Gilbert Féraud

Plateau ages and excess argon in phengites: an 40Ar39Ar laser probe study of Alpine micas (Sesia Zone, Western Alps, northern Italy)

Abstract Bulk samples and single grains of high-pressure white micas and a biotite from two restricted areas (Mucrone and Marine) 10 km apart in the Sesia Zone (Western Alps, Italy) were analysed by the conventional (step-heating) and continuous laser probe (step-heating and spot fusion ) 40 Ar 39 Ar techniques, respectively. The analysed minerals crystallized during the Eoalpine, eclogitic facies metamorphism. The phengites from the Mucrone area display ages which scatter between ∼ 104 and ∼ 180 Ma. In the Marine area (a 50-m-wide outcrop) the phengites from four samples with different bulk-rock compositions display precise plateau ages, which scatter between 69.4 ± 0.7 and 76.9 ± 0.6 Ma, and homogeneous age maps, whereas two minerals (phengite and biotite) from the same rock sample provide two very “discordant” plateau ages, 69.4 ± 0.7 and 140.5 ± 0.6 Ma, respectively. In the light of the present knowledge of the history of the Alps, ages as old as 180 Ma are unrealistic and are related to excess argon, whereas ages in the range 70–80 Ma, perhaps even 140 Ma, could be significant. We suggest that excess argon, probably conveyed by a fluid phase and trapped in defects within the structure of the phengite grains at the time of their isotopic closure, is the best explanation for the observed spread in apparent ages on a regional to single grain scale, in spite of a nearly universal existence of plateau ages and homogeneous age maps. We propose that the age of closure of the phengites to argon loss is ⩽ 69.4 ± 0.7 Ma.

RbSr and 40Ar39Ar laser probe dating of high-pressure phengites from the Sesia zone (Western Alps): underscoring of excess argon and new age constraints on the high-pressure metamorphism

Abstract The combined use of the RbSr and 40Ar39Ar laser probe methods allows confirmation of the existence of excess argon in phengites from eclogites from the Sesia zone (Western Alps) despite systematic 40Ar39Ar plateau ages. 40Ar39Ar phengite ages from two different areas show integrated age spreads between 65.4 ± 0.3 Ma and 109.5 ± 0.4 Ma. The youngest 40Ar39Ar plateau age (65.9 ± 0.4 Ma) is concordant with two RbSr phengite-whole-rock isochron ages from the same outcrop which display a mean age at 64.2 ± 2.5 Ma and older than a Rb-Sr biotite-whole-rock isochron age at 53.0 ± 1.0 Ma. This age concordance, being incompatible with the concept of isotopic closure temperature, is probably fortuitous and probably results from an excess argon contamination. However, the concordance of the RbSr phengite isotopic closure temperature with the temperature reached during the eclogite-facies event suggests that the age at 64.2 ± 2.5 Ma could be a crystallization age probably contemporaneous with the eclogite-facies event in the Sesia zone. When compared to recent age data from the Piemont zone and the Dora Maira Massif, the entire data set (which needs to be confirmed by extra measurements) support diachronous high-pressure (HP) events in the Western Alps.

40Ar/39Ar dating of the Jurassic volcanic province of Patagonia: migrating magmatism related to Gondwana break-up and subduction

The Mesozoic large igneous province (LIP) of Patagonia (southern South America), which is one of the largest silicic provinces on Earth has been investigated by the 40Ar/39Ar method. Twenty-seven ages considered as valid, including twenty plateau ages, show that the volcanic activity, ranging from 187 to 144 Ma, occurred between and contemporaneously with the initial break-up of Gondwana (starting with the Karoo-Antarctic-Tasmanian (KAT) flood basalt province) in the east, and a subduction in the west. The data display a regular decreasing of ages from the ENE (187 Ma) to the WSW (144 Ma) along about 650 km, apparently related to the tectonic structure in half-grabens oriented NNW-SSE. The good fitting of this trend with the opening of the Rocas Verdes-Sarmiento marginal basin favors a space time evolution of this continental volcanism culminating towards the SSW in a continental disruption behind the magmatic arc. The observed age progression of volcanism may be the result of the variations of the physical characteristics of the subduction. The spreading and thermal effect of the KAT plume may have an additional effect and also could account for the unusually large volume of magma.

