Cenozoic magmatism of the valencia trough (western mediterranean): Relationship between structural evolution and volcanism∗

145

Tectonophysics, 203 (1992) 145-165 Elsevier Science Publishers B.V., Amsterdam

Cenozoic magmatism of the Valencia trough (western Mediterranean) : relationship between structural evolution and volcanism * J. Marti a, J. Mitjavila b, E. Rota ’ and A. Aparicio d aInstitut de C&~ies de la Terra (JaumeAbnera), C.S.I.C., Marti i Fran&s s/n, 08028 Barcelona, Spain b Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964, USA c Departament de Geologia Diruimica, Geofisica i Paleontologia, Universitat de Barcelona, 08071 Barcelona, Spain d Departamento de Geologia, Muse0 National de Ciencias Naturales, C.S.I.C., C/ J. Gutierrez Abascal2, 28006 Madrid, Spain (Received October 8, 1990; revised version accepted April 10,199l)

ABSTRACT Marti, J., Mitjavila, J., Rota E. and Aparicio, A., 1992. Cenozoic magmatism of the Valencia trough (western Mediterranean): relationship between structural evolution and volcanism. In: E. Banda and P. Santanach (Editors), Geology and Geophysics of the Valencia Trough, Western Mediterranean. Tectonophysics, 203: 145-165. The Cenozoic magmatism of the Valencia trough is characterized by the existence of two volcanic cycles. The first cycle (Early Miocene-Middle Miocene) comprises talc-alkaline rocks which have been recognized in the central to eastern offshore areas and in Mallorca. The second cycle (Middle Miocene to Recent) is represented mainly by poorly differentiated alkaline rocks which are predominantly distributed along the Iberian margin of the Valencia trough. In accordance with the inferred structural evolution of the Valencia trough, petrological and geochemical data, as well as volcanological evidence, we suggest that two different geodynamic environments existed during these magmatic cycles. The Early to Middle Miocene talc-alkaline volcanism developed under compressive tectonic conditions and seems to be associated with the northwestern dipping subduction of the African plate. In contrast, the Middle Miocene to Recent alkaline volcanism is associated with extensional tectonics and can be explained within the framework of a low volcanicity rift model. Three zones-Catalan, Valencian and Columbretes-each located at different parts of the rift structure, can be distinguished in the Middle Miocene to Recent volcanism. This intraplate volcanism results from partial melting, due to extension-driven decompression, with the largest degrees of melting in the centre of the Valencia trough (Columbretes volcanic zone). In the Catalan volcanic zone a progressive increase in the extension rate from the first to the last volcanic episode has occurred, favouring the interaction of two mantle sources.

Introduction

The present structure of the western Mediterranean results mainly from the convergence of the African and Eurasian plates. This convergence was initiated during the Late Cretaceous (Dewey et al., 1973; Dercourt et al., 1986; Dewey et al., 1989). However, under this predominantly compressional regime, which lead to the con-

Correspondence to: J. Marti, Institut de Cibncies de la Terror (Jaime AImera), C.S.I.C., c/Marti i Franques s/n, 08028 Barcelona, Spain. 0040-1951/92/$05.00

struction of a complex system of thrust belts, local extensional environments developed and allowed the formation of large basins. Several different hypotheses have been proposed to explain the origin of these western Mediterranean extensional basins including: mantle diapirism combined with gravity tectonics (Van Bemmelen, 1969, 1973; Weijermars, 1985), back-arc basins (Boccaletti and Guazzone, 1972; Auzende et al., 1973; Biju-Duval et al., 1978; Horvath and Berckhemer, 19821, lateral expulsion of microplates (Brunn, 1976; Tapponnier, 19771, and collapse of thrust edifices (Dewey, 1988; Platt and Vissers, 1989).

0 1992 - Elsevier Science Publishers B.V. All rights reserved

146

J. MART1

The integration of geophysical and geological data is necessary in order to understand the origin of the western Mediterranean basins. The abundant and varied magmatic manifestations that exist in the western Mediterranean (Bellon and Brousse, 1977; Bellon and Letouzey, 1977; Girod and Girod, 1977; Bellon, 1981) are thus significant for the understanding of the geodynamic evolution of the basins. However, despite the fact that the geodynamic evolution of the Valencia trough has been associated with important extrusive activity, relatively few studies (Mauffret, 1977; Marti and Rota, 1989) have been focused on the magmatic evolution of this basin. The volcanic activity in the Valencia trough, recognized since the last century (Hermite, 1879; Ryan et al., 1972; Donville, 1973), is represented by a complex system of volcanic edifices, that range in age from the Early Miocene to the Present and offer a wide diversity of petrological and geochemical features. The Cenozoic magmatism of the Valencia trough is characterized by the existence of two volcanic cycles, which can be clearly separated in

0

Alpine

thrust

and

fold

El’ AL.

time and by their petrology and tectonic setting (Marti and Rota, 1989). This study, based on a review of published data and new data focuses on the characterization of both volcanic cycles and proposes a model of the magmatic evolution of the Valencia trough on the basis of geochronology, tectonic setting, volcanology, petrology and geochemistry of the volcanic rocks. In addition, significant advances in the understanding of the lithospheric structure and geodynamic evolution of the Valencia trough, acquired during the last few years from geophysics and geological studies (Dafiobeitia et al., 1990, 1992-this volume; FontbotC et al., 1990; Gallart et al., 1990; Watts et al., 1990; Rota and Desegaulx, 1992; Maillard et al., 1992-this volume; Torn6 et al., 1992-this volume) allow the relationship between magmatic and structural evolution to be established, helping to constrain models of the geodynamic evolution of this basin. Geologic setting

The Valencia trough is a NE-SW-oriented Neogene basin, located between the Iberian

belts

Fig. 1. Location of the Valbncia trough showing the main physiographic areas of the western Mediterraneanand the distributionof Cenozoic magmatism. Data from Clercq et al. (1975), Girod and Girod (1977), Di Girolamo (1978), BRGM (1980); Bellon et al. (1984, 1985), Torres-Roldin et al. (1986), Ancochea et al. (1987), HernAndez et al. (1987), Savelli (1988) and this work. Black squares= talc-alkaline magmatism from 30 to 15 Ma; black circles = alkaline magmatism from 30 to 15 Ma; outlined squares = talc-alkaline magmatism from 15 Ma to present; circles = alkaline magmatism from 15 Ma to present; triangles= shoshoniticand ultrapotassic magmatism from 15 Ma to present.

