Marc Poujol

Correlation of the nappe stack in the Ibero-Armorican arc across the Bay of Biscay: a joint French–Spanish project

Abstract A correlation between allochthonous units exposed in the NW Iberian Massif and the southern Armorican Massif is carried out based on lithological associations, structural position, age and geochemistry of protoliths and tectonometamorphic evolution. The units on both sides of the Bay of Biscay are grouped into Upper, Middle and Lower allochthons, whereas an underlying allochthonous thrust sheet identified in both massifs is referred to as the Parautochthon. The Lower Allochthon represents a fragment of the outermost edge of Gondwana that underwent continental subduction shortly after the closure of a Palaeozoic ocean which, in turn, is represented by the Middle Allochthon. The latter consists of supra-subduction ophiolites and metasedimentary sequences alternating with basic, mid-ocean ridge basalt (MORB)-type volcanics, with inheritances suggesting the proximity of a continental domain. Seafloor spreading began at the Cambro-Ordovician boundary and oceanic crust was still formed during the Late Devonian, covering the lifetime of the Rheic Ocean, which is possibly represented by the Middle Allochthon. The opening of the oceanic domain was related to pulling apart the peri-Gondwanan continental magmatic arc, which is represented by the Upper Allochthon.

The Johannesburg Dome, South Africa: new single zircon U–Pb isotopic evidence for early Archaean granite–greenstone development within the central Kaapvaal Craton

Abstract The Johannesburg Dome, located in the central part of the Kaapvaal Craton, constitutes one of the key areas to better understand the Archaean crustal evolution of this part of the craton. The dome comprises a variety of Archaean granitic rocks intruded into mafic–ultramafic greenstone remnants. This study presents new precise U–Pb single zircon dating for seven different granitoid samples and an amphibolite dyke collected from the Johannesburg Dome. A trondhjemitic gneiss sampled on the northwestern part of the dome yielded an age of 3340±3 Ma and represents the oldest granitoid phase recognized so far. This result has important implications with regard to the age of the mafic and ultramafic greenstone remnants scattered throughout the dome as it implies that the greenstone remnants are older than c.3.34 Ga. This initial magmatic episode, involving early greenstone development and the intrusion of trondhjemitic and tonalitic granitoids on the northern half of the dome, was followed by the emplacement of a 3201±5 Ma hornblende–biotite–tonalite gneiss in the south. Following the trondhjemite–tonalite gneiss emplacement a further period of magmatism took place on the dome, which resulted in the intrusion of mafic dykes that are manifest as hornblende amphibolites. The age of these dykes has yet to be determined quantitatively, but they fall within the time constraints imposed by the age of the trondhjemitic gneisses (3340–3200 Ma) and later, crosscutting, potassic granitoids. These rocks, consisting dominantly of granodiorites constitute the third magmatic event and occupy an area of batholithic dimensions extending across most of the southern portion of the dome. The southern and southeastern parts of the batholith consist mainly of medium-grained, homogeneous, grey granodiorites dated at 3121±5 Ma. Their porphyritic granodiorite equivalents in the southwestern part of the dome yielded an age of 3114±2.3 Ma. An age of 3117±12 Ma, from zircons extracted from one of the mafic dykes possessing granitic microveins, provided confirmation of the timing of this third magmatic event. Lastly, pegmatites that crosscut all these earlier granitoid events are younger than 3114 Ma and might be at least 3.0 billion-years old. These new data provide confirmation of the conclusion that the Witwatersrand Basin was deposited after c.3074 Ma on an Archaean basement as young as c.3120 Ma. The data, combined with that from other parts of the Kaapvaal Craton, further supports the view that the evolution of the Craton was long-lived and episodic, and that it grew by accretionary processes, becoming generally younger to the north and west of the c.3.5 Ga Barberton-Swaziland granite–greenstone terrane situated in the southeastern part of the Craton.

Episodic granitoid emplacement in the Archaean Amalia–Kraaipan terrane, South Africa: confirmation from single zircon U–Pb geochronology

