Monte Gariglione Complex.
Marbles occur as lenses and layers (100 m to 1 km scale; Fig. 2) within the metapelites but were not collected systematically for this study.
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PETROGRAPHY AND MINERAL CHEMISTRY |
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To constrain the P–T conditions of metamorphism of the Monte Gariglione Complex, nine metapelites and four metabasic rocks were studied with the electron microprobe (see Table 1 for assemblages), using the CAMECA CAMEBAX microprobe at the University of Kiel in the wavelength-dispersive mode with 15 kV acceleration potential and 15 nA beam current. Corrections were carried out with the PAP correction program (Pouchou & Pichoir, 1984
). Representative analyses are presented in Tables 2–6.
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Metapelitic migmatites
Texturally, two types of garnet can be distinguished in melanosomes: coarse 1–5 mm anhedral to subhedral grains and small euhedral ones (0·1–0·5 mm). Both types display the same chemical composition. The larger grains contain inclusions of biotite, quartz, ilmenite and sometimes plagioclase in their cores, and sillimanite and minor cordierite near the rims (Fig. 3a), suggesting garnet growth at the expense of these phases. Coronas of cordierite (Fig. 3a) or cordierite–spinel symplectites replacing garnet (rare) and the breakdown of garnet to aggregates of biotite ± sillimanite ± plagioclase (Fig. 3b and c) are late-stage formations.
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The garnet cores are essentially unzoned, pointing to intracrystalline diffusional homogenization as a result of the high metamorphic grade (Fig. 4a–d; Table 2). Garnet core compositions are in the range of Alm63–82, Prp13–32, Grs2–4, Sps1–3. The XFe values [XFe = Fe/(Fe + Mg)] are 0·66–0·86 and decrease from east to west of the study area, which is towards the structurally lower part of the complex. Commonly, garnet shows retrograde zoning patterns along fractures, biotite inclusions and near the rims (Fig. 4b and c). Garnet in the structurally higher part of the unit is characterized by wider zones of retrogression (
1–1·5 mm; Fig. 4a) than those from the structurally deeper part (<0·25 mm; Fig. 4c and d). Increasing spessartine content at the rims suggests resorption during retrogression (Fig. 4b). Some garnet of the western part is characterized by a slight rimward increase of grossular component (e.g. sample 42-96; Fig. 4c).
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Three textural types of biotite can be distinguished in the melanosomes: inclusions in garnet, matrix grains and grains formed as a breakdown product of garnet. Symplectitic intergrowths of skeletal biotite with quartz near the leucosome veins (Fig. 3d) suggest dehydration melting of matrix biotite. The lowest XFe values are found in biotite inclusions in garnet. Within a single sample, compositional differences between matrix biotite and grains in contact with garnet formed during retrogression are small, suggesting retrograde re-equilibration of the prograde matrix biotite. XFe values of matrix grains range from 0·34 to 0·61 (Table 3). Most biotite has lower Ti contents and XFe values than biotite of comparable metamorphic grade from New England (except for sample 57-1-96; Fig. 5). This can be explained by intensive re-equilibration of the matrix minerals during retrogression, an interpretation that is supported by the presence of exsolved ilmenite coronas around biotite grains.
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Coarse-grained sillimanite is a major constituent of the melanosomes and is oriented parallel to the matrix foliation. Retrograde prismatic sillimanite intergrown with biotite and replacing garnet is common (Fig. 3b). Late-stage fibrolitic sillimanite is much more abundant in the eastern part of the Monte Gariglione Complex than in the western part. Mats of fibrolite embay garnet and are often found as knots of tiny fibres within the leucocratic segregations. Retrograde andalusite replacing cordierite is also common in the eastern part of the study area (Figs 2 and 3e) and is formed always later than fibrolitic sillimanite, which is found as inclusions in andalusite (Fig. 3f). Kyanite was found in only two thin sections of a sample from the western part of the study area (sample C1-22 II at the road at Colle d’Ascione, east of the M. Paganella). In these thin sections Dubois (1976) regarded kyanite as a pre-Hercynian relic phase. However, here it is interpreted as a retrograde mineral formed together with chlorite and biotite at the expense of cordierite (Fig. 3g).
In melanosomes two textural types of cordierite can be distinguished: matrix grains and late-stage coronas around garnet (Fig. 3a). The matrix grains show inclusions of sillimanite and biotite and, in quartz-deficient rocks, also of spinel (Fig. 3h and i). Retrograde breakdown of cordierite to biotite + sillimanite + quartz or andalusite + chlorite + quartz is common in the eastern part of the Monte Gariglione Complex; in the western part, cordierite breakdown to kyanite-bearing assemblages is locally evident (Fig. 3e and g). The cores of matrix grains and of coarse-grained cordierites at the margin of leucosomes are unzoned and uniform in composition within each sample (Table 4). The XFe range is 0·21–0·51 decreasing systematically from east towards the west of the Monte Gariglione Complex, which, when coexisting with Grt + Sil + Qtz, suggests increasing metamorphic pressures (or changing aH2O) in this direction (e.g. Thompson, 1976; Holdaway & Lee, 1977). Rimward decrease of XFe is explained by retrograde Fe–Mg exchange with matrix minerals. The Na content of the unzoned cores decreases to the west (from 0·05 to 0·01 cations p.f.u.) suggesting rising metamorphic temperatures in this direction (Mirwald, 1986). XFe values of late-stage cordierite coronas surrounding garnet are similar to or lower than those of the rims of the matrix grains, which is attributed to late-stage cation exchange.