From flood basalts to the inception of oceanization: Example from the 40Ar/39Ar high‐resolution picture of the Karoo large igneous province

The Jurassic Karoo large igneous province consists of vast quantities of basaltic lava flows, giant dike swarms, and poorly dated silicic magmatism. The Karoo magmatism occurred over more than 3 × 106 km2 and was associated with the opening of the Indian Ocean. We present new 40Ar/39Ar geochronological data concerning the easternmost part (i.e., the Mwenezi and Lebombo areas) of the Karoo province, close to the future rifted margin. These data allow documenting the final history of the province and the time elapsed between the end of the Karoo magmatism and the inception of oceanization. This important type of information is poorly constrained for most of the large igneous provinces. Eighteen plagioclase separates yielded 14 plateau and “mini-plateau” ages obtained on 3 basaltic dikes (178.1 ± 1.1 to 177.2 ± 2.4 Ma; 2σ), 3 gabbroic (178.2 ± 1.7 to 176.8 ± 0.7 Ma) and 2 silicic (175.8 ± 0.7 to 174.4 ± 0.7 Ma) plutons, and 1 rhyolitic lava flow (177.8 ± 0.7 Ma). We also obtained three concordant plateau and mini-plateau ages ranging from 173.9 ± 0.7 Ma to 172.1 ± 2.3 Ma on the atypical E-MORB-like N–S striking Rooi Rand dikes. One dike from the Save-Limpopo N70°-oriented giant dike swarm provides a mini-plateau age of 177.7 ± 0.8 Ma in agreement with the dates previously determined on this branch and possibly assessing the same age and short duration (≤1–2 Myr) as for the 179 Ma giant Okavango dike swarm. New and previous selected age data obtained on the Karoo province show that the magmatism was active over ∼10 Myr from 184 to 174 Ma. The main basaltic phase occurred mostly over the first ∼5 Myr and was progressively followed by a more differentiated and less voluminous magmatism over the last 4 Myr. The easternmost Lebombo-Mwenezi long-lasting magmatism is interpreted as being triggered by the progressive lithospheric extension preceding the continental disruption. The transition from rifting to oceanization is probably illustrated by the E-MORB-like Rooi Rand dikes which are likely to be emplaced during or shortly after the final stage of the Karoo magmatism. A geodynamic evolution of the province is proposed. These data indicate that the Karoo magmatism represents an atypical province (admitting that other large igneous provinces are sufficiently known) with a long-lasting and relatively low-emission rate magmatic activity. It appears from a reevaluation of major Phanerozoic Continental Flood Basalts (CFBs) that for most of them, including the Karoo, the onset of oceanization shortly follows (or is coeval to) the latest CFB-related activity more closely than previously thought.

The history of intrusive activity on the island of La Palma (Canary Islands)

Abstract The island of La Palma, located in the northwest of the Canary archipelago, consists of two main geological units. (1) The subaerial extrusive Coberta Series covers most of the island and consists of alkali basaltic lavas, phonolitic plugs, and pyroclastic rocks, erupted between 1.61 Ma and the Recent. (2) The basement complex is exposed in the Barranco de Las Angustias and the Caldera de Taburiente and consists of the submarine intrusive and extrusive Seamount Series and a basal plutonic complex. Alkali basaltic dikes of different ages and orientations intruded in both units. Three groups of dikes and sills can be distinguished, based on their field relationships, petrography, geochemistry and ages. The oldest Group I, represent the feeder dikes to the Seamount Series, intruded between 2.9 and 4.0 Ma B.P. They were originally steep, with a N05°W strike and perpendicular to the originally nearly flat bedding of the layered Seamount Series. Subsequent to emplacement, Group I dikes and the extrusives of the Seamount Series were rotated into their present orientation. Group II sills, intruded parallel to the bedding of the Seamount Series ( 45 230 ), are coeval with or younger than Group I, but older than the erosional unconformity separating the Coberta and the basement complex (i.e. > 1.6 Ma; 50 vol % of the volcanic edifice of the La Palma seamount and highlight the importance of intrusive processes for island growth. Intrusions into central oceanic volcanoes like La Palma may result in an uplift, and rotation of more than 30 km 3 by up to 45°.

