Tectonic structure and post-Hercynian evolution of the Serre, Calabrian Arc, southern Italy: Geological, petrological and radiometric evidences

Tectonophysics, 124 (1986) 223-238 Elsevier Science Pub&hers B.V., Amsterdam - Printed in The Netherlands

223

TECTONIC STRUCTURE AND POST-HERCYNIAN EVOLUTION OF THE SERRE, CALABRIAN ARC, SOUTHERN ITALY: GEOLOGICAL, PETROLOGICAL AND RADIOMETRIC EVIDENCES

ALDO DEL MORO t, ANTONIO PAGLIONICO 2, GIUSEPPE PICCARRETA * and ALESSANDRO ROTTURA 3

’ Istituto di Gevcronologia e Geochimica Isotopica del C.N. R., 56100 Pisa (Italy) ’ Dipartimento Geomineralogico, Universith di Bari, 70121 Bari (Italy) ’ Istituta di Minera~ogia e Petrografia, university df Bologna, 40127 Boiogna (Ita&) (Received April 26, 1985; revised version accepted September 25, 1985)

ABSTRACT Del Moro, A., Paghonico, A., Piccarreta, G. and Rottura, A., 1986. Tectonic structure and post-Hercynian evolution of the Serre, Calabrian Arc, southern Italy: geological, petrological and radiometric evidences. Tectonophysic, 124: 223-238. Conflicting opinions exist concerning the structure and the post-Hercynian evolution of the Serre. The present paper deals with these topics on the basis of new geological, petrological and radiometric evidence. The ~rn~~tion of the so-called Stifo and Polia-Cop~ello units has been redefined. The above domains-former sections of upper and lower Palaeozoic continental crust respectively-came into contact, due to transcurrent movements 130-140 Ma ago. A significant vertical component during the transcurrent movements, probably, exhumed the former section of lower crust. The above domains, juxtaposed, were successively involved as a single kinematic body in the Alpine orogenesis. The results enable us to make inferences for the Caiabrian Arc evolution and call attention to similarities between an Austro-Alpine element (Stile + Polia-Copanello) of the Calabrian chain and a South-Alpine sector of the Alps (Ivrea + Ceneri zones).

INTRODUCTION

neological setting

and purpose of the work

The Calabrian Arc is a complex nappe structure located at the boundary of the Adriatic microplate and connecting the Apenmne with the Maghrebide chain of Sicily (Fig. 1). It is made up largely of Palaeozoic basement terrains (low-grade to granulite-facies rnet~o~~cs and granitic to tom&tic plutonics), with minor Jurassic ophiolite sequences and Mesozoic to Recent cover series (Amodio More% et al., 1976). The geodynamic evolution and the geologic-tectonic framework of the Arc are

~-1951/86/~3.50

0 1986 Elsevier Science Publishers B.V.

Fig. 1. Structural composite

(Austro-Alpine cene-lowermost Peloritani,

sketch of the Calabrian

belt of Alpine

and ophiolitiferous Miocene

Garighone

most Miocene

units),

terrigenous

6 = Long-Tao~na

due to juxtaposition 8 = Monte

1s. age (from

Arc according Bonardi

2 = Longobucco

deposits,

of Alpine Is, and Hercynian

terrigenous

9 = Tiriolo

deposits,

Unit,

opinions.