CENOZOIC

MAGMATISM

OF THE

VALENCIA

147

TROUGH

(Hinz, 1972; Banda et al., 1980; Gallart et al., 1990, Daiiobeitia et al., 1992~this volume), deep seismic reflection (Mauffret et al., 1992~this vol-

Peninsula and the Balearic promontory (Fig. 1). Geophysical data, comprising gravimetric (Morelli et al., 1975; Haxby, 19831, seismic refraction

NEOGENE PYRENEES EBRO

BASIN

c

INFILLING

PRE-COMPRESSIVE

s-’ %,,c. % 00 020:

ROCKS

/c,

NEOGENE ACTIVE FAULTS

El

(Early-Mlddle

SETIC THRUST BELT

Miocene)

C\

+

N ^ o_ 0”%3

0 PROVENCAL BASIN

SE

NW A

CATALAN-VALENCIAN

DOMAIt&BETIC-BALEARIC I-

A’

DOMAIN

4o1....................................................~(

I m

WATER LOWER

CRUST

m

NEOGENE

(IIIIIIIID MESOZOIC

MANTLE

@$gg

BETIC

THRUST

c:::l

UPPER

CRUST

SHEETS

Fig. 2. Structural map of the Valencia trough (modified from Rota and Desegaulx, 19921, showing the distribution of the Catalan-Valencian and Betic-Balearic domains. The A-A’ line represents a simplified crustal-scale cross-section from the Ebro basin to the Algerian basin.

148

1. MART1

VOLCANISM a

Volcanic

rocks

.i e

Probable

volcanic

E’I‘ AL.

.+i

rocks 3C

0

Alkaline

l

Calc-alkaline

*

Unknown character

magmatic

A

Unknown

aae

Middle

Miocene

Early -Middle

17) - Present

Miocene

100

km

Fig. 3. Distribution and composition of the main outcrops of the Cenozoic magmatism of the Valencia trough. Data from Ryan et al. (1972), Donville (1973), Mauffret (19771, RiviCre et al. (19811, Fourcade et al. (19821, Araiia et al. (19831, Ancochea et al. (1984), Lanaja (19871, Wadsworth and Adams (19691, Mitjavila et al. (1990), Mitjavila (unpublished data), and this work. 1 = Puig de I’Ofre; 2 = Cap de Formentor; 3 = Unidad Ibiza; 4 = DR 21; 5 = DSDF site 123; 6 = DSDP site 122; 7 = KS 08; 8 = Barcelona marino Bl; 9 = R&pita-l; 10 = PeiGscoia-1; II = Benicarld C-l; 12 = Delta J-l; 13 = Castellcin L-l; 14 = Sagunto 1; 15 = Golf0 de Valencia D-l; 16 = Columbretes A-l; 17 = Columbretes Islands; 18 = Picasent; 19 = Cofrentes; 20 = Ports de Beseit; 21= Hostalric; 22 = Sant Corneli; 23 = Caldes de Malavella; 24 = Massanet de la Selva; 25 = L’Esparra; 26 = Sant Dalmai; 27 = Puig de Banya de Boc; 28 = Traiter-Artigues Rojes; 29 = Santa Pau sector; 30 = Olot sector; 31 = Castelifollit de la Rota; 32 = Puig d’Adri; 33 = Puiig Moner; 34 = La BisbalRupi$ 35 = Fla@-Sant Marti Veil; 36 = Arenys; 37 = Vilacolum; 38 = Calamar A-l; 39 = Alcanar-Ma~no A-l; 40 = Vaiencia 3-1; 41 = Riumors-1.

ume; Tom6 et al., 1992~this volume), heat flow (Foucher et al., 1989) and aeromagnetic data (Galdeano and Rossignol, 1977) show that the Valencia trough is characterized by: (1) A thinned continental crust, 7-9 km thick in the axis of the basin, which overlies mantle with anomalous low P velocities (7.2-7.9 km/s>. (2) The existence of magnetic anomalies of low wavelength and high amplitude which reach values of 500 nT, and which have been interpreted as being generated by volcanic edifices (Mauffret, 1977).

(3) A high heat flow in the central zone of the basin (70-90 mW/m*). In the Valencia trough two well-differentiated domains can be distinguished (Soler et al., 1983; FontbotC et al., 1990, Rota and Desegaulx, 1992): the Catalan-Valencian domain and the BeticBalearic domain (Fig. 2). The Cat~an-Valencian domain includas the Catalan Coastal Ranges, the southeastern part of the Iberian Range, and the north and west se&o-rs of the offshore baas& (Fig. 2). This domain is characterized by extensional tectonics which produced a widespread system of

CENOZOIC

MAGMATISM

OF THE

VALENCIA

149

TROUGH

Neogene horsts and grabens bounded by ENEWSW- to N-S-oriented normal faults. The secular evolution of the structure of the CatalanValencian domain has been divided into two stages (FontbotC et al., 1990; Bartrina et al., 1992-this volume). The first stage, ranging in age from the Late Oligocene to the early Burdigalian, is characterized by extensional tectonics which produced the main morphological and structural features of this domain. The second stage ranges from the Langhian to the Present and shows a reduction in tectonic activity; it became restricted to the main faults located in the western boundary of the domain. The Betic-Balearic domain is represented by the Balearic promontory, the eastern Betics, and the eastern and southern sectors of the offshore basin (Fig. 2). Its structure is more complex and consists of stack of WNW trending thrust sheets affected by a system of listric extensional faults. Two main stages can also be identified in the kinematic evolution of the Betic-Balearic domain. During the first stage (from Late Oligocene to Langhian), the emplacement of a thrust system took place (Ramos-Guerrero et al., 1989). Thrusting propagated progressively toward the northwest and induced a crustal-scale flexure of the previously thinned Valencia trough due to the load of the thrust pile. The major part of the foreland sediments were progressively accreted on to the Betic allochthon. During the second stage (from Serravalian to the Present) compressional tectonics were replaced by an extensional regime that may have originated due to the collapse of the thrust edifice (FontbotC et al., 1990). Thus, the tectonic evolution of the Valencia trough can be divided into two stages (FontbotC et al., 1990). The first stage ranges from the Late Oligocene to the Middle Miocene and was a period of high tectonic activity in which the basin was formed as a result of two main factors. Firstly, the thinning produced by the progression of a horst and graben system in the Catalan-Valencian domain and, secondly, the loading flexure generated by the thrust pile in the Betic-Balearic domain. The second stage ranges from the Middle Miocene (Serravallian) to the present and is char-

acterized in the Betic-Balearic domain by extensional tectonics related to the opening of the Algerian basin, and in the Catalan-Valencian domain by a significant reduction of extensional tectonics. The locations of the main centres of volcanism are related to the structure of the basin (Fig. 3). The first volcanic cycle is represented by calc-alkaline rocks which appear along the thrusts of the Betic-Balearic domain and in the offshore areas of the Catalan-Valencian domain. The second volcanic cycle is represented by alkaline volcanic rocks which are distributed along the main extensional faults at the margins of the Valencia trough: the ENE-WSW to N-S faults of the Iberian margin and the North Balearic fault zone, which is represented by a set of NW-SE oriented faults, that run from the eastern Pyrenees to the north of Menorca and which put a northern limit on the Provenqal basin and the Valencia trough (Mauffret, 1977). Geochronology The geochronology of the volcanic rocks of the Valencia trough is well established, mainly on the basis of K-Ar data (Donville, 1973; Saez-Ridruejo and Lopez-Marinas, 1975; Araiia et al., 1983; Mitjavila et al., 1990; this work) and plagioclase thermoluminescence data (Guerin et al., 1986) (Table 1). Both volcanic cycles are well-differentiated in time (Fig. 3). The age of the talc-alkaline volcanism is between 24 and 18.6 Ma; the rocks from the DSDP site 123 being slightly older than the Mallorca rocks. Therefore, the Valencia trough talc-alkaline volcanism is not temporally related to the Cabo de Gata volcanism, which ranges in age from 12 to 7.5 Ma (Di Battistini et al., 1987) and preceded the Albordn volcanism (19-7 Ma) (Table 1 and Fig. 3). However, the talc-alkaline volcanism of the Valencia trough is contemporaneous with the talc-alkaline volcanism of Sardinia (32.3-11.6 Ma, after Coulon (1977) and Beccaluva et al. (198511 (Table 1, Fig. 3). The alkaline volcanism ranges in age from 10 Ma to Recent (Table 1). As we will show, three different zones can be distinguished within this

