Search Results (6)

The Late Paleozoic strata on the northwestern margin of the Tarim Block provide valuable insights into the subduction and collision processes that formed the Southwest Tianshan Orogenic Belt. This study integrates detrital zircon U-Pb dating and sandstone geochemical analysis of the Balikelike and Kalundaer formations to examine sedimentary provenance and tectonic settings during the Cisuralian–Guadalupian Epoch in the Keping area on the northwestern margin of the Tarim Block. Three of five Precambrian detrital zircon U-Pb age populations, 2500–2300 and 2000–1800 Ma and 900–600 Ma, are likely related to the fragmentation of the Columbia supercontinent and Rodinia’s assembly, respectively. Two Paleozoic detrital zircons, 500–380 Ma, are associated with Paleozoic magmatism. Among them, ~295 Ma zircons are associated with post-collisional extension and emplacement of the Tarim Large Igneous Province. Geochemical analysis of sandstones, coupled with tectonic reconstruction, indicates a passive continental margin setting in the northwestern margin of the Tarim Block during the Silurian Period, later transitioned to a foreland basin from the Pennsylvanian to the Guadalupian Epochs. The crustal transformation from the Middle-late Devonian to Early Mississippian marked the closure of the South Tianshan Ocean (STO), involving a soft collision and significant uplift, with major orogenesis occurring in the Late Guadalupian. Five key stages are identified in the evolution of the foreland basin: (1) Middle-late Devonian to Early Mississippian initiation (remnant ocean basin stage); (2) Late Mississippian to Early Pennsylvanian early stage; (3) Late Pennsylvanian to Early Cisuralian middle stage; (4) the Late Cisuralian stage; and (5) the terminal Guadalupian stage. These findings provide new constraints on when STO closed and propose an innovative foreland basin evolution model during the late post-collisional phase from the Late Mississippian to Guadalupian. Collectively, the data advance our understanding of the tectonic processes that shaped the northwestern Tarim Block, with broader implications for Paleozoic geodynamics. Full article

The magmatic rocks of the Ben Giang-Que Son complex exposed in the southern part of the Truong Son belt have petrographic compositions including gabbro, gabbrodiorite, granodiorite, and granite. Geochemically, the magmatic rocks are of subalkaline affinity and belong to the high-K calc-alkaline series. All analyzed samples contain hornblende and biotite as mafic minerals and are defined as metaluminous with a aluminum saturation index of <1.1. Chondrite-normalized rare earth element diagrams are characterized by fractionation between light and heavy rare earth elements and show small to moderate negative Eu anomalies (Eu/Eu* = 0.81–0.44). Primitive mantle-normalized trace element patterns exhibit enrichment in LILEs such as Rb, K, U, and especially Pb and depletion of HFSEs (Nb, Ta, and Ti), indicating arc-magma. The BG-QS gabbro and gabbrodiorite have low Cr, Co, and Ni and display enrichment in ⁸⁷Sr/⁸⁶Sr (0.7084 to 0.7147), Ꜫ_Nd(t) (−0.5 to −1.9), and positive Ꜫ_Hf(t) (+2.4 to 4.5), suggesting generation from the enriched mantle source. The BG-QS diorite and granodiorite contain small mafic enclaves, have a wide range of SiO₂ contents and enrichment in Sr-Nd isotopes (⁸⁷Sr/⁸⁶Sr = 0.7109–0.7178; Ꜫ_Nd(t) = −3.3 to −3.7), and display high Mg# (43–51). All these features indicate that they were formed by the mixing of magmas, which originated from an enriched mantle source and the pre-existing juvenile mafic lower crust. The whole-rock Nd and zircon Hf model ages are of 1160–760 Ma. The Ben Giang-Que Son complex yields LA-ICP-MS zircon U-Pb ages of 285 ± 3.1 and 278 ± 3.5 Ma that corresponds to the Cisuralian epoch (early Permian), which is linked to the subduction and amalgamation of the Indochina and South China blocks due to the closing a branch of the Paleotethys along the Song Ma suture. Full article

Calamitaceae was the predominant arborescent element of the Late Paleozoic plant assemblages. However, there is currently controversy surrounding the evolutionary relationships of their four reproductive organs, and there is no unified understanding of the geographical distribution and species richness characteristics. This paper is based on the detailed description of the macro- and microstructures of Calamites and Macrostachya discovered in the Shanxi Formation of the Cisuralian in the eastern Hexi Corridor of northwestern China, and it discusses the evolutionary patterns of calamitean reproductive organs. The results indicate that the current specimens can be identified as the Calamites cistii and Macrostachya sp., and the in situ spores should exhibit distinct trilete marks, indicating that they belong to the Punctatisporites punctatus. The abundant fossil records suggest that the “Calamostachys” type should be considered as the ancestral type of strobili. Additionally, Calamitaceae was widely distributed in mid to low latitudes globally from the Pennsylvanian to the Cisuralian and coupled with paleogeographic distribution characteristics. Full article

