Compte tenu de l'erreur, la datation effectuée sur la biotite de la granodiorite calcoalcaline du massif de Segur fournit un âge comparable à ceux obtenus par Solé (1993) dans le massif du Montnegre, d'environ 285 Ma. Nous interprétons cette valeur comme l'âge de refroidissement en-dessous de -350°C, qui est la température de fermeture de l'Ar pour la biotite (McDougall et Harrisson, 1988). L'âge que nous avons obtenu montre que dans le bloc côtier du batholite Catalan, des intrusions distantes de plus de 100 km (entre le massif du Montnegre et le massif de Segur) ont le même âge de refroidissement. Ceci apporte une première indication en faveur d'un refroidissement homogène à l'échelle régionale dans la portion étudiée du batholite.
Les trois ~iotites des leucogranites alcalins ont fourni des âges très différents, compris entre 281.6±5.8 Ma (leucogranite de Salions) et 234.3±4.9 Ma (leucogranite du Cadiretes). Le pourcentage d'argon radiogénique mesuré est compris entre 86% et 98%, ce qui en principe est un bon indicateur de la fiabilité des analyses.
Les deux âges plus vieux, obtenus pour le leucogranite de Salions (281.6±5.8 Ma) et pour le leucogranite du stock de Cabreres (272.5±5.8 Ma), qui sont pareils dans l'intervalle de l'erreur peuvent s'interpréter comme l'âge de refroidissement des intrusions en-dessous de 350 °C. Or leur mise en place à un niveau très superficiel suggère un refroidissement rapide et donc, les valeurs obtenues pourraient être très proches des âges de cristallisation.
Par contre, l'âge obtenu sur la biotite du leucogranite du Cadiretes (234.3±4.9 Ma) est beaucoup trop jeune pour considérer qu'il enregistre le refroidissement du pluton qui, en tout cas, se met en place avant le stock de Cabreres. L'âge obtenu doit donc refléter une perturbation du système de l'Ar dans la biotite de cet échantillon.
La plupart des datations K-Ar sur biotite obtenues par Solé (1993) dans le massif du Montnegre, fournissent des âges de 285 Ma. Solé {1993) remarque que si le concentré de biotite comprend de la chlorite, des âges plus vieux de l'ordre de 300 Ma sont obtenus. Cet auteur conclut que la chlorite piège de l'Ar*. Ce phénomène n'a pas été observé dans les analyses que nous avons effectuées, malgré la présence de chlorite, mise en évidence par les faibles teneurs en K des concentrés. Au contraire, l'âge que nous avons obtenu sur la biotite du leucogranite du Cadiretes est la première datation K-Ar sur biotite qui, dans le batholite Catalan, montre des signes de perturbation.
La corrélation entre l'âge des feldspaths potassiques et leur structure, mise en évidence dans le massif du Montnegre par Solé (1993), est aussi observée sur les feldspaths provenant des massifs du Cadiretes et de Segur. L'âge de 263.5±5.6 Ma de l'orthose pure M5-2-Kfs (massif de Segur) peut s'interpréter comme l'âge de refroidissement de l'intrusion en-dessous de -150°C, température de fermeture du système Ar dans les feldspaths (McDougall et Harrisson, 1988).
Pour cet échantillon, avec l'âge de la biotite M5-2-St obtenu, un taux de refroidissement moyen de -7°C/Ma entre 292 Ma et 263 Ma peut être estimé. Les âges obtenus sur les orthoses-microclines intermédiaires du massif du Cadiretes sont compris entre 161.8±3.5 Ma et 137.1±2.9 Ma. Ils sont environ 100 m.y. plus jeunes que les biotites des mêmes échantillons.
