Geological Setting and Genesis of Coticules (Spessartine Metapelites) in the Lower Ordovician of the Stavelot Massif, Belgium

Journal of the Geological Society, London, Vol. 143, 1986, pp. 253-258, 8 figs, 2 tables Printed in Northern Ireland

Geological setting and genesis of coticules (spessartine metapelites) in the Lower Ordovician of the Stavelot Massif, Belgium

J. LAMENS’, F. GEUKENS’ & W. VIAENE2 Instituut voor Aardwetenschappen, Katholieke Universiteit Leuven, Redingenstraat 16, B-3000 Leuven, Belgium ’Afdeling Fysico-Chemische Geologie, Katholieke Universiteit Leuven, Celestijnenlaan 200C, B-3030 Heverlee, Belgium

Abstract: The LowerOrdovician in the CaledonianStavelot Massif (Belgium) containsa coticule member, probablyArenig in age, composed of redhematite-rich shales alternating with thin yellowish coticulebeds. These coticules are fine-grained highly manganiferousmetamorphic rocks mainly composed of spessartine garnet, mica and quartz. Similar rocks of about the same age occur in several localities along the Appalachian and Caledonian fold belts. The Belgian coticules are thought to haveoriginated from marly sediments which had beendeposited rapidly, probably by density currents, and in which CaCO, had been diagenetically replaced by MnCO,. During metamorphism, spessartine garnets- were formed from this MnCO, and mica. Manganese and iron are thought to have originated from volcanic-exhalative processes.

The Stavelot Massif is one of the five Caledonian Massifs in could be used as a key lithology for correlation along the Belgium. It consists of MiddleCambrian to Lower Appalachian and Caledonian fold belts. Similar rocks have Ordovician strata (Fig. 1, Table 1). The southeastern partof also been described from older (Precambrian) and younger the Stavelot Massifis at a low metamorphicgrade. The strata(Roy & Purkait 1968; Vearncombe 1983; Minnigh LowerOrdovician of the Stavelot Massifis called 1979). Most of these spessartine-rich rocks, however, have “Salmian”.It has been subdivided into Salmian 1 (Sml), morequartz and less mica than the Belgian coticules. Salmian 2 (Sm2) and Salmian 3 (Sm3) by Geukens (1965); Several origins havebeen proposed for these quartz-rich these units are lithostratigraphic. The Sml is Tremadoc in coticules: Mn-rich sand layers (Clifford 1960); cherts ageasindicated by the presence of the graptolite originating from chemical precipitation of silica and Dictyonema flabelliforme. The Sml consists mainly of black, manganesesalts (Schiller & Taylor 1965; Doyle 1984); grey andgreen shales, siltstones and sandstones; it is detrital sediments rich in manganese oxides (Roy & Purkait thought to representa gradual transition from a basin plain, 1968); chemically precipitatedMnCO, (Woodland 1939); characterized by turbidite deposits, through adepositional and Mn-rich volcanic tuffs (Kramm 1976). slope to a shelf (Lamens 1985). The Sm2 and Sm3 contain This paper discusses the origin of coticules in the Belgian no macrofossils; judging from acritarchs they are attributed type area based on their sedimentological and geochemical tothe Arenig and Llanvirn, possibly also the Llandeillo characteristics and on a study of stratigraphically equivalent (Vanguestaine 1973). The Sm2, which is present in the strata in the non-metamorphic area of the Stavelot Massif. Stavelot Massif only, consists mainly of reddish pelitic rocks rich in iron and manganese. Geukens (1965) subdivided the Sm2 of the metamorphic area into Sm2a, Sm2b and Sm2c. The coticule-bearing Sm2b strata in the The Sm2a is composed of redsiltstones and shales. The metamorphic area of the Stavelot Massif Sm2b, also known as the coticule member, consists of red The Sm2b in the metamorphic area consists of red shales shales alternating with coticule beds of 1-15 cm; these are alternating with coticule beds of 1-15 cm (Fig. 2). The red fine-grained highly manganiferous metamorphic rocks which red shales are composed of fine-grained mica, quartzand are mainly composed of spessartine (Mn-garnet), mica and hematite; they also contain small amounts of spessartine quartz. The Sm2c consists of violet and green chloritoid-rich garnet and some andalusite and kaolinite blasts. They show shales. The Sm3 is composed of grey and black shales and a regular horizontal millimetre-scale lamination. Coticules siltstones. are on average composed of about 40 wt% garnet, 30 wt% Coticules werepetrographically described forthe first mica and 30 wt%quartz. They are almost devoid of time in Belgium by Renard (1878). They used to be mined hematite. The rock is very fine grained;the garnets are and sold as whetstones; they owe this quality to their high mostly less than 20 microns diameter. There are also more content of very small garnets. Similar spessartine-rich rocks quartz-richvarieties of this coticule: these consist of a havesince been found inseveral other countries. These millimetre-scale alternation of greenish quartz-rich laminae rocks occur mainly in the lowerOrdovician of the and yellow garnet-rich laminae. Rarer varieties are rich in Appalachian and Caledonian fold belts;they have been andalusite or chloritoid. found in New England, Massachusetts,Newfoundland, The contact between the coticules and the shales is usually Nova Scotia, Ireland, Wales and Norway (Kennan Lk very sharp. In contrast to the shales, the coticules and their Kennedy 1983). Kennan & Kennedy suggest that coticules quartz-rich varieties show many sedimentary structures. The 253