Left: The Calabrian sector

Unit; southern

sector,

4 = Stilo Unit, 5 = crystalline

Unit, 7 = Apenninic

Unit,

to current

et al., 1980): northern

and Maghrebian

12 = Stile Unit,

chain”,

13 = Apenninic

3 = uppermost

of Serre-Aspromonte

units. Right:

ranges (from Lorenzoni 10 = “Alpine

The Calabrian

and Zanettin

II = uppermost chain,

Arc is a

I = Eo-Alpine

Lorenzoni.

chain Oligoand Arc is 1983):

Oligocene-lower-

14 = Hercynian

chain.

controversial (Amodio Morelli et al., 1976; Zanettin Lorenzoni, 19821, due to (I) the isolated position of the crystalline nappes forming the Calabrian Arc in the scenario constituted by the sedimentary terrains of their Apennine-Maghrebide mountain system and” to (2) its complex history, resulting from the involvement in the Hercynian, the Alpine and the Apemrinic orogeneses. In current opinion (Fig. 1) it is considered a nappe structured belt of Alpine 1-s. age, including a fragment (the so-called uorthem sector) representing the former southward continuation of the Alps, thrust on the Apennine during Early Miocene times (Haccard et al., 1972; Amodio ikrelli et al., 1976; Bonardi et al., 1980). Otherwise a chain built up during

225

the Hercynian orogeny (the Hercynian Range after Zanettin Lorenzoni 1982) and only slightly displaced during the Alpine tectogenesis could be present in the Calabrian Arc, in addition to the Alpine and the Apenninic Ranges (Zanettin Lorenzoni, 1982; Ferla et al., 1982). A nappe structure has been recognized in the Serre area (Figs. l-3). The sequence of tectonic units (bottom to top) from north to south, is: (1) mainly Palaeozoic phyllites and metagraywackes in greenschist facies (Bagni unit); (2) a phyllonitic unit derived by Alpine transformation of older amphibolite-facies augen gneisses, and micaschists with minor granitoids (Castagna unit); (3) paragneisses granulite-amphibolite facies terrains-at places phyllonitized in Alpine times along E-W-trending bands localized especially in the lower part of the unit, tonalitic to dioritic gneisses and finally tonalites having more or less foliated texture which outcrop in the upper part of the sequence and belong to the Cardinale subunit according to Borsi et al., 1976 (Polia-Copanello unit); (4) late Hercynian granodiorites, granites and tonalites intruded, at high crustal levels, into Palaeozoic phyllites and paragneisses covered by Mesozoic deposits (Stilo unit). A large consensus about a later emplacement of unit 4 onto the underlying and earlier structured edifice exists (Amodio Morelli et al., 1976; Zanettin Lorenzoni, 1982; Bonardi et al., 1984). Whether the underlying units belong to an Alpine structure or to a Hercynian one, is still debated. In addition some authors (Bonardi et al., 1982; Tortorici, 1982; Boccaletti et al., 1984) suggest that near or in the Serre area, two sectors (northern and southern) of the Calabrian Arc, which evolved differently during the Alpine orogeny, juxtapose. A better knowledge of this region, one of the puzzling areas of the Calabrian Arc, might give important information on the evolution of the Arc. The present study deals with Rb-Sr ages of biotites from various lithologies of the units 3 and 4 (i.e. the Stilo and the Polia-Copanello units, including the Cardinale sub-unit) and with the new geological, geochemical and petrological data now available. On this basis a new interpretation of the tectonic structure and of the post-Hercynian evolution of the Serre will be proposed. Previous geochronological

data

Many mineral ages relative to the various rock-types making up the Stilo and the Polia-Copanello units are available (Fig. 2). The plutonics of the Stilo unit display radiometric Rb-Sr biotite ages ranging from 200 Ma to 289 Ma (Borsi et al., 1976). The rock-types belonging to the Polia-Copanello unit (including the Cardinale sub-unit) show a more complex age-pattern. In fact U-Pb ages ranging from 283 Ma to 298 Ma for monazites and zircons, Rb-Sr ages ranging from 145 Ma to 200 Ma for muscovite and feldspars (Schenk, 1980) and Rb-Sr biotite ages ranging from 85 Ma to 149 Ma (Borsi et al., 1976; Schenk, 1980), have been determined. In addition Schenk (1980) has pointed out that along a section through the Polia-Copanello unit the biotite ages are somewhat older in the upper than in the lower part.

