Intercalibration of the Hb3gr 40Ar/39Ar dating standard.

The 40Ar/39Ar dating technique is based on the knowledge of the age of neutron fluence monitors (standards). Recent investigations have improved the accuracy and precision of the ages of most of the Phanerozoic-aged standards (e.g. Fish Canyon Tuff sanidine (FCs), Alder Creek sanidine, GA1550 biotite and LP-6 biotite); however, no specific study has been undertaken on the older standards (i.e. Hb3gr hornblende and NL-25 hornblende) generally used to date Precambrian, high Ca/K, and/or meteoritic rocks.In this study, we show that Hb3gr hornblende is relatively homogenous in age, composition (Ca/K) and atmospheric contamination at the single grain level. The mean standard deviation of the 40Ar/39ArK (F-value) derived from this study is 0.49%, comparable to the most homogeneous standards. The intercalibration factor (which allows direct comparison between standards) between Hb3gr and FCs is RFCsHb3gr = 51.945 0.167. Using an age of 28.02 Ma for FCs, the age of Hb3gr derived from the R-value is 1073.6 5.3 Ma (1σ, internal error only) and 8.8 Ma (including all sources of error). This age is indistinguishable within uncertainty from the K/Ar age previously reported at 1072 11 Ma [Turner G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. 40Ar-39Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19-35]. The R-value determined in this study can also be used to intercalibrate FCs if we consider the K/Ar date of 1072 Ma as a reference age for Hb3gr. We derive an age of 27.95 0.19 Ma (1σ, internal error only) for FCs which is in agreement with the previous determinations. Altogether, this shows that Hb3gr is a suitable standard for 40Ar/39Ar geochronology

Modelling effect of sericitization of plagioclase on the 40K/40Ar and 40Ar/39Ar chronometers: implication for dating basaltic rocks and mineral deposits

The 40Ar/39Ar technique is the most commonly used technique to date basaltic rocks. For basaltic rocks older than about 30 Ma, the dating of plagioclase separates is preferred over groundmass as the latter is susceptible to containing cryptic alteration due to fluid circulations, difficult if not impossible to remove during sample preparation. Alteration under such metamorphic conditions progressively forms K-rich sericite after plagioclase. Owing to its transparency, plagioclase allows a distinction to be made optically between partially–completely altered grains and fresh grains. However, practice shows that grains that contain less than about 1% of sericite are hard to identify under the stereomicroscope. Owing to the high K2O content (c. 10 wt%) of sericite, such compromised grains can have dramatic effects on the age determination of plagioclase.Here, we investigate and quantify the effect of sericite on the 40Ar/39Ar age determination of plagioclase using a numerical model with multiple variable parameters. We show that the most influential parameter is the time difference between the crystallization of plagioclase and the sericitization event. We also show that for some continental flood basalts, even 0.1 wt% of sericite can bias the apparent age of a plagioclase separate by several hundred thousand years. The presence of sericite can be identified using a combination of Ca/K ratios, age spectra, and 39Ar and 37Ar degassing curves obtained during a conventional 40Ar/39Ar step-heating procedure. When the age of the fresh plagioclase and its Ca/K ratio are known, the percentage of sericitization and the age of the alteration event can be estimated. Ultimately, above approximately 65% of sericitization, the apparent age measured on the altered plagioclase is within ±1% of the age of the alteration event, with implications for accurately dating low-temperature metamorphism and mineral deposit formations

40Ar/39Ar ages and duration across the Triassic-Jurassic boundary of the Central Atlantic Magmatic Province volcanism in Morocco and Portugal.

