Chlorite crystallinity as an indicator of metamorphic ... · structure and chemistry, de Caritat et al. ... and .Arkai et al. (1995a). Thus the Darn6 Hill ... (Downes et al., 1990)

Clay Minerals (1997) 32, 205-222

Chlorite crystallinity as an indicator of metamorphic grade of low-temperature

meta-igneous rocks: a case study from the Btikk Mountains, northeast Hungary

P. A R K A I AND D. S A D E K G H A B R I A L

Laboratory for Geochemical Research, Hungarian Academy of Sciences, H-1112 Budapest, BudaOrsi tit 45, Hungary

(Received 15 January 1996; revised 19 June 1996)

A B S T R A C T : X-ray diffraction chlorite crystallinity (ChC) indices and major element chemical compositions of chlorites and bulk rocks were determined and correlated in meta-igneous rocks from different Mesozoic formations in various tectonic units of the Biikk Mountains, NE Hungary. The rocks, of basic to acidic compositions, range from ocean-floor metamorphic prehnite-pumpellyite facies (diagenetic zone) through regional metamorphic prehnite-pumpellyite facies (anchizone) up to the regional metamorphic pumpellyite-actinolite and greenschist facies (epizone). As in the case of meta-sedimentary rocks, chlorite crystallinity can be applied as an empirical, complementary petrogenetic tool to determine relative differences in grades of low-temperature meta-igneous rocks. Electron microprobe and XRD data show that ChC is controlled mainly by the decreasing amounts of contaminants (mixed-layered components or discrete, intergrown phases of mostly smectitic composition) in chlorite with advancing metamorphic grade, up to the epizone. The apparent increase in calculated AI iv content-0f chlorite with increasing temperature is related to the decrease of these contaminants, as stated earlier by Jiang et al. (1994). On the basis of the significant correlations found between ChC and temperatures, derived by the chlorite-AP v geothermometer of Cathelineau (1988), both methods may be used for estimating the approximate temperatures of metamorphism, in spite of the contrasting interpretation of chemical data from chlorites obtained by electron microprobe analyses. After determining the effects of changing bulk chemistry on chlorite composition and ChC, the chlorite crystallinity method may complement the correlation of the illite crystallinity-based zonal classification of meta-sediments and the mineral facies classification of meta-igneous rocks.

Determination of grade at subgreenschist facies has utilized a variety of approaches ranging from metamorphic facies to the illite crystallinity zonal scheme. The illite crystallinity technique (Ktibler, 1967, 1968) has been widely applied to sequences with abundant pelitic rocks. For igneous rocks of basic-intermediate composition, a variety of facies has been defined (Liou et aL, 1987). Various attempts have been made to correlate these two approaches (for a review see Kisch, 1987). However, the occurrences of diagnostic mineral assemblages are strongly restricted, not only by the whole-rock chemistry but also by the composition

of the fluid phase. Thus, in the case of CO2- containing fluids, non-diagnostic mineral assemblages consisting of chlorite, albite, quartz, carbonate minerals _+ white K-mica, titanite and opaque minerals occur commonly in basic-intermediate meta-igneous rocks. As these assemblages are stable over a relatively broad temperature range (from late diagenesis through the very low-grade realm up to the low-temperature part of the greenschist facies), they are inadequate for determination of metamorphic conditions.

Recently, increased attention has been paid to the structural and chemical variations of chlorite as a

�9 1997 The Mineralogical Society

206 P. Arkai and D. Sadek Ghabrial

function of temperature. There has been particular interest in the relationship of changes in A1 iv and ER vi in chlorite to increasing temperature, initially noted by McDowell & Elders (1980). Subsequently, Cathelineau & Nieva (1985) and Cathelineau (1988) calibrated empirically a chlorite geothermometer based on tetrahedral A1 ~-* Si [T(~ = -61.92 + 321.98(AliV)] that was believed to be independent of bulk rock chemistry. Bevins et al. (1991) applied this thermometer, originally defined for meta- andesites and sandstones, to meta-basic rocks, and obtained meaningful, acceptable results. Trends similar to that established by Cathelineau (1988) were found for sedimentary chlorites by Hillier & Velde (1991) and Jahren (1991). Kranidiotis & MacLean (1987) and Jowett (1991) modified the linear equation of Cathelineau & Nieva (1985) and Cathelineau (1988), by making allowance for variation in the Fe/(Fe+Mg) ratio of chlorite. However, the temperature-related trends in chlorite chemistry were found to be strongly modified by the bulk rock composition, thus hindering application of the chlorite-Al iv geothermometer (Hillier & Velde, 1991). Aagard & Jahren (1992) presented a thermodynamic model for the chlorite-A1 iv thermometer of Cathelineau (1988), calculating the reaction muscovite + clinochlore = MgAl-celadonite + amesite. On evaluating the effects of fo2, pH, Fe/(Fe+Mg) and bulk rock composition on chlorite structure and chemistry, de Caritat et al. (1993) concluded that none of the published chlorite thermometers provides satisfactory estimates of T over the whole range of natural conditions, namely over wide ranges of temperature, A1 content, Fe/(Fe+Mg) ratio and various coexisting minerals. Therefore, they should be applied with caution, in combination with other tools.

In a similar manner to the diagenetic evolution from smectite to illite through mixed-layers, a continuous change from tri-smectite (<180~ through random mixed-layer smectite-chlorite (200-240~ random and regular mixed-layer chlorite-smectite (245-265~ up to discrete chlorite (>270~ has been described in hydro- thermally altered meta-basic suites (Schiffmann & Fridleifsson, 1991; see also Bevins et al., 1991; Robinson & Bevins, 1992; Robinson et al., 1993). Other works have identified discontinuous change in smectite content with the formation of discrete corrensite (Shau et al., 1990; Inoue & Utada, 1991; Bet t i son-Varga et a l . , 1991; Hillier, 1993; Schiffman & Staudigel, 1995). Shau et al. (1990)

also identified mixed-layer chlorite-corrensite as a transitional phase. The diversity of these aggrada- tion processes can be explained by the effect of changing fluid/rock ratio (Schiffman & Staudigel, 1995)

Comparing AEM and EMPA data on chlorites, Jiang et al. (1994) have demonstrated that the continuous decrease in octahedral occupancy and increase in tetrahedral A1 with increasing metamorphic grade shown by EMPA data are attributed to decreases in the proportion of mixed-layers and/ or fine-grained intergrown minerals that commonly occur as a result of increasing crystal size and homogeneity. They concluded that the use of the chlorite thermometer of Cathelineau (1988) may therefore lead to inaccurate T estimates.