150

J. MARTI

area of volcanism: Valencia, Columbretes and the Catalan zones (Fig. 3). An age of 2-1.3 h4a has been estimated for the Valencia volcanic rocks (SBez-Ridruejo and Upez-Marifias, 1975) and an age of l-O.3 Ma has been obtained for the Columbretes rocks. The Catalan zone contained the major centre of volcanic activity during the second volcanic cycle. The age of these volcanic rocks ranges from 10 Ma to Recent and three sub-zones, also identified by petrological and geochemical differences, can be distinguished (Donville, 1973; Araiia et al., 1983): L’Empordi (lo-8 Ma), La Selva (7.9-1.7 Ma) and La Garrotxa (0.1-0.01 Ma) (Table 1, Fig. 3). In addition to these data, a K-Ar age of 0.3 Ma was obtained by Mauffret (1977) for a submerged volcano located in the North Balearic fault zone (Fig. 3, location 4).

E-F Al..

Early to Middle Miocene volcanism Volcanology and petrology

The Early to Middle Miocene volcanism is represented only by a few outcrops with a limited area1 extent (Fig. 3, locations I, 2 and 3). Only the DSDP site 123 (Ryan et al., 1972; Rivibre et al., 1981) and two small outcrops on Mallorca Island (Alvaro et al., 1987; Wadsworth and Adams, 1989; Mitjavila et al., 1990) could be studied in detail. All these volcanic rocks show similar lithological features. They are fine-grained pyroclastic rocks composed of vitric ash and some phenocrysts of sanidine, sodium plagioclase, biotite and quartz. Occasionally, small-sized pumice fragments have also been identified. These pyroelastic rocks contain few lithic clasts. The Mal-

TABLE 1 Radiometric and radiation-damage Location Early Miocene volcankn

ages of the Cenozoic volcanic rocks of the Valencia trough and neighbouring areas. Lithology

Age (Ma)

Reference

Dacite Dacite Rhyolite

21.9-20.8 * 24.4-19.4 19-18.6

Ryan et al. (1972) Riviire et al. (1981) Mitjavila et al. (1990)

Basanite

2-1.3

SCez-Ridruejo and tipez-Marinas

Basalt

l-0.3

This work

Basanite, trachyte Basanite, basalt Basanite, basalt

10-8 7.9-l .7

Donville (1973), Araiia et al. (1983) Donville (19731, Arabiaet al. (1983) Donville (19731, Araiia et al. (19831, Guerin et al. (1986)

Andes&e-Dacite-Rhyolite Andesite Silicic tuffs Andesite-Dacite-Rhyolite Lamproites Alkaline basalts Basanite, Basalt

32.3-11.6 19-7 21-22 12-7.5 7-5.5 2.7 (17.5)-1.75

(Fig. 3)

DSDP site 123 DSDP site 123 Puig de 1’Ofre

Middle Miocene to Recent volcanism (Fig. 3)

Valbncia Volcanic Zone Cofrentes Columbretes Volcanic Zone Columbrete Grande Catalan Volcanic Zone L’Empordl sub-zone La Selva sub-zone La Garrotxa sub-zone

Neighbowing areas (Fig.

Sardinia Alboran Betic Cordillera SE Spain SE Spain SE Spain Campos de Calatrava

0.1-0.01

(197%

1) Coulon (19771, Beccaluva et al. (1985) Bellon (1981), this work Clercq et al. (1975) Di Battistini et al. (1987) Bellon et al. (1983) Bellon et al. (1983) Ancochea qt al. (1979)

* Recalculated with the decay constants of Steiger and Jaeger (1977). All ages are K-Ar data except those from Guerin et al. (1986) which are plagioclase thermoluminiscene

data.

CENOZOIC

MAGMATISM

OF THE

VALENCIA

151

TROUGH

fluidal textures have been observed in some of these rocks (Alvaro et al., 1987; Mi~a~la et al., 1990). Therefore, a pyroclastic flow origin is considered more likely for the pyroclastic rocks of Mallorca and the DSDP site 123. In addition to the outcrops described above, several outcrops of talc-alkaline andesitic and silicic rocks have also been identified in the offshore area, in several exploration wells (Shell Espaiia, N.V., 1975, and REPSOL, unpubl. data, 1972, 1980) (Fig. 3, locations 4-16 and 38-40). The lack of radiometric data means that these rocks can not be dated with any certainty. However, stratigraphic~ly, they can be placed roughly within the Early to Middle Miocene (Shell

lorca outcrops are only a few meters thick. Nevertheless, in the DSDP site 123 more than 100 m of the same unit of silicic tuffs have been reported (Ryan et al., 1972). The absence of epiclastic components in these volcanic rocks indicates a primary pyroclastic origin. Stratigraphical relationships (Wadsworth and Adams, 1989; Mitjavila et al., 1990) and the characteristics of the alteration products (Riviere et al., 1981) suggest that these rocks were emplaced in a subaerial or very shallow sub-aqueous environment. The origin of these volcanic rocks is not clear. However an air-fall origin (see Wadsworth and Adams, 1989) seems unlikely when the thickness of the outcrops is considered. In addition,

TABLE 2 Chemical ambition Component

of the Early Miocene volcanic rocks of the Valencia trough.

