Christian Coulon

Post-collisional transition from calc-alkaline to alkaline volcanism during the Neogene in Oranie (Algeria): magmatic expression of a slab breakoff

Abstract During the Neogene, a magmatic change from calc-alkaline to alkaline types occurred in all the regions surrounding the western Mediterranean. This change has been studied in Oranie (western Algeria). In this area, potassic to shoshonitic calc-alkaline andesites (with La/Nb ratios in the range 4–6) were mainly erupted between 12 and 9 Ma. They were followed (between 10 and 7 Ma) by basalts displaying geochemical features which are transitional between calc-alkaline and alkaline lavas (La/Nb=1–1.7). After a ca. 3-Ma quiescence period, volcanic activity resumed, with the eruption of OIB-type alkaline basalts (La/Nb=0.5–0.6), from 4 to 0.8 Ma. A combined geochemical approach, using incompatible elements and Sr, Nd and O isotopes, allows us to conclude that the transitional basalts derived from the melting of a heterogeneous mantle source, at the boundary between lithosphere and asthenosphere. We propose that melting of a previously subduction-modified lithospheric mantle occurred between 12 and 10 Ma, in response to the upwelling of hot asthenosphere flowing up into an opening gap above a detached sinking slab. As a result, calc-alkaline magmas were formed. From 10 to 7 Ma, the transitional basalts were generated through melting of the boundary mantle zone between the lithosphere and the upwelling asthenosphere. During that stage, the contribution of the lithospheric source was still predominant. Then, as sinking of the oceanic slab progressed, the increasing uprise of the asthenosphere led to the formation and emplacement (from 4 to 0.8 Ma) of typical within-plate alkaline basalts derived from a plume-modified asthenospheric mantle.

40Ar/39Ar dating of a tephra layer in the Pliocene Senèze maar lacustrine sequence (French Massif Central): constraint on the age of the Réunion–Matuyama transition and implications on paleoenvironmental archives

Abstract The sedimentary sequence cored in the Seneze maar (Velay, France) is very favourable for magnetostratigraphy of the Late Pliocene and for studying the paleoenvironmental changes that occurred during that period. The magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the Matuyama reverse period, except for the upper part where a short normal polarity episode was recorded. A tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2.10±0.01 Ma by the 40 Ar/ 39 Ar method. Using this chronological marker, it can be inferred that the normal polarity episode recorded in the Seneze sequence corresponds to the Reunion event and that the age of the normal–reverse transition closing this event is ca. 2.09 Ma. These chronological constraints also permit to (1) demonstrate that the ‘Villafranchian’ mammal fauna found in the Seneze maar is younger than 2.09 Ma and (2) correlate the climatic events recorded in the Seneze sequence to the marine δ 18 O records. This suggests that the Seneze pollen sequence (5–120 m depth) ranges from isotopic stage 85 to 76.

40Ar/39Ar dating on tephra of the Velay maars (France): implications for the Late Pleistocene proxy-climatic record

Abstract Pollen contents and magnetic records of the Pleistocene sedimentary sequences cored in the Praclaux and Lac du Bouchet maars (Velay region, French Massif Central) demonstrated that they have high potential as detailed climatic and environmental archives. Tephra layers interbedded in these sequences have been studied in order to perform isotopic dating. First, petrographic studies and 40 Ar/ 39 Ar ages have shown that these tephra layers are the witness of the same eruption from the Sancy volcano centre, located 100 km to the north. According to fourteen 40 Ar/ 39 Ar ages, carried out on sanidine and biotite grains, this eruption occurred 275±5 ka ago. This result allows to construct a time scale for a composite high resolution record of the climate covering the last 300,000 years and to establish an absolute chronology for the palaeoclimatic events recorded in these sequences. These tephra layers are interbedded in organic layers, deposited during an interstadial stage called the Amargiers interstadial which, according to its age, can be considered as equivalent to isotopic stage 9a. Moreover, the geographical extension of the tephra and its well-constrained age give to this tephra the interest of a chronostratigraphic marker for upper Quaternary sequences in southeastern France.

