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<p>As one of the micro-blocks dispersed in the South China Sea (SCS), the Xisha Islands are covered by thick Cenozoic sedimentary layers, making it challenging to obtain magmatic rocks. Well CK-2 is a kilometer-scale scientific drilling project on the Xisha Islands in the northwestern SCS. It penetrates the thick reef limestone and reaches basaltic pyroclastic rocks. This study presents the whole-rock and olivine compositions of the basaltic volcaniclastic rocks from Well CK-2. These rocks exhibited ocean island basalt signatures characterized by the enrichment of light rare earth elements and high-field-strength elements. Compared with partial melting products derived from mantle peridotite, whole-rock compositions showed elevated Fe/Mn and Zn/Fe mass ratios. Additionally, olivines were characterized by a lower Ca content, higher Ni content, elevated Fe/Mn mass ratios, and moderate Mn/Zn mass ratios compared to those crystallized from peridotitic melts. The compositions of both the whole-rock and olivine phenocrysts indicate the presence of pyroxenite in the mantle source, which likely formed through the reaction of recycled oceanic crust with the surrounding mantle peridotite. Using the olivine-liquid Mg-Fe exchange thermometer, this study derived mantle potential temperatures (<i>T</i><sub>p</sub>) ranging from 1 502 °C to 1 756 °C, which is consistent with those of plume-related ocean island basalts. Furthermore, the basaltic volcaniclastic rocks exhibit low H<sub>2</sub>O contents (0.01%–1.47%), which were significantly lower than those found in the primary magmas of Large Igneous Provinces. These results suggest that the basaltic volcaniclastic rocks on the Xisha Islands originated from a volatile-poor mantle plume source.</p>

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Geochemical characteristics and petrogenesis of the basaltic pyroclastic rocks from the Xisha Islands, northwestern South China Sea

  • 期刊论文

  • 发表日期：

    2025年9月1日

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  • Yu Zhang,
  • Yaqi Yuan,
  • Kefu Yu

摘要

As one of the micro-blocks dispersed in the South China Sea (SCS), the Xisha Islands are covered by thick Cenozoic sedimentary layers, making it challenging to obtain magmatic rocks. Well CK-2 is a kilometer-scale scientific drilling project on the Xisha Islands in the northwestern SCS. It penetrates the thick reef limestone and reaches basaltic pyroclastic rocks. This study presents the whole-rock and olivine compositions of the basaltic volcaniclastic rocks from Well CK-2. These rocks exhibited ocean island basalt signatures characterized by the enrichment of light rare earth elements and high-field-strength elements. Compared with partial melting products derived from mantle peridotite, whole-rock compositions showed elevated Fe/Mn and Zn/Fe mass ratios. Additionally, olivines were characterized by a lower Ca content, higher Ni content, elevated Fe/Mn mass ratios, and moderate Mn/Zn mass ratios compared to those crystallized from peridotitic melts. The compositions of both the whole-rock and olivine phenocrysts indicate the presence of pyroxenite in the mantle source, which likely formed through the reaction of recycled oceanic crust with the surrounding mantle peridotite. Using the olivine-liquid Mg-Fe exchange thermometer, this study derived mantle potential temperatures (Tp) ranging from 1 502 °C to 1 756 °C, which is consistent with those of plume-related ocean island basalts. Furthermore, the basaltic volcaniclastic rocks exhibit low H2O contents (0.01%–1.47%), which were significantly lower than those found in the primary magmas of Large Igneous Provinces. These results suggest that the basaltic volcaniclastic rocks on the Xisha Islands originated from a volatile-poor mantle plume source.