Sample 123-2

123-3

123-4

123-5

123-6

123-7

F

PO-5

PO-6

PO-8

SiO, (%o)

68.17

66.71

67.75

68.08

70.47

70.11

77.90

71.00

71.00

TiO, (%I

0.23

0.23

0.22

0.24

0.24

0.22

0.14

0.21

0.13

0.01

12.75

12.61

12.59

12.49

12.37

12.49

12.20

12.08

10.98

11.20

2.75

2.59

2.43

2.63

1.99

1.98

1.30

0.47

0.47

0.45

0.84

0.65

0.64

0.01

0.01

0.02

0.22

0.41

Al,o,

(%o)

Fe,03 (%) Fe0 (%I

71.10

MnO (%I

0.02

0.02

0.02

0.02

0.02

0.02

MgO (%I

2.02

1.68

1.53

1.31

1.19

1.09

0.04

0.24

CaO (%I

0.73

0.75

0.76

0.66

0.69

0.62

0.44

0.71

1.09

1.41

Na,O (%I

2.40

2.58

2.58

2.52

2.98

2.93

3.50

3.13

2.35

2.71

K,O (%o)

3.22

3.44

3.60

3.60

3.76

3.67

4.30

4.84

5.33

3.76

0.01

P,O, (%I H*o+

(%I

4.14

4.71

4.54

3.87

3.70

3.37

H,O-

(%)

4.23

3.79

3.43

3.70

3.00

3.87

100.66

99.11

99.45

99.12

100.40

100.37

Total (%) Th

@pm)

99.82

0.01

6.28

7.42

8.09

99.82

99.65

99.81

31.2

29.70

31.70

Ta (ppm)

1.55

2.30

2.70

Hf (ppm)

9.4

8.70

8.40

Cs (ppm) La (ppm) Ce fppm)

73.9 148

73.4 146

67.8 134

66.8 134

58.2 115

65.5 129

Nd (ppm)

57.5

62.6

53.2

57.4

44.5

43.2

Sm (ppm)

10.9

11

11

10.2

8.6

9.3

14.04

11.70

14.00

65.2

72

67

135

139

126 45.2

54

44.5

Eu (ppm)

0.2

0.17

0.12

0.19

0.11

0.21

0.2

0.2

Tb (ppm)

2.3

2.3

2.1

2.1

1.7

1.9

1

1.4

1.2

Yb (ppm)

5.5

5

5

5

4.1

4.5

4.48

4.49

4.8

Lu (ppm)

0.84

0.81

0.78

0.78

0.62

0.69

0.2

Samples 123-2, 123-3, 123-4, 123-5, 123-6 and 123-7, DSDP site I23 (Rivibre et al., 1981). Sample F, mean of 34 microprobe analyses of glassy groundmass of the Formentor rocks (recalculated as water-free) (Wadsworth and Adams, 1989). Samples PO-5, PO-6, PO-8, Puig de l’Ofre rocks (this work; analyses of major elements are by X-ray fluorescence spectrophotometry (XRFS), and Th to Yb by (INAA)).

152

.I MAR’I‘I

Espaiia, N.V., 1975, and REPSOL, unpubl. data, 1972, 1980). Lithological characteristics suggest that many of these volcanics are pyroclastic rocks which normally exhibit sequences several tens of metres thick. This confirms the signifi~nce of explosive activity during the Early to Middle Miocene volcanism of the Valencia trough. Geochemistry Only a few analyses are available from the Early to Middle Miocene silicic volcanic rocks (Table 2). These rocks are homogeneous in composition. Major and trace elements indicate a rhyolitic composition for the Mallorca rocks and a dacite to rhyodacite composition for the DSDP

ETA?

site 123 rocks, as well as a talc-alkaline affinity for the Early Miocene volcanics. Chondrite-normalized rare earth element (REE) profiles for Mallorca and DSDP site 123 rocks are similar (Fig. 4). The strong negative Eu anomaly observed in these rocks is explained by high level fractionation of calcic plagioclase. The REE distributions of the Early Miocene volcanics also show affinities with patterns developed in highly silicic magma chambers in continental volcanoplutonic belts (see, for example, Hildreth, 1979; Noble et al., 1979). In addition, the Early to Middle Miocene volcanics of the Valencia trough have compositions similar to the contempora~ subduction-related talc-alkaline silicic rocks of Sardinia (see Coulon, 1977; Bellon, 1981; Savelli 1988). Late Miocene to Recent volcanism

Puig de I’Ofre

La

Ce

-

Nd

-

-

Eu

-

Tb

-

-

-

-

Yb

DSDP -f 23

The Valencia volcanic zone

-

I

loo ? f

I f

10 :

Fig. 4. Chondrite-normalized REE profiles for the Early Miocene volcanic rocks of the Valencia Trough. Circles = Puig de I’Ofre rocks; crosses = DSDP site 123
The Valencia volcanic zone (WZ) includes only two small groups of volcanic rocks located at Cofrentes and Picasent (Fig. 3, locations 18 and 19). The WZ rocks are represented by strombolian pyroclastic deposits and lava flows, which are normally found as parts of eroded cinder cones. The Cofrentes rocks are composed of olivine nephelinites and basanites. In contrast, the Picasent volcanic rocks have a hawaiitic com~sit~n. The presence of small, mantle-derived xenoliths is common in the Cofrentes lavas @ncochea et al., 1984). Chemical analyses of the VVZ rocks are shown in Table 3. Different degrees of evolution can be observed in the VVZ volcanic rocks. The Cofrentes rocks are strongly depleted in HREE (heavy rare earth elements) relative to LREE (light rare earth elements) and have a more primary character than the Picasent rocks. Moreover, compared with the rest of the basaltic rocks of the Late Miocene to Recent volcanism of the Valencia trough, the Cofrentes rocks have the most primitive and most undersaturated character. Ancochea et al. (1984) suggested that the WZ rocks were derived from a mantle enriched in

CENOZOIC

MAGMATISM

OF THE

VALENCfA

153

TROUGH

TABLE 3 Chemical composition of the Middle Miocene to Recent volcanics of the Valencia trough: the Valencia volcanic zone Component

Sample Cof-1

SiO, (%) TiO, (%I Al,o, i%) FerO, f%) Fe0 (%) MnO (%o) MgO (%) CaO (%I Na,O (%I K,O (%I P,O, f%f H,O (o/o) Total Ni (ppm) Zr (ppm) Nb (ppm) Y @pm) Rb (ppm) Sr (ppm) Ba @pm)

41.53 2.97 12.64 4.42 8.41 0.21 10.49 10.79 5.01 1.91 1.21 0.30 99.79 145 249 71 31 53 1127 801

Cof-2 41.16 2.90 13.07 3.34 9.31 0.21 10.31 10.82 5.28 2.03 1.27 0.41 100.13

40.10 2.37 12.55 4.43 6.80 0.21 11.73 12.58 4.60 1.77 1.31 1.50 99.95

166 243 70 30 51 1143 868

Th (ppm) Ta (ppm) Hf (ppm) La (ppml Ce (ppm) Nd (ppm) Eu fppm) Yb (ppm)

Cof-3

Pit4 49.47 2.70 14.78 4.93 5.88 0.15 5.30 6.98 4.30 1.24 0.74 3.95 100.42

Pit-5 48.54 2.87 14.65 5.48 5.27 0.17 5.68 6.56 4.16 1.33 0.71 3.60 99.02

65 345 68 34 33 1042 676

67 359 69 32 41 1050 630

90 163

86 152

7.81 3.9 5.38 101 126

125 123

65.6 116 48.6 4.98 1.99

Confrentes rocks, samples 1 and 2 (Ancochea et al., 19841, and 3 (this work; analyses of major elements are by XRFS, and Th to Yb by INAA). Picasent rocks, samples 4 and 5 (Ancochea et al., 1984).

