广西涠洲岛晚新生代玄武岩地幔源区及岩浆成因

涠洲岛作为我国最年轻的第四纪火山岩岛，其火山活动表现出多期、多旋回和多喷发中心的特征，但其地幔源区特征和岩浆成因依然存在争议。本文对涠洲岛玄武岩开展了详细的矿物学和全岩主、微量元素及Sr-Nd-Pb同位素研究，以揭示其地幔源区特征和岩浆成因。涠洲岛玄武岩主要为碱性玄武岩，在岩浆上升过程，几乎未受到地壳物质的混染，经历了橄榄石和单斜辉石的分离结晶作用。轻稀土（LREE）富集、重稀土（HREE）亏损，轻、重稀土强烈分馏（（La/Yb）_N=14.42~28.64），Nb、Ta明显正异常，显示出与洋岛玄武岩（OIB）相似的微量元素和Sr-Nd-Pb同位素特征。Sr-Nd-Pb同位素比值变化较均一，且呈现出亏损地幔端元（DM）与富集地幔端元（EM2）的二元混合趋势。其中，EM2端元可能源于海南地幔柱。Sr/Sr^*（1.21~2.36）和Eu/Eu^*（1.01~1.11）正异常，指示源区存在再循环辉长岩洋壳组分。结合已有的地震层析成像结果和岩石地球化学数据，得出南海及周缘地区的晚新生代玄武岩的形成受控于海南地幔柱。伴随着海南地幔柱的上升，再循环的辉长岩洋壳经部分熔融与地幔橄榄岩反应生成石榴石辉石岩（贫硅辉石岩），石榴石辉石岩和未反应的地幔橄榄岩混合部分熔融形成涠洲岛玄武岩。

Mantle sources and magma genesis of Late Cenozoic basalts in Weizhou Island, Guangxi, China

As the youngest Quaternary volcanic island in China, the volcanic activities of Weizhou Island are characterized by multi-stage, multi-cycle and multi-eruption centers, however, whether the volcanic rocks in this island have mantle source and what is the genesis of the magma remain controversial. Here, we present a comprehensive study of mineral chemistry, whole-rock major and trace elements, and Sr-Nd-Pb isotopes of basalts from Weizhou Island, aiming to reveal their mantle sources and magma genesis. The basalts from Weizhou Island are mainly alkali basalts, which were barely contaminated by crustal material and experienced fractional crystallization of olivine and clinopyroxene, showing oceanic island basalt (OIB)-like trace elements and Sr-Nd-Pb isotopic signatures as follows: with a light rare earth elements (LREEs) enrichment and a heavy rare earth elements (HREEs) depletion, and highly fractionation of LREEs and HREEs ((La/Yb)N=14.42~28.64), significant positive abnormalities in Nb and Ta. Furthermore, they have relatively homogeneous Sr-Nd-Pb isotope ratios that show a trend of binary mixing between depleted mantle (DM) and enriched mantle Ⅱ (EM2). Moreover, the EM2 component may be originated from the Hainan mantle plume. The positive Sr/Sr* (1.21~2.36) and Eu/Eu* (1.01~1.11) anomalies indicate the existence of recycled gabbro oceanic crust in its source region. Combining the previous seismic tomography results and geochemical data, we suggest that the Late Cenozoic magmatism of the South China Sea and its surrounding areas may be controlled by the Hainan mantle plume. In the upwelling mantle plume, the recycled gabbro oceanic crust is partially melted and reacts with mantle peridotites to produce garnet pyroxenites (silica-deficient pyroxenite). Therefore, we suggest that basalts from Weizhou Island may be originated from a mixing source of garnet pyroxenites and unreacted peridotites.