THORSTEN GRAESSNER AND VOLKER SCHENK

Transcription

1 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 PAGES 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 THORSTEN GRAESSNER AND VOLKER SCHENK INSTITUT FÜR GEOWISSENSCHAFTEN DER CHRISTIAN-ALBRECHTS-UNIVERSITÄT ZU KIEL, KIEL, GERMANY RECEIVED AUGUST 20, 1999; REVISED TYPESCRIPT ACCEPTED AUGUST 15, 2000 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 am- phibolite- to greenschist-facies upper crust during regional meta- morphism. In 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 hightemperature 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 nearisobaric 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 KEY WORDS: Northern Calabria; lower-crustal cross-section; Hercynian medium-pressure high-temperature metamorphism; migmatitic granulites INTRODUCTION 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 km 2 ) of granulite-facies, dominantly metapelitic migmatites, which was metamorphosed dur- ing the Hercynian orogeny. This unit (the Monte Gari- glione 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 Corresponding author. Fax: vs@min.uni-kiel.de Oxford University Press 2001

2 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Hercynian nappe boundary within the Monte Gariglione the same Hercynian crustal segment may store valuable Complex. The granulite-facies metapelites of the Sila information about the former thermal conditions at massif resemble those of the southern Calabrian (Serre) different levels of that Hercynian crust. massif, where they form the lowermost part of an exposed The relatively simple tectonic model for the Sila nappe continuous profile through the Hercynian continental described above, which assigns the granulite-facies rocks crust (Schenk, 1984, 1990). to a single Hercynian unit ( Monte Gariglione Complex), Until now, modern petrological methods have not been is, however, debated (e.g. Dubois, 1970, 1976; Messina applied to the high-grade gneisses of northern Calabria. et al.; 1991; Borghi et al., 1992; and references therein). Therefore, the principal aim of this study is to check According to, for example, Amodio-Morelli et al. (1976), whether mineral chemistry and phase relations of metapelites Zanettin Lorenzoni (1980) and Lorenzoni & Zanettin point to a continuous metamorphic gradient Lorenzoni (1983), the high-grade metamorphic rocks are through the Monte Gariglione Complex or to the existence subdivided into two Alpine nappes of which only the of nappe slices of different metamorphic grade. upper one is characterized by a considerable amount of The second aim of this paper is to evaluate the thermal migmatite. These workers proposed a Hercynian nappe and baric conditions of the granulite-facies segment so boundary within the migmatitic unit separating highgrade as to reconstruct its former structural level within the metamorphic rocks from overlying intermediateas continental crust. Furthermore, the P T paths of these grade rocks. As will be discussed below, on the basis of rocks are deduced to gain information on the geodynamic metamorphic phase equilibria and mineral chemistry causes of metamorphism and to compare it with that of systematics of the granulite-facies rocks, we favour the the lower crust exposed in southern Calabria. simpler of the two tectonic models for the Monte Gari- glione Complex. In addition, we will deduce that the high-grade rocks of this unit represent an intermediate GEOLOGICAL SETTING OF crustal level in relation to the upper- and the lowercrustal rocks exposed in southern Calabria (Schenk, 1984; NORTHERN CALABRIA Graessner & Schenk, 1999). 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. FIELD RELATIONS AND MINERAL The Alpine mountain system became dismembered as a result of later movements of microplates in the western ASSEMBLAGES Mediterranean (e.g. Alvarez, 1976). Some workers (Amo- To evaluate the metamorphic field gradient in the highgrade dio-morelli et al., 1976; Scandone, 1979; Bonardi et al., rocks of the Sila nappe, metapelitic and metabasic 1982) have regarded the Calabrian massif as a piece of rocks were collected along two SW NE-trending traverses Adriatic crust that was thrust first (Alpidic stage) onto through the southern part of the Monte Gariglione ophiolitic units in the west and later (Appenninic stage), Complex ( Fig. 2). A few additional thin sections were together with its underlying base, back onto the Appenninic provided by N. Le Breton (see Le Breton, 1983). carbonate platform in the east. In contrast, The Monte Gariglione Complex ( Fig. 2) consists mainly Dietrich (1988) regarded the northern Calabrian massif of metapelites as well as subordinate garnet biotite as derived from the European continent, thrust eastwards gneisses. Both rock types contain numerous, mainly concordant over the ophiolitic units and resting now with its ophiolitic leucosomes. The amount of these is roughly base upon the Adriatic platform units. In contrast to estimated at 10 vol. % but reaches locally 20 vol. % in the situation in southern Calabria, the supposition of the upper part of the section. These leucosomes form Hercynian basement rocks on ophiolites is undisputed in lenses and bands mostly parallel to the foliation (stromatic northern Calabria. leucosomes) and are interpreted as crystallized partial The tectonically higher units of the Sila massif in melts of the host metapelites. Silica-undersaturated aluminous northern Calabria ( Monte Gariglione, Bocchigliero and restitic rocks are found as small layers on a Mandatoriccio Complexes) form the so-called Sila thin-section scale but only rarely on a hand-specimen nappe of Dubois (1970, 1976), which consists in its deeper scale. In most of the coarse-grained gneisses a well- part of granulite-facies gneisses, the Monte Gariglione developed foliation affected by isoclinal folding is recognizable. Complex. Late Hercynian granitoids intruded these It dips >20 30 towards the east. underlying high-grade gneisses as well as the overlying The metapelites have been metamorphosed at sillimanite amphibolite-facies upper-crustal gneisses and low-grade + K-feldspar grade, so that prograde muscovite Palaeozoic rocks (Bocchigliero and Mandatoriccio Com- + quartz was not stable during peak metamorphism. plexes) (Fig. 1). The Sila nappe thus consists of rocks Fourteen mineral assemblages have been observed in from very different crustal levels and if they were part of the metapelites, among which the most common are 932