Puisqu'il s'agit d'intrusions refroidies rapidement (car mise en place superficielle) on peut exclure qu'il s'agisse d'âges de refroidissement, et donc, ils témoignent de pertes d'Ar qui font penser à des perturbations thermiques. Les âges obtenus sont nettement inférieurs aux âges K-Ar minima de -200 Ma enregistrés par les feldspaths tricliniques du Montnegre. Cette valeur de -200 Ma a été interprétée (Solé, 1993) comme l'âge de la perturbation thermique qui rajeuni les
88 CHAPITRE V
feldspaths. Les âges plus jeunes que nous avons obtenus dans le massif du Cadiretes suggèrent que des événements thermiques plus tardifs ont pu affecter ce massif.
Les âges obtenus sur les felsites, de 179.5±3.6 Ma et de 157 .3±3.2 Ma, sont comparables aux âges obtenus sur les feldspaths potassiques du massif du Cadiretes. Il est probable que, dans ces roches composées de quartz, feldspath potassique et muscovite, les datations K-Ar soient contrôlées par la systématique de l'Ar dans le feldspath potassique. Les âges très jeunes obtenus seraient donc indicatifs de perturbations tardives.
V.3 GÉOCHRONOLOGIE 40Arf39Ar
Voir article annexe:
40ArJ39Ar chronological constraints on late-Hercynian magmatism:
a case study in the Catalan batholith (NE Spain).
GEOCHRONOLOGIE K-Ar, 40Arf39Ar et Rb-Sr
40ArJ39Ar CHRONOLOGICAL CONSTRAINTS ON LATE HERCYNIAN MAGMATISM:
A CASE STUDY IN THE CATALAN BATHOLITH (NE SPAIN)
1Ferrés-Hernàndez, M., 1•2Singer, S.S., 3Solé, J. and 4Enrique, P.
1. Dépt. de Minéralogie, Université de Genève. Rue des Maraîchers 13, 1211 Genève 4, Switzerland.
2. 40Ar/39Ar Geochronology Laboratory, Université de Genève. Rue des Maraîchers 13, 1211 Genève 4, Switzerland.
3. lnstituto de Geologfa. Apdo. Postal 70-296. Ciudad Universitaria. 04510 México D.F., Mexico.
4. Dept. de Petrologia, Universitat de Barcelona. Pedralbes, 08071-Barcelona, Spain.
Abstract
Twenty-five 40Arf39Ar incremental-heating analyses were performed on hornblende, biotite, and K-feldspar from multiple epizonal to hypabyssal intrusions of the late-Hercynian Catalan batholith. The collected data constrain the magmatic history of three intrus ive events: (1) sparse calc-alkaline gabbroic and dioritic small plutonic bodies are the oldest intrusions, which cool below about 550°C between 296.1±3.2 Ma and 291.1±1.6 Ma in different massifs of the batholith; (2) widespread voluminous epizonal calc-alkaline granitoids and genetically related cross-cutting hypabyssal dykes intruded and rapidly cool below 350°C in a brief period between 288.4±2.8 Ma and 284.5±2.8 Ma; and (3) a shallow alkaline granitic ring complex comprises plutons that cool below 350°C at 286.5±1.8 Ma, i.e. simultaneous to the more widespread calc-alkaline granites. Minor alkaline stocks yielded a minimum cooling age of -277 Ma, most probably underestimated. The data indicate that the whole of magmatic activity in this region lasted no more than 20 m.y.
Field relationships constrain batholithic uplift prior to 250 Ma. Younger and heterogeneous ages were obtained on biotite with interlayered chlorite (234.6±3.4 Ma to 178.4±2.0 Ma) and on orthoclase-microcline (177.6±1.4 Ma to 134.3±1.6 Ma) separates from both calc-alkaline and alkaline samples. We interpret these results to reflect low Ar retentivity of those minerais that suffered deuteric alterations, e.g. chloritization of biotite and structural inversion of K-feldspar, that would have favored later selective 40Ar* loss over geological times. ln neighboring areas, Triassic and Jurassic-Cretaceous rifting events with related hydrothermal fluid circulation are recognized. These are reasonable geological events that may have reset the K-Ar system. However, collected samples for this isotopie study preserve magmatic textures and magmatic stable isotopes signatures. Therefore, we suggest that rifting-related hydrothermal fluids circulated through numerous fissures that affect the batholithic rocks, inducing conductive heat for 40Ar* loss. Collected samples would have preserved unaltered features.