Abstract The Amalia–Kraaipan granite–greenstone terrane, located in the western part of the Kaapvaal Craton, South Africa consists of metamorphosed mafic volcanic rocks and interlayered ferruginous and siliceous metasediments (mainly banded iron formations), intruded by a variety of granitoid rocks comprising tonalitic and trondhjemitic gneisses, granodiorites and adamellites. This study presents new single zircon dating for several granitoid rocks in order to define the chronology of magmatic events influencing this terrane. New TIMS and SHRIMP U–Pb data demonstrate episodic granitoid emplacement events, which occurred over a time-span of approximately 250 Ma. The oldest granitoid rocks so far recognized are dated at ca. 3008 Ma and are represented by biotite–trondhjemite gneisses located southwest of the town of Amalia. Homogeneous leuco-trondhjemite dykes intruding these gneisses yield an age of ca. 2940 Ma. Two granodiorite samples from the central part of the Amalia–Kraaipan terrane were dated at 2913±17 and 2915±12 Ma, respectively, while a further two granodiorite samples from the central and northern parts of the study area yielded ages of 2879±9 and 2879±11 Ma. The youngest granitoid body, the Mosita adamellite, which is exposed in the northwest sector of the Amalia–Kraaipan terrane, was dated at 2791±8 Ma. Since the 3008 Ma biotite–trondhjemite gneisses contain conformably interlayered amphibolite xenoliths, at least some of the Kraaipan Group volcano-sedimentary rocks are probably older than 3008 Ma. Based on available age data the 3010–2920 Ma Amalia–Kraaipan granitoids described in this paper could represent a possible source for some of the Witwatersrand sediments 100 km to the east and their contained gold. The ca. 2791 Ma age obtained for the Mosita adamellite permits a possible genetic link between this granitoid body and the ca. 2781 Ma Gaborone Granite Complex exposed approximately 120 km north in neighbouring Botswana.

Nb-Ta fractionation in peraluminous granites: A marker of the magmatic-hydrothermal transition

In their late stages of evolution, peraluminous granitic melts exsolve large amounts of fluidswhich can modify the chemical composition of granitic whole-rock samples. The niobium/tantalum (Nb/Ta) ratio is expected to decrease during the magmatic differentiation of graniticmelts, but the behavior of both elements at the magmatic-hydrothermal transition remainsunclear. Using a compilation of whole-rock geochemical data available in the literature, wedemonstrate that fractional crystallization alone is not sufficient to explain the distribution ofNb-Ta in most peraluminous granites. However, we notice that most of the granitic samplesdisplaying evidence of interactions with fluids have Nb/Ta < 5. We propose that the decreaseof the Nb/Ta ratio in evolved melts is the consequence of both fractional crystallization andsub-solidus hydrothermal alteration. We suggest that the Nb/Ta value of ~5 fingerprints themagmatic-hydrothermal transition in peraluminous granites. Furthermore, a Nb/Ta ratio of ~5appears to be a good marker to discriminate mineralized from barren peraluminous granites.

Regional-scale Cretaceous albitization in the Pyrenees: evidence from in situ U–Th–Pb dating of monazite, titanite and zircon

Abstract: Albitization is a common metasomatic process active in various geodynamic contexts. In the northern Pyrenees, there are several occurrences of albitites but, until now, only one occurrence has been dated (117 Ma, Ar–Ar dating). This paper presents new U–Th–Pb ages for several albitite occurrences throughout the Pyrenees to test whether they are contemporaneous and, if so, to specify the regional extent of the albitization event. Ages obtained from large euhedral titanite and monazite grains from distinct albitites are 110 ± 8 and 98 ± 2 Ma, respectively. The zircon U–Th–Pb isotopic system did not record this Cretaceous metasomatic event, even when grains were selected in metasomatically Zr-enriched rocks or in hydrothermal structures (millimetre-sized veins cross-cutting granitoids). We argue that the total time span of 20 Ma recorded by albitites corresponds to a long-lived hydrothermal system that was active during the rotation of Iberia around Europe, along the North Pyrenean Fault. Because albitization and talc mineralization have the same spatial and temporal distribution in the Pyrenees, we argue that these two metasomatic phenomena are two independent records of this single, regional-scale, long-lived hydrothermal system.

Transcurrent shearing, granite sheeting and the incremental construction of the tabular 3.1 Ga Mpuluzi batholith, Barberton granite–greenstone terrane, South Africa

Structural, petrographic and geochronological studies show that the tabular 3.1 Ga Mpuluzi batholith in the Barberton granite–gneiss terrane in South Africa was emplaced via a combination of external and internal processes. External structural controls are indicated by systematic variations in intrusive relationships and strain along the margins of the Mpuluzi batholith and are consistent with an emplacement of the granite in a dilational jog within a NE–ENE-trending system of dextral transcurrent synmagmatic shear zones. Internally, the Mpuluzi batholith is essentially made up of granite sheets. The structurally higher parts of the granite are made up of shallowly dipping sheets that are underlain by an anastomosing network of steeply dipping, variably deformed dykes and sheets. These granite sheets at lower structural levels intruded either into the actively deforming shear zones or into extensional sectors between and along the bounding shear zones. Multiple intrusive relationships and geochronological evidence suggests that granite sheeting and the assembly of the pluton occurred over a period of 3–13 Ma. The spatial and temporal relationship between deformation and magma emplacement reflects episodes of incremental dilation related to deformation along the bounding shear zones and granite sheeting. The transition to the mainly subhorizontal granite sheets at higher structural levels of the tabular Mpuluzi batholith indicates the intrusion of the granites during subhorizontal regional shortening, where the reorientation of the minimum normal stress to vertical attitudes at the shallow levels of emplacement allowed for vertical dilation and subhorizontal emplacement of the granite sheets.