(nephelinites) originated from low degrees of partial melting of peridotite at depth (in excess of 60 km) in the presence of volatile components and probably garnet, as seems to be suggested by the large ion lithophile plus LREE-enriched character of these rocks. The Col~retes

uolcan~czone

The Columbretes volcanic zone (COVZ) includes a group of small islands and represents the only subaerial exposure of the Middle Miocene to Recent voIcanism in the offshore area of the Valencia trough (Fig. 3, location 17). The COVZ mainly consists of alkaline basaltic volcanic rocks, even though the presence of more differentiated rocks in some islands has also been observed by previous authors (Parga Pondal, 1935; Alonso Mantilla, 1985). The Columbrete Grande Island is the largest island of the Columbretes archipelago and represents a partially eroded marine volcano, which is formed only of basaltic rocks. Basaltic rocks from the Columbrete Grande are homogeneous in composition and correspond to Na-rich basanites with low SiO, contents (Table 4). They have a lower incompatible element content than the basalts from the Catalan volcanic zone and also from the WZ (Tables 4 and 5). The Columbrete Grande rocks also show lower contents of some compatible elements such as Fe, Mg, Mn, Ni and Cr, as well as higher contents of P, than the Catalan and Valencia rocks. This is

1000 f Cofrentes

incompatible elements, and proposed a low degree of partial melting (5% from a pyrolitic mantle) for the Cofrentes rocks. They also suggest that the Cofrentes rocks originated at pressures higher than 10 kbar, while the Picasent rocks were derived from higher degrees of partial melting (9.4% to 10.2% from a pyrolitic mantle), at shallower depths. According to Ancochea et al. (19&Q, the chond~te-no~alized REE pattern (Fig. 5) and the La/Yb ratio suggest that the Cofrentes rocks are undersaturated basalts

14

i_eCo-I&J-

Fig. 5. Chondrite-normalized

-

Eu-Tb-

-

-

-

Yb-

REE profiles for the Cofrentes rocks.

154

_I. MART1

TABLE

4

Chemical volcanic

of the

Middle

Miocene

rocks of the Valencia

composition

trough:

the Columbretes

to Recent vol-

canic zone. Com~nent

Sample Co&2094

Cal-2120

Cal-2105 47.52

(%)

46.94

46.40

TiO, (%I

2.33

2.36

2.10

f%)

15.25

15.25

13.98

Fe,O, (%I Fe0 (%‘c)

3.97

4.65

3.95

6.10

5.27

5.63

MnO (%I

0.15

0.15

0.16

MgO (%o)

7.25

7.5

I

7.67

CaO (%I

9.23

9.66

9.61

Na,O

3.68

90, Al,O,

ET AI..

4.11

4.11

K,O

(%)

(%I

2.40

2.30

2.37

P,O,

(%I

0.75

0.74

0.62

H,O

(%I

Total

1.24

1.25

2.34

99.72

99.65

99.63

Cr (ppm)

207

221

135

Ni (ppm)

128

123

115

Zr (ppm)

105

106

104

Nb (ppm)

7

6

2

23

24

20

Y (ppm) Rb (ppm)

32

41

27

Sr (ppm)

839

838

741

Ba (mm)

838

879

748

Th (ppm)

5.71

4.25

5.74

(porn) Hf (ppm)

4

3.1

3

5.8

4.56

5.7

Ta

La (ppm)

45.9

39.8

40.6

Ce ippml

66.6

76

75.1

Nd (ppm)

30

35.9

24.7

(porn) Tb (ppm) Yb (ppm) Eu

Analyses

2.1

2.39

2.1 0.68

0.7 2.08

from SiO,-Ba

1.6 are by XRFS,

1.78 and Th-Yb

by INAA.

consistent with their less sili~-undersaturated nature. This suggests that the Columbretes volcanic rocks resulted from a higher degree of partial melting than the Catalan and Valencia volcanic rocks, which are possibly of a similar source. This larger degree of partial melting is also corroborated by the LREE/HREE ratio ((La/Yb), = 14.57-16.43) (Fig. 6). Differences within that group of volcanic rocks may in part result from fractional crystallisation.

Volcanology

The Catalan volcanic zone (CVZ) exhibits the greatest concentration of Middle Miocene to Recent volcanism in the Valencia trough. Three different sub-zones have classically been established in the CVZ: L’Empordri (Fig. 3, locations 34-371, La Selva (Fig. 3, locations 21-26 and La Garrotxa (Fig. 3, locations 27-33) (Sole Sabaris, 1962; Tournon, 1968; Donville, 1973; Arabia et al., 1983). Moreover, an outcrop of basaltic rocks in the offshore area, contemporaneous with those from the La Garrotxa sub-zone, was described by Mauffret (1977). The CVZ is characterized by cinder cone eruptions along fissure zones. The existence of phreatomagmatic events is also common (Marti and Mallarach, 1987). Calculations of the rate of magma ascent, based on the presence of ultramafic xenoliths in some deposits, indicates that only a relatively short time was required for the magmas to reach the surface. Velocities of 0.2 m/s have been obtained for some volcanoes of the La Garrotxa sub-zone using Spera’s (1984) method. This is in agreement with the undifferentiated character of most of these volcanic rocks, suggesting that conditions suitable for the formation of shallow magma chambers did not occur during the ascent of these magmas. However, as we will see, the presence of crustal contaminants and the occurrence of partially resorbed granitic xenoliths, suggests that the rising of magmas was sometimes slow enough to permit some crustal assimilation. Small cinder cones were produced during monogenetic short-lived eruptions associated with widely dispersed fractures of short lateral extent. This suggests that each eruption was caused by an individual batch of magma, which was transported rapidly from the source region. According to Guerin et al. (19861, each batch would represent the products of an individual partial melting event. The total volume of magma extruded was relatively small. This suggests a low magma supply rate (see Fedotov, 1981; Wadge, 1981). Nevertheless, the volume of extruded magma seems to increase progressively from the early episodes

CENOZOIC

MAGMATISM

OF THE

VALENCIA

155

TROUGH

loo0

Columbretes

1-I

I

Lace-Nd-

I

I

.

1

Eu-Tb-

I

I

-

-

1

I

-.Yb-

Fig. 6. Chondrite-normalized REE profile for the Columbretes rocks.

