Palaeoproterozoic arc magnatism and collision in Liaodong Peninsula (north-east China).

In the north-eastern part of the North China Block, a afic magmatic belt consisting of mafic–ultramafic rocks and marine sedimentary rocks crops out between the northern Archean Anshan Block and a southern Palaeoproterozoic Block. 40Ar ⁄ 39Ar amphibole ages around 1.9 Ga from gabbros, and trace element analyses of gabbros, pyroxenite and shale show that these rocks formed along a Palaeoproterozoic active continental margin. The mafic magmatic belt is interpreted as an arc developed above a south-directed subduction zone, which was subsequently overthrust to the north upon the Anshan Archean Block. This study provides a new example agreeing with increasing evidence supporting plate mobility and thrust tectonics during the Palaeoproterozoic. These new insights must be considered with regard to the formation of the North China Block by magmatic accretion and tectonic collision

Contrasted tectonic styles for the Paleoproterozoic evolution of the North China Craton. Evidence for a ~2.1 Ga thermal and tectonic event in the Fuping Massif

International audienceStructural analysis along with 40Ar–39Ar and U–Pb datings in the Fuping massif provide new insight into the evolution of the eastern part of the Trans-North China Belt (North China Craton), from 2.7 Ga to 1.8 Ga. D1 is responsible for the development of a dome-and-basin structure coeval with crustal melting giving rise to migmatite and Nanying gneissic granites at 2.1 Ga. This dome-and-basin architecture resulted from the interference between a N–S compression of a weak ductile crust and gravity-driven vertical flow, in a high thermal regime. The next events involved flat lying ductile thrusting (D2) and normal faulting (D3) dated at around 1880 Ma and 1830 Ma, respectively. The D2 and D3 events belong to the Trans-North China Orogeny that results in the final amalgamation of the North China Craton. The D1 deformation is considered as evidence for an earlier orogen developed around 2.1 Ga prior to the Trans-North China Orogeny. The change in the deformation style between the 2.1 Ga and 1.8 Ga could be viewed as a consequence of the cooling of the continental crust in the North China Craton

Polyorogenic evolution of the Paleoproterozoic Trans-North China Belt, new insights from the in Lüliangshan-Hengshan-Wutaishan and Fuping massifs

http://www.episodes.org/backissues/302/abstract2.htmInternational audienceThe Trans-North China Belt (TNCB) is a Paleoproterozoic collisional orogen (ca. 1.9-1.8 Ga) responsible for the amalgamation of the North China Craton. Detail field works in Lüliangshan, Hengshan, Wutaishan and Fuping massifs where the belt is well exposed, allow us to draw new tectonic map and crustal-scale cross sections. The available petrologic, radiometric, geochronologic data are integrated in a geodynamic evolution scheme for this orogen. The Low Grade Mafic Unit (LGMU) is interpreted as an ophiolitic nappe rooted in a suture zone located in the western part of the Lüliangshan. This ophiolitic nappe overthrusts to the SE upon the Orthogneiss-Volcanites Unit (OVU) that consists of a bimodal volcanicsedimentary series metamorphosed under amphibolite facies conditions intruded by calcalkaline orthogneiss. The OVU is a composite Neoarchean-Paleoproterozoic magmatic arc developed during two stages (ca. 2500 and 2100 Ma) upon a continental basement corresponding to the western extension of the Neoarchean Fuping massif. The OVU overthrusts to the SE the Fuping massif along the Longquanguan shear zone. This stack of nappe, coeval with an amphibolite facies metamorphism, is dated at ca 1880 Ma. Subsequently, the metamorphic series experienced a widespread migmatization at 1850 Ma and was intruded by post-orogenic plutons dated at 1800 Ma. The weakly to unmetamorphosed Hutuo Supergroup unconformably overlies the metamorphosed and ductilely deformed units (OVU and LGMU), but it is also involved in a second tectonic phase developed in subsurface conditions. These structural features lead us to question the ca 2090 Ma age attributed to the Hutuo supergroup. Moreover, in the Fuping massif, several structural and magmatic lines of evidence argue for an earlier orogenic event at ca 2100 Ma that we relate to an older west-directed subduction below the Fuping Block. The Taihangshan Fault might be the location of a possible suture zone between the Fuping Block and an eastern one. A geodynamic model, at variance with previous ones, is proposed to account for the formation of the TNCB. In this scheme, three Archean continents, namely from West to East, the Ordos, Fuping and Eastern Blocks are separated by the Lüliang and Taihang Oceans. The closure of the Taihang Ocean at ca 2100 Ma by westward subduction below the Fuping Block accounts for the arc magmatism and the 2100 Ma orogeny. The second collision at 1900-1880 Ma between the Fuping and Ordos blocks is responsible for the main structural, metamorphic and magmatic features of the Trans-North China Belt

