This paper reports Rb-Sr isotope dating of Yangjiapeng MORB gabbro which constitutes Huashan ophiolitic melange. A whole-rock sample and the minerals (plagioclase and clinopyroxene) picked out from the sample give a Rb-Sr isochron age of 435±49(2σ) Ma, with I_ Sr being 0.70516±28(2σ). The age is consistent with the Rb-Sr isochron age (422 Ma) of the adamellite from Sanligang area. The results indicate that intense metamorphism must have taken place in this area in Caledonian period, and that the Huashan ocean basin was probably formed older than the previous prediction. The previous deduction about the age limit of Huashan ocean cannot be conclusively proved.
The hydrochemical characteristics of watersheds are influenced by many factors, with chemical weathering and human activities exerting the most substantial influence. Performing a quantitative evaluation of the factors contributing to the chemical weathering of rocks is of significant scientific importance. This research zeroes in on the Qingtang River basin to elaborate on the hydrochemical characteristic, explore the origins of ions, and quantify the influence of anthropogenic discharges amidst cation interferences, thus improving the accuracy of chemical weathering rate estimations. The samples encompassed surface water, groundwater, and water from dripping in karst caves. The findings indicate that human-induced alterations significantly influence hydrogeochemical dynamics, although chemical weathering of rocks in their natural state is the controlling factor. The mean contributions of cations from atmospheric deposition, human inputs, carbonate weathering, and silicate weathering were 17.56%, 21.05%, 51.77%, and 9.54%, respectively. The chemical weathering rate for carbonate rocks was 62.4 t·km−2·a−1, which increased by 27.87% due to the influence of exogenous acids. The anthropogenic impact is predominantly evident in two aspects: (1) the alteration of hydrochemical processes within the watershed through direct input of ions, and (2) the acceleration of rock weathering rates in the watershed due to the exogenous acids.
ABSTRACTThe lithosphere beneath the eastern part of the North China Craton (NCC) is widely recognized as having undergone extensive thinning and destruction since the Mesozoic. Although most models propose that the destruction was related to the Paleo-Pacific subduction, the timing and mechanism of the destruction remains controversial. The Tanlu Fault is the largest deep strike-slip fault zone in eastern China. It is an ideal object to study the destruction of the NCC and the subduction history of the Paleo-Pacific. In this review, we compile the ages and geochemical data of the Mesozoic magmatic rocks along the Tanlu Fault zone, in combination with evidence related to the tectonic evolution of the Tanlu Fault during this time. We further discuss the relationship between subduction of the Paleo-Pacific and the thinning and destruction processes affecting the NCC lithosphere. In the Late Triassic period, adakitic rocks, A-type granites, and mafic rocks generated from depleted asthenosphere were distributed in Liaoning and Shandong provinces along the Tanlu Fault. These magmas were related to an extensional environment caused by exhumation of the Yangtze block after subduction. The magmatic characteristics indicate that the lithospheric mantle began to change from cold and refractory to a hot and active, suggesting that the NCC began to undergo cratonic destruction at this time. The magmatic lull ranging from 200–185 Ma represents the transition for the Tanlu Fault zone tectonic domain from Paleo-Asian Ocean subduction in the north to the Paleo-Pacific subduction in the east. During the Jurassic, the NCC was also affected by subduction and compression of the Mongol – Okhotsk domain in the north and the influence of the Tethys tectonic domain in the south. Under this multi-directional compression, the crust thickened and the subducted slabs were dehydrated and melted, triggering partial melting of the overlying lithospheric mantle, providing a heat source for partial melting of the crust. At the end of the Jurassic, due to the steepening of the subduction angle of the Paleo-Pacific plate and the roll-back of the plate at that time, another magmatic lull (155–145 Ma) occurred in the Tanlu Fault area. In the early Early Cretaceous, due to a change of the Paleo-Pacific subduction direction (from NW to NNW), a large-scale strike-slip movement took place along the Tanlu Fault zone, and the study area began to experience extensive magmatism. At ca. 125 Ma, A-type granites were