Acta Geologica Sinica ‐ English Edition最新文献

High-Mg andesite/diorite (HMA) is useful for identifying subduction-related processes in orogenic belts, including the identification of ophiolites formed in suprasubduction zone (SSZ) environments. The E'rentaolegai high-Mg diorite from the Diyanmiao ophiolite in central Inner Mongolia, North China, has been investigated revealing low-K tholeiitic–calc-alkaline characteristics and have SiO₂ contents of 53.44–54.92 wt%, MgO contents of 8.44–9.54 wt%, and Mg^# of 54.35–57.60, with variable Fe₂O₃ (7.51–8.61 wt%), Al₂O₃ (11.95–15.09 wt%), and Na₂O (3.42–3.94 wt%) contents, low K₂O (0.34–0.97 wt%), TiO₂ (0.35–0.67 wt%), and P₂O₅ (0.12–0.15 wt%) contents, and high Ni (43–193 ppm) and Cr (189–556 ppm) contents. Samples collected have low total rare earth element (REE) contents (30.58–77.80 ppm), with flat or slightly right-dipping REE patterns (La_N/Yb_N = 2.19–3.11) and a lack of pronounced Eu anomalies. The samples are also enriched in large-ion lithophile elements (LILEs, e.g., K, Rb, Ba, U, and Sr) and depleted in high field strength elements (e.g., Ta, Nb, Ti, and P). The E'rentaolegai high-Mg diorite has characteristics typical of HMA, and are similar to those of sanukites from the Setouchi Arc in SW Japan. They also display high positive ε_Nd(t) values (+6.32 to +7.80), comparable to the values of their host rocks. Petrogenetic analyses suggest that the E'rentaolegai HMA was probably formed by the interaction of partial melts and aqueous fluids from subducted sediments with mantle peridotite. Zircon U-Pb dating reveals that the high-Mg diorite crystallized at 313.6 ± 2.4 Ma, i.e., late Carboniferous. Combining our data with the temporal and spatial distribution of the Diyanmiao SSZ-type ophiolite, we propose that the eastern Paleo-Asian Ocean had not closed by the late Carboniferous, but intra-oceanic subduction was ongoing. A new model of the initiation of subduction in the eastern PAO during the late Paleozoic.

The Dabaoshan porphyry Cu deposit (420 kilotons (kt) of Cu @ 0.36%) is located in South China. The newly discovered Cu orebodies are hosted in the dacite porphyry adjacent to a granodiorite porphyry. The alteration and mineralization timing and stages of the porphyry Cu deposit were not well-constrained. In this study, we combine field mapping, petrography, whole-rock geochemistry, hydrothermal rutile U-Pb dating and Cu isotopes to synthesize an ore model at Dabaoshan. In situ hydrothermal rutile U-Pb dating yields an age of 159 ± 13 Ma, which brackets the timing of porphyry Cu mineralization. From top to bottom, the alteration zones in Dabaoshan are divided into quartz-sericite, biotite, chlorite-epidote, and chlorite-sericite subzones. Veins are classified into four stages (Stage 1 to 4) with Stage 4 quartz-sericite-chalcopyrite veins being the main Cu ore-bearing veins. The mineralized dacite porphyry has high SiO₂, but low MgO, CaO, and Na₂O contents. The chalcopyrite hosted in veins exhibits δ⁶⁵Cu = values ranging from –1.29‰ to 0.51‰. Such copper isotope fractionation is attributed to vapor-brine phase separation, and mixing of fluids from different geochemical reservoirs. The timing of Cu mineralization and hydrothermal alteration support that the Jurassic granodiorite porphyry is an ore-forming intrusion at Dabaoshan.

The relationship between marine transgression and the distribution of lacustrine organic matter has restricted shale oil and gas exploration for decades. In this study, the research objective is to analyze the sedimentary environment and evaluate its influence on organic matter in transgressive lacustrine shale. The study uses various analyses including total organic carbon (TOC), Rock-Eval pyrolysis, gas chromatography-mass spectrometry (GC-MS), trace element and isotope analysis. Finally, the study proposes an enrichment model for organic matter. The lacustrine shale of the second member of the Funing Formation (E₁f²) is divided into three sequences. The results indicate that the depositional environment of the organic matter during this period was an arid and humid, reduced, closed, rift lake basin. In the first sequence, high salinity resulted from increased evaporation, leading to low primary biological productivity. At this time, the lake basin belonged to a salinized closed lake basin. Intermittent transgressions began in the second sequence, with the deep lake area still being dominated by a reducing environment. The third sequence saw the environment evolve into a closed lake basin characterized by a warm and humid freshwater environment with high primary productivity. Marine transgressions introduce a substantial amount of marine plankton, nutrient elements, as well as more CO₂ and CO₃^2– into the lake, leading to increased primary productivity. The sedimentary model for transgressive lacustrine source rocks proposed here serves as an example for similar transgressive lake basins.