Plagioclase, like perthitic K-feldspar, is a main constituent of the quartzofeldspathic leucosomes but volumetrically more important than the latter. In the commonly concordant leucocratic lenses and veins both feldspars crystallize mainly at the margins. K-feldspar is usually embayed and replaced by late-stage myrmekitic plagioclase–quartz symplectites. This has led in many samples to complete replacement of K-feldspar, mainly in the eastern part of the Monte Gariglione Complex. The cores of plagioclase are essentially unzoned and uniform within a sample. No compositional difference was found between plagioclase in leucosomes and that in melanosomes. The analysed core compositions are in the range of An31–44 (Table 5). In most analysed plagioclase grains the rims (<0·1 mm) are characterized by a moderate decrease of Ca suggesting retrograde re-equilibration of the rims during isobaric cooling. This would correspond to the rimward increasing grossular content of some garnets if the Pl–Sil–Grt–Qtz equilibrium controls this zonation. K-feldspar is commonly perthitic microcline. The composition of the exsolved host is in the range of Kfs91–94.
Green to brown spinel may occur as tiny inclusions in garnet (sample 33-97), as symplectitic intergrowth within large (<5 mm) euhedral sillimanite porphyroblasts or rarely in symplectitic intergrowth with cordierite replacing garnet. More common are inclusions in cordierite in quartz-deficient layers of the migmatites (Fig. 3h and i). The spinel is rich in hercynite (Hc51–66) and gahnite (Gh10–34) components. XFe varies between 0·76 and 0·85, but the compositions are relatively constant in any one specimen. Cr2O3 content is generally between 0·25 and 0·32 wt % but reaches 2·6–4·1 wt % (sample 46-3-96). Site and charge-balance calculations indicate a low magnetite component (Fe3+ = 0·02–0·12 cations p.f.u. based on 32 oxygens). Zonation trends have not been identified.
Staurolite has been found only as fine-grained retrograde spherical to euhedral crystals. It occurs at the margins of biotite or in the vicinity of garnet, and as small needles at the seams of cordierite in biotite–sillimanite-rich layers. In the analysed sample 50-4-96, XFe ranges between 0·80 and 0·88 and is therefore lower than that of associated garnet rims (XFe = 0·88–0·89). MnO and ZnO contents are 0·16–0·28 wt % and 2·6–3·2 wt %. No regular zonation has been identified in individual grains.
Muscovite + quartz as a retrogressive assemblage is very common in the structurally higher part of the unit. Its formation after K-feldspar + sillimanite is indicated by the symplectitic intergrowth of muscovite with quartz or by inclusions of fibrolitic sillimanite and/or pseudomorphs after sillimanite (Fig. 3j) (Ashworth, 1972). Phengite component is low (Si = 3·08–3·11 cations p.f.u.).
Ilmenite and rutile are common accessory minerals and occur as primary matrix phases and as inclusions in garnet. Measured MnO contents are in the range of 0·14–1·57 wt % and 0·01–0·08 wt %, respectively. Additional accessories are monazite, zircon, apatite, pyrrhotite and graphite.
Metabasites and intermediate rock types
Garnet is present in 19 of the 41 samples studied. Four textural types can be distinguished: (1) net-like to subhedral porphyroblasts (0·6–3·0 mm); (2) smaller euhedral grains (<0·3 mm). The net-like grains, enclosing quartz, plagioclase and biotite, suggest that no penetrative deformation took place after garnet growth. The two textural types together occur only in two of the analysed samples (79-2-96, 84-96). (3) Late-stage garnet formation is indicated by coronas around plagioclase separating it from orthopyroxene (sample 29-96, Fig. 3k) and from hornblende–cummingtonite. (4) Late-stage poikiloblastic garnet grows at the expense of biotite + quartz symplectites, which replace orthopyroxene in a sample (84-96) of enderbitic composition (Fig. 3l).
The cores of textural garnet types (1) and (2) are essentially unzoned and are in the range of Alm62–67, Prp19–30, Grs5–12, Sps2–3 (Fig. 6, Table 2). Only the net-like garnet (sample 79-2-96) is zoned and shows a slightly higher grossular content in the core, interpreted as a relic of the prograde growth zoning. The XFe values are 0·67–0·78 and, like grossular, increase towards the rims. It is obvious from the Ca–Fe–Mg ternary in Fig. 7 that garnet–orthopyroxene-bearing rocks without hornblende as a Ca-buffering phase contain garnet poorer in grossular component than those with hornblende. The compositions of the late-stage coronitic garnet are higher in grossular but lower in pyrope contents.
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Plagioclase is a major constituent of all the metabasic rocks. The cores of the anhedral grains (0·1–0·7 mm) are unzoned with uniform compositions within a thin section (An39–85; Table 5). Most of the analysed plagioclase grains are characterized by an increase of anorthite contents at the rims.
