Jean-Louis Paquette

Experimental resetting of the U–Th–Pb systems in monazite

Abraded fragments (200–400 µm) of a large, chemically homogeneous, and non-metamict Brazilian monazite crystal, characterised by a concordant U–Pb ages of 474 +/- 1 Ma (208Pb/206Pb = 19.5), were hydrothermally treated at varying temperatures with solutions of different compositions. Product monazites were analysed with Scanning Electron Microscope (SEM), Electron Microprobe (EMP), Secondary Ion Mass Spectrometer (SIMS) and Isotope Dilution–Thermal Ionisation Mass Spectrometer (ID-TIMS). Experiments with pure water over a temperature range of 800–1200°C, at 700 MPa and durations ranging from 5 to 60 days showed that even at 1200°C any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain. Neither Pb diffusion at the EMP scale, nor significant discordancy were observed. We performed experiments at 800 and 1000°C for different durations using different fluid compositions at quartz saturation: a 10 wt.% CaCl2 fluid, a 10 wt.% SrCl2 fluid, a 10 wt.% NaCl fluid and a fluid containing NBS 982 Pb standard (208Pb/206Pb = 1). For all runs, EMP traverses revealed no Pb-diffusion profiles. Significant overgrowths of newly formed monazite are documented by SEM analyses. They occurred only in the 1000°C experiments when either CaCl2 or Pb-bearing fluids were present. In the CaCl2 experiment, two zones could be distinguished within the crystal: a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution/precipitation, which was enriched in Ca and Pb-free. ID-TIMS dating of single grains treated with SrCl2 and CaCl2 solutions at 1000°C are significantly discordant. Experiments employing the NBS Pb-standard produced sub-concordant monazite, for which the 207Pb/206Pb apparent age has become older than prior to the experiment (544 Ma at 800°C and 495 Ma at 1000°C). The newly grown monazite rim had obviously incorporated Pb from the fluid. None of our reaction products contained a detectable diffusion profile. The only resetting mechanism we detected involved dissolution/precipitation. The extent of the dissolution/precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks.

U-Pb, Single Zircon Pb-Evaporation, and Sm-Nd Isotopic Study of a Granulite Domain in SE Madagascar

Madagascar is a poorly known Precambrian domain at the eastern border of the Mozambique belt, strongly affected by the Pan-African orogeny. This large island represents a key part of Gondwanaland and the accurate dating of the major metamorphic event is critical for the reconstruction of that supercontinent. In the southeastern zone of the island, metamorphism has reached high-temperature and low-pressure conditions (850°C and 5 kbar) with moderate retrogression. A U-Pb conventional multigrain, single zircon stepwise Pb-evaporation and Sm-Nd geochronological study on meta-felsic rocks has shown that basement of at least lower Proterozoic age was strongly metamorphosed around 570-580 Ma. This tectometamorphic event nearly obliterated any pre-Pan-African zircon isotopic memory. The granulite-facies metamorphism is associated with the syn-metamorphic emplacement of the Anosyan charnock-ites and granites. A strong channelized fluid contribution at 545 Ma has produced the (re)crystallization of biotite-apatite lenses containing giant monazites. Finally, the emplacement of zircon-bearing calcite veins at 520 Ma represents the latest Pan-African event in this SE Madagascar domain. This precise determination of the Pan-African overprint in this region facilitates comparative discussion of the geodynamic evolution of Madagascar and surrounding areas at the end of the Proterozoic. There is good agreement between the single zircon Pb-evaporation results and conventional isotopie dating; nevertheless slightly younger apparent ages are obtained by the former method. This discrepancy, which may have some important implications, could be related to the discordancy of the analyzed zircon populations.

Large-scale geometry, offset and kinematic evolution of the Karakorum fault, Tibet

The total offset, lifespan and slip rate of the Karakorum fault zone (KFZ) (western Tibet) are debated. Along the southern fault half, ongoing oblique slip has exhumed dextrally sheared gneisses intruded by synkinematic leucogranites, whose age (V23 Ma, U/Pb on zircon) indicates that right-lateral motion was already in progress in the late Oligocene. Ar/Ar K-feldspar thermochronology confirms that rapid cooling started around 12 Ma, likely at the onset of the present dextral normal slip regime. Correlation of suture zones across the fault requires a total offset greater than 250 km along the active ^ northern ^ fault branch. An average long-term slip rate of 1 ? 0.3 cm/yr is inferred assuming that this offset accrued in a time span of 23^34 Ma. Southwest of the Ladakh-Karakorum Range, the large-scale boudinage of ophiolitic units suggests that an offset of several hundreds of kilometers exists along another ^ southern ^ branch of the KFZ. Towards the southeast, in the Mount Kailas region, the fault zone does not end at Gurla Mandatha, but continues eastwards, as a transpressive flower structure, along the Indus^Tsangpo suture. Our new data thus suggest that the KFZ contributed to absorb hundreds of kilometers of India^Asia convergence.

