Zircon U-PB and LU-HF isotopic data from some peri-gondwana variscan terranes (Sardinia-Corsica block and calabropeloritan arc): new insights on the cenozoic geodynamic evolution of the central mediterranean basin

The reconstruction of pre-Variscan plates configuration in the Mediterranean area, is still poorly understood. Different hypotheses have been proposed to explain the relationships between peri-Tyrrhenian blocks such as Sardinia-Corse block (SCB) and Calabrian-Peloritain Arc (CPA), which before the opening of South Tyrrhenian Basin belonged to a branch of Southern Variscan Realm (Alvarez & Shimabukuro, 2009). Is still ambiguous if the CPA in the early time was formed by an amalgamation of two or more continental terranes that collided during Tertiary (Bonardi etal., 1980; Scandone, 1982) or was a single terrane during the middle and late Tertiary (Amodio-Morelli et al., 1976). With the aim to provide a contribution in unraveling the peri-Tyrrhenian area evolution of the peri-Gondwana terranes, amalgamated during Variscan and widespread during Cainozoic opening (and closing) of Tethys, both Variscan and Tertiary rocks cropping out in the two Blocks have been investigated, either magmatic or sedimentary. The first part of the research was focused on the magmatic bodies supposed belonging to the European Variscides with the aim to provide more constrains to the Variscan magmatic evolutionand so to draft the analogies between the present-day, separated blocks. In detail, the geochemical data from magmatic and detrital zircons of the basement rocks highlight analogies or differences between the tectono-sedimentary sectors of the Sardinian Variscides (nappe and foreland zones) and between them and the Calabria Variscides. The geochemical whole rocks analyses and U-Pb data obtained show that the Variscan basement which crops out in the Northern part of the CPA (Sila region) is characterized by the similar features of that in the SCB: the Middle Ordovician and Late Palaeozoic magmatic products recognized in the two blocks look similar. Some of these data can provide new insights also about the Ordovician evolution of the Northern Gondwana margin. To compare the and better constrain the relationships between the two blocks, detrital zircons were sampled in Tertiary rocks in NE Sardinia, E Corsica and Calabria, commonly regarded as closely related to the tectonic evolution of the Central-western Mediterranean. The analyses performed suggest some differences on the classical paleo-geographic positions of the SCB respect to the CPA and some assumptions can be made also about their relationships with other peri-Gondwana terranes. The association of U-Pb and Lu-Hf study performed on Tertiary detrital zircon populations is a good tool to furnish more data for recognize the pre-Variscan inputs suffered by the peri-Tyrrhenian area, and to highlight differences and similarities between them. The first consideration that can be made is that the two blocks suffered different crustal evolution. The main difference between them has given by the Grenvillian zircons input studied in the Tertiary deposits. The Southern part of CPA (Aspromonte and Peloritani) is characterized by the lacking of Mesoproterozoic age, whereas the Northern part of CPA (Sila) together with the SCB, are characterized by an important Grenvillian input. These considerations have a geodynamic implications about the misunderstood early evolution of the studied zones The Southern part of CPA shows strong similarities with Cadomiam terranes, which permit to correlate it with the evolution of Variscides now cropping out in North Africa. The SCB and the Northern part of CPA could be associated to the other European Variscan terranes (e.g. Iberia) characterized by widespread Grenvillian zircons. A supplementary assumption that stem out from these analyses is that the early position of SCB and Northern part of CPA, were possibly close to the Amazonian craton or nearness to the West African craton (Arabian-Nubian shield and the Sahara Metacraton), quite different from more of the proposed reconstructions. Finally, considering all these data in the complicated evolution of southern Tyrrhenian basin, here is supported the “two-terrane model” (slightly different from that proposed by Alvarez and Shimabukuro, 2009) which considers the CPA composed by a northern CPA terrane connected with the SCB from the end of the Variscan orogeny until the almost the Eocene. The southern CPA terrane suffered a different evolution before the opening of the Tyrrhenian Sea. According to the roll-back model of the westward subduction of Adria plate under Europe during Neogene-Pliocene the whole CPA, likely amalgamated in the early Miocene, was then tear apart from SCB and drifted away eastwards during the open of the South Tyrrhenian basin

San Diablo Front at Miraflores region, Sunsas Belt, Bolívia: tectonics and stratigraphics implications

