Добірка наукової літератури з теми "U-Th-Pb and 40Ar-39Ar dating"

Detrital zircon U-Pb and muscovite 40Ar/39Ar dating are useful tools for investigating sediment provenance and regional tectonic histories. However, the two types of data from same sample do not necessarily give consistent results. Here, we compare published detrital muscovite 40Ar/39Ar and zircon U-Pb ages of modern sands from the Yangtze River to reveal potential factors controlling differences in their provenance age signals. Detrital muscovite 40Ar/39Ar ages of the major tributaries and main trunk suggest that the Dadu River is a dominant sediment contributor to the lower Yangtze. However, detrital zircon data suggest that the Yalong, Dadu, and Min rivers are the most important sediment suppliers. This difference could be caused by combined effects of lower reaches dilution, laser spot location on zircons and difference in closure temperature and durability between muscovite and zircon. The bias caused by sediment laser spot targeting a core or rim of zircon and zircon reworking should be considered in provenance studies.

In this study, we present zircon U/Pb, plagioclase and K-feldspar 40Ar/39Ar and apatite fission track (AFT) data along the South Tannuol Fault Zone (STFZ). Integrating geochronology and multi-method thermochronology places constraints on the formation and subsequent reactivation of the STFZ. Cambrian (~510 Ma) zircon U/Pb ages obtained for felsic volcanic rocks date the final stage of STFZ basement formation. Ordovician (~460–450 Ma) zircon U/Pb ages were obtained for felsic rocks along the structure, dating their emplacement and marking post-formational local magmatic activity along the STFZ. 40Ar/39Ar stepwise heating plateau-ages (~410–400 Ma, ~365 and ~340 Ma) reveal Early Devonian and Late Devonian–Mississippian intrusion and/or post-magmatic cooling episodes of mafic rocks in the basement. Permian (~290 Ma) zircon U/Pb age of mafic rocks documents for the first time Permian magmatism in the study area creating prerequisites for revising the spread of Permian large igneous provinces of Central Asia. The AFT dating and Thermal history modeling based on the AFT data reveals two intracontinental tectonic reactivation episodes of the STFZ: (1) a period of Cretaceous–Eocene (~100–40 Ma) reactivation and (2) the late Neogene (from ~10 Ma onwards) impulse after a period of tectonic stability during the Eocene–Miocene (~40–10 Ma).

The north–northwest-striking Bathurst fault in the northeastern Slave craton displaced the 1.9 Ga Kilohigok basin and the ca. 2.02–1.96 Ga Thelon tectonic zone, and projects beneath the 1.7 Ga Thelon basin where unconformity-associated uranium deposits are spatially associated with basement faults. Here we investigate the deformation–temperature–time history of the Bathurst fault rocks using structural and microstructural observations paired with U–(Th–)Pb and 40Ar/39Ar geochronology. Highly strained hornblende-bearing granitoid rocks, the predominant rock type on the northeastern side of the Bathurst fault in the study area, show ambiguous sense of shear suggesting flattening by coaxial deformation. Quartz and feldspar microstructures suggest ductile deformation occurred at ≥500 °C. Along the main fault trace, brittle features and hydrothermal alteration overprint the pervasive ductile flattening fabric. In situ U–Th–Pb dating of synkinematic monazite suggests ductile fabric formation at ca. 1933 ± 4 Ma and ca. 1895 ± 11 Ma, and zircon from a cross-cutting dyke constrains the brittle deformation to ≤1839 ± 14 Ma. 40Ar/39Ar dating of fabric-defining minerals yield cooling ages of ca. 1920–1900 Ma and ca. 1900–1850 Ma for hornblende and muscovite, respectively, and a maximum cooling age of ca. 1840 Ma for biotite. We suggest the ca. 1933–1895 Ma ductile flattening fabric developed during orthogonal collision and indentation of the Slave craton into the Thelon tectonic zone and Rae craton. Brittle deformation on the Bathurst fault was localised parallel to the ductile flattening fabric after ca. 1840 Ma and preceded Thelon basin deposition. Brittle deformation features in Bathurst fault rocks preserve evidence for fluid–rock interaction and enhanced basement permeability, suggesting the fault is a possible conduit structure for mineralising fluids.

High-precision isotope dilution – thermal ionization mass spectrometry (ID–TIMS) U–Pb zircon and baddeleyite ages from the PX1 vertically layered mafic intrusion Fuerteventura, Canary Islands, indicate initiation of magma crystallization at 22.10 ± 0.07 Ma. The magmatic activity lasted a minimum of 0.52 Ma. 40Ar/39Ar amphibole dating yielded ages from 21.9 ± 0.6 to 21.8 ± 0.3, identical within errors to the U–Pb ages, despite the expected 1% theoretical bias between 40Ar/39Ar and U–Pb dates. This overlap could result from (i) rapid cooling of the intrusion (i.e., less than the 0.3 to 0.6 Ma 40Ar/39Ar age uncertainties) from closure temperatures (Tc) of zircon (699–988 °C) to amphibole (500–600 °C); (ii) lead loss affecting the youngest zircons; or (iii) excess argon shifting the plateau ages towards older values. The combination of the 40Ar/39Ar and U/Pb datasets implies that the maximum amount of time PX1 intrusion took to cool below amphibole Tc is 0.8 Ma, suggesting PX1 lifetime of 520 000 to 800 000 Ma. Age disparities among coexisting baddeleyite and zircon (22.10 ± 0.07/0.08/0.15 Ma and 21.58 ± 0.15/0.16/0.31 Ma) in a gabbro sample from the pluton margin suggest complex genetic relationships between phases. Baddeleyite is found preserved in plagioclase cores and crystallized early from low silica activity magma. Zircon crystallized later in a higher silica activity environment and is found in secondary scapolite and is found close to calcite veins, in secondary scapolite that recrystallised from plagioclase. close to calcite veins. Oxygen isotope δ18O values of altered plagioclase are high (+7.7), indicating interaction with fluids derived from host-rock carbonatites. The coexistence of baddeleyite and zircon is ascribed to interaction of the PX1 gabbro with CO2-rich carbonatite-derived fluids released during contact metamorphism.