Episodic and short-lived granitic pulses in a post-collisional setting: evidence from precise U–Pb zircon dating through a crustal cross-section in Corsica

Abstract In western Corsica, a large granitoid batholith was emplaced during Carboniferous and Permian times. These rocks display an evolution through time from high Mg–K calc-alkaline, to calc-alkaline, then alkaline affinities. Moreover, the magmatic processes can be observed at different crustal levels. New and precise Isotope Dilution and Thermo-Ionisation Mass Spectrometry (ID-TIMS) U–Pb dating suggests that granite magmatism in Corsica did not simply reflect the continuous thermal evolution of a thickened crust. A precise timing for the post-collisional magmatism is documented with four distinct short-lived episodes, respectively at 345, 338, 305 and 280 Ma, separated by periods of inactivity. These four magma pulses display variable Nd isotopic signatures related to mantle–crust interactions. Granite magma generation and emplacement is strongly connected to the thermal and tectonic activity of the lithosphere. An increasing contribution of a depleted mantle to the magmatic processes is documented for the last igneous event. This is interpreted to reflect the emplacement of depleted mantle materials in the lower crust, which enhanced heat flow and triggered partial melting of previously underplated mafic rocks. This increasing contribution of mantle materials marks the onset of a new geodynamic cycle characterised by an extensional tectonic regime, preluding to the opening of the Alpine orogenic tract.

New U-Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault

[1] Zircons and monazites from 6 samples of the North Ayilari dextral shear zone (NAsz), part of the Karakorum fault zone (KFZ), have been dated with the U-Th-Pb method, using both ID-TIMS and SIMS techniques. The ages reveal (1) inheritance from several events spanning a long period between the late Archean and the Jurassic; (2) an Eocene-Oligocene magmatic event (similar to 35-32 Ma); (3) an Oligo-Miocene magmatic event (similar to 25-22 Ma), at least partly synkinematic to the right-lateral deformation; and (4) a period of metamorphism metasomatism (similar to 22-14 Ma) interpreted as thermal and fluid advection in the shear zone. The Labhar Kangri granite located similar to 375 km farther Southeast along the KFZ is dated at 21.1 +/- 0.3 Ma. Such occurrence of several Oligo-Miocene granites along the KFZ, some of which show evidence for synkinematic emplacement, suggests that the fault zone played an important role in the genesis and /or collection of crustal melts. We discuss several scenarios for the onset and propagation of the KFZ, and offset estimates based on the main sutures zones. Our preferred scenario is an Oligo-Miocene initiation of the fault close to the NA range, and propagation along most of its length prior to similar to 19 Ma. In its southern half, the averaged long-term fault-rate of the KFZ is greater than 8 to 10 mm/a, in good agreement with some shorter-term estimates based on the Indus river course, or Quaternary moraines and geodesy. Our results show the KFZ cannot be considered as a small transient fault but played a major role in the collision history.

A new insight into Pan-African tectonics in the East–West Gondwana collision zone by U–Pb zircon dating of granites from central Madagascar

Abstract The assembly of Gondwana was the result of a major collision orogen, the East African Orogen, between East and West Gondwana during Neoproterozoic times. Madagascar, which represents a fragment of East Gondwana, is located in a key area of this Pan-African orogen. Granites of unambiguous tectonic setting have been dated using the U–Pb zircon method in order to constrain the timing of orogenic events. The central part of Madagascar is characterized by syntectonic alkaline granitic sheets, referred to as “stratoid” granites. These are of both mantle and crustal derivation. Their U–Pb zircon ages are well defined between 627 and 633 Ma for both plutonic suites, regardless of either mainly mantle or crustally origin. It is not surprising that the crustally-derived suite contains inherited zircons in the 2.2–2.4 Ga range attesting to the existence of Lower Proterozoic crust in northern central Madagascar. The generation of huge amounts of granitic magma is regarded as the result of post-collision extension under a high heat flow regime. Therefore, an age between 700 and 650 Ma is inferred for the beginning of Gondwana assembly along the collision zone between central Madagascar and Kenya, i.e., in the central part of the East African Orogen. Following this, brittle fracturing of the stratoid granite series permitted the emplacement of the Ambatomiranty granitic dyke swarm at a minimum age of 560 Ma, in possible connection with a nearby shear belt. The strike–slip tectonic regime at ∼570–560 Ma is well known in southern Madagascar and in its Gondwana connections. This stage corresponds to intracontinental reworking and the final suturing of Gondwana.