(L’Emporda) to the later ones (La Garrotxa). Thus, a progressive and concomitant increase in the volume of magma generated, as well as an increase in the degree of partial melting, can be observed in the CVZ. The tectonic setting during the Middle Miocene-Recent volcanism suggests that the cause of melting was decompression in the mantle caused by extensional tectonics. The volumes and composition of melts generated from the mantle by adiabatic decompression during the extension of continental lithosphere are directly related to the degree of extension (McKenzie and Bickle, 1988; Latin and White, 1990). Thus, in the CVZ the variations in the extrusion rate of magmas would also indicate a progressive increase in the rate of extension from the oldest sub-zone (L’Empordl) to the youngest one (La Garrotxa). Petrology and geochemistry

The CVZ is mainly represented by poorly differentiated alkaline basalts. Lopez-Ruiz and Rodriguez-Badiola (1985) distinguished three groups of Basaltic rocks: leucite basanites, basanites and olivine basalts. In addition, an outcrop of trachyte is also present.. A detailed study of the petrology of the CVZ rocks can be found in Arabia et al. (19831 and Lopez-Ruiz and Rodriguez-Badiola (1985). The presence of ultrabasic xenoliths is characteristic of some volcanoes of the La Garrotxa sub-zone. The xenoliths may be divided into: py-

melanogabbros, amphibolites and roxenites, spine1 lherzolites (Arafia et al., 1983; Llobera, 1983), the pyroxenites being the most abundant. Llobera (1983) suggested that the pyroxenites from the Rota Negra volcano (Fig. 3, location 29) crystallized in the mantle at 10 kbar and 1100”c. The Catalan rocks are homogeneous in their major element contents (Table 5). Only Ti shows significant variation amongst the samples, which may be interpreted as reflecting differences in the temperature of formation of these basaltic rocks (see Walker et al., 1979). Strong variations can be observed in the contents of some compatible elements such as Ni, Co, Cr and REE’s. The chondrite-normalized REE patterns exhibit significant variations, which indicate a heterogeneous source region and different evolution trends for the CVZ rocks (Fig. 71. The (La/Yb), ratio is also very variable, from 16.65 to 21.12 from the Emporda sub-zone basalts, from 16.36 to 21.28 for La Selva and from 11.56 to 22.61 for La Garrotxa basalts. The higher values (L’Empordl and La Selva sub-zones) indicate that these rocks are undersaturated basalts (nephelinites), while the lower values shown by the rest of the rocks (mainly from La Garrotxa sub-zone) indicate that they are alkaline basalts originating by higher degrees of partial melting, probably of a spine1 peridotite. The Vilacolum trachyte has a more evolved composition than the basaltic rocks of the same sub-zone. This could suggest that the trachyte was derived by fractional crystallization from the undersaturated basalts of that sub-zone. However, the ratio between incompatible elements (Rb, La, Zr, Th, Ta), the Sr isotopic composition (0.704, after Araiia et al., 1983) and the (La/Yb), ratio (26.7) (Fig. 81 suggest that this silicic rock is a fractional crystallization derivative of alkali basalts rather than having any relation to undersaturated basalts. Petrogenesis The petrogenesis

of the volcanic rocks of the CVZ has been studied previously, and various hypotheses have been proposed to explain the causes and history of this magmatic episode

16.85

16.10

15.75

(%)

Al,o,

0.98

0.39

1.05

0.43

2.65

K,O (%)

PzO5 (o/o)

Hz0 (%)

0.36 0.35

0.31 0.65 99.90

2.40

99.79

72

24 457

218

59

187

92

29

240

61

189

145

32

0.3

2.34

1.2

2.76

146

3.61

0.9

3.19

63.8

256

4.22

2.5

2

83

205

98

13.9 8

7.15

780

2.05

0.3

3.2

37.2

17.4

2.5

1.8

3.3

57.9

150

75

6.8

3.7 36

5.7

7.21

1040

2.62

2.87

560

905

54

33

94

232

84

340

351

99.53

0.30

0.39

2.34

3.68

8.30

8.61

0.20

8.10

3.19

14.90

2.71

47.20

o-174

2.19

1.1

3.37

61.5

153

75

8

4.3

8.23

840

930

63

35

97

301

62

322

293

99.53

0.55

0.58

2.38

4.03

8.90

9.97

0.19

7.08

3.75

15.55

2.48

44.70

O-180

2.07

0.85

2.3

45

91.8

51.3

5.2

4

6.58

865

760

54

34

89

210

64

173

177

99.97

0.65

0.31

2.00

3.04

10.69

7.83

0.16

6.14

4.11

16.40

2.45

46.50

O-183

1.96

0.3

2.6

33.7

100

61

5.3

4.3

9

1370

755

45

31

105

199

54

263

302

99.92

1.90

0.24

1.35

4.56

9.83

7.17

0.17

6.65

3.06

16.10

2.03

47.10

O-184

f.83

0.7

2.25

46

105

58

2.7

4.3

7.69

965

640

67

28

73

139

61

182

390

99.96

0.85

0.23

2.12

3.45

10.13

8.14

0.17

7.14

3.88

16.05

1.83

46.20

O-185

2

0.7

2.1

40.8

116

64.5

5.5

4.18

7.52

1065

920

74

37

92

202

70

137

171

99.82

1.80

0.27

1.89

4.32

9.94

6.79

0.15

6.91

2.98

16.55

1.84

46.70

O-186

and O-165 (Vitacolum~. La Selva sub-zone: samples O-183 (L’Esparra), O-184 (Massanet), O-185 (Hostalric), and O-186

1.3

0.4

2

23.6

63.3

30.6

2.9

2

3.05

785

31 635

-

(by INMJ

this work.

La Garrotxa sub-zone: samples O-168 (St. Joan les Fonts), O-169 (Sta. Paul, O-173 (St. Feliu de Pallerols), O-174 (Banya de Boc), and O-180 (Adri). Major and

samples 0-162 (RupiB), O-163 (Rupii),

2.79

43.8

60

2.9

59

112

5.2

5

88.3

4.2

8 10.2

18 30

6.57

9.52

1520

815

1090

830

36

705

49

102 1250

30

1330

33

168

43

71 35

66 31 28

159

163

80

173 82

162

loo.04

0.50

0.26

155

134

99.44

0.45

0.43

1.53

3.42

3.69 1.96

10.14

9.87

0.18 9.16

32

1310

3.76 8.64

8.08

0.19

210

177

204

8 59

309

335

196

99.88

2.09

3.61

3.65

6.72

10.12

trace elements from SiO, to Ba from Arabia et al. (19831, and from Th-I’h

(Sam &r&i).

L’Empord&zone:

La bpm) Ce Crud Nd (ppm) Eu (ppm) Tb (ppml M (ppm)

Th @fxn) Ta (ppm) Hf (ppm)

Cr Imd Ni CObwm) 25 @pm) Nb (tirn) Y hsm) Rb (rwm) Sr (ppm) Ba (ppml

99.55

3.52

3.34

Na,O (%I

Total

3.76

10.19

10.86

9.21

Cao (%I

0.17

1.23

8.72

MS0 (%I

0.14

0.18 7.98

0.18

MnO (%Io)

3.95

3.75 8.10

15.90

14.60 7.48

3.05

2.61 15.00

45.10

46.00 2.75

o-173

44.50

O-169

2.50

O-168

3.37

3.20

2.87

7.71

FezO, (%I

Fe0 f%)

-

0.98

2.39

2.12 2.85

59.70

44.80

44.30

O-165

SiO, (%)

O-163

TiO, (%)

0-162

Chemical composition of the Middle Miocene to Recent volcanic rocks of the Valencia trough: the Catalan volcanic zone. -

TABLE 5

m

z

CENOZOIC

MAGMATISM

OF THE

VALENCIA

157

TROUGH

Vilacolum trachyte

L’EmpordA

La&-

Nd

-

-

Eu-

lb

-

-

-

-

Yb-

Fig. 8. Chondrite-normalized REE profile of the Vilacolum trachyte (Catalan volcanic zone). La Selva

ratio in some of the oldest rocks. These rocks (L’Emporda sub-zone) also show a higher Sr content that could be explained by crustal contamination. However the high Sr content in the basalts 0.7048

A #

0.7oa

,4 La

1

Co

-

Nd

-

-

Eu

-

lb

-

-

-

-

Yb

-

85

0.7042 -

& 6

0.7040 -

+

n

0.7038 -

La Garroba im

0.7a34 4 600

loo0

a00

11 10

12M)

: Sr

f 0.7048

g c

B 0.7044 -

10 :

*

I L

14

I

La

Co

.