Abstract The main products of volcanic activity in the teschenite-picrite association (TPA) are shallow, sub-volcanic intrusions, which predominate over extrusive volcanic rocks. They comprise a wide range of intrusive rocks which fall into two main groups: alkaline (teschenite, picrite, syenite, lamprophyre) and subalkaline (dolerite). Previous 40Ar/39Ar and 40K/40Ar dating of these rocks in the Polish Outer Western Carpathians, performed on kaersutite, sub-silicic diopside, phlogopite/biotite as well as on whole rock samples has yielded Early Cretaceous ages. Fluorapatite crystals were dated by the U-Pb LA-ICP-MS method to obtain the age of selected magmatic rocks (teschenite, lamprophyre) from the Cieszyn igneous province. Apatite-bearing samples from Boguszowice, Puńców and Lipowa yield U-Pb ages of 103± 20 Ma, 119.6 ± 3.2 Ma and 126.5 ± 8.8 Ma, respectively. The weighted average age for all three samples is 117.8 ± 7.3 Ma (MSWD = 2.7). The considerably smaller dispersion in the apatite ages compared to the published amphibole and biotite ages is probably caused by the U-Pb system in apatite being less susceptible to the effects of hydrothermal alternation than the 40Ar/39Ar or 40K/40Ar system in amphibole and/or biotite. Available data suggest that volcanic activity in the Silesian Basin took place from 128 to 103 Ma with the the main magmatic phase constrained to 128-120 Ma.

Abstract Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene–Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ∼20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109–108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ∼37 °C/m.y.); in these terranes, 8–12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene–early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ∼1.4 km of volcaniclastic strata, including ∼20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene–Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up.

Abstract During the Grenvillian assembly of Rodinia, the Namaqua-Natal Metamorphic Province (NNMP) was formed as a result of the convergence of the Laurentia and Kalahari cratons. A detailed model for this accretion along the south-eastern margin of the Kalahari Craton has been established, but the tectonic history of the NNMP along the western margin of the Kalahari Craton has remained highly controversial. U-Pb SHRIMP zircon age dating of gneiss in the Kakamas Domain of the NNMP, as well as U-Pb SHRIMP age dating of detrital zircons and 40Ar/39Ar dating of metamorphic muscovite from sediments overlying the gneiss, confirms the presence of at least two separate events during the Namaqua-Natal Orogeny at ~1 166 Ma and 1 116 Ma. These events occurred after the Areachap Terrane was accreted onto the western margin of the Proto-Kalahari Craton during the Kheis Orogeny. 40Ar/39Ar ages derived from metamorphic muscovite formed in the metasediments of the Kheis terrane does not provide evidence for the timing of the Kheis Orogeny but suggests that it most likely only occurred after ~1 300 Ma and not at 1 800 Ma as commonly accepted. A U-Pb concordia age of ~1 166 Ma was derived from granitic gneiss in the Kakamas Domain of the Bushmanland Subprovince, possibly reflecting subduction and the initiation of continent-continent collision between the Proto-Kalahari Craton and the Bushmanland Subprovince. This granitic gneiss is nonconformably overlain by the metasediments of the Korannaland Group that contains metamorphic muscovite with 40Ar/39Ar ages of ~1 116 Ma. This age suggest that complete closure of the ocean between the Proto-Kalahari Craton and Bushmanland Subprovince probably occurred about 50 Ma after the intrusion of the ~1 166 Ma granitic gneisses.

New U/Pb and 40Ar/39Ar isotopic dating in the Babine porphyry copper district of central British Columbia documents three distinct magmatic events at 107–104, 85–78, and 54–50 Ma. The earliest event involved emplacement of rhyolite domes into submarine volcanic rocks of the Rocky Ridge Formation. The rhyolite domes and related dacitic to basaltic volcanic rocks gave a U–Pb age of 107.9 ± 0.2 Ma and an 40Ar/39Ar age of 104.8 ± 1.2 Ma. The rhyolites, which were previously mapped as Eocene, are reinterpreted to be part of a previously unrecognized mid-Cretaceous cauldron subsidence complex. The regionally extensive Late Cretaceous magmatic event is also recognized in the Babine district and is represented by 40Ar/39Ar ages of 85.2 ± 2.8 and 78.3 ± 0.8 Ma on two Bulkley intrusions, one of which has associated porphyry copper mineralization. The final magmatic event is the most widespread and involved emplacement of the Babine intrusions and formation of numerous porphyry copper deposits including the Bell and Granisle past producers. Twenty-one new 40Ar/39Ar isotopic ages for these intrusions and coeval andesites of the Newman Formation have a narrow range from 53.6 ± 0.9 to 49.9 ± 0.6 Ma, whereas previous K–Ar isotopic dating had a possible range of 15 Ma. The mid-Cretaceous, Late Cretaceous, and Eocene magmatic suites in the Babine district are interpreted to be part of a long-lived volcano-plutonic complex that was the site of periodic magmatism and porphyry copper mineralization over a 60 Ma time period. This complex may have evolved within a zone of extension (pull-apart basins) situated between dextral strike-slip faults that were active during periods of rapid oblique plate convergence.

Published time scales provide discrepant age estimates for Jurassic stage boundaries and carry large uncertainties. The U-Pb or 40Ar/39Ar dating of volcaniclastic rocks with precisely known stratigraphic age is the preferred method to improve the calibration. A radiometric age database consisting of fifty U-Pb and 40Ar/39Ar ages was compiled to construct a revised Jurassic time scale. Accepted ages have a precision of ±5 Ma (2σ) or better and are confined to no more than two adjacent stages. The majority of these calibration points result from integrated bio- and geochronologic dating in the western North American Cordillera and have not been previously used in time scales. Direct dates are available only for the Triassic-Jurassic boundary and the initial boundary of the Crassicosta chron and the Callovian stage. The chronogram method was used to estimate all Early and early Middle Jurassic zone boundaries (attempted here for the first time), late Middle Jurassic substage boundaries, and Late Jurassic stage boundaries. Significant improvement is achieved for the Pliensbachian and Toarcian, where six consecutive zone boundaries are determined. The derived zonal durations are disparate, varying between 0.4 and 1.6 Ma. The latest Jurassic isotopic database remains too sparse, therefore chronogram estimates are improved using interpolation based on magnetochronology. The initial boundaries of Jurassic stages are proposed as follows: Berriasian (Jurassic-Cretaceous): 141.8+2.5&#150 1.8 Ma; Tithonian: 150.5+3.4&#150 2.8 Ma; Kimmeridgian: 154.7+3.8&#150 3.3 Ma; Oxfordian: 156.5+3.1&#150 5.1 Ma; Callovian: 160.4+1.1&#150 0.5 Ma; Bathonian: 166.0+3.8&#150 5.6 Ma; Bajocian: 174.0+1.2&#150 7.9 Ma; Aalenian: 178.0+1.0&#150 1.5 Ma; Toarcian: 183.6+1.7&#150 1.1 Ma; Pliensbachian: 191.5+1.9&#150 4.7 Ma; Sinemurian: 196.5+1.7&#150 5.7 Ma; Hettangian (Triassic-Jurassic): 199.6 ± 0.4 Ma.