New time constraints on dyke swarms related to the Paraná-Etendeka magmatic province, and subsequent South Atlantic opening, southeastern Brazil

Abstract 40 Ar 39 Ar incremental heating analyses were carried out on tholeiitic dyke swarms of Santa Catalina (Florianopolis) and Rio de Janeiro and alkaline dykes from the proximity of Rio de Janeiro City, southeastern Brazil. Most of the plagioclases from the tholeiitic dykes display disturbed age spectra, with younger apparent ages most often clearly correlated with alteration phases. However, a calculated weighted mean age from the less disturbed released fractions of the NE-oriented Santa Catarina dyke swarm gives an age of 129.4 ± 0.3 Ma which is contemporaneous with the Ponta Grossa Dyke Swarm (PGDS) and at least the northern Parana Continental Flood Basalts (CFB). The Rio de Janeiro tholeiitic dykes display age spectra affected by alteration and probably excess argon, but with low-temperature ‘mini-plateau ages’ between 130.6 and 129.8 Ma, which are concordant with previous data obtained on the PGDS and Rio de Janeiro dyke swarms by Renne et al. (1996a) and Turner et al. (1994). These new geochronological data when added to the previous ones show that the initiation of Gondwanas breakup in the region was characterized by a volcano-tectonic system with a geometry more complicated than the rift-rift-rift system suggested by many authors. The different rift arms of this structural feature were operating during a brief time period, mainly from 131.5 to 129 Ma, and were contemporaneous at least with the northern part of the Parana-Etendeka Province (PEP). Single grain and bulk sample biotite from the alkaline, mainly coast-parallel dyke swarm near Rio de Janeiro City display plateau ages around 80 Ma (81.8 ± 1.8 and 78.8 ± 2.8 Ma) and 70 Ma (72.5 ± 0.5 and 69.7 ± 0.2 Ma). These magmatic intrusions refer to Late Cretaceous flexuring and vertical movements and rift basin formations (Almeida, 1976) due the northward progressing rifting motions separating South America and Africa during South Atlantic Ocean formation.

40Ar/39Ar geochronology and structural data from the giant Okavango and related mafic dyke swarms, Karoo igneous province, northern Botswana

Abstract In NE Botswana, the Karoo dykes include a major N110° dyke swarm known as the Okavango giant dyke swarm (ODS/N110°) and a second smaller set of N70° dykes belonging to the Sabi-Limpopo dyke swarm (SLDS/N70°). New 40Ar/39Ar plagioclase dating of Karoo dolerites of the giant ODS/N110° and the SLDS/N70° in NE Botswana yield plateau ages between 179.6±1.2 and 178.4±1.1 Ma. Our data are concordant with previous 40Ar/39Ar ages for Northern Karoo dykes and lava flows exposed in western Zimbabwe. The data are tightly clustered, indicating a short-lived (179–181 Ma) flood basalt magmatism in this region. The new radiometric dates allow the definition of a diachronous Jurassic flood basalt activity in southern Africa. A significant south to north younging at the scale of the Karoo igneous province correlates with a chemical zonation from low-Ti (south) to high-Ti (north) mafic rocks. Structural measurements on the ODS/N110° and SLDS/N70° Karoo dykes of NE Botswana suggest that: (1) most of the host fractures are inherited Precambrian structures; (2) dyke emplacement occurred under unidirectional tensional stresses; (3) significant syn- and post-volcanic extensional tectonics are lacking. Combined with regional geology, these geochronological and structural data do not confirm unambiguously the triple-junction hypothesis usually put forward to support a mantle plume model for the evolution of the Karoo igneous province, prior to Gondwana breakup.

40Ar/39Ar dating of alkaline and tholeiitic magmatism of Saudi Arabia related to the early Red Sea Rifting

40Ar/39Ar plateau-ages at 27–28 Ma obtained on feeder dykes and one lava flow of the alkaline massif of Harrat Hadan are in agreement with the assumed age partly deduced from the conventional K/Ar data of the early activity of other alkaline volcanic massifs from Saudi Arabia. This magmatic activity is spatially distributed over a large area, along the western edge of the Arabian plate, and their N-S to NW-SE volcano-tectonic directions are similar to those of the future Red Sea Rift. Preliminary results obtained on tholeiitic lava flows, dykes and plutons gave 17 plateau-ages which, combined with 6 ages deduced from more disturbed age-spectra, display a main histogram peak from 24 to 21 Ma, much narrower than that obtained with conventional K/Ar ages on the same formations. Therefore, a strong tholeiitic activity affected a narrow linear area following the actual eastern Red Sea coast, over nearly 1700 km, during a brief period of time, and without showing any apparent migration. The main volcano-tectonic features of the future Red Sea are strongly outlined during this event. Such brief magmatic episodes related to continental rifts have also been documented by precise 40Ar/39Ar analyses on the British Tertiary Igneous Province, the Deccan traps and the eastern Central Atlantic.