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Fig. 1. (a) Caledonian Massifs in Belgium. (b) Geological map of the Stavelot Massif (after Geukens 1984).

most frequent structures are recumbent folds and low-angle slump structures. The slump direction is towards the south. faults(Fig. 3), which areinterpreted here as small-scale Primary small-scale cross-lamination is also a typical structure. Figure 4 shows garnet-rich oblique laminae in a quartz-richlayer between horizontal quartz-rich and garnet-rich laminae. Figure 5 shows a sequence starting with 1 a cross-laminated quartz-rich lens atthe base, passing Table 1. Lithostratigraphic units inthe Stavelot upwards into horizontal lamination and recumbent folds. It Massif in correlation with the Cambro-Ordovician stratigraphic scale. After Geukens (in Robaszynski is noteworthy that the lower part of a coticule bed is often & Dupuis 1983) quartz-rich whereas the upper part is garnet-rich. This may reflect primary compositional differences caused by graded Ashgill Caradoc bedding. The bases of the beds sometimes show small load Llandeillo casts or flame structures or may be slightly erosional. Some Llanvirn SalmianLlanvirn 3 beds contain small red mudstone fragments (Fig. 6). Arenig Salmian 2 Salmian Arenig The coticules are typically very rich in manganese and Tremadoc SalmianTremadoc 1 poor in iron when compared to the associated shales which are much poorer in MnO and richer in Fe,03 (Table 2). The UPPER Revinnian contact between shales and coticules is also geochemically CAMBRIAN (f1900m) very sharp (Fig. 7). The coticule member is thickest (40-50m) near Vielsalm MIDDLE Devilliarl (Fig. 1). It thins towards the north and towards the west. It CAMBRIAN (+600 m) is only a few metres thick near MaImCdy in the north and about 10metres near Lierneux in the west; it seems to be

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Fig. 6. Coticule bed showing slightly erosional base and small red mudstone fragments. totally absent further west. NearLierneux some thin (ca 1m) volcanic rock layers interstratified with Sm2b red shales and coticules havebeen recently discovered (Lamens & Geukens 1985). Theyare greenish porphyritic rocks of intermediate composition which belong to the calc-alkalic series. Fig. 2. Red shales and yellow coticule bed. Note the sharp contact between the two lithologies. The Sm2b strata in the non-metamorphic area of the Stavelot Massif The Chevron syncline in the non-metamorphic area (Fig. 1) contains strata which, with respect to the well-defined Sml-Sm2 boundary,appear to be situated atthe same stratigraphic level as the coticule member (Sm2b) in the metamorphic area. This stratigraphic interval consists in its lower part of an alternation of red shales, siltstones and coarse-grained beds of 1-20 cm thick. These beds are either structureless or show horizontal lamination andgraded bedding. They are composed of fragments 1-5 mm in size in Fig. 3. Coticule bed showing recumbent slump fold and low-angle a hematite-rich matrix (Fig. 8). Most fragments are wholly fault. or partlycomposed of carbonates. The most conspicuous

Fig. 4. Thin section of coticule showing oblique garnet-rich laminae (dark) in quartz-rich layer between horizontal quartz-rich and garnet-rich laminae.

1 3 5 7 9 % 2'6 'lO'%

Fig. 5. Coticule bed showing cross-laminated quartz-rich lens at the Fig. 7. Geochemical profile in the Sm2b showing the sharp contrast base, horizontal lamination in the central part andrecumbent folds in Fe and Mn contents between coticule beds (white) and shales at the top. (black).