1. INTRODUCTION
The late~Hercynian Catalan batholith (1500 km2, Catalan Coastal Ranges, NE Spain) escaped Alpine metamorphic events and thus, is a suitable terrane for geochronological research on Hercynian magmatism. The batholith comprises severa! blocks (Coastal, Guilleries, and Tarragona) bounded by recent Neogene grabens (Fig. 1 ). On the basis of conventional 40Ar/39Ar step-heating experiments, we investigate the magmatic history and thermal evolution of the late-Hercynian intrusions occurring on the major, Coastal block
89
90 CHAPITRE V
(1 000 km2 ). This comprises the Montnegre, Cadiretes, and Segur massifs (Fig. 1) which mainly consist of multiple post-orogenie calc-alkaline intrusions. ln the Cadiretes massif, the calc-alkaline intrusions are post-dated by an alkaline granitic ring-complex with A2-type (Eby, 1992) affinities (Ferrés et al., 1996). Previous geochronological studies have been undertaken in the Montnegre massif (Salé, 1993; Salé et al., 1998 and Salé et al., in prep). ln this paper, data are presented from twenty five 40Arf3 9Ar incremental-heating experiments on mineral separates of hornblende, biotite, and K-feldspar from calc-alkaline and alkaline samples collected in the Cadiretes and Segur massifs.
Experiments on hornblende and biotite from calc-alkaline intrusions and from one alkaline sample yielded precise cooling ages derived from isochrons. The data constrain the magmatic history of these intrusions. However, most experiments on biotite from alkaline granites yield.ed geologically meaningless total fusion ages. The data reflect a selective disturbance of Ar systematics in biotite from the alkaline intrusions. These yielded undulating age spectra with a characteristic mid-temperature plunge, whereas undisturbed biotites from calc-alkaline intrusions yielded concordant age spectra with a systematic mid-temperature hump. Similar spectra have been previously reported and discussed (humps: Tetley and McDougall, 1978; Berger and York, 1981; Ruffet et al. 1991; Scaillet et al., 1996; and plunges: Berger, 1975; Lo and Onstott, 1989). Our contribution illustrates good examples of these particular spectra from biotite separates that suffered the same geological thermal evolution and the same experimental treatment. Thus, we discuss sorne hypotheses on the significance of these spectra. Bath calc-alkaline and alkaline samples yielded heterogeneous 4DArf39Ar K-feldspar ages that correlate with the structural state of the crystals. Orthoclases yielded time since cooling of intrusions, whereas orthoclase-microclines gave younger ages. Our results suggest that alpine thermal events reset the K-Ar system of biotite from the localized alkaline complex, and of orthoclase-microclines over a regional scale. On the basis of the diffusive studies of Ar in biotite and K-feldspar, and considering the tectono-sedimentary alpine evolution of NE Spain, we present a preliminary discussion on which diffusive processes and which geological events may be good candidates to reset the geochronometers.
2. GEOLOGICAL SETTING
2.1 The late-Hercynian Catalan batholith
The Catalan batholith was built during the waning stages of the Hercynian orogeny. lt intruded within Paleozoic rocks that had suffered prior Hercynian deformation and regional metamorphism. The batholith is therefore post-tectonic in character, or late-Hercynian as defined by Arthaud and Matte (1975). lt is of composite type, and mainly consists of calc-alkaline plutons and related dykes that form a voluminous, high-K, calc-calc-alkaline suite, intrusions ranging from hornblende gabbros to leucogranites (Enrique, 1990). ln the Cadiretes massif, between the towns of Tossa de Mar and Sant Feliu de Gufxols, a late, alkaline granitic ring-complex encompasses an area of 67 km2 and comprises severa!
intrusions.
