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
THORSTEN GRAESSNER and
VOLKER SCHENK,*
INSTITUT FÜR GEOWISSENSCHAFTEN DER CHRISTIAN-ALBRECHTS-UNIVERSITÄT ZU KIEL, 24098 KIEL, GERMANY
Received
August 20, 1999;
Revised typescript accepted
August 15, 2000
 |
ABSTRACT
|
|---|
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–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
|
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 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.
|
GEOLOGICAL SETTING OF NORTHERN CALABRIA
|
|---|
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.
View larger version (39K):
[in this window]
[in a new window]
|
Fig. 1. Simplified geological map of the Sila massif in northern Calabria [modified from Ayuso et al. (1994)]. Box shows area of Fig. 2.
|
|
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
|
|---|
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
).
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.
|
PETROGRAPHY AND MINERAL CHEMISTRY
|
|---|
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.
View this table:
[in this window]
[in a new window]
|
Table 1: Mineral assemblages of the analysed samples; sample locations are given in Fig. 2; minerals in square brackets are retrograde
|
|
View this table:
[in this window]
[in a new window]
|
Table 6: Representative analyses of orthopyroxene, hornblende and cummingtonite from metabasites (total iron as Fe2+)
|
|
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.
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
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).
View larger version (43K):
[in this window]
[in a new window]
|
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.
|
|
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.
View larger version (41K):
[in this window]
[in a new window]
|
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.
|
|
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.
View larger version (36K):
[in this window]
[in a new window]
|
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.
|
|
View larger version (19K):
[in this window]
[in a new window]
|
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.
|
|
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.
|
MINERAL REACTION HISTORY
|
|---|
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
Some of the clots of fibrolitic
sillimanite within the leucosomes may be prograde products of
the latter reaction. Because many metapelitic rock types may
not contain enough muscovite to produce >10 vol. % of melt
by this reaction (Clemens & Vielzeuf, 1987
; 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
This biotite-melting
reaction may produce a large amount of H
2O-understurated melt
in rocks of pelitic composition (e.g. 20–30 vol. %; Clemens
& Vielzeuf, 1987
; 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
was reached. This would lead
to coarse-grained peak-metamorphic garnet–cordierite–K-feldspar–biotite–sillimanite–quartz-bearing
assemblages coexisting with leucosomes, an assemblage that is
found throughout the study area. In consequence, biotite-dehydration
reactions (2) and (3) account for the formation of garnet +
cordierite-bearing leucosomes. The rare cordierite inclusions
in garnet rims (Fig. 3a) reflect the reaction
which
points to a pressure increase during prograde metamorphism.
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)
A Ti phase may have
participated in this reaction as described for intermediate
and felsic rock types of the Serre massif in southern Calabria
(Schenk, 1984
). 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
which
is nearly independent of temperature (Spear
et al., 1999
).
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)
 |
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
Again,
a subsequent temperature fall is indicated by the replacement
of garnet and locally of cordierite by aggregates of biotite
+ sillimanite ± plagioclase. This rehydration may be
due to infiltration of aqueous fluids either released from crystallizing
leucosomes or from external sources. A back-reaction involving
melt [reverse of reactions (2) and (3)] instead of vapour might
also be possible at some higher temperatures. Mats of fibrolitic
sillimanite intergrown with biotite replacing garnet and/or
cordierite may have formed by these reactions (Fig. 3b). In
aggregates of biotite and plagioclase in which sillimanite is
absent, garnet has been consumed by the reaction (Fig. 3c)
Some coarse-grained randomly oriented muscovite
flakes in leucosomes surrounding fibrolite and/or pseudomorphs
after sillimanite may have been formed as a result of the reversal
of reaction (1a) and (1b) (Kfs + Sil + L/V = Ms + Qtz ±
Pl; Fig. 3j). In the western part of the study area, further
temperature fall was accompanied by the formation of small staurolite
grains in the vicinity of garnet and in cordierite rims suggesting
the reaction
In rock samples with
retrograde muscovite, exclusively in those of the eastern part,
staurolite was formed at biotite rims and in the vicinity of
garnet by the reaction
Further cooling led to the decomposition of cordierite via the reactions (Fig. 3e)
and
or (Fig. 3g)
Reactions
(12) and (13) are documented in the eastern part of the study
area, whereas the kyanite-producing reaction (14) is found only
in one sample from the western part (Fig.