-

I

Nd

1

-

-

.

Eu

I

-

I

lb

-

I

-

-

I

-

Yb

.

-

I

Fig. 7. Chondrite-normalized REE profiles for the Catalan basaltic rocks. Stars = L’Empordi sub-zone; triangles = La Selva sub-zone; squares = La Garrotxa sub-zone.

(Arafia et al., 1983; Mpez-Ruiz and RodrfguezBadiola, 1985; Guerin et al., 1986; Upez-Ruiz et al., 1986; Hertogen and Mpez-Ruiz, 1989). The isotopic composition of Sr in the Catalan basaltic rocks (Arabia et al., 1983) shows a higher

t$

0.7042 -

5 k

0.7040 0.7033 -

n

0.7033

A A

i 0.70344., 0.02

0.03

, 0.04

. 0.03

, 0.03

, 0.07

, 0.03

0.03

FIbtar

Fig. 9. 87Sr/s%r versus (a) Sr and (b) Rb/Sr diagrams for the rocks from the Catalan volcanic zone.’ Stars = L’Emporda sub-zone; triangles = La Selva sub-zone; squares = La Garrotxa sub-zone; diamonds = Vilucolum trachyte (data from Araha et al., 1983).

158

J. MAKTI

requires a large amount of contamination to change the isotopic composition. The mafic character of the high s’Sr/*‘Sr basalts makes this process very unlikely. High 87Sr/86Sr may be best explained by a disequilibrium mechanism during partial melting (Arabia et al., 1983) or by heterogeneity in the source (Arafia et al., 1983). This disequilibrium mechanism could have affected the Sr-rich zones in the source region during the formation of L’Emporda volcanic rocks. The variations in Sr isotopic composition shown by the rest of the Catalan basalts could be explained by a model incorporating partial melting from a heterogeneous source and very low amounts of crustal contamination. The crustal contamination would be not very significant as is indicated by the presence of only partially resorbed crustal nodules and by the relatively high ascent rates deduced for these basalts. The heterogeneity and low degrees of crustal contamina0.7046

.

0.7044

-

A

o.m34

rnA A

4 40

1

T

A

2

060 -

AAA #

0

0 :

#

0.60 R

n

0.40-1 20

#

40

I

.

60

1

80

I

100

La

Fig. 11. La/Cc-La diagram for the basaltic rocks from the Catalan volcanic zone, showing that the existence of crustal contamination is not significant (except for La Selva sub-zone) and that the samples from the same sub-zone are related by partial melting processes. 1 = Partial melting trend; 2 = partial melting and crustal contamination trend; stars = L’Empordl sub-zone; triangles = La Selva sub-zone; squares = La Garrotxa sub-zone.

#

wo

0.7036

ET AL.

.

45

I

66

I

1

1

60

’

30,

Fig. 10. s’Sr/?Sr versus (a) Mg and (b) Siiz diagrams for the rocks from the Catalan volcanic zone. Stars = L’Empordl sub-zone; diamonds = Vilacolum trachyte; triangles = La Selva sub-zone; squares = La Garrotxa sub-zone (data from Arafia et al., 1983).

tion in the Catalan basaltic rocks are also suggested in some REE plots. The “Sr/%r against Sr and 87Sr/86Sr against Rb/Sr diagrams (Fig. 91, show the typical correlations of an ACF model (DePaolo, 1981). Nevertheless, the same Sr isotopic ratio plotted against Mg and SiO, contents (Fig. lo), shows the opposite correlation for some of the samples. This suggests that crustal contamination is not playing a significant role, as is indicated by the trends on the La/Cc-La diagram (Fig. 11) (see Defant and Nielsen, 1990), where crustal contamination causes a decrease in the La/Cc ratio while La content remains essentially unchanged. Only La Selva samples may be related by a process of partial melting phrs crustal contamination. Thus, the observed variations could indicate heterogeneities in the source region, as well as lithospheric and asthenospheric upper mantle sources for the oldest and youngust rocks, respectively. The heterogeneity of the source for the basaltic rocks from the CVZ is also confirmed in the Th/Ta-Th/Hf diagram (Fig. 12), where different straight iines representing different magmatic

CENOZOIC

MAGMATISM

OF THE

VALENCIA

0

159

TROUGH

2

1

3

m/m Fig. 12. Th/Ta-Th/Hf diagram for the basaltic rocks of the Catalan volcanic zone. Stars = L’Empordi sub-zone; diamonds = Vilacolum trachyte triangles = La Selva sub-zone; squares = La Garrotxa sub-zone. Numbers correspond to isotopic values (after Arabia et al. 1983) (see text).

sources can be drawn. The position of points along these lines should correspond to different degrees of partial melting for the same source. However, the lines do not relate samples from the same geographical area, or with the same isotopic ~m~sition or radiometric age, confirming the heterogeneity of the source for the Catalan rocks. Discussion and conclusion Despite the need for more work to characterize precisely the Cenozoic ma~atism of the Valencia trough, a first approximation based on the evidence presented above can be outlined here. The Cenozoic magmatic evolution of the Valencia trough is defined by the existence of two well-differentiated volcanic cycles. The first cycle is of Early to Middle Miocene age and is mainly represented by talc-alkaline andesitic and silicic pyroclastic rocks. The silicic character of the samples studied makes it difficult to relate this volcanism to mantle processes. However, the geochemistry and volcanology of these volcanic rocks indicate that highly silicic magma chambers are developed in the upper part of the continental crust. From the geochronological data shown in Table 1 the Early to Middle Miocene volcanism of the

Valencia trough cannot be related to the main talc-alkaline volcanism of the southeast of Spain, which was related to extensional (Doblits and Oyarztin, 1989; Platt and Vissers, 1989) or to directional (Montenat et al., 1987) tectonics. However, in the Betic Cordillera the presence of silicic pyroclastic rocks of Burdigalian age has been observed by Clercq et al. (1975). Moreover, Torres-Rold5n et al. (19861 show the presence of subduction-related tholeiitic dykes of Early Miocene age within the Betic Cordillera and also explain the post-collision Middle Miocene-Pliocene talc-alkaline to ultrapotassic volcanism of southeastern Spain as being connected with the occurrence of traverse strike-slip and extensional deep faulting. In addition, the early-Middle Miocene volcanism of the Valencia trough is contemporaneous with the long period, subduction-related talc-alkaline volcanism of Sardinia (see Coulon, 1977; Girod and Girod, 1977; Rehault et al., 1984; Beccaluva et al., 1985; Savelli, 1988). As indicated by Girod and Girod (19771, if the Corsica-Sardinia block is put back into its original position before rotation (see Alvarez, 1972; Bayer et al., 1973; Burrus, 1984; Rehault et al., 1984) (Fig. 13), it can be seen that the Early-addle Miocene volcanism forms a straight band oriented NE-SW. This fact, as well as the existence