Unlike the illite crystallinity (IC) method, chlorite crystallinity (abbreviated as ChC, i.e. the full width value of the XRD peak measured at the half-height of the basal reflection) has been applied as a metamorphic grade ( ~ temperature) indicator only in recent years, and exclusively for sedimentary (pelitic-marly) rocks. For the references to early, sporadic applications of ChC see the brief review of Frey (1987). A, rkai (1991) pointed to the analogies between the diagenetic-metamorphic evolution paths of illite-muscovite and chlorite and their precursor phases. He showed strong correlation between IC and ChC, and attributed the decreasing values of ChC with increasing grade to increase in the crystallite size, and to decrease in the lattice strain of the crystallites (see also Arkai & T6th, 1990). Yang & Hesse (1991) explained the similar trends of IC and ChC during anchizonal metamorphism of meta-pelites by decreasing amounts of expandable, mixed-layers. ChC(002), i.e. the width of the 7 A reflection, expresses the 'real' chlorite crystallinity, because the smectite- chlorite and vermiculite-chlorite mixed-layers affect the 14 A reflection of chlorite, characterized by ChC(001), but have little or no effect on the 7 A peak. A correlation between the ChC and IC scales (and zonal classification), meta-basite facies and coal rank was given by Arkai (1991). Using a statistically evaluated, large database, ,~rkai et al.

(1995b) proved the applicability of ChC for determining metamorphic zones in fine-grained clastic metasediments.

It seems reasonable to suppose that the diagenetic and low-T metamorphic evolution of chlorite and its precursor phases is complex in nature. In the low-T (diagenetic) range, the structural and chemical

Chlorite crystallinity as an indicator o f metamorphic grade 207

changes in tri-smectite to chlorite reflect varying proportions of mixed-layered components or intergrown discrete phases (see also Jiang & Peacor, 1990, 1994; Velde et al., 1991; Jiang et al., 1994), while in high-T (anchi- and epizonal metamorphic) conditions the isomorphic substitutions (see also Bevins et al., 1991; Jahren, 1991 and Jahren & Aagard, 1992), the increase of crystallite size and the decrease of structural strain within the chlorite structure predominate. Thus, presumably both chlorite crystallinity and A1 iv geothermometric methods are empirical and use these composite changes for estimating metamorphic grades ( ~ temperatures).

Although recently there have been numerous papers devoted to the characterization of chlorite and other phyllosilicates in mafic meta-igneous rocks, none of them has applied the chlorite crystallinity method to the material studied. The aims of the present paper are: firstly to test whether the chlorite crystallinity method can be used to determine relative differences in metamorphic grade of meta-igneous rocks; and secondly, to determine

the relationship between the crystallinity index and chemical composition of chlorites and to evaluate the effect of bulk rock composition on chlorite chemistry in a suite of meta-igneous rocks of strongly varying composition. The Mesozoic igneous formations of the B~ikk Mountains, northeastern Hungary provide a suitable suite for this work.

G E O L O G Y A N D R E G I O N A L M E T A M O R P H I S M

The various sequences of the B~Jkk Mountains (northeastern Hungary) together with those of the Uppony and Szendr6 Mountains comprise the Btikkium, that is the innermost tectonic unit of the Western Carpathians (Fig. 1). According to the palaeogeographic reconstruction of Kov~ics (1989), the BOkkium is related to the South-Gemer (Aggtelek-Rudab~inya) Unit, which is characterized by fragments of Middle Triassic and Middle-Upper Jurassic oceanic crust, as indicated by the dismembered ophiolitic units of the so called

I I I

1 I I 1 5 ~ 2 0 ~ 2 5 ~

5 0 ~ N

- 4 5 ~

FIG. 1. Tectonic setting of the BiJkkium within the Alpine-Carpathian-Dinaric framework. SG = South Gemeric unit; NG = North Gemeric unit; the black rectangle indicates the position of the Btikk Mountains (see Fig. 2);

dash-dotted line = state border of Hungary.


Meliata Ocean. The mobile, outer shelf-type Btikkium represented the southern (African) border of this oceanic basin. Recent plate tectonic reconstructions restore the Palaeozoic and Mesozoic sequences of the BOkkium to the north-western part of the Inner Dinarides. These sequences were juxtaposed to their present position by meso- Alpine , l a rge-sca le hor izonta l d i sp lacement (Kov~cs, 1989; K~zm6r & Kov~cs, 1989).

The Btikk Mounta ins consis t of Middle Carboniferous to Late Jurassic sequences in several tectonic units, in which not only the stratigraphic sequences but also the metamorphic conditions vary (Fig. 2, see also Arkai, 1983, 1991 and Csontos, 1988). Figure 2 shows the main geological and tectonic features of the B~ikk Mountains, and also the sequences/areas where the meta-igneous rocks were studied. These are: metabasalts of the Darn6 Hill (BtDH), members of the Szarvask6 ophiolite-like complex (SZ), namely meta-basa l t s (BtSZ), meta-d iabases (DbSZ), meta-gabbros (GbSZ) and meta-granites (GrSZ); and the meta-andesite tuff (AdEB) and meta-basalt (BtEB) formations of the Eastern B~ikk.

The conditions of the Alpine, Cretaceous (pre- Senonian) regional metamorphism of these formations are summarized below, based on the detailed accounts of ,~rkai (1983, 1991) and .Arkai et al.

(1995a). Thus the Darn6 Hill (southwestern end of the B~ikk Mountains), which is separated by a strike-slip fault from the other parts of the mountains, belongs to the oceanic Meliata series of the South Gemer Unit. Here the Middle Triassic pillow basalts show ocean-floor hydrothermal metamorphism with prehnite-pumpellyite facies assemblages, whilst the interbedded pelitic, cherty, and marly sedimentary rocks lacking any signs of deformation, are nonmetamorphic, showing only diagenetic alteration.