226

221

Borsi et al., (1976) propose relating the Alpine ages of biotite from the rocks of the Polia-Copanello unit, with a cooling following an Early Alpine uplift of the unit. Schenk (1980, 1984) instead suggests relating the overall pattern of mineral ages with a Hercynian uplift of the former lower crust which reached middle crustal levels and successive cooling due to erosion, until the last Tertiary uplift brought it to the surface. Following the first authors, the Stilo and the Polia-Copanello units are two distinct Alpine kinematic bodies, as suggested by the different age patterns of biotites and by geological considerations. Following Schenk, the Polia-Copanello and the Stilo units came into contact owing to the Hercynian uplift of the former lower crust, synchronously with the granitoid intrusions. GEOLOGICAL,

PETROLOGICAL

AND NEW RADIOMETRIC

DATA

Stilo unit in the Serre

North (Sila Piccola) and south (Aspromonte) of the Serre, the Stilo unit clearly forms a distinct tectonic unit, with its own cover, lying upon medium-high (Aspromonte) and high grade (Sila) metamorphics. In central Serre the situation is not so clear. Moresi and Paglionico (1975) and Borsi et al. (1976) have mapped a thrust contact (Fig. 3a) along a zone of tectonization which should separate plutonics of the Stilo unit (i.e. tonalites and K-feldspar megacrysts bearing-granites) from those of Polia-Copanello unit (i.e. tonalites of the Cardinale sub-unit). This interpretation also seems to be supported by radiometric considerations. In fact south of the contact, biotites older than 200 Ma have been found in the plutonics; north of the contact, more or less foliated tonalites outcrop instead, containing rejuvenated biotite with an age quite similar to that of biotites from gneisses present further to the north (Table 1). Goerler (1978) and Crisci et al. (1985) contest that the contact between plutonics in the central Serre is by overthrusting. In fact the contact mapped by Moresi and Paglionico (1975) and Borsi et al. (1976), largely extrapolated and only locally directly observable, is nearly vertical and, at places, is clearly fault-controlled. According to Goerler (1978) and Crisci et al. (1985) quite similar lithologies crop out on both sides of the postulated overthrust contact. In particular two masses of tonalites occur which show geochemical features (major and trace elements) which allow us to consider them as genetically related (Crisci et al., 1985). In fact the diversities existing in and between the two masses are fairly compatible with the heterogeneities generally occurring in the same plutonic complex. All these facts seem to be hardly compatible with overthrusts; more likely- the mapped contact coincides with zones affected by minor tectonics. If this is accepted, the Cardinale tonalites must be considered as an integral part of the plutonic complex of the Serre (Crisci et al., 1985). Geological, petrological and

TYRRHENlAN

20km

0

A

f

TYRRHENIAN SEA

+++ l Cap0 Vatican0

nrcad’Assr+

+++

%

0

10

ZOkm

f

Fig. 3. a Geological

sketch

map of the Serre-Capo

Vatican0

area according

to Borsi et al. (1985) and

Crisci et al. (1985). A = Polia-Copanello

Unit; B = Stilo Unit. I = granulite-amphibolite

phics;

3 = Copanello

2 = Cardinale

grade metamorphics; mica microgranodiorites, b. Geological B = Stilo Unit phics;

subunit

tonalites;

5 = main granodiorites;

auct.

3 = Hercynian

tonalites;

facies metamor4 = low-medium

7 = two mica granites;

4 = tectonic

8 = two

contacts.

to Crisci et al. (1985) and this work. A = Polia-Copanello

I = granulite-amphibolite plutonics;

Vatican0

6 = Bocca d’Assi tonalites;

9- 10 = Alpine tectonic

sketch map according

and Capo

facies metamorphics;

2 = low-medium

grade

Unit auct.; metamor-

contact.

geochemical data currently obtained for the tonalites outcropping in eastern Serre (Caggianelli, 1984) suggest that they must also be considered as part of the same plutonic complex (Fig. 3b). This complex is in tectonic contact with the

229

TABLE 1 Rb-Sr analytical data of biotite for plutonics of the Serre and Capo Vatican0 (CV) of the Stilo unit auct. and other plutonics probably related to that* I?b

_1!wm)

St.

