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摘要:Identification and anatomy of oceanic arcs within ancient orogenic belt are significant for better understanding the tectonic framework and closure process of paleo-ocean basin. This article summarizes the geological, geochemical, and geochronological characteristics of upper crust of Proto-Tethyan Lajishan intra-oceanic arc and provides new data to constrain the subduction evolution of the South Qilian Ocean. The intra-oceanic arc volcanic rocks, including intermediate–mafic lava, breccia, tuff, and minor felsic rocks, are distributed along southern part of the Lajishan ophiolite belt. Geochemical and isotopic compositions indicate that the intermediate–mafic lava were originated from depleted mantle contaminated by sediment melts or hydrous fluids, whereas the felsic rocks were likely generated by partial melting of juvenile mafic crust in intra-oceanic arc setting. Zircons from felsic rocks yield consistent and concordant ages ranging from 506 to 523 Ma, suggesting these volcanic rocks represent the relicts of upper crust of the Cambrian intra-oceanic arc. Combined with the Cambrian forearc ophiolite and accretionary complex, we suggest that the Cambrian intra-oceanic arc in the Lajishan ophiolite belt is belonging to the intra-oceanic arc system which was generated by south-directed subduction in the South Qilian Ocean at a relatively short interval between approximately 530 and 480 Ma.

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摘要:Three eusauropod teeth (SDUST-V1064, PMOL-AD00176, PMOL-ADt0005) are reported from the Lower Cretaceous Yixian Formation of Ningcheng, southeastern Inner Mongolia, China. Two of them (SDUST-V1064, PMOL-AD00176) are assigned to early-diverging titanosauriforms in having slightly mesiodistal expansion at the base of the tooth crown, a slenderness index value >2.0 and <4.0, and D-shaped cross section. Furthermore, SDUST-V1064 and PMOL-AD00176 are referred as an Euhelopus-like titanosauriform on the basis of having a sub-circular boss on the lingual surface and an asymmetrical crown-root margin which slants apically, respectively.

摘要:The Jiangchuan Biota from the Jiucheng Member (Mb.) of the Dengying Formation (Fm.), discovered in Jiangchuan, eastern Yunnan, China, is marked by copious macrofossils at the apex of the Ediacaran strata. This fauna features benthic algae with varied holdfasts and other fossils of indeterminate taxonomic affinity and is compositionally unique compared to the Shibantan and Gaojiashan biotas of the Dengying Fm. and the Miaohe and Wenghui biotas of the Doushantuo Fm., elsewhere in China. One novel benthic saccular macroalgal fossil, named here Houjiashania yuxiensis gen. and sp. nov., from the Jiangchuan Biota is based on fossils that are sausage-shaped, elongate, tubular, ranging from 0.3 to 4 cm in length, and up to 0.8 cm in diameter. One terminus is blunt and rounded to an obtuse angle, the other is bent with a spread-out surface resembling a holdfast, suggesting a three-dimensional thallus. Thin, stipe-shaped outgrowths, likely vestiges of sessile saccular life forms, are prevalent in macroalgal fossils of analogous size and shape, as well as present brown algae Scytosiphonaceae, such as Colpomenia and Dactylosiphon. The new findings augment the diversity of benthic algae, such as those known from the Early Neoproterozoic Longfengshan Biota in North China. The benthic algal macrofossils in the Jiucheng Mb. add to knowledge of Late Ediacaran metaphyte diversification and offer more clues about the evolutionary positioning of primitive macroalgae. The co-occurrence of numerous planktonic and benthic multicellular algae and planktonic microbes might have facilitated ecologically the more extensive later Cambrian explosion evidenced by the Chengjiang Biota in Yunnan.

摘要:In this study, zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone (YZSZ) in southern Xizang (Tibet) yielded an age of 247 ± 3 Ma. According to whole rock geochemical and Sr-Nd-Pb isotopic data, the Early Triassic samples could be divided into two groups: Group 1 with P-MORB affinity, showing initial 87Sr/86Sr ratios of 0.70253–0.70602, εNd(t) values of 4.2–5.3, (206Pb/204Pb)t ratios of 16.353–18.222, (207Pb/204Pb)t ratios of 15.454–15.564, and (208Pb/204Pb)t ratios of 35.665–38.136; Group 2 with OIB affinity, showing initial 87Sr/86Sr ratios of 0.70249–0.70513, εNd(t) values of 4.4–4.9, (206Pb/204Pb)t ratios of 17.140–18.328, (207Pb/204Pb)t ratios of 15.491–15.575, and (208Pb/204Pb)t ratios of 36.051–38.247. Group 2 rocks formed by partial melting of the mantle source enriched by a former plume, and assimilated continental crustal material during melt ascension. The formation of Group 1 rocks corresponds to the mixing of OIB melts, with the same components as Group 2 and N-MORBs. The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang (Tibet).