The Szarvask6 Complex is the uppermost tectonic unit of the Btikk Mountains, i.e. the Szarvask6-M6nosb61 Nappe, and is located in the southwestern area (Fig. 2). It consists of an incomplete, dismembered, Jurassic MORB-type, ophioli te- l ike ul trabasic-basic-acidic sequence (Downes et al., 1990) and clastic sedimentary rocks. The Jurassic clastic sedimentary rocks are found above and below the pillow basalts, and also

0 5 l O k m ~

4iskolc

~ SZ A R V A S K O ~ -o J C O M P L E X ~

~'N 6 HILL (~ E;er

[-'----] ]

F;-q7

- " - - " 9

" ' " 10

J L I l l

FIC. 2. Geological and metamorphic sketch map of the Btikk Mountains after Csontos (1988), Kov~ics (1989) and ,~rkai (1983, 1991), also showing the distribution of the investigated meta-igneous rocks. 1 = Tertiary and Quaternary (diagenetic zone); 2 = South Gemer Unit (Upper Permian - Mesozoic): diagenetic zone; 3 = Szarvask6-M6nosb61 Nappe (mostly Jurassic): diagenetic, partly low-T anchizonal; 4 = Kisfenns~ Nappe (Triassic): diagenetic zone; 5 - 8 = Fenns~ (Btikk Plateau) Parautochthon: 5 = Triassic and Jurassic sedimentary formations: mostly anchi-, partly epizonal; 6 = Ladinian meta-andesite: greenschist facies; 7 = Carnic metabasalt: pumpellyite-actinolite facies; 8 = Middle Carboniferous-Permian sedimentary formations: mainly anchi-, partly epizonal; 9 = strike-slip fault; 10 = nappe boundary with teeth on hangingwalls; 11 = major imbrications within

the nappes.

Chlorite crystaUinity as an indicator of metamorphic grade 209

have contacts with the sheeted dyke and cumulate gabbro members. Apart from traces of high thermal gradient hydrothermal metamorphism, the existence of which in the gabbros has been proved by Sadek Ghabrial et al. (1996), the meta-igneous rocks have prehnite-pumpellyite facies assemblages, that are compatible with the dynamothermal (orogenic) metamorphic character of the clastic sedimentary rocks, such that alteration ranges from late diagenesis to the lower anchizone.

The Middle Triassic meta-andesite (AdEB) and Upper Triassic meta-basalt (BtEB) formations are part of the lowest tectonic unit, i.e. the Fennsfk (Btikk Plateau) Parautochthon of the Biikk Mountains. This unit suffered mostly anchizonal, partly epizonal, low- to medium-pressure regional metamorphism. The Ladinian meta-andesite formation comprises stratovolcanic lavas and largely fine- grained pyroclastic rocks of mostly andesitic composition. Their secondary mineral assemblages correspond to the chlorite zone of the greenschist facies (~ epizone). The Upper Triassic (Carnic) bimodal volcanism produced voluminous basaltic tufts and lavas and subordinate amounts of rhyolitic volcanic rock, with secondary mineral assemblages corresponding to the pumpellyite-actinolite facies. The clastic sedimentary formation, separating the two magmatic units, is characterized by epizonal and upper anchizonal IC and ChC values. On the basis of the strong syn- and post-crystallization deformation of the volcanic and sedimentary rocks, a regional (dynamothermal) metamorphic event is implied.

M E T H O D S

Mesoscopic and microscopic petrographic, micro- structural and mineral paragenetic investigations were undertaken along with X-ray diffraction (XRD), electron microprobe and major element bulk chemical analyses.

The XRD measurements of chlorite crystallinity (ChC) were carried out following similar techniques to those for illite crystallinity (IC). The procedure of preparing <2 p.m grain size-fraction samples was similar to that used by Kfibler (1968). The meta- igneous rock samples were disaggregated under standard conditions, using a jaw crusher followed by crushing in a mortar mill (type Pulverisette 2, Fritsch) for 3 min. The <2 I-tm grain size-fraction was separated from aqueous suspension by differ- ential settling. The aqueous suspension of the given

fraction was pipetted and dried at room temperature on glass slide sample holders to produce highly oriented preparations with material amounts of ~ 2 mg/cm 2. The ChC and IC values were measured on <2 gm fraction air-dried (AD) and ethylene glycol (EG) solvated, highly oriented samples. The sedimented <2 gm samples were solvated in an atmosphere saturated with EG for 4 h at 80~ A Philips PW 1730 diffractometer was utilized at 45 kV/35 mA, with Cu-K~ radiation, graphite monochromator, proportional counter, divergence and detector slits of 1 ~ goniometer speeds of 2~ and ~A~ time constant of 2 s, and chart speed of 2 cm/min. At these conditions, the standard deviations were s = 0.012 ~ (n = 10) for a full width at half maximum (FWHM) value of 0.259 ~ using 2~ and s = 0.008 ~ (n = 10) for a FWHM of 0.215 ~ using �89176 goniometer speed. As there are no major differences in errors of measuring the FWHM, only the results obtained at 2~ goniometer speed are given, in order to assure direct comparability with the earlier results (the results measured at �89176 as well as the whole data set used in the present study can be obtained upon request from the first author).

The calibration of IC and ChC values against Kfibler's IC scale, where the anchizone ranges between 0.25 and 0.42" A20, was made using standard rock slab series (nos. 32, 34 and 35) kindly provided by B. Ktibler. Smaller scale, temporary instrumental changes were corrected by the repeated use of another also calibrated standard rock slab series (nos. A-l, -2 and -3) of the Laboratory for Geochemical Research, Budapest. Applying the least squares' method, the calibration equation is:

IC(Ktibler) = 1.124*lC(present work) - 0.069.

The actual boundary ranges of IC(002), ChC(001) and ChC(002) of the present paper, which correspond to Kfibler's original anchizone, are: 0.284-0.435 ~ 0.310-0.427 ~ and 0.262-0.331 ~ A20, respectively (for the ChC boundaries see also ,~rkai, 1991). All of these boundary values refer to air-dried mounts. A detailed discussion of the problems and uncertainties of the determination of zone boundaries has been presented by Kisch (1983, 1990) and Arkai (1983, 1991).

Electron microprobe mineral analyses (EMPA) were carried out by a JEOL JCXA-733 instrument equipped with three wavelength dispersive X-ray


spectrometers, using the correction method of Bence & Albee (1968). Measuring conditions were: 15 kV/30 nA, electron-beam diameter of 5 gm. For quantitative major element analyses, orthoclase (for Si, A1 and K), synthetic glass (for Mg, Fe and Ca), albite (for Na), spessartite (for Mn) and rutile (for Ti) were applied as standards. The errors in quantitative analyses (expressed in __ 1~ values) are: SiO2: -I-0.3, TiO2: -t-0.05, A1203: -t-0.05, FeO: -I-0.2, MgO: _0.1, MnO: -I-0.05, CaO: +0.1, Na20: -t-0.03, and K20: -t-0.02%.