87Rb q-1_

L

Bd

25+

biotite !

313

(wm) 2.1

L

Bd

22+

#I

341

2.1

535.15

L

Ed

8+

I,

329

1.9

551.55

L

Cit

58+

0

363

1.4

910.60

L

Cit

47+

,,

302

2.2

440.65

L

Bd

14+

I,

364

2.9

398.01

L

Cit

62+

,I

362

1.6

800.86

L

Cit

71+

I,

386

1.4

L

Cit

64+

I,

314

1.4

K-82-21+

I,

393

I,

1.6

87 Sr 86S,

+_16

age

fib

(Ma) 28

1442

2

1.8608f

15

151'

2

1.9620*

36

159*

2

2.9966i

27

176*

3

1.7074*

18

159'

2

1.6320*

20

163*

3

2.6383'

38

169*

3

942.16

2.9084'

28

164'

2

768.68

2.1882f140

135'

2

861.42

2.7147+

47

163*

2

469.03

1.6725i

368

1.8

711.30

2.8231=

30

209*

3

K-82-13

0

374

2.8

422.93

1.8013+

27

181'

3

K-82-16

II

346

4.1

267.29

1.5649?: 23

2252

3

I,

603

5.7

345.77

2.1314*

16

289*

4

K-82-17

I,

269

19.2

41.29

0.8785*

11

285*

5

CV

33+

I,

355

1.9

628.60

2.2608?

54

173*

3

CV

43+ o

I,

358

2.2

503.40

1.69752

17

138'

2

I,

299

4.1

152?

4 7

L

Cit

K-82-17

19 20 12 11 13 10 7

56

0 I,

355

1.6

142*

I,

507

3.5

200+- 6

I,

524

2.6

203?

4

6.3

230*

8

0 0 0 ,I

434

,,

563

6.9

236'

7

I,

348

4.3

238+

7

0 o 0

680

6.8

243-+ 6

6

o

I,

374

4.3

244+

7

14

o o

I,

667

3.8

261+

5

I,

465

8.4

268-+ 9

15

2

I,

5

a

I,

363

2.5

27Oi

5

8

o

n

695

2.6

271+

4

9

a

I,

697

1.6

276i

4

3

o

,I

330

4.5

278'

8

4

o

,,

390

1.1

281f

5

,,

378

1.9

289i

5

lo

* Crosses: more or less foliated tonalites; circles: model ages from Borsi et al. (1976). recalculated using 87Rb=1.42~10-1’ y-l.

