SHRIMP U–Pb zircon dating of anatexis in high-grade migmatite complexes of Central Spain: implications in the Hercynian evolution of Central Iberia

U–Pb SHRIMP ages obtained in zircons from the Sotosalbos and Toledo anatectic complexes in Central Spain give new constraints to the evolution of the inner part of the Hercynian Iberian belt. Pre-Hercynian ages in zircons from the Sotosalbos complex (∼464 Ma) are well preserved and reveal that an age diversity of the Lower Paleozoic magmatism in the area exists, as previous data on westernmost orthogneisses yield significant older ages. Zircon ages in the pelite-derived granites from the Toledo complex also show an important Neoproterozoic age component which points to a metasedimentary protolith deposited maximally 560 Ma ago. Younger zircon populations in both complexes at ∼330 Ma in the Sotosalbos region and ∼317 Ma in the Toledo complex indicate an important diachronism between the anatectic processes in both areas but also that these processes are mainly unrelated to the generation of the later Hercynian granite batholith of Central Spain, which could be of deeper crustal derivation. In addition, as migmatization occurred late in the metamorphic cycle, after peak conditions were attained, the age of anatexis is younger than the age of the main Hercynian metamorphic event, which still is not well constrained. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sr isotope excursion across the Precambrian–Cambrian boundary in the Three Gorges area, South China

The Precambrian/Cambrian (PC/C) boundary is one of the most important intervals for the evolution of life. However, the scarcity of well-preserved outcrops across the boundary leaves an obstacle in decoding surface environmental changes and patterns of biological evolution.In south China, strata through the PC/C boundary are almost continuously exposed and contain many fossils, suitable for study of environmental and biological change across the PC/C boundary. We undertook deep drilling at four sites in the Three Gorges area to obtain continuous and fresh samples without surface alteration and oxidation. ⁸⁷Sr/⁸⁶Sr ratios of the fresh carbonate rocks, selected based on microscopic observation and geochemical signatures of Mn/Sr and Rb/Sr ratios, aluminum and silica contents, and δ¹³C and δ¹⁸O values, were measured with multiple collector-inductively coupled plasma–mass spectrometric techniques.The chemostratigraphy of ⁸⁷Sr/⁸⁶Sr ratios of the drilled samples displays a smooth curve and a large positive anomaly just below the PC/C boundary between the upper part of Baimatuo Member of the Dengying Formation and the lower part of the Yanjiahe Formation. The combination of chemostratigraphies of δ¹³C and ⁸⁷Sr/⁸⁶Sr indicates that the ⁸⁷Sr/⁸⁶Sr excursions preceded the δ¹³C negative excursion, and suggests that global regression or formation of the Gondwana supercontinent, possibly together with a high atmospheric pCO₂, caused biological depression.     

Cambrian Burgess Shale-type Lagerstätten in South China: Distribution and significance

The final assembly of the supercontinent Gondwana during the Pan-African orogenic episodes (ca. 550–520 Ma) almost simultaneously took place with the Cambrian explosion that is best manifested by a number of Cambrian Burgess Shale-type Lagerstätten in South China. The relationship between South China and Gondwana during the Cambrian is far from consensus. Burgess Shale-type Lagerstätten may have potential importance for the paleogeographic reconstruction. However, such Lagerstätten have been known in large number only in Laurentia and South China, far less common in Gondwana and other continents. Burgess Shale-type Lagerstätten in South China are not evenly spaced through the Cambrian. They appear to be concentrated in the Lower Cambrian, particularly in the Canglangpuian and Qiongzhusian stages, much reduced in number from the uppermost Lower Cambrian. Of ten reported such Lagerstätten, only the Kaili biota (basal Middle Cambrian) is known to be younger than Early Cambrian. This reduction could be explained by the fact that vast areas of siliciclastic facies in both the western plate interior (platform) and the eastern slope basin during most time of Early Cambrian (Meishucunian to Canglangpuian) is evolved into carbonate facies at the very end of Early Cambrian (Longwangmiaoian). It has been known from this study that both siliciclastic platform facies and slope basin facies (shale basin) could preserve soft-bodied fossils. Cambrian Burgess Shale-type Lagerstätten in South China are of great significance for providing a sequences of exceptionally preserved biota in a chronological succession. Comparison of such Lagerstätten in a chronological framework may give us more details on the Cambrian explosion events.     