Major e lement composit ions of bulk rock samples were determined using a Perkin Elmer 5000 atomic absorption spectrophotometer, after digestion with lithium metaborate. In addition to the AAS technique, permanganometric (FeO), gravi- metric (SiOz, TiOz, H20 and P205) and volumetric (CO2) methods were also applied.

R E S U L T S A N D D I S C U S S I O N

M e t a b a s i t e m inera l f a c i e s

Secondary mineral assemblages of meta-basalts of Darn6 Hill (BtDH) consist of Qtz, Ab, Chl __+ Pmp, Prh, Act, Cc, which indicate prehnite- pumpellyite facies (mineral abbreviations after Kretz, 1983). Fissure filling analcime was found only in one sample, representing a possible product of late (cooler) stage hydrothermal activity. In the cumulate gabbros of the Szarvask6 complex, traces o f l o w - P a m p h i b o l i t e f ac i e s o c e a n - f l o o r (hydrothermal) metamorphism were detected recently (Sadek Ghabrial et al., 1996)�9 This event was overprinted by an orogenic (dynamothermal) metamorphism, resulting in prehnite-pumpellyite facies assemblages in both the intrusive and

! ~ Lint i Co e rh ~N C l t z Sd A/X/V' Bt

�9 Pml : S d ; Prh BI I O . t z D b - -

Gb i Gr -

Pmp Bt Ac I Sd . . . A r

C hC (001),~ 20

0.3 0.4

I J I I '. e : 1 0

I 71

^ t l l I ' + ~ 7 [ i I

E A D

V3 I

| I -4 1 I i I I

ChC (002),~ 20

0.3 O4 I l r

l t ~ 6

l i I l ~ 2 4

i I ~ 9

] I I

IC(002), 0A28

0.3 0.4 0.5 I I l

I l I 10: = :

I I 1 ,--<)---427

I I vl

I F-*-~ 6 I

i f [ 4 [

| I Ma& I

I I

0 E I A D

FIG. 3. Variations of chlorite and illite crystallinity indices of the meta-igneous rocks and surrounding clastic rocks in the Btikk Mountains. Act = actinolite; Lmt = laumontite; Pmp = pumpellyite; Prh = prehnite; Qtz = quartz. Ad = meta-andesite tuff; Bt = meta-basalt; Db = meta-diabase; Gb = meta-gabbro; Gr = meta-granite (plagiogranite); Sd = clastic (meta)sedimentary rocks. Illite crystallinity zones: D = diagenetic zone; A = anchizone; E = epizone in the sense of Ktibler (1968, 1990). The corresponding ChC boundaries of these zones are given after Arkai (1991). The measurements were carried out on <2 p_m fraction air-dried samples. The horizontal bars crossing the symbols correspond to 2s (standard deviations), the adjoining number indicates the

number of the samples.

Chlorite crystallinity as an indicator ()f metamorphic grade

extrusive members. The mineral assemblages of the metabasic rocks consist of Qtz, Ab, Chl -t- Pmp, Prh, Mg-Ep, Act, Cc, Ms, while those of the plagiogranites are Qtz, Ab, Chl + Prh, Ep, Cc, Ms. The secondary mineral assemblages of meta-basalts from the Eastern B~kk (BtEB) are Qtz, Ab, Chl _+ Pmp, Ep, Act, Cc, Ms and indicate pumpellyite- actinolite facies metamorphism. The IC data from the interbedded meta-sedimentary rocks indicate that the highest grade (greenschist facies, chlorite zone) is developed in the meta-andesite tuff of the Eastern Btikk (AdEB). The parageneses are Qtz, Ab, Ms, Chl __+ Ep, Cc. For further details on the facies conditions of the meta-igneous rocks of the Biikk Mountains see Arkai (1983) and Sadek Ghabrial et al. (1994).

Chlorite and illite crystallinity

Figure 3 shows the mineral facies, ChC and IC- zone classification of the three areas investigated. The IC averages of the interbedded sedimentary rocks (right hand column in Fig. 3) indicate diagenetic conditions for the Darn6 Hill, lower anchizonal conditions for the Szarvask6 complex, and epizonal conditions for the Eastern B(ikk, using the IC zone boundaries of Kabler (1968, 1990). In certain meta-igneous rock types the IC was also measurable: its values proved to be comparable to those of the corresponding meta-sediments.

Similar trends are found when comparing the ChC(001) and ChC(002) averages of the three areas (the boundary values of diagenetic, anchi- and epizones determined for meta-clastic rocks by Arkai, 1991 are also indicated). The ChC indices of the Darn6 Hill sedimentary rocks and meta- basalts fall in the ancbizone [ChC(001)] and on the d i a g e n e t i c / a n c h i z o n e boundary [ChC(002)] . Epizonal ChC values were obtained for the Eastern B~kk, where the ChC averages of the meta-basalts are somewhat greater than those of the meta-andesite tuff, although there are considerable overlaps between the standard deviations, especially in the case of ChC(001). Fairly good agreement was found between the ChC data of meta-sedimentary and meta-volcanic rocks of the Eastern Bakk. The variation of ChC averages calculated for the various rock types of the Szarvask6 ophiolite-like complex is rather complicated. From the pillow basalt through the diabase (sheeted dyke complex) down to the cumulate gabbro, there is a tendency for a decrease of ChC average values. However, the

211

differences in ChC average values proved to be significant at P = 5% only between the basalt and diabase and between the gabbro and plagiogranite. In contrast, there are no systematic changes either in IC or ChC indices of the closely associated clastic sedimentary rocks. Disregarding the slight contact metamorphic effects found only in very restricted aureoles around the intrusive bodies, the ChC averages of the sedimentary rocks of the Szarvask6 complex correspond to the grade of alteration of the pillow basalt. Consequently, the steep thermal gradient hydrothermal metamorphism might be restricted only to the igneous complex of high permeability, while the impermeable pelitic- silty complex escaped this effect.

Precursor phases and impurities o f chlorite influencing ChC and correlations between crystallinity and chemistry o f chlorite

Figure 4 shows the variations of intensity ratios of the first three basal reflections of chlorite. The decrease in subordinate amounts of the smectitic component is reflected by decreasing lom/looz ratios with increasing metamorphic grade. This trend is superposed by the effects of chemical variability of chlorites; with increasing Fe content (increasing Fe/ Mg ratio), the Iool/lc~2 ratio decreases. The chlorites of the AdEB, GrSZ and GbSZ are located near to the 1o02 corner, having high Fe contents, while the chlorites of the BtDH, BtSZ, DbSZ and the BtEB are characterized by high Mg contents. The superposition of these two effects may be the reason why no significant correlations were found between the lool/ loo2 and the Mg/(Mg+Fe) ratios, and between the lool/loo2 ratio and the apparent octahedral vacancies and the total interlayer charge (Na+K+2Ca), characteristic of the amount of the smectitic component or phase forming impurities in chlorite.