3 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA Fig. 1. Simplified geological map of the Sila massif in northern Calabria [modified from Ayuso et al. (1994)]. Box shows area of Fig. 2. Grt Sil Crd [mineral abbreviations are from Kretz comprise the primary magmatic assemblage Ol + Pl + (1983)] and Grt Sil Kfs, both in equilibrium with Qtz Bt Opx ± Cpx and late-stage reaction rims around olivine ± Pl. The univariant KFMASH assemblage Grt of Opx + Cpx + Spl and Prg Hbl + Spl. This rock Bt Crd Kfs Sil Qtz, forming an isograd in the Serre type is similar to the Rovale Gabbro (Acquafredda et al., massif further to the south (Schenk, 1990), has been 1992; Caggianelli et al., 1994) in the northern part of the found at 24 localities and at very different structural study area and resembles metagabbroic rocks described levels of the Sila massif. A discontinuity in metamorphic by Dubois (1976) in the west of the Monte Gariglione grade from high-grade granulite-facies to intermediategrade Complex. rocks, which would support the model of tectonic Marbles occur as lenses and layers (100 m to 1 km slices in the Monte Gariglione Complex as proposed by scale; Fig. 2) within the metapelites but were not collected Zanettin Lorenzoni (1980) and Lorenzoni & Zanettin systematically for this study. 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 PETROGRAPHY AND MINERAL (Fig. 2). CHEMISTRY Metabasites and some intermediate rock types of en- To constrain the P T conditions of metamorphism of derbitic composition are mainly restricted to two large the Monte Gariglione Complex, nine metapelites and bodies near the village of Villaggio Mancuso and to a four metabasic rocks were studied with the electron few small lenses and layers within the metapelites ( Fig. microprobe (see Table 1 for assemblages), using the 2). Leucosomes in metabasites have not been observed CAMECA CAMEBAX microprobe at the University in the field or on a thin-section scale but cannot strictly of Kiel in the wavelength-dispersive mode with 15 kv be ruled out, because of the scarcity of outcrops. In acceleration potential and 15 na beam current. Cormetabasic rocks seven different mineral assemblages have rections were carried out with the PAP correction probeen distinguished, not considering amphibole as a crit- gram (Pouchou & Pichoir, 1984). Representative analyses ical mineral for subdivision ( Fig. 2). The granulite facies are presented in Tables 2 6. 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 Metapelitic migmatites medium-pressure type of Green & Ringwood (1967). Texturally, two types of garnet can be distinguished Olivine metagabbros are common as river pebbles in in melanosomes: coarse 1 5 mm anhedral to subhedral the southwestern part of the study area (Fig. 2). They grains and small euhedral ones ( mm). Both types 933

4 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Fig. 2. Regional distribution of mineral assemblages of metapelitic and metabasic rocks of the Monte Gariglione Complex in the southern part of the Sila massif ( Sila Piccola ). The outlines of the main geological units are modified from Lorenzoni & Zanettin Lorenzoni (1983) and Ayuso et al. (1994). Location of map area is given in Fig. 1. 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. 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 Alm 63 82, Prp 13 32, Grs 2 4, Sps 1 3. The X Fe values [X Fe = Fe/(Fe + Mg)] are 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). 934

