Acta Geochimica最新文献

An enhanced understanding of the history of the western Qinling-Dabie orogen is pivotal in reconstructing geological processes of the east Asian mainland. However, less attention has been paid to its early-stage uplift-erosion history after closure of surrounding oceanic basins at the mid-Paleozoic. In this study, we undertook a comprehensive study including paleocurrent reconstruction, sandstone petrology, and detrital zircon U–Pb dating on Late Carboniferous to Early Permian successions in the southern Ordos neighboring the northern Qinling-Dabie. New provenance data reveal a significant provenance shift at the Carboniferous-Permian transition. The older Benxi Formation was sourced southerly from the North Qinling Terrane that provided detritus mostly of Neoproterozoic and Early Paleozoic ages. In contrast, Early Permian samples yield age relation dominated by Neoarchean, Paleoproterozoic, Early Paleozoic, and Late Paleozoic age populations, with a significant gap of ca. 1600–550 Ma, implying a sediment derivation from the Inner Mongolia Continental Arc. This shift is further verified by paleocurrent transition from south to north then. We suggest that the North Qinling Terrane experienced a significant uplift history from ca. 500 Ma and remained as a highland until end-Carboniferous. From Early Permian, the North Qinling Terrane was submerged, covered by widespread deltaic sedimentation there. Northerly source from the Inner Mongolia Continental Arc began to be accumulated in the northern flank of the North Qinling Terrane, before termination approximately along the southern North Qinling Terrane, where shallow-water carbonate shelf sedimentation sustained from Devonian to Triassic. This new finding indicates that uplift of the North Qinling Terrane lasted about 150 Ma after the Proto-Tethys Ocean closure.

Elemental concentrations of the siliciclastic sediments from a sedimentary basin provide clues on paleo-weathering, paleoclimate, provenance, and tectonic setting of the basin. Records for Permo–Triassic mass extinction and climatic fluctuations are commonly traced from the sediments in the Gondwana basins. Nevertheless, our understanding on sedimentation, provenance, and regional tectonics of the Raniganj Basin, a Gondwana basin in the eastern India is poor. Minerals including clay particles and major and trace element concentrations of the siliciclastic sediments from different formations of the Raniganj Basin have been studied to establish the paleo-weathering, paleoclimate, provenance, and tectonic settings of the basin. This study suggests that the Talchir Formation experienced cold and dry climatic conditions at the sediment source area, while the Barakar, Raniganj, and Panchet formations had prevailing semiarid climates. The sources of the siliciclastic sediments are from the felsic rocks of the Chotanagpur Granite Gneissic Complex (CGGC). Further, the geochemical results suggest a rift-like (passive) tectonic setting for the Raniganj Basin, while few samples represent the collision tectonic setting of the basement CGGC, formed due to collision of major Indian blocks during the Paleo-Neoproterozoic time.

The majority (up to 90%) of riverine materials is transported from the continent to the ocean mainly in flood events. It is thus crucial to characterize the geochemistry of elements and their flux in river system in order to better constrain their global biogeochemical cycling and impact on the oceanic ecosystem. However, the geochemical behavior including the distribution, migration and partitioning of typical metal elements amongst different phases, during hydrodynamic flood event remains still to be well explored. Here, we investigated the geochemical behaviors of typical metal elements in dissolved phase and suspended particulate matter collected from a single flood event in the natural Chishui River, Southwest China. The results showed clearly that the geochemistry of metal elements was largely controlled by the hydrodynamic effect, of which the different flowrates introduce a natural sorting of different mineral particles transported at different flood stages, depending on their shape, size and density. The maximum concentrations of alkaline and alkaline earth metals (Li, Mg, K, Rb and Sr) in SPM appeared before the flood peak, which was largely controlled by aluminosilicate minerals. However, transition metals (Cr, Mn, Fe, Ni and Cu) showed their abundance peaks lagging behind the flowrate summit, as a result of the late arrival of coarse particles or heavy minerals, evidenced by the mineralogical phase analysis. In addition, the distribution coefficient (K_d) between particulate and dissolved loads were lower and stable for soluble alkali/alkaline earth metals which could be affected by pH, while higher and fluctuant for transition metals that were largely influenced by SPM content. Overall, the present study reveals clear effects of hydrodynamic sorting on the geochemistry of metal elements during the flood event of the natural Chishui River, which should be taken into account when characterizing the riverine flux and their impact on marine ecosystem.

