Anchizone Metamorphism in Sedimentary Sequences of the Northern Apennines (Preliminary Results)

RENDICONTI Socida !to/lono di Mlnero!OgI4 e Pdro/ogI4 ~ 33 (1) . log-IU (l977)

GIAMPIERO VENTURELU ·, MARTIN FREY··

ANCHIZONE METAMORPHISM IN SEDIMENTARY SEQUENCES OF THE NORTHERN APENNINES (PRELIMINARY RESULTS)

ABSTRACT. - More than 45 samples from predominantly shall' rocks from the Northern Apennines have been investigated by X-ray diffraction methods. Samples from the Monte Caio flysch (Monte Caio unit) are unmetamorphosed if based on evidence of illite crystallinity data and the presence of irregular mixed·layer illile/montmorillonite. All the other investigated fonnations (i.e.; Palombini shales, Giariene shales, Gottero sandstones, Calpionella limestone, Lavagna shales, Sopralacroce shales, Casanova complex), belonging to the Gottero unit, Bracco unit and Casanova complex, show unvarying ill ite+chlorite assemblage with illite crystallinity characteristic of the deep diagenetic zone or of the anchizone, while nearby ophiolitic rocks belong mainly to the prehnite.pumpellyite fades.

R IASSUNTO. - Durante gli uhimi anni 10 studio delle trasformazioni metamorfiche subite dalle formazioni aliioranti nell'Appennino Senentrionale c stato rivolto quasi esclusivamente alle rocce di associazione ofiolitka mentre le sequenze sedimen ta rie sono state molto trascurate sotto questo punto di vista. I risultati preliminari qui riportati si pongono nella prospettiva di contrbuire a colmare tale lacuna e di pottare un contributo alia soluzione dei complessi problemi geologid riguardanti l'Appennino Settenuionale. In via preiiminarc sono state prese in considerazione varie fonnazioni di origine sedimen laria appartenenti a distinte unita tettoniche affio1'1lnti in un'area a cavallo tra la Liguria orien tale e l'Emilia; di esse vengono riponati dali mineralogici _ Ira cui la cristallinita dell'i\lite - molm importanti per la comprensione dei fenomeni diagenetici e di debole metamorfismo intervenuti dopo la deposizione. Le formazioni appartenenti all'unita del Gottero, del Bracco e almeno alcune costituenti iI Complesso di Casanova hanno sublto un metarnorfismo di basso grado, cio cbe puo essere dedouo dai valori ddla cristallinita dell'illile - il cui indice I.K. raggiunge valori piuttosto bassi negH argilloscisti di Val Lavagna _ e dalla tipica par1lgenesi ad illite e clorite. In parlicolare alcune fonnazioni come, ad esempio, gli argilloscisli di Val Lavagna, gli argilloscisti a Palombini, i calcari a Calpionella, le marne di Sopralacroce e parziaImente i1 Complesso di Casanova, sono riferibili all'anchizona. Le temperature a cui tali fonnazion i sono state solloposte dovrebbero essere comprese Ita 200 e J()()O C cin:a. Viceversa il flysch di Monte Caio (Unila di Monte Caio), analogarnente aI flysch dell'Antola ad ovest della linea Varzi-Levanto, non pare aver sublto fenomeni metarnorfici di basso grado ed e riferibile ad un ambiente di diagenesi avanzata; tale formazione e caralterizzala dalla presenza di strati misti illite/montmoril lonite, cio che consellle di ipotizzare un evento termico con temperature comprese, in prima approssimazione, Ira 100 e 2()()<> C .

• Istituto di Mineralogia ddl'Universita di Patma . •• Mineralogisch-Petrographisches Institut, Universitiit _ Basd. 110 G. VENTUR£LLI, M. FREY