granulite-amphibolite facies metamorphics cropping out northward (see also Borsi et al., 1976). The contact is sharp. steeply dipping up vertically, and everywhere is marked by phyllonites and milonites of the adjacent rock types. A magmatic contact between the foliated tonalites and the metamorphics of the Polia-Copanello unit has never been encountered from western to eastern Serre. A similar geological situation is present in the Vibo Valentia area, west of the Serre (Ioppolo et al.. 1978). It must be pointed out, however. that in the high grade terrains of the Polia-Copanello unit granitoid-looking gneisses occur (tonalitic to dioritic in composition) which might lead to misunderstandings. These, however, belong to a very heterogeneous lithologic complex, (i.e. the tr~sitional complex after Lorenzoni et al.. 1976) with migmatic character, clearly distinguishable from the tonalitic plutonics of the Stile unit, which are characterized by: (1) notable homogeneity on a large and small scale, (2) lack of granulitic basement enclaves and (3) I locally, by the presence of small plugs of muscovite and fibrolite-bearing peraluminous granitoids (Caggianelli, 1984) found, up to now, only as high level intrusions in the basement of the Calabrian Arc (e.g. D’Amico et al., 1982). All these elements allow us to think that the contact between the plutonics and the granulitica basement in the Serre is tectonic. Only in the Capo Vatican0 area some quartz-diorites and tonalites include large blocks of high-grade metamorphics similar to some rock-types belonging to the granulitic basement of the Serre. Further investigation is needed to clarify the original relationships between plutonics and metamo~hics. Polin-CopaneiIo unit in the Serre On the basis of the previously exposed arguments for the Stilo unit it appears that the Polia-Copanello unit needs also revision in that some rock bodies must be referred to the Stilo unit. It is made up only of granulite-amphibolite facies rocks (metasedimentary and metaigneous lithologies) representing a section through a former lower continental crust. This structural unit is in contact southward and eastward with the plutonics of the “Stilo unit” along a major very dipping or vertical tectonic line (Fig. 3b) and northward lies upon, by overth~sting, former amphibolite facies lithologies more or less deeply retrogressed in greenschist facies in which one biotite age of 43 Ma has been determined (Schenk, 1980). Radiometric analyses Sampling Metamorphics and plutonics cropping out in northern and central Serre have been collected to obtain an overall radiometric picture. In particular the samples have been collected along profiles transversal to the regional structures in order to detect any possible regular data distribution. The location of the samples is given in Fig. 4.

231

Fig. 4. Location of the samples and map distribution of biotite ages. A = Polia-Copanello unit auct.; B = Stilo unit I = granulite-amphibolite facies metamorphics; 2 = low-medium-grade metamorphics; 3 = Hercynian plutonics; 4 = Early Alpine tectonic contact; 5 = “age bands”.

Analytical procedures

Fresh samples of about lo-15 kg were collected; they were completely crushed then the specimens representative of the “whole rock” were extracted. Biotite concentrates were obtained using conventional magnetic separation techniques and purified by grinding under ethanol. Rb and Sr were determined using a Varian Mat TH 5 mass spectrometer combined with a Laben computer adapted to collect and elaborate spectrometric data. The whole rock analyses of metamorphics were performed with a VG Isomass 54 E. The concentration of Rb and Sr were prepared with isotopic dilution method employing “Sr (98%) and 84Sr (99.9%) spikes; the Sr isotopic compositions were measured on portions containing the 84Sr spike. The precision of 87Rb/86Sr is lesser than .f 1.5%; the uncertainty of 87Sr/86Sr is reported to f 1 standard deviation of the analysis. The biotite ages and the errors were determined using York’s method (1966) after the making of biotite-whole rock isochrons (Del Moro et al., in prep.). The used decay constant is “Rb = 1.42 x lo-” and

-1

Y

.

Geochronological

results

Rb-Sr radiometric ages of biotites from both plutonics and high grade metamorphics range from 118 to 289 Ma (Tables 1 and 2). These results are in agreement

237 TABLE 2 Rb-Sr

analytical

northern

data of biotite

Serre: Polia-Copanello

K-82-2'

WI?

for lithotypes

WI? biotltr

c.7ts73: C.0276

240 5.X

1.50 230.74

c\.725ii6’ 6 1.I?25 220

K-82-6b

291 10. i

0.11 65.06

?.7l’Yhi_ 4 il.8292 ilb

1261

4

116

WI? biotitp

K-~32-8~

524 14.’

a.11 G3.52

c. 70899i 3 LT.7652 t 4

1lRl

2

WI? hiotitr

K-82-9’

WR hiotite

L\. 50 169.1411

$.71486? 3 1.0170 217

126i

‘2

K-82-2Pd

WK

0.7126 0.9441

1322

2

0.9; t.0

444

49 510 63.5

section of lower crust cropping

122.04

““0

hiotltr

K-82-58

from a former

out in the

unit*

0.52

35i

biotitc=

286

6.8

biotitr

2.1 1

7.5

12.3.92

1 +14

i_ 2 ‘25

d

16a

I3Ofl3

d

II

395

:,

d

I,

425

R. 2

137+- 4

2gd

I,

312

3.2

139111

24d

II

39 I

i.4

1375

6

17b

I,

56 1

6 .