2). Retrograde andalusite
locally overgrows fibrolite and sillimanite and is therefore
clearly of late-stage origin (Fig. 3f). Magnesite instead of
chlorite has never been observed among the decomposition products
of cordierite. This reflects lower
XCO2 values of the fluid
phase during retrogression than in metapelites of southern Calabria,
where magnesite is very common (Schenk, 1990
).
In metabasites textures such as garnet rims between plagioclase and orthopyroxene suggest a nearly isobaric cooling path during which the reaction (Fig. 3k)
has
been crossed. This texture was found in the west and in one
sample only. Rehydration reactions are widespread in the study
area and have commonly affected ortho- and clinopyroxene. Cummingtonite
and/or green hornblende surrounding orthopyroxene may have formed
as a result of the reactions
and
Partial replacement of clinopyroxene by hornblende–quartz
symplectites reflects the reaction
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
This reaction has been described by Hollocher (1991)
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
)
Biotite–quartz symplectites and poikiloblastic
garnet overgrowing orthopyroxene in the enderbitic sample 84-96
record a rehydration reaction such as (Schenk, 1984
, fig. 3l)
|
PHASE RELATIONS OF METAPELITES
|
|---|
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.
View larger version (21K):
[in this window]
[in a new window]
|
Fig. 8. Al2O3–FeO–MgO projection from quartz of the assemblage garnet–cordierite–sillimanite–quartz in metapelitic rocks along the two studied profiles through the Monte Gariglione Complex. In both profiles the three-phase fields show a shift towards the Mg-side of the triangle from the top (in the east) towards the base (in the west). For comparison, the ranges of XFe values of garnet and cordierite in metapelites of the lower crust in the Serre massif of southern Calabria (Schenk, 1984) are shown as heavy bars.
|
|
|
P–T CONDITIONS ALONG THE CRUSTAL SECTION
|
|---|
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.
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).
View this table:
[in this window]
[in a new window]
|
Table 7: Peak-metamorphic pressure and temperature estimates
(a) Metapelites at 700°C and 5 kbar, respectively, and TWEEQU results (all core compositions and matrix biotite)
|
|
View this table:
[in this window]
[in a new window]
|
Table 7: (b) Metabasites and intermediate rock types at 700°C and 5 kbar, respectively, and TWEEQU results (all core compositions)
|
|
View larger version (58K):
[in this window]
[in a new window]
|
Fig. 11. TWEEQU results for two of the analysed metapelitic samples (50-4-96, 57b/85) in the KCFMASH system. Sample locations are given in Fig. 2. Activities of end-members were calculated using the models given below Table 7a. P–T results for 1·5 SD calculated with INTERSX program (Berman, 1991); outlier is reaction (8). The plotted mineral equilibria are listed in Table 9. (For further explanation see text.)
|
|
View larger version (39K):
[in this window]
[in a new window]
|
Fig. 12. TWEEQU results for metabasic samples in the KCFMASH system. Sample locations are given in Fig. 2. Activities of end-members were calculated using the models given below Table 7b. P–T results for 1·5 SD calculated with INTERSX program (Berman, 1991); outliers are reactions (3) and (6) in sample 79-2-96, and reaction (2) in sample 84-96. The plotted mineral equilibria are listed in Table 9. (For further explanation see text.)
|
|
View this table:
[in this window]
[in a new window]
|
Table 8: Retrograde metamorphic temperature and pressure estimates
(a) Metapelites at 5 kbar and 700°C, respectively (all rim compositions of grains in contact)
|
|
View this table:
[in this window]
[in a new window]
|
Table 8: (b) Metabasites and intermediate rock types at 5 kbar and 700°C, respectively, and TWEEQU results (all rim compositions)
|
|
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.
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.
View larger version (40K):
[in this window]
[in a new window]
|
Fig. 10. Estimates of peak-metamorphic P–T conditions and temperatures for mineral rim compositions of metabasites and intermediate rock types. Pressures estimated with the GOPQ geobarometer (PC, Perkins & Chipera, 1985; ENK, Eckert et al., 1991) and the Al solubility in orthopyroxene (HG, Harley & Green, 1982). Temperatures based on the Fe–Mg partitioning between garnet and orthopyroxene (H, Harley, 1984; B, Bhattacharya et al., 1991) and garnet and hornblende (GP, Graham & Powell, 1984). (1) Green & Ringwood (1972), silica-undersaturated rocks; (2) Green & Ringwood (1967), Grt-in for quartz tholeiites.
|
|
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 (GCS
TOP
ABSTRACT
INTRODUCTION