As part of the wider European GTS Next project, I propose new constraints on the ages of the Late Cretaceous, derived from a multitude of geochronological techniques, and successful stratigraphic interpretations from Canada and Japan. In the Western Canada Sedimentary Basin, we propose a new constraint on the age of the K/Pg boundary in the Red Deer River section (Alberta, Canada). We were able to cyclostratigraphically tune sediments in a non-marine, fluvial environment utilising high-resolution proxy records suggesting a 11-12 precession related cyclicity. Assuming the 40Ar/39Ar method is inter-calibrated with the cyclostratigraphy, the apparent age for C29r suggests that the K/Pg boundary falls between eccentricity maxima and minima, yielding an age of the C29r between 65.89 ± 0.08 and 66.30 ± 0.08 Ma. Assuming that the bundle containing the coal horizon represents a precession cycle, the K/Pg boundary is within the analytical uncertainty of the youngest zircon population achieving a revised age for the K/Pg boundary as 65.75 ± 0.06 Ma. The Campanian - Maastrichtian boundary is preserved in the sedimentary succession of the Horseshoe Canyon Formation and has been placed ~8 m below Coal nr. 10. Cyclostratigraphic studies show that the formation of these depositional sequences (alternations) of all scales are influenced directly by sea-level changes due to precession but more dominated by eccentricity cycles proved in the cyclostratigraphic framework and is mainly controlled by sand horizons, which have been related by autocyclicity in a dynamic sedimentary setting. Our work shows that the Campanian - Maastrichtian boundary in the Western Canada Sedimentary Basin coincides with ~2.5 eccentricity cycles above the youngest zircon age population at the bottom of the section and ~4.9 Myr before the Cretaceous - Palaeogene boundary (K/Pg), and thus corresponds to an absolute age of 70.65 ± 0.09 Ma producing an ~1.4 Myr younger age than recent published ages. Finally, using advances with terrestrial carbon isotope and planktonic foraminifera records within central Hokkaido, Northwest Pacific, sections from the Cretaceous Yezo group were correlated to that of European and North American counterparts. Datable ash layers throughout the Kotanbetsu and Shumarinai section were analysed using both 40Ar/39Ar and U-Pb methods. We successfully dated two ash tuff layers falling either side of the Turonian - Coniacian boundary, yielding an age range for the boundary between 89.31 ± 0.11 Ma and 89.57 ± 0.11 Ma or a boundary age of 89.44 ± 0.24 Ma. Combining these U-Pb ages with recent published ages we are able to reduce the age limit once more and propose an age for the Turonian - Coniacian boundary as 89.62 ± 0.04 Ma.

Unidades litológicas, em particular arenitos, muitas vezes, carecem de um posicionamento cronoestratigráfico preciso. Como os arenitos são importantes rochas-reservatório de hidrocarbonetos e aquíferos, a falta de exatidão nestas informações dificulta a exploração destes bens minerais. A datação relativa de rochas sedimentares pode ser obtida por análise do conteúdo fossilífero ou por correlação estratigráfica. Entretanto, em algumas rochas sedimentares, esta análise não é possível ou tem um caráter duvidoso. Este é o caso da Formação Marizal (Bacia do Recôncavo) que apresenta um histórico controverso sobre a real idade deposicional. A Formação Marizal é um arenito flúvio-eólico cuja idade é discutível e, por isso, sua posição na coluna estratigráfica (aproximadamente Albiniano/Aptiniano), ainda é questionável. Em algumas amostras são encontrados overgrowths de K-feldspatos e nos quais é possível aplicar a técnica de datação 40Ar-39Ar visando obter idades que possam ser relacionadas com processos ocorrentes nestes arenitos (em geral, deposição e/ou diagênese). Entre os minerais pesados existentes nas amostras da Formação Marizal, foram encontrados grãos de zircões. A datação U-Pb de zircões detríticos pode fornecer informações sobre a proveniência desta unidade. Assim, zircões da Formação Marizal foram analisados visando complementar as informações sobre esta unidade, permitindo uma melhor interpretação. Os overgrowths de K-feldspatos indicaram valor de 159.89 ± 23.96 Ma e, para o núcleo detrítico, 432.57 ± 11.89 Ma. O valor médio obtido em torno de 160 Ma, considerando-se que todos os cuidados analíticos e de seleção de amostra foram considerados, é mais antigo do que o esperado. Assim, este valor foi interpretado como indicativo de que o overgrowth teria sido desenvolvido numa rocha fonte sedimentar sendo posteriormente transportado. Esta idade pode ser relacionada a fase pré rifte da Bacia do Recôncavo. O valor confirma ideias existentes de remobilização do substrato da bacia durante a fase rifte. Como tem sido discutido, overgrowths de K-feldspato são estáveis e possíveis de serem transportados por pequenas distâncias, o que corrobora a interpretação acima. Já o valor obtido para o núcleo mostra a contribuição do Paleoprotrozóico adjacente à bacia, retrabalhado no Brasiliano. Em relação ao zircão, a idade do núcleo detrítico de 432,53± 6,54 Ma pode ser associada com a cobertura sedimentar do Paleoproterozóico retrabalhada no ciclo Brasiliano, também observada nos valores U-Pb definidos para os zircões. Em relação ao zircão, os dados indicam ausência aparente de fontes arqueanas. Os resultados mostram duas fontes principais para a sedimentação: uma Rhyaciana (Paleoproterozóico onde ± 53 % dos grãos são “Transamazônicos”) e outra Neoproterozóica-Cambriana (30% dos zircões são “Brasilianos”).
Sandstones represent the most important reservoir rocks and aquifers in many sedimentary basins. It is necessary to have a precise chronostratigraphic position in order to provide a better explotation of water or hydrocarbons. Traditionally, the relative dating of sedimentary units is obtained with fossil content or stratigraphic correlation. But in many sedimentary rocks these analyses are not possible and sometimes have a dubious interpretation. This is the case of the Marizal Formation (Recôncavo Basin) where many questions arise when the age of the unit is questioned. The Marizal Formation is a fluvio-eolic sandstone which has been associated with an Albian/Aptian age in the stratigraphic column, although very discussible. Samples of sandstones of Marizal Formation present an important diagenetic overgrowths around K-feldspar detrital cores and they are suitable to 40Ar-39Ar dating concerning the identification of processes in the sandstones (as diagenesis or depositional ages). Among the heavy mineral suite in the Marizal Formation, zircon grains are identified. The U-Pb dating of detrital zircons can provide information about the provenance of the unit allowing better interpretation to the Marizal Formation. The overgrowths of K-feldspar indicated a value of 159.89 ± 23.96 Ma and to the detrital core, 432.57 ± 11.89 Ma. The mean value obtained around 160 Ma, considering that all care and analytical sample selection were considered, is older than expected. So, this was interpreted as indicating that the overgrowth, have been developed in a sedimentary source rock being transported latter to the depositional site. This age may be related to pre-rift stage of the Recôncavo Basin. The value confirms previous ideas of remobilization of the substrate during the rift basin stage. As has been extensively discussed, overgrowths of K-feldspars are stable and can be transported by small distances, which corroborates the above interpretation. The value obtained to the detrital core can be associated with a Paleoproterozoic sedimentary cover reworked in the Brazilian cycle. For zircon U-Pb dating, the data indicate no apparent Archean sources. The results show two main sources for sedimentation: a Rhyacian (Paleoproterozoic where ± 53% of the grains are "Transamazonian") and another Neoproterozoic-Cambrian (30% of zircon are "Brazilian").

