The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive ε_Nd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average ε_Nd(2050 Ma) of Archean TTGs (–10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The ε_Nd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (–0.9 to +0.6 and –3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (∼2–4 kbar, ∼7–15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.

非造山构造环境中铁质 A 型花岗岩的起源仍然是一个长期存在的岩石学难题。提出的模型范围从地幔岩浆的极端分异结晶到地壳岩石的部分熔融，或涉及两者的组合。在这项研究中，我们应用全岩化学和 Sm-Nd 同位素组成以及热力学约束模型（岩浆室模拟器，MCS）来破译一套 A1 型过碱性到过铝质花岗岩和相关中间岩（二长闪长岩-二长岩）的成因，正长岩）来自芬兰中部芬诺斯坎迪亚地盾的太古代卡累利阿克拉通西南边缘。这些深成岩在大约 2 处就位。05 Ga 在卡累利阿克拉通西缘破裂的早期阶段，显示出与海岛玄武岩型岩浆的微量元素亲和力。中间岩石显示正ε_Nd (2050 Ma) 值（+1.3 至 +2.6），仅略低于估计的同期耗竭地幔值（+3.4），但远高于太古代 TTG 的平均 ε _Nd（2050 Ma）（-10 ) 在周围的基岩中，表明这些岩石基本上来自地幔源。ε _Nd(2050 Ma) 过碱性和过铝质花岗岩样品的值重叠（分别为–0.9 到+0.6 和–3.2 到+0.9）并且略低于中间岩中的值，表明花岗岩的母体基性岩浆必须在分馏过程中已经同化了一些较旧的太古代大陆地壳，这与花岗岩成员的类大陆地壳微量元素特征一致。MCS 模型表明地幔源岩浆的分异结晶可以解释中间岩的主要元素特征。花岗岩的产生需要这些岩浆的进一步分步结晶以及太古代地壳的同化作用。这些过程发生在地壳中上层（~2-4 kbar，7-15 公里），涉及大量单斜辉石、斜长石和橄榄石的结晶。我们的研究结果强调了耦合 FC-AFC 过程在 A 型岩浆成因过程中的重要性，并支持了 A 型铁质花岗岩的岩浆通过地壳同化而变得更含铝的普遍看法。他们进一步表明，同化开始时岩浆的可变分馏路径可能解释了 A 型长英质和中间火成岩的种类繁多，这些岩浆经常被观察到在同一火成岩复合体中在时间和空间上紧密排列。