摘要:The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt. In this study, we present SHRIMP zircon U-Pb ages, Hf isotopic and geochemical data on the Xingxingxia biotite granite, amazonite granite and granitic pegmatite in Central Tianshan, NW China. Zircon U-Pb dating yielded formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite. These granitoid rocks have high K2O with low MgO and CaO contents. They are enriched in Nb, Ta, Hf and Y, while being depleted in Ba and Sr, showing flat HREE patterns and negative Eu anomalies. They have typical A-type granite geochemical signatures with high Ga/Al (8–13) and TFeO/(TFeO + MgO) ratios, showing an A2 affinity for biotite granite and an A1 affinity for amazonite granite and granitic pegmatite. Zircon εHf(t) values of the granitoids are 0.45–2.66, with Hf model ages of 0.99–1.17 Ga. This suggests that these A-type granites originated from partial melting of the lower crust. We propose that Xingxingxia Triassic A-type granites formed under lithospheric extension from post-orogenic to anorogenic intraplate settings and NE-trending regional strike-slip fault-controlled magma emplacement in the upper crust.

摘要:Post-collisional magmatism contains important clues for understanding the reworking and growth of continental crust, as well as lithospheric delamination and orogenic collapse. Early Devonian magmatism has been identified in the North Qilian Orogenic Belt (NQOB). This paper reports an integrated study of petrology, whole-rock geochemistry, Sm-Nd isotope and zircon U-Pb dating, as well as Lu-Hf isotopic data, for two Early Devonian intrusive plutons. The Yongchang and Chijin granites yield zircon U-Pb ages of 394–407 Ma and 414 Ma, respectively. Both of them are characterized by weakly peraluminous to metaluminous without typical aluminium-rich minerals, LREE-enriched patterns with negative Eu anomalies and a negative correlation between P2O5 and SiO2 contents, consistent with geochemical features of I-type granitoids. Zircons from the studied granites display negative to weak positive εHf(t) values (?5.7 to 2.1), which agree well with those of negative εNd(t) values (?6.4 to ?2.9) for the whole-rock samples, indicating that they were derived from the partial melting of Mesoproterozoic crust. Furthermore, low Sr/Y ratios (1.13–21.28) and high zircon saturation temperatures (745°C to 839°C, with the majority being >800°C) demonstrated a relatively shallow depth level below the garnet stability field and an additional heat source. Taken together, the Early Devonian granitic magmatism could have been produced by the partial melting of ancient crustal materials heated by mantle-derived magmas at high-temperature and low-pressure conditions during post-collisional extensional collapse. The data obtained in this study, when viewed in conjunction with previous studies, provides more information about the tectonic processes that followed the closure of the North Qilian Ocean. The tectonic transition from continental collision to post-collisional delamination could be constrained to ~430 Ma, which is provided by the sudden decrease of Sr/Y and La/Yb ratios and an increase in zircon εHf(t) values for granitoids. A two-stage tectonic evolution model from continental collision to post-collisional extensional collapse for the NQOB includes (a) continental collision and crustal thickening during ca. 455–430 Ma, characterized by granulite-facies metamorphism and widespread low-Mg adakitic magmatism; (b) post-collisional delamination of thickened continental crust and extensional collapse of orogen during ca. 430–390 Ma, provided by coeval high-Mg adakitic magmatism, A-type granites and I-type granitoids with low Sr-Y ratios.

摘要:The Shenxianshui granites in the western Gejiu area were formed in the Late Cretaceous. Laser ablation inductively coupled plasma mass spectrometry indicates zircon U-Pb ages ranging from 90.67 ± 0.7 to 85.97 ± 0.6 Ma. The intrusive rocks are peraluminous (A/CNK = 1.03 to 1.33) and calc-alkaline, showing an affinity towards I-type granite. Large ion lithophilic elements are enriched in K and Rb, while high field strength elements are depleted. Moreover, light rare earth elements are significantly enriched, showing a slight negative Eu anomaly (Eu/Eu* = 0.39 to 0.58). Shenxianshui granite has a relatively high initial Sr isotope ratio (87Sr/86Sr)i (0.7098–0.7105), negative εNd(t) values (?7.99 to ?7.44) and negative εHf(t) values (?8.37 to ?2.58). Combined with previous studies, these characteristics suggest that the Shenxianshui alkaline granites were formed in a post-collision extensional environment. The alkaline granitic magma possibly originated from the partial melting of the lower crust during the Mesoproterozoic era and may have contained mantle source materials. Shenxianshui alkaline granite was formed from mixed magma with a high degree of crystal differentiation. The abundance of ore-forming elements indicates that Shenxianshui granite has the potential to mineralize key metals and rare earth elements.