Située à l'extrême-ouest de la Méditerranée occidentale, la chaîne bético-rifaine s’est formée au travers d’une histoire orogénique alpine complexe, impliquant des processus de subduction liés à la convergence entre l’Afrique et l’Eurasie depuis le Crétacé. Une découverte importante de ces quatre dernières décennies d'investigations géologiques, a été la mise en évidence des vestiges d’un événement varisque dans les zones internes de la chaîne. Ces résultats soulignent bien la présence de deux systèmes orogéniques superposés, les zones internes de la chaîne bético-rifaine demeurent donc des zones privilégiées pour étudier l’importance de l’héritage structural et métamorphique dans les réactivations partielles ou totale par les évènements les plus récents. Ce travail est localisé dans le secteur de Beni Bousera, ou affleurent les roches crustales et mantéliques qui forment les unités les plus internes de la chaîne. Il s’appuie sur une étude menée à partir des analyses structurales et pétrologiques, des datations U-Th-Pb sur monazite et 40Ar-39Ar sur des micas et des amphiboles. Il nous permet de résumer l’histoire de la chaîne bético-rifaine de la manière suivante : 1) un événement de HP-HT affecte la base du domaine interne à ca 281 ± 3 Ma. Ces nouvelles données pétrologiques et géochronologiques obtenues dans le Rif interne sont corrélées avec les Bétiques, les Kabylies et le massif de l'Edough en Algérie, la ceinture mauritanienne et les Appalaches. Elles attestent d'un domaine convergent au cours du Carbonifère supérieur- Permien inférieur. Tous ces segments orogéniques font partie des Variscides nord-africains construits à la marge nord-ouest du Gondwana en réponse à une convergence entre cette dernière et la Laurentia. 2) autour de 29-26 Ma, un événement métamorphique avec un gradient de type Barrovien à Abukuma affecte les Sebtides (les unités les plus internes de la chaîne), et il est interprété comme résultant de l'évolution de la plaque supérieure d'une zone de subduction. Cet évènement alpin est caractérisé par un chemin prograde marqué par un réchauffement à la base des Sebtides entre 26 et 22 Ma. De telles conditions reflètent un amincissement et un réchauffement de la croûte liée à la remontée asthénosphérique due au retrait de la plaque plongeante ; cet événement marque le début d’un évènement extensif majeur. 3) Au Miocène inférieur à 22-20 Ma, les zones internes (ou domaine d’Alboran) sont affectées par une extension E-W contemporaine de l’ouverture du bassin d'Alboran dans un contexte arrière-arc, et par l’intrusion de filons granitiques dans les péridotites et les unités métamorphiques crustales du domaine interne. Cet évènement a permis l’exhumation finale des Sebtides. 4) Du Miocène inférieur au Miocène moyen, la chaîne bético-rifaine a acquis sa géométrie arquée (l’arc de Gibraltar) suite à la collision entre les zones internes et les zones externes, attestée par une phase de raccourcissement majeur de direction NE-SW à E-W, et 5) l’arc de Gibraltar est affecté par une phase de raccourcissement N-S ante-Pliocène de direction N-S, qui a modifié considérablement sa géométrie
Located at the extreme tip of the western Mediterranean, the Betic-Rif orogenic system is built through a complex alpine orogenic history involving processes of subduction related to convergence between Africa and Eurasia since the Cretaceous. A remarkable discovery during the last four decades of geological investigations, has been the remains of a variscan event in the internal zones of the belt. These results underline the presence of two superimposed orogenic systems, the internal zones of the belt thus remain a privileged area to study the importance of the structural and the metamorphic heritage in the partial or total reactivation by the most recent events. This work is located in the Beni Bousera sector, where crustal and mantle rocks that form the innermost units of the chain are exposed. Based on structural and petrological analyses, U-Th-Pb dating on monazite and 40Ar-39Ar dating on micas and amphiboles. The history of the Betic-Rif belt can be summarized as it follows: 1) a HP-HT event affects the base of the internal domain at around 281 ± 3 Ma. These new petrological and geochronological data obtained in the internal Rif, are correlated with the Betics, the Kabyle, the Edough massif of Algeria, the Mauritanian, and the Appalachian belts, attesting a convergent domain during the late Carboniferous – early Permian. All of these orogenic segments are part of the North African Variscides built at the north-western margin of Gondwana in response to convergence between the later and Laurentia. 2) at around 29-26 Ma, a Barrovian to Abukuma metamorphic event affects the Sebtides (the innermost units of the chain) and interpreted as the evolution of the upper plate of a subduction zone. This alpine event is typically characterized by a prograde metamorphic path marked by heating affecting the base of the Sebtides between 26 to 22 Ma, such conditions reflect thinning and heating of the crust related to the asthenosphere upwelling due to slab roll-back. This event marks the beginning of a major extensive event. 3) In the Miocene around 22-20 Ma, the internal zones are affected by an E-W extension contemporary to the opening of the Alboran Basin in a back-arc context, and the intrusion of granitic dykes into the peridotites and crustal metamorphic units, the exhumation of the Sebtides was complete at this time. 4) From early to middle Miocène, the Betic-Rif belt acquired its arcuate geometry (the Gibraltar Arc) during the collision between the Internal and the external zones, attested by de NE-SW to E-W shortening phases across the arc. 5) more lately prior to Pliocene, the Gibraltar arc was subjected to contractional possess related to a N-S shortening phase, which drastically altered its geometry

