北部湾涠洲岛及斜阳岛火山岩微量元素和同位素地球化学及其构造意义

Trace-element and isotopic geochemistry of volcanic rocks and it's techtonic implications in Weizhou Island and Xieyang Island,Northern Bay.

本文在北部湾内一对姊妹火山岛即涠洲岛及斜阳岛火山地质研究基础上,进一步开展火山岩微量元素和 Sr-Nd-Pb同位素,以及地幔橄榄岩 Re-Os 同位素地球化学研究。岛上早晚两期火山岩均为碱性玄武岩,分别属于碱性橄榄玄武岩和碧玄岩。碧玄岩为玻基斑状结构,含地幔橄榄岩碎块(一般<1cm),表明为地幔岩浆快速喷出地表冷凝而成,岩浆上升过程中极少演化。火山岩微量元素和 Sr-Nd-Pb 同位素资料表明,涠洲岛及斜阳岛玄武岩与雷琼及北部湾周边、南海海盆玄武岩类似,具有亏损地幔的 Sr、Nd 同位素组成与 Pb 同位素显示的 EMII 富集地幔特征的 Dupal 异常,表明岩浆并非来自单一地幔源区,不可与 OIB 或 MORB 源区简单类比,也非地幔柱成因,而是由两个不同的地球化学组分混合而成。Re-Os 同位素特征也指示地幔橄榄岩捕虏体来源于亏损的岩石圈地幔,而非核幔边界。推测涠洲岛及斜阳岛与雷琼及北部湾周边的岩浆可能是由于南海扩张后大陆裂解一软流圈地幔热物质上涌,与上覆薄而年轻的岩石圈地幔相互作用的产物。

Weizhou Island and Xieyang Island are twin volcanoes in Northern Bay. Based on detailed geological researches, in this paper we report geochemical researches on trace elements and Sr-Nd-pb isotopes in volcanic rocks and researches on Re-Os isotopes in mantle xenoliths. In the islands, volcanic rocks of the two stages, early stage and late stage, are all alkaline basalt, including alkaline olivine basalt and picrite. The pierite has porphyritic texture, with glassy matrix and fragments of mantle xenoliths （usually less than lcm）. This indicates that ascending speed of the magma from the mantle chamber was very quick, and hence the magma evolution was not significant for these volcanoes. Geochemistry of trace elements and Sr-Nd-Pb isotopes in the volcanic rocks in the Weizhou Island and Xeiyang Island show similar characteristics with the surrounding volcanics in Leiqiong, circum Norhtern Bay and Southern Sea Basin. Sr, Nd isotopes show characteristics of DMM （ depleted mantle member）, while Pb isotopes show characteristics of Dupal abnormity which means mixing of EMIl source. The geochemical characteristics reveals that the basalts in the two islands are not similar with MORB or OIB, and the magma is not derived from a simple mantle source, but from a mixture of two different types of mantles sources. Geochemistry of volcanic rocks indicate that the magma genesis shows no signal of mantle plume, and geochemistry of Re-Os isotopes of the mantle xenoliths also suggest that the magma is from depleted lithospheric mantle, not core-mantle boundary. However we suggest the story of the magma genesis of Northern Bay area is that the extension of South China Sea and disaggregation of the continent induced the asthenosphere upwelling, then mixed and reacted with the young and thin lithospheric mantle, and magma formed.