Chiare tracce di tale fenomenologia sono ampiamente diffuse nella porzione settentrionale dell'area esaminata e ad esse sono da attribuire le discrepanze ...
Miner. Petvogr. Acta Vol. XXXVII, pp. 37-49 (1994)
Relation between isotopic and structural characters in calcite-graphitepairs from Peloritani Mts (Sicily) PAOLO CENSI, PAOLO FERLA and SILVIO G. ROTOLO Istituto di Mineralogia, Petrografia, Geochimica. Universita di Palermo, via Archirafi 36, 90123 Palermo
RIASSUNTO - Viene affrontato in questa nota 10 studio di paragenesi calcitegrafite in terreni metamorfici affioranti nelle immediate vicinanze dell'intrusione pegmatitica di Capo Calava (Sicilia nord-orientale). Su tali associazioni e stato condotto uno studio mineralogico e geochimico-isotopic0 ed i dati ottenuti sono stati paragonati a risultati analoghi registrati in sequenze metamorfiche carbonatiche presenti nella porzione meridionale dei Monti Peloritani. I risultati hanno posto in risalto la presenza di una relazione, fra grado di ordine strutturale della grafite, in termini di dooz,e composizione isotopica della grafite stessa, che sembra non essere influenzata da episodi successivi a1 metamorfismo. L'andamento dei valori di composizione isotopica della calcite sembra invece essere talvolta interessato dagli effetti di scambi isotopici, verificatisi per l'azione di fluidi idrotermali esplicatasi in condizioni di sistema aperto e variabile rapport0 acqua/roccia. Chiare tracce di tale fenomenologia sono ampiamente diffuse nella porzione settentrionale dell'area esaminata e ad esse sono da attribuire le discrepanze osservate fra grado metamorfico proprio di alcune delle paragenesi osservate e temperature isotopiche per esse calcolate in base ai valori di A'7C~,,.g,).Invece le temperature isotopiche stimate su coppie calcite-grafite provenienti da campioni affioranti nella porzione meridionale dell'area studiata variano fra 384°C e 526°C e risultano quindi in buon accord0 con le osservazioni petrografiche. Parole chiave: Grafite, indice di cristallinita, composizione isotopica del carbonio, metamorfismo, Capo Calava, Monti Peloritani. ABSTRACTThis study mainly deals with the calcite-graphite assemblage in metamorphic rocks outcropping near the pegmatoid intrusion of Capo Calava (north-eastern Sicily). A mineralogical and isotopic geochemical study was carried out on these associations. The results were compared with analogous data measured in carbonate sequences in the southern part of the Peloritani Mountains. They indicate a generalized relationship between 6I3C and dOo2in graphite which seems to be refractory to subsequent isotopic exchange phenomena due to water-rock interactions under hydrothermal conditions which affected the whole area. The isotopic temperatures, mainly ranging between 384°C and 526"C, are obtained from A"C(,,.g,-) in pairs coming from the southern part of the area. They are in agreement with the regional metamorphism undergone by the rocks. On the other hand, in the northern part of the area the isotopic data do not agree with petrological evidence, by which the original isotopic ratios in carbonates should have been altered by hydrothermal fluids. Key words: Graphite, crystallinity index, carbon isotopic composition, metamorphism, Capo Calava, Peloritani Mts.
Introduction
T h e Capo CalavA area (northern coast
of Sicily, Fig. 1) contains metamorphic rocks of the old Peloritani Cambrian or pre-Cambrian basement, with effects of static crystallization attributed to a ther-
F? Censi, F? Fevla, S.G. Rotolo
Fig. 1 - Geological sketch-map of Capo Calava-Gioiosa Vecchia area (redrawn from Ferla, 1970). Mandanici Unit: 3,5,6,7,8,9. Lower phyllitic Unit (Atzori and Ferla, 1979). 2. Ali-Gioiosa Vecchia Unit (Ferla et al. 199 1). Lithologies and Tectonics: 1) Beach deposits; 2) Conglomerates, sandstones and shales (((Verrucano faciesn); 3) Pegmatites; 4) Phyllites with S1 schistosity; 5) Paragonite-bearing marbles (i.e. marbles I1 of Censi and Ferla,1983); 6) Graphite-bearing phyllites with S2; 7) Two micas?grt?gr schists; 8) bt-tstikv schists; 9) Marbles (i.e. marbles I of Censi and Ferla,1983). 10) Overthrusts. 11) Faults. Static metamorphic assemblages induced by thermal overprint: a)Sil+Bt; b)And+Bt; c)Bt.
