Clay Minerals (1996) 31, 391-401

G E O C H E M I S T R Y AND M I N E R A L O G Y AS INDICATORS OF P A R E N T A L AFFINITY FOR CENOZOIC BENTONITES:

A CASE S T U D Y FROM S. CROCE DI M A G L I A N O ( S O U T H E R N A P E N N I N E S , ITALY)

R. L A V I A N O AND G. M O N G E L L I *

Dipartimento Geomineralogico, Universitft degli Studi di Bari, via E. Orabona 4, 70125 Bari, Italy, and *DiSGG, Universith della Basilicatu, via della Tecnica 3, 85100 Potenza, Italy

(Received 17 August 1995; revised 12 January 1996)

A B S TRACT: The major and trace element contents and mineralogical composition of Cenozoic bentonites from the southern Apennines (Italy) have been determined, for the whole-rocks and the <2 ~tm size-fractions, in order to constrain parental affinities. The main mineralogical and chemical differences have been recognized in eleven samples allowing them to be grouped into two distinct subsets. The differences are based on smectite abundance, occurrence or lack of detrital clay phases, different contents of Ti, Fe, Mn, K, P, Rb, Sc, V, Cr and Ni and differences in the Eu/Eu* and Ti/AI elemental ratios. These ratios indicate an affinity for felsic volcanics for the subset showing high smectite contents. The low smectite subset shows, instead, an affinity for Cretaceous-Oligocene southern Apennine shales. A similar result is obtained using the La-Th-Sc and Th-Sc-Zr/10 diagrams. We suggest that during the deposition of the southern Apennine shales, episodic volcanic events took place. These were associated with the suture stage of the Tethyan ocean that promoted accumulation of felsic ash in the related basin and the diagenetic alteration of these materials produced bentonitic layers interbedded with shales.

Bentonites are clay-rich rocks, mineralogically smectite dominated, resulting from the early diagenetic alteration of vitric fallout ash in a subaqueous, mainly marine, environment (Millot, 1970; Grim & Gtiven, 1978; Fisher & Schmincke, 1984). A major goal in studying bentonites is to determine the parent material because it may provide information on ancient tectonic settings. The composition of authigenic smectite in these sediments is related to both the parent material and the water composition, making it difficult to recognize the source, a difficulty often enhanced by the occurrence of detrital smectite.

Several investigators have suggested that some elemental ratios such as Fe, Ti, Cr, Zr and Ni to A1 (Hein & Scholl, 1978; Spears & Kanaris-Sotiriou, 1979) and also the contents of some conservative elements such as Zr, Nb and Th (Pacey, 1984) may discriminate basic derived beds from more silicic derived ones. On the other hand, although the role played by clay phases in hosting trace elements is regarded as major, their control on trace element

distribution can be modified during diagenetic processes (e.g. Crichton & Condie, 1993). Consequently, the use of trace elements as source indicators, in fine sediments experiencing diagen- esis, has to be carefully evaluated.

In this study we report the mineralogy and the distribution of major and trace elements, including rare earth elements (REE) in Cenozoic bentonites of the southern Apennines, both in the whole-rocks and in the <2 Ixm size-fractions where smectite is concentrated, in order to decipher their parental affinity.

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

The geological evolution of the Mediterranean area involves (Ogniben, 1985): (I) continental rifting during the Triassic; (2) oceanic spreading in the Jurass ic -Ear ly Cretaceous; (3) suture of the Tethyan ocean, from Late Cretaceous to Tertiary; and (4) continental collision between the African

~) 1996 The Mineralogical Society

392 R. Laviano and G. Mongel l i

and the European plates, coupled with the deformation of the domains giving rise to the Apennine chain.

Several volcanoclastic deposits of Oligocene- Miocene age crop out in the Appenines. These deposits derive from calc-alkaline magmas having rhyolites and andesites as their dominant products and are linked to volcanic arcs developed on continental crust (Guerrera & Veneri, 1989). In the eastern area of the southern Apennines bentonite beds of Cattian• age occur in a

succession characterized by continuity of sedimen- tation and composed mainly of Cretaceous- Oligocene shales (Argille Varicolori Formation), bentonite beds, Serravallian flysch (Faeto Formation) and Tortonian muds (Toppo Capuana Formation) (Dazzaro & Rapisardi, 1984). A set of 11 bentonite samples, ranging in colour from pale yellow to green and interbedded with calcareous mads and calcarenites, has been collected in a quarry near the village of S. Croce di Magliano (Fig. 1).