formed, representing the large-scale extension. At ca. 122 Ma, OIB-like mafic rocks began to intrude, which indicate that the geochemical properties of the lithospheric mantle of the NCC underwent a fundamental transformation at this time. Hence, the lithospheric mantle of the NCC was replaced by new lithospheric mantle. Slab subduction certainly weakened the NCC. However, the studies of mantle peridotite xenoliths in Cenozoic basalts indicate that the large fault zone (Tanlu Fault) was the priority area for lithospheric mantle transformation and replacement. Taking the time and space distribution characteristics of the NCC destruction into consideration, thermochemical erosion was an indispensable destruction model. According to magmatic and structural evidence, the destruction of the NCC was indeed related to the Paleo-Pacific movement. Besides, it should be noted that the Tanlu Fault was also a factor that cannot be ignored.KEYWORDS: North China CratonTanlu FaultPaleo-Pacific platelithospheric thinningMesozoic AcknowledgmentsThe first author would like to thank Professor Simon Wilde for his patient revision to improve the early draft. We really appreciate Professor Stern for his suggestions and patience. We are very grateful to the two reviewers (Professor Wen-Liang Xu and Professor Ross N. Mitchell) for their comments and suggestions, which have greatly improved the manuscript. We thank members of the Beijing SHRIMP Center for their help with SHRIMP analysis and zircon CL imaging.Disclosure statementNo potential conflict of interest was reported by the author(s).Supplementary materialSupplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2269221Additional informationFundingThis study was financially supported by Key Laboratory of Gold Mineralization Processes and Resource Utilization, MNR, Shandong Provincial Key Laboratory of Metallogenic Geological Process and Resource Utilization (KFKT202101), and China National Space Administration (Grant no. D020205).
In this contribution, we report new U-Pb SHRIMP zircon ages and geochemical data for volcanic rocks from the Dianzhong Formation. The unit represents the lower part of the Linzizong volcanic succession located in the Shiquanhe area, western part of the Gangdese belt. Zircon U-Pb dating of three trachytes yields 206Pb/238U crystallization ages of 71.5±0.6 Ma, 70.8±1.0 Ma and 68.9±1.0 Ma. Whole rock major and trace element analyses indicate a main trend of calc-alkalic to high-potassic calc-alkalic, as well as an enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), with negative Nb, Ta and Ti anomalies. In addition, these volcanic rocks are enriched in LREE and depleted in HREE without a clear Eu anomaly. These geochemical features are similar to those of active continental margin volcanic rocks. In combination with previously published zircon U-Pb ages and geochemical data of the Dianzhong Formation, we suggest that the Linzizong volcanism started as early as the late Cretaceous (∼71 Ma) and the volcanic rocks of the Dianzhong Formation in the Shiquanhe area are the product of rollback of the Neo-Tethyan oceanic plate.
We report geochemical data and zircon SHRIMP U-Pb ages for Late Mesozoic granitoids from the western Zhejiang province and southern Anhui province (the WZSA region) from southeast China. In combination with published geochronological and geochemical data, the granitoids in the region can be divided into three stages: 171–141 Ma, 140–121 Ma, and 120–95 Ma. The first stage of these granitoids is mainly composed of granite porphyry and granodiorite which are similar to I-type granitoids, including having weakly negative Eu anomalies with enrichment in light rare earth elements (LREE), Rb, Th, and U. The second stage of granitoids consists of monzogranite, syenogranite, and granite with the characteristics of both A-type and I-type granitoids including strongly negative Eu anomalies; depletion of Ba, Sr, and Ti; and enrichment of K, Rb, and high field strength elements (HFSEs) (such as Th and U). The third stage of granitoids is mainly composed of granite, quartz monzonite, quartz diorite, and mafic rocks with weakly negative Eu anomalies and also enrichment in LREE, Rb, Th, U, and K. From our work, we propose a transition from compressional to extensional magmatism at ~141 Ma. Based on the geochemical characteristics of these granites and coeval mafic rocks, we propose that the formation of the A-type magmatism in the WZSA region formed as the result of lithospheric extension and asthenospheric upwelling during the Early Cretaceous.
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