Orthopyroxene occurs in nine of the 41 rock samples and only in the western part of the study area, and forms net-like to anhedral blasts (0·1–0·6 mm). In the enderbitic sample 84-96 net-like grains, enclosing quartz, plagioclase and small euhedral garnet (Fig. 3l), suggest that no penetrative deformation took place after peak metamorphism. TheXFe and Al(VI) values are very similar in the analysed samples [XFe = 0·48–0·49 and Al(VI) = 0·02–0·05 cations p.f.u.; Fig. 7, Table 6], except for sample 27-97, which has lower XFe and higher Al(VI) values (0·42 and 0·08). Where in contact with garnet, orthopyroxene shows narrow rims slightly lower in XFe as a result of retrograde Fe–Mg exchange. The moderate rimward decrease of Al content in orthopyroxene and the increase of grossular component in coexisting garnet reflect cooling (Bégin & Pattison, 1994; Fitzsimons & Harley, 1994). Common retrograde breakdown products of orthopyroxene are cummingtonite and/or hornblende and biotite + poikiloblastic garnet (sample 84-96).
Amphibole occurs as brown or green hornblende and as cummingtonite. The latter two are in most cases retrograde breakdown products of pyroxene. Brown and dark brown–green hornblende, which occurs locally as a matrix mineral and as small inclusions in plagioclase, might be a relic of the prograde stage. Patchy intergrowths of hornblende and cummingtonite are common; less common are hornblende lamellae within cummingtonite, possibly exsolved from formerly homogeneous clinoamphiboles (e.g. Klein et al., 1996). Seams of hornblende around cummingtonite also occur. The calcic amphiboles range in composition from actinolite to mainly magnesiohornblende. The generally anhedral grains are unzoned and their Si, Al and Ti contents are in the range of 6·8–7·7, 0·56–1·79 and 0·03–0·1 cations p.f.u. (Table 6). Their XFe values are always lower (0·34–0·43) than those of coexisting cummingtonite (0·36–0·45) and orthopyroxene (0·48). Unexsolved hornblende has higher XFe values than that intergrown with cummingtonite.
Biotite occurs in most of the studied samples, but its modal abundance is low (<3 vol. %). Three textural types of biotite can be distinguished: matrix phase, inclusions in garnet and late-stage breakdown product of orthopyroxene. The lowest XFe values are found in biotite inclusions in garnet. XFe varies from 0·25 to 0·46 and the Ti contents range between 0·14 and 0·29 cations p.f.u. (Table 3) and are thus higher than in metapelitic biotite.
Clinopyroxene occurs in few metabasites of the western part of the study area. The grains are euhedral to subhedral and 1 mm in diameter. Some grains contain hornblende lamellae, which seem to have been formed after exsolved pyroxene. Larger porphyroblasts (2 mm) show retrograde replacement rims of poikilitic green hornblende with quartz inclusions.
Ilmenite is a common accessory mineral and occurs as a primary matrix phase and as inclusions in garnet. Measured MnO contents range between 0·30 and 0·39 wt %. Additional accessories are titanite (intergrown with ilmenite in some samples), apatite, allanite and zircon.
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MINERAL REACTION HISTORY |
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Prograde reactions
The prograde reaction history in metapelites is best constrained by mineral inclusions in garnet and the presence of quartzofeldspathic leucosomes. The presence of abundant leucosomes in most of the rock samples (
10–20 vol. %) and biotite–quartz symplectites at the margins of the leucocratic segregations (Fig. 3d) indicates that dehydration melting has occurred. Sillimanite inclusions in garnet and cordierite are evidence that prograde metamorphism proceeded within the stability field of sillimanite. The absence of muscovite and the presence of the assemblage sillimanite + K-feldspar + quartz in all samples indicates temperature conditions above the dehydration melting of muscovite according to the reaction
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; Spear et al., 1999), further melt reactions involving biotite have to be taken into account for the Calabrian metapelites.
Inclusions of biotite and plagioclase in the cores and sillimanite in the rims of garnet point to the divariant garnet + K-feldspar-producing melting reaction
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; Le Breton & Thompson, 1988) depending on the initial modal amount of biotite and the P–T conditions attained during melting. The garnet rims, which contain numerous sillimanite inclusions parallel to the matrix foliation as a result of an overgrowth of the pre-existing sillimanite fabric, display in some samples slightly higher grossular contents. This might be due to crossing of the muscovite dehydration melting curve [reaction (1a)] through which both plagioclase and melt will become more calcic. When garnet resumes growth as a result of reaction (2), subsequent to muscovite dehydration melting, the Ca content in the rim of garnet will become higher to adjust to the new plagioclase composition (Spear & Parrish, 1996; Spear et al., 1999).
In rocks with low XFe values containing cordierite instead of garnet (Fig. 2), cordierite presumably was produced by a melting reaction analogous to reaction (2). Further melting would have proceeded with changing P–T conditions until the univariant reaction
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Prograde reaction textures in metabasites and intermediate rock types are poorly preserved. Dehydration melting of hornblende, which occurs at higher temperatures than any of those involving biotite (Clemens & Vielzeuf, 1987), was not observed in the studied rocks. In the enderbitic sample 84-96, dehydration, which may have taken place at the metamorphic peak, led to the formation of large net-like orthopyroxene porphyroblasts together with small euhedral garnet as a result of a reaction of the form (Fig. 3l)
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). K-feldspar has not been found in the sample described here, but a K-bearing fluid or potassic feldspar must have been involved in this dehydration of biotite.
Retrograde reactions
The metapelitic and metabasic rocks were strongly affected by retrograde metamorphism. This is obvious by the abundance of decompression textures and the even more common rehydration reactions. Decompression after peak metamorphism is documented by textures such as cordierite rims around garnet in metapelites, which were formed via the reverse of the continuous reaction (4) (Fig. 3a) or as a result of
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).