U-Pb zircon age of Cape Granite Suite ignimbrites: characteristics of the last phases of the Saldanian magmatism.

The end of the Saldanian magmatic event is marked by a short period of peraluminous silicic magmatism, evident as a series of ignimbrite flows, quartz porphyry and tuffisite intrusions. There is a close relationship between high-level granitic intrusions present as granite porphyries in the Langebaan area and the volcanic event. The subvolcanic-volcanic rocks under discussion ended the fourth and final phase of the Cape Granite Suite magmatism, thus starting and ending with a peraluminous phase. U-Pb ID-TIMS zircon age determinations on the second of three mappable ignimbrite flows yielded an age of 515.5 ± 3.1Ma. Two single zircon grains with 207 Pb/ 206 Pb ages of ~1875Ma and ~1575 Ma respectively are interpreted to represent xenocrysts extracted from older crust or from sedimentary contaminants.

U-Pb isotopic evidence for episodic granitoid emplacement in the Murchison greenstone belt, South Africa

This study presents new single-zircon U-Pb data that constrain the timing of granitoid emplacement within and to the south of the Murchison greenstone belt, South Africa. Four main magmatic events are identified at ca 3020, 2970, 2820 and 2680 Ma for the Baderoukwe, Discovery, Willie and Mashishimale intrusions, respectively. The age of 3020 Ma, found for the Baderoukwe Pluton, which is intrusive into the Letaba, Bawa and Sable schist belts, suggests that the schist belts are older than 3020 Ma. The age of the Discovery Granite is identical to that of the rhyodacitic Rubbervale Formation, suggesting that granite magmatism was associated with volcanism in the Murchison greenstone belt at around 2970 Ma. The emplacement of the Willie Granite is comtemporaneous with several other granites on the Kaapvaal Craton, suggesting that there was a major period of felsic magmatism on the craton between 2820 and 2880 Ma. The 2700 Ma Mashishimale Granite reflects, furthermore, the age of many of the late-stage potassic granite plutons on the Kaapvaal Craton, indicating that this period was also an important crust-forming event.

New U-Pb data on zircons from the Amalia greenstone belt Southern Africa: insights into the Neoarchaean evolution of the Kaapvaal Craton

Evidence for the existence of Neoarchaean greenstone rocks in the western part of the Kaapvaal Craton is provided by ID-TIMS and SHRIMP U-Pb age data on zircons from supracrustal rocks of the Amalia greenstone belt. Although all the units described from the Amalia greenstone belt might not be coeval, an age of 2750.1 ± 4.6 Ma found for two samples appears to be the best estimate for the deposition of this part of the Amalia greenstone belt sequence. These samples also contain younger zircons, the majority of them yielding a 1099 ± 32 Ma upper intercept age. This date of ~ 1.1 Ga might indicate that these rocks have been affected by the crustal thickening and magmatism event coeval with the peak of metamorphism during the Namaqua orogeny ~1.1 Ga. These data provide the first evidence for the development of a Neoarchaean greenstone sequence in the western part of the Kaapvaal Craton. They also demonstrate that the Namaqua orogeny affected rocks within the craton itself.

U-Pb LA-ICP-MS dating of apatite in mafic rocks: Evidence for a major magmatic event at the Devonian-Carboniferous boundary in the Armorican Massif (France)

Abstract Apatite is a ubiquitous accessory mineral found in most magmatic rocks and is often the only U-bearing mineral available to date mafic rocks because primary zircon and/or baddeleyite are not present. In this paper, U-Pb LA-ICP-MS dating of apatite was applied to seven different dike and sill samples of dolerite from the Variscan belt of Brittany (Armorican Massif, western France). These dolerites, which are characterized by a within-plate tholeiite geochemical signature, are organized in several dense swarms across the belt. Their geochemical compositions are homogeneous although they intrude a large geographical area subdivided into several domains each characterized by different tectonic-metamorphic settings. Their emplacement ages were so far poorly constrained due to the difficulty to date these mafic rocks using either the 40Ar/39Ar or the U-Pb methods on classical minerals like mica, plagioclase, or zircon. Although the closure temperature of apatite is lower than the emplacement temperature of the magma, physical models show that the time needed to solidify and cool these mafic dikes and sills below the apatite closure temperature is basically of the order of 100 years or less. Consequently, the U-Pb dates obtained on apatite can be interpreted as the emplacement ages for these mafic intrusions. Our results demonstrate that, in all cases, the apatite grains do carry enough radiogenic Pb to be dated by in situ U-Pb analyses and yield a 207Pb-corrected mean age of 363.4 ± 5.8 Ma. These results reveal the existence of a major and short-lived magmatic event in the Variscan belt of Brittany during the Devonian-Carboniferous transition, a feature further highlighted by field evidence. Beyond the geological implications of these results, U-Pb LA-ICP-MS dating of apatite appears to represent an ideal tool to date small size mafic intrusions.