Early Cretaceous Surtseyan volcanoes of the Baño Nuevo Volcanic Complex (Aysén Basin, eastern central Patagonian Cordillera, Chile)

Tens of Surtseyan tuff cones are exposed in the Río Coichel valley, between Ñireguao and Estancia Baño Nuevo (Southern Chilean Andes). The Early Cretaceous products of the submarine eruptions rest on, or are interbedded with, shallow marine sandstones of the Hauterivian-early Aptian Apeleg Formation. The Early Cretaceous rocks typically contain large amphibole phenocrysts, clinopyroxene and plagioclase, and have compositions that range from relatively primitive basalts to andesites. The basalts are slightly enriched in LREE and incompatible trace elements. They have low Ti-contents and negative anomalies in Nb and Ta, characteristic of subduction-related magmas. Centimetre-sized amphibole megacrysts and an amphibole-bearing clinopyroxenite xenolith have been found in one of the tuff cones located near Estancia Baño Nuevo. The amphibole megacrysts are pargasite (low Si and Ti, high Mg). The xenolith has diopsidic clinopyroxene with abundant inclusions, and pargasite veins enclosing smaller clinopyroxene neoblasts. Geochemical data show that the megacrysts and the xenolith have the same origin and correspond to underplated igneous bodies crystallized at high pressure

Discovery of an albite gneiss from the Ile de Groix (Armorican Massif, France): geochemistry and LA-ICP-MS U-Pb geochronology of its Ordovician protolith

For the first time, an albite orthogneiss has been recognised and dated within the HP-LT blueschist facies metabasites and metapelites of the Ile de Groix. It is characterised by a peraluminous composition, high LILE, Th and U contents, MORB-like HREE abundances and moderate Nb and Y values. A U-Pb age of 480.8±4.8Ma was obtained by LA-ICP-MS dating of zircon and titanite. It is interpreted as the age of the magmatic emplacement during the Early Ordovician. Morphologically different zircon grains yield late Neoproterozoic ages of 546.6-647.4Ma. Zircon and titanite U-Pb ages indicate that the felsic magmatism from the Ile de Groix is contemporaneous with the acid, pre-orogenic magmatism widely recognised in the internal zones of the Variscan belt, related to the Cambro-Ordovician continental rifting. The magmatic protolith probably inherited a specific chemical composition from a combination of orogenic, back-arc and anorogenic signatures because of partial melting of the Cadomian basement during magma emplacement. Besides, the late Devonian U-Pb age of 366±33Ma obtained for titanite from a blueschist facies metapelite corresponds to the age of the HP-LT peak metamorphis

A polyorogenic model for the Paleoproterozoic Trans-North China Belt: Insights from an integrated structural, metamorphic and geochronological study.

In the North China Craton, the Paleoproterozoic Trans-North China Belt (TNCB) is a nearly north-south trending zone, of 1200 km long and 300 km wide, that separates two Archean blocks. Previous tectonic models assumed that the TNCB is the result of a 1850 Ma collision between the two Archean eastern and western blocks with an intervening 2500 Ma magmatic arc

Étude pétrographique et géochimique comparative du support en mortier de deux tables à marqueterie de pierres

Les tables à marqueterie de pierre ont été créées comme catalogues pour présenter les pierres marbrières disponibles sur le marché au xviie et xviiiesiècle. Notre étude porte sur deux tables dont le plateau est une marqueterie de pierres marbrières provenant très majoritairement de la région Languedoc-Roussillon. Dans les deux cas, la marqueterie de pierre est montée sur un support en mortier, technique de fabrication connue dans les ateliers français. Le plus célèbre de ces ateliers est l’atelier des Gobelins, ouvert entre 1645 et 1715, et dont on pense que la table de Versailles dite «table du Roi» est issue. Les résultats des observations et des analyses des mortiers des deux tables à notre disposition (observation macroscopique, microscope polarisant, microsonde électronique, diffraction X, éléments en traces par ICP-MS) montrent qu’il s’agit de mortiers dont l’agrégat est formé de tufs volcaniques importés de la province magmatique romaine et toscane en Italie, lié par de la phillipsite. La similitude extrême des résultats obtenus permet de conclure à la fabrication contemporaine et dans un même atelier des tables étudiées.Stones inlaid tables have been created to present the available marbles on the market at 17th or 18th centuries. Our study deals with two tables from which tops are inlaid of marbles coming in the majority of cases from Languedoc-Roussillon. The two tables show a mortar made support; a technique applied in French workshop the Atelier des Gobelins between 1645 and 1715. The table call “king table” localised to the Versailles castle was probably made in the Atelier des Gobelins. Results of mortars analysis show that the aggregate is made of volcanic tuffs imported from Roman and Toscan magmatic province, bound with Phillipsite. The extreme similarity of the results allows concluding that the two tables making have been contemporaneous and done in the same workshop