5 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA Table 1: Mineral assemblages of the analysed samples; sample locations are given in Fig. 2; minerals in square brackets are retrograde Sample Qtz Pl Bt Grt Sil Crd Kfs Ilm Rt Spl [Crd] [Myrm] [St] [And] [Ms] Metapelites b/ C8F Sample Qtz Pl Bt Grt Opx Ilm Hbl Cum [Grt] [Hbl] [Cum] [Myrm] [Bt] Metabasites 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 in- clusions 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 as- semblages is locally evident ( Fig. 3e and g). The cores 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). 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 X Fe 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. X Fe values of matrix grains range from 0 34 to 0 61 ( Table 3). Most biotite has lower Ti contents and X Fe values than biotite of comparable metamorphic grade from New England (except for sample ; 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. Coarse-grained sillimanite is a major constituent of the melanosomes and is oriented parallel to the matrix foliation. Retrograde prismatic sillimanite intergrown 935

6 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Table 2: Representative garnet analyses from metapelites and metabasites (total iron as Fe 2+ ) Rock type: Metapelite Metabasite Sample b/ C8f no.: core core core core core core core core core core core core seam outer core core euhedral leucosome core elongated idiomorph SiO Al 2 O FeO MgO MnO CaO Total Structural formulae on a basis of 24 oxygens Si Al Fe Mg Mn Ca Total Fe/(Fe+Mg) X Alm X Grs X Prp X Sps

7 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA Table 3: Representative biotite analyses from metapelites and metabasites (total iron as Fe 2+ ) Rock type: Metapelite Metabasite Sample b/ C8f no.: matrix matrix matrix matrix matrix matrix matrix matrix cont.grt matrix matrix matrix SiO TiO Al 2 O Cr 2 O FeO MgO MnO CaO Na 2 O K 2 O Total Structural formulae on a basis of 11 oxygens Si Al(IV) Al Ti Cr Fe Mg Mn Total Ca Na K Total Fe/(Fe+Mg) of matrix grains and of coarse-grained cordierites at grains, which is attributed to late-stage cation exchange. the margin of leucosomes are unzoned and uniform in Plagioclase, like perthitic K-feldspar, is a main con- composition within each sample (Table 4). The X Fe range stituent of the quartzofeldspathic leucosomes but volumetrically is decreasing systematically from east towards more important than the latter. In the the west of the Monte Gariglione Complex, which, when commonly concordant leucocratic lenses and veins both coexisting with Grt + Sil + Qtz, suggests increasing feldspars crystallize mainly at the margins. K-feldspar is metamorphic pressures (or changing ah 2 O) in this dir- usually embayed and replaced by late-stage myrmekitic ection (e.g. Thompson, 1976; Holdaway & Lee, 1977). plagioclase quartz symplectites. This has led in many Rimward decrease of X Fe is explained by retrograde samples to complete replacement of K-feldspar, mainly Fe Mg exchange with matrix minerals. The Na content in the eastern part of the Monte Gariglione Complex. of the unzoned cores decreases to the west (from 0 05 The cores of plagioclase are essentially unzoned and to 0 01 cations p.f.u.) suggesting rising metamorphic uniform within a sample. No compositional difference temperatures in this direction (Mirwald, 1986). X Fe values was found between plagioclase in leucosomes and that of late-stage cordierite coronas surrounding garnet are in melanosomes. The analysed core compositions are in similar to or lower than those of the rims of the matrix the range of An (Table 5). In most analysed 937