160

J. MARTI

of compressional tectonics in the Betic-Balearic Domain, suggest that the Early Miocene volcanism of the Valencia trough might also have been related to the development of a marginal basin associated with the northwestern dipping subduction of the African plate (see Banda and Channell, 1979; Cohen, 1980; Rota et al., 1990). The second volcanic cycle ranges from Late Miocene-Recent and is mainly represented by poorly differentiated alkaline basaltic rocks of the intraplate type. The geochemistry of these volcanic rocks confirms the regime of lithospheric extension in the Valencia trough. Partial melting of mantle rocks, caused by decompression during

n

Calc-alkaline Subduction

Fig. 13. Geodynamic

reconstruction

occcurring and the relationship

extensional tectonics is the origin of this volcanism. Differences observed in the three volcanic zones might be related to the different position of each zone with respect to the rift structure. For the Catalan volcanic zone (CVZ), we suggest that a lithospheric mantle affinity is characteristic of the oldest basaltic rocks, while an oceanic island basalt source region is more adequate to explain the origin of the rest (Fig. 14). In the CVZ, the existence of both lithospheric and asthenospheric affinities can be explained by a progressive increase in the rate of extension from the early to the late episodes, causing decompression at deeper levels, which favoured the ascent

045’

magmatism zone

of the northwestern

PI’ AL

Main

Underthrusting

IZZI Mediterranean

between tectonics and magmatism.

active

faults plate

during the Early Miocene

In this reconstruction,

(25-18

Ma); the events

the relative position of continental

blocks

is based on Burrus (1984): Corsica (Co) and Sardinia (Su); Rehault (1981): Kabylies (Kn) and Calabria (Cal; Rota et al. (1990): Balearic thrust units. The location of the inferred subduction

zone is approximate 1984).

(modified from Rehault,

1981 and Rehault et al.,

CENOZOIC

MAGMATISM

OF THE VALENCIA

2201

161

TROUGH

I

l

im -

# 3

/

120AA 70 -

n n

201

.

I

I

I

I

0.7034 0.7OS60.7038

0.7040

0.7042

I

0.7044

.

0.7046

Fig. 14. 87Sr/s6Sr versus Nb for the Catalan volcanic rocks. The arrow indicates the increase in the asthenospheric character towards the youngest rocks. Stars = L’Emporda basaltic rocks; diamonds = Vilacolum trachyte; triangles = La Selva basaltic rocks; squares = La Garrobta basaltic rocks (data from Araiia et al., 1983).

of asthenospheric material and a progressive crease in the degree of partial melting. In Valencia volcanic zone and the Columbretes canic zone, however, these differences in

LATE OLIGOCENE

inthe volthe

source region may be related to differences in lithospheric structure, significantly thinner in the centre of the rift (Columbretes) than in the margin area (Cofrentes) (Marillier and Mueller, 1985; Fernindez et al., 1990). In addition, in the CVZ, crustal contamination also caused some compositional variations amongst the basaltic rocks of the La Selva sub-zone. The geodynamic evolution of the Valencia trough, can be explained in terms of the igneous rock record, which is characterized by a first volcanic cycle developed under compression and a second cycle associated with extensional tectonics. This geodynamic evolution is mirrored in the structural evolution of the region, where two well-differentiated stages can be identified (Fig. 15): a first stage (ranging from the Late Oligocene to the Middle Miocene) in which the basin developed as a result of simultaneous extension in the Catalan-Valencian domain and the emplacement of a thrusting system in the Betic-Balearic domain; and a second stage (ranging from the Middle Miocene to Recent), in which the Valencia trough was dominated by extensional tectonics.

(7) - MIDDLE MIOCENE

MIDDLE

VOLCANISM

n

MIOCENE

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Fig. 15. Scheme showing the evolution of the Valencia trough in two different stages on the basis of the structural and petrological analyses of this basin. N&f = North Balearic fault zone.

162

We wish to thank REPSOL and the SHELL ESPA&A oil company for its generous release of data. The radiometric data of Columbretes and Alboran were worked out at the Istituto di Geocronologia e Geologia Isotopica, CNR, Piss, Italy. This work has partially been supported by CICYT project GE089-0831. E. Banda is thanked for his helpful review of an early draft. We would also like to thank D.M. Latin, J.I. Gil Ibarguchi and an anonymous reviewer for constructive comments which greatly improved the paper. References Alonso Mantilla, LLA., 1985. Variaciones petroquimicas en el volcanismo de las islas Columbretes (Caste%& Publ. Cat. Geol. Aplic. Obras PhbI. Univ. Politec. ValBncia, ETSICCP, pp. 59-101. Alvarez, W., 1972. Rotation of the Corsica-Sardinia microplate. Nature, 235: 103-105. Alvaro, M., de1 Olmo, P. and Aguilar, M.J., 1987. Ignimbritas palecigenas en Mallorca (islas Baleares). Geogaceta, 2: 7-9. Ancochea, E., Giuliani, 0. and Villa, I.M., 1979. Edades radiometricas K-Ar de ta regi6n central espairola. Estud. Geol., 35: 131-135. Ancochea, E., MuBoz, M. and Sagredo, J., 1984. Las manifestaciones volcinicas de Cofrentes y Picasent (Prov. Valkncia). Congr. Espaiiol de Geologia, Ist, Vol. 2, pp. 1-13. Ancochea, E., Muiioz, M. and Sagredo, J., 1987. Las rotas volcLnicas de Nuhalos (provincia de Zaragoza). Geogaceta, 3: 7-10. Araiia, V., Aparicio, A., Martin-Escorza, C., Garcia-Cache, L., Ortiz, R., Vaquer, R., Barberi, F., Ferrara, G., Albert, J. and Gassiot, X., 1983. El volcanismo Nebgeno-Cuaternario de Catalunya: caracteres estructurales, petrolcigicos y geodinlmicos. Acta Geol. Hisp., 18: 1-17. Auzende, J.M., Bonnin, J. and Olivet, J.L., 1973. The origin of Western Mediterranean basins. J. Geol. Sot. London, 129: 607-620. Banda E. and Channell, J.E.T., 1979. Evidencia geofisica para una modelo de evoluci6n de las cuencas de1 MediterrAneo occidental. Estud. Geol., 35: 5-14. Banda, E., Ansorge, J., Boloix, M. and Cbrdova, D., 1980. Structure of the crust and upper mantle beneath the Balearic Islands (Western Mediterranean). Earth Planet. Sci. Let&, 49: 219-230. Bartrina, M.T., Cabrera, Li., Jurado, M.J., Guimerk, J. and Rota, E., 1992. Evolution of the centrai Catalan Margin (Valencia trough, western Mediterranean). In: E. Banda

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