广西北海涠洲岛(含斜阳岛)第四纪玄武质火山岩的地球化学性质及其源区特征

Geochemistry of Quaternary basaltic volcanic rocks of Weizhou island in Beihai City of Guangxi and a discussion on characteristics of their source

广西北海涠洲岛 (含斜阳岛 )是我国最大的第四纪火山岩海岛 ,岛内出露中更新世 (Q2 )溢流相玄武质火山岩、晚更新世 (Q3 )空落沉积相沉火山碎屑岩和全新世 (Q4)爆发溢流相玄武质火山岩 ,其中中更新世为玄武岩 ,全新世多为碧玄岩或苦橄质碧玄岩。两者比较 ,前者相对高SiO2 、低MgO ;后者相对高MgO、低SiO2 。在地球化学上 ,这些玄武质火山岩均属无Eu异常的轻稀土元素富集和无Nb异常的强不相容元素富集类型 ,较之原始地幔和N_MORB具较低的Th/La值 (0 .0 4～ 0 .2 8)、La/Nb值 (0 .5 2～ 0 .91)、Rb/Nb值 (0 .4 5～ 1.0 9)、87Sr/ 86Sr值 (0 .70 35 2～0 .70 4 4 3)以及较高的εNd值 (+3.2～ +5 .2 )。上述特征说明 ,晚更新世火山岩经历了以橄榄石和斜长石为主的分离结晶 ,全新世火山岩则代表了部分熔融形成的原生岩浆 ,同化混染作用对这两种岩浆演化的影响程度很低。根据微量元素和同位素示踪可知 ,涠洲岛地区火山岩的源区类似于洋岛 ,其岩浆来自更深的软流圈 ,且无大陆岩石圈组分的卷入。为此进一步推测 ,我国雷琼和北部湾地区深部存在一个从属于南海盆地地幔热柱的亚热柱 。

Weizhou island in Beihai City of Guangxi is the largest Quaternary volcanic oceanic island in China. The surface of the island is covered with Middle Pleistocene (Q_2) basaltic volcanic rocks of effusive facies, Late Pleistocene (Q_3) sedimentary pyroclastic rocks of air_fall accumulation facies and Holocene (Q_4) basaltic volcanic rocks of explosive_effusive facies. Among them, Middle Pleistocene is dominated by basalt whereas Holocene by basanite or picritic basanite, with the former having higher SiO_2 and lower MgO than the latter. In geochemistry, these basaltic rocks are enriched in light rare earth elements without Eu anomaly and strong incompatible elements without Nb anomaly. These rocks have lower Th/La (0.14～0.28), La/Nb (0.52～0.91), Rb/Nb (0.45～1.09) and ^(87)Sr/^(86)Sr (0.703?52～0.704?43) ratios and higher εNd values (+3.2～+5.2) than other Cenozoic basalts in South China. These characteristics show that Late Pleistocene volcanic rocks were formed through fractional crystallization of olivine and plagioclase, that Holocene basaltic volcanic rocks represent primary magma formed by partial melting, and that crustal contamination seems not important in the formation and evolution of these two sorts of magma. Based on tracing of trace elements and isotopes, the authors infer that the source of volcanic rocks in Weizhou island is the same as that of the oceanic island. The magma must have originated from asthenosphere without the involvement of lithosphere components. Weizhou island represents asthenosphere window in epicontinental lithosphere of South China. Therefore, it is inferred that there exists a sub_plume in the depth of Leiqiong region and Beibu Bay, which belongs to Nanhai sea mantle plume whose activity lasted until Holocene.