摘要:Feldspar Pb isotopes have been widely used to trace magmatic formation and evolution processes. However, it remains unclear whether post-magmatic thermal events can affect feldspar Pb isotopic ratios. Here, the in situ Pb isotopic composition of feldspar hosted in granitic rocks (thirteen Archean and one Paleoproterozoic) from the northern Kongling terrane, Yangtze Craton, South China, is analyzed. The samples reveal a substantial variation in their Pb isotopic composition, spanning the gap between the 1.9 Ga and present-day geochrons, which indicates extensive resetting by later tectonothermal events. This resetting was interpreted to have likely resulted from Paleoproterozoic and Neoproterozoic tectonothermal events related to the assembly and breakup of the Columbia and Rodinia supercontinents. These results suggest that Pb isotopes should be used cautiously when tracing magma sources and petrogenesis in magmatic rocks that have experienced post-magmatic reworking. However, the in situ Pb isotopic composition of feldspar in ancient granitoids may also potentially be used to reveal later tectonothermal events. The extensive resetting of the Pb isotopic composition in feldspar by regional thermal events may also provide new insights into our understanding of the Pb isotope paradox.

摘要:The Jiama deposit, a significant porphyry-skarn-type copper polymetallic deposit located within the Gangdese metallogenic belt in Tibet, China, exemplifies a typical porphyry metallogenic system. However, the mineral chemistry of its accessory minerals remains under-examined, posing challenges for resource assessment and ore prospecting. Utilizing electron microprobe analysis and LA-ICP-MS analysis, this study investigated the geochemical characteristics of apatite in ore-bearing granite and monzogranite porphyries, as well as granodiorite, quartz diorite, and dark diorite porphyries in the deposit. It also delved into the diagenetic and metallogenic information from these geochemical signatures. Key findings include: (1) The SiO2 content, rare earth element (REE) contents, and REE partition coefficients of apatite indicate that the dark diorite porphyry possibly does not share a cogenetic magma source with the other four types of porphyries; (2) the volatile F and Cl contents in apatite, along with their ratio, indicate the Jiama deposit, formed in a collisional setting, demonstrates lower Cl/F ratios in apatite than the same type of deposits formed in a subduction environment; (3) compared to non-ore-bearing rock bodies in other deposits formed in a collisional setting, apatite in the Jiama deposit exhibits lower Ce and Ga contents. This might indicate that rock bodies in the Jiama deposit have higher oxygen fugacity. Nevertheless, the marginal variation in oxygen fugacity between ore-bearing and non-ore-bearing rock bodies within the deposit suggests oxygen fugacity may not serve as the decisive factor in the ore-hosting potential of rock bodies in the Jiama deposit.

摘要:A polymetallic layer is usually developed at the bottom of the early Cambrian black shale in Guizhou Province. The mineral that makes up the polymetallic layer is related to the sedimentary facies. To analyze the differentiation mechanism between polymetallic deposits (Ni-Mo and V), the Zhijin Gezhongwu profile located in the outer shelf and the Sansui Haishan V deposit located in the lower slope are selected to study the in situ sulfur isotopes and trace elements of pyrite. The results show that δ34S values of pyrite vary widely from ?7.8 ‰ to 28 ‰ in the Gezhongwu profile, while the δ34S values are relatively uniform (from 27.8 ‰ to 38.4 ‰) in the Haishan profile. The isotopic S composition is consistent with the transition that occurs in the sedimentary phase from the shelf to the deep sea on the transgressive Yangtze platform; this indicates that the δ34SO42? values in seawater must be differently distributed in depositional environments. The sulfur in the Ni-Mo layer is produced after the mixing of seawater and hydrothermal fluid, while the V layer mainly originates from seawater. Overall, the Ni-Mo and V deposits have been differentiated primarily on the basis of the combined effect of continental weathering and hydrothermal fluid.

摘要:Xiazhuang uranium ore field, located in the southern part of the Nanling Metallogenic Belt, is considered one of the largest granite-related U regions in South China. In this paper, we contribute new apatite fission track data and thermal history modeling to constrain the exhumation history and evaluate preservation potential of the Xiazhuang Uranium ore field. Nine Triassic outcrop granite samples collected from different locations of Xiazhuang Uranium ore field yield AFT ages ranging from 43 to 24 Ma with similar mean confined fission track lengths ranging from 11.8 ± 2.0 to 12.9 ± 1.9 μm and Dpar values between 1.01 and 1.51 μm. The robustness time-temperature reconstructions of samples from the hanging wall of Huangpi fault show that the Xiazhuang Uranium ore field experienced a time of monotonous and slow cooling starting from middle Paleocene to middle Miocene (~60–10 Ma), followed by relatively rapid exhumation in the late Miocene (~10–5 Ma) and nearly thermal stability in the Pliocene–Quaternary (~5–0 Ma). The amount of exhumation after U mineralization since the Middle Paleogene was estimated as ~4.3 ± 1.8 km according to the integrated thermal history model. Previous studies indicate that the ore-forming ages of U deposits in the Xiazhuang ore field are mainly before Middle Paleocene and the mineralization depths are more than 4.4 ± 1.2 km. Therefore, the exhumation history since middle Paleocene plays important roles in the preservation of the Xiazhuang Uranium ore field.