147:

6

2Sa

I,

431

149*

9

ld

II

304

12 .

2f

I,

300

I1 .Q

21 22

* a: kinzigites, gneisses;

with

b: coarse-grained

114?

1

5

131116

6

granofelses,

4

137214

c: granulite,

d: tonalitic

and dioritic

gneisses,

e: augen

o: model ages from Borsi et al. (1976) recalculated.

those of Borsi et al. (1976) and of Schenk (1980) and all the data permit us to draw a complete and coherent geochronological picture. On the whole the biotite ages decrease toward the north and display a quite regular distribution (Figs. 4 and 5). A “band” SW-NE trending characterized by age clustering between 131 and 139 Ma bestrides the contact between the plutonics and high-grade metamorphics, i.e. between the Stilo and Polia-Copanello units.

233

North and west of this band only lower Cretaceous and Upper Jurassic ages are recorded, and some biotite ages between 118 and 126 Ma are distributed along a SW-NE trending band cutting across the stratigraphic section (Fig. 4). Southward, however, ages growing older up to 289 Ma have been determined for biotites; they depict less defined “age bands”. The “age bands” change their SW-NE trends in the eastern Serre where they seem to turn to the north. This trend-variation is particularly evident for the band of ages clustering between 131 and 139 Ma. It is noteworthy that in the Capo Vatican0 area K-Ar and Rb-Sr biotite ages ranging from 128 to 173 Ma, have been found (Civetta et al., 1973; this paper), which are very comparable with those determined in the Serre. DISCUSSION

To avoid ambiguities, the data presented here have been integrated with those of Borsi et al. (1976) since they have been performed in the same laboratory. In fact some systematic differences appear when our data are compared with Schenk’s ones (1980). On the whole they contribute to reconstruct the post-Hercynian history of the Serre and the tectonic evolution of the Stilo and the Polia-Copanello domains. The conclusions are of more than local interest allowing us to make inferences for the Calabrian Arc evolution. Post-Hercynian history of the Serre

The occurrence of several Early Alpine biotite ages clearly indicates that the studied area was affected, at these times, by geological events geochronologically recorded. The Early Alpine ages become more and more northward, until they became exclusioe in central and northern Serre, displaying preferential distribution. In this area the most striking feature is the clustering of biotite ages between 131-139 Ma and 118-126 Ma (Fig. 5) without significant spread. These clustered biotite ages are localized, respectively: (1) along the tectonic contact between the plutonics and the high-grade metamorphic terrains and (2) along a band which, from Vibo Valentia to Curinga, cuts across the granulite facies terrains (Fig. 4). This age clustering is strong evidence for a tectonic event at eo-Alpine times. A likely explanation for this discovery is that it is a consequence of the shear tectonics which affected the European and the African continental margins during Tethys evolution. Regional and other pieces of information support this hypothesis. Mylonitic structures attributed to shear tectonics, probably eo-Alpine in age (K-Ar and Rb-Sr biotite ages ranging from 89 to 135 Ma), are widespread in the tonalitic gneisses outcropping in the Palmi-Bagnara area, South of the Serre (Rottura, 1985). Crisci et al. (1985) attributed the tectonic contact among the late-Hercynian plutonits in the Serre to shear tectonics. An intense shear tectonic deformation, affecting

‘34

-------b

SOUTH

10

3

t-t

Fig. (arrow

5. Distribution = x = 186.939;

of biotite interval

ages of various

rock-types

from

Polia-Copanello

and

Stilo domains

= o = 16.1909.

