东非裂谷系统(EARS)地幔柱成因的新生代火山作用地球化学标志

The geochemical criteria of mantle plume origin for Cenozoic volcanism, Eastern African Rift System(EARS)

本文应用岩浆岩地球化学图解对东非裂谷系统(EARS)火山岩的岩石地球化学数据进行处理,重点回顾和讨论埃塞俄比亚大裂谷(MER)、阿法(Afar)盆地及肯尼亚裂谷火山作用的构造环境和地幔柱成因。MER火山岩的岩石类型具双峰式火山岩套特征,以高Ti的大陆溢流玄武岩(CFB)-大陆洋岛玄武岩(OIB)-流纹岩系列为主,缺少中性岩,是来自地幔柱岩浆分异的结果,与板块俯冲作用无关。阿法(Afar)盆地和红海为CFB-MORB系列。肯尼亚裂谷(KR)及埃塞俄比亚大裂谷最南端图尔卡纳盆地只见以大陆OIB,缺乏流纹岩。EARS是一个主动型地幔柱,由地幔上涌冲击地壳底部而成,其火山岩以富集常量元素Ti、Fe和Mg,富集高场强元素Nb、Ta和相容元素V、Cr、Co和Ni为特征。绝大多数EARS火山岩的Nb/Zr>0.04和Ta/Hf比值>0.1,在地球化学-构造环境判别图上落在板内火山岩的范围内(大洋板内和地幔柱),集中在Nb/Zr>0.15和Ta/Hf比值>0.3范围内的火山岩样品可能为主动地幔柱成因。根据La/Nb、Ce/Pb和Ba/La比值,地幔柱成因的MER火山岩可分为受地壳混染地幔和未受地壳混染的原始地幔两种类型。La/Nb(≤1)、Ce/Pb(30~50)、Ba/Nb比值(>10)和La/Yb≤12是来源于未受地壳混染原始地幔的重要判别标志。地球化学证据(微量元素,Sr-Nd-Pb-He同位素)表明,MER-阿拉伯-也门地幔柱起源于深部核-幔边界的HIMU。MER-Afar的前裂谷和同裂谷火山岩(50-12 Ma)具有高的3He同位素标志(R/Ra比值>16.4)说明其非洲板块之下存在一个深藏的地幔岩浆源深度大于670 km,位于石榴子石-尖晶石橄榄岩过渡带(图6b、c、d)。MER-Afar大火山岩省及其最南端图尔卡纳湖和肯尼亚地体(克拉通,改造的克拉通边缘和活动带)深部地壳底部之下的HIMU地幔可能是导致裂谷型的OIB和CFB异常发育的岩浆源区,而Afar洼地(吉布提)、红海和亚丁湾形成于后裂谷期(5~0 Ma)的MORB和亏损LREE玄武岩则归因于HIMU、富集地幔(EM1-EM2)与DM混合地幔源的熔融,其岩浆源区位地壳拉张减薄带之下的尖晶石橄榄岩区。

Based on the compilation of published geochemical data by utilizing the geochemical software of igneous petrology to deal with litho-geochemical data of Eastern African Rift System（ EARS）, this paper discusses and reviews the tectonic setting and mantle plume origin for the volcanism of Main Ethiopia Rift（MER）,Afar depression and Kenya Rift. The volcanic types of MER are composed of high-Ti continentalflood basalt（CFB）-continental ocea island basalt（OIB）-rhyolite series, which are bimodal volcanic suiteswith lack ofintermediate rocks. These rock series may be considered to be generated by grammatical differentiation of mantle plume,and therefore their originis not related to slab subduction. Afar depression and Red Sea display CFB-MORB series. Kenya Rift and the Turkana depression（the most southern part of MER）show continental OIB-type only with lack of rhyolites.EARS is a result of a large, active mantle plume impinging upon the base of continental crust. All of their volcanic rocksare characterized by enrichment of main elements of Ti、Fe and Mg, trace elements（HFSE）of Nb and Ta,and metal elements of V、Cr、Co and Ni.Almost EARS＇s volcanic rocks have ratios of Nb/Zr 〉0.04 and Ta/Hf〉 0.1,and present on the discrimination diagrams of geochem–tectonic setting in the intraplate basalt area（ within oceanic plate and mantle plume）,in which the volcanic rocks with high ratios of Nb/Zr〉0.15 and Ta/Hf〉0.3 are related to mantle plume origin.According to La/Nb 、Ce/Pb and La/Ba ratios,however,the rocks derived from mantle plume may be classified into tow types： primary mantle and mantle contaminated by continental crust. The important criteria to determine the uncontaminatedprimary mantle mainly depend on the ratios of La/Nb（≤1）,Ce/Pb（30～50）,Ba/Nb（10）and La/Yb≤12.Geochemical evidence（trace elements, Sr-Nd-Pb-He isotopic ratios） display that the mantle plumes of MER-Saudi Arabia-Ymen are originated from the core-mantle boundary. The MER-Afar＇s pre-and syn-rift volcanic rocks（50～12 Ma）with a high-3 He component（up to 16.4 Ra）originate from HIMU suggesting that there are a large,deeply sited mantle source at deeps below 670 km beneath the African plate,which is located within a mixture area of spinel-garget peridotite transition zone. The HIMU mantle below the deeply sited base of the lithosphere,underlain MER-Afar large igneous province, Kenya dome（craton,reworked craton margin and mobile belt） and Turkana depression, maybe a igneous source to create a number of rift-type of OIB and CFB＇s development,whereas the post-rift MORB and depleted LREE basalt erupted in Afar depression（Djibouti）,Red Sea and Aden Gulf are interpreted as a result of plume-derived melts from mixing of HIMU、enriched mantle（EM1-EM2） and DM, whose igneous source originates from a area of spinel-garget peridotite beneath the crust thinning and extension zone.