8 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Table 4: Representative cordierite analyses from metapelites Sample no.: b/ C8f core core core core core core core core core SiO TiO Al 2 O FeO MgO MnO CaO Na 2 O K 2 O Total Structural formulae on a basis of 18 oxygens Si Al Al Ti Total Fe Mg Mn Total Ca Na K Total Fe/(Fe+Mg) plagioclase grains the rims (<0 1 mm) are characterized and 0 32 wt % but reaches wt % (sample by a moderate decrease of Ca suggesting retrograde reequilibration 96). Site and charge-balance calculations indicate a low of the rims during isobaric cooling. This magnetite component (Fe 3+ = cations p.f.u. would correspond to the rimward increasing grossular based on 32 oxygens). Zonation trends have not been content of some garnets if the Pl Sil Grt Qtz equilibrium identified. controls this zonation. K-feldspar is commonly perthitic Staurolite has been found only as fine-grained retro- microcline. The composition of the exsolved host is in grade spherical to euhedral crystals. It occurs at the the range of Kfs margins of biotite or in the vicinity of garnet, and as Green to brown spinel may occur as tiny inclusions in small needles at the seams of cordierite in biotite garnet (sample 33-97), as symplectitic intergrowth within sillimanite-rich layers. In the analysed sample , large (<5 mm) euhedral sillimanite porphyroblasts or X Fe ranges between 0 80 and 0 88 and is therefore lower rarely in symplectitic intergrowth with cordierite replacing than that of associated garnet rims (X Fe = ). garnet. More common are inclusions in cordierite MnO and ZnO contents are wt % and 2 6 in quartz-deficient layers of the migmatites ( Fig. 3h and 3 2 wt %. No regular zonation has been identified in i). The spinel is rich in hercynite (Hc ) and gahnite individual grains. (Gh ) components. X Fe varies between 0 76 and Muscovite + quartz as a retrogressive assemblage is 0 85, but the compositions are relatively constant in any very common in the structurally higher part of the unit. one specimen. Cr 2 O 3 content is generally between 0 25 Its formation after K-feldspar + sillimanite is indicated 938

9 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA Table 5: Representative plagioclase analyses from metapelites and metabasites Rock type: Metapelite Metabasite Sample b/ no.: core core core core core core core core core rim core core core SiO Al 2 O Fe 2 O CaO Na 2 O K 2 O Total Structural formulae on a basis of 8 oxygens Si Al(IV) Total Fe Ca Na K Total An Ab Kfs by the symplectitic intergrowth of muscovite with quartz is indicated by coronas around plagioclase separating it or by inclusions of fibrolitic sillimanite and/or pseudomorphs from orthopyroxene (sample 29-96, Fig. 3k) and from after sillimanite ( Fig. 3j) (Ashworth, 1972). hornblende cummingtonite. (4) Late-stage poikiloblastic Phengite component is low (Si = cations garnet grows at the expense of biotite + quartz symp.f.u.). plectites, which replace orthopyroxene in a sample (84- Ilmenite and rutile are common accessory minerals 96) of enderbitic composition ( Fig. 3l). and occur as primary matrix phases and as inclusions in The cores of textural garnet types (1) and (2) are garnet. Measured MnO contents are in the range of essentially unzoned and are in the range of Alm 62 67, wt % and wt %, respectively. Ad- Prp 19 30, Grs 5 12, Sps 2 3 (Fig. 6, Table 2). Only the netlike ditional accessories are monazite, zircon, apatite, pyrrhohigher garnet (sample ) is zoned and shows a slightly tite and graphite. grossular content in the core, interpreted as a relic of the prograde growth zoning. The X Fe values are and, like grossular, increase towards the rims. Metabasites and intermediate rock types It is obvious from the Ca Fe Mg ternary in Fig. 7 that garnet orthopyroxene-bearing rocks without hornblende Garnet is present in 19 of the 41 samples studied. as a Ca-buffering phase contain garnet poorer in grossular Four textural types can be distinguished: (1) net-like component than those with hornblende. The comto subhedral porphyroblasts ( mm); (2) smaller positions of the late-stage coronitic garnet are higher in euhedral grains (<0 3 mm). The net-like grains, enclosing grossular but lower in pyrope contents. quartz, plagioclase and biotite, suggest that no penetrative Plagioclase is a major constituent of all the metabasic deformation took place after garnet growth. The two rocks. The cores of the anhedral grains ( mm) are textural types together occur only in two of the analysed unzoned with uniform compositions within a thin section samples ( , 84-96). (3) Late-stage garnet formation (An ; Table 5). Most of the analysed plagioclase grains 939