Petrology of basaltic xenoliths in andesitic to dacitic host lavas from Martinique (lesser antilles): Evidence for magma mixing

AbstractSome recent calc-alkaline andesites and dacites from southern and central Martinique contain basic xenoliths belonging to two main petrographic types:The most frequent one has a hyalodoleritic texture (« H type ») with hornblende + plagioclase + Fe-Ti oxides, set in an abundant glassy and vacuolar groundmass.The other one exhibits a typical porphyritic basaltic texture (« B type ») and mineralogy (olivine + plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxides and scarce, or absent hornblende). Gradual textural and mineralogical transitions occur between these two types (« I type ») with the progressive development of hornblende at the expense of olivine and pyroxenes. Mineralogical and chemical studies show no primary compositional correlations between the basaltic xenoliths and their host lavas, thus demonstrating that the former are not cognate inclusions; they are remnants of basaltic liquids intruded into andesitic to dacitic magma chambers. This interpretation is strengthened by the typical calc-alkaline basaltic composition of the xenoliths, whatever their petrographic type (« H », « I » or « B »). The intrusion of partly liquid, hot basaltic magma into colder water-saturated andesitic to dacitic bodies leads to drastic changes in physical conditions. The two components; the basaltic xenoliths are quenched and homogeneized with their host lavas with respect to To;fO2 andpH2O conditions. « H type » xenoliths represent original mostly liquid basalts in which such physical changes lead to the formation of hornblende and the development of a vacuolar and hyalodoleritic texture. The temperature increase of the acid magma depends on the amount of the intruding basalt and on the thermal contrast between the two components. The textural diversity which characterizes the xenoliths reflects the cooling rate of the basaltic fragments and/or their position relative to the basaltic bodies (chilled margins or inner, more crystallized, portions). In addition to physical equilibration (T, fO2) between the magmas, mixing involves:mechanical transfer of phenocrysts from one component to another, in both directions;volatile transfer to the basaltic xenoliths, with chemical exchanges. It is here demonstrated that a short period of time (some ten hours to a few days) separates the mixing event from the eruption, outlining the importance of magma mixing in the triggering of eruption. The common occurrence of basaltic xenoliths (generally of « H » type) in calc-alkaline lavas is emphasized, showing that this mechanism is of first importance in calc-alkaline magma petrogenesis.

Magmatology of Mt. Pelée (Martinique, F.W.I.). III: Fractional crystallization versus magma mixing

The mineralogical and chemical compositions of the magmas emplaced during the three successive stages of Mt. Pelees activity (ca. 0.4-0.2 Ma; ca. 0.1 Ma-19,500 y.B.P.; < 13,500 y.B.P.) remained relatively constant. All magmas are cogenetic in a broad sense and result from the combination of two major petrogenetic processes: fractional crystallization witnessed by gabbroic and dioritic cumulates (Fichaut et al., 1989, this issue), and magma mixing evidenced by textural, mineralogical and chemical heterogeneities (Gourgaud et al., 1989, this issue). The study of heterogeneous eruptions, e.g. the ash-and-scoria flows (second stage) and the 1902–1929 block-and-ash flows, demonstrates that the basic and acid magmas involved have small but significant geochemical differences; the distribution of compatible (Mg, Sc, Co) and hygromagmaphile (Th, La, Ta, Hf) elements indicates that fractional crystallization played a dominant role in the genesis of the second stage products, whereas magma mixing was dominant in the 1902–1929 lavas. Geological, mineralogical and geochemical constraints are used to propose models of magma chambers. The second stage deposits are considered to be the result of the discharge of the magmas stored in a chemically stratified, ca. 2–3 km3 magma chamber, tentatively described as a cylinder or a parallelepiped of large vertical extent (tens of km) but small horizontal section (some hundreds of meters). By contrast, the young ( < 13,500 y.B.P.) deposits of Mt. Pelee are thought to originate from a small (< 1 km3) and shallow (ca. 10 km in depth) chamber filled with andesitic to dacitic magmas. The periodic intrusion of basic magmas, presumably from a deeper chamber, causes magma mixing which triggers the eruption of heterogeneous and hybrid products.

Mélanges magmatiques à la Montagne Pelée (Martinique) Origine des eruptions de type Saint-Vincent