maloverprintoflateHercynianage (Ferla, 1968; 1970; 1983). This basement consists of a rock sequence ranging between the biotite-in isograd in the Scoglio Nero phyllites (Ferla and Lucido, 1973) not far from staurolite isograd in Monte Pizzicalori, slightly south of Capo Calava. The post-ki'nematic crystallization records northward-increasing temperatures, with prograde crystallization of biotite, andalusite, cordierite, sillimanite and orthoclase. The occurrence of mylonites and pseudotachylites suggested that these rocks should be studied further to verify the real presence of a thermal effect, without ignoring the possibility that it was caused by deformations.
Anisotopicstudywasthereforecarried out on coexisting calcite and graphite, and the structural ordering in graphite was also investigated by means of the crystallinity index. The aim of this study was to calculate isotope temperatures or to identify disequilibrium conditions, in order to obtain useful informations about the above mentioned questions.
Geopetrographic framework At Capo Calava, there are outcrops of prealpine crystalline rocks of the Peloritani Mountains. This area consists of a large pegmatoid body embedded in phyllites and micaschists. These rocks belong to various tectonic units overlying
Relatiorz h e t t v e e ~isotopic a n d stvuctural charactevs i n calcite-gvapl7ite paivs ...
Triassic sediments (((Verrucano))facies) which outcrop slightly south of Monte Pizzicalori on the eastern side of Gioiosa Vecchia mount(Fig. 1). The complexity and great variety of rock types in this relatively small area have been variously interpreted by the authors. In general geological studies the complexity has been ascribed to the existence of various tectonic units (Duek, 1969; Ferla 1970; Amodio Morelli et al., 1976). The sequences, believed to be of medium-high metamorphic grade, are presumed to be part of the Aspromonte Unit (Atzori and Vezzani, 1974; Bonardi et al., 1976; Bonardi and Giunta, 1982), i.e. the highest tectonic Unit of the Peloritani chain. Studies based on geopetrographic data lead to completely different conclusions, according to which the metamorphites with S, schistosity of this area are polymetamorphic and belong to the shallowest portion of an old pre-Hercynian basement which, in Alpine times, was thrust into the Mandanici Unit (Ferla, 1983). In the Peloritani area the Mandanidi Unit is overthrust by the Aspromonte Unit, which is the deepest part of the old basement. At Gioiosa Vecchia, the Mandanici Unit overthusts another tectonic unit, i.e. the palaeozoic cover of the old basement, with phyllites with S , schistosity: i.e. a unit of South Peloritani Complex of Ferla (1972; 1983) or a lower Unit of Atzori and Ferla, (1979). These two units also overthrust on the Verrucano terrains of the lowest unit of the Peloritani Mts, i.e. the Ali-Gioiosa Vecchia Unit of Ferla et al. (1991) (Fig. l). Polymetamorphism is characterized by the presence of a late static crystallization over phyllitic rocks in which the effects of
39
at least two older deformational events are recorded. The original upper portion of the old basement, now overturned and thus located in the lower part of the sequence, contains three thick marble horizons outcropping in the eastern sector of the Peloritani. Isotope temperatures between 350°C and 400°C, based on 6°C of graphite-ankerite-siderite parageneses, have been estimated between the upper horizon of marble (111) and marble (11). These temperature values are compatible with the petrographic data (Censi and Ferla, 1983; 1989). According to Ferla (1970; 1983), the portions between marble (11) of Gioiosa Vecchia a n d the deeper marble (I) of Gioiosa Marea o u t c r o p in the Capo Calava area. The pegmatoid intrusions of Capo Calava crosscut these metamorphic rocks which, in the original deepest layers near marble (I), contain kyanite and staurolite. The metamorphic grade in the Aspromonte Unit is even higher. The Capo Calava rocks mainly consist of pelitic and pelitic-psammitic metamorphites, often containing graphite. These schists sometimes show intercalations of metabasites. This portion of the Mandanici Unit in the Peloritani Mts. consists of phyllites (frequently chloritoidbearing and garnetiferous phyllites), two mica garnet schists, as well a s minor amphibole schists, quartzites, biotite schists and marbles. The sequences show a westward monoclinal trend, the metamorphic grade increasing upwards (Fig. l). Clear traces of a post-kinematic thermal event are recorded near Capo Calava, overprinted on the mineral' assemblages from the preceding kinematic event. The former clearly took place under lower
