Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
The Early Palaeozoic Orogen of SE China consists of three litho-tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite-granite domes, high-temperature metamorphism (M2) and granitic plutons related to post-orogenic crustal melting. We date here the syn-M1 ductile shearing at 453 +/- 7 Ma by U-Th/Pb method on monazite. Previous ages and our new
ISSN: 0954-4879

hal-00420901
https://hal.archives-ouvertes.fr/hal-00420901
https://hal.archives-ouvertes.fr/hal-00420901/document
https://hal.archives-ouvertes.fr/hal-00420901/file/Faureetal-TerraNova-2009.pdf
DOI : 10.1111/j.1365-3121.2009.00888.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : A Ar-40/Ar-39 geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapa Block and the Carecuru Domain. In the AmapA Block, an Archean continental block involved in the Transamazonian orogenesis (2.26-1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 +/- 3 to 2047 +/- 20 Ma, and biotite ages spread mainly between 2079 +/- 18 and 2033 +/- 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 +/- 17 Ma, and biotite ages of 1928 +/- 19 Ma and 1833 +/- 13 Ma. These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U-Th-Pb monazite and Pb-Pb zircon geochronological records and petro-structural observations, the new Ar-40/Ar-39 data delineate contrasting cooling and exhumation histories for the tectonic domains. In the AmapA Block, the data suggest nearly vertical T-t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.
ISSN: 0895-9811

hal-00420075
https://hal.archives-ouvertes.fr/hal-00420075
DOI : 10.1016/j.jsames.2008.08.010

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This paper presents a reappraisal of the tectonic evolution of the Zanhuang Massif that lies at the eastern margin of the Trans-North China Orogen, a continent–continent collision belt that marked the amalgamation of the North China Craton in Late Paleoproterozoic. Detailed field work with focus on geometries of structures and kinematics was performed. This was completed with LA-ICP-MS U–Pb analyses on zircon, EPMA U–Th/Pb dating on monazite and 40Ar/39Ar dating on amphibole. These studies led us to propose a new three-fold litho-tectonic subdivision of the massif: The Western Zanhuang Domain (WZD) made of TTG, migmatite and pink anatectic granite is correlated to the Fuping Massif that crops out to the north-west. Both areas represent a continental block, called the Fuping Block, which acquired most of its architecture around 2100 Ma ago. The Eastern Zanhuang Domain (EZD) made of TTG and migmatite represents the western edge of an Eastern Neoarchean Block. In between, the Central Zanhuang Domain (CZD) is a NE–SW trending stack of supracrustal, gneiss and mafic magmatic rocks thrust sheets displaced toward the ESE upon the Eastern Block. The lithological features suggest that the CZD represents the remnant of an oceanic basin, called the Taihang Ocean that closed during the amalgamation of the Eastern Block and the Fuping Block around 1880–1850 Ma. In agreement with recent work done along the western margin of the belt, in the Lüliang Massif, this study documents the amalgamation of the North China Craton in response to the closure of two oceanic basins, namely the Lüliang Ocean and the Taihang Ocean. West-dipping subductions and collisions involving three distinct continental blocks, called the Western, the Fuping and the Eastern Blocks, took place around 1880–1850 Ma.
ISSN: 0301-9268

insu-00403871
https://hal-insu.archives-ouvertes.fr/insu-00403871
https://hal-insu.archives-ouvertes.fr/insu-00403871/document
https://hal-insu.archives-ouvertes.fr/insu-00403871/file/Trap-PrecambrianResearch-2009.pdf
DOI : 10.1016/j.precamres.2009.03.011

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Unravelling the detailed pressure–temperature–time-deformation (P–T–t-D) evolution of magmatic and metamorphic rocks provides essential insights into the timing and duration of partial melting and related plutonism during crustal flow and migmatitic dome formation. The Montagne Noire Axial Zone (MNAZ) is a migmatitic dome located within the Variscan orogen in the southern French Massif Central. The timing of the main thermal event that was responsible for intense partial melting is still highly debated. In this study we present new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) age data on micaschists, migmatites and granites that clarify the P–T–t-D evolution of the MNAZ. Structurally controlled samples were collected in order to constrain the timing of metamorphism, migmatization and plutonism regarding the main structural pattern D1, D2 and D3. D1 and D2 correspond to nappe stacking and dextral transpression, respectively. D3 is related to vertical shortening and coaxial thinning with a preferential NE–SW- to E–W-directed stretching. LA-ICP-MS analyses on the syntectonic Anglès, Soulié and Martys granites yielded U–Th/Pb monazite ages of 305 ± 1.5, 306 ± 1.9 and 314 ± 2 Ma, respectively. Five migmatitic rocks sampled in the eastern and central Espinouse area yielded in situ ages ranging between 312 ± 2 and 301 ± 2 Ma. Along the dome envelope, two garnet–staurolite-bearing micaschists near Saint-Pons-de-Thomières village gave in situ U–Th–Pb ages of 312.1 ± 2.1 and 309.0 ± 3.1 Ma. A fine-grained gneiss with a D3 fabrics in the eastern dome envelope yield a 208Pb/232Th mean age at 305.7 ± 3.9 Ma. All ages obtained in this study for the micaschists, migmatites and granites range between 315 and 301 Ma. We interpret this time span as the record of the high thermal event responsible for intense crustal partial melting within the lower and middle crust. The onset of partial melting occurred at ca. 315 Ma that marked the beginning of transpressional deformation D2. Based on structural and petrological studies, our new U–Th–Pb results suggest that (1) partial melting may have started at ca. 315 Ma and lasted 15–10 Myr and (2) D2 et D3 developed between 315 and 300 Ma and were synchronous. D1 deformation ended at 315 Ma. The onset and duration of D1 related to nappe stacking and crustal thickening is still uncertain.
ISSN: 1437-3254

hal-01526942
https://hal.archives-ouvertes.fr/hal-01526942
DOI : 10.1007/s00531-016-1417-x

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : Upper Carboniferous heating and melting of the middle orogenic crust associated with the emplacement of syntectonic granitoids are documented in the Upper Gneissic Unit of the Livradois area (central part of the French Massif Central). Crustal melting post-dates peak metamorphism conditions (800-625 degrees C, 10-8 kb) dated at 360 +/- 4 Ma (U-Th-Pb on monazite). The P-T evolution of the metamorphic series indicates that Barrovian metamorphism was followed by a decompression (from 10 +/- 1 kbar to 6 +/- 1 kbar) associated with either a decrease in temperature in the southern part of the series or with an increase in temperature (of about 150 degrees C) in the northern part of the series. This evolution records the first step of the exhumation of the series coeval with granitoids intrusion, of which the emplacements were dated at 315 +/- 4 and 311 +/- 18 Ma (U-Pb on zircon). The final stage of the exhumation is associated with an isobaric cooling of the whole series. Similarity of Ar-40/Ar-39 ages for biotite in the paragneisses (307-300 Ma) and K-feldspar in the granitoids (306-300 Ma) document rapid cooling for this stage. Moreover dextral reverse mylonites, at the border and the northern part of the metamorphic series indicate north-south compression coeval with the unroofing of the series. Youngest Ar-40-Ar-39 ages on K-feldspar (274.6 +/- 5 Ma) combined with normal shearing in mylonites limiting the Carboniferous Brassac-les-Mines basin document the late Carboniferous-early Permian stage of extension coeval with the upwelling of the Velay granitic dome.
EISSN: 1777-5817

hal-00617429
https://hal.archives-ouvertes.fr/hal-00617429
DOI : 10.2113/gssgfbull.182.1.5