Hervé Bertrand

Synchrony of the Central Atlantic magmatic province and the Triassic-Jurassic boundary climatic and biotic crisis

The evolution of life on Earth is marked by catastrophic extinction events, one of which occurred ca. 200 Ma at the transition from the Triassic Period to the Jurassic Period (Tr-J boundary), apparently contemporaneous with the eruption of the worlds largest known continental igneous province, the Central Atlantic magmatic province. The temporal relationship of the Tr-J boundary and the provinces volcanism is clarified by new multidisciplinary (stratigraphic, palynologic, geochronologic, paleomagnetic, geochemical) data that demonstrate that development of the Central Atlantic magmatic province straddled the Tr-J boundary and thus may have had a causal relationship with the climatic crisis and biotic turnover demarcating the boundary.

Global warming of the mantle at the origin of flood basalts over supercontinents

Continents episodically cluster together into a supercontinent, eventually breaking up with intense magmatic activity supposedly caused by mantle plumes ([Morgan, 1983][1]; [Richards et al., 1989][2]; [Condie, 2004][3]). The breakup of Pangea, the last supercontinent, was accompanied by the emplacement of the largest known continental flood basalt, the Central Atlantic Magmatic Province, which caused massive extinctions at the Triassic-Jurassic boundary ([Marzoli et al., 1999][4]). However, there is little support for a plume origin for this catastrophic event ([McHone, 2000][5]). On the basis of convection modeling in an internally heated mantle, this paper shows that continental aggregation promotes large-scale melting without requiring the involvement of plumes. When only internal heat sources in the mantle are considered, the formation of a supercontinent causes the enlargement of flow wavelength and a subcontinental increase in temperature as large as 100 °C. This temperature increase may lead to large-scale melting without the involvement of plumes. Our results suggest the existence of two distinct types of continental flood basalts, caused by plume or by mantle global warming. [1]: #ref-21 [2]: #ref-26 [3]: #ref-5 [4]: #ref-17 [5]: #ref-19

40Ar/39Ar dating and geochemistry of tholeiitic magmatism related to the early opening of the Central Atlantic rift

Tholeiitic effusive and intrusive magmatism from Iberia, Morocco, Algeria and Mali, related to the early opening of the Central Atlantic rift, was dated by the 40Ar/39Ar step-heating method. Four plateau ages, ranging from 203.7 ± 2.7 to 197.1 ± 1.8 Ma, were obtained on plagioclase from dykes from the Taoudenni area (Mali) and two lava flows from Morocco. The Messejana dyke (Iberia), which previously yielded discrepant conventional KAr dates, did not furnish any 40Ar/39Ar plateau dates. However, there is a clear inverse relationship between apparent age and K/Ca ratio for gas fractions from a plagioclase separate (proportional to the alteration degree) which, combined with dates obtained on amphibole, biotite and pyroxene, allows us to determine an age of around 200 Ma for this body. These data, and those obtained on the Foum Zguid (Morocco) and the Ksi Ksou (Algeria) dykes, give evidence of a brief magmatic event (between 206 and 195 Ma ago) which affected a large area ca. 2500 km long. Trace-element modelling shows that most of these formations originated from a homogeneous, enriched, source material. Such a brief magmatic episode related to the opening of a continental rift is in agreement with findings in other magmatic provinces (e.g. the Deccan traps and the Red Sea rift, precisely dated by the 40Ar/39Ar method as well).

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.

AGE OF JURASSIC CONTINENTAL THOLEIITES OF FRENCH GUYANA, SURINAM AND GUINEA : IMPLICATIONS FOR THE INITIAL OPENING OF THE CENTRAL ATLANTIC OCEAN

Abstract A detailed 40Ar/39Ar study, of mineral separates from the Jurassic Atlantic Continental Tholeiites (JACT) of Guyana (French Guyana and Surinam, South America), and Guinea (West Africa) related to the initial opening of the Central Atlantic, has been carried out. In French Guyana, plateau ages of 196.0 ± 5.7 Ma and 196.1 ± 7.5 Ma were obtained on single, small amphibole grains from NNW—SSE trending dykes. In Guinea, single biotite grains from intrusive formations from the Kakoulima and Fouta Djalon areas yielded plateau ages of 200.4 ± 0.2 Ma and 194.8 ± 0.5 Ma, concordant with high temperature apparent ages on other biotites. The bulk plagioclase samples display disturbed age spectra due to alteration and excess argon. However, intermediate temperature, weighted mean plagioclase ages are similar in both regions of Guyana and Guinea, ranging from 200.2 ± 2.4 Ma to 188.7 ± 1.9 Ma, partly in agreement with the amphibole and biotite data. These data, combined with previous 40Ar/39Ar and U/Pb results from the northern part of the Central Atlantic margins, indicate intense magmatic activity distributed over a large area from Iberia to Liberia (ca. 4500 km long) for a short period of time (204-195 Ma, perhaps less for the bulk of the magmatism) during the initial break-up of Pangea continent. These data do not support an initiation of the magmatism from a radial volcano-tectonic system centred in the south of the region, as suggested by May [1], and the initial break-up seems to affect the whole Central Atlantic during a period of 9 Ma.