Arc–continent collision is a significant plate boundary process that results in crustal growth. Since the early stages of evolution are often obscured in mature orogens, more complete understanding of the processes involved in arc–continent collision require study of young, active collision settings. The Banda Arc presents an exceptional opportunity to study a young arc–continent collision zone. This thesis presents aspects of the geology and geochronology of Ataúro and the Aileu Complex of Timor-Leste, and the tectonics of the Banda Arc.
U–Pb dating of detrital zircons from the Aileu Complex by LA-ICPMS show major age modes at 270–440 Ma, 860–1240 Ma and 1460–1870 Ma. The youngest zircon populations indicate a maximum depositional age of 270 Ma. The detrital zircon age populations and evidence for juvenile sediments within the sequence favours a synorogenic setting of deposition of sediments sourced from an East Malaya – Indochina terrane.
Previous uncertainty in aspects of the cooling history for the Aileu Complex is resolved with 39Ar/40Ar geochronology of hornblende. Cooling ages of 6–10 Ma are established, with the highest metamorphic grade parts of the Complex yielding the older ages. Cooling ages of 10 Ma imply that metamorphism of the Aileu Complex must have commenced by at least ~12 Ma. Metamorphism at this time is attributed to an arc setting rather than the direct result of collision of the Australian continent with the Banda Arc, an interpretation consistent with the new provenance data.
Geological mapping of Ataúro, an island in the volcanic Banda Arc north of Timor, reveals a volcanic history of bi-modal subaqueous volcanism. 39Ar/40Ar geochronology of hornblende from dacitic lavas confirms that volcanism ceased by ~3 Ma. Following the cessation of volcanism, coral reef marine terraces have been uplifted to elevations of 700 m above sea level. Continuity of the terraces at constant elevations around the island reflects regional-scale uplift most likely linked to sublithospheric processes such as slab detachment.
North of Timor, the near complete absence of intermediate depth seismicity beneath the inactive segment of the arc is attributed to a slab window that has opened in the collision zone and extends to 350 km below the surface. Differences in seismic moment release around this slab window indicate asymmetric rupture, propagating to the east at a much faster rate than to the west. If the lower boundary of this seismic gap signifies the original slab rupture then the slab window represents ~4 m.y. of subsequent subduction and implies that collision preceded the end of volcanism by at least 1 m.y.
Variations in seismic moment release and stress state across the transition from subduction of oceanic crust to arc–continent collision in the Banda Arc are investigated using earthquake catalogues. It is shown that the slab under the western Savu Sea is unusual in that intermediate depth (70–300 km) events indicate that the slab is largely in down-dip compression at this depth range, beneath a region of the arc that has the closest spacing of volcanoes in the Sunda–Banda arc system. This unusual state of stress is attributed to subduction of a northern extension of the Scott Plateau. Present day deformation in the Savu Sea region may be analogous with the earliest stages of collision north of Timor.

La tsavorite, grossulaire vert à V-Cr-Mn, est contenue dans des gneiss et roches calco-silicatées graphiteux, souvent associés à des marbres dolomitiques, et appartenant à la ceinture métamorphique néoprotérozoïque mozambicaine. La tsavorite se trouve soit dans des nodules ou des veines de quartz (gisements primaires), soit dans des placers (gisements secondaires). L'étude minéralogique des tsavorites propose un nouveau protocole de certification de leur origine géographique, à partir du rapport V/Cr, de la teneur en Mn et du [delta]18 O. L'étude des gisements de Lemshuku et Namalulu en Tanzanie montre que le métamorphisme des protolithes sédimentaires riches en matière organique et évaporites s'est effectué à P = 7,0 ± 0,4 kbar et T = 677 ± 14°C, à 634 ± 22 Ma (datation U-Th-Pb sur monazite). Le bâti métamorphique s'est refroidi vers 500 Ma (datation 40Ar-39Ar sur muscovite). Deux stades de métasomatose sont reliés à la formation de la tsavorite : (i) une métasomatose de diffusion formant les nodules à P = 5,0-7,4 kbar et T = 580-691°C; (ii) une métasomatose calcique d'infiltration contemporaine de la formation des veines de quartz à P = 3,6-4,9 kbar et T = 505-587°C. Ces dernières sont datées in situ par la méthode Sm-Nd à 606 ± 36 Ma. Les évaporites continentales, déposées dans une sabkha de côte marine avec des sédiments silico-calcaires, sont transformées en tsavorite dans le cas des nodules, alors que les sels fondus sont associés à la formation des veines de quartz. Les minéralisations sont contrôlées par la lithostratigraphie et la tectonique
Tsavorite, a (V, Cr, Mn)-bearing green grossular, is hosted by graphitic gneisses or calc-silicates, often asssociated with dolomitic marbles, and belonging to the Metamorphic Neoproterozoic Mozambique Belt. Tsavorite is found either as nodules or in quartz veins (primary deposits), or in placers (secondary deposits). The mineralogical study of tsavorites suggests a new protocol to certificate their geographical origin, based on the V/Cr ratio, Mn content and delta18O. The study of the Lemshuku and Namalulu deposits in Tanzania has shown that the metamorphism of organic matter-rich and evaporites-rich sedimentary protoliths occurred at P = 7.0 ± 0.4 kbar and T = 677 ± 14°C, at 634 ± 22 Ma (U-Th-Pb dating on monazite). The metamorphic series cooled down at around 500 Ma (40Ar-39Ar dating on muscovite). Two metasomatic stages are linked to the formation of tsavorite : (i) diffusion metasomatism forming nodules at P = 5.0-7.4 kbar and T = 580-691°C; (ii) calcitic infiltration metasomatism forming quartz veins at P = 3.6-4.9 kbar and T = 505-587°C. These last have been dated in situ with Sm-Nd dating at 606 ± 36 Ma. Continental evaporites, deposited in a coastal marine sabkha with (Si, Ca)-bearing sediments, transformed into tsavorite in the case of the nodules, while the molten salts are associated with the formation of the quartz veins. The mineralisations are controlled by lithostratigraphy and structure