10 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Table 6: Representative analyses of orthopyroxene, hornblende and cummingtonite from metabasites (total iron as Fe 2+ ) Sample no.: Mineral: opx opx opx opx hbl hbl cum cum core core core core core core core core SiO TiO Al 2 O Cr 2 O FeO MgO MnO CaO Na 2 O K 2 O Total Structural formulae on a basis of 6 oxygens (opx) and 23 oxygens (amphibole), ignoring H 2 O Si Al(IV) Al Ti Cr Fe Mg Mn Ca Na K Total Fe/(Fe+Mg) are characterized by an increase of anorthite contents at the increase of grossular component in coexisting garnet the rims. reflect cooling (Bégin & Pattison, 1994; Fitzsimons & Orthopyroxene occurs in nine of the 41 rock samples Harley, 1994). Common retrograde breakdown products and only in the western part of the study area, and forms of orthopyroxene are cummingtonite and/or hornblende net-like to anhedral blasts ( mm). In the enderbitic and biotite + poikiloblastic garnet (sample 84-96). sample net-like grains, enclosing quartz, plagioclase Amphibole occurs as brown or green hornblende and and small euhedral garnet ( Fig. 3l), suggest that no as cummingtonite. The latter two are in most cases penetrative deformation took place after peak metamorphism. retrograde breakdown products of pyroxene. Brown and The X Fe and Al(VI) values are very similar in dark brown green hornblende, which occurs locally as the analysed samples [X Fe = and Al(VI) = a matrix mineral and as small inclusions in plagioclase, cations p.f.u.; Fig. 7, Table 6], except for might be a relic of the prograde stage. Patchy intergrowths sample 27-97, which has lower X Fe and higher Al(VI) of hornblende and cummingtonite are common; less values (0 42 and 0 08). Where in contact with garnet, common are hornblende lamellae within cummingtonite, orthopyroxene shows narrow rims slightly lower in X Fe possibly exsolved from formerly homogeneous clino- as a result of retrograde Fe Mg exchange. The moderate amphiboles (e.g. Klein et al., 1996). Seams of hornblende rimward decrease of Al content in orthopyroxene and around cummingtonite also occur. The calcic amphiboles 940

11 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA Fig. 3. (a) (d) 941

12 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Fig. 3. (e) (h) 942

13 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA Fig. 3. (i) (l) 943

14 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Fig. 4. Representative garnet zoning profiles from metapelites of structurally higher (a, b) and deeper parts (c, d) of the Monte Gariglione Complex. Each profile extends from rim to rim through the core of the garnet grains. Location of samples is given in Fig. 2. range in composition from actinolite to mainly mag- garnet and late-stage breakdown product of orthopyroxene. nesiohornblende. The generally anhedral grains are unzoned The lowest X Fe values are found in biotite and their Si, Al and Ti contents are in the range inclusions in garnet. X Fe varies from 0 25 to 0 46 and the of , and cations p.f.u. ( Table Ti contents range between 0 14 and 0 29 cations p.f.u. 6). Their X Fe values are always lower ( ) than (Table 3) and are thus higher than in metapelitic those of coexisting cummingtonite ( ) and orthopyroxene biotite. (0 48). Unexsolved hornblende has higher Clinopyroxene occurs in few metabasites of the western X Fe values than that intergrown with cummingtonite. part of the study area. The grains are euhedral to Biotite occurs in most of the studied samples, but its subhedral and >1 mm in diameter. Some grains contain modal abundance is low (<3 vol. %). Three textural types hornblende lamellae, which seem to have been formed of biotite can be distinguished: matrix phase, inclusions in after exsolved pyroxene. Larger porphyroblasts (>2 mm) Fig. 3. (a) Prograde sillimanite and cordierite inclusions in garnet rim. Garnet shows resorption by a late-stage corona of cordierite formed during decompression (sample ). (b) Retrograde decomposition of garnet to an aggregate of biotite, sillimanite and quartz in leucosome (leuc) (sample 69c/85). (c) Garnet at the rim of a melanosome partly resorbed by biotite quartz symplectites and plagioclase (sample 49-97). (d) Biotite dehydration melting: skeletal biotite at the margin of leucosome (leuc), partly rotated into the former melt (sample 69c/85). (e) Andalusite + chlorite grown at the expense of cordierite during cooling (sample C 47 B). (f ) Undeformed late-stage andalusite overgrowing foliated matrix of fibrolitic sillimanite and biotite (sample ). (g) Late-stage kyanite + chlorite embaying cordierite (sample C1-22 II). (h) Inclusions of sillimanite, spinel and rounded relics of biotite in cordierite. Spinel has overgrown the sillimanite inclusions (sample ). (i) Late-stage sillimanite seams and retrograde biotite formed between spinel and cordierite during near-isobaric cooling. Reverse of reaction seen in (h) (sample C47 B). ( j) Coarse-grained randomly oriented muscovite quartz pseudomorphs after sillimanite, next to myrmekite in leucosome. Knots of fibrolite possibly formed during dehydration melting of muscovite (sample ). (k) Retrograde reaction rims of garnet developed between orthopyroxene and plagioclase during near-isobaric cooling (sample 29-96). (l) Biotite quartz symplectites replacing net-like orthopyroxene, and showing overgrowth by late-stage poikilitic garnet (Grt poik ). Small euhedral garnet grains (Grt id ) and orthopyroxene are interpreted as the peak metamorphic assemblage (sample 84-96). 944