In small quartz-deficient domains in metapelites specific reaction textures occur: symplectitic intergrowths of spinel and cordierite and inclusions of sillimanite and spherical relics of prograde biotite in cordierite, and in some cases also in spinel. These point to the divariant KFMASH reaction (Fig. 3h)
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This reaction has been proposed by Brown (1998) and Whittington et al. (1998) for low-pressure rocks of the Ryoke Belt and the Nanga Parbat. As the reaction has a positive slope the observed textures also indicate decompression after peak metamorphism. Importantly, in some cases the reverse of reaction (7) took place in the same sample, which suggests falling temperatures subsequent to decompression. This late-stage reaction is documented by sillimanite seams around spinel inclusions in cordierite and newly grown biotite (Fig. 3i). In one quartz-deficient rock (sample 28-1-97) decompression is documented by development of spinel–cordierite symplectites replacing garnet, which can be explained by the reaction
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Further cooling led to the decomposition of cordierite via the reactions (Fig. 3e)
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In metabasites textures such as garnet rims between plagioclase and orthopyroxene suggest a nearly isobaric cooling path during which the reaction (Fig. 3k)
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Textural features such as garnet seams separating cummingtonite from plagioclase, all in close association with biotite and hornblende, could be attributed to the prograde CNFMASH reaction
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in Fe-rich metabasic rocks of central Massachusetts. However, in the Sila massif, the garnet coronas are associated with rare relics of orthopyroxene as inclusions in cummingtonite, which suggest retrograde formation of cummingtonite by rehydration of the granulite-facies orthopyroxene. In this case the garnet reaction rims are presumably formed like cummingtonite at a retrograde stage via a reaction such as (Hollocher, 1991)
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, fig. 3l)
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PHASE RELATIONS OF METAPELITES |
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The mineral assemblages Grt + Sil + Crd + Qtz and Grt + Sil + Kfs + Bt + Qtz are most common in the studied metapelites (Fig. 2) and can be used to identify regional differences in metamorphic grade within the Monte Gariglione Complex. In these assemblages the ferromagnesian minerals become more Mg rich with increasing metamorphic grade (Thompson, 1976
). From the regional variations of the XFe values of coexisting minerals, a metamorphic gradient along the two east–west traverses through the Sila massif can be deduced. This relationship can be shown in an AFM diagram of the model KFMASH system, projected from the appropriate excess components. As MnO and CaO contents in garnet are very low (1–3 mol % spessartine, 2–4 mol % grossular) and are also minor in the coexisting minerals, their influence on the projected positions of the minerals can be ignored. Figure 8 shows two Al2O3–FeO–MgO triangles for rocks with the Grt + Sil + Crd + Qtz assemblage (projected from quartz) from the northern and southern profiles. A decrease in the XFe values of cores of garnet and matrix cordierite from east to west of the study area results in a shift of the three-phase field Grt–Sil–Crd towards the Mg side of the AFM triangle. From this a continuous pressure increase (and a smaller temperature rise) from east to west through the Monte Gariglione Complex can be inferred. A corresponding temperature gradient from decreasing XFe in coexisting garnet–biotite pairs is not well established by garnet–biotite thermometry. Despite the strong retrogression leading to relatively homogeneous biotite compositions in individual samples (Fig. 5), XFe of biotite decreases, from east to west through the Monte Gariglione Complex, as does that of coexisting garnet. The preservation of systematic phase relations is surprising in view of the retrogression that affected mineral textures and chemistry. A possible explanation is that unzoned cores of garnet and cordierite porphyroblast retained their peak-metamorphic composition and retrogressive effects are restricted to the mineral rims. However, the influence of retrograde metamorphism on biotite chemistry cannot be neclected (see Fig. 5) and estimates of metamorphic conditions on this mineral are less reliable.
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P–T CONDITIONS ALONG THE CRUSTAL SECTION |
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First indications on the P–T conditions during peak metamorphism are obtained from mineral assemblages of the high-grade gneisses in the western part of the Monte Gariglione Complex. The stability of the assemblage Opx + Pl + Qtz ± Grt and the absence of Grt–Cpx–Otz in metabasites point to the P–T field of medium-pressure granulites (700–850°C at 5–7 kbar; Green & Ringwood, 1967
, 1972). Corona textures in carbonate rocks suggest crossing of the reaction Fo + Cal + V = Di + Dol during cooling from peak-metamorphic temperatures. Therefore, the latter must have been higher than 690–720°C at 5–7 kbar according to the experiments of Käse & Metz (1980). Peak-metamorphic conditions must have been below Bt + Qtz- or Bt + Grt + Qtz-breakdown (<820–840°C at 5–7 kbar; Spear et al., 1999; Fig. 15), as orthopyroxene is absent in metapelites of the Sila massif.
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P–T conditions along the two studied east–west profiles are first constrained by conventional geothermobarometry, applying well-calibrated net-transfer and cation-exchange reactions uniformly at 5 kbar for temperature estimates and 700°C for pressure estimates (Table 7). These results will be compared with results obtained from multi-reaction calculations using the TWEEQU program (Berman, 1991
). For estimates of peak-metamorphic conditions, the core compositions of coexisting matrix minerals were used, as those are assumed to preserve the highest metamorphic grade. For the enderbitic sample 84-96, peak conditions were estimated using the composition of the small euhedral garnet grains, which are thought to have been formed together with orthopyroxene at the peak of metamorphism. Retrograde conditions were calculated using rim compositions of matrix minerals that are in contact with each other (Table 8).