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A região sudeste do Escudo das Guianas é parte de uma das mais expressivas faixas orogênicas paleoproterozóicas do mundo, cuja evolução está relacionada ao Ciclo Orogênico Transamazônico (2,26 – 1,95 Ga). Neste segmento foram estudados distintos terrenos tectônicos, denominados Jari, Carecuru e Paru, reconhecidos em estudos anteriores em função de seus notáveis contrastes em termos de idade, conteúdo litológico e assinatura geofísico-estrutural. O Domínio Jari é constituído por uma assembléia de embasamento do tipo granulito-gnaissemigmatito com protólitos arqueanos, enquanto o Domínio Carecuru é composto basicamente por rochas cálcio-alcalinas e seqüências metavulcano-sedimentares, com evolução relacionada ao Evento Transamazônico. O Domínio Paru foi delimitado no interior do Domínio Carecuru, e é formado por gnaisses granulíticos com protólitos arqueanos, que hospedam plútons charnoquíticos paleoproterozóicos. Neste estudo, quatro métodos geocronológicos foram empregados em rochas provenientes dos distintos domínios tectônicos, com o objetivo de entender significado tectônico de cada um deles, definir os processos de evolução crustal que atuaram no Arqueano e no Paleoproterozóico e avaliar a extensão de crosta arqueana neste setor da faixa orogêncica em questão. Os métodos de evaporação de Pb em zircão e Sm-Nd em rocha total demonstram que a evolução do Domínio Jari envolve vários estágios de acresção e retrabalhamento crustal, do Arqueano ao Paleoproterozóico. Atividade magmática ocorreu principalmente na transição Meso- Neoarqueano (2,80-2,79 Ga) e durante o Neoarcheano (2,66-2,60 Ga). O principal período de formação de crosta continental ocorreu a partir do final do Paleoarqueano e ao longo do Mesoarqueano (3,26-2,83 Ga), enquanto retrabalhamento crustal prevaleceu no Neoarqueano. Durante o Evento Transamazônico, dominaram processos de retrabalhamento de crosta arqueana, com vários pulsos de magmatismo granítico, datados entre 2,22 Ga e 2,03 Ga, que marcam distintos estágios da evolução orogenética. Os dados geocronológicos obtidos neste estudo, conjugados aos disponíveis na literatura, indicam que o Domínio Jari é parte do mais expressivo segmento de crosta arqueana conhecido no Escudo das Guianas, aqui definido e denominado de Bloco Amapá. No Domínio Carecuru foram definidos dois pulsos de magmatismo cálcio-alcalino, entre 2,19 e 2,18 Ga e entre 2,15 e 2,14 Ga, enquanto magmatismo granítico foi datado em 2,10 Ga. Acresção crustal juvenil cálcio-alcalina foi reconhecida em torno de 2,28 Ga. No entanto, idades TDM (2,50-2,38 Ga), preferencialmente interpretadas como idades mistas, e εNd < 0, indicam a participação de componentes arqueanos na fonte das rochas paleoproterozóicas. Os dados isotópicos, somados à associação litológica deste domínio, sugerem uma evolução relacionada a sistema de arco magmático em margem continental ativa, que foi acrescido ao Bloco Amapá durante o Evento Transamazônico. No Domínio Paru, magmatismo neoarqueano datado em torno de 2,60 Ga, foi produzido por retrabalhamento de crosta mesoarqueana, assim como no Bloco Amapá. Adicionalmente, acresção crustal juvenil e magmatismo cálcio-alcalino foram reconhecidos, em torno de 2,32 Ga e 2,15 Ga, respectivamente, além de magmatismo charnoquítico em 2,07 Ga. Idades U-Th-Pb obtidas em monazitas provenientes da assembléia de alto grau do sudoeste do Bloco Amapá, revelaram dois estágios distintos da evolução orogenética transamazônica. O primeiro ocorreu em torno de 2,09 Ga, que marca a idade do metamorfismo de fácies granulito, contemporâneo ao desenvolvimento de um sistema de cavalgamento oblíquo, relacionado ao estágio colisional da orogênese. O outro ocorreu em torno de 2,06 Ga e 2,04 Ga, e é consistente com o estágio tardi-colisional, marcado por migmatização do embasamento e colocação de granitos ao longo de zonas de cisalhamento transcorrentes. Finalmente, análises 40Ar/39Ar em anfibólios e biotitas de unidades estratigráficas representativas, principalmente do Bloco Amapá e do Domínio Carecuru, revelam distintos padrões de resfriamento e exumação para estes dois segmentos crustais. No Bloco Amapá, as idades de anfibólios variam entre 2,13 e 2,09 Ga, enquanto as biotitas forneceram idades principalmente entre 2,10 e 2,05 Ga. No Domínio Carecuru, anfibólios e biotitas apresentaram idades entre 2,16 e 2,06 Ga e entre 1,97 e 1,85 Ga, respectivamente. Taxas de resfriamento da ordem 67 °C/Ma e 40 °C/Ma foram calculadas para o Bloco Amapá, indicando resfriamento rápido e exumação controlada por tectonismo, possivelmente relacionada ao estágio colisional do Evento Transamazônico. Em contrapartida, no Domínio Carecuru, as taxas de resfriamento regional variam em torno de 3-2,3 °C/Ma, sugerindo resfriamento lento e exumação gradual, o que é consistente com o modelo de arco magmático, no qual, crescimento de crosta continental resulta principalmente de acresção magmática lateral, sem espessamento crustal significativo.
The southeastern portion of the Guiana Shield is part of a large Paleoproterozoic orogenic belt, with evolution related to the Transamazonian Orogenic Cycle (2.26 – 1.95 Ga). In this area, previous works defined distinct tectonic domains, named Jari, Carecuru and Paru, which present outstanding differences in terms of age, lithological content, structural pattern and geophysical signature. The Jari Domain is constituted of a granulite-gneiss-migmatite basement assemblage derived from Archean protoliths, and the Carecuru Domain is composed mainly of calc-alkaline rocks and metavolcano-sedimentary sequences, developed during the Transamazonian Event. The Paru Domain is an oval-shaped granulitic nucleous, located within the Carecuru Domain, formed by granulitic gneisses with Archean precursors and Paleoproterozoic charnockitic plutons. In this study, distinct geochonological methods were employed in rocks from the distinct domains, in order to define their tectonic meaning and crustal evolution processes during Archean and Paleoproterozoic times. Pb-evaporation on zircon and Sm-Nd on whole rock dating were provided on magmatic and metamorphic units from the Jari Domain, defining its long-lived evolution, marked by several stages of crustal accretion and crustal reworking. Magmatic activity occurred mainly at the Meso-Neoarchean transition (2.80-2.79 Ga) and during the Neoarchean (2.66-2.60 Ga). The main period of crust formation occurred during a protracted episode at the end of Paleoarchean and along the whole Mesoarchean (3.26-2.83 Ga). Conversely, crustal reworking processes have dominated in Neoarchean times. During the Transamazonian Event, the main geodynamic processes were related to reworking of older Archean crust, with minor juvenile accretion at about 2.3 Ga, during an early orogenic phase. Transamazonian magmatism consisted of syn- to late-orogenic granitic pulses, which were dated between 2.22 and 2.03 Ga. Most of the εNd values and TDM model ages (2.52-2.45 Ga) indicate an origin of the Paleoproterozoic granites by mixing of juvenile Paleoproterozoic magmas with Archean components. The new geochronological results, added to data from previous studies, revealed that the Jari Domain represents the southwestern part of the most expressive Archean continental landmass of the Guiana Shield, here defined and named Amapá Block. The recognition of an extended Archean block precludes previous statements that the Archean in the southeast of the Guiana Shield, was restricted to isolated remnants or inliers within Paleoproterozoic terrains. In the Carecuru Domain the widespread calc-alkaline magmatism occurred at 2.19-2.18 Ga and at 2.15-2.14 Ga, and granitic magmatism was dated at 2.10 Ga. Crustal accretion was recognized at about 2.28 Ga, in agreement with the predominantly Rhyacian crust-forming pattern of the Guiana Shield. Nevertheless, TDM model ages (2.50-2.38 Ga), preferentially interpreted as mixed ages, and εNd < 0, point to some participation of Archean components in the source of the Paleoproterozoic rocks. The lithological association and the available isotopic data registered in the Carecuru Domain, suggests a geodynamic evolution model based on the development of a magmatic arc system during the Transamazonian Orogenic Cycle, which was accreted to the southwest border of the Archean Amapá Block. In the Paru Domain, Neoarchean magmatism at about 2.60 Ga was produced by reworking of Mesoarchean crust, as registered in the Amapá Block. Crustal accretion events and calc-alkaline magmatism were recognized at 2.32 Ga and at 2.15 Ga, respectively, as well as charnockitic magmatism at 2.07 Ga. U-Th-Pb chemical ages in monazites from high-grade rocks of the southwestern part of Amapá Block, dated two main tectono-thermal events. The first one was revealed by the monazite ages of about 2.09 Ga and marks the age of the granulite-facies metamorphism. These data, added to petro-structural information, indicate that the granulite-facies metamorphism was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian Orogeny. The later event was testified by monazite ages at about 2.06 Ga and 2.04 Ga, and is consistent with a late-orogenic stage marked by granitic emplacement and coeval migmatization of the Archean basement along strike-slip zones. Finally, 40Ar/39Ar geochronological study on amphibole and biotite from representative units of the Amapá Block and of the Carecuru Domain delineated contrasting cooling and exhumation stories. In the former amphibole vary from 2.13 to 2.09 Ga, and biotite ages range mainly between 2.10 and 2.05 Ga. In the later, amphibole and biotite ages are between 2.16 and 2.06 Ga, and 1.97 and 1.85 Ga, respectively. In the Amapá Block, fast cooling rates around 67 °C/m.y. and 40 °C/m.y indicate a tectonically controlled exhumation, related to collisional stages of the Transamazonian Event. Conversely, in the Carecuru Domain, regional cooling rates in the order of 3-2.3 °C/m.y. suggest slow cooling and gradual uplift, which is consistent with the magmatic arc model, where continental growth results mainly from lateral magmatic accretion, precluding significant tectonic crustal thickening.