Inlroduction

In recent years new petrographic and geochemical data on the c eugeosy n e1ine sequences :t C) of the Northern Apennim:s has been collected. However, the interest of the various authors was particularly concerned with ocean-floor basalts - mainly belonging to the prehnitc-pumpellyite facies -, gabbros and ultramafics of ophiolitic association (GALL I -and CoRUSOGNO. 1970; BEZZI and PIlX: .... RDO, 1971; G .... LLI et alii, 1972; SPOONER and FVFE, 1973; BECCALUVA et alii, 1973, 1975, 1976; CokTESOGNO and OLIVIER1, 1974; FERRARA et alii, 1976; BoccHI ct alii, in press; etc.), the sedi mentary scquencc::s being mostly disregarded from this points of view. Although some geologists already suspected a number of M:dimentary formations as havi ng undergone a weak metamorphic process (e.g.: ABBATE et alii, 1970; ELTEIl and PERTUSATI, 1973; MALE.SANI, 1966) only recently mineralogical data have been reported (ScHAMEL, 1974): these are concerned with the low-grade metamorphic transformations which occurred in the geological units outcropping between Voltri Group and Monte Ramaceto area (central-western Liguria). Thus this work aims to provide new data on the late diagenesis-low metamorphism processes that took place in sedimentary sequences of the Northern Apennines and, panicularly, on those sequences jointly outcrop pi ng with ophioli te complexes. In this preliminary paper, mineral data and ilIite crystallinity values are reported on about fifty sa mples of pelitic, marly and - in minor amounts - sandy rocks belonging to the Antola fo rmation (Monte Caio Aysch) r), Giariette shales. Gottero sandstones, Lavagna shales, Cal pionella limestones, Palombini shales, Sopralacroce shales and Casanova complex (F;g. 1 and 2).

Geological setting

The Alps-Apennines boundary has been placed fo r long time along the Sestri Voltaggio line (north-south trending, near to Genoa); according to ScHNEIOER (1935), LANTEAuM£ (1962). ABOUIN (1961) and others, this line .separates the Alps (to the west) from the Apennines (to the east) because of the opposite polariry of the thrusts. More recently ELTER and P£RTUSATI (1973) and 5cHAMEL (1974). mainly on the ground of tectonic evidence, have shifted the limit eastward along the Varzi -Levanto line. All the formations occurring in the Northern Apennines are geologicall y well known and have been described mainly by Italian Authors. Readers unfamiliar

(I) In this paper eugeosyncline sequence is used as a comprehensive term to include all the formations attributed to the cugcosyncline environment by ASBATE et alii (1970) without any other speci fic geological meaning. (~) Eastward of the ~ Bracco ridge. (DECANDlA and ELT£l, 1969) the Monte Antob formation is called Monte Caio Bysch (ZAmUCQlI, 1%3). ANCHIZONE (SUB--<:REENSCHlST) METAMORPHISM IN SEDIMENTARY £CC. III

)~.J7 )'·40•" .. , T... .. I... .. "

"• • u MIC ~ .,. . u ... o u ·

•. -

Fig. I. - Simplified ,ketch map showing 5:lmple loc:ulon in the Northern Apennines. The: number of the "'mpla 3fe referred to Table I. SIImple number. for the Mt. Pcnna ,.,."" arc not ,hown. with italian complex nomenclature can find a detailed description of this area, on the basis of the classical geosyncl ine theory, in c Development of the Northern A~nnines Gcosyncli ne, (1970). As far as more recent paleogeographic and tectonic reconstructions are concerned, one can see BARRIERI et alii (1968), GUJr.Tl and PASQUARE (1970), Bocc.u.£TTl et al ii (1971), GlF.8£ et alii (1970), L AUIISCHER (1971), HACCAJU) et alii . (1972). BItAGA et alii (1973), D ECANDIA and ELTER (1973), ZANZUCCHI (1972), $ T UItANI (1973), ELTER and PERTuSATI (1973), ScHAMEL (1974), DAL PIAZ (1974). 112 G. VENT URELLI, M. FIlEY

The investigated area IS located NW of Chiavari on the boundary between Liguria and Emilia just east of the zone recently investigated by $CHAMEL (1974). Many fo rmations occur in the region (see ABRAn and SAGRT, 1970) which have very different lithological characters and which constitute several sequences. According to the present geological interpretations, these formations may be grouped into different tectonic units all showing an eastward polarity typical of the Ap peninic area (ELTER and PERTUSATI, 1973). The formations sampled are as follows :

" "\

Go,uro ""It 1,,«. "nit eo ...... "",, 1• • A

GOTWIC U~II SR,.\OC(l U" IT MO ~ lE U-,1 D U/HI 2 " (6. 1) " (5.7.7 .l) (6.1-1.7) " (19 .9. ZI.G) !i 11 · 11 "" (4 .7. I.l) " '" Mt. I_,I O GMIII RI