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Conventional geothermometry
In metapelites calculated peak-metamorphic temperatures based on the Fe–Mg partitioning between coexisting garnet and cordierite (Thompson, 1976
; Bhattacharya et al., 1988; Dwivedi et al., 1998) as well as garnet and biotite (Hodges & Spear, 1982) are similar in both profiles, ranging from 660 to 820°C and 630 to 830°C (Table 7a, Fig. 9). Within each rock specimen the calculated garnet–cordierite temperatures scatter over 20–60°C. The presence of a temperature gradient across the Monte Gariglione Complex as deduced from phase relations is not supported by the absolute data obtained by geothermometry. The highest temperatures (e.g. 740–830°C) were calculated for rock samples from the eastern part of the study area, where a lower metamorphic grade than in the western part has been deduced from AFM phase relations. Overestimates of temperatures occur if, as a result of retrograde sliding reactions, cores of matrix biotite and cordierite attained higher XFe ratios than they had at the metamorphic peak, but garnet cores remained unchanged (see Spear & Florence, 1992, fig. 1). Peak temperature conditions of 730 to 770°C for metapelites of the western part of the study area (samples 57-1-96, 57b/85 and 86-96) might be more realistic than those obtained in the east. However, in the west also some unrealistic low temperatures (630–710°C) were determined, presumably as a result of late-stage cation exchange.
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Temperatures calculated for garnet rims and adjacent cordierite coronas or biotite are 140–300°C and 60–130°C lower than peak estimates for the same sample (Table 8a, Fig. 9), which can be attributed to retrograde Fe–Mg exchange between minerals in contact.
In metabasites and intermediate rocks, temperatures could be calculated only for three samples from the structurally lower, western part of the study area. The estimates are based on the Fe–Mg partitioning between coexisting garnet and orthopyroxene (Harley, 1984; Lee & Ganguly, 1988; Bhattacharya et al., 1991) and between garnet and hornblende (Graham & Powell, 1984) and on the Al solubility in orthopyroxene coexisting with garnet (Harley & Green, 1982). The garnet–orthopyroxene thermometer calibrated by Bhattacharya et al. (1991) yields the highest peak temperatures (700–720°C, Table 7b, Fig. 10) among the applied geothermometers (with the only exception of 785°C for sample 27-97 based on the Al content in orthopyroxene) but the results are still lower than those calculated for metapelites.
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Temperature estimates for late-stage garnet coronas separating orthopyroxene and plagioclase (sample 29-96) range from 555 to 642°C (Table 8b). Rims of isolated single garnet and orthopyroxene grains as well as rims of grains in contact yield temperatures in the range of 500–645°C (Table 8b and Fig. 10). The garnet–hornblende estimate of 556°C calculated for garnet rim and the highest XFe ratio of a hornblende, which has been formed during retrogression around cummingtonite (sample 79-2-96), is also in the range of retrograde temperatures obtained from garnet–orthopyroxene thermometry. The low temperatures are supported by the low Ti content of hornblende (<0·1 cations p.f.u.). Similar low Ti content in hornblende from southern India (Raase et al., 1986) and Sri Lanka (Schumacher et al., 1990) indicate amphibolite-facies conditions (570 and 660°C).
Conventional geobarometry
The presence of a pressure gradient during peak metamorphism across the Monte Gariglione Complex has been deduced from AFM phase relations and is supported by the results of geobarometry on metapelitic assemblages. The garnet–cordierite–sillimanite–quartz (GCSQ) geobarometer reveals a westward increase of peak-metamorphic pressures. Using the TWEEQU program of Berman (1991) with thermodynamic data and the garnet activity model of Berman (1988, 1990; assuming ideal mixing of anhydrous Mg-cordierite) pressures range between 4–5 kbar (northern profile) and 4·2–5·3 kbar (southern profile; Table 7a, Fig. 9). The calibration by Martignole & Sisi (1981) of the GCSQ barometer gives slightly lower pressures for the northern (3·2–4·7 kbar) and southern profiles (3·6–4·9 kbar; Table 7a). As volatiles (mainly H2O and CO2) may be important constituents of cordierite in granulite terranes, pressures were also calculated with the TWEEQU program assuming H2O-saturated cordierite (ideal mixing model of hydrous Mg-cordierite; Berman, 1988). These estimates yield pressures of 5·2–6·4 kbar (northern profile) and 5·5–6·7 kbar (southern profile; Table 7a, Fig. 9), which are 1 kbar higher than those calculated with anhydrous cordierite.
Coexisting rutile and ilmenite have been observed only in two of the analysed metapelites (42-96, 57-1-96) of the southern profile. For these the GRAIL calibration of Bohlen et al. (1983) gives pressures of 4·8 and 5·7 kbar, similar to the GCSQ estimates for the same samples (Table 7a).
Calculations with the GASP barometer, using the calibration of Koziol & Newton (1988), yield peak-metamorphic pressures in the range of 2·9–5·3 kbar (northern profile) and of 4·2–5·3 kbar (southern profile). However, the GASP estimates do not vary significantly between the structurally upper and lower parts (Fig. 9, Table 7a).
Retrograde pressures were calculated for garnet rims and cordierite coronas, which are 1–2 kbar lower than peak pressures (Table 8a, Fig. 9).
