Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : Present Address : ISTO - UMR7327
International audience
The pre-rift Mesozoic sequences of the Cretaceous passive margins fossilized in the North Pyrenean Zone (NPZ) are characterized by high temperature deformation in relation with thinning of the continental basement. Our compilation of chronological and geological data confirms a clear correlation between the distribution of the highest paleotemperatures in the pre-rift sedimentary cover and the loci of extreme crustal stretching. Geological evidences such as the occurrence of peridotite bodies directly underlying metamorphic pre-rift sediments indicate an early attenuation of the rifted continental crust. This leads us to propose a mechanism of rifting involving boudinage of the continental crust. The lateral extraction of the Paleozoic basement occurred under the pre-rift cover that is decoupled on the Triassic clays and evaporates. The thermal conditions allowing coeval ductile deformation of the crust and of the pre-rift sediments leaded to the widening of basins devoid of large faulted blocks. We discuss the implications on the origin and significance of the granulites and the relations between flysch deposition and high temperature metamorphism of the pre-rift sediments. In the NPZ, Albian-Cenomanian flysch sequences were deposited synchronously with the syn-metamorphic ductile deformation of the pre-rift sequences. Since the base of the flysch deposits also recorded locally the high-temperature tectonic event, we propose an original mechanism for the evolution of the basins involving continuous deformation of the pre-rift metamorphic sediments. At the scale of the Pyrenean domain, our results suggest a strong lateral variability in the tectonic style of passive margins, in direct link with their thermic pattern
ISSN: 0278-7407

insu-01017432
https://hal-insu.archives-ouvertes.fr/insu-01017432
https://hal-insu.archives-ouvertes.fr/insu-01017432/document
https://hal-insu.archives-ouvertes.fr/insu-01017432/file/tect20164.pdf
DOI : 10.1002/2013TC003471

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study presents new geochemical data on 26 mafic lavas from the Middle Atlas and Central Morocco volcanic provinces, including Miocene nephelinites and Pliocene-Quaternary (3.9-0.6Ma) nephelinites, basanites, alkali and subalkaline basalts. Most of them represent near-primary magmas, although some alkali basalts were derived from the minor fractionation of olivine and diopside phenocrysts. These evolved samples and the subalkaline basalt display higher 207Pb/204Pb and Zr/Nb ratios and lower εNd consistent with their contamination by lower crustal granulites during an open fractionation process. The progressive enrichment in incompatible elements observed from alkali basalts to nephelinites suggests their derivation from decreasing partial melting degrees of an enriched mantle source located at the garnet-spinel transition zone. The strong negative spikes observed for K in multielement patterns indicate that this source contained a residual pargasitic amphibole. We propose that partial melting occurred at around 2GPa, i.e. near the lithosphere-asthenosphere boundary beneath the Middle Atlas (60-80km).The trace element and isotopic Sr-Nd-Pb-Hf signature of the uncontaminated lavas displays a geochemical flavour intermediate between those of high μ (HIMU), "C", and enriched mantle components. It is very similar to that of abundant metasomatic amphibole- and clinopyroxene-rich lithospheric peridotites and pyroxenites carried by Middle Atlas lavas, which likely represent an analog of the source of these lavas. It is therefore not necessary to postulate the contribution of a "fresh" asthenospheric mantle to their genesis. We propose that they resulted from the partial melting of the base of a veined lithospheric mantle metasomatised during the late Cretaceous by alkaline melts from the Central Atlantic plume, the ancestor of the Canary plume. Melting was probably triggered by the flux of a hot mantle within a regional SW-NE sub-lithospheric channel, in response to either the Alboran slab retreat or edge-driven convection around the West African craton.
ISSN: 0024-4937

insu-01056523
https://hal-insu.archives-ouvertes.fr/insu-01056523
DOI : 10.1016/j.lithos.2014.07.009

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
Highly restitic rocks from the Antananarivo Block in northern Madagascar are investigated in this study in order to unravel processes of H2O-rich biotite formation in HT rocks. Polyphase metamorphism and melt migration occurred at 0.6 GPa and 850 °C. Biotite remains stable together with orthopyroxene and makes up to 45 vol% of the rock. In addition, three well-characterised and delimited microdomains having different textural, chemical and petrological characteristics are preserved. Thermodynamic models using the specific bulk compositions of the domains are in agreement with petrological observations. These rocks provide evidence that the lower crust may be strongly heterogeneous, locally associated to the formation of hydrous restites controlled by episodes of melt production and melt escape. This has significant consequences for understanding of the lower crust.
ISSN: 1437-3254

hal-01356313
https://hal.archives-ouvertes.fr/hal-01356313
DOI : 10.1007/s00531-015-1265-0