La chaîne Bético-Rifaine, située dans la partie occidentale de la Méditerranée, est le résultat de la convergence des plaques européenne et africaine et des processus de subduction qui en résultent. Il s’agit donc d’un lieu d’exception pour étudier les processus liés à la dynamique de la subduction. Cette thèse se focalise sur l’étude du domaine interne de la chaine du Rif. Deux zones d’étude ont été sélectionnées (Ceuta et Cabo Negro) où affleurent des roches crustales de haute pression-basse température (HP-BT) (seulement à Ceuta) mais aussi des roches de basse pression-haute température (BP-HT) ainsi que des roches ultrabasiques dont la signification tectonique, le mode et les âges de mise en place restent encore très débattus. Une approche multi-pluridisciplinaire a été effectuée pour réaliser ce travail, avec une étude structurale et pétrographique détaillée, ainsi que des datations U/Th/Pb sur zircon, monazite et xénotime et 40Ar/39Ar sur mica blanc. Les résultats obtenus démontrent que : (i) à environ 29 Ma un métamorphisme de moyenne pression-haute température (MP-HT) affecte les unités des Sebtides inférieures et Ghomarides. Cette phase est contemporaine d’un épaississement de la croûte et de la mise en place de sills de diorite ayant une signature géochimique d’arc magmatique calco-alcalin fortement potassique. Cette phase est d'autre part contemporaine du métamorphisme de HP-BT observé dans les unités crustales des Sebtides supérieures, (ii) à environ 21 Ma une phase d’extension contribue à l’exhumation finale de ces roches. Cet évènement est associé à un épisode métamorphique qui se développe à la limite des conditions des facies amphibolites et schistes verts, sous des conditions de 400-550°C et 1-3 kbar. La combinaison quasi-contemporaine d’unités métamorphiques de gradients thermiques radicalement différents est caractéristique des « ceintures métamorphiques appariées ». Dans ce cas nous proposons le modèle suivant : à 29 Ma, pendant la subduction Alpine, les unités des Ghomarides et Sebtides inférieures se situent au niveau de la plaque supérieure du système de subduction où se développe le métamorphisme de MP-HT. En parallèle le métamorphisme de HP-BT se manifeste dans le panneau plongeant constitué par les Sebtides supérieures. La déshydratation de la plaque plongeante induit un magmatisme calco-alcalin dans la plaque supérieure. A 21 Ma, le recul de la plaque plongeante produit une phase d’extension créant l’ouverture du bassin d’Alboran et l’exhumation des unités de HP-BT
The Betic-Rif Cordillera, located in the western part of the Mediterranean, results from the convergence of the European and African plates and related subduction processes. It is therefore an exceptional site to study the processes induced by subduction dynamics. This thesis focuses on the study of the internal domain of the Rif chain. Two key areas have been selected (Ceuta and Cabo Negro) where high pressure-low temperature crustal rocks (HP-LT) (present only in Ceuta) but also low pressure-high temperature rocks (LP-HT) and ultrabasic rocks outcrop. Their tectonic significance, placement mode and ages are still heavily debated subjects. A multidisciplinary approach was developed to carry out this work, with a detailed structural and petrographic study, as well as U/Th/Pb dating on zircon, monazite and xenotime and 40Ar/39Ar on white mica. The results obtained show that: (i) at circa 29 Ma a medium pressure-high temperature metamorphism (MP-HT) affects the lower Sebtides and Ghomarides. This phase is simultaneous with a thickening of the crust and the placement of diorite sill having a geochemical signature of calc-alkaline to high-K magmatic arc. This phase is contemporaneous to the HP-LT metamorphism observed in the crustal units of the upper Sebtides (ii) at circa 21 Ma an extension phase contributes to the final exhumation of these rocks. This event is associated with a metamorphic episode developed at the boundary between Amphibolite and Greenschist facies conditions, at 400-550°C and 1-3 kbar. The quasi-contemporary emplacement of metamorphic units with radically different thermal gradients is characteristic of “paired metamorphic belts”. In this case we propose the following model: at 29 Ma, during Alpine subduction, the units of the Ghomarides and Lower Sebtides are located within the upper plate of the subduction system where the MP-HT metamorphism develops. In parallel, the HP-LT metamorphism occurs within the lower plate constituted by the upper Sebtides. Dehydration of the lower plate induces calc-alkaline magmatism in the upper plate. At 21 Ma, the slab rollback produces an extension phase creating the opening of the Alboran basin and the exhumation of the HP-LT units