In metabasites and intermediate rocks peak-metamorphic pressures were calculated using the garnet–orthopyroxene–plagioclase–quartz (GOPQ) geobarometer of Eckert et al. (1991; Mg-end-member reaction) and Perkins & Chipera (1985; Fe-end-member reaction). The resulting pressure values for sample 27-97 (3·7–4·4 kbar) are lower than for the other samples 79-2-96 and 84-96 (4·6–5·4 kbar; Table 7b, Fig. 10), which is consistent with the higher structural position of the former. The peak-metamorphic pressure estimates of the metabasites agree well with those of the GCSQ barometer using the thermodynamic data of Berman (1988) and those of the GASP barometer.
Retrograde pressures were also calculated for the late-stage garnet coronas between orthopyroxene and plagioclase in sample 29-96. The results of the GOPQ geobarometer range between 4·4 and 6·7 kbar depending on the calibration used (Table 8b, Fig. 10). This is not significantly different from estimates of peak pressures.
TWEEQU thermobarometry
Pressure–temperature estimates for metapelites and metabasites were also obtained with the TWEEQU computer program (version 1.02) of Berman (1991). Using the internally consistent thermodynamic dataset of Berman (1988) and Mäder et al. (1994) all possible equilibria (stable and metastable) are simultaneously calculated for the set of end-member phases (given in Figs 11 and 12) and their activities within the simplified chemical system K2O–CaO–FeO–MgO–Al2O3–SiO2–H2O. The calculations were restricted to H2O-conserving equilibria, thus constraining the P–T conditions independently of water activity.
Among metapelites only two of the analysed samples (50-4-96, 57b/85) show a tightly constrained set of intersections suggesting a close approach to equilibrium (Fig. 11). The observed scatter of equilibria in the other metapelitic samples might be explained by retrograde exchange among cordierite and biotite with garnet. Figure 11a–c shows improving convergence for equilibria in sample 57b/85 with a decreasing number of equilibria as a result of a progressive exclusion of end-members [Fig. 11: (a) all phases, inclusive reactions depending on aH2O; (b) excluding K-feldspar; (c) excluding K-feldspar and biotite; see Table 9 for computed equilibria]. An equivalent figure is shown for sample 50-4-96 but only the equilibria without the phases biotite and K-feldspar are given (Fig. 11d). Conspicuous in both samples is the shift of the garnet–cordierite Fe–Mg exchange reaction (8) to the low-temperature side of most intersections. The P–T estimates of 743 ± 12°C at 4·7 ± 0·2 kbar (50-4-96) and 769 ± 7°C at 5 ± 0·1 kbar (57b/85) agree with those derived from conventional thermobarometry (Table 7a).
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For metabasites and intermediate rocks the TWEEQU software computed seven (samples 27-97, 84-96; Fig. 12c and d) and eight (samples 29-96 and 79-2-96; Fig. 12a and b) anhydrous equilibria (see Table 9 for reactions). All analysed samples show a good convergence of calculated equilibria with pressure estimates (4·4–5·5 kbar) falling in the range of those obtained by conventional geobarometry (Tables 7b and 8b). This is also true for sample 79-2-96, for which it was concluded previously that the hornblende and garnet core might not be in equilibrium because of the late-stage formation of hornblende. The calculated low temperatures (560–680°C) correspond to the results determined with the garnet–orthopyroxene thermometer of Harley (1984)
or are even lower (sample 27-97). Despite the good convergence of the calculated equilibria, the P–T estimates for peak metamorphism (samples 27-97, 79-2-96, 84-96) are lower than those of the metapelites and are also too low for the peak of the granulite-facies metamorphism. The only reasonable explanation seems to be that minerals re-equilibrated during the slow cooling.
Calculation of water activity
In metabasites the water activities were calculated with the TWEEQU program for the retrograde stage (samples 29-96 and 79-2-96) of metamorphism. The pressure estimates are taken as a fixed reference and the H2O-dependent mineral equilibria are plotted on T–aH2O diagrams (Fig. 13). The results of water activity estimates for sample 29-96, showing late-stage garnet rims and retrograde hornblende, and sample 79-2-96, also containing a retrograde hornblende, are 0·04 and 0·32. Despite the evidence for late-stage fluid infiltration in both samples suggested by abundant retrograde growth of hornblende–cummingtonite at the expense of orthopxyroxene, the calculated water activities are very low.
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The water activities for the two metapelitic samples (50-4-96, 57b/85) were also calculated with the TWEEQU program, but the procedure was different from that described above. Seven possible equilibria were computed within the simplified chemical system KCFMASH including dehydration reactions which are dependent on water activity (Fig. 14, Table 9). The diagrams were calculated by lowering the water activity until all intersections of reaction curves converged and the P–T results corresponded to those calculated by H2O-absent equilibria. The water activities of the samples 50-4-96 and 57b/85 range from 0·38 to 0·45 and from 0·43–0·50, respectively. These low aH2O values and the widespread occurrence of quartzofeldspathic leucosomes are in agreement with dehydration melting. A sequence of melting reactions [reactions (1a)–(3)] would lower the water activity, as the H2O component of the fluid phase is partitioned into the generated melt. The calculated water activities for the metapelitic samples agree fairly well with experimentally determined solidus curves of the haplogranitic system (Qtz–Ab–Or–H2O ± CO2) for aH2O 0·5 at 740–770°C and 5 kbar (Ebadi & Johannes, 1991; Holtz & Johannes, 1994; Vielzeuf & Montel, 1994).
