The subduction system in southern Patagonia provides direct evidence for the variability of the position of an active continental arc with respect to the subducting plate through time, but the consequences on the arc magmatic record are less well studied. Here we present a geochemical and geochronological study on small plutons and dykes from the upper crust of the southern Patagonian Andes at ~ 51°S, which formed as a result of the subduction of the Nazca and Antarctic plates beneath the South American continent. In situ U–Pb geochronology on zircons and bulk rock geochemical data of plutonic and dyke rocks are used to constrain the magmatic evolution of the retro-arc over the last 30 Ma. We demonstrate that these combined U–Pb and geochemical data for magmatic rocks track the temporal and spatial migration of the active arc, and associated retro-arc magmatism. Our dataset indicates that the rear-arc area is characterized by small volumes of alkaline basaltic magmas at 29–30 Ma that are characterized by low La/Nb and Th/Nb ratios with negligible arc signatures. Subsequent progressive eastward migration of the active arc culminated with the emplacement of calc-alkaline plutons and dikes ~ 17–16 Ma with elevated La/Nb and Th/Nb ratios and typical subduction signatures constraining the easternmost position of the southern Patagonian batholith at that time. Geochemical data on the post-16 Ma igneous rocks including the Torres del Paine laccolith indicate an evolution to transitional K-rich calc-alkaline magmatism at 12.5 ± 0.2 Ma. We show that trace element ratios such as Nb/Ta and Dy/Yb systematically decrease with increasing SiO2, for both the 17–16 Ma calc-alkaline and the 12–13 Ma K-rich transitional magmatism. In contrast, Th/Nb and La/Nb monitor the changes in the source composition of these magmas. We suggest that the transition from the common calc-alkaline to K-rich transitional magmatism involves a change in the source component, while the trace element ratios, such as Nb/Ta and Dy/Yb, of derivative higher silica content liquids are controlled by similar fractionating mineral assemblages. Analysis of a global compilation of Nb/Ta ratios of arc magmatic rocks and simple geochemical models indicate that amphibole and variable amounts of biotite exert a major control on the low Dy/Yb and Nb/Ta of derivative granitic liquids. Lastly, we suggest that the low Nb/Ta ratio of silica-rich magmas is a natural consequence of biotite fractionation and that alternative models such as amphibolite melting in subduction zones and diffusive fractionation are not required to explain the Nb/Ta ratio of the upper continental crust.

中文翻译：

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巴塔哥尼亚南部（托雷斯德尔潘恩地区）与俯冲相关的岩体和堤群以及上地壳火成岩的低 Nb/Ta 的源和分异控制

巴塔哥尼亚南部的俯冲系统为活动大陆弧相对于俯冲板块的位置随时间的变化提供了直接证据，但对弧岩浆记录的影响研究较少。在这里，我们对南巴塔哥尼亚安第斯山脉南部上地壳的小岩体和岩脉进行了地球化学和地质年代学研究，该岩脉位于南美洲大陆下方，由纳斯卡和南极板块俯冲形成，约 51°S。锆石的原位 U-Pb 年代学和深成岩和岩脉岩石的大块岩石地球化学数据被用来限制过去 30 Ma 中弧后的岩浆演化。我们证明这些岩浆岩的 U-Pb 和地球化学组合数据跟踪了活动弧的时空迁移，和相关的弧后岩浆作用。我们的数据集表明，弧后区域的特征是 29-30 Ma 的少量碱性玄武岩岩浆，其特征是低 La/Nb 和 Th/Nb 比率，弧特征可忽略不计。随后活动弧的逐渐向东迁移以钙碱性岩体和岩脉的就位而告终，大约 17-16 Ma，La/Nb 和 Th/Nb 比率升高，典型的俯冲特征限制了当时巴塔哥尼亚南部岩基最东端的位置. 包括 Torres del Paine laccolith 在内的后 16 Ma 火成岩的地球化学数据表明，在 12.5 ± 0.2 Ma 处演化为过渡性富钾钙碱性岩浆作用。我们表明，随着 SiO2 的增加，微量元素比率如 Nb/Ta 和 Dy/Yb 对于 17-16 Ma 钙碱性和 12-13 Ma 富钾过渡岩浆作用。相比之下，Th/Nb 和 La/Nb 监测这些岩浆源成分的变化。我们认为，从普通钙碱性到富钾过渡岩浆作用的转变涉及源成分的变化，而衍生的较高二氧化硅含量液体的微量元素比，如 Nb/Ta 和 Dy/Yb 由以下因素控制：类似的分馏矿物组合。对弧形岩浆岩 Nb/Ta 比率的全球汇编和简单地球化学模型的分析表明，角闪石和可变数量的黑云母对衍生花岗岩液体的低 Dy/Yb 和 Nb/Ta 起到主要控制作用。最后，