As the world’s largest accretionary orogen, the Central Asian Orogenic Belt (CAOB) underwent continuous juvenile crustal growth in the Phanerozoic. The northern margin of the North China Craton (NCC) and its adjacent area form the eastern segment of the CAOB, which is a key area for learning about the geological evolution of the Paleo-Asian Ocean (PAO). In the Permian, the west of the northern margin of the NCC was a post-collision extensional environment, while the east was in a subduction stage. As a connecting area, the Permian evolution of the PAO in the middle of the northern margin of the NCC has not been systematically studied. In order to fill the gap and understand the continuous temporal and spatial evolutionary process of the PAO, this paper focuses on the Permian granitic rocks in the Chifeng area. Zircon U-Pb dating and the geochemical analysis of whole-rock major and trace elements were conducted to build a granite chronological framework, and to discuss the genesis and tectonic background of the granitic rocks, along with tectono-magmatic evolutionary history in the Chifeng area. The respective LA-ICP-MS zircon U-Pb dating results from eight samples are 269 ± 1, 268 ± 3, 260 ± 4, 260 ± 1, 260 ± 1, 255 ± 2, 254 ± 2 and 256 ± 1 Ma, respectively. These results, combined with previous data, revealed that the Permian granitic rocks had undergone three events of magmatism: (1) monzogranitic-syenitic phase (294–284 Ma; Cisuralian); (2) monzogranitic phase (269–260 Ma; Guadalupian) and (3) late monzogranitic-syenitic phase (256–254 Ma; Lopingian). From the Early Permian (294–284 Ma) to the Middle Permian (269–260 Ma), granites with fine-medium-grained locally porphyritic texture and massive structure showed a high-potassium calc-alkaline series formed in a compressional setting, indicating a continuous collision between the Xing’an-Mongolian Orogenic Belt (XMOB) and the NCC. During the Late Permian-Early Triassic (256–248 Ma), granites with massive structure and medium-grained texture in the Chifeng area were magmatism dominated by A- and I-type granites of high-potassium calc-alkaline series, combined with the coeval basic rocks, which constituted a typical “bimodal” rock assemblage. This suggests that the Chifeng area was located in an extensional setting where the subducting slab broke off during the collision between the XMOB and NCC. These granitic plutons from the Permian are believed to have been generated by the subduction-collision of the Paleo-Asian oceanic crust beneath the NCC, according to emplacement time and occurrence location. Our findings provide strong evidence for Permian continuous temporal and spatial tectonic evolution and the characterization of the eventual closure of the PAO in Chifeng area at the northern margin of the NCC. Full article

An assessment performed using raman spectroscopy has found space in the black shales of the Cisuralian-age rocks of the Karoo Basin in South Africa, particularly those from the Guadalupian Ripon, Cisuralian Whitehill and Prince Albert Formations. It is used in conjunction with geochemical screening techniques such as organic petrology and programmed pyrolysis. In turn, the combination of these techniques is used for the assessment of the thermal maturity of the sedimentary organic matter from the perspective of hydrocarbon generation, retention, and expulsion. To provide further understanding of the black shales in the Cisuralian-age rocks of the Karoo Basin in South Africa, this study focuses on the characterization of samples from the KWV−01 borehole drilled in the southeastern Karoo Basin. In addition, the USA Devonian/Carboniferous Berea Sandstone project samples were included for comparison, and were used as a quality assurance measure. Organic petrology was utilized to assess the organic quality and thermal maturity of the black shales. The results obtained showed that the Karoo Basin shales are overmature, containing an abundance of solid bitumen, and this often characterizes a shale reservoir with moveable hydrocarbons (shale gas). The programmed pyrolysis analysis conducted on the black shales of the Karoo Basin yielded artifact results, as they were determined from a very low and poorly defined S2 peak. This indicated the shales to be overmature and categorized them to be of poor hydrocarbon generation potential. Raman spectroscopy was used to gain insights about the molecular structure of the black shales and to assess if this technique could be used as a complimentary tool to determine the thermal maturity of the shale samples. Raman parameters such as G–D1 Band separation, G and D1 band full width at half maximum (G_fwhm and D1_fwhm) and G band position were successfully correlated with vitrinite reflectance (R_oV), demonstrating a good potential for Raman spectroscopy to predict the thermal maturity of the shales. Overall, the study provides valuable information and knowledge concerning black shale sample characterization (particularly the thermal maturity and molecular structural characterization) in the Karoo Basin, South Africa. Full article

The Ojén nappe, an allochthonous unit that underlies the Ronda peridotites (Betic Cordilleras), is composed of Triassic marbles overlying a metapelitic sequence. In order to carry out an LA-ICP-MS U-Pb age determination and provenance analysis, detrital zircon grains from two quartzites interlayered within marbles and metapelites were extracted. The obtained results yield the youngest zircon population of 254 ± 3 Ma (Late Permian) with a mean Th/U ratio of 0.34 that supports both the Permian–Triassic age of the marble member and the felsic magma nature for zircon-bearing protoliths. The Permian zircons exhibit tight age peaks in three main populations of 292 ± 2 Ma, 278 ± 3 Ma, and 254 ± 3 Ma, in agreement with the main age populations reported from rhyolites and shallow crustal basaltic–andesite subalkaline rocks emplaced in transtensional Permian basins of the Variscan Orogen during the break-up of Pangea. Moreover, the analyzed detrital zircon grains yield age distributions with clear Tonian–Stenian (ca. 950–975 Ma), Ediacaran–Cryogenian (ca. 560–615 Ma), Ordovician (ca. 460–465 Ma), and Cisuralian peaks (ca. 280–290 Ma) and smoothed Mesoproterozoic peaks that point to a weak and residual Cadomian peri-Gondwanan terrane inheritance. The identified Middle Ordovician detrital zircon populations (460–465 Ma) strengthen the hypothesis that the Alborán domain would be located along the southern passive margin of the European Hunic superterrane. Full article