The presence of a subordinate smectite component in chlorite is also supported by the effects of glycolation: for BtDH, BtSZ and DbSZ, small scale broadening of the 14 A peak and, simultaneously, a small decrease in its intensity compared to the 7 ,& peak can be observed. In the case of GbSZ and GrSZ, moderate to small scaleosharpening and a decrease in intensity of the 14 A peak was found, whereas no observable effects caused by glycolation could be demonstrated for the higher anchizonal BtEB and epizonal AdEB.

Table 1 contains the average compositions of chlorites from representative samples. No relation-

212 P. i~rkai and D. Sadek Ghabrial

I(002)

-A !

,,0o,, ,,00,, /

S ..1../. / ! ;~._'

. . , ....... ) , I

I(0011

m e o n

ChC(O02) volues in ~

0.342 .............. meta-bosal t

80

70

60

50

Dorn~ Hitt 0.372 meta-basatt 0.359 meta-diabase 0.337 meta-gobbro 0,282 meta-granite 0.248 meta-basatt } O. 221 mela-ondesite

Szarvask~ complex

Eastern B~kk

FIG. 4. Intensity ratios of the first three basal reflections of chlorite from meta-igneous rocks, measured on sedimented (highly-oriented) <2 gm size-fraction, air-dried mounts.

ship is seen between metamorphic grade and anhydrous oxide sum, as the latter changes irregularly within a very narrow range (86.4- 89.5 wt%). The X values (last row in Table 1), calculated by the method of Bettison & Schiffman (1988), are characteristic of the proportions of chlorite in a chlorite-smectite (saponite) system, supposing that both chlorite and smectite are trioctahedral (X = 1 means pure trioctahedral chlorite, while X = 0 pure trioctahedral smectite). Disregarding the meta-granites of the Szarvask6 complex, the proportions of the chloritic phase or component increase with advancing metamorphic grade in the chlorite analysed by the electron beam of relatively large (c. 51xm) diameter. Thus, for the diagenetic BtDH 20-15%, the anchizonal

Szarvask6 meta-basic rocks 12-4%, the epizonal BtEB 11-0%, and the also epizonal AdEB 8-1% smectitic (saponitic) contributions can be supposed. This trend is proved also by the increasing Z octahedral cations, the decreasing apparent octahedral vacancies (F-l) and the decreasing totals of interlayer charges (Na+K+Ca) with advancing metamorphic grade (Table 1). Consequently, the effects of impurities (smectite, vermiculite, illite- muscovite, etc) should be taken into consideration when the relations between ChC and chlorite chemistry determined by EMPA are evaluated, although the amounts of these impurities are subordinate compared to the chlorite, not exceeding c. 20% in the chloritic mixture analysed. Due to the small proportions of these impurities, and also

Chlorite crystallinity as an indicator of metamorphic grade 213

because of the polymineralic character of the samples, the nature of these impurities (discrete, intergrown phases or irregular or regular mixed- layers with chlorite) could not be determined exactly by the available methods.

The very strong, significant, positive correlations between the ChC indices and the total interlayer charge of chlorite (rows 3a and 4a in Table 2), as well as the moderate, but still significant correlations between the ChC indices and the apparent octahedral vacancies (rows la and 2a in Table 2) in chlorites from meta-basic rocks indicate that the chlorite crystallinity is significantly influenced by the smectitic precursor phases being present even in small quantities. In the case of meta-igneous rocks of acidic to intermediate compositions, these relations proved to be weak and not significant, especially for the total interlayer charge (rows lb to 4b in Table 2). Thus, special attention should be paid to the discrimination between the chemical effects caused by the impurities in chlorite measured by EMPA, the amounts of which tend to decrease with advancing metamorphic grade, and the eventual crystal chemical changes within the chlorite structure itself.

Figure 5 shows that there are strong, significant positive correlations between the total interlayer charge, being proportional to the amount of impurities, and the apparent octahedral vacancies, implying that the changes in the latter parameter are controlled mainly by the proportion of the non-

0 5 -

0 . 4 -

0 . 3 -

N

0 .2 -

0.1

.(1~" /

a o 0.0 0.1 0.2 0.3 0.4 05 0.6 0.7

opporent octahedrol vctconcJe:i

FIG. 5. Plot of total interlayer charge vs. apparent octahedral vacancies assuming 12 octahedral sites. For symbols see Fig. 3. 1 = basic rocks; 2 = acidic and intermediate rocks; 3 = all meta-igneous rocks. The parameters of the regressions are given in rows 5a -c

in Table 2.

chloritic components/phases being mixed-layered or intergrown with the dominant chlorite phase. The statistical parameters of these relations are given in rows 5 a - c of Table 2. For meta-basic rocks, the decreases in apparent octahedral vacancy and Si values are the consequences of the decreasing amounts of smectitic (saponitic) contributions to chlorite with advancing metamorphic grade (Fig. 6). The acidic and intermediate meta-igneous rocks display a similar trend, which, however, is shifted parallel to that of the meta-basic rocks in directions of lower Si (higher A1 iv) and lower octahedral divalent cation values. The quasi-parallel shift of these trends is demonstrated also in Fig. 7b, showing

R3o=4 R3"=2

'b / ' /

;~ t T~c Sapi= // Kfs /

' / i / ~'~++ v I "

/ / , //"

/ ' § / i

11 / ~' Rt & Cal

/I / ii 5 i

6 8 10 12 octahedra[ divalent cations

Fla. 6. Si vs. octahedral divalent cations plotted after Jiang et al. (1994). For symbols see Fig. 3. Arrows indicate the schematic trends of compositional deviations generated from mixtures of an assumed composition of chlorite [Fe5.sMg4A12.sSis 5Ala.502o(OHho] and other minerals. V = apparent octahedral vacancies; R 3+ = octahedral trivalent cations. Ab = albite; Kfs = K-feldspar; Qtz = quartz; Col = calcite; Ill = illite; Kln= kaolinite; Ms = muscovite; Rt =rutile; Sap =

saponite and T l c = talc.