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DISCUSSION |
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P–T path reconstruction and peak-metamorphic conditions
The documented prograde metamorphic P–T path is characterized by heating accompanied by loading and proceeded in the stability field of sillimanite. The details of this prograde path, however, are not known but a clockwise P–T path is very likely.
The peak-metamorphic conditions for the rocks of the study area have been deduced by combining P–T calculations with AFM phase relations. The peak temperature conditions are best defined by the stability fields of the observed mineral assemblages in the NaKFMASH system (Fig. 15). Peak temperatures for the upper and lower part of the Monte Gariglione Complex are fairly close to each other and must have been in the range of the univariant dehydration melting reaction (3) in metapelites, which is above 740–770°C at the pressure range of interest, and below the final breakdown of Bt + Grt + Qtz or Bt + Qtz (<800–820°C, Spear et al., 1999; Fig. 15). These temperatures coincide with results using the TWEEQU method in two of the metapelites and the occurrence of the peak assemblage Fo–Cal–Dol in carbonate rocks in the structurally lower part of the study area. Peak-metamorphic pressure estimates for the temperatures discussed above range between 4 and 5·5 kbar (GCSQ and TWEEQU barometry) and 4·4–6·5 kbar (GASP equilibrium), increasing from the upper to the lower part of the section (Fig. 9). Comparable P–T data for peak metamorphism were determined by Le Breton (1983; T > 650°C and P > 4·5–5 kbar) and Althaus & Istrate (1990; T = 690–740°C and P < 6–8 kbar). The higher pressures of the latter workers are deduced from studies of CH4- and N2-rich fluid inclusions in gneisses of the western part of the Monte Gariglione Complex.
The combination of reaction textures preserved in metapelitic and metabasic rock types leads to retrograde paths characterized by more or less isothermal decompression from peak-metamorphic conditions to mid-crustal levels (10–15 km) followed by a stage of near-isobaric cooling (Fig. 15). Decompression from 4–6 kbar to 2·4–4·8 kbar is supported by cordierite coronas around garnet in metapelites and silica-undersaturated aluminous rocks as well as cordierite–spinel symplectites replacing garnet and textures suggesting growth of cordierite + K-feldspar + spinel at the expense of biotite + sillimanite. Textural support in metapelites for isobaric cooling after decompression includes the reversal of the latter reaction, indicated by newly grown sillimanite and biotite separating cordierite from spinel. Further indications of cooling are the reversal of dehydration melting reactions (2) and (3) and reaction (9), which consume garnet or form muscovite at the expense of sillimanite/fibrolite + K-feldspar + melt/vapour [reversal of reactions (1a) and (1b) in Fig. 15]. Reaction (9) is only observed in rocks at structurally high levels. As the retrograde P–T trajectory of this part crosses reaction (1b) below the invariant point IP1 (Fig. 15), retrograde muscovite could not have formed if the water released during crystallization of melts had already left the rock before the stability field of Ms + Qtz was reached (Spear et al., 1999). However, in the Sila massif, water either remained in the rocks during retrogression or may have been introduced from outside. In the lower part of the section rehydration by biotite formation is common. However, the reversal of the muscovite dehydration melting reaction (1a) is not observed although the P–T trajectory crosses the reaction above IP1. Further cooling is petrologically documented by andalusite and kyanite (+ Chl + Qtz) overgrowing even late-stage cordierite in metapelites of the upper and lower part of the study area, respectively (Fig. 3e and g). In metabasites textural support for near-isobaric cooling at 4–5 kbar is the formation of late-stage coronal garnet separating orthopyroxene from plagioclase (sample 29-96; Fig. 3k). Secondary seams of Opx + Cpx + Spl and subsequent Prg–Hbl + Spl around Ol + Pl in olivine metagabbros are in agreement with this interpretation. The interpretation of an isobaric-cooling stage is also supported by mineral zoning patterns; garnet zoning in metabasites and metapelites is characterized by a rimward increase of the grossular content, and orthopyroxene zoning by a corresponding rimward decrease in Al. In metapelites, there is a slight decrease of anorthite component at the rims of plagioclase. The very low Grt–Opx temperatures obtained from metabasic rock sample 29-96 support the late-stage formation of coronal garnet during the near-isobaric cooling stage.
The deep crustal sections in northern and southern Calabria: a comparison
The striking similarities between the lithostratigraphic sequences exposed in the Sila (north) and the Serre (south) sequences of the Calabrian massif have been known at least since the pioneering work of Quitzow (1935), who compiled a geological map of the Calabrian massif and adjoining areas. In both massifs, high-grade gneisses (‘kinzigites’) are overlain by granitoids (Qtz diorites, tonalites, granites) which display intrusive contacts with the overlying low-grade rocks of the upper crust. Despite these far-reaching similarities, different geological histories and internal tectonic subdivisions have been proposed for both parts of the Calabrian massif. The tectonic models of the Sila massif that assume that this lithological sequence is part of an Alpine nappe pile, composed of Hercynian basement slices thrust onto Mesozoic ophiolitic rocks, are undisputed in the literature (e.g. Dubois, 1970, 1976; Amodio-Morelli et al., 1976, Lorenzoni & Zanettin Lorenzoni, 1983). In this respect the Sila massif differs from southern Calabria, where no underlying ophiolites occur. As discussed above, the internal tectonostratigraphic subdivision of the highest unit in the Sila massif, the so-called ‘Sila nappe’, is a matter of debate. A relatively simple tectonic model that attributes the entire Hercynian rock pile to a single Alpine nappe (Dubois, 1970, 1976; resumed by Messina et al., 1991) contrasts with the nappe-stack model of, for example, Amodio-Morelli et al. (1976), Zanettin Lorenzoni (1980) and Lorenzoni & Zanettin Lorenzoni (1983), according to which the high- and low-grade rocks define distinct Alpine tectonic crustal slices. In addition, the latter two studies complicated the tectonic subdivision even more, by proposing incorporation of Hercynian nappe boundaries within the Alpine units.
