Abstract The magmatic-hydrothermal transition in granite-related, rare-metal metallogenic systems has received great attention as economic rare metal (including rare earth) minerals reach saturation and trigger mineralization at this stage. However, deciphering the details of the melt-fluid evolution process and the distribution behavior of rare metals remains difficult. Here, we applied tourmaline chemistry and B isotopes to unravel processes at the magmatic-hydrothermal transition that are responsible for rare-metal partitioning in the Huoshibulake (HS) and Tamu (TM) REE-Nb-mineralized intrusions in Southern Tianshan, SW Central Asian Orogenic Belt. Three types of tourmaline are identified in the plutons: (1) disseminated tourmaline in the granite, with a brown-yellow core (HS-DB) and blue-green rim (HS-DG); (2) orbicular tourmaline, with a brown-yellow core (HS-OB and TM-OB) and blue-green rim (HS-OG and TM-OG); and (3) vein tourmaline (HS-V and TM-V). Compositionally, all these tourmalines exhibit extremely low Ca and Mg contents and are classified as schorl. The substitution processes of major-element variations are dominantly caused by (Al,☐)(Fe,Na)−1 exchange vectors. Four generations of tourmaline crystallization are established based on the petrographic, compositional, and B isotopes evolution of the tourmaline. First, the HS-DB crystals crystallized from the highly evolved residual melt, and then HS-OB and TM-OB precipitated from immiscible B-rich aqueous melts during the magmatic-hydrothermal transition. Subsequently, the blue-green overgrowths (HS-DG, HS-OG, and TM-OG) crystallized from exsolved hydrothermal fluids. Finally, the formation of HS-V and TM-V resulted from another melt pulse from a deeper magma chamber. The magmatic tourmaline exhibits a narrow range of δ11B values between –12.6 to –10.0‰, while the hydrothermal tourmaline shows significantly heavier and variable δ11B values ranging from –10.2 to –4.9‰. The fractionation of B isotopes is reproduced by Rayleigh fractionation modeling. Lower Nb and Sn contents in the orbicular tourmaline relative to those precipitated from the residual melt, along with the lack of rare-metal minerals in the orbicules, indicate that B-rich melt/fluid exsolution does not necessarily contribute to the rare-metal mineralization. In comparison, the veins contain abundant rare-metal and REE minerals in close paragenesis with fluorite, and the vein tourmaline shows high-Nb and -Sn contents. These observations suggest that saturation of fluorite triggered the precipitation of rare metals, and fluorine played a critical role in rare metal concentration and mineralization. This study highlights the potential of tourmaline to trace the magmatic-hydrothermal transition and provide insights into rare-metal mineralization in the granitic systems.
Abstract Earth history is punctuated by voluminous magmatism and the formation of large igneous provinces (LIPs). Although anomalous mantle temperatures are known to be involved in many LIPs formation, the potentially critical role of fluids remains elusive. Here we apply machine learning methods (e.g., random forest, deep neural network, and support vector machines) to train models based on global datasets of basalts associated with different settings. The trained models predict that the basalts of Tarim LIP in northwestern China show a spatial decrease in their island arc affinity from northeast to southwest, which can be correlated to fluids released from earlier southward oceanic subduction. Temporally, the fluid activity declined from 290 Ma basalts to 270 Ma mafic dykes, suggesting that the fusible components in the mantle source were waning over time and ultimately a strengthened lithosphere was generated. Our study provides new insights into the crucial role of fluids in the generation of LIPs, particularly those related to ancient subducted slabs.
The differentiation process of ultrapotassic magmas is enigmatic and poorly understood. The Yaojiazhuang ultrapotassic complex is concentrically zoned by late-intruded syenite in the core and early emplaced clinopyroxenite in the periphery, combining a "bi-modal" lithology. Spatially, apatite and iron oxide-apatite (IOA) ores, glimmerite and pseudoleucite occur in the upper part of clinopyroxenite. The syenite and clinopyroxenite are composed of variable amounts of clinopyroxenite, biotite, K-feldspar, magnetite, apatite with minor analcite, titanite and primary calcite. The pseudoleucite clinopyroxenite contains mainly clinopyroxene, biotite and garnet in the matrix, and nepheline–K-feldspar intergrowth with muscovite and minor celestine in the leucite pseudomorph. Geochemically, rocks of the Yaojiazhuang complex are significantly enriched in potassium (K), light rare earth elements (LREE) and large ion lithophile elements (LILE). Crustal contamination by Archean tonalite–trondhjemite–granodiorite (TTG) gneisses basement may play an important role to convert the syenitic melts from silica-undersaturation to saturation. Fractionation crystallization is supported by the mineral crystallization sequence to explain the bimodal lithologies instead of silicate liquid immiscibility. During the magmatic evolution, decompression, fractionation of volatile-poor clinopyroxene and the enhancement by CO2 may result in the exsolution of an aqueous fluid. The late-stage interactions between existing minerals and magmatic fluids in the crystal mush could be a key process in the generation of both leucite pseudomorphs and apatite/IOA ores.
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