The Central Atlantic Magmatic Province (CAMP) is the world's largest known continental igneous province and is contemporaneous with the Triassic\u2013Jurassic (T\u2013J) boundary that marks one of the five largest biological extinctions in the last 600 Ma. Although constraints on the duration of the CAMP volcanism remain yet poorly defined, the assumption that the CAMP may have had a causal relationship with the climatic and biotic crisis was suggested [Marzoli, A., Bertrand, H., Knight, K., Cirilli, S., Buratti, N, Verati, C., Nomade, S., Renne, P.R., Youbi, N., Martini, R., Allenbach, K., Neuwerth, R., Rapaille, C., Zaninetti, L., Bellieni, G., 2004. Synchrony of the Central Atlantic magmatic province and the Triassic\u2013Jurassic boundary climatic and biotic crisis. Geology 32, 973\u2013976.]. In this paper, we present new 40Ar/39Ar ages on basaltic flows from Moroccan (central High Atlas, Oujda and Argana) and Portuguese (Algarve and Santiago do Cac\ue9m) basins where CAMP lava flows are interlayered with T\u2013J sedimentary sequences. In the Moroccan basins, where the basalts were erupted across the T\u2013J boundary, well defined 40Ar/39Ar plateau ages (n=12) for more than 90% of the total erupted volume of CAMP basalts range from 197.8\ub10.7 Ma to 201.7\ub12.4 Ma and display a main restricted peak at 199.1\ub11 Ma providing a precise estimate of the 40Ar/39Ar age of the T\u2013J boundary, concordant with the current U\u2013Pb age [199.6\ub10.3 Ma; P\ue1lfy, J., Mortensen, J.K, Carter, E.S., Smith, P.L., Friedman, R.M., Tipper, H.W., 2000. Timing the end-Triassic mass extinction: First on land, then in the sea? Geology 28, 39\u201342.]. The topmost lava flows (recurrent unit) yield a significantly younger mean age of 196.6\ub10.6 Ma, highlighting late episodic eruption of CAMP magmas. These late basalts represent a small magma volume (b10% of the total), which was erupted most likely due to asthenospheric upwelling and lithospheric extension within the T\u2013J central High Atlas basins. In the Portuguese basins, stratigraphical and palynological studies of the associated continental sedimentary sequence underlying the volcanic units suggest an age close to the T\u2013J boundary for the CAMP basaltic flows. This is confirmed by new 40Ar/39Ar data for the Portuguese basalts, which provide the first accurate radio-isotopic ages for lava flows in the European CAMP, with two plateau ages yielding a mean value of 198.1\ub10.4 Ma. These plateau ages (plus three concordant mini-plateau ages) suggest synchronous rifting and volcanism both in south-western Europe and Morocco (central High Atlas and Oujda basins)

Discovery of Lower Cretaceous synmetamorphic thrust tectonics in French Lesser Antilles (La Désirade Island, Guadeloupe): Implications for Caribbean geodynamics

International audienceLocated east of Guadeloupe, the island of La Désirade exhibits the oldest rocks of the Lesser Antilles arc and the eastern Caribbean plate. An old magmatic basement is composed of (1) late Jurassic ophiolitic complex with meta-basaltic pillow lavas and interbedded radiolarites, (2) acid igneous complex comprising meta-quartz-diorite and meta-rhyolitic lavas flows, and (3) meta-diabasic/microdioritic dyke swarm complex. We present and discuss the discovery of synmetamorphic thrust tectonics in this island. Based on detailed structural analysis we evidenced two main compressive events. The first event (D1) is characterized by pervasive folding associated with thrust development. In the northeastern part of the island, a major thrust fault, the Grand Abaque Thrust, has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit. The second deformation event (D2) is featured by a main pervasive cleavage S2 observed at the regional scale associated with upright folding (F2) and by the development of two sets of conjugated steeply dipping strike-slip shear zones, dextral 130°N and sinistral 20°N striking, respectively. Microstructural observations indicate that Greenschist facies metamorphism is clearly contemporaneous with the development of D1 and D2 superimposed tectonic structures. Ar/Ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a D2 event. These analyses yield well-defined and concordant plateau ages at 106.2 ± 1.7 Ma and 107.2 ± 1.8 Ma. Thus, shortening and thickening tectonics occurred during Lower Cretaceous (Albian). In the available framework of Caribbean geodynamics, the synmetamorphic thrust tectonics we discovered in the French Lesser Antilles is related to the collision between an Aptian-Albian oceanic plateau and the Andean-Cordilleran east-dipping subduction zone. Our results support subduction polarity reversal during Lower Cretaceous