Fig. 2. - A - Simplified sketch of (he tectolllC rdationship. among 5<)mC geological units occ urring in Ih" Northern Apennines (modified after EI-TE", 1972, and ZANZUCCIIl, per •. comm.). - S - Sch"maoc columnar """lions (not 10 scale) of the various tectonic units describc:O in the text. The ranges of illite CI}·s!allinity values are reported in bracket •. _ Symools: GS = GiariclI" shales; GSJ = Goucro undilones; LS = Lavagna shales; PS = P310mbini shales: CL = Ca lpiondla limestone; SS = Supra bcrocc ,hale.; CC = eas.anon complex; MCI = Monte Caio HYKh; OF = ophiolil"s; 01 = ophiolile, mainly aI oli,tolilh.; 0 = o1istomnmcs; I/M = mixed-layer i!litc/mommorillonile.

1. c: GO'lTERO UNIT :t MT. GHIFFI (GHIFFI RIDGE). MEZZANEGO AREA Giariette shales (Paleocene). Marl y, shaly and si hy rocks; the type area is near Giariette, a village between the Bocco pass and Santa Maria deJ T aro. Gottero (Monte Zatta) sandstonn (Upper Cretaceous· Paleocene). Graded, thick-bedded grey sandstones interbedded with dark shaly and marly rocks. iAvagna shales (Upper Cretaceous). Slates, shales, laminated siltston~ and small amounts of limestones essentially constitute the formation. ANCHIZONE (SUB.cREENSCHIST) METAMORPHISM IN SEDIM ENTARY ECC. 113

FORMATiONS AREAS

Antola fomation Mt . TOIn

Mt.Agugiaia

o Mt.TOIn

CHanova complex 01 istostromes: I ,near So .. pra I acrtlce; 2, Mt . AgugiaTa ZO NE Matrix of po l igenlc brec OIAGENESIS cia:Mt.Agugi ala - • Basalts:Mt.PeMa

Gia riette shales ill

Gottero sands tones Iil Ghiffl ridge ANCH1ZON£ Mezzanego lavagna shales la!

Pa lombini shales Gravegli a valley EP1ZONE Biscla pass Calplonella limestone Vara valley

S, sandstones; C, CaCI 2 sa turated

Fig. 3. - H istogram of illite cr)"Stalli ni ty (I.K.) va lues (after KUHI..EII, 1968) for the different inve\tigated formations. Two main groups Can be defined: 1 _ the Mon te Caio Rysch, which belongs to the diagenetic zone, 2. all other formations which are typiul of low-grade metamorphism environment. T he lowest average value of '-K. is found in the Lavag na shales.

2. C BRACCO UNIT . GRAVEGLlA YALLEY - BISCtA PASS - UPPER V ARA YALLEY AREA Lavagna shal~s (see above). Palombini shal~s (Neocomian - Cenomanian ?). The formation consists of alternating layers of very-fine grained limeslones (c palombi ni.) wilh interbedded shaly sedimems; the frequency of limestones decreases upwards. Calpion ~lla lim ~s ton~ (Barremian - Vala nginian). Grey and/or white Radiolarian bearing limestone with concoidal fracture; the calcareQus beds are separated by marly layers. Ch~ts (Maim ?). In thin beds generally red, morc rarely greenish; at the transition with the Calpiondla limestone, interlayerings of limestone and chert can be found. Oph ;olit~ rocks. Serpentinized ultramafics, gabbros and ocean-floor basalts.

3. C C ASANOVA COMPLEX . In the Monte Agugiaia area this complex includes the Sopralacroce shales formation. 114 G. VENTURELLI, M. FREY

3a. MONTE ACUGIAIA - SOPRALACROCE AREA Sopralacroce shaleJ (Paleocene). This formation is very similar to that of the Lavagna shales; it includes mainly shaly, silty and marly rocks. Casanova complex (Upper Cretaceous). It consists of laminated siltstones and shales, graded sandstones (Dragonale sandstones), sometimes with interbedded limestones, breccias and congiomermes, olistostromcs and olistoliths including slabs of basaltic and ultraman.c rocks. In the Monte Agugiaia area grey sandstones interbedded with shaly beds are abundant. 3b. MONTE TOMARLO - ANZOLA - M ONU P ENNA AREA Casanotla complex (Upper Cretaceous). The lithology is reported above. In the Monte T omarlo area the sandstones are abundant and clearly visible.