15 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA Fig. 5. Plot of Ti (per 11 oxygens) as a function of Fe/(Fe + Mg) for metapelitic matrix biotites along the northern (a) and southern profile (b) through the Monte Gariglione Complex. Open and closed symbols refer to core and rim compositions of biotite. Unshaded fields: compositional ranges of biotite from metamorphic zones in New England (Robinson et al., 1982). Location of samples is given in Fig. 2. Fig. 6. Zoning profiles of garnet porphyroblasts from metabasites of the Monte Gariglione Complex. Each profile extends from rim to rim through the core of the garnet grains. Location of samples is given in Fig. 2. 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. 945

16 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 Fig. 7. Lower portion of a Ca Mg Fe ternary (inset) showing the compositions of coexisting garnet, orthopyroxene and hornblende from metabasic rock samples of the Monte Gariglione Complex. Location of samples is given in Fig. 2. 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 X Fe 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 Bt + Sil + Qtz = Grt + Crd + Kfs + L (3) was reached. This would lead to coarse-grained peak- MINERAL REACTION HISTORY metamorphic garnet cordierite K-feldspar biotite Prograde reactions sillimanite quartz-bearing assemblages coexisting with The prograde reaction history in metapelites is best leucosomes, an assemblage that is found throughout the constrained by mineral inclusions in garnet and the study area. In consequence, biotite-dehydration reactions presence of quartzofeldspathic leucosomes. The presence (2) and (3) account for the formation of garnet + corof abundant leucosomes in most of the rock samples dierite-bearing leucosomes. The rare cordierite inclusions (>10 20 vol. %) and biotite quartz symplectites at the in garnet rims ( Fig. 3a) reflect the reaction margins of the leucocratic segregations (Fig. 3d) indicates that dehydration melting has occurred. Sillimanite inclusions Crd = Grt + Sil + Qtz (4) in garnet and cordierite are evidence that pro- which points to a pressure increase during prograde grade metamorphism proceeded within the stability field metamorphism. of sillimanite. The absence of muscovite and the presence Prograde reaction textures in metabasites and interof the assemblage sillimanite + K-feldspar + quartz in mediate rock types are poorly preserved. Dehydration all samples indicates temperature conditions above the melting of hornblende, which occurs at higher temdehydration melting of muscovite according to the re- peratures than any of those involving biotite (Clemens action & Vielzeuf, 1987), was not observed in the studied rocks. Ms + Pl + Qtz = Sil + Kfs + L. (1a) In the enderbitic sample 84-96, dehydration, which may have taken place at the metamorphic peak, led to the Some of the clots of fibrolitic sillimanite within the formation of large net-like orthopyroxene porphyroblasts leucosomes may be prograde products of the latter re- together with small euhedral garnet as a result of a action. Because many metapelitic rock types may not reaction of the form ( Fig. 3l) contain enough muscovite to produce >10 vol. % of melt by this reaction (Clemens & Vielzeuf, 1987; Spear et al., Bt + Qtz = Opx + Grt id + Kfs + V/L. (5) 1999), further melt reactions involving biotite have to be A Ti phase may have participated in this reaction as taken into account for the Calabrian metapelites. described for intermediate and felsic rock types of the Inclusions of biotite and plagioclase in the cores and Serre massif in southern Calabria (Schenk, 1984). K- sillimanite in the rims of garnet point to the divariant feldspar has not been found in the sample described garnet + K-feldspar-producing melting reaction here, but a K-bearing fluid or potassic feldspar must Bt + Sil + Pl + Qtz = Grt + Kfs + L. (2) have been involved in this dehydration of biotite. This biotite-melting reaction may produce a large amount of H 2 O-undersaturated melt in rocks of pelitic composition (e.g vol. %; Clemens & Vielzeuf, 1987; Retrograde reactions Le Breton & Thompson, 1988) depending on the initial The metapelitic and metabasic rocks were strongly affected modal amount of biotite and the P T conditions attained by retrograde metamorphism. This is obvious by the during melting. The garnet rims, which contain numerous abundance of decompression textures and the even more sillimanite inclusions parallel to the matrix foliation as a common rehydration reactions. Decompression after result of an overgrowth of the pre-existing sillimanite peak metamorphism is documented by textures such as fabric, display in some samples slightly higher grossular cordierite rims around garnet in metapelites, which were 946