214 P. f~rkai and D. Sadek Ghabrial

~2

;>

.'.!,

~2

�9

. ?

o

o

;< o

r..)

<

~ 0

t'r I,~

0

0

c5

. ~ ~ g. N

8

<

~ d ~ d d d d ~

~ d ~ d d d d d d r

~ O O o ~ o ~ O 0 ~ ~ = ~ ~

�9

o

~0

v

'~ ~o

i

o~

0 o

II "

t ' q ~

~ o

" a

~ , z . , ~ II

~0 " ~

II II

~ o.

d d

~ o

d

m

~ <

Chlorite crystallinity as an indicator of metamorphic grade

M M M & ~ d d d d m d d ~

. . . . . < q ~ . . . . ~ < .

>.~

�9

d cq

~ ~ ~ +

2 1 5

0

.r-~ ~

0 , - ~

o , ~

H ~

m b a , g

L I . , II

* 13Q I1

216 P. S~rkai and D. Sadek Ghabrial

TABLE 2. Statistical parameters of the linear regression (y = ax+b) and the corresponding correlation coefficient (r) of the chlorite and bulk rock properties [with bold: the r value differs significantly from zero at P = 5% (*), at

P = 1% (**) and at P = 0.1% (***) levels].

y x a b r n

la lb lc

2a 2b 2c

3a 3b 3c

4a 4b 4c

5a 5b 5c

6a 6b 6c

7a 7b 7c

8a 8b 8c

9a T~C 9b T~C 9c ~ C

10a T~ 10b 7~C

ChC(001), AD ChC(001), AD ChC(001), AD




Na+K+2Ca, chlorite Na+K+2Ca, chlorite Na+K+2Ca, chlorite

Na+K+2Ca, chlorite Na+K+2Ca, chlorite Na+K+2Ca, chlorite

[5], chlorite [5], chlorite [5], chlorite

7~C Cathelineau, 1988) 70C ~athelineau, 1988) 7~C Cathelineau, 1988)

Cathelineau, 1988) Cathelineau, 1988) Cathelineau, 1988)

Cathelineau, 1988) Cathelineau, 1988)

10c T~C (Cathelineau, 1988)

l la 70C (Cathelineau, 1988) l ib 70C (Cathelineau, 1988) l lc 70C (Cathelineau, 1988)

12a T~ (Cathelineau, 1988) 12b T~C (Cathelineau, 1988) 12c 7~C (Cathelineau, 1988)

13 SiO2 , chlorite 14 A1203 , chlorite 15 A1203/(A1203+SiO2), chlorite 16 MgO/(MgO+FeO), chlorite 17 Al203/(Al203+FeO+MgO), chlorite 18 Al 'v, chlorite 19 Al iv, chlorite 20 MgO, chlorite 21 FeO, chlorite

Fq, chlorite 0.331 0.255 0.66* 10 [S], chlorite 0.117 0.250 0.56 6 IS], chlorite 0.165 0.279 0.41 16

[S], chlorite 0.268 0.239 0.69* 10 [S], chlorite 0.116 0.224 0.57 7 [7, chlorite 0.180 0.242 0.50* 17

Na+K+2Ca, chlorite 0.715 0 .211 0.92*** 10 Na+K+2Ca, chlorite 0.112 0.265 0.44 6 Na+K+2Ca, chlorite 0.229 0.283 0.43 16

Na+K+2Ca, chlorite 0.623 0.193 0.92*** 10 Na+K+2Ca, chlorite 0.096 0.241 0.38 7 Na+K+2Ca, chlorite 0.199 0.259 0.40 17

[S], chlorite 0.425 0.074 0.79** 11 l-I, chlorite 0.713 -0.001 0.90** 7 [S], chlorite 0.600 0.022 0.84*** 18

Al TM, chlorite 0.269 0.717 -0 .66* 11 A1TM, chlorite -0.560 1.587 -0.87** 7 Al TM, chlorite 0.144 0.525 -0.40 18

Al TM, chlorite 0.621 1.498 --0.90*** 11 A1TM, chlorite 0.787 2.229 -0.97*** 7 Al TM, chlorite -0.296 0.955 -0.60** 18

IS], chlorite -210.25 310.26 -0.90*** 11 IS], chlorite -193.98 390.73 -0.97*** 7 IS], chlorite -193.10 337.90 0.60** 18

Na+K+2Ca, chlorite -261.12 295.36 -0.66* 11 Na+K+2Ca, chlorite -218.45 377.13 -0.87** 7 Na+K+2Ca, chlorite -182.60 313.42 -0.40 18

Mg/(Mg+Fe), chlorite 67.03 206.3 0.29 11 Mg/(Mg+Fe), chlorite 194.20 262.8 0.30 7 Mg/(Mg+Fe), chlorite -109.98 318.4 -0.30 18

ChC(001), AD -277.90 344.74 -0.62* 10 ChC(001), AD -708.46 527.19 -0.71" 6 ChC(001), AD -532.00 450.85 -0.71"* 16

ChC(002), AD -373.75 359.83 -0.62* 10 ChC(002), AD -670.88 500.54 -0.68* 7 ChC(002), AD -743.54 498.62 0.78*** 17

SiO2 , bulk rock -0.176 37.117 -0.69** 17 A1203 , bulk rock 0.765 5.970 0.71'* 17 AlzO3/(Al203+SiO2), rock -0.124 0.418 -0.10 17 MgO/(MgO+FeO), rock 0.8591 0 .051 0.89*** 17 Al203/(Al203+FeO+MgO), rock 0.1201 0.233 0.58* 17 A1203 , bulk rock 0.117 0.256 0.63** 17 A1203/(A1203+SiO2), rock -0.727 2.231 -0.07 17 MgO, bulk rock 1.412 6.406 0.82*** 17 FeO, bulk rock 0.356 26.495 0.14 17

a = basic rocks; b = acidic and intermediate rocks, c and simple numbers = all rocks; n = number of data-pairs; AD = air-dried preparations; f-l, chlorite = apparent octahedral vacancies in chlorite. As the XRD parameters could not be measured on all of the investigated samples, the number of the measurements varies.

Chlorite crystallinity as an indicator o[ metamorphic grade 217

very strong, significant, negative correlations within each rock group (see also rows 7 a - c in Table 2). Similar, but somewhat weaker, negative correlations were found between the A1 iv contents and the total interlayer charge values (Fig. 7a, rows 6 a - c in Table 2).