4 2 c -

41 ~ -

Santa Croce diMagliano (

Potenza

Bah

20 Kin i !

15 ~ 1 6 ~ 17"

, , ,0 .

5-

10

15 B

~...~ ~ 6"

FIG. 1. Sampling location and detailed lithostratigraphy of the sampled quarry: (1) bentonites; (2) calcarenites; and (3) calcareous marls.

lnd&ators of parental affinity for bentonites 393

The <2 gm size-fraction was separated by gravity settling using deionized water. The semi-quantita- t ive clay mineralogy was obtained by X-ray diffraction (XRD) using Ni-filtered Cu-K0t radiation according to Schultz (1964) and Shaw et al. (1971), modified by Laviano (1987). Smectite crystal- lochemical formulae have been calculated by the method of Marshal l (1949). Because of the dif f icul ty in ca lcula t ing formulae from bulk chemical data and in order to minimize the effects of impuri t ies on the stoichiometry, we have performed the calculation on the <2 Ixm size- fraction of samples in which smectite is the only clay phase, after subtraction of SiO2 due to quartz and/or cristobalite. Elemental analyses for major, V, Cr, Ni, Rb, Sr, Y, Zr, Nb and Ba concentrations were obtained by X-ray fluorescence on pressed powder disks. X-ray counts were converted into concentrations using a computer program based on the matrix correction method according to Franzini et al. (1972, 1975) and Leoni & Saitta (1976). The CaCO3 and MgCO3 contents were determined by

titration with EDTA after HC1 (2%) dissolution. Loss on ignition (LOI) was determined, after heating the sample for 3 h, at 900~ The Sc, REE, Th and U concentrations were determined by instrumental neutron activation analysis (INAA) at the Activation Laboratories, Ancaster, Canada. The precision of the determination of all trace elements, except Yb and Lu, is better than 5%. The precision of the Yb and Lu determinations is better than 7%.

M I N E R A L O G Y

The most important mineralogical component in bentonites is smectite and its chemistry may reflect the chemistry of the parent ash (Hein & Scholl, 1978). Some caution is due, however, both because the chemistry of authigenic smectite is affected by water composition, and because smectite can also be detrital in marine sediments.

The semi-quantitative clay mineralogy (Table 1) shows that the samples can be grouped according to smectite contents into two subsets: a subset with

TABLE 1. Whole-rock and clay fraction (<2 gm) mineralogical composition of the S. Croce di Magliano pelites.

Sample S I+M K Ch Ca Cr Qz Zeol FI CM

HSS 6a 75 tr tr tr 4 10 6 2 3 75 6af 79 tr tr tr 4 9 5 / 3 79 6b 80 tr tr tr tr 4 7 5 4 80 6bf 85 tr tr tr tr 5 6 / 4 85 2a 85 tr tr tr tr 8 3 / 4 85 2af 94 tr tr tr tr 6 tr / tr 94 2b 78 tr tr tr tr 9 5 2 6 78 2bf 90 tr tr tr tr 5 2 / 3 90 2d 63 5 tr tr tr 24 5 1 2 68 2df 82 2 tr tr tr 16 tr / tr 84 2e 67 6 tr tr tr 19 3 / 5 73 2ef 86 tr tr tr tr 10 2 / 2 86

LSS 5b 60 9 5 3 6 5 10 1 1 77 5bf 63 9 7 4 6 3 8 / tr 83 5a 58 8 7 4 7 tr 12 3 1 77 5af 68 8 6 2 4 tr 8 2 2 84 ld 71 5 3 3 tr 6 9 2 1 82 ldf 79 4 4 3 tr 5 3 / 2 90 3a 52 tr tr tr 22 10 13 / 3 52 3af 60 tr tr tr 20 9 11 / tr 60 7a 45 11 5 5 18 tr 12 / 4 66 7af 60 9 6 5 5 tr 10 / 5 80

Symbols: S = smectite; I+M = illite+muscovite; K = kaolinite; Ch = chlorite; Ca = calcite; Cr = cristobalite; Qz = quartz; Zeol = zeolites; FI = feldspars; CM = total clay minerals; tr = trace; / = absent.

