Fucheng Yu

In recent years, a series of pegmatite-type rare metal deposits and occurrences—including Chakabeishan, Shaliuquan, Jinshuikou, Adatan, and Gaduo—have been reported in Qinghai, sparking a wave of rare metal exploration. The Southern Kunlun Convergence Zone is a key tectonic unit of the Eastern Kunlun Orogenic Belt; however, systematic geochronological and geochemical constraints on the granitic–pegmatitic rare metal mineralization system within this region have long been lacking. This study focuses on the newly discovered beryllium ore body at Wulugou in the Southern Kunlun Mountains. Samples from the ρ10 and ρ38 beryllium-bearing pegmatite veins were selected for petrological analysis, LA-ICP-MS zircon–monazite U-Pb geochronology, and whole-rock major and trace element analyses. Zircon Lu-Hf and monazite Sm-Nd isotope analyses were also conducted on the ρ10 and ρ38 beryllium-bearing pegmatite veins. This study precisely determined the diagenetic and mineralization ages of the pegmatites; systematically clarified the genesis and evolutionary relationships among the ore-bearing pegmatite, the surrounding granite, and the barren pegmatite; and elucidated the characteristics of the magma source region and the mechanisms of beryllium mineralization. LA-ICP-MS dating results for monazite and zircon in the pegmatites indicate that the ore-bearing pegmatite formed during the Early Devonian (404.9–408.3 Ma). Whole-rock geochemical analysis indicates that the albitized granites, barren pegmatite, and beryllium-bearing pegmatites in the study area constitute a continuous, homologous, highly differentiated evolutionary sequence, all of which are highly differentiated granites. As the degree of crystallization increases, the rock undergoes continuous enrichment in SiO₂, Na₂O, MnO, P₂O₅, and Be, while K₂O, Ca, and Sr are progressively depleted. The effects of the four rare earth groups, along with the Zr/Hf and K/Rb ratios, collectively indicate that saturation and dissolution of volatile fluids in the late stage are the key controlling factors in the precipitation of beryllium ore. For primary magmatic zircon, ε_Hf (t) = −2.69 to +1.95 (mean + 0.76); the two-stage Hf model age is 1.28 to 1.57 Ga; isolated ε_Nd (t) = −6.71 to −5.44; and the Nd two-stage model age is 1.57 to 1.68 Ga. The source-area discrimination diagrams all fall within the muddy clastic-melt zone, indicating that the parent magma originated from low-degree partial melting of the Middle Proterozoic Wanbaogou Group metamorphic basement, accompanied by contamination by mantle-derived magma. This study discovered Early Devonian Caledonian-period pegmatite-type beryllium deposits in the Southern Kunlun Mountains, providing a theoretical basis and practical reference for the exploration of granitic–pegmatite-type beryllium deposits in the Southern Kunlun Mountains.

Mafic microgranular enclaves (MMEs) are widespread in granitic plutons and provide valuable insights into mush–magma mixing processes in crustal magma reservoirs. In this study, we characterize chemical zoning and Sr isotopic compositions of plagioclase in the MMEs, gabbro and host monzogranite from the Late Triassic Xiuyan pluton in East China, to constrain the origin of MMEs and the role of crystal mushes in magma mixing. The MMEs in the Xiuyan pluton are angular and range from centimeters to several meters in size. They exhibit sharp contacts with the host monzogranite and show diverse disequilibrium textures. Plagioclase in MMEs occurs as fine-grained antecryst with normal zoning (An_46–66 in the core and An_17–29 in the rim). The cores are commonly characterized by coarse sieve textures, patchy zoning, and resorption surfaces at core–rim boundaries. In situ Sr isotopic compositions show subtle but systematic core–rim variations, with (⁸⁷Sr/⁸⁶Sr)_i increasing slightly from cores (~0.70639) to rims (~0.70664), and rim values overlapping the whole-rock (⁸⁷Sr/⁸⁶Sr)_i of MMEs. These features suggest that the rim was crystallized from locally hybridized melts produced by interaction between interstitial melts in a basaltic mush and granitic magma. Plagioclase in the gabbro occurs as medium-grained phenocryst with normal zoning (An_46–65 in the core and An_18–27 in the rim) but shows nearly homogeneous (⁸⁷Sr/⁸⁶Sr)_i across individual grains (0.70612–0.70637), comparable to whole-rock gabbro values of 0.70623. The plagioclase cores in gabbro also show coarse sieve texture and patchy zoning with the resorption surface in the margin of the core and rim. We interpret the sieve textures in plagioclase cores from both MMEs and gabbro to record partial dissolution during rapid ascent and decompression of an initially H₂O-undersaturated, crystal-bearing basaltic magma, during which increased effective water activity reduced plagioclase stability prior to the growth of the rim. Plagioclase in the host monzogranite is medium- to coarse-grained, compositionally homogeneous, and characterized by low An contents (An_12–24) and elevated (⁸⁷Sr/⁸⁶Sr)_i of ~0.70828. We propose that MMEs in the Xiuyan pluton formed when semi-consolidated mafic mush was mechanically disaggregated into angular fragments and subsequently entrained into coexisting granitic melt. This study reveals that MMEs formed by mechanical disaggregation of a semi-consolidated mafic mush into angular fragments, followed by their entrainment into the granitic melts.