4. ( MONTE CAIO UNIT ~ Antola formation (Monte Caio flysch: ZASZUCC HI, 1963) (Upper Cretaceous). Calcareous-sandy turbidites with interbedded shales; the formation belongs to the Helminthoid Aysch sequence. A schematic and very semplified sketch of the tectonic relations among the different units is reported in Fig. 2.

From diagenesis to metamorphism

The transformations undergone by the sheet silicates in daystones and mariy rocks can indicate the degree of diagenesis or low-grade metamorphism which these rocks heve undergone (e.g.: DUNOYER et alii, 1976; KUBLER, 1967, 1968; FREY, 1970; ZEN and THOMPSON, 1974; FOSCOLOS et alii, 1976). In the diagenetic zone typical day minerals include smectites, kaoli nite, irregular mixed-layers minerals and IMd illite; in the anchizone IMd and 2M illite and chlorites are predominant over pyrophyllite, mixed-layer paragonitejphengite (FREY, 1969) and paragonite. METHODS About fifty samples of the previously melllioned formations have been studied by X-ray methods. From each sample the whole rock powder and the fraction < 2 I.l. (air-dried, glycolated or heat treated where necessary) were analyzed with X-ray diffractometer. The relative abundance of the sheet silicates was determined semi quantitatively from the intensity ratios of some basal reRections of the clay fraction. A Guinier camera was used to determine the d(06O) values of illite and chlorite while the chlorite basal spacings were determined by X-ray diffraClometry. In both cases quartz was used as the internal standard. T he illite crystallinity has been determined as width at half.height of the 10 A peak according to KUBLER (1967). T he intensi ty ratio between the peaks at d = 2.80 and d = 2.58 A was used to obtain relative amounts of the 2M and IMd illite polymorphs (MAXWELL and H oWE.R, 1967). T he composition of the chlorites was estimated from the basal spaci ngs and d(06O) values using the regression equations given by W U7.EL (1973). ANC H IZONE (SU B-<: RE £NSCHIST) METAMORPHISM IN SEDIMENTARY ECC. 11 5

28 i

DIAGE NESIS " ",' 22 ",'

, 20 '" ~ ,>=, '" < 0; 0 0 ~8 u 0 On v 0 I!l - 7 0 DD " ANCHllO N E I!l 6 Iil Ii1~I!l OJ I!l

.r------~

EPllONE

Fig. 4. - ll1 ite crystallin;I, ... illile d(060) nlues. 11w: sample,; from the Monte Caio AyKh and from the c.... nova wmp l e~ (Mt. Tornado un) define IWO well KplITale fieWs. Symbol. as in Fig. 3.

RE SU LT S The sample locations are shown in Fig. 1 and the results are presented m Table 1 and in Figs. 108. 116 C. VENTURELLI, M. FR£Y

TABLE 1 Mineralogical data on clay minerals from some unmelamorphosed and anchimelamorphosed formations occurring in the northern Apenninu

_"",,-..-... <11\, N ...... ,."., ..... ,~". "",.. "'" ...... <"".,-...... ,." ..... ,.'" _. •• 1." .....t"'''''''' ., nun ",,_., ...... IU'" ., ,_ , "', In ...... , ~ ,_• ••", . ... T.. In "" """DO ...... - " p ...... A." I.', 10 ...... " !." ,.;...... ,.... ,- ...... ,_ ...... • .. .., ...... ~...... -,._", .. _,- .. • • ... nm.-•• .-._. ... • ...... 3-<00 __," ...... ,._-, • • .••• .. • .. • ... ftIl4.,,_._, • .. .., ... • .. .., ! ... • • ... ,i .. • .. '.' _.. • .. ... ~. • • .. , =. .. .. •• ,t =, ... H.'...... "", .. :,,,,, ..,,,, .. ""e...... , •• .. .. '.' ." .",,<1.' • • •• • .. ... i ..,,, .... , ••..- ,It· ...... • .... .-1 '.' ••• • . """' .... , ...... U- ... • .. .." ... •.... _ .... ,..'_1 • ...... • • '.' • • '.' 1 ... .. • .. '.' ..=...... =. • .. .. ••• ...... • ...... '- .... _1.. .. '.' ...... " .. ... ! ... .. " .. ... !, ,._•• <-0>. fL_" ... • • ...... n.,' ,I .. • .. ... " _. ...'...... r/' . .···lO.-I!. ". •• ... ..,. .. >l .a" '! ...... ' ...... ' H =,-'- .. " .. '.' rJ