The calc-alkaline suite
Calc-alkaline intrusions mainly comprise severa! km2-sized plutons. Granodiorite (66-72 wt.% Si02) is the most common composition, followed by granite (72-77 wt.%
Si02 ), and tonalite (60-65 wt.%. Si02 ). Intrusion of plutons induced a contact
GEOCHRONOLOGIE K-Ar, 40ArJ39Ar et Rb-Sr
metamorphism that overprinted regional greenschist facies metamorphism, which indicates an epizonal levet of emplacement (Enrique, 1984). Thermobarometric studies in the Montnegre massif report depth of plutons emplacement between 5.5 km (Gil lbarguchi, 1988) and 8 km (Salé, 1993). A genetically related, NE- to E-trending, hypabyssal dyke-swarm, dioritic to granitic in composition, intruded the plutons. The dykes are -5-20 m wide, can be followed over severa! km, and show well-developed chilled margins indicating that their intrusion was under thermal contrast into plutonic host-rocks. Minor and sparse plutonic bodies of diorites and hornblende gabbros represent prior calc-alkaline intrusions.
Field relationships show brittle intrusive contacts of epizonal plutons into these minor bodies of diorites and gabbros. Ali varieties of mentioned intrusions contain biotite, and hornblende occurs only in sorne granodioritic or Jess acidic rocks. Linear trends in Harker diagrams suggest that the intrusions may be related by simple magmatic differentiation (Enrique, 1985). The suite is slightly peraluminous except the most basic compositions which are metaluminous. Previous greochronological studies report 40ArJ39Ar and Rb-Sr data for the calc-alkaline plutons from the Montnegre massif. ln the following data, errors are quoted to 2cr. Two Rb-Sr mineral isochrons on biotite, K-feldspar, plagioclase and whole-rock yielded crystallization ages of 289.5±5.3 Ma (27-point isochron including whole-rock and mineral separates analyses from 13 calc-alkaline intrusions, Salé, 1993) and 284±7 Ma (4-point mineral isochron on one plutonic tonalite, del Moro and Enrique, 1996). Initial 87Sr/86Sr ratios are 0.709 for gabbros and 0.711 for granitoids, implying that the magmas acquired a significant quantity of continental crustal material (Salé, 1993). One amphibole separate from one gabbro included in the former Rb-Sr mineral isochron yielded a total fusion age of 291.1 ±1.6 Ma (Salé, 1993), which was interpreted to record cooling below -550°C. Four 4°ArJ39Ar analyses on biotite separates from epizonal granitoid plutons included in the Rb-Sr isochrons yielded plateaus over 80% of cumulative 39Ar released. The experiments gave an average of 285.6±1.5 Ma interpreted as time since cooling below -350°C (Salé, 1993). The data indicate synchronous closure of the Rb-Sr and biotite and amphibole K-Ar isotope systems, thus suggesting rapid cooling of plutons. Seven K-feldspar 40ArJ39Ar analyses yielded discordant age spectra. Total fusion ages range from 277.1±1.4 Ma to 191.8±1.0 Ma, with a cluster around 200 Ma. The oldest age was obtained in one pure orthoclase, and was interpreted to record magmatic cooling; whereas the younger ages correspond to orthoclases partially inverted to microcline. Thus, it was stated (Solé, 1993) that Ar Joss would be related to an orthoclase-microcline inversion probably occurring at around 200 Ma in response to a regional heating during Triassic rifting.
However, it has not been precised which related process (i.e. lithospheric thinning, burial metamorphism, or fracturing with associated hydrothermal fluid circulation) outgased these K-feldspars.