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ponents also occur: they are carbonate-richmudstone fragments,probably intraclasts, and well-rounded quartz, quartzite and volcanic rockfragments. The carbonate mineral present in such high amounts in this rock is however notCaCO, but MnCO,; it is nearly pure rhodochrosite containing on average only 3%Ca (Berger 1965). The chemical analysis of the rock shows a high MnO and a low CaO content (Table 2). The rest of this stratigraphic interval consists of red shales alternating with whitish beds 1-15 cm thick. The red shales are composed of fine-grained mica, quartzand hematite. They also contain small amounts of rhodochrosite. The whitish beds are composed of fine-grained quartz, mica and variable amounts of rhodochrosite. Some beds are very rich in rhodochrosite and contain very little hernatite. They show horizontallamination or cross-lamination. The chemical composition of these whitish beds and thered shales is given in Table 2.

Discussion The Sm2rocks inthe metamorphic area of the Stavelot Massif are rich in spessartine garnets whereas those in the non-metamorphic areaare rich inrhodochrosite. Such mineralogical composition strongly suggests thatthe spessartineoriginated from rhodochrosite.Formation of spessartine from rhodochrosite and mica is a well-known metamorphic reaction (Choubert 1973; Dorr 1973; Shannon 1977; Woodland 1956). We consider the whitish beds composed of rhodochrosite, mica andquartz as the non-metamorphicequivalents of coticules. However, the origin of these highly manganiferous carbonate beds and the nearly perfect separation of manganese from iron remain a Fig. 8. Thin section of coarse-grained bed in the non-metamorphic problem. area showing carbonate fragments in a hematite-rich matrix. The sedimentary structures observed in the coticules are in sharp contrast with the regular horizontal laminations in components have been identified as echinoderm fragments; the shales. They indicate that the coticules must originally they form 10-30% of the rock. Other carbonate fragments have been very water-saturatedsediments which were could bealgae (oncolites). Elongate calcium phosphate rapidly deposited under relatively high-energy conditions fragments are probablyfrom shells. Non-biogenic com- compared to those existing during shale deposition. The red shalesprobably originated from slow sedimentation of Table 2. Chemical analyses of Sm2 rocks in the metamorphic suspended clay, silt and trivalentiron compounds. If the (A,B, C)and the non-metamorphic (D, E, F) areas. A;coticule; B: ‘proto’- coticules were deposited rapidly the iron would be quartz-richcoticule; C: red shales; D: coarse-grained fossiliferous considerably diluted. The separation of ironfrom rock; E: white rhodochrosite-bearing bed; F: red shales. B and C are from Kramrn (1976) manganese may be explained by their important difference in chemical mobility: starting from a solution rich in both Metamorphic area Non-metamorphic area elements,iron can be precipitatedquantitatively whereas A B C D E F manganeseremains in solution (Krauskopf 1956, 1957). Furthermore, during diagenesis, manganese is more readily Si02 48.83 63.75 51.00 43.12 51.01 57.64 remobilized than iron. This manganese was concentrated, as TiO, 0.99 1.10 1.04 0.55 0.77 0.83 a carbonate, in the thiniron-poor beds. Wesee three A1203 19.77 18.67 20.77 5.42 17.34 20.17 possible ways whereby this may have happened. 0.47 0.47 15.70 26.92 0.09 8.64 First, by direct chemical precipitation of MnCO, from the Fe0 0.70 0.54 0.47 0.35 7.28 0.72 sea-water concurrent with sedimentation of clay and quartz. MnO 24.63 8.32 0.61 11.01 5.88 0.91 Since coticules appear to have been deposited rapidly, this MgO 0.72 0.61 1.51 1.04 1.62 1.58 would require high instantaneous manganese concentrations CaO 0.54 0.62 0.42 1.47 1.40 0.37 in the water, which is unlikely. Moreover such a hypothesis Na,O 0.36 1.42 1.52 0.29 0.85 0.76 cannot explain the traction and erosion features observed. K20 1.12 1.75 2.32 1.16 2.74 4.86 Second, by diageneticreprecipitation of manganese. H,O? 1.13 2.12 3.58 1.48 2.13 2.59 During diagenesis manganese could be remobilized in the 6.82 7.65 0.56 CO2 <0.04 0.00 0.00 sediment,migrate upwards with interstitialwater and 0.29 0.28 0.40 0.53 0.19 p205 0.08 reprecipitate atthe sediment/waterinterface (or more generally at the oxiclanoxic redox boundary). This process 99.38 99.66 99.22 100.03 99.29 99.82 has been invoked by several authors for the formation of