From Variscan to Alpine tectonics: magmatic and metamorphic evolution of the Argentera-Mercantour dioritic dyke swarm

In the Argentera-Mercantour Massif (External Crystalline Massifs, ECMs) Variscan migmatites and late Carboniferous granites are crosscut by a dioritic dyke swarm. More than 60 years after the first report (Faure Muret, 1955), these dykes have been mapped in detail at Valscura and Val du Bor\ue9on, located, respectively, N and S of the main post-collisional granitic pluton, the \u201cCentral Granite\u201d. NE-SW trending and steeply dipping dioritic dykes show chilled margins and degassing bubbles, while, in the innermost parts, soft contacts and magmatic breccias mark the boundary between different igneous types, testifying a complex intrusive history. As inferred by whole rock and EMPA analyses, these dykes are magnesian, calc-alkalic to alkali-calcic, and peraluminous; an alkaline affinity only for the latest magmatic pulse is otherwise suggested by amphibole composition. Intrusive structures and textures indicate a shallow emplacement level, consistently with the sub-aerial condition locally experienced by the Argentera-Mercantour basement from late Carboniferous to Lower Triassic. Thermobarometric estimates on the first metamorphic assemblage developed within these dykes (T~300\ub0C, P<0.1 GPa), and interpreted as related to a post-magmatic hydrothermal system not affecting the host rocks, confirm the shallow intrusion. At a later time, dykes were intersected by the mylonitic shear zones that reworked the massif during the Alpine cycle: syn-kinematic mineral assemblages within dykes confirm the metamorphic peak conditions already inferred for this tectonic event (Sanchez et al., 2011). The occurrence of this dioritic dyke swarm implies an extension of the magmatic activity in the Argentera-Mercantour Massif beyond the Permo-Carboniferous boundary: intrusives similar in field relationships and compositions are not yet known in the other ECMs (Debon &Lemmet, 1999), but are commonly reported in other parts of the southern European Variscan belt (e.g. Bonin et al., 1998). Moreover, thermobarometric constrains on metamorphic assemblages have allowed for the first time to define a P-T-d-t path relating to the Alpine evolution of Argentera-Mercantour Massif that include both prograde and retrograde stages

The late- to post-Varsican dioritic dyke swarm of the Argentera-Mercantour Massif and its tectono-metamorphic record

The progressive emplacement of igneous bodies of various sizes was a major process that ruled the renewal of the continental crust after the apex of the Variscan collision. Although late- to post-Variscan magmatic intrusions have been locally reported as intermittent events, if considered at the scale of the whole European Variscan belt, their age range of tens of millions years refects the long lasting perturbed thermo-mechanical state that has been affecting the lithosphere since the end of the Variscan cycle. In the southern Variscan belt, two main magmatic sequences are described, the first of which, markedly calc-alkaline, records Lower Permian ages and has been accommodated by transpressive to transtensive structures, while the second, which comprises alkaline granitoids and lesser tholeiites, has emplaced since the Middle Permian under a pure extensional tectonic regime. Conversely, in most of the External Crystalline massifs (ECMs), which are segments of the southern Variscan belt reworked by the Alpine collision, late- to post-collisional magmatism have ended at the down of Permian time. But there are notable exceptions: in particular, a dioritic dyke swarm crosscuts both migmatitic basement and late- to post-collisional granitoids in the Argentera- Mercantour ECM. These NE-SW trending and steeply dipping dykes display well-developed chilled margins and vesicular texture; within them, soft contacts and magmatic breccias involving different igneous lithotypes are the finnal outcome of complex intrusive processes. WR analyses show that these dykes are magnesian, calc-alkalic to alkali-calcic and peraluminous, while intrusive relationships and igneous textures indicate a shallow emplacement level. The occurrence of these dioritic dykes implies an extension of the magmatic activity in the Argentera-Mercantour ECM beyond the Permo-Carboniferous boundary, in line with what is reported in the southern Variscan belt. Moreover, thermobarometric constrains on their superposed metamorphic assemblages have allowed to dene a complete P-T-t path from the magmatic emplacement to the Alpine exhumation