40
F! Censi, F! Ferla, S.G. Rotolo
pressure and with a different zonation than the latter event, which was responsible for the destabilization of garnet and muscovite and for the static crystallization of reddish biotite, andalusite and sillimanite plus orthoclase; there are also traces of an incipient anatexis (Ferla, 1968; 1970).According to this author, the source of the heating involved in this static event was not the Capo Calava pegmatoid intrusion, but the result of anomalous heat flows of late-Hercynian age, which presently are widespread in the Calabrian-Peloritan Arc. The area also contains late mylonites and pseudotachylites. Lastly, this part of the Tyrrhenian coast of the Peloritani Mts. is sometime subject to strong hydrothermal activity, probably linked to the volcanic activity of the nearby Aeolian Islands (Ferla, 1968; 1985). These hydrothermal phenomena might have affected some of the samples examined.
Sampling and analytical procedures The isotopic study was carried out on coexisting graphite and calcite sampled from rocks of different metamorphic grade. The rocks range from phyllites to andalusite-garnet schists, diopside and Ca-silicate felses, sillimanite schists, and biotite gneissic schists, all mylonitized to varying degrees or with pseudotachylites. The graphite-calcite pairs coming from enclaves in the pegmatoid rocks were also analysed excluding local kaolin-rich bodies with sulphates and arsenates, located along recent faults with active hydrothermalism. Sampling sites are shown in Fig. 2. Metamorphic assemblages of the host rocks are reported in Table 1. The calcitegraphite pairs occurring in marbles or calcschists, characterized by a large excess of calcite over graphite, were not considered for the estimates of isotopic
Fig. 2 - Geological sketch-map of Capo Calava area (redrawn from Ferla,1968), showing sampling sites. Same legend as Fig. l .
Relation between isotopic and structural characters in calcite-graphite pairs ...
temperatures (Morikiyo, 1984; Censi and Ferla, 1989). The amount of graphite in the examined samples is not greater than 2.00 wt% with a value of 1.79 in the SN phyllite, one of the richest in graphite. Calcite is always lower than 6 wt% and the examined samples have a graphitelcalcite molar ratio close to 1. For X-ray investigations, graphite-bearing samples were gently powdered and reacted with concentrated HCl to remove the carbonate fraction. The samples were then reacted with a HF+HCl mixture in a platinum capsule in a water bath (Landis, 1971))and the suspended graphite in the solution was separated. The treatment was repeated up to five times without the materials attaining dryness. The structural ordering or crystallinity index, CI, of graphite was estimated by Xray measurements of the peak, as A20°, at half-height (Landis, 1971; Petrov et al., 1974; Itaya, 1981). The higher the graphite CI, the more the 002 peak shifts to greater 20" angles and the sharper it becomes. For isotopic treatments, the graphite was separated by hand-picking and gently washed with HC1 0.1 M at room temperature for one hour to remove any possible carbonate impurity. Then samples were converted to CO, by reaction with CuO at 1000°C in a quartz tube. The isotopic composition of carbonates was investigated by standard procedures (McCrea, 1950). Isotopic compositions measured with a Varian MAT 250 mass spectrometer are expressed in 6%0 units versus PDB-1 for carbon and versus SMOW for oxygen. The reproducibility measured from duplicate analyses was +0.2%0 for carbon, and ?0.05%0 for carbon and +O.l%o for oxygen, in graphite and carbonate respectively.