the southern sector of the Calabrian Arc in eo-Alpine time, has been hypothesized by Tortorici (1982). On a larger scale there are more indications suggesting the effectiveness of Mesozoic transcurrent tectonics with SW-NE trends in the Northern Apennines (e.g. Fazzini and Gelmini, 1982) which derive by deformation of calcareous platforms belonging to the same continental margin as the Palaeozoic basement of the Calabrian Arc. In the northern Serre a nappe structure is evident, and for the structural unit underlying the Polia-Copanello domain Schenk (1980) gives a biotite age of 43 Ma, which is fairly comparable with mineral ages found in deeper structural units of the Palaeogenic Alpine chain (Borsi and Dubois, 1968; Beccaluva et al., 1981). From Fig. 4 it is evident that the contact between plutonics and high grade metamorphics, here related to Early Alpine shear tectonics affecting the African continental margin, changes its trend from SW-NE to S-N in the eastern Serre. This is due, possibly, to the movements which have caused the arcuate physiography of the Calabrian Arc (e.g. Ghisetti and Vezzani, 1979; Scandone, 1982). According to some authors (e.g. Bonardi et al., 1982; Boccaletti et al., 1984) in the Serre the northern and the southern sectors of the Calabrian Arc juxtapose along a SW-NE Neogene shear zone, from Capo Vatican0 to Soverato (Fig. 4). Our data do not reveal such ages; only Cretaceous-Upper Jurassic ages are recorded by biotites along the contact between the Stilo and the Polia-Copanello domains. Possibly the Neogene shear movements reactivated the Mesozoic shear zones, in a thermal regime which did not involve the Rb-Sr resetting of biotites. Tectonic evolution of the Stile domain in the Serre The late-Hercynian plutonics of the Serre must be considered as a whole (Crisci et al., 1985). They were already in contact, owing to shear tectonics, with former lower continental crust rocks (Polia-Copanello domain) since 130 Ma-140 Ma. Whether the two domains were adjacent or not before the shearing is a matter of speculation.

235

If this is accepted, the following considerations are plausible: (1) the palaeogeographic location of the Stilo domain was on the southern continental margin (African) of the Jurassic Tethys; (2) the fragments of the Stilo domain occurring north and south of the Serre, whose emplacement has been dated at Langhian-Upper Tortonian (Bonardi et al., 1984) might represent the rest of larger fragments detached from the Stilo domain and pushed over other structural units in this time; (3) the Stilo and Polia-Copanello domains behaved as a single kynematic body during the building-up of the Palaeogenic chain. Post-Hercynian tectonic evolution of the Polia-Copanello domain in the Serre

Biotite ages of the rocks from this section of former lower continental crust, are invariably Lower Cretaceous-Upper Jurassic: relict older ages have never been found. This fact is hardly compatible with a structurally high position of this domain in the Hercynian chain, as claimed by some authors (Lorenzoni and Zanettin Lorenzoni, 1983; Zanettin Lorenzoni, 1982; Ferla et al., 1982). More likely only during Lower Cretaceous-Upper Jurassic times, the closing temperature for Rb-Sr in biotites was reached and overcome. From the data given by Schenk (1980) it clearly appears that a gap of about 10 Ma exists between the oldest and the youngest biotite ages which, according to the same authors, characterize the structurally higher and lower parts respectively, of the Polia-Copanello domain. This gap seems to argue against the gradual cooling effects due to erosion suggested by Schenk (1980,1981,1984) and to account for the Mesozoic age-pattern. This gap of ages (confirmed by our dates) and their concentration (1) along the tectonic contact between plutonics and high grade metamorphics and (2) along a band which cuts across the stratigraphy of the lower crust section (Fig. 4) allows us to consider that the overall early Alpine age pattern might be related to shear tectonics through transcurrent movements with significant vertical component too. Along these bands the movements continued probably later, as suggested by same biotite ages of about 80 Ma (Schenk, 1980). Successively the Polia-Copanello domain was involved in the formation of the Palaeogenic chain as suggested by the biotite age of 43 Ma in the underlying structural unit (Schenk, 1980). INFERENCES FOR AND FROM OTHER AREAS

