Abstract Geochronology of continental flood basalts sampled from the Emei large igneous province (LIP) on the western margin of the Yangtze platform was investigated by the laser microprobe 40 Ar/ 39 Ar dating technique. These basalts yield a fairly wide range of 40 Ar/ 39 Ar ages, varying from 259 to 135 Ma. One basalt sample, at least altered, recorded the oldest 40 Ar/ 39 Ar age of about 259 Ma, corresponding to a peak eruption age of the Emei LIP continental flood basalts. Most of the samples yield much younger ages from 135 to 177 Ma, which are consistent with the K‐Ar ages for the same samples (122.8–172.1 Ma). The dating data suggest that these Permian basalts had been widely affected by the regional tectonothermal event at 177–135 Ma. The event was probably caused by the convergence and collision among the Laurasia, Yangtze and Qiangtang‐Qamdo continental blocks on the eastern margin of the Qinghai‐Tibet plateau after the late Triassic. The age of the event reflects the timing of the peak collisional orogeny.
Abstract Post-collisional ultrapotassic rocks (UPRs) in the Tibetan Plateau exhibit extreme enrichment in incompatible elements and radiogenic isotopes. Such enrichment is considered to be either inherited from a mantle source or developed during crustal evolution. In this study, to solve this debate we combined mineral textures and in situ geochemical composition of clinopyroxene phenocrysts in UPRs from southern Tibet to reveal their crustal evolution, enrichment cause and constrain metasomatism in their mantle source. Results show that the UPRs experienced an array of crustal processes, i.e., fractional crystallization, mixing, and assimilation. Fractional crystallization is indicated by decreases in Mg# and Ni and enrichment in incompatible elements (e.g. rare earth element (REE), Sr, Zr) toward the rims of normally zoned clinopyroxene phenocrysts (type-I). Magma mixing is evidenced by the presence of some clinopyroxene phenocrysts (type-II, -III) showing disequilibrium textures (e.g. reversed and overgrowth zoning), but in situ Sr isotope and trace element analysis of those disequilibrium zones indicate that late-stage recharged mafic magmas are depleted (87Sr/86Sr: 0.70659–0.71977) compared with the primitive ultrapotassic magmas (87Sr/86Sr: 0.70929–0.72553). Assimilation is revealed by the common presence of crustal xenoliths in southern Tibetan UPRs. Considering the much lower 87Sr/86Sr values (0.707759–0.709718) and incompatible element contents of these crustal xenoliths relative to their host UPRs, assimilation should have resulted in geochemical depletion of southern Tibetan UPRs rather than enrichment. The diluting impact of both assimilation and mixing is also supported by the modeling results based on the EC-E′RAχFC model combining the growth history of clinopyroxene. Trace elements ratios in clinopyroxenes also imply that the mantle source of southern Tibetan UPRs suffered an enriched and carbonatite-dominated metasomatism. Thus, we conclude that enrichment of southern Tibetan UPRs was inherited from the mantle source.
Abstract Crustal growth is commonly associated with porphyry deposit formation whether in continental arcs or collisional orogens. The Miocene high-K calc-alkaline granitoids in the Gangdese belt in southern Tibet, associated with porphyry copper deposits, are derived from the juvenile lower crust with input from lithospheric mantle trachytic magmas, and are characterized by adakitic affinity with high-Sr/Y and La/Yb ratios as well as high Mg# and more evolved isotopic ratios. Researchers have argued, lower crust with metal fertilization was mainly formed by previous subduction-related modification. The issue is that the arc is composed of three stages of magmatism including Jurassic, Cretaceous, and Paleocene–Eocene, with peaks of activity at 200 Ma, 90 Ma, and ca. 50 Ma, respectively. All three stages of arc growth are essentially similar in terms of their whole-rock geochemistry and Sr-Nd-Hf isotopic compositions, making it difficult to distinguish Miocene magma sources. This study is based on ~430 bulk-rock Sr-Nd isotope data and ~270 zircon Lu-Hf isotope data and >800 whole-rock geochemistry analyses in a 900-km-long section of the Gangdese belt. We found large scale variations along the length of the arc where the Nd-Hf isotopic ratios of the Jurassic, Cretaceous, and Paleocene–Eocene arc rocks change differently from east to west. A significant feature is that the spatial distribution of Nd-Hf isotopic values of the Paleocene–Eocene arc magmas and the Miocene granitoids, including metallogenic ones, are “bell-shaped” from east to west, with a peak of εNd(t) and εHf(t) at ~91°E. In contrast, the Jurassic and Cretaceous arc magmas have different isotopic distribution patterns as a function of longitude. The isotopic spatial similarity of the Paleocene–Eocene and Miocene suites suggests that the lower crust source of the metallogenic Miocene magmas is composed dominantly of the Paleocene–Eocene arc rocks. This is further supported by abundant inherited zircons dominated by Paleocene–Eocene ages in the Miocene rocks. Another important discovery from the large data set is that the Miocene magmatic rocks have higher Mg# and more evolved Sr-Nd-Hf isotopic compositions than all preceding magmatic arcs. These characteristics indicate that the involvement of another different source was required to form the Miocene magmatic rocks. Hybridization of the isotopically unevolved primary magmas with isotopically evolved, lithospheric mantle-derived trachytic magmas is consistent with the geochemical, xenolith, and seismic evidence and is essential for the Miocene crustal growth and porphyry deposit formation. We recognize that the crustal growth in the collisional orogen is a two-step process, the first is the subduction stage dominated by typical magmatic arc processes leading to lower crust fertilization, the second is the collisional stage dominated by partial melting of a subduction-modified lower crust and mixing with a lithospheric mantle-derived melt at the source depth.
