The South Eastern Futuna Volcanic Zone (SEFVZ; 50 km × 80 km) is located in the NW of the Lau back-arc basin at the northern boundary of the Futuna and YAO microplates. Fatu Kapa is an area (∼15 km × 10 km) of diffuse volcanic activity at the center of the SEFVZ and at the intersection of two volcanic axes with the Futuna fracture zone. At this place, the volcanic, tectonic, and hydrothermal activities are enhanced. The most differentiated volcanic series of the whole Futuna region are found in the Fatu Kapa area. Based on major and trace element data, four groups of volcanic rocks are distinguished in the Fatu Kapa area. The main groups form a well-defined transitional suite from basalts to andesitic and dacitic lavas that display enrichments in Zr, Cl and heavy rare earth elements. The most mafic lavas of this group are extracted from an Indian-type depleted mantle mixed with a component with trace element and isotopic characteristics similar to the nearby Samoa mantle plume. There is no evidence of subduction input. Petrological modelling shows that the mafic melts of the main group (Group 1A) evolve towards trachyandesites through fractional crystallization coupled with assimilation of 10 % of a Cl-rich and Th-poor melt generated by low-degree (≤2 %) partial melting of a hydrothermally altered basaltic crust. Rocks of Groups 1B(a) and 1B(b) were then generated by mixing of a mafic melt from Group 1A with a melt obtained by partial melting of andesitic and hydrothermally altered wallrock in shallow magma chambers. Extremely high Cl concentrations can be explained by assimilation of interstitial K- and Cl-rich brines in the pore water of the hydrothermally altered volcanic rocks before melting of these rocks. This is in good agreement with unusual extension of the Fatu Kapa hydrothermal area and indicates that, in the north of the Lau basin, the microplate boundaries are privileged zones to generate a complex felsic magmatism within areas of widespread hydrothermal alteration. Our study confirms the importance of assimilation and fractional crystallization and mixing processes in generating high-silica lavas in back-arc environments decoupled from subduction.

中文翻译：

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法图卡帕岩浆系统（西北刘弧后盆地）的成因与演化：高硅质熔岩成因的洞见

东南富图纳火山带 (SEFVZ; 50 km × 80 km) 位于富图纳和 YAO 微板块北部边界的 Lau 弧后盆地的西北。Fatu Kapa 是 SEFVZ 中心和两个火山轴与富图纳断裂带相交处的扩散火山活动区域（~15 km × 10 km）。在这个地方，火山、构造和热液活动得到加强。在法图卡帕地区发现了整个富图纳地区最具差异化的火山系列。根据主量元素和微量元素数据，法图卡帕地区火山岩分为四组。主要组形成了一个明确的过渡套件，从玄武岩到安山岩和英安岩熔岩，显示出富含 Zr、Cl 和重稀土元素。该组中镁铁质含量最高的熔岩是从印度型贫化地幔中提取的，该地幔中混合了一种与附近萨摩亚地幔柱相似的微量元素和同位素特征的成分。没有俯冲输入的证据。岩石学模型表明，主要组（1A 组）的基性熔体通过分级结晶以及由低度（≤2%）部分熔融产生的 10% 的富 Cl 和贫 Th 熔体同化向粗面长安岩演化。一种热液蚀变的玄武质地壳。然后，通过将来自 1A 组的镁铁质熔体与在浅岩浆房中部分熔融安山岩和热液蚀变围岩获得的熔体混合，生成 1B(a) 组和 1B(b) 组岩石。极高的 Cl 浓度可以通过在热液蚀变的火山岩熔融之前同化间隙中富含 K 和 Cl 的盐水来解释。这与 Fatu Kapa 热液区的异常延伸非常吻合，并表明在 Lau 盆地北部，微板块边界是在广泛的热液蚀变区域内产生复杂长英质岩浆的特权区。我们的研究证实了同化和分级结晶以及混合过程在与俯冲脱钩的弧后环境中生成高硅熔岩的重要性。在 Lau 盆地北部，微板块边界是在广泛的热液蚀变区域内产生复杂的长英质岩浆作用的特权区。我们的研究证实了同化和分级结晶以及混合过程在与俯冲脱钩的弧后环境中生成高硅熔岩的重要性。在 Lau 盆地北部，微板块边界是在广泛的热液蚀变区域内产生复杂的长英质岩浆作用的特权区。我们的研究证实了同化和分级结晶以及混合过程在与俯冲脱钩的弧后环境中生成高硅熔岩的重要性。