The tectonic model of distinct Alpine nappe slices has also been adopted to the Serre massif of southern Calabria (Amodio-Morelli et al., 1976). However, within the high-grade gneisses no further tectonic subdivision has been made, but an Alpine nappe boundary has been proposed between the lower-crustal rocks and the overlying tonalites. An alternative model for the Serre massif, based on petrological analyses, geological mapping and isotopic dating, has been proposed by Schenk (1980, 1984, 1990). According to these data, the Serre massif exposes a continuous profile (8 km) through a Hercynian granulite-facies lower crust on top of which the overlying Hercynian granitoids intruded. Our present petrological study on the high-grade gneisses of the Sila nappe shows that the Monte Gariglione Complex of the Sila massif probably represents a single lithological unit characterized by a continuous change in metamorphic grade and thus resembles the lower-crustal ‘metapelite unit’ of the Serre massif (Schenk, 1984). However, coexisting garnets and cordierites in metapelites of the Sila massif are higher in XFe (0·87–0·66 and 0·51–0·20) than in the Serre section (0·81–0·62 and 0·28–0·13; Fig. 8; Schenk, 1984) indicating that a shallower crustal level (4–6 kbar) is exposed in the lower part of the Sila nappe than in the Serre massif (5·5–7·5 kbar). These petrological results are in agreement with the observation that below the unit of migmatitic metapelites in the Serre massif an even deeper lithostratigraphic unit, the ‘granulite–pyriclasite unit’ (Schenk, 1984), is exposed, which is missing in the Sila nappe. The lower-crustal sections of the Sila and the Serre massifs are also different with respect to the described undersaturated olivine-bearing rock type, as this lithology seems to be absent from the latter region (Schenk, 1984). The ‘univariant’ assemblage Grt–Bt–Crd–Kfs–Sil–Qtz–L, which forms an isobar in the Serre massif (5·5–6 kbar), and which therefore supports the model of a continuous cross-section through a lower crust, occurs in the Monte Gariglione Complex at all structural levels. This difference is possibly due to the fact that only in the Sila massif does the geotherm follow the slope of the ‘univariant’ melt reaction (3). The estimates of peak-metamorphic temperatures in both crustal sections are nearly the same, 800–770°C at the base and 740–700°C at the top. The slight systematic differences in temperature values obtained in the two sections probably can be attributed to the application of different thermometric methods and calibrations. However, an unusually high geothermal gradient at the time of Hercynian metamorphism, in the range of 50–35°C/km, is obvious from both sections.
Both deep crustal sections in northern and southern Calabria experienced similar P–T histories. After prograde metamorphism, characterized by strong heating accompanied by a moderate pressure increase, a stage of isothermal uplift of the granulite-facies rocks to mid-crustal levels was followed by nearly isobaric cooling. The geodynamic causes for decompression after peak metamorphism might be extensional tectonics, which occurred between the Adriatic crust and the European continent as a result of Mesozoic rifting. This scenario has been envisaged by Brodie & Rutter (1987) and Handy & Zingg (1991) to explain the post-Hercynian uplift of the Ivrea zone, which was in a similar palaeogeographical position to Calabria. For Calabria, however, the present authors prefer a crustal thickening event before peak metamorphism as the cause of metamorphic heating and the subsequent crustal uplift and isobaric cooling. The large calcalkaline (qtz diorite and tonalite) to metaluminous granitoid intrusions into the middle crust of the Sila and Serre massifs certainly provoked metamorphic heating and contributed to the loading and crustal thickening that accompanied Hercynian granulite-facies metamorphism (Schenk, 1980, 1990; Graessner & Schenk, 1999). A palaeo-geotectonic setting of a continental arc above a subduction zone would satisfy the petrological and isotopic data for the Calabrian massif (Schenk, 1990; Graessner et al., 2000). Metamorphic textures in metabasites and metapelites of different structural levels of the deep crustal sections in northern and southern Calabria provide evidence that prograde heating outlasted penetrative deformation. We attribute this textural relationship to the magmatic heat source inducing the unusually high geothermal gradients in the crust of the Calabrian continental arc.
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ACKNOWLEDGEMENTS |
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Constructive comments of G. Droop on an earlier version of the manuscript, and thoughtful reviews of M. Raith, S. Harley and an anonymous reviewer helped to improve the presentation. E.-R. Neumann is thanked for the careful editorial handling. We thank B. Mader and D. Ackermand for help with the microprobe. A. Fehler made many excellent thin sections. The Deutsche Forschungsgemeinschaft supported the project through grant Sche 265/9-1, 9-2.
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FOOTNOTES |
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*Corresponding author. Fax: +494318804457. E-mail: vs{at}min.uni-kiel.de
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