The file contains the P-wave velocity model and related plotting scripts for the manuscript entitled " Eastward growth of Tibetan Plateau controlled by Cenozoic Indian slab tearing". The P wave velocity model is stored in Vp.xyz, it contains four columns, which are longitude, latitude, depth (unit: km) and velocity perturbation relative to AK135 model, respectively. The plotting scripts are stored at " Figure1(b)" and "Figure2", which are used to plot figure 1(b) and figure 2 in the manuscript.
Abstract Collision‐related porphyry Cu deposits (PCDs) are restricted to previous magmatic arcs, in which sulfide‐rich lower crust occurred. Fertile adakite‐like porphyries associated with PCDs have higher K 2 O contents and K 2 O/Na 2 O ratios than barren porphyries emplaced in the same arc. The elevated K 2 O/Na 2 O ratios of fertile porphyries reflect substantial inputs of coeval hydrous, oxidized ultrapotassic melts. Input of such melts could increase the water content and oxygen fugacity of the lower‐crust‐derived melts, which in turn would promote the breakdown of sulphides in the lower crust and increase the contents of Cu and S in the melts, making them favourable for the formation of large PCDs. The relatively low K 2 O contents and K 2 O/Na 2 O ratios of barren porphyries indicate limited input of ultrapotassic melts; these magmas have low potential to form PCDs. Thus, the input of ultrapotassic melts into the sulfide‐rich juvenile lower crust drives the formation of collision‐related PCDs.
Abstract The Laowangzhai gold deposit, located in the Ailaoshan gold belt (SW China), is hosted in various types of rocks, including in quartz porphyry, carbonaceous slate, meta‐sandstone, lamprophyre, and altered ultramafic rocks. In contrast to other wall rocks, the orebodies in altered ultramafic rocks are characterized by the occurrence of a large amount of Ni‐bearing minerals. The ore‐forming process of the orebodies hosted by altered ultramafic rocks can be divided into two stages: pyrite‐vaesite‐native gold and gersdorffite‐violarite stages. The contents of As and Sb increased during the evolution of ore‐forming fluid based on the mineral assemblages. Thermodynamic modeling of the Ni‐Cu‐As‐Fe‐S system using the SUPCRT92 software package with the updated database of slop16.dat indicates the f S 2 in ore‐forming fluid decreases significantly from stage I to stage II. The decreases of f S 2 due to crystallization of sulfides and f O 2 due to fluid–rock reaction were responsible for ore formation in altered ultramafic rocks of the Laowangzhai gold deposit. Geological evidence, the in situ sulfur isotope values of pyrite, and the other published isotopic data suggest that the ore‐forming fluid for ultramafic rock ores was dominantly composed of evolved magmatic fluid with the important input of sediments.
Carbonatite-associated rare earth element (REE) deposits (CARD) are mainly found at the edges of cratons in the rift or post-collisional settings, but only a minority carbonatite can form large to giant REE deposit. Fluids rich in volatiles (such as F, Cl, S) are critical for the extraordinary enrichment of REE, but the manner in which volatiles and REE concentrate in the mantle source of CARD remains unclear. This study focuses on the Lizhuang syenite from the Mianning–Dechang REE belt, southwest China, to investigate its petrogenesis, source characteristics, and volatiles evolution, through in situ Hf–O isotope analysis, coupled with mineral chemistry, bulk-rock geochemistry, and geochronology. The Lizhuang syenite was generated in a post-collisional setting at about 27 Ma, resulting from partial melting of an enriched lithospheric mantle, followed by fractional crystallization and immiscibility. When contrasted with barren carbonatite-syenite complexes worldwide, the syenite in REE deposit has a relatively higher content of F and S, with no significant difference in Cl content, and there is no clear correlation between magma oxygen fugacity and REE mineralization. The Hf–O isotopic signatures of zircon and apatite, along with bulk-rock Sr–Nd isotopic modeling, reveal that the mantle source of the Mianning–Dechang CARD experienced metasomatic processes related to the melts/fluids derived from both marine sediments and altered oceanic crust during subduction. The cratonic root, enriched in volatiles and REE, is reactivated during the upwelling of asthenosphere, and the released volatiles are able to participate in the formation of CARD.
Abstract Rare earth elements (REEs) are essential metals for modern technologies. Recent studies suggest that subcontinental lithospheric mantle (SCLM) remelting, previously fertilized by subducted marine sediments, leads to formation of REE-bearing rocks. However, the transfer mechanism of REE-rich sediments from the subducted slab to the overlying mantle wedge is unclear. We present high-pressure experiments on natural REE-rich marine sediments at 3–4 GPa and 800–1000 °C to constrain the phase relations, sediment melting behavior, and REE migration during subduction. Our results show recrystallization into an eclogite-like assemblage, with melting only occurring at 4 GPa, 1000 °C, experiments. Regardless of melting behavior, REE are refractory and mostly hosted by apatite. Buoyancy calculations suggest that most of the eclogite-like residues would form solid-state diapirs, ascending to the SCLM, resulting in the REE-fertilized source. Such flux may be required for substantial REE transport during subduction, as a foundation for economic-grade mineralization.
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