Cenozoic trachytes are characteristic of some active volcanic fields in China. In particular, the origin and mechanisms of the evolution of trachytes from the Tianchi (Changbaishan) volcano (TV, China/North Korea) are poorly known. Here, we present new geochronological, geochemical and isotopic data on two trachytic suites outcropping on the northern and southern upper slopes of TV. Detailed zircon laser ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) U-Pb dating, Rb-Sr isochron dating of plagioclase and hornblende, ⁴⁰Ar/³⁹Ar chronology with mineral chemistry, whole-rock element and Sr-Nd-Pb isotope data are used to explore their origins and evolutionary mechanisms during the late Middle Pleistocene. Our data indicate that the trachytes mainly consist of sanidine, orthoclase and plagioclase, with minor albite, quartz and hornblende. They formed at 0.353–0.346 Ma (lava flow from the northern slope) and 0.383–0.311 Ma (lava flow from the southern slope), respectively. The TV trachytes are characterized by high K₂O/Na₂O and AR values, with low A/CNK and Mg^# values. They are enriched in rare earth elements (REEs; except Eu), depleted in Sr and Ba, crystallizing at 742–858°C. The TV trachytes have high (⁸⁷Sr/⁸⁶Sr)_i values (0.70776–0.71195), positive ε_Nd(t) values (0.61–2.93) and radiogenic ²⁰⁶Pb/²⁰⁴Pb values (17.515–17.806). These values are similar to those of the Pleistocene and Holocene TV trachytes. Geochemical data indicate that they were formed by fractional crystallization from a basaltic melt and assimilated upper crust material. The trace element pattern of the studied trachytes is consistent with an evolution from basaltic melts representative of an enriched mantle source. The vent from which the trachytic magma was erupted probably collapsed inside the caldera during the TV ‘Millennium’ eruption (ME) in 946 AD. The contemporaneous emission of basaltic and trachytic magma during the Middle–Late Pleistocene suggests the coexistence of fissural basaltic volcanism and central-type trachytic volcanism, the latter of which was associated with a magma chamber in the upper crust during the TV cone-construction stage.

Knowledge of the seismogenic environment of fault zones is critical for understanding the processes and mechanisms of large earthquakes. We conducted a rock magnetic study of the fault rocks and protoliths to investigate the seismogenic environment of earthquakes in the Motuo fault zone, in the eastern Himalayan syntaxis. The results indicate that magnetite is the principal magnetic carrier in the fault rocks and protolith, while the protolith has a higher content of paramagnetic minerals than the fault rocks. The fault rocks are characterized by a high magnetic susceptibility relative to the protolith in the Motuo fault zone. This is likely due to the thermal alteration of paramagnetic minerals to magnetite caused by coseismic frictional heating with concomitant hydrothermal fluid circulation. The high magnetic susceptibility of the fault rocks and neoformed magnetite indicate that large earthquakes with frictional heating temperatures >500°C have occurred in the Motuo fault zone in the past, and that the fault maintained an oxidizing environment with weak fluid action during these earthquakes. Our results reveal the seismogenic environment of the Motuo fault zone, and they are potentially important for the evaluation of the regional stability in the eastern Himalayan syntaxis.

The pervasively distributed granitoids in South China contributed greatly to regional polymetallic mineralization, including tungsten, tin, copper, gold, rare metals, and rare earth elements (REEs). To ascertain the dynamic backgrounds, rock types and genesis of the parent rocks related to the Early–Middle Jurassic ionic rare earth mineralization, typical deposits at Muzishan, Xiahu, and Zudong were investigated by conducting petrographic, geochronologic, whole-rock geochemical, and Sr-Nd-Pb isotope analyses, which found that the parent rocks from the Muzishan deposit were the A1-type K-feldspar granite (~195 Ma), from the Zudong deposit were the A2-type monzogranite (~171 Ma), and from the Xiahu deposit were the I-type monzogranite (~167 Ma). All the three granitic rocks underwent different degrees of fractionation, with the Xiahu granite experiencing the highest degree, followed by the Zudong granite, and the Muzishan granite undergoing the lowest degree. The Muzishan granite was concluded to be formed under an intraplate extensional tectonic regime influenced by the hotspots or the mantle plume. The Zudong granite was formed in a post-arc extensional setting related to subduction–collision–rollback of the paleo-Pacific Plate, which caused upwelling of the asthenosphere, thinning of the lithosphere, and partial melting of crustal materials. The Xiahu granite was generated under a transitional tectonic setting of extension and compression, triggered by delamination and rollback of the paleo-Pacific Plate.