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Hengshan–Wutaishan area forms part of the Trans-North-China Belt developed in response to the final amalgamation of the North China Craton. The Hengshan–Wutaishan domain and the adjacent Fuping massif constitute the largest and the most representative rock exposure of the belt. A structural study allows us to redefine the lithological and tectonic units that compose the architecture of the Hengshan–Wutaishan domain and to propose that the Trans-North-China Belt was built up through a polyphase tectonic evolution within the period 1900–1800 Ma. The first event (D1) corresponds to the emplacement of lower and upper nappes herein called the Orthogneiss and Volcanites Unit (OVU) and the Low Grade Mafic Unit (LGMU), respectively. The OVU mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the LGMU represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism. The syn-metamorphic D1 deformation is characterized by a NW–SE stretching and mineral lineation with a top-to-the SE sense of shear that corresponds to the sense of nappe motion. U–Th–Pb EPM chemical dating on monazite from three metapelites of the OVU gives isochron ages of 1884 ± 11 Ma, 1886 ± 5 Ma and 1887 ± 4 Ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the D1 event. The D2 event corresponds to a widespread crustal melting of the OVU and underlying basement rocks in Hengshan related to the exhumation of deep structures about 20–30 Ma after nappe emplacement. The migmatite contains blocks of retrograded eclogite or HP-granulitic restites, and is characterized by the development of a pervasive foliation and lineation. A late D2 event represents the folding stage of the migmatitic foliation during the completion of a dome. The Orthogneiss and Volcanics Unit and the Low Grade Mafic Unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the Hutuo Supergroup. These rocks were deformed by a ductile D3 event, characterized by a NW–SE trending stretching lineation, south verging folds associated with an axial planar slaty cleavage and an E–W crenulation lineation. Lastly, a left-lateral wrenching along the Zhujiafang Shear Zone separates the northern and southern parts of Hengshan. Our structural study shows that there is no significant difference between the southern part of Hengshan and the lower part of Wutaishan, both areas belong to OVU and show a structural continuity. We suggest that this polyphase deformation developed in response to a north-westward subduction of an old ocean, named the Lüliang Ocean, beneath the Western Block, followed by collision with a micro-continent named the Fuping Block at not, vert, similar1890–1880 Ma.
ISSN: 0301-9268

insu-00151594
https://hal-insu.archives-ouvertes.fr/insu-00151594
https://hal-insu.archives-ouvertes.fr/insu-00151594/document
https://hal-insu.archives-ouvertes.fr/insu-00151594/file/Trap-PrecambrainResearch-2007.pdf
DOI : 10.1016/j.precamres.2007.03.001

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The Ronda orogenic peridotite (southern Spain) contains a variety of pyroxene-rich rocks ranging from high-pressure garnet granulites and pyroxenites to low-pressure plagioclasespinel websterites. The 'asthenospherized' part of the Ronda peridotite contains abundant layered websterites ('group C' pyroxenites), without significant deformation, that occur as swarms of layers showing gradual modal transitions towards their host peridotites. Previous studies have suggested that these layered pyroxenites formed by the replacement of refractory spinel peridotites. Here, we present a major- and trace-element, and numerical modelling study of a layered outcrop of group C pyroxenite near the locality of Tolox aimed at constraining the origin of these pyroxenites after host peridotites by pervasive pyroxene-producing, refertilization meltrock reactions. Mg-number [=Mg/(Mg + Fe) cationic ratio] numerical modelling shows that decreasing Mg-number with increasing pyroxene proportion, characteristic of Ronda group C pyroxenites, can be accounted for by a melt-consuming reaction resulting in the formation of mildly evolved, relatively low Mg-number melts (similar to 0.65) provided that the melt fraction during reaction and the time-integrated melt/rock ratio are high enough (>0.1 and >1, respectively) to balance Mg-Fe buffering by peridotite minerals. This implies strong melt focusing caused by melt channelling in high-porosity domains resulting from compaction processes in a partial melted lithospheric domain below a solidus isotherm represented by the Ronda peridotite recrystallization front. The chondrite-normalized rare earth element (REE) patterns of group C whole-rocks and clinopyroxenes are convex-upward. Numerical modeling of REE variations in clinopyroxene produced by a pyroxene-forming, melt-consuming reaction results in curved trajectories in the (Ce/Nd)(N) vs (Sm/Yb)(N) diagram (where N indicates chondrite normalized). Based on (Ce/Nd)(N) values, two transient, enriched domains between the light REE (LREE)-depleted composition of the initial peridotite and that of the infiltrated melt may be distinguished in the reaction column: (1) a lower domain characterized by convex-upward REE patterns similar to those observed in Ronda group C pyroxeniteperidotite; (2) an upper domain characterized by melts with strongly LREE-enriched compositions. The latter are probably volatile-rich, small-volume melt fractions residual after the refertilization reactions that generated group C pyroxenites, which migrated throughout the massifincluding the unmelted lithospheric spinel-tectonite domain. The Ronda mantle domains affected by pyroxenite- and dunite- or harzburgite-forming reactions (the 'layered granular' subdomain and 'plagioclase-tectonite' domain) are on average more fertile than the residual, 'coarse granular' subdomain at the recrystallization front. This indicates that refertilization traces the moving boundaries of receding cooling of a thinned and partially melted subcontinental lithosphere. This refertilization process may be widespread during transient thinning and melting of depleted subcontinental lithospheric mantle above upwelling asthenospheric mantle.
ISSN: 0022-3530

hal-00412200
https://hal.archives-ouvertes.fr/hal-00412200
DOI : 10.1093/petrology/egn014