- According 10 the present geo!ogial knowledge" ;. the nudied v .. the above mentioned formal;onl belong to: I . Gonero unit, 2 . Bracco unit, 3 . C

• , ,

, ·," , o, ,-< o < --,-, , , , - • •o , r ...... I - • .1 ~- ·; l zo • ; " ! -•o • " ,l "Z · o -• < • o , , < •, Z r---'-,.:---J, -~-~, • o " u• •<

2• , • •< • • • -o• • " • •u , •< , , · , · .r--'. • 118 C. VJ.;NT URELLI, M. FREY , , • • SQmpl~s from lh~ anchizon~ C) and '" from th~ Irantition diag~lIesis-anch izone I ....., 10'1 With the exception of the Monte <> . Ca io fl ysch sa mples, all the analy~d rocks belong to this group. T he main ,., mineralogical assemblage of the shaly portion is constituted by illitc, chlorite, quartz and albitc; calcite is also present is variable amounts; dolomite has been detected only in a few samples from the 8 Lavagna shales, Palombini shales and Sopralacroce shales. A large gap in the illite crystallinity separates the Monte Dio Aysch from the other investigated formations (Figs. 2, 3 and 4). The lowest values in illite ,., A crystallinity index (I.K.) were found in the Lavagna shales of the Gottero unit. The samples from the Casanova complex of Monte T omario, on the other hand, show the highest values and belong to the deepest part of the diagenetic zone. Moreover, these samples are characterized by high illite d (06O) val ues suggesting or ~ r , , , , •, . relativel y high iron and/or magnesium 29 CuK .. /NI contents. In Fig. 4 the Monte Tornario Fill_ 6. - l)itfr~'10mCl(r (raccs obtained from a $ample of the MOlltc Caio Aysch (A). The positiol! samples define an area c1e.uly separated of the illitc/mommOf"illollh" peak shifts 10 Iowa fro m the rocks of all other formations. angle "alu,. after trnuncnt with e,hy]"" gly_ ,0/ (8). The chlorite composition (as esti mated from X-ray studies) shows a rela tively narrow Si distribution (53-5.9) and a somewhat larger range in the Fe-Mg distribution (Fe~·/(F e2 ' + Mg) = 0.18-0.5 1), Fig. 8. Following HEY'S classification these chl.orites should be termed ripidolile, pycnochlorite and clinochlore. It is noteworthy that no penninite was fo und in metabasalts from Monte Penna (BECCALUVA et al ii, 1975. 1976) alt hough this ch lorite variety is mentioned in many petrographic studies on basahic rocks of the Northern Apennines. The chlorites from the Casanova complex (with the exception of the Monte Agugiaia area) show the highest Si contents observed.

(3) Note added in proof. Recently P£RTUSATI and HORRENBI!RGER (1 9n) have reported structural evidences of anchimetamorphism for the La vagna shall'S belonging to the Gouero unit (Boil. Soc. Genl. It., 94. \375-1436 ). ANCHlZONE (SUB--<:REENSCHlST) METAMORPHISM IN SEDIMENTARY ECC. 119

" .. <2".. " • i'~ 3 • •u" · 0 - ,

Fig. 7. - JIlite aystallinity VI intensity ratio of the peaks at 2.80 and 2.58 A arcally computed (JCe MuwEL.l. and How .., 1967); the ratio va lues are dirccd)' proportional 10 2M illile content. In the plot, the 2M pol)'morph content ranges roughly from 30 10 100 % as the illite UYlullinily inaoses (i.e. the numerica l values ol I.K. decrease). Symbols a. in FIg. 3.