Implications of widespread high-μ volcanism on the Arabian Plate for Afar mantle plume and lithosphere composition

Abstract We report on 55 Nd–Sr–Pb and 23 Hf isotopic compositions for late Miocene to Plio–Quaternary basalts from the Arabian Plate. The sampling profile represents a 2500 km N–S transect along the Red Sea margin from Syria to Yemen. 206Pb/204Pb displays a wide range (from 18.60 to 19.55) from south to north, indicating a pervasive high-μ distribution that questions the previous attribution of this component to the Afar plume. We rather suggest that the high-μ signature resides within the Arabian lithospheric mantle. Compared to basalts from northern and central Arabia (Saudi Arabia to Syria), basalts from southern Arabia (Yemen) display similar ranges for 143Nd/144Nd, 176Hf/177Hf, and 206Pb/204Pb, but are shifted towards compositions more radiogenic in Sr and 208Pb. These distinct time-integrated Rb/Sr and Th/U ratios between north and south are believed to reflect heterogeneous development of the Arabian lithosphere through time and/or result from the thermal effect of the Afar plume on the southern part of the lithosphere.

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 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.

Mantle sources and magma‐continental crust interactions during early Red Sea‐Gulf of Aden rifting in southern Yemen: Elemental and Sr, Nd, Pb isotope evidence

Large-scale magmatic activity, ranging from late Oligocene to Quaternary, is associated with the Red Sea-Gulf of Aden rifting throughout the Arabian passive margin. The Southern Yemen area represents the southernmost extremity of this magmatic range, facing the Afar area, and provides a means of studying the magmatic records of early stages of rifting (30–16 Ma) in a plume-related context. We investigate major and trace elements, and Nd, Sr and Pb isotopes of a bimodal series of transitional affinity consisting of (1) thick olivine-basalt traps overlain by ignimbritic rhyolites, (2) basaltic, rhyolitic, trachytic and peralkaline dykes with a prevailing N120–140° E orientation, and (3) gabbroic, syenitic and granitic plutons. Major and some trace element variations from basalts to felsic rocks are consistent with low-pressure fractional crystallization. Mass balance calculations using major elements suggest the fractionation of clinopyroxene (6–9%), olivine (∼6%), plagioclase (42–43%), magnetite (∼12%), apatite (1–2%) ± alkali feldspar (16%). However, LILE (large ion lithophile element) enrichment and high initial 87Sr/86Sr ratios (up to 0.7074 and 0.710 in rhyolites and pantellerites, respectively) require the felsic rocks to be generated through significant crustal assimilation. Nd and Sr isotopic ratios of the rhyolitic traps can be reproduced by bulk mixing between magmas similar to the underlying basaltic unit and the Arabian Proterozoic basement. On the other hand, an assimilation-fractional crystallization process is required to account for the isotopic diversity of the rhyolitic and peralkaline dykes. Rhyolites can be derived from a basaltic liquid by a moderate fractionation rate (F = 0.47) and a high crustal assimilation rate (R = 0.45), whereas the pantellerites require more significant fractionation rate (F = 0.07) and a very low assimilation rate (R = 0.05). Elemental and isotopic signatures of the basalts do not support a significant crustal contribution in their formation and their isotopic diversity (87Sr/86Sr from 0.7034 to 0.7051, 143Nd/144Nd from 0.512676 to 0.513045 and 206Pb/204Pb from 17.96 to 18.66) mainly reflects mantle source heterogeneities. Sr-Nd-Pb isotopic data are consistent with a binary mixing between depleted and enriched source regions. The depleted end-member corresponds to an asthenospheric reservoir approaching that producing mid-ocean ridge basalts at Gulf of Aden/Red Sea spreading centers. The enriched reservoir, intermediate between enriched mantle I and II end-members, is supposed to be located within old subcontinental lithosphere related to the Pan-African orogenic events. Unlike the modern volcanics from Afar, no HIMU (high U/Pb ratio) signature has been recognized in our sampling. This rules out any significant chemical influence of the Afar plume upon early rift-related volcanism in Southern Yemen and suggests a continental rift initiation of passive type. However, one can suspect that the Afar plume may have supplied the excess heat required to produce so voluminous traps and to trigger melting in the lithospheric mantle, making the distinction between passive and active rifting more ambiguous.