394 R. Laviano and G. Mongelli

high smectite content (HSS, average = 74.7%; samples: 6a, 6b, 2a, 2b, 2d and 2e) and a subset with low smectite content (LSS, average = 57.2%; samples: 5a, 5b, ld, 3a and 7a).

In the LSS samples, illite, kaolinite and chlorite are always recognized, with the exception of sample 3a. In the HSS samples, kaolinite and chlorite are never recognized and illite is observed in the samples 2d and 2e only. The amount of quartz and feldspars is generally higher in the LSS samples. Diagenetic cristobalite is always present in the HSS samples with a maximum content of

20% (samples 2d and 2e) whereas its occurrence is minor or absent in the LSS samples. Authigenic clinoptilolite is detected in some samples of the high-smectite subset. Detrital carbonate is generally present in the LSS samples whereas it is observed in the HSS in sample 6a only. The mineralogical assemblage of the <2 p.m size-fraction relative to the whole-rock one is characterized by a higher smectite content. The average smectite structural formula for the HSS samples is: (Ko.o9Ca0.47Nao.25) (Tio.o3Fe0.n9A12.71Mgo.68)(OH)4(Si7.64Alo.36)O2o and indicates a prevailing montmorillonitic affinity.

Concerning the LSS, the smectite crystallochem- ical formula, because of the occurrence of other clay phases, has been calculated for the 3a sample only and shows compositional affinity for the beidellite-nontronite series: (Ko.37Cal.ooNa0.15) (Tio. l oFeo.74A12.o2Mgo.sz)(OH)4(Si7.17A10.83)O20

The scanning electron microscopy (SEM) obser- vations of the samples of the low-smectite subset and reported in Laviano & Melidoro (1994), show typical detrital micromorphology for the clay phases. Further, the interlayer K and octahedral Ti, Fe and Mg contents observed for smectite of the 3a sample suggest affinity for the smectitic phase occurring in Cretaceous-Oligocene Apennine shales, deriving from weathering of a dioctahedral micaceous precursor (Fiore & Mongelli, 1991; Mongelli, 1995). This evidence suggests that LSS samples have been affected by a certain degree of detrital supply.

G E O C H E M I S T R Y

The chemical composition of both subsets is characterized, with respect to the post-Archaean Australian shales (PAAS, Taylor & McLennan, 1985), by higher contents of Ca and Sr, probably due to carbonate minerals and lower contents of the other major and trace elements (Fig. 2). This trend

is probably caused by the diluting effect exerted by carbonate phases. Only Mg, probably acquired from seawater (Hein & Scholl, 1978), is enriched in some samples. In detail, however, the LSS and HSS samples are different: the LSS are higher in Ti, Fe, Mn, K, P, Rb, Sc, V, Cr, Ni and display a positive Eu anomaly with respect to the PAAS, (Eu/Eu* from 1.01 to 1.23) whereas HSS are characterized by a negative Eu anomaly (Eu/Eu* from 0.79 to 0.99) (Table 2). The LaN/YbN fractionation index (relative to the PAAS) ranges from 0.87 to 2.37 for the HSS and from 1.05 to 1.56 for the LSS (Tables 2, 3).

In the HSS samples, where no detrital contribution is observed, the contents of Fe, Mg, Na, V, Cr and Ni are generally greater in the <2 ~m size-fraction with respect to the whole-rock (Tables 2, 3). The contents of P, Rb, Ba, Y and REE are lower whereas Si, Ti, AI, Th and Sc show minor variation (Tables 2, 3). Inverse relationships exist between AIzO3 and Na20, Ba, La, Eu and Yb (Fig. 3a,b,c,d,e) suggesting that these elements are not incorporated in the smectite structure (Al-rich phase) during the bentonitization process. Sodium, which is enriched in the fine fraction with respect to the whole-rock, is probably concentrated in clinoptilolite and plagio- clase. Barium and the REE, on the other hand, are probably concentrated in size-fractions other than the <2 lam, probably in accessory phases. To a lesser extent, the REE trend with lower contents in the <2 ~tm size-fraction relative to the whole-rock, is observed also in the LSS samples (Table 3). In this subset a similar trend occurs for Th and the positive Ce-Th relationship (Fig. 31) suggests that light REE (LREE) and Th share a common mineralogical control, according to the suggestions of Mongelli et al. (1996) that stress the relevance of accessory phases in distributing REE and Th in siliciclastic fine-sediments.