NWA 16080 is a representative reduced CV carbonaceous chondrite (CV_red), consisting mainly of chondrules (47 vol%) and matrix (42 vol%), along with minor quantities of calcium- and aluminum-rich inclusions (CAI) and amoeboid olivine aggregates (AOA) (CAI + AOA, 6 vol%) and opaque minerals (5 vol%). The chondrules exhibit well-preserved outlines and can be categorized into Type I (Fa < 10) and Type II (Fa > 10). They primarily consist of magnesium-rich olivine, along with both low-Ca and high-Ca pyroxenes, and contain minor amounts of secondary plagioclase. Olivines present in chondrules display compositional zoning characterized whereas the matrix is composed of fine-grained olivine. Nickel-rich metal and nickel-poor sulfides are also present, along with trace amounts of magnetite. In contrast to standard oxidized CV chondrites (CV_ox), the presence of high metal, Ni-poor sulfides, and reduced magnetite in NWA 16080 indicates a more reduced parent-body environment. Shock metamorphism is classified as mild (S1), while terrestrial weathering is characterized as low (W2). Raman spectroscopy indicates a diverse spectrum of organic matter (OM) maturity: certain areas exhibit characteristics akin to other CV_red chondrites, whereas others reach maturity levels comparable to those observed in CV_ox chondrites. The Raman parameters indicate that this meteorite is classified as approximately type 3.4 to 3.5. The overlapping OM maturity with certain CV_ox chondrites provides a contradiction to the anticipated depth-thermal layering outlined in the onion-shell model. This suggests that the CV parent body probably experienced more intricate processes, including impacts and fluid-rock interactions, rather than merely depth-dependent heating.

Subduction polarity reversal typically occurs in intra-oceanic arc settings; the existence of an ancient intra-oceanic arc and its associated back-arc system within the Neotethyan plate has been deliberated. In this study, we investigate the possible role of subduction initiation of polarity reversal in the formation of Nagaland-Manipur ophiolite (NMO), evaluate the petrological and geochronological data and compare it with the neighboring natural examples of subduction polarity reversal of the Andaman-Nicobar ophiolite (ANO). The ancient intra-oceanic arc, namely the Incertus-Woyla Arc, and its associated back-arc remnant have been correlated with the back-arc mafic of the ANO. We found that the geochemical signatures of mafic rocks of NMO and ANO are comparable, and the available geochronology data of ~ 145 Ma from the NMO basalt and chert fit well with the evolution and formation of the intra-oceanic arc, i.e., Incertus-Woyla Arc. The evolution and age of the Incertus-Woyla Arc are between 135 and 150 Ma. Although the oldest age of the ANO has been reported from metamorphic sole at about 106.4 and 105.3 Ma, the back-arc affinity of the amphibole has been credited to the back-arc spreading that occurred behind the Woyla Arc. Previous paleomagnetic and geochronological studies have suggested that the development of the back-arc basin behind the Incertus-Woyla Arc was a result of divergent double subduction. Therefore, we have inferred a similar scenario for the development of the back-arc affinity rocks of the NMO behind the Incertus-Woyla Arc and the reinterpretation for the evolution of the supra-subduction zone affinity rocks of NMO and ANO during subduction initiation after subduction polarity reversal.

A comprehensive examination of detrital sandstone modes from the Sylhet Trough reveals a diverse range of sub-lithic to sub-feldspathic quartz arenites. Soil samples were gathered from Dupi Gaon (Jaintiapur) in Bangladesh, followed by a thorough analysis using field examination, X-ray diffraction (XRD), X-ray fluorescence (XRF), petrography, and heavy mineral concentration analyses. Field observations revealed the soil sample varying from white to yellowish to variegated, with thicknesses ranging from 15 cm to about 4 m, and exhibiting moderate softness. XRF analysis revealed significant SiO₂, Al₂O₃, and Fe₂O₃ levels in the samples, with zirconium (Zr) and copper (Cu) showing consistently high concentrations. XRD analysis identified quartz as predominant, along with muscovite, biotite, and accessory minerals like rutile and magnetite. Petrographic analysis highlighted quartz as dominant, with fractures suggesting tectonic influences, while heavy mineral separation techniques identified zircon, garnet, goethite, rutile, and magnetite. These findings provide insights into sediment provenance, depositional processes, and environmental conditions during the formation of the Dupi Tila Formation. The comprehensive geochemical data of the entire rock indicates that most of the sediments originated from felsic igneous sources, and also suggests a moderate to high level of weathering in the source region. Overall, the analyses suggested an in situ origin of the Dupi Tila Formation, with parent materials being predominantly detrital rather than authigenic, supported by the presence of detrital quartz and an assessment of the depositional environment, providing insights into the geological conditions of the era and potential modes of sediment transportation.

