Lithospheric to asthenospheric transition in Low-Ti flood basalts from southern Paraná, Brazil
Lithospheric to asthenospheric transition in Low-Ti flood basalts from southern Paraná, Brazil
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DOI：
10.1016/0009-2541(95)00086-0
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发表时间：
1996-01
期刊：
影响因子：
3.9
通讯作者：
D. Peate;C. Hawkesworth
中科院分区：
文献类型：
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作者：
D. Peate;C. Hawkesworth
根据某些元素丰度和比率，在巴拉那州南部的溢流玄武岩序列中区分了两种地球化学上不同的低钛岩浆类型（格拉玛多和埃斯梅拉达）。与格拉玛多岩浆相比，埃斯梅拉达岩浆具有较低的 Ce/Sm 和 (87Sr86Sr )，以及较高的 Ti Zr 和 ϵNd。详细的地层学研究表明，存在从格拉玛多型岩浆到埃斯梅拉达型岩浆的时间演化过程。这种成分变化不能简单地用软流圈熔融物的地壳污染程度随着时间的推移而下降来解释，相反，这两种岩浆类型似乎是从不同的母体岩浆演化而来的。尽管格拉玛多岩浆的成分变化主要是地壳同化和分异结晶作用，但推断的母岩浆的地球化学特征[低Ti Y < 300和低Nb La < 0.8，负εNd值~ -3和高(87Sr86Sr )i0.708−0.710]表明大陆地幔岩石圈的显着贡献。格拉玛多岩浆还表现出明显的区域变化，这可能意味着地幔源在数百公里的长度范围内成分不均匀。有人建议，Esmeralda 岩浆类型内的变化是通过在相对较浅的地壳水平上混合 ϵNd>+4 的软流圈熔体和 Gramado 型岩浆，然后进行分级结晶来控制的。软流圈成分具有与 MORB 相当的相对贫化的不相容元素特征。因此，巴拉那州南部的低钛熔岩序列记录了随着时间的推移，从岩石圈为主向软流圈特征的转变。巴拉那溢流玄武岩似乎是由于岩石圈扩张而产生的，该扩张与南大西洋在异常热地幔区域上的开口有关，归因于特里斯坦地幔柱的存在。然而，在特里斯坦达库尼亚岛、高夫岛和伊纳塞西布尔岛看到的现代特里斯坦羽流的独特地球化学特征在巴拉那州南部的低钛岩浆成分中并不明显。
Two geochemically distinct Low-Ti magma types (Gramado and Esmeralda) are distinguished within the flood basalt sequences of southern Paraná on the basis of certain element abundances and ratios. Esmeralda magmas have lower Ce/Sm and (87Sr86Sr ),iand higher Ti Zr and ϵNd, than Gramado magmas. Detailed stratigraphical work indicates that there was a temporal progression from Gramado- to Esmeralda-type magmas. This compositional shift cannot simply be explained in terms of a declining extent of crustal contamination of an asthenosphere-derived melt with time, and instead it seems that the two magma types evolved from distinct parental magmas. Although compositional variations of Gramado magmas are dominated by crustal assimilation and fractional crystallisation, geochemical features of the inferred parental magmas [low Ti Y of < 300 and low Nb La of < 0.8, negative ϵNd-values ∼ −3 and high (87Sr86Sr )i0.708−0.710] are indicative of a significant contribution from the continental mantle lithosphere. Gramado magmas also show a marked regional variation which might imply that the mantle source was compositionally heterogeneous over a length scale of a few hundred km. It is suggested that variations within the Esmeralda magma type are controlled by mixing between an asthenospheric melt with ϵNd> + 4 and a Gramado-type magma, at relatively shallow crustal levels, followed by fractional crystallisation. The asthenospheric component has relatively depleted incompatible element characteristics comparable to MORB. Thus, Low-Ti lava sequences of southern Paraná record a shift from a predominantly lithospheric to an asthenospheric signature with time. Paraná flood basalts appear to have been generated in response to lithospheric extension associated with the opening of the South Atlantic over a region of anomalously hot mantle, attributed to the presence of the Tristan mantle plume. However, the distinctive geochemical signature of the modern Tristan plume, seen on the islands of Tristan da Cunha, Gough and Inaccessible, is not evident in the compositions of Low-Ti magmas in southern Paraná.