The a/kaline granitic camp/ex
The alkaline granitic complex consists of a ring structure of about 10 km in diameter.
lt comprises severa! shallow intrusions of high-Si02 (73-77 wt.%), weakly alkaline granites. These comprise two major, central, coarse- to fine-grained plutons emplaced via subsequent cauldron subsidence processes, post-dated by severa! peripheral porphyry stocks of fine-grained (2-0.5 mm), aplitic (150-500 11m) and microaplitic (50-150 Jlm) groundmass. The latter contain miarolitic cavities and granophyric intergrowths. The distribution and texture of the stocks suggest that these may belong to an incomplete ring-dyke that bounds the alkaline complex. Ali these alkaline intrusions are cross-eut by related, E-trending, porphyry, granophyric and felsitic dykes. The dykes are 0.5 to 20 m wide, and show chilled margins indicating intrusion under thermal contrast into host-rocks.
Felsites contain spherulitic textures due to devitrification of rhyolitic glass matrix. Green 91
92 CHAPITRE V
biotite in very small amounts (often <3% modal composition) is the only matie mineral in the alkaline complex. The granites are slightly peraluminous with A/CNK ratio between 1.04 and 1 .14. Higher values obtained from felsitic and granophyric dykes reflect alkali mobility which, in the former, may be associated with glass devitrification. The alkalinity of ali these intrusions is expressed by lower CaO content (<0.5 wt.%) and higher (Na20+K20)/Ca0 ratio when compared to the granites from the calc-alkaline suite of equal Si02 content (with Ca0>0.5 wt.%). ln addition, granites from the alkaline complex are characterized by higher Y, Nb, Th, and Ga concentrations relative to the calc-alkaline granites. ln the trace-element discrimination diagrams, the alkaline granites essentially plot as within plate granites (WPG, after Pearce et al., 1984); as A-type granites (after Whalen et al., 1987); and according to Eby's (1992) classification they are A2-type granites. Petrologic and geochemical features of these granites are very close to those of slightly peraluminous, subsolvus, alkaline, biotite-granites of post-orogenie ring-complexes from the neighboring island of Corsica, in France (Bonin, 1990), for which a Rb-Sr crystallization age of 276±3 Ma has be en determined by Poitrasson et al. (1994) in the Tana-Peloso ring complex.
2.2 Alpine evolution of the Catalan domain
The Paleozoic basement of the Catalan Coastal Ranges is covered by a Mesozoic-Cenozoic sedimentary sequence. The sequence starts with continental red conglomerates and sandstones (German Buntsandstein facies) that unconformably overlie the previously peneplained Paleozoic basement (Anad6n et al., 1979), and locally lay over batholithic calc-alkaline granitoids. Palynological studies on these sediments report Lundbandispora sp. and Cycapodites as the oldest fossil record, attributed to the lower Triassic (Calvet and Marzo, 1994). This indicates uplift and erosion of calc-alkaline intrusions prior to about 250 Ma (Lower-Triassic boundary). ln contrast, no stratigraphie or superposition relations constrain an upper limit for the age of emplacement, cooling, or erosion of the younger, alkaline granitic complex. Alpine evolution of the Catalan domain is relatively simple, comprising three major geodynamic stages (Roca, 1994):
(1) Du ring the whole Mesozoic, the Catalan domain was extended via NE- to NW-trending basement faults inherited from the late-Hercynian (Arthaud et Matte, 1975;
Ziegler, 1993), that bounded several blocks of distinct tectonic and sedimentary evolution.
Hence, it is suggested (Anadon et al., 1979) that the NE portion of the Catalan domain, where most of the late-Hercynian batholithic rocks are presently exposed, remained as part of an emergent black preventing post-Triassic sedimentation. ln contrast, Triassic to Cretaceous mainly shallow marine sediments were deposited in large basins in the SW portion of the Catalan domain, and probably extended northeastwards, along the present coast. Subsidence analyses of Mesozoic basins located 300 km southwest of the batholith, record two periods of rifting with rapid subsidence in the Triassic ( -255-200 Ma) and the Upper Jurassic-Lower Cretaceous ( -145-100 Ma). These rifting stages are related to the opening of the Tethys Sea and the Atlantic Ocean, respectively, and each is followed by a post-rift period of slower subsidence by thermal relaxation (Roca et al., 1994). Upper Triassic alkaline volcanism is reported in the south westernmost portion of the Catalan Coastal Ranges (Salas and Casas, 1993).