17 GRAESSNER AND SCHENK HERCYNIAN DEEP CRUSTAL SECTION, N CALABRIA formed via the reverse of the continuous reaction (4) Crd + Grt + Sil + V = St +Qtz. (10) (Fig. 3a) or as a result of In rock samples with retrograde muscovite, exclusively Grt + Sil + L = Crd + Kfs (6) in those of the eastern part, staurolite was formed at which is nearly independent of temperature (Spear et al., biotite rims and in the vicinity of garnet by the reaction 1999). Bt + Sil ± Grt + V = St + Ms + Qtz. (11) In small quartz-deficient domains in metapelites spe- Further cooling led to the decomposition of cordierite cific reaction textures occur: symplectitic intergrowths of via the reactions (Fig. 3e) spinel and cordierite and inclusions of sillimanite and spherical relics of prograde biotite in cordierite, and in Crd + Bt = Chl + Ms + Qtz (12) some cases also in spinel. These point to the divariant KFMASH reaction ( Fig. 3h) and Bt + Sil = Crd + Spl + Kfs ± Ilm + L. (7) Crd = Chl + And + Qtz (13) This reaction has been proposed by Brown (1998) and or (Fig. 3g) Whittington et al. (1998) for low-pressure rocks of the Crd = Chl + Ky + Qtz. (14) Ryoke Belt and the Nanga Parbat. As the reaction Reactions (12) and (13) are documented in the eastern has a positive slope the observed textures also indicate part of the study area, whereas the kyanite-producing decompression after peak metamorphism. Importantly, reaction (14) is found only in one sample from the western in some cases the reverse of reaction (7) took place in part ( Fig. 2). Retrograde andalusite locally overgrows the same sample, which suggests falling temperatures fibrolite and sillimanite and is therefore clearly of latesubsequent to decompression. This late-stage reaction is stage origin (Fig. 3f ). Magnesite instead of chlorite has documented by sillimanite seams around spinel inclusions never been observed among the decomposition products in cordierite and newly grown biotite (Fig. 3i). In one of cordierite. This reflects lower X CO2 values of the fluid quartz-deficient rock (sample ) decompression is phase during retrogression than in metapelites of southern documented by development of spinel cordierite sym- Calabria, where magnesite is very common (Schenk, plectites replacing garnet, which can be explained by the 1990). reaction In metabasites textures such as garnet rims between Grt + Sil = Spl + Crd. (8) plagioclase and orthopyroxene suggest a nearly isobaric cooling path during which the reaction ( Fig. 3k) Again, a subsequent temperature fall is indicated by the replacement of garnet and locally of cordierite by Opx + Pl = Grt + Qtz (15) aggregates of biotite + sillimanite ± plagioclase. This has been crossed. This texture was found in the west and rehydration may be due to infiltration of aqueous fluids in one sample only. Rehydration reactions are widespread either released from crystallizing leucosomes or from in the study area and have commonly affected ortho- and external sources. A back-reaction involving melt [reverse clinopyroxene. Cummingtonite and/or green hornblende of reactions (2) and (3)] instead of vapour might also be surrounding orthopyroxene may have formed as a result possible at some higher temperatures. Mats of fibrolitic of the reactions sillimanite intergrown with biotite replacing garnet and/ or cordierite may have formed by these reactions ( Fig. Opx + Pl + Ilm + V = Hbl + Qtz (16) 3b). In aggregates of biotite and plagioclase in which sillimanite is absent, garnet has been consumed by the and reaction (Fig. 3c) Opx + Qtz + V = Cum. (17) Grt + Kfs + L/V = Bt + Pl +Qtz. (9) Partial replacement of clinopyroxene by hornblende Some coarse-grained randomly oriented muscovite flakes quartz symplectites reflects the reaction in leucosomes surrounding fibrolite and/or pseudomorphs Cpx + Pl + Ilm + V = Hbl + Qtz. (18) after sillimanite may have been formed as a Textural features such as garnet seams separating result of the reversal of reaction (1a) and (1b) (Kfs + Sil cummingtonite from plagioclase, all in close association + L/V = Ms + Qtz ± Pl; Fig. 3j). In the western with biotite and hornblende, could be attributed to the part of the study area, further temperature fall was prograde CNFMASH reaction accompanied by the formation of small staurolite grains in the vicinity of garnet and in cordierite rims suggesting Cum + Bt + Pl + Ilm = Grt + Hbl + Qtz + V. the reaction (19) 947