Realistic, geologically acceptable temperature values were obtained for the investigated meta- igneous rocks of the BOkk Mountains by the empirically calibrated chlorite-A1 iv geothermometer of Cathelineau & Nieva (1985) and Cathelineau (1988). The T values obtained by this method are lower in most cases (maximum of 10~ than those calculated by the method of Jowett (1991), this method being a slight modification of that of Cathelineau (1988). Fairly close agreements were found between the earlier rough, indirect T estimates (/krkai, 1983, 1991) and the chlorite-A1 iv T values. The latter T values, calculated by the method of Cathelineau (1988), scatter between 186 and 384~ Thus, the prehnite-pumpellyite facies meta-basalts of Darn6 Hill were probably metamorphosed at c. 200~ (186-205~ The prehnite- pumpellyite facies Szarvask6 Complex yields temperatures between 230 and 350~ (meta-basalt: 236~ meta-diabase: 224-252~ meta-gabbro: 2 3 0 - 2 4 8 ~ meta-grani te : 2 7 6 - 3 5 0 ~ The pumpellyite-actinolite facies meta-basalt of the Eastern Btikk indicates T~280~ (265-296~ and the greenschist facies meta-andesite tuff of the Eastern Btikk T ~ 3 7 5 ~ (368-384~

The very strong, significant, negative correlations between the T values calculated by the method of Cathelineau (1988) and the apparent octahedral vacancies, as well as the somewhat weaker, but significant correlations between the T and the total interlayer charge values (rows 8 a - c and 9a -c , respectively, in Table 2), prove that the increase in A1 iv values, calculated from the chlorite EMPA data is not a consequence of increasing A1--*Si isomorphic substitution within the chlorite structure itself, as Cathel ineau & Nieva (1985) and Cathelineau (1988) originally stated. Instead, the present data are in agreement with and support the earlier conclusions of Jiang et al. (1994), suggesting that the increase in the apparent A1 iv values reflects the decreasing amounts of contaminants (mixed- layers or discrete, intergrown phases) analysed together with the host chlorite phase. No significant correlations were found between the T estimates and the Mg/(Mg+Fe) ratios of chlorite (rows 10a-c in Table 2).

0.5- ~\ + \

0.4- �9 0

0.2- �9 " ' ~ - o , " ~ - - 2 a

0.1 - �9 � 9 x ' ~ , \

x~.

1.6 1.8 2.0 2.2 2.4 2.6 2,8 AIIV

0.7-

1 " 0,6 +\~ b

~ "' , , + ~ 0.5

~- o.4 ~ ~^\ ~ ~ ',%

\.,.... -<<% ,,, A ~0 ~ '~

~o ~ ", 0.z- ~ " ~ , x

1 i i 0.0 115 I;8 2'.0 2.2 2.4 2.6 2'.8

A[ Iv

FIG. 7. Relations between total interlayer charge and tetrahedral A1 (a), apparent octahedral vacancies and tetrahedral A1 (b). For symbols see Fig. 3. The parameters of the regression lines of the basic (1), acidic and intermediate (2) and all of the investigated meta-igneous rocks (3) are given in rows 6 and 7 in

Table 2.

It seems obvious, however, that in spite of the above mentioned fundamental difference in the interpretations of the chlorite chemical data obtained by EMPA, the chlorite-A1 iv method of Cathelineau (1988), as an empirically calibrated, semiquantitative thermometer can be applied for rough estimates of temperature for rocks of appropriate bulk chemistry. Figure 8 shows moderately strong, significant negative correlations between the T values calculated by the chlorite-A1 iv contents and the chlorite crystallinity indices (see also rows l l a - c and 12a-c in Table 2). Note that the differences in the


*C 400-

300-

200-

K Q

": '2"0 " %~, |

AA

oC

400-

300 -

200-

0'.2 o13 o14 ~;.5 ChC (001), ~

x

§ |

2 0.3 0.4 0.5 ChC (002), nA 20

FIG. 8. Relations between chlorite crystallinity indices and temperatures calculated by the chlorite-Al iv geothermometer of Cathelineau (1988). For symbols see Fig. 3. The parameters of the regression lines of the basic (1), acidic and intermediate (2) and all of the investigated meta-igneous rocks (3) are given in rows

11 and 12 in Table 2.

temperature values calculated for rocks of contrasting bulk chemistries increase with decreasing ChC values (i.e., with increasing metamorphic grade). Thus, surprisingly large differences in the T estimates exist between the meta-gabbro and meta-granite of the Szarvask6 complex (c. 240 and 300~ and between the meta-basalt and meta-andesite tuff of the Eastern B0kk (c. 280 and 375~ respectively). The plagiogranite occurs as dykes intruded into the previously solidified gabbro. The metamorphism of the two rock types should have been synchronous and of similar grade. Similarly, the meta-andesite tuff and the meta-basalt of the Eastern Biikk probably experienced synchronous metamorphism at similar T. Consequently, the great T difference may only be explained by the effect of whole rock chemistry.

Correlations between chlorite and whole rock chemistries

The average chemical compositions of the investigated rock types are given in Table 3. However, the statistical analyses (Table 2) were carried out using the whole-rock and chlorite chemical data of individual samples.

As chlorite is the only (or dominant) Fe-Mg- bearing mineral in the investigated samples, its Mg/Fe ratio is not buffered by other minerals. This may serve as an explanation why a positive, strongly significant correlation exists between the Mg/(Mg+Fe) ratios of chlorite and the whole rock (row 16 in Table 2). While the positive correlation between the MgO contents of chlorite and bulk rock is strongly significant (row 20), there is practically no relationship between the FeO contents (row 21). As the Mg/(Mg+Fe) ratio of chlorite proved to be unrelated to geothermometry using either the chlorite-A1 iv thermometric or the chlorite crystallinity methods, its strong dependence on bulk rock Mg/(Mg+Fe) ratio does not influence the thermometric application of chlorite. Positive, moderately significant correlations were found between the A1203 contents of chlorite and bulk rock (row 14), and between the chlorite A1 iv and bulk rock A1203 contents (row 18), while the bulk rock A1203/ (A1203+SIO2) ratio does not correlate with the AI iv of chlorite (row 19). Consequently, the A1203 content should be taken into consideration when chlorite is applied for thermometric purposes. Moderately significant negative correlation was found between the SiO2 contents of chlorite and host rock (row 13). While no correlation exists for the A1203/(A1203+SiO2) ratio (row 15), weak significant positive correlations have been established for the A1203/(A1203+FeO+MgO) ratio between chlorite and bulk rock (row 17).