In the western segment of the East Kunlun Orogen (WEKO), muscovite granite and garnet granite from the Hureguole intrusion exhibit zircon U-Pb ages of 435–442 Ma. Geochemically, these rocks contain high concentrations of SiO₂, K₂O and Al₂O₃, with low concentrations of TiO₂ and MgO, indicating a peraluminous high-K calc-alkaline affinity. They are enriched in Rb, Th, U and LREE, depleted in Eu, Ba, Sr and Ti, being classified as S-type granites. Negative whole-rock ε_Nd(t) values (–9.8 to –9.1) and zircon ε_Hf(t) values (–11.6 to –8.2) for those granites indicate that they were derived from partial melting of pelitic rocks in the Paleoproterozoic Baishahe Formation of the Jinshuikou Group. Based on the collected zircon ages, Cambrian–Devonian magmatic activity in the WEKO was divided into three stages: early (446–520 Ma), middle (427–441 Ma) and late (372–424 Ma) stages. Statistically, whole-rock Nd and zircon Hf isotope data (ε_Nd(t)/T_DM^Nd, ε_Hf(t)/T_DMC^Hf) from Paleozoic igneous rocks in the WEKO reveal a magma source that was initially dominated by depleted mantle components in the northward subduction stage of the Proto-Tethyan Ocean plate (446–520 Ma), shifting to predominantly crustal sources during the closure period of the North Qimantagh back-arc basin (427–441 Ma), then to crust-mantle mixed sources in the post-collision stage (372–424 Ma).

The Duanfengshan deposit is a newly discovered large pegmatitic-type Nb-Ta deposit in the central section of the Jiangnan orogenic belt, South China. There are three types of pegmatite in the Duanfengshan area: microcline pegmatite, microcline-albite pegmatite and albite pegmatite. Although several geological, geochronological and geochemical studies of this deposit have been carried out, the relationships between the evolution degree of different types of pegmatites and mineralization are still unclear. We address this problem through systematic petrographic and geochemical studies of muscovite and feldspars from two representative pegmatite veins, the No. 328 microcline-albite pegmatite vein, and the No. 610 albite pegmatite vein. The results of electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of muscovite and K-feldspar reveal that K/Rb ratios decrease with increasing Rb, Cs, Ga, Nb and Ta contents alongside decreasing Ba and Sr contents, suggesting that magmatic differentiation played a dominant role in rare metal mineralization. A comparison of the analytical results of this study with those from rare metal pegmatites globally suggests that the No. 610 vein has a high mineralization potential, whereas the No. 328 vein has relatively low mineralization potential. The results from this study may be applied to the evaluation of mineralization potential for other pegmatite veins in the Duanfengshan area and other rare metal pegmatite fields with similar geological settings.

In a re-examination of the Neogene stratigraphy of the Uşak, Güre, and Selendi basins of western Anatolia, western Turkey, the stratigraphic position of the previously defined İnay Group is revised, which was previously considered to be of the Middle Miocene age. Based on mammalian biochronology and stratigraphic relationships, two sequences are identified, separated by conformable/transitional contacts within the former group: the Middle Miocene Güre Group is composed of the Fakılı Formation, characterized by alluvial fan deposits, and the lacustrine Derbent Formation. Radiometric dating of alkaline volcanics laterally associated with Güre Group sediments in the Uşak and Güre basins, and mammalian fossils re-evaluated into the MN5-6 biozones in the Selendi Basin, indicate early Middle Miocene. Considering the stratigraphic relationship with the overlying early Late Miocene İnay Group as redefined, we estimate that the Güre Group was deposited in the Middle Miocene. The İnay Group is characterized by a lateral–vertical transition from alluvial deposits of the Ahmetler Formation to the overlying lacustrine Ulubey Formation. An MN9-10 fauna, containing deinotheres, was found within the latter formation.

The Late Cretaceous Jiepailing granitoids, located at the central Nanling Range in South China, are closely associated with significant Sn-Li-Be-F polymetallic metallogeny. The Jiepailing granitoids mainly consist of granitic porphyry and zinnwaldite granite. The two granitoids have an A-type affinity, showing elevated Rb/Sr ratios and significant depletions in Ba, Sr and P. Integrated zircon and monazite U-Pb dating results suggest that granitic porphyry and zinnwaldite granite were emplaced at ~89 Ma and ~94 Ma, respectively. The low Ce⁴⁺/Ce³⁺ ratios of the Jiepailing granitoids, together with significant negative Eu anomalies of the zircons, indicate that their formation occurred under conditions of reduced oxygen fugacity. Through the analysis of zircon Hf-O and whole-rock Nd isotopes, it has been determined that both stages of the Jiepailing granitoids originated in the lower-middle Mesoproterozoic crustal basement [ε_Nd(t) = –5.33 to –4.96, t^C_DM(Nd) = 1289–1234 Ma, ε_Hf(t) = –4.13 to +2.22, t^C_DM(Hf) = 1418–1015 Ma and δ¹⁸O_Zrc = 6.33‰–7.72‰], with the involvement of mantle-derived materials. Both granitic porphyry and zinnwaldite granite exhibit elevated concentrations of fluorine (F), with the positive correlation between F and Sn emphasizing the crucial role of high F sources in tin mineralization. Drawing upon the study of the Late Cretaceous magma systems in southern Hunan and through comparison with the mineralized granites observed in coastal regions during the Late Cretaceous, a genetic model for the mineralized granites in the Nanling region is developed. When the Paleo-Pacific Plate retreated to the coastal region, the continental crust in southern China underwent extensional thinning and asthenospheric upwelling due to gravitational collapse. Such processes resulted in the partial melting of the middle–lower crustal metamorphic sedimentary basement and the subsequent formation of F-rich granitic magmas, related to tin mineralization.