Le Limousin (NW du Massif Central) est caractérisé par de larges massifs granitiques mis en place entre 360 et 290 Ma. Ils présentent d'étroites relations spatiales avec de grands accidents ductiles en faille normale et décrochement qui prolongent vers le SE la zone de cisaillement Sud Armoricaine.
Le volumineux (~ 10000km3) complexe granitique N-S de Millevaches, limité par des décrochements et failles normales, est un exemple type de granite mis en place dans un contexte tectonique décrochant.
Le modèle de mise en place des granites de Millevaches prend en compte l'analyse structurale, microstructurale, magnétique (A.S.M.), gravimétrique et géochronologique (40Ar/39Ar et U/Pb). L'ascension des magmas se fait par des conduits verticaux étroits sous forme d'injections successives qui se relaient le long de l'axe principal N-S des Pradines. Les magmas sont ensuite piégés puis canalisés par la foliation précoce, anisotropie mécanique sub-horizontale majeure de la croûte moyenne. Les magmas syntectoniques du décrochement dextre N-S des Pradines enregistrent des trajectoires de déformation orientées N-S dans la faille et NW-SE de part et d'autre. La poussée du magma au toit du laccolite induit une déformation par aplatissement relaxée par le développement de failles d'échappement sub-horizontales et normales. La mise en place syntectonique des leucogranites du Millevaches, datée à 313 ± 4 Ma est contemporaine du métamorphisme granulitique subi par les roches encaissantes.
Le fonctionnement des décrochements du Limousin débute vers 350 Ma et finit vers 300 Ma. Nous proposons que les deux générations de granites (granodiorite-monzogranite et leucogranite) se mettent en place dès 350 Ma, dans une ceinture tectonique résultant d'un contexte en transpression. Les cisaillements ductiles constituent les branches d'un large, long (~700 km), et unique système décrochant lithosphérique analogue à une « pop-up structure » NW-SE dextre allant du Massif Sud Armoricain au Limousin.

Isotopic ratios of elements in natural materials on the earth either have been constant in time and space or have varied as a result of radioactive decay or geochemical fractionation. Elements which show variations in isotopic abundances in different samples and the reasons for these variations have helped resolve many geological and archaeological problems. Radioactive decay has provided absolute dating clocks: for archaeology, the most useful systems have been associated with 14C, 40Ar, and U-disequilibrium series. Variations in isotopic ratios of the stable elements H, C, O, N, S, Sr, and Pb have helped solve problems of provenance, paleoenvironments, and paleodiets. The rationale for isotopic variations of individual elements will determine the types of applications to archaeological geology. The most important applications are the determinations of artifact signatures, paleodiet, and paleoenvironment. Isotopic fractionation of light elements by physical, chemical, and biological processes is controlled by those thermodynamic properties which are determined by atomic weight and electronic configuration. Thermodynamic properties of molecules that are mass and temperature dependent include energy, which decreases with decreasing temperature, and vibrational frequency, which varies inversely in proportion to the square root of the reduced mass. Easily measurable isotopic separation is generally restricted to the lighter elements, that is, with atomic weights less than 40. Because isotopic fractionation is mass dependent, the separation is greater for elements with the greater mass difference between isotopes. The greatest separation is expected for hydrogen (mass 1) versus deuterium (mass 2); the other light elements commonly have isotopic differences closer to 10%. Thus, the lighter isotopes have higher vibrational energy and their chemical bonds are more easily broken. The different reactivity of lighter versus heavier isotopes of an element is responsible for their separation during geochemical and biological processes. Thermodynamic behavior has been considered a principal cause for variations, not in isotopic abundances of the heavier elements Sr and Pb, but rather in abundances of their parent radionuclides: Rb for Sr and U and Th for Pb. Recently, however, P. Budd and others suggested that under nonequilibrium conditions, fractionation could theoretically take place among the lead isotopes.