P A R E N T A L A F F I N I T Y A N D P A L A E O T E C T O N I C S E T T I N G S

The chemical and mineralogical differences existing between the HSS and the LSS samples are indicative of different geological histories. To establish which of these differences can be confidently assumed as parental indicator(s) and which are superimposed by secondary processes is an important question. Hein & Scholl (1978) suggested the use of elemental ratios such as Fe/ A1 and Ti/AI to monitor the source compositions of

Indicators of parental affinity ./:or bentonites 395

Si Ti AI Fe Mn Mg Ca Na K P

1-

o ~

0.17

0 . 0 5 - - -

Si Ti AI Fe Mn Mg Ca Na K P

O !

0

sample 2d

o.1-:1 �9 ~ l m m ~ o.1-

0.01'

2

0.5

0 2

0.01 Rb Sr Ba Th U Zr Nb Y Sc V Cr Ni

l..a Ce Nd Sm Eu Tb Yb Lu La Ce

HSS subset

Rb Sr Ba Th U Zr Nb Y Se V Cr Ni

Nd Sm Eu Tb

LSS subset

1 i

Yb L

FIG. 2. Major and trace element compositional ranges of the bentonites normalized to Post-Archaean Australian Shales (PAAS, Taylor & McLennan, 1985).

bentonites. The Ti and A1 are thought to be chemically immobile during weathering, sedimenta- tion, diagenesis and metamorphism (Slack & Stevens, 1994) and their ratio can be significant as a parental index for bentonites. On the other hand, it is known that during diagenesis, redox changes due, e.g., to the occurrence of intrastratal solutions, can preferentially destroy iron bearing

minerals (Milliken & Mack, 1990), making the use of the Fe/A1 parameter suspect as a source indicator. In addition, this process may potentially mobilize conservative elements, such as Sc and Cr, which are probably hosted in Fe-rich phases. However, unlike Fe, Sc is trivalent only and in a large range of alkaline conditions (from near neutral to about pH 12, assuming 10 -6 as the activity of

396 R. Laviano and G. Mongelli

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Indicators o f parental affinity for bentonites

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397

398 R. Laviano and G. Mongelli

~ t a "~nn-I , I , , I , , J , , , /.rl.#~ .

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A1203 , I i I , I ~ I i I , I ,

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FIG. 3. Relationships between A1203 and selected oxides and trace elements for the HSS samples (from a to e) and relationship between Th and Ce (f) for LSS samples.

dissolved Sc) it exists as a hydroxide (Brookins, 1979), although a certain degree of fractionation 1988), whereas Cr can occur under oxidizing and between LREE and H R E E causing changes in the alkaline conditions as Cr 6+, soluble as the CrO,~- (La/Yb)~h ratio, has been observed in studies of species (e.g. Middelburg et al., 1988). As a pore-water profiles and sediments (Elderfield & consequence, elemental ratios involving Cr should Sholkowitz, 1987; Sholkowitz et al., 1989; Murray not be readily accepted as parental indicators for et al., 1991). The Eu anomaly is generally regarded sediments experiencing diagenesis, as a suitable parental indicator for sediments.

Diagenetic effects on R E E have long been Europium can occur both as Eu 3§ and Eu 2+ and assumed to be minor (Chaudhuri & Cullers, the Eu 3§ ~ Eu 2§ redox reaction can take place,

Indicators of parental affinity for bentonites 399

during sedimentary process, only under highly reducing conditions, with alkaline pore waters of anoxic sediments (Sverjensky, 1984). Consequently the Eu aqueous chemistry is dominated by Eu 3+ and the Eu fractionation from the other trivalent REE is an unlikely event.