O Front San Diablo trata-se de uma megaestrutura tectônica desenvolvida nas rochas do Terreno Paraguá e pela cobertura metassedimentar do Grupo Sunsás. Os dados geológicos e geocronológicos obtidos neste trabalho permitiram correlacionar os litotipos descritos na região estudada a três unidades litoestratigráficas: o Complexo Chiquitania, formado por biotita gnaisses de composição monzogranítica a granodiorítica; a Suíte Intrusiva Pensamiento, representada pelo Granito Colmena, um corpo intrusivo milonitizado de composição monzogranítica, localizado na zona de maior deformação do front; e o Grupo Sunsás, formado por micaxistos e metarenitos. Foram identificadas três fases de deformação: F1 gerou o bandamento gnáissico S1 com trend N-S nas rochas do Complexo Chiquitania; F2 promoveu a transposição do bandamento S1, e deu origem a dobras e à foliação S2 segundo o trend regional E-W; F3 foi marcada por dobras suaves e foliações espaçadas do tipo crenulação (S3) ortogonais ao trend geral das estruturas. Os dados geocronológicos U-Pb (SHRIMP) indicaram idade de cristalização em 1.685 ± 4,9 Ma e provável idade de metamorfismo em 1.111 ± 10 Ma para o gnaisse do Complexo Chiquitania; já os dados U-Pb (LA-ICP-MS) obtidos a partir de zircões detríticos sugerem idade máxima de sedimentação em torno de 1,2 Ga para o Grupo Sunsás, com fontes de idade paleo e mesoproterozoicas. Os dados estruturais caracterizam o Front San Diablo como uma larga zona de cisalhamento com direção NE e movimento direcional destral, gerada durante a Orogênese Sunsás em torno de 1,0 Ga, como indicam os dados K-Ar prévios. A partir dos dados apresentados, sugere-se que o Front San Diablo não se trata de um limite tectônico entre os terrenos alóctones Paraguá e San Pablo, mas sim de uma larga faixa de cisalhamento desenvolvida no Toniano, responsável pela justaposição por meio de movimentos direcionais de blocos crustais no interior do Terreno Paraguá.The Front San Diablo is a tectonic structure of the rocks in the Paraguá Terrane and the metasedimentary cover of the Sunsás Group. The geologic and geochronologic data obtained in this work defined three lithostratigraphic units: the Chiquitania Complex composed of biotite gneiss with monzogranitic to granodiorite composition; the Suite Pensamiento represented by the Colmena Granite, a mylonitic monzogranitic intrusive body mylonatized with monzogranitic composition, exposed in the major deformation zone; and the Sunsás Group, consisting of micaschist and quartzite. Three phases of deformation were identified: F1 generated the gneissic banding and S1 with a N-S trend in the Chiquitania Complex rocks; F2 resulted in the transposition of the S1 banding, and gave rise to folds, and S2 foliation parallel to the regional E-W trend; phase F3 is marked by open folds and spaced foliation of crenulation type (S3) orthogonal to the regional structures. The SHRIMP U-Pb geochronological data indicated the crystallization age of 1,685 ± 4.9 Ma for the Chiquitania complex gneiss; on the other hand, the LA-ICPMS U-Pb data obtained for detrital zircon grains suggest maximum age of sedimentation of ca.1.2 Ga for the deposition of the Sunsás Group. Paleo- and mesoproterozoic sources are also recognized. The structural data characterize the San Diablo Front as a dextral NE shear zone generated during the Sunsás Orogeny around 1.0 Ga, as indicated in the K-Ar previous data. Based on the data reported here, it becomes clear that the Front San Diablo does not represent a boundary between the Paraguá and San Pablo allochthonous terrane, but rather, a Neoproterozoic structure separating crustal blocks within the Paraguá Terrane.

Provenance of Ediacaran-Ordovician sediments of the Medio Armorican Domain, Brittany, West France: Constraints from U/Pb detrital zircon and Sm Nd isotope data

International audienceThe temporal evolution of the sedimentary source areas of the Armorican Massif, involving Ediacaran to Upper Ordovician strata, is investigated to gain insight into the palaeogeographic affinities and changes that occurred as a result of Cadomian orogenesis. Until now, palaeogeographic reconstructions based on geodynamic, stratigraphic and paleontological data have shown geological continuity between the Armorican Massif and the Iberian and Bohemian massifs and have allowed researchers to locate the Armorican Massif near the West African Craton and the Trans-Saharan Belt. This study goes beyond the interpretations based on lithostratigraphic correlation, which may be influenced by allocyclic factors (e.g., sea-level change) or fauna assemblages that have a wide provincial distribution, to provide a correct assessment of sediment flux. To determine the palaeogeographic location more accurately, the provenance of the siliciclastic sediments was examined in this study using Usingle bondPb LA-MC-ICP–MS geochronology on detrital zircons coupled with whole-rock Smsingle bondNd and zircon Lusingle bondHf isotope analysis. This work was carried out on the sedimentary succession of the Medio Armorican Domain. The oldest studied sedimentary rocks were shown to belong to the Brioverian succession, which contains mainly 519–781 Ma old zircons, likely derived from sources that are still present in the Armorican basement. Successively, the lower Paleozoic succession was deposited in the rift stages of the Rheic Ocean, with contributions from a new source of 827–1120 Ma old zircons.A comparison of the zircon populations showed an increase in negative εNd(t) and εHf(t) values of the sedimentary supply in the post-Cadomian samples. Moreover, it revealed that the Medio and North Armorican domains had different locations during the Lower Ordovician, and that some areas of the Iberian Massif and the Medio Armorican Domain close to the Sahara Metacraton and Arabian-Nubian Shield were contiguous