(2) ln the upper-Cretaceous, a topographie inversion reflects the beginning of the Alpine Orogeny. Paleogene collision between the lberian Plate and Europe generated a N-S compression in lberia (Guimerà, 1984). This lead to formation of the lberian Chain and the Catalan Coastal Ranges, which can be regarded as a unique intermediate intracontinental
GEOCHRONOLOGIE K-Ar, 40Arf39Ar et Rb-Sr
range (Salas and Casas, 1993). ln the Catalan Coastal Ranges, where neither magmatism nor metamorphism occurred, tectonics was controlled by reverse sinistral reactivation of the NE-trending late-Hercynian basement faults.
(3) These faults were later reactivated again as !istrie normal faults during the Neogene extension (Roca and Guimerà, 1992) that built the Valencia trough, between the Balearic Islands and the NE Spanish coast (Banda and Santanach, 1992). The Catalan Coastal Ranges represent the emergent NW margin of the trough. ln this sector, a first stage of rifting from late Oligocene to early Miocene was followed by a period of thermal subsidence lasting up to present (Roca and Guimerà, 1992). Rifting stage in the Catalan Coastal Ranges is expressed by formation of NE-trending grabens, giving rise to the present "basin and range" morphology of the Catalan Coastal Ranges. Related to extension, middle Miocene to recent alkaline volcanic activity occurs onshore and offshore of the Valencia trough (Marti et al., 1992). ln the Catalan Coastal Ranges, eruptive cent ers occur in its north-easternmost sector, along NW-trending deep faults. Also related to extension, hydrothermal fluid circulation persists up to date (Fernandez and Banda, 1989).
3. SAMPLES AND ANALYTICAL TECHNIQUES 3.1 Samples
Analyzed samples in this study were collected in the Cadiretes and Segur massifs (Figs.
2 and 3) from the coastal black of the batholith. Location and petrological-mineralogical characteristics of collected samples are summarized in table 1.
40Arf39Ar incremental-heating experiments were performed on 25 mineral separates th at include 4 hornblende, 10 biotite, and 4 K-feldspar separates from 13 calc-alkaline granitoids; and 6 biotite and 2 K-feldspar separates from 6 alkaline granites. Rock-samples caver ali patrologie types. Thus, calc-alkaline samples were mainly collected from plutons that range in composition from quartz-diorites to leucogranites (54.6 to 77.3 wt.% Si02 ).
ln the Cadiretes massif, these show intrusive contacts. For example, sample M2-25 is from an Hbi-Bt-granodiorite intrusion which can also be found as angular xenoliths up to severa!
m 3 in Bt-granodiorite M2-17. This is cross-eut by M2-1 Hbl-quartz-dioritic dyke. The dyke is about two meters wide and shows well-developed chilled margins over about 5 cm, grading to a porphyry texture that comprises a few plagioclase phenocrysts in a micro-crystalline groundmass. ln its innermost portion, the dyke has a micro-crystalline, equidimensional, fine-grained texture mainly composed of plagioclase, amphibole and minor
m 3 in Bt-granodiorite M2-17. This is cross-eut by M2-1 Hbl-quartz-dioritic dyke. The dyke is about two meters wide and shows well-developed chilled margins over about 5 cm, grading to a porphyry texture that comprises a few plagioclase phenocrysts in a micro-crystalline groundmass. ln its innermost portion, the dyke has a micro-crystalline, equidimensional, fine-grained texture mainly composed of plagioclase, amphibole and minor