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Mn-rich crusts and nodules at the bottom of seas and lakes. comment as to their origin. Considering the trace element In thepresent case,however, it is incompatible with the composition of the rocks,Kramm (1980) favoureda geochemically very sharp lower surfaces of the coticules and volcanic-exhalative source. We doubt whether it is possible with the sedimentarystructures observed. A selective to distinguish volcanic-exhalative FeMn-deposits from reprecipitation in previously depositedpermeable (sandy) terrigenous ones on the sole basis of trace elements. Both layers is contradicted by the observation thatthe types tend to be enriched in trace elements: Fe and Mn are manganese-rich coticules are typically very fine-grained and known to be powerful scavengers, whatever their origin rather poor in quartz. (Sayles et al. 1975). Nevertheless we agree with Kramm's Third, by diageneticreplacement of calcite by MnC0,. conclusion. The MnC0,-rich fossiliferous beds in the non-metamorphic continentalA weathering origin in warm climatic area clearly indicate a diagenetic replacement of CaC03 by conditions is hardly consistent with recentpalaeomagnetic MnCO,. Replacement of Ca2+ by MnZ+in carbonates is a datafor the Lower Ordovician which place Belgium at well-known phenomenon. Bostrom (1967) suggested that about 60" south of the Equator (Van der Voo et al. 1980; MnCO, originated by replacement of CaCO, : xCaCO, + Perroud et al., 1984). Iron and manganese enrichments and, yMnZ+= (Ca,_,Mn,)CO, + yCa2+. Due to its similar ionic on metamorphism, formation of spessartine-rich rocks have radius, electronegativity and charge, MnZ+is able to replace occurred on a large scale during Lower Ordovician (mostly Ca2+ in calcite (Shanmugam & Benedict I11 1983). The Arenig)times. These deposits occur along thenorthern partition coefficient of Mn2+ between calcite and solution is margin of the former Armorican tectonic plate, correspond- greaterthan unity so that calcite in contact with a ing to the southern margin of the Lower Palaeozoic Iapetus manganiferoussolution is enrichedin Mn whereasCa is Ocean and Medio-European Sea (Kennan & Kennedy 1983; dissolved (Pingitore 1978). This process is enhanced if cf. also Perroud et al., 1984 and references therein). They manganese concentrations in the water are high; Milliman are often spatially associated with volcanic rocks; this is also (1971) mentions very high manganesecontents in calcite the case in Belgium (Lamens & Geukens 1985). This associated with submarine volcanism. A similar replacement laterally extensive occurrence of FeMn-depositsand of Ca by Mn in carbonates associated with volcanic rocks is volcanic rocks near aformer plate margin and in a fairly also described by Bischoff (1969) in Red Sea sediments and narrow stratigraphic interval seems to indicate a large-scale by Shanmugam & Benedict I11 (1983) in the American production of ironand manganese by volcanic-exhalative Ordovician.We can apply this replacement hypothesis to sources, probably related to continental margin processes. the fine-grained rhodochrosite-rich beds of the non- The Sm2 in the Stavelot Massifis lithologically and metamorphic area and to the coticules themselves. We think geochemically fairly homogeneous. It is, therefore, unlikely thesewere originally composed of clay, quartz and lime thatthe iron and manganesecompounds represent direct mud, corresponding to a marl. precipitates around exhalative centres, since such deposits The Sm2 was depositedin quieta basin deepening tend to be very irregular and lensoid. We think exhalations towards the southeast and limited in the west by a platform. in Belgium took place at a certain distance to the south of This platform is documented by an important stratigraphic the present Stavelot Massif. Metal enrichments can form at gap (corresponding to the Arenig) observed in the WCpion aconsiderable distance from the exhalative centres; this borehole in the Condroz Massif (cf. Graulich 1961). In the obscures all primary variations and leads to homogenization basin, slow suspension sedimentation of clay, silt and iron (Toth 1980; Sayles et al. 1975). Iron and manganese would and manganesecompounds took place. Onthe platform, have been transported in thewater, as colloids or in calcium carbonate was being produced, at least partly solution, deposited slowly from suspension and enriched in organically. Now andthen carbonate sedimentfrom the the mud. On diagenesis manganese was remobilized and platform was brought intothe basin, possibly by density eventually concentratedin carbonates by replacement of currents. The coarse material reached no farther than the calcite. Chevron area whereas the fine lime mud extended into the Vielsalm area; it was deposited rapidly so that it contained Theauthors wish tothank P. S. Kennan(University College almost no iron. Manganesereplaced calcium in the Dublin) for interesting discussions and for showing US the Irish carbonates so that the beds consisted eventually of a mixture coticules in the field. B. Mamet and A. Prtat (Universitt Libre de Bruxelles) were helpful with the identification of the fossils. D. of clay, quartzand rhodochrosite. 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Received 10 July 1985; revised typescript accepted 12 September 1985

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