Results A) Graphite: Carbon isotopic compositions in graphite from schists are between -10.1%0 and -20.7%0; the mean value being - 15 + 1.5%0.Samples from pegmatites have similar values (Tab. l ) , from -10.3%0and -20.6%0,with a mean value of -13.9+0.64%0. In general, graphite of the Capo Calava area is relatively well-ordered, with a crystallinity index (CI) ranging from 0.2 1 to 0.50, while the do,, effect ranges from 3.354 A to 3.380 A. According to the terminology of Landis (197 l ) , analyzed graphites with crystallinity between d, and the totally ordered do are shown. The do type from Capo Calava is characterized by d,o,=3.354?0.002~and CI> of the volatiles a structural change of the residue takes place. This occurs with an increase in length a n d in n u m b e r of the carbon layers, while the number of layers in the crystalline stacks increases together with the planarity of layers and the interplanar distance do,, converges to 3.350 A (Buseck and Huang, 1985; Wopenka and Pasteris, 1993).
Relation between isotopic and structural characters i n calcite-graphite pairs ...
Therefore the graphitization process is probably responsible for the relationship between the isotopic composition and the structural characters of graphite. This relation is verified in the examined samples (Fig. 3) although it is not always well determined. This may be due to the presence in the same sample of graphites with different grades of structural order, especially for intermediate do,, values. The hypothesis of a possible coexistence of different graphites was questioned by Grew (1974) according to X-ray investigations, but it proved correct after HRTEM observations carried out by Buseck and Huang (1985). This may be the reason for the unclear relationship between C1 and do,, or 613C,the meaning of which will be discussed later. B) Isotope g e o t h e r n z o m e t ~ :Geothermometric estimates from the calcite-graphite carbon fractionations were obtained according to Wada and Suzulu (1983) and Morikiyo (1984). The two equations used
Lower phyUitic
Unit
Fig. 3 - Relation between b"C (PDB-I) and basal spacing dooz (A) of graphites from Capo Calava area (Mandanici Unit) in l ) schists and 2) pegmatites . Also shown are data from Censi and Ferla (1988). 3) g ? s i d ? a n k schists from the Ionian part of the Peloritani Mts. near Fiumedinisi-Ali area. 4) S,-phyllites from the lower phyllitic Units.
43
give almost identical temperature values close to 575°C. However, the isotopic temperature data obtained from the fractionation of Wada and Suzuki (1983) better fit those estimated petrographically on the same materials (Ferla, 1968; 1970; Ferla and Lucido, 1973). The first interesting result is that most of the calculated temperatures range between 400-500°C, although a few reach 680°C. Notably, the sample showing the lowest temperature (i.e. 358"C, sample no 48) was collected slightly south of the examined area and belongs to the layers underlying the Gioiosa Vecchia marbles (Fig. l). The temperature values are in good agreement with those expected according to the thickness of sequences, although some samples show mineralogical associations induced by static crystallization. The 613Cvalues are compared with the calculated temperatures in Fig. 4. The -12 -14 -16 $ -18-20 U . -22 a -24 -26 -28 -
5 ; o
Unit
Fig. 4 - 6I3cof graphite versus isotopic temperature. 1) schists and 2) pegmatites. Data from Censi and Ferla (1989) also shown: 3) Gr?sid+ank schists from the Ionian part of the Peloritani Mts. near Fiumedinisi-Ali area. 4) S,-phyllites from the lower phyllitic Unit. 5 ) petrographically estimated temperature.
l? Censi, l? Ferla, S.G. Rotob
44
TAB.1 Carbon and oxygen isotopic composition in the studied samples. Isotopic temperatures are calculated according to Wada and Suzuki (1983). The mineralogical assemblages in the host rocks are also reported. Asterisks mark temperature values which do not agree with metamorphic assemblages.