In a Ti/AI vs. Eu/Eu* diagram, the HSS and LSS samples are clearly discriminated (Fig. 4). In a comparison between analysed pelites and volcanic average lithologies (Condie, 1993), the Ti/A1 and Eu/Eu* ratios emphasize a felsic affinity for the samples of the high smectite subset, also stressed by the low contents of compatible trace elements such as Sc, V and Cr (Table 3) and the high values of the Th/Sc index (average 2.7). This result is consistent with the available chemical data for the Apennine volcanoclastic deposits of Oligo-Miocene age (Guerrera & Veneri, 1989), and it seems to exclude the basic affinity suggested, (Dazzaro & Rapisardi, 1984) on a merely palinspastic basis. The samples of the low smectite subset, instead, have Ti/A1 and Eu/Eu* values similar to those of the average southern Apennine shales forming the bed of the bentonite deposit studied here.

Trivariate diagrams, such as La-Th-Sc and Th- Sc-Zrll0, involving immobile trace elements, are also used to discriminate between different provenances and palaeotectonic settings (Bhatia & Crook, 1986). These diagrams are based on the

observation that different tectonic environments have distinctive geochemical signatures. They have largely been used for siliciclastic sediments although some limitations arise from the following considerations: (1) the tectonic fields are based on data from Palaeozoic sediments, for which palaeo- tectonic settings are not fully known (e.g. Slack & Stevens, 1994) and (2) some sediments may have been transported from their tectonic setting of origin into a sedimentary basin related to a different tectonic environment (McLennan et al., 1990). However, the diagrams, if carefully used and constrained with other chemical, mineralogical and geological data, are generally useful. They may also be potentially useful for sediments experiencing diagenesis, because the discriminant elements are regarded as little or uninvolved in diagenetic processes (e.g. Taylor & McLennan, 1985).

In the La-Th-Sc and in the Th-Sc-Zr/10 diagrams, the two subsets are well discriminated (Fig. 5) and the samples of the high smectite subset fall in the field of the active and passive margin (La-Th-Sc) or are included in the fields of both active and passive margins (Th-Sc-Zr/10). These samples clearly show a silicic affinity and rule out an oceanic source, i.e. a basic affinity. The studied bentonites, Chattian-Langhian in age, were formed during the suture stage of the Tethyan ocean due to the convergence between the European and the

1.7

1.6

1.5

1.4

1.3

1.2

Basalt I

A n d e ~

fl "% ] LSS

1 . 0 - / ~ Apennine Shales 2 0.9 ~ /oo / o 0.8 ~ ~ " l ~ - ~ ' FelsicVolcanic

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.090.10 0.1

Ti/AI

FIG. 4. Relationship between the Ti/A1 and Eu/Eu* ratios, i From Condie (1993). Felsic volcanic, andesite and basalt: Meso-Cenozoic. 2 From Caggianelli & Fiore (1994): Cretaceous-Oligocene, errors as standard

deviations. The Eu/Eu* is calculated with respect to the PAAS.

400 R. Laviano and G. Mongelli

La

Th

/ / ' \

/; ' \ /

/ / / Active m~gms

- - o \ ~ ~ /Oceanic island / o o \ ' ~ \ ' \ \

/ ,/" \ ) Continental mar~ins \. / L j / " o S c [ arc \ Zr/10

FIG. 5. La-Th-Sc and Th-Sc-Zr/10 palaeotectonic discriminant diagrams (after Bhatia & Crook, 1986).

�9 HSS 0 LSS.

African plates. At this stage the tectonic settings are related to active continental margins (e.g. Ogniben, 1985, and references therein) and the results obtained by the discrimination diagrams appear to be consistent.

In both diagrams the samples of the low smectite subset fall in or near the field of the continental arc. Mongelli et al. (1996) observed the same behaviour for Cretaceous-Oligocene shales of the southem Apennines (Argille Varicolori Formation). This is consistent with the previous mineralogical and geochemical evidence and confirms that the LSS samples are not 'true bentonites', but detrital pelites with an affinity to the southern Apenninic shales.

On the basis of mineralogical and geochemical data, consistent with the palinspastic models, we suggest that during the deposition of the southern Apenninic shales, volcanic events occurred episo- dically, associated with the suture stage of the Tethyan ocean. These events promoted an accumu-

lation of felsic ash in the related basin and diagenetic alteration of these materials produced bentonitic layers interbedded with shales.

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