Lamprophyres and phonolite intrusions were sampled in the so-called Pre-Salt sequence of well 1-BRSA-905-RJS in northern Santos Basin nearby the Cabo Frio Structural High in SE Brazil. These alkaline rocks indicate the existence of a strongly silica-undersaturated trend. However, fractional crystallization modeling as well as Sr-Nd-Pb-Hf isotope data shows that the alkaline lamprophyres are not suitable parental compositions to the evolved felsic phonolitic magmas. The Sr-Nd-Pb-Hf isotope compositions of the lamprophyric magmas represent mixing of EM1 and Trindade plume mantle components. The EM1 is possibly related with the melt- and/or fluid-induced metasomatic processes on the subcontinental lithospheric mantle (SCLM) that occurred beneath the South American Platform during the Neo-Proterozoic-Cambrian subduction processes associated with the final stages of the Gondwana amalgamation, as recorded by the T ages (601-499 Ma) of the studied intrusions in Santos. Ar/Ar analysis presented integrated age of 41.06±0.02Ma and 38.62±0.02Ma for phonolite and lamprophyre samples, respectively. A “plateau” age assignment for the true crystallization age of the sample could not be obtained due to the discordance in the ages of individual steps. This discordance was attributed to loss of radiogenic Ar that can happen in nature due to reheating events or metasomatism. An interaction between the Trindade plume head and SCLM is proposed for the geodynamics of Santos Basin at 40 Ma. The presence of an anomalously hot mantle represented by the deflected Trindade plume head beneath Santos at the Eocene is supported by the petrological and geochronological data. Therefore, it may be relevant to consider high geothemal flow regimes in basin evolution models proposed for Santos during the Eocene.

中文翻译：

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巴西桑托斯盆地盐下沉积序列始新世碱性侵入岩的岩石成因和地球动力学

在巴西东南部卡波弗里奥构造高点附近桑托斯盆地北部的 1-BRSA-905-RJS 井所谓的盐下层序中，对灯岩和响岩侵入体进行了采样。这些碱性岩石表明存在强烈的二氧化硅不饱和趋势。然而，分步结晶模型以及Sr-Nd-Pb-Hf同位素数据表明，碱性煌斑岩不是演化的长英质声石岩浆的合适母体成分。煌斑岩浆的 Sr-Nd-Pb-Hf 同位素组成代表了 EM1 和 Trindade 羽流地幔成分的混合。EM1可能与次大陆岩石圈地幔（SCLM）上的熔融和/或流体诱发的交代过程有关，该交代过程发生在与冈瓦纳合并最后阶段相关的新元古代-寒武纪俯冲过程期间南美地台下方，根据所研究的桑托斯侵入岩的 T 年龄（601-499 Ma）记录。Ar/Ar 分析显示响岩和煌斑岩样品的综合年龄分别为 41.06±0.02Ma 和 38.62±0.02Ma。由于各个步骤的年龄不一致，无法获得样品真实结晶年龄的“平台”年龄分配。这种不一致归因于自然界中由于再加热事件或交代作用而可能发生的放射性 Ar 的损失。针对 40 Ma 桑托斯盆地的地球动力学，提出了 Trindade 羽流头和 SCLM 之间的相互作用。岩石学和地质年代学数据支持了始新世桑托斯下方偏转的特林达德地幔头部所代表的异常热地幔的存在。因此，在为桑托斯提出的始新世盆地演化模型中考虑高地热流态可能是相关的。