From a regional point of view there is further radiometric evidence of events which affected the basement rocks from the Calabrian Arc in early Alpine times. In Sila, north of the Serre, several biotite ages recording Cretaceous-Jurassic geochronological events have been determined (Borsi and Dubois, 1968; B. Wieland, pers. commun., 1980). The possibility exists that they have the same meaning as that supposed for the Serre. A K-Ar isochron at 210 Ma for amphiboles from amphibolites of former lower continental crust, cropping out at the boundary between

Calabria

and Lucania,

rock isochrone metabasites

has been established

from the Peloritani

203 Ma has been determined texture,

by Delaloye

at 217 Ma has been proposed

occurring

Mountains

whole

et al. (1984) for some

of Sicily. A comparable

by Schenk (1980) for muscovite

in the metapelites

Biotite ages ranging

et al. (1983). A K-Ar

by Zuppetta

Rb-Sr

age of

of aplite with schistose

from the Polia-Copanello

domain

from 200 to about 240 Ma have been determined

granodiorites from the Stilo domain (Borsi et al., 1976). This radiometric evidence suggests that, also in the Calabrian

in the Serre.

in granites

Arc, tectonic

and

events

are recorded, Triassic in age, such as those occurring in Peri-Adriatic chains and whose geodynamic significance is very controversial (e.g. Ferrara and Innocenti. 1974; Castellarin et al., 1979; Rau and Tongiorgi, references therein).

1980; Zuppetta

et al., 1984 and

From a more general point of view the results of this study support to a greater extent, the analogies pointed out between the Serre region and the Ivrea and Ceneri zones of the Southern Alps, in respect to their structure and geological evolution (e.g. Moresi et al., 1979; Schenk, 1980, 1981). In particular the geological and structural setting of the Serre, where a major tectonic contact exists between late-Hercynian plutonics intruded into low- to medium-grade metamorphics with a Mesozoic cover and lithologies of a section of a former lower continental crust, appears

comparable with that occurring in the Southern Alps along the Pogallo and the Ivrea zone from the the Brissago-Mergozzo-Cossago lines, separating Strona-Ceneri zone (Boriani and Sac&i, 1973). The similarity is even more evident if the Pogallo line is post-Variscan and referable to early Mesozoic rifting or Alpine tectonics, as hypothesized by Zingg (1983). It must be kept in mind, however, that the Stilo + Polia-Copanello complex is an Austro-Alpine element, whereas the Ivrea and Ceneri zones belong CONCLUDING

to the South-Alpine

realm.

REMARKS

The data presented

here together with those already available,

allow us to draw an

overall picture relative to the tectonic structure of the Serre and to the reconstruction of the post-Hercynian history of the so-called Stilo and Polia-Copanello units: (1) They were in contact along a major tectonic line running from the southern slopes of Capo Vatican0 promontory to Copanello (Fig. 4) since 130-140 Ma ago. (2) The features of the contact zone together with the pattern of biotite ages, indicate that a wrench tectonic was responsible for such juxtaposition. (3) During the building up of the eo-Alpine chain, the Stilo and Polia-Copanello domains behaved as a single kinematic body; only in Langhian-Upper Tortonian times some fragments of the Stilo domain were detached and pushed over other structural units. (4) The pattern of the biotite ages for lithologies of the Polia-Copanello domain - a former section of lower continental crust - indicates it overcame the closing

231

temperature of biotite in upper Jurassic - Lower Cretaceous times; transcurrent movements with vertical components, too, seem a’likely explanation for its exhumation. (5) The results of this study substantiate further the already suggested analogies between the Austro-Alpine Serre region and the Ivrea and Ceneri zones of the Southern Alps. ACKNOWLEDGEMENTS

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