613c
2
3
83 f
.g .S
3 d
3
3
calcite
graphite
sample -
(T
SN 9 18 19 21 23 26 33 33A 37 38 40 41 43 44 48
-13.7 -16.2 -17.1 -16.6 -11.8 -14.0 -20.7 -16.1 -18.7 -15.7 -16.2 -17.0 -10.1 -14.8 -11.4 -18.0
f0.23 f0.21 f0.28 f0.16 f0.15 f0.17 f0.15 f0.12 f0.22 fO.ll f0.12 f0.14 f0.08 f0.15 fO.ll f0.20
5 6 24 25 28 31 32 35 36 42 46
-13.5 f0.28 -15.0f0.17 -12.8 f0.15 -14.1 f0.16 -20.6f0.11 -14.0 f0.12 -12.2 fO.ll -10.3 f0.21 -12.0 f0.09 -18.4M.11 -14.4 fO.10
dOo2(A) 613c 0 3.360 - 4.86 M.15 3.365 - 6.79 M.04 3.363 - 6.98 M.05 3.369 - 7.03 M.10 3.355 - 7.29 39.12 3.359 - 7.34 M.06 3.379 3.360 - 7.51 M.08 3.374 - 5.39 M.09 3.363 - 4.56 33.09 3.365 - 9.36 M.05 3.363 - 6.55 M.06 3.354 - 6.44 M.06 3.365 - 7.28 M.07 3.361 -3.26M.03 3.363 - 6.35 M.04
3.360 3.367 3.354 3.362 3.373 3.362 3.355 3.359 3.359 3.365 3.360
- 3.84 M.04 -5.59M.05 - 4.91 M.12 - 7.73 M.08 -9.43M.04 - 5.18 M.05 - 4.31 M.05 +0.26 M.03 - 3.29 M.10 -11.76M.06 - 9.18 M.03
result is difficult to interpret, and only tentative considerations can be made. Graphite becomes isotopically ((heavier)) with increasing temperature, following two different linear trends. Trend A, relative to temperatures ranging from about 350°C to about 475"C, is characterized by sharp 6I3C variations. Trend B is representative of the highest calculated temperatures starting from T-400°C and 613C=17%0. Furthermore the similarities of paths of isotopic data of graphite in schists and pegmatites suggest that the mineral in the last case is reworked from the country rocks.
isotopic temperme
metamorphic assemblage (host rock)
g180
G
OC
19.76 25.47 27.04 28.03 27.84 19.77
M.10 M.08 M.ll M.10 M.09 f0.08
433 415 396* 410 627 513*
19.35 21.76 27.55 26.00 26.31 19.77 21.11 21.52 16.68
M.12 f0.09 M.09 f0.14 M.ll f0.09 f0.04 fO.10 f0.05
441 324* 370* 505* 387. 688* 478* 456* 358
m+chl m+chl ms+grt+bt+and ms+grt+bt+and bt+ms+st+ky bt+sil ms+grt+bt+and ms+grt+bt ms+chl bt+sil bt+sil bt+sil bt+sil bt+sil bt+sil ms+chl
29.03 fO.10 20.93f0.15 18.20 33.08 30.84 f0.06 24.29f0.12 21.25 M . l l 23.03 f0.14 24.97 f0.09 25.54 fO.10 25.12f0.03 26.40 f0.05
408 415 465 526 369 433* 465* 384* 437 514 584
ms+chl ms+chl ms+grt+bt+and ms+grt+bt+and m~+chl bt+sil bt+sil ms+grt+bt ms+grt+bt ms+grt+bt bt+m+st+ky
C) Carbon and oxygen isotopic composition in calcites: The 6IXOand 6I3Cof calcites associated to graphite are reported in Tab. 1. In Fig. 5a calcites whose isotopic temperature is not in agreement with metamorphic paragenesis are reported. The spread shown by these 6180and 6I3C values can be understood if one consider that calcite may have undergone extensive water-rock interaction under hydrothermal conditions. This phenomenon can be analytically described with the following equations (Taylor, 1977):
Relation between isotopic and structural characters i n calcite-graphite pairs ...