18 JOURNAL OF PETROLOGY VOLUME 42 NUMBER 5 MAY 2001 This reaction has been described by Hollocher (1991) samples (Fig. 5), X Fe of biotite decreases, from east to in Fe-rich metabasic rocks of central Massachusetts. west through the Monte Gariglione Complex, as does However, in the Sila massif, the garnet coronas are that of coexisting garnet. The preservation of systematic associated with rare relics of orthopyroxene as inclusions phase relations is surprising in view of the retrogression in cummingtonite, which suggest retrograde formation that affected mineral textures and chemistry. A possible of cummingtonite by rehydration of the granulite-facies explanation is that unzoned cores of garnet and cordierite orthopyroxene. In this case the garnet reaction rims are porphyroblast retained their peak-metamorphic compresumably formed like cummingtonite at a retrograde position and retrogressive effects are restricted to the stage via a reaction such as (Hollocher, 1991) mineral rims. However, the influence of retrograde metamorphism on biotite chemistry cannot be neclected (see Opx + Pl + V = Grt + Cum + Hbl + Qtz. (20) Fig. 5) and estimates of metamorphic conditions on this mineral are less reliable. Biotite quartz symplectites and poikiloblastic garnet overgrowing orthopyroxene in the enderbitic sample record a rehydration reaction such as (Schenk, 1984, fig. 3l) Opx + Kfs + Ilm + V = Bt + Grt poik + Qtz. P T CONDITIONS ALONG THE (21) CRUSTAL SECTION First indications on the P T conditions during peak metamorphism are obtained from mineral assemblages PHASE RELATIONS OF of the high-grade gneisses in the western part of the Monte Gariglione Complex. The stability of the assemblage Opx METAPELITES + Pl + Qtz ± Grt and the absence of Grt Cpx Qtz The mineral assemblages Grt + Sil + Crd + Qtz and in metabasites point to the P T field of medium-pressure Grt + Sil + Kfs + Bt + Qtz are most common in granulites ( C at5 7 kbar; Green & Ringwood, the studied metapelites ( Fig. 2) and can be used to identify 1967, 1972). Corona textures in carbonate rocks suggest regional differences in metamorphic grade within the crossing of the reaction Fo + Cal + V = Di + Dol Monte Gariglione Complex. In these assemblages the during cooling from peak-metamorphic temperatures. ferromagnesian minerals become more Mg rich with Therefore, the latter must have been higher than increasing metamorphic grade (Thompson, 1976). From C at5 7 kbar according to the experiments of the regional variations of the X Fe values of coexisting Käse & Metz (1980). Peak-metamorphic conditions must minerals, a metamorphic gradient along the two east west have been below Bt + Qtz- or Bt + Grt + Qtztraverses through the Sila massif can be deduced. This breakdown (< C at5 7 kbar; Spear et al., 1999; relationship can be shown in an AFM diagram of the Fig. 15), as orthopyroxene is absent in metapelites of the model KFMASH system, projected from the appropriate Sila massif. excess components. As MnO and CaO contents in garnet P T conditions along the two studied east west profiles are very low (1 3 mol % spessartine, 2 4 mol % grossular) and are also minor in the coexisting minerals, their are first constrained by conventional geothermo- influence on the projected positions of the minerals can barometry, applying well-calibrated net-transfer and be ignored. cation-exchange reactions uniformly at 5 kbar for tem- Figure 8 shows two Al perature estimates and 700 C for pressure estimates 2 O 3 FeO MgO triangles for rocks with the Grt + Sil + Crd + Qtz assemblage ( Table 7). These results will be compared with results (projected from quartz) from the northern and southern obtained from multi-reaction calculations using the profiles. A decrease in the X Fe values of cores of garnet TWEEQU program (Berman, 1991). For estimates of and matrix cordierite from east to west of the study area peak-metamorphic conditions, the core compositions of results in a shift of the three-phase field Grt Sil Crd coexisting matrix minerals were used, as those are as- towards the Mg side of the AFM triangle. From this a sumed to preserve the highest metamorphic grade. For continuous pressure increase (and a smaller temperature the enderbitic sample 84-96, peak conditions were es- rise) from east to west through the Monte Gariglione timated using the composition of the small euhedral Complex can be inferred. A corresponding temperature garnet grains, which are thought to have been formed gradient from decreasing X Fe in coexisting garnet biotite together with orthopyroxene at the peak of meta- pairs is not well established by garnet biotite therrim morphism. Retrograde conditions were calculated using mometry. Despite the strong retrogression leading to compositions of matrix minerals that are in contact relatively homogeneous biotite compositions in individual with each other ( Table 8). 948