40Ar39Ar chronology of tertiary magmatic activity in Southern Yemen during the early Red Sea-Aden rifting

Abstract The early opening of the Red Sea rift was accompanied by magmatism that formed lava flows, plutons and dyke swarms. Some of these formations were sampled in Southern Yemen during a preliminary field survey and were investigated by geochemistry and 40 Ar 39 Ar geochronology. Ten plateau ages and more disturbed age spectra on plagioclase, alkali feldspar, biotite and whole rocks are discussed in detail, and allow us to establish the first chronological succession of magmatic events in this area immediately adjacent to the well-studied Afar region. An approximately 2000-m-thick section from the middle part of the traps gives ages ranging from 28.9 ± 0.1 Ma at the bottom to 26.5 ± 0.8 Ma at the top (lower part of rhyolitic lava flows). These ages are concordant with the oldest precisely dated alkaline lava flows from the harrats (Harrat Hadan) of Saudi Arabia. The majority of the analysed samples from two distinct dyke swarms mainly trending N120–150 °E give ages of around 25.5 Ma and 16–18.5 Ma, respectively. These dyke swarms do not reflect any drastic change in stress regime in this region during this period, but show, in the vicinity of the Gulf of Aden, the existence of volcano-tectonic trends similar to those of the Red Sea Rift in Saudi Arabia. Geochemical analysis (trace element and isotopic ratio) of these samples show a break at 21–22 Ma contemporaneous with a major magmatic event of tholeiitic affinity occurring over nearly 1700 km along the Red Sea Rift in Saudi Arabia.

Excess argon incorporation in phengite of the Mulhacen Complex: submicroscopic illitization and fluid ingress during late Miocene extension in the Betic Zone, south-eastern Spain

Abstract 40 Ar/ 39 Ar induction furnace and laser step-heating of well-crystallised post-tectonic phengitic mica single grains from gneisses of the Mulhacen Complex with an early Alpine tectonic fabric has resulted in: (1) highly scattered integrated ages, (2) an abnormally high atmospheric contamination and (3) often anomalously old apparent ages during early 39 Ar release that is associated with a high 36 Ar AIR and 37 Ar Ca contamination. This low-temperature excess argon ( 40 Ar XS ) component is probably released from carbonate formed during slight alteration of the mica. More than 50% of the samples yielded plateau ages ranging from 15.8±0.4 to 90.1±1.0 Ma. Samples taken only a few metres apart may differ in age by as much as 50 Ma; a grain that was split over the basal plane yielded plateau ages for each half that differ by 12%. The age variation on these different scales is explained by heterogeneous 40 Ar XS incorporation during a period with a high transient partial argon pressure in the metamorphic fluid, resulting from a late stage reheating event. The very swift cooling of 50–100°C/Ma during exhumation of the Mulhacen Complex concomitant with late Miocene extension may have prevented the equilibration of different 40 Ar XS levels in the mica. HRTEM images of the oldest and youngest phengite specimens show that at least 20% of the lattice is affected by submicroscopic illitisation, which is concentrated in several micrometer wide zones and veins that cross-cut the basal cleavage. These are made up of aggregates of 0.07–0.30 μm thick crystallites of three illitic micas types, which are chemically and structurally progressively closer to pure illite and occur in different textures. The oldest specimen is affected most severely as the veins contain newly formed (pseudo)illite that does not inherit its crystallographic orientation and chemistry from the host mica, in contrast to the youngest sample. HRTEM–AEM analyses revealed that phengite and the different illitic micas may be depleted in K. The oldest sample is derived from a coarse-grained augen gneiss with extensively developed hydraulic cracks, which are lacking in the youngest sample, a fine-grained mylonitic gneiss. Fluid–rock interaction and consequently sub-microscopic illitization were therefore more intense in the coarser-grained rocks. Growth of the illitic micas in equilibrium with a high partial 40 Ar pressure could account for 40 Ar XS incorporation in K-vacancies and other lattice imperfections. Variation in illitisation and associated textural dissimilarities between the oldest and youngest mica permit the different levels of 40 Ar XS incorporation that account for the observed age discordance. The finding of 40 Ar XS plateau ages, despite the degassing of intimately intergrown micaceous minerals, is interpreted by gas release invoked by in-vacuo chemical and structural changes that led to a joint collapse of the lattices of phengite and the illitic micas between 800°C and 1000°C.