Abstract The Lebyazhka acid metavolcanics occupying 40% of the Keivy Terrane in the Arctic Region of the Fennoscandian Shield belong to mainly alkali-calcic and calc-alkaline series and are classified as peraluminous to metaluminous metadacites, metarhyodacites, metarhyolites and metatrachyrhyolites. Geochemical characteristics of most of these metavolcanics are very close or identical to those discriminative of A-type granites. Compositionally these rocks are similar and often identical to the Keivy A-type peralkaline granites. On variation plots and geochemical spider and discriminant diagrams the Lebyazhka acid metavolcanics and Keivy peralkaline granites share the same trends and patterns and fall into fields of post-orogenic, within-plate and A-type granites. A U–Pb (SHRIMP II) zircon age of the Lebyazhka acid metavolcanics is 2678 ± 7 Ma. The first thermal event recorded by the U–Pb isotope system of magmatic zircon occurred at ca. 1.90 Ga, an age corresponding to the termination of the Paleoproterozoic Lapland-Kola collisional orogeny. The youngest zircon generation is dated at 1805 ± 23 Ma ago and is assumed to have grown during metasomatic or hydrothermal alteration. The finding of relics of baddeleyite, a mineral crystallizing only in mafic and ultramafic igneous rocks, in zircon from the Lebyazhka acid metavolcanics along with the chemical similarity of these metavolcanics and the Keivy peralkaline granites and their emplacement simultaneously with the Keivy gabbro-anorthosites 2.68–2.66 Ga ago, obviously suggests that these three types of rocks should have a common origin. Based on new data and existing tectonic models, the origin of the Lebyazhka acid metavolcanics as well as the Keivy peralkaline granites, gabbro-anorthosites and nepheline syenites is concluded to be linked with underplating by basaltic magma of the lower crust of the eastern Kola Province 2.68 Ga ago. This underplating led to anorogenic bimodal magmatism including the 2678 Ma Lebyazhka acid metavolcanics, the 2674–2663 Ma Keivy peralkaline granites, the 2674–2659 Ma Keivy gabbro-anorthosites, and the 2645 Ma Sakharjok nepheline syenites and subalkaline gabbro. The underplating also caused partial melting of the Mesoarchean continental crust and involvement of this older crustal component into felsic melts parental for the Lebyazhka acid metavolcanics and the Keivy peralkaline granites.

中文翻译：

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芬诺斯坎迪亚地盾东北部 Keivy Terrane 中的新太古代 A 型酸性变质火山：地球化学、年龄和起源

摘要 Fennoscandian地盾北极地区占Keivy Terrane 40%的Lebyazhka酸性变火山岩主要属于碱-钙和钙-碱性系列，分为过铝质至准铝质变英安岩、变水英石、变流纹岩和变质流纹岩。大多数这些变质火山的地球化学特征与区分 A 型花岗岩的地球化学特征非常接近或相同。这些岩石在成分上与 Keivy A 型高碱性花岗岩相似且通常相同。在变异图和地球化学蜘蛛和判别图上，Lebyazhka 酸性变质火山和 Keivy 过碱性花岗岩具有相同的趋势和模式，属于后造山、板内和 A 型花岗岩领域。Lebyazhka 酸性火山岩的 AU-Pb (SHRIMP II) 锆石年龄为 2678 ± 7 Ma。岩浆锆石 U-Pb 同位素系统记录的第一次热事件发生在大约 1.90 Ga，对应于古元古代拉普兰-科拉碰撞造山运动结束的年龄。最年轻的锆石年代为 1805 ± 23 Ma，推测是在交代或热液蚀变过程中生长的。在来自 Lebyazhka 酸性变质火山的锆石中发现了斜斜辉石遗迹，一种仅在镁铁质和超镁铁质火成岩中结晶的矿物，以及这些变质火山和 Keivy 过碱性花岗岩的化学相似性以及它们与 Keivy 辉长岩-斜长岩的同时侵位 2.6 2.66 Ga 以前，显然表明这三种岩石应该有一个共同的成因。基于新数据和现有构造模型，Lebyazhka酸性变质火山以及Keivy过碱性花岗岩、辉长斜长岩和霞石正长岩的成因被认为与2.68 Ga前东部科拉省下地壳玄武质岩浆的底侵有关。这种底板导致了非造山双峰岩浆作用，包括 2678 Ma Lebyazhka 酸性变质火山、2674-2663 Ma Keivy 过碱性花岗岩、2674-2659 Ma Keivy 辉长岩-斜长岩和 2645 Ma Sakharjokline 辉长岩 底镀层还导致中太古代大陆地壳部分熔融，并使这一较老的地壳成分卷入 Lebyazhka 酸性变质火山和 Keivy 高碱性花岗岩的母长英质熔体中。辉长斜长岩和霞石正长岩被认为与2.68 Ga前东部科拉省下地壳玄武质岩浆的底侵有关。这种底板导致了非造山双峰岩浆作用，包括 2678 Ma Lebyazhka 酸性变质火山、2674-2663 Ma Keivy 过碱性花岗岩、2674-2659 Ma Keivy 辉长岩-斜长岩和 2645 Ma Sakharjokline 辉长岩 底镀层还导致中太古代大陆地壳部分熔融，并使这一较老的地壳成分卷入 Lebyazhka 酸性变质火山和 Keivy 高碱性花岗岩的母长英质熔体中。辉长斜长岩和霞石正长岩被认为与2.68 Ga前东部科拉省下地壳玄武质岩浆的底侵有关。这种底板导致了非造山双峰岩浆作用，包括 2678 Ma Lebyazhka 酸性变质火山、2674-2663 Ma Keivy 过碱性花岗岩、2674-2659 Ma Keivy 辉长岩-斜长岩和 2645 Ma Sakharjokline 辉长岩 底镀层还导致中太古代大陆地壳部分熔融，并使这一较老的地壳成分卷入 Lebyazhka 酸性变质火山和 Keivy 高碱性花岗岩的母长英质熔体中。2674-2663 Ma Keivy 过碱性花岗岩、2674-2659 Ma Keivy 辉长岩斜长岩和 2645 Ma Sakharjok 霞石正长岩和亚碱性辉长岩。底镀层还导致中太古代大陆地壳部分熔融，并使这一较老的地壳成分卷入 Lebyazhka 酸性变质火山和 Keivy 高碱性花岗岩的母长英质熔体中。2674-2663 Ma Keivy 过碱性花岗岩、2674-2659 Ma Keivy 辉长岩斜长岩和 2645 Ma Sakharjok 霞石正长岩和亚碱性辉长岩。底镀层还导致中太古代大陆地壳部分熔融，并使这一较老的地壳成分卷入 Lebyazhka 酸性变质火山和 Keivy 高碱性花岗岩的母长英质熔体中。