04
I
-
15
20
25
30
Fig. 5 - Carbon and oxygen isotopic compositions of hydrothermal carbonates from C a m CalavB area (a) and from marbles (Censi and ~ e r l a ,1983) (h). The exchange curves describe the isotopic variations promoted by water-rock interactions. S.C.: starting isotopic composition. Numbers on the curves indicate C021H20 molar ratios in fluids. For more details see text.
for oxygen isotopes
for carbon isotopes
This approach forecasts that the original isotopic ratios of carbonates can be modified by interaction with fluid along 6180-613Cpaths depending on the waterlrocks and the CO,/H,O molar ratio. The model is based on the knowledge of the initial isotopic composition of both carbonate (6180i,c and 6"CiCc)and fluid (61801,and 613Cco,for water carbon dioxide). A'", ,-, and A13C,,,o, are the isotopic fractionation factors for calcite-H,O and calcite-CO, from O'Neil et al. (1969) and
Bottinga (1968), respectively. Tentatively, the family of curves which best models the isotopic behaviour of carbonates reported in Fig. 5a is calculated for 6l3CiC,=-4%0 and 6180i,,=+30%~, for T=250°C,6'80',=+ 10%0,613Cico,=-1 5%0and the CO,/H,O molar ratio ranging between 0.1 1 and 99. The 6"CI,, value was chosen as high as -4%0 because it is close to that measured in the hydrothermal aragonite (SN sample). A temperature value not higher than 250°C is in agreement with presence of hydrothermal minerals such as scorodite, nathrojarosite, Tc-kaolinite and barite reported in the studied area (Ferla, 1968). In the case of paragonite-bearing marbles (calcite) the arrays that best represent the isotopic values in marbles are calcula11%0,613C'co,=ted for T=408"C, 6180i,=+ 16%0,813C',,=+4%~, 6180'cc=+29.2%~, with CO,/H,O ranging from 0.01 to 1 (Fig. 5b). value is a mean value The 6'TiCc=+4%o well representing marbles outcropping at Gioiosa Vecchia (Censi and Ferla, 1983). The final isotopic values would represent an interaction process under low water to rock ratio conditions (between 0.005 and 30 molar ratio) and they could represent the effect of fluids linked to the regional metamorphism to which the rock was subjected at T-400°C and 2sPs3 Kb (Ferla and Lucido, 1973).
Conclusive remarks In the Capo Calava area, graphite has a rather large variation as regards its structural state: it ranges between d, and the totally ordered do types, consistent with the range of metamorphic grades in the rocks sequence. The graphite shows also a variable S13Csignature.
46
l? Censi, l? Fevla, S.G. Rotolo
According to mineralogical and petrographical data, calcite-graphite carbon isotope geothermometry allows the following considerations to be made: 1 - In graphite, increased isotope temperature is accompanied by increased "C contents, greater structural ordering and larger crystal size. 2 - 6°C variations in the calcite and graphite seem to be also influenced by inherited diagenetic features, e.g. the original ratio between organic matter and carbonates (Eichmann and Schidlowski, 1975). The positive relation between graphite 6I3Cand temperature is probably due to progressive loss of CH, (Valley and O'Neil, 1981) and formation of a "Cricher residue, as may occur during prograde metamorphism in a hydrous environment at low f 0 2 (Rumble and Horing, 1986). It should be recalled that low-grade metamorphism is accompanied by a higher CH,/CO, ratio than high-grade metamorphism (Ohmoto and Kerrick, 1977). However, according to Arneth et al. (1985), a 6°C increase in graphite may be due to a progressive isotope exchange between isotopically kerogenic substances and isotopically
