Fe–Ti–P-rich mafic to intermediate rocks (monzodiorites and oxide–apatite–gabbronorites, OAGNs) are found as small intrusions in most AMCG (anorthosite–magnerite–charnokite–granite) suites. The origin of the monzodioritic rocks is still debated, but in many studies, they are presumed to represent residual liquid compositions after fractionation of anorthositic cumulates. In the 1.64 Ga Ahvenisto complex, SE Finland, monzodioritic rocks occur as minor dike-like lenses closely associated with anorthositic rocks. We report new field, petrographic, and geochemical (XRF, ICP-MS, EMPA) data complemented with crystallization modeling (rhyolite-MELTS, MAGFRAC) for the monzodioritic rocks, apatite–oxide–gabbronorite, and olivine-bearing anorthositic rocks of the Ahvenisto complex. The presented evidence suggest that the monzodioritic rocks closely represent melt compositions while the apatite–oxide–gabbronorite and olivine-bearing anorthositic rocks are cumulates. The monzodioritic rocks seem to form a liquid line of descent (LLD) from primitive olivine monzodiorites to more evolved monzodiorites. Petrological modeling suggests that the interpreted LLD closely corresponds to a residual melt trend left after fractional crystallization (FC) and formation of the cumulate anorthositic rocks and minor apatite–oxide–gabbronorite in shallow magma chambers. Consequent equilibrium crystallization (EC) of separate monzodioritic residual magma batches can produce the observed mineral assemblages and the low Mg numbers measured from olivine (Fo25–45) and pyroxenes (En48–63, Mg#cpx 60–69). The monzodioritic rocks and apatite–oxide–gabbronorites show similar petrological and geochemical characteristics to corresponding rock types in other AMCG suites, and the model described in this study could be applicable to them as well.

中文翻译：

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地块型斜长岩残余熔体的平衡结晶：芬兰东南部 1.64 Ga Ahvenisto 复合体的案例研究

在大多数 AMCG（斜长岩-菱镁矿-charnokite-花岗岩）套件中，富含 Fe-Ti-P 的镁铁质至中间岩石（二长闪长岩和氧化物-磷灰石-辉长岩，OAGNs）被发现为小侵入体。二长闪长岩的起源仍有争议，但在许多研究中，它们被假定为代表斜长岩堆积物分馏后的残余液体成分。在芬兰东南部的 1.64 Ga Ahvenisto 杂岩中，二闪长岩以与斜长岩密切相关的小堤状透镜体形式出现。我们报告了新的现场、岩相学和地球化学（XRF、ICP-MS、EMPA）数据，并辅以结晶模型（流纹岩-熔体、MAGFRAC），用于阿韦尼斯托的二长闪长岩、磷灰石-氧化物-辉长岩和含橄榄石的斜长岩复杂的。所提供的证据表明，二闪长岩非常接近熔体成分，而磷灰石-氧化物-辉长岩和含橄榄石的斜长岩则是堆积物。二闪长岩似乎形成了从原始橄榄石二闪长岩到更进化的二长闪长岩的液态下降线 (LLD)。岩石学模型表明，解释的 LLD 与分步结晶 (FC) 和在浅层岩浆房中形成堆积斜长岩和少量磷灰石-氧化物-辉长岩后留下的残余熔体趋势密切对应。随后分离的二闪长岩残余岩浆批次的平衡结晶 (EC) 可以产生观察到的矿物组合和从橄榄石 (Fo25-45) 和辉石 (En48-63, Mg#cpx 60-69) 测量的低镁值。