Summary and conclusioDs

Most of the sedimentary formations studied from the Northern Apennines constituting the Gonero unit, the Bracco unit and the Casanova complex Ixlong to the low-grade metamorphism (sub.greenschist). IIlite crystallinity data and the predominance of ilIite and chlorite suggest the samples from the Giarictte shales, the Gottero sandstones, the Lavagna shales. the Palombini shales, the Calpionella limestone and the Sopralacroce shales of the studied area all Ixlong to the anchimetamorphic zone, while the samples from the Casanova complex Ixlong both to the anchizone (Mt. Agugiaia) and to the deepest part of the diagenetic zone (Mt. Tornado area). The Monte Caio Rysch (the equivalent of the Antola formation in the east), on the other hand, is unmeta· morphosed. tn the Monte Tomarlo-Anzola area, the Monte Caio Rysch and the Casanova complex are in tectonic contact and have distinctly different values in illite crystal linity. This suggests that the Monte Caio Rysch as well as the Monte Antola formation (see SCHAMEL, 1974) had a somewhat different geological histories in comparison to the other investigated formations. In the field the Casanova complex is chaotic and lithologically inhomogeneous 120 c. VENTURELLI. M. FREY

III W • C • • .. I • •, ~ • !iO 0 .. , " u •· • 0 ~ • 0 .' 0 ""Ji , ":OA u • • • • • ~ • • III ,. ... _ ..,. " " "

• 0 • • 0 0 • ," 0 u • 0 • • • • ,• • • u •

Si "

Fig. 8. - Chlor;!e classification ~heme after Htv (1954) (modifinl con.idcring only th., divalent iron) with po:t:;lion of "" mpln from the sludit(] formation •. The Fe'·, Mg and Si values were determined using the regression equa{ion~ prOflO$C'd by WKTZIiI.,. (1 973). The scattering of m~gne $i um and iron is very wide; moreover, Ihe chlor;tc. from the Monte I'cnna rnclabasalu and from the C:u."ooYa complex of the MOnle Tornado un show the high.,,! si lica contcnlS. Symbols a. in Fig. 3. characterized by olistostromes and olistoliths of basaltic and ultramafic composition. The pe1itic rocks from this for mation, sampled in the Mt. Agugiaia and Mt. To rnarlo areas, show some differences in iIlite and chlorile composition as well as in ilIite crystallinity. Comparisons with other areas (e.g.: MUFFLER and WHITE., 1%9; ZEN and THO'-IPSON, 1974) suggest that the Monte Caio Aysch and the Monte Antola ANCHIZONE (SUB-GREENSCHlST) METAMORPH ISM IN SEDIMENTARY ECC . I2l formation (east and we:st of the Varzi-uvanto line: rispectivdy) rc=achro te:mpe rature:s in the range: of loo-2Q()'> C, while: the Olhe: r fo rmations experie:nce:d tem peratures of about 200-300" C at some time: of their geological history. Such metamorphic imprint, which agrees to the prehnite·pumpellyite facie:s of the: metabasalts, could be referre:d to oroge:nic metamorphism; K jAr age dating already in progre:ss on the illite:s of the: Lavagna shales and on the me:tabasa.lts from the Casanova complex, would be very useful to connect this metamorphism to the tectonic e"cnts of the Northern Apennines. Obviously further investigations are: nt':(:ded to get the: geological and tc=c:tonic meaning of the mineralogical characters re:ported above:; in any case the: prc=.sen t results should be take:n into account for future interpretations of the geological history of the Northern Apennines.

Aknowledgmentr. - !be authors are graleful to prof. E. NIGGLI, University 04: Bern, 10 prof. V. BoRTOLOTTI, University of Firenze, to pro£. G. V. nAL PIU, University of Padova, to profs. F. EMILlANI, G. MEZZADRI, G. ZAmuccHI and 10 dn. R. VALLONI and L. VEIlNIA, Un iversity of Parma, to dr. R. KLlGFlELD, Colombia University for criticism on the manucript and/or useful assistance during the field work. The researt:h ha5 been carried out at the Mineraiogisch-Per:rographisches Inslitut, Univenily of Berne. Financial support was by Consiglio Narionale dd1e Ricert:be (It.lia) . Gruppo di Lavoro sulle Ofioliti Meditemmet':.

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