The Bainiuchang Ag-polymetallic ore deposit, located in southeastern Yunnan, China, is one of the region’s largest deposits. However, the hypabyssal granite porphyry within this mining area has yet to be comprehensively investigated. In this study, we conducted geochemical, geochronological, whole-rock Sr–Nd isotope, and zircon Hf isotope analyses on granite porphyry samples collected from the Bainiuchang deposit. The results indicate that the granite porphyry formed between 87.5 and 87.4 Ma in the Late Yanshanian period. Geochemically, the granite is strongly peraluminous, with high silica and alkali contents consistent with S-type granite characteristics. The granite porphyry is enriched in large-ion lithophile elements (Rb, Th, U, and K) and is relatively depleted in Ba and Sr. The initial ⁸⁷Sr/⁸⁶Sr ratios are high (0.71392–0.71585), accompanied by low ε_Nd(t) values (− 8.9 to − 8.2). The zircons exhibited similarly low ε_Hf(t) values (− 9.31 to − 3.6). These data suggest that the porphyry-forming magma originated from a continental crustal source. The two-stage Hf and Nd model ages are estimated at 1534–1216 Ma and 1615–1561 Ma, respectively. Thus, the granite porphyry likely formed under a strike-slip extensional setting in the Late Yanshanian period and resulted from the re-melting of Proterozoic basement metagreywackes. This porphyry shares a similar magmatic origin with concealed granite bodies within the deposit and is associated with structural reactivation during the Yanshanian. The findings of this study provide valuable insights into the tectonomagmatic mineralization processes in the Bainiuchang area.

The sedimentary geochemistry of St. Martin’s Island is important to determine the origin of the source rock, paleo weathering, tectonic setting, sediment recycling, maturity, sorting, redox condition, and paleo salinity of the sediments. Major oxides, trace elements, and rare earth elements (REEs) obtained from the INAA technique are presented by analyzing the sediment samples collected from the shoreline of St. Martin’s Island, Bangladesh. The elemental ratios, comparison with average upper continental crust (UCC), binary diagrams (Th/Sc vs. Sc, La/Th vs. Hf, Th/Co vs. La/Sc), and chondrite normalized REE patterns exhibit substantial LREE enrichment, relatively flat HREE fractionation, considerable negative Eu anomalies (average: 0.72), indicates the derivation from a source dominated by felsic rock, with contribution from intermediate source and mafic component. Sediments from St. Martin’s Island exhibit the deposition of sediments in transitional environments of active and passive continental margin settings. Weathering indices value of CIA, PIA, CIW, CIX, and K₂O/Rb ratio show moderate chemical weathering, indicating that the sediments are chemically mature. Sedimentary redox indicative proxies, such as U/Th, V/Cr, and V/Sc, show an oxic depositional environment during sediment deposition. The intermediate CIA and other weathering index values of the St. Martin’s sediments show that the area had semiarid and humid climatic conditions throughout the deposition. The Rb/K ratio of the St. Martin’s sediments suggests that the development and deposition of the sedimentary sequence of St. Martin’s Island mainly occurred in a brackish water environment during the geological past.

The Gabo lithium deposit represents a newly discovered pegmatite-type lithium deposit within the Himalayan metallogenic belt. The tourmaline-muscovite granite, the largest leucogranite in the mining area, displays a close spatial correlation with the Li-pegmatite veins. This study aims to examine the genesis of tourmaline and evaluate the significance and potential of pegmatite lithium deposits. Tourmaline is extensively distributed in tourmaline-muscovite granite at Gabo deposit in Luozha county (Xizang). Investigation of the compositional and in situ boron isotopes of the tourmaline revealed that the tourmalines mainly belong to the schorl group and exhibit uniform elevated Li–Sn contents and δ¹¹B values (− 11.6 ‰ to − 10.5‰). This indicates that the tourmaline mainly crystallized from a boron-rich granitic magma undergoing enrichment in elemental lithium during the tourmaline crystallization process. Compared with the principal rare metal leucogranite-pegmatites in the Himalayan orogen, it is proposed that the elevated lithium (Li) content of tourmaline serves as an effective mineral indicator for the highly evolved pegmatite-type rare metal deposits.

The Shichuanhe River, a major tributary of the lower Weihe River, is situated in Xi’an, Shaanxi Province, China. It holds significant information regarding the evolution of the Northern Weihe River, making its study crucial for understanding environmental changes in the region. Despite its importance, research on the Shichuanhe River basin has thus far been primarily focused on riverbank construction, with limited exploration of its sediment characteristics. Although river sediments hold potential for guiding agricultural practices in the area, comprehensive studies on their composition and provenance remain scarce. To address this gap, a systematic sediment sampling campaign was conducted in the lower reaches of the Shichuanhe River, and detailed mineralogical analyses were performed. The results show that the sediment is predominantly composed of detrital quartz and feldspar (albite and orthoclase), with heavy minerals constituting approximately 10%–12% of the total sediment volume. The analysis of the heavy mineral assemblage reveals an absence of significant contributions from igneous rocks in the sedimentary profile. Furthermore, the distribution patterns of major, trace, and rare-earth elements in the Shichuanhe River’s argillaceous sediments exhibit similarities to those found in the Weihe River’s sediments. Elemental fractionation patterns suggest that the Shichuanhe River sediments are primarily derived from Loess Plateau sediments, aligning with findings from both the Weihe River and the middle reaches of the Yellow River.