Tasmante cruise: Swath-mapping and underway geophysics south and west of Tasmania

The 1994 Tasmante swath-mapping and reflection seismic cruise covered 200 000 km² of sea floor south and west of Tasmania. The survey provided a wealth of morphological, structural and sedimentological information, in an area of critical importance in reconstructing the break-up of East Gondwana.The west Tasmanian margin consists of a non-depositional continental shelf less than 50 km wide and a sedimented continental slope about 100 km wide. The adjacent 20 km of abyssal plain to the west is heavily sedimented, and beyond that is lightly sedimented Eocene oceanic crust formed as Australia and Antarctica separated. The swath data revealed systems of 100 m-deep downslope canyons and large lower-slope fault-blocks, striking 320° and dipping landward. These continental blocks lie adjacent to the continent ocean boundary (COB) and are up to 2500 m high and have 15°–20° scarps.The South Tasman Rise (STR) is bounded to the west by the Tasman Fracture Zone extending south to Antarctica. Adjacent to the STR, the fracture zone is represented by a scarp up to 2000 m high with slopes of 15–20°. The scarp consists of continental faultblocks dipping landward. Beyond the scarp to the west is a string of sheared parallel highs, and beyond that is lightly sedimented Oligocene oceanic crust 4200–4600 m deep with distinct E-W spreading fabric. The eastern margin of the bathymetric STR trends about 320° and is structurally controlled. The depression between it and the continental East Tasman Plateau (ETP) is heavily sedimented; its western part is underlain by thinned continental crust and its central part by oceanic crust of Late Cretaceous to Early Tertiary age. The southern margin of the STR is formed by N-S transform faults and south-dipping normal faults.The STR is cut into two major terrains by a N-S fracture zone at 146°15E. The western terrain is characterised by rotated basement blocks and intervening basins mostly trending 270°–290°. The eastern terrain is characterised by basement blocks and intervening strike-slip basins trending 300°–340°. Recent dredging of basement rocks suggests that the western terrain has Antarctic affinities, whereas the eastern terrain has Tasmanian affinities.Stretching and slow spreading between Australia and Antarctica was in a NW direction from 130–45 Ma, and fast spreading was in a N-S direction thereafter. The western STR terrain was attached to Antarctica during the early movement, and moved down the west coast of Tasmania along a 320° shear zone, forming the landward-dipping continental blocks along the present COB. The eastern terrain either moved with the western terrain, or was welded to it along the 146°15 E fracture zone in the Early Tertiary. At 45 Ma, fast spreading started in a N-S direction, and after some probable movement along the 146°15E fracture zone, the west and east STR terrains were welded together and became part of Australia.     

华南地块的地极移动曲线及其地质意义

结合最近获得的古地磁数据,作者重新计算编制了华南地块的地极移动曲线,进而讨论了华南和华北地块在早古生代的地理位置,二者最后拼合的时代,以及华南地块和欧亚大陆北部两条极移曲线之间的系统偏差等问题。     

藏北羌多——查布地区下二叠统划分与早二叠世动物群特征

下二叠统分角木茶卡组和肖茶卡组。角木茶卡组为碳酸盐岩夹玄武岩,局部夹含砾板岩,厚1000-2500m。含(蜒)类组合Monodiexodina-Parafusulina,珊瑚组合 Lytvolasma Szechuanophyllum,腕足类组合Costiferina-Neospirifer-Spiriferella等。属冷、暖动物群混生类型。肖茶卡组为碳酸盐夹玄武岩,800-2900m。含(蜒)Neoschwagerina,珊瑚组合Iranophyllum-Waagenophyllum,腕足类组合Neospirifer-Spiriferella-Marginifera等,属暖水型动物群。这些反映了早二叠世时冈瓦纳大陆北缘浅海台地深拗陷带碳酸盐和火山岩的沉积特征。     

Shear zone evolution and timing of deformation in the Neoproterozoic transpressional Dom Feliciano Belt,Uruguay

New structural, microstructural and geochronological (U-Pb LA-ICP-MS, Ar/Ar, K-Ar, Rb-Sr) data were obtained for the Dom Feliciano Belt in Uruguay. The main phase of crustal shortening, metamorphism and associated exhumation is recorded between 630 and 600 Ma. This stage is related to the collision of the Río de la Plata and Congo cratons at ca. 630 Ma, which also involved crustal reworking of minor crustal blocks such as the Nico Pérez Terrane and voluminous post-collisional magmatism. Subsequent orogen-parallel sinistral shearing gave rise to further deformation up to ca. 584 Ma and resulted from the onset of the convergence of the Kalahari Craton and the Río de la Plata-Congo cratons. Sinistral shear zones underwent progressive strain localization and retrograde conditions of deformation during crustal exhumation. Dextral ENE-striking shear zones were subsequently active at ca. 550 Ma, coeval with further sinistral shearing along N- to NNE-striking shear zones. The tectonothermal evolution of the Dom Feliciano Belt thus recorded the collision of the Río de la Plata and Congo cratons, which comprised one of the first amalgamated nuclei of Gondwana, and the subsequent incorporation of the Kalahari Craton into Western Gondwana.