During the last 40,000 years B.P. the eruptive activity of Mont Pelée (Martinique) has been exclusively pyroclastic, including mainly pumice flow deposits, Pelean-type and Merapi-type nuée ardente deposits, characterized by an andesitic to dacitic magma composition. In addition, a few Saint-Vincent-type nuée ardente deposits are present. Their products are compositionally more basic (basalt, basaltic andesite) and show some characteristic magma mixing features.Two well defined Saint-Vincent type eruptions, named SV1 and SV2 have been studied here. They have been dated by the C14 method respectively at 25,700±1,200 and 22,300±1,200 years B.P. Both follow a similar eruptive pattern, evolving from an andesitic to a more basic magma composition, through an intermediate stage of magma mixing. The volume of ejected products is extensive (1 km3 or more), compared with other deposits such as the Pelean-type nuée ardente. The moderate and progressive variations of magma composition (3 to 6 % SiO2), mineralogy and crystallization pressure-temperature conditions (T: 920°–930° to 950°970°C, using Fe-Ti oxides geothermometer) demonstrate the cogenetic nature of these various magmas.These results, as well as the study of the recent activity of Mont Pelée suggest that during a former period (about 40,000 to 20,000 years B.P.), two magmatic chambers existed rather close to one another. The triggering of these Saint-Vincent type nuée ardente eruptions might involve injections of less-differentiated magma from a lower to a shallower reservoir, followed by the emptying of both reservoirs.During the recent period (less than 13,500 years B.P.), the cyclic eruptive activity of Mont Pelée Volcano has been controlled mainly by a relatively shallow and permanent magmatic chamber. The triggering of eruptions has depended on two processes: volatile overpressure and periodic replenishment of this superficial reservoir by deeper and less-differentiated magma injections. This change in eruptive character results perhaps from succession of SV1 and SV2 Saint-Vincent type eruptions; the volcano deep-structure might have changed, as a consequence of the extensive volume of ejected products.ResumeAu cours des 40.000 dernières années, l’activité de la Montagne Pelée a consisté en l’émission de coulées de ponces, nuées ardentes peléennes et de type Mérapi mettant en place des produits de composition andésitique à dacitique, ainsi que de quelques nuées ardentes de type Saint-Vincent responsables de l’émission de produits plus basiques (andésites basaltiques, basaltes) accompagnés par des phénomènes de mélanges magmatiques. Deux éruptions de ce type, SV1 et SV2 datées à 25700±1200 et 22300±1200 ans B.P., ont été étudiées au point de vue du déroulement de l’éruption, de la composition chimique des produits, et de la minéralogie. Elles suivent toutes les deux un même scénario éruptif, évoluant depuis un pôle andésitique vers un pôle plus basique, avec un stade intermédiaire de mélange magmatique. Les volumes émis sont très importants, de l’ordre de 1 km3 ou plus. Les différences chimiques peu marquées (3 à 6% SiO2), les variations modérées et progressives de la minéralogie ainsi que des conditions physiques de cristallisation (T: 920°–930° à 950°–970°C) indiquent le cogénétisme des différents magmas.Ces résultats, ainsi que l’étude de l’activité éruptive récente de la Montagne Pelée, suggèrent qu’au cours de la période intermédiaire (20.000 à plus de 40.000 ans B.P.), deux chambres magmatiques existaient sous le volcan, à des profondeurs peu différentes. Le déclenchement de ces éruptions de type Saint-Vincent peut s’expliquer par l’injection de magma moins différencié originaire de la chambre inférieure, dans la chambre superficielle, accompagnée par l’émission des magmas des deux réservoirs.Au cours de la période récente (13.500 ans B.P. à maintenant), l’activité cyclique de la Montagne Pelée serait sous l’influence prédominante d’une chambre magmatique assez superficielle et permanente. Le déclenchement des éruptions associerait des processus de surpression en volatil et de réalimentation périodique par injection de magma profond. On peut supposer que ce changement de caractère dans l’activité de la période récente est une conséquence des deux éruptions SV1 et SV2, susceptibles d’avoir perturbé la structure profonde du volcan en raison de l’importance des volumes émis.

Petrology of tholeiitic lavas from Tanna Island (New Hebrides): Importance of cumulative processes in Island arc magmatism

Tanna, one of the southernmost islands of the New Hebrides volcanic arc, is made of Late Pliocene to Recent island arc tholeiitic basalts and andesites, with SiO2 contents ranging from 45 to 57%. These lavas are highly porphyritic (30–50% in volume): phenocrysts of plagioclase are the most abundant, together with olivine and clinopyroxene. The groundmass contain plagioclase, augite, olivine, magnetite and glass; pigeonite, tridymite, sanidine and, rarely, biotite may also occur. The olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals, but their compositional variations are not correlated with the Mg/Fe ratio of bulk host rocks, the most Fe-rich compositions being found in Mg-rich lavas. Plagioclase compositions range from An95 to An60 in the basalts and An60 to An50 in the andesites, but, within each group, they are not correlated with SiO2 or Na2O contents of host lavas. Consequently, the bulk major element compositions of Tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids. The oxyde-SiO2 variations diagrams, and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions. However, this does not exclude fractional crystallization: the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite.

Rifting actif ou passif en Éthiopie ? éléments de réponse apportés par l'étude des xénolites péridotitiques de la région du lac Tana

The comparative study of peridotitic xenoliths from Oligocene and Quaternary volcanic formations of north-western Ethiopia shows that the textural, mineralogical, thermal and geochemical characteristics of the lithospheric mantle of this region were acquired in Oligocene times. These data suggest that the impinging of the Afar plume beneath the Ethiopian lithosphere occurred prior to the traps emplacement and the Plio-Quaternary rifting. This favours a model of active rifting for the Afar triple junction.