U-Pb zircon, Rb-Sr and Sm-Nd geochronology of high- to very-high-pressure meta-acidic rocks from the western Alps

Three meta-acidic rocks from the western Italian Alps, a magnesiochloritoid-bearing “metapelite” from the Monte Rosa massif, a coesite-pyrope-“quartzite” from the Dora Maira massif and the Monte Mucrone granite in the Sesia Zone, have been studied by U-Pb zircon, Rb-Sr on whole-rock, apatite and phengite and Sm-Nd wholerock methods. The mineral parageneses of the investigated rocks indicate high- to very-high-pressure and medium-to-high-temperature metamorphism. This combined isotopic study has enabled us to constrain the ages of magmatic and metamorphic events and also to compare the behaviour of U-Pb zircon systems in three intensely metamorphosed areas of the Pennine domain. The U-Pb zircon data have yielded a magmatic age for the Monte Mucrone granite at 286±2 Ma. This result confirms the occurence of late-Hercynian magmatism in the Sesia Zone, as in other Austro-Alpine units and in other areas of the European crystalline basement. In the Monte Rosa massif, a geologically meaningless lower intercept age of 192±2 Ma has been interpreted as an artefact due to a complex evolution of the U-Pb zircon system. The magmatic shape of the zircons implies a magmatic or volcano-sedimentary protolith for this rock, originally considered as a metasediment. The very-high-pressure metamorphism in the Dora Maira “quartzite” has produced an opening of the U-Pb zircon system at 121+12−29 Ma. The Rb-Sr data support the occurence of high-grade metamorphism during Cretaceous times. Phengites model ages are slightly younger than the U-Pb zircon lower intercept ages due to cooling phenomena or possible response of the phengites to a later deformation. The Nd model ages from the whole-rock samples, as well as the U-Pb upper intercept ages from zircons of all three investigated rocks, indicate the presence of Proterozoic crustal components inherited from the precursors of these meta-acidic rocks. The studied zircon populations and their U-Pb systems apparently showed open-system behaviour only when affected by extreme metamorphic conditions (700–750° C, > 28 kbar), whereas eclogite-facies conditions of 500–550° C and 14–16 kbar were not enough to disturb significantly the U-Pb zircon evolution. It is also probable that the sedimentary or magmatic origin of the protoliths of these meta-acidic rocks, which involved different characteristics such as grain-size and fluid phase concentration and composition, could be another important factor controlling the U-Pb zircon system behaviour during metamorphic events.

Early Devonian suprasubduction-zone ophiolite related to incipient collisional processes in the Western Variscan Belt: The Sierra de Careón unit, Ordenes Complex, Galicia

The Careon unit consists of imbricate tectonic sheets of serpentinite, metagabbro, and diabase dikes interpreted as a dismembered ophiolitic sequence. Zircons from leucogabbro intruding serpentinites point to an igneous crystallization at 395 ± 3 Ma, in accord with previous results. Combined with a reported 377 ± 1 Ma Ar/Ar cooling age, this date implies that ophiolite generation occurred shortly before its tectonic emplacement. Most samples are enriched in light rare earth elements (REE) relative to heavy REEs and show distinct fractionation of the heavy REEs. Enrichments in Th and La relative to Nb are ubiquitous, while negative anomalies of Zr and Ti occur in most samples. Initial e Nd values (+6.4 to +9.1), even for samples with high light REE/heavy REE ratios, point to a time-integrated mantle source strongly depleted in Nd relative to Sm, and preclude significant contamination of mafic melts during their ascent through the crust. Combined trace element and Nd-isotope data favor an intraoceanic, suprasubduction-zone setting, where hydrous fluids and silicate melts metasomatized a wedge of depleted mantle and triggered its partial melting. On the basis of magmatic affinities, age constraints, and broad tectonometamorphic context, ophiolite generation is interpreted to reflect oceanic spreading above a subduction zone dipping away from a passive continental margin. Suprasubduction extension might have been associated with subduction hinge retreat caused by instability of old, cold oceanic lithosphere entering the subduction zone. It is suggested that this occurred in the context of early stage arc collision documented at that epoch in the Variscan realm.

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 elusive Hadean enriched reservoir revealed by 142Nd deficits in Isua Archaean rocks

The first indisputable evidence for very early differentiation of the silicate Earth came from the extinct 146Sm–142Nd chronometer. 142Nd excesses measured in 3.7-billion-year (Gyr)-old rocks from Isua (southwest Greenland) relative to modern terrestrial samples imply their derivation from a depleted mantle formed in the Hadean eon (about 4,570–4,000 Gyr ago). As dictated by mass balance, the differentiation event responsible for the formation of the Isua early-depleted reservoir must also have formed a complementary enriched component. However, considerable efforts to find early-enriched mantle components in Isua have so far been unsuccessful. Here we show that the signature of the Hadean enriched reservoir, complementary to the depleted reservoir in Isua, is recorded in 3.4-Gyr-old mafic dykes intruding into the Early Archaean rocks. Five out of seven dykes carry 142Nd deficits compared to the terrestrial Nd standard, with three samples yielding resolvable deficits down to −10.6 parts per million. The enriched component that we report here could have been a mantle reservoir that differentiated owing to the crystallization of a magma ocean, or could represent a mafic proto-crust that separated from the mantle more than 4.47 Gyr ago. Our results testify to the existence of an enriched component in the Hadean, and may suggest that the southwest Greenland mantle preserved early-formed heterogeneities until at least 3.4 Gyr ago.