As certain chemical components and ratios do influence the quantities and ratios of those components of chlorite (more precisely, chlorite and its impurities measured by EMPA) that serve as a base for thermometric application of this mineral or which affect the metamorphic zone-indicating crystallinity indices of chlorite, these thermometric methods may be applied to meta-igneous rocks of similar chemical compositions (or of limited range of chemical variability). In the case of rocks characterized by higher AI203 content and A1203/ (AI=O3+Mg+FeO) ratio, as compared with those rocks for which the ch!orite-A1 iv thermometric or

Chlorite crystaIlinity as an indicator o f metamorphic grade

TABLE 3. Average major element compositions (wt%) of the investigated rock types.

219

Rock type/locality BtDH BtSZ DbSZ GbSZ GrSZ BtEB AdEB Number of samples 4 2 2 2 5 4 3

SiO2 45.78 48.90 51.03 48.29 64.97 45.26 57.69 TiO2 1.93 1.59 1.60 3.03 0.37 1.42 1.24 A1203 13.60 14.56 15.79 13.88 15.14 17.27 17.57 Fe203 3.22 2.68 3.79 3.28 0.89 3.15 2.69 FeO 6.87 8.02 6.51 11.06 3.74 5.48 4.00 MnO 0,13 0.18 0.18 0.24 0.22 0.13 0.08 MgO 5,91 6.79 5.50 4.01 0.79 7.79 1.73 CaO 10.58 9.98 5.51 7.19 4.00 6.66 3.11 Na20 3,63 3.73 5.70 5.13 6.49 3.26 3.60 K20 0,31 0.27 0.22 0.24 0.27 2.91 2.90 - H 2 0 0.46 0.15 0.28 0.08 0.09 0.22 0.11 +H20 3.62 2.95 3.64 3.04 1.93 4.50 2.97 CO2 3.43 0.14 0.06 0.14 0.49 1.62 1.82 P205 0.27 0.15 0.16 0.27 0.19 0.32 0.35

Total 99.74 100.09 99.97 99.88 99.58 99.99 99.86

BtDH -- meta-basalt, Darn6 Hill; BtSZ = meta-basalt, DbSZ = meta-diabase, GbSZ = meta-gabbro and GrSZ = meta-granite from the Szarvask6 complex; BtEB = meta-basalt and AdEB = meta-andesite tuff from the Eastern Biikk Mountains.

the chlorite crystallinity methods were elaborated, the apparent temperatures and metamorphic zones will be systematically higher. In order to determine the ranges of these shifts, systematic research on a larger sample population of different bulk rock chemistry representing varying metamorphic grades is required.

C O N C L U S I O N S

(1) As determined on a meta-igneous rock series with bulk rock chemistry varying from basic through intermediate to acidic, the chlorite crystallinity (ChC) indices determined by XRD on <2 lam size-fraction samples can be applied for determining relative differences in metamorphic grades ( ~ temperature) in the interval ranging from diagenetic conditions through the anchizone (very low-grade realm), up to the epizone (chlorite zone of the greenschist facies). Fairly good agreements were found between the ChC values of the meta- igneous rocks investigated and the interbedded meta-sedimentary rocks.

(2) The distribution of the intensity ratios of the first three basal reflections of chlorite and the effects of ethylene glycol solvation vary systematically with metamorphic grade, indicating that discrete phases and/or mixed-layer components of

smectitic composition, being present in subordinate proportions, contribute to the changes of chlorite crystallinity with metamorphic grade.

(3) Electron microprobe data show the presence of impurities (mixed- layered components or discrete, intergrown phases of mainly smectitic composition) in chlorites analysed. Chlorite crystallinity is controlled mainly by the decreasing amounts of these impurities, traces of which are present even in chlorites from meta-igneous rocks belonging to the epizone. The significant negative correlations found between the calculated A1 iv content of chlorite and the apparent octahedral vacancies and total interlayer charge values (the latter two being proportional to the amounts of smectitic contaminants) imply that the apparent increase in AI iv content of chlorite with increasing metamorphic grade (temperature) is related to the decrease in amounts of impurities, rather than to the increasing isomorphic substitution of Si by AI within the chlorite structure itself. This conclusion is in agreement with the earlier statements of Jiang et al. (1994).

(4) Despite the considerable difference in interpretations of Cathelineau (1988) on the one hand, and Jiang et al. (1994) and the present work, on the other, the chlori te-A1 ~v g e o t h e r m o m e t e r of


Cathelineau (1988) as an empirical tool can be applied for the determination of the approximate tempera ture of metamorphism. As s ignif icant correlations exist between the ChC indices and the temperature values determined by the chlorite-A1 iv

geothermometer, the ChC may be used for indirect est imation of metamorphic temperatures, after checking the effects of bulk rock chemistry on chemical composition and crystallinity of chlorite.

(5) Based on statistically evaluated relations between the chemical compositions of chlorites and the host rocks, the bulk rock chemistry (mainly the contents of A1203, SiO2) and to a lesser extent, the A1203/(AlzO3+FeO+MgO) ratio of the rocks, do influence the amounts of those components in chlorite which are used for T estimates, and also influence the ChC indices. Further research on a large data-set is needed to determine the absolute ranges of shifts caused by various bulk chemistries.

Thus, the chlorite crystallinity method can be applied for the determination of metamorphic grade of meta-igneous rocks lacking indicative mineral assemblages. In addition, considering that chlorite is one of the most common rock forming minerals, occurring both in meta-sedimentary and meta-igneous rocks of the very low-grade metamorphic realm, the chlorite crystallinity scales, correlated earlier with illite crystallinity, coal rank and mineral facies classifications by Arkai (1991) and Arkai et al. (1995b), may serve as a 'bridge' between the illite crystallinity based metamorphic zones and the meta- basite mineral facies classifications in the future.

ACKNOWLEDGMENTS

The authors are indebted to Dr G. Nagy and Mr Z. Wieszt (Laboratory for Geochemical Research, Hungarian Academy of Sciences) for the electron microprobe and major element bulk chemical analyses. Thanks are due to Mrs O. Komor6czy and Ms K. Temesv~iri and V. Varga for their technical assistance. Reviews by Drs B. Roberts (London, UK), R. E. Bevins (Cardiff, UK), H. Rice (Vienna, Austria) and D. Robinson (Bristol, UK) considerably improved the manuscript. The present work forms a part of the metamorphic petrological research programme of P.A. sponsored by the Hungarian National Research Fund (OTKA, Budapest), Project No. T007211/1993-1996.

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