Abstract Chromitite occurs in many Archean anorthosite complexes. The Neoarchean Sittampundi Anorthosite Complex (SAC) in southern India is composed of mainly (amphibole-rich) anorthosite, minor gabbro, two pyroxene granulite, and many anorthosite-related chromitite layers and lenses. High-Mg# orthopyroxene megacrysts are present in the chromitite but absent in the anorthosite. These megacrysts are disturbed and/or corroded by Cr-spinel and interstitial amphibole into fragment, but are still largely optically continuous. The absence of orthopyroxene in the anorthosite indicates that the mineral was formed in an earlier stage in the deep magma chamber(s), and then partially remelted when it was entered into the extant magma chamber. Two types of amphiboles are recognized based on textural and trace elemental features. Amphibole-1 in the chromitites is coarse-grained, and commonly surrounded by Cr-spinel and amphibole-2, which forms resorption texture along amphibole-1 rims. Chondrite-normalized REE patterns of the amphibole-1 in chromitite and that in anorthosite have similarly positive Eu* anomalies (1.53–3.04) and low total REE contents (ΣREE = 2.90–4.42 ppm). This implies that amphibole-1 in the chromitite was captured from the anorthosite. Meanwhile, amphibole-2 is fine-grained and occurs as interstitial infill among Cr-spinel, plagioclase, and amphibole-1. Amphibole-2 shows negative to weakly positive Eu anomalies (Eu/Eu* = 0.67–1.17), and slightly elevated total REE contents (ΣREE = 5.42–14.20 ppm). The resorption textures on orthopyroxene, amphibole-1, and plagioclase, and the close paragenetic relations between Cr-spinel and amphibole-2, suggest that Cr-spinel was saturated through the mixing of the anorthositic mush with a replenished, more primitive magma. The amphibole-2 REE patterns indicate that the injected magma had partially corroded the pre-existing plagioclase and amphibole-1, and mixed with the interstitial melt in the mush. Addition of Cr2O3 from the replenished magma and SiO2 from the anorthositic crystal mush had likely pushed the melt toward the Cr-spinel saturation and triggered chromitite formation. The high Al2O3 content of Cr-spinel is likely led by plagioclase remelting, which increased the Al3+ content in the mixed magma. Comparable Cr-spinel compositions of the Archean anorthosites worldwide suggest that their chromitites were possibly formed via similar magma injection and mixing processes.

中文翻译：

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熔体注入和混合在太古代巨晶斜长岩中铬铁矿形成中的作用：来自印度南部 Sittampundi 斜长岩复合体的证据

摘要 铬铁矿存在于许多太古代斜长岩复合体中。印度南部的新太古代 Sittampundi 斜长岩复合体 (SAC) 主要由（富含角闪石的）斜长岩、次要辉长岩、两个辉石麻粒岩和许多与斜长岩相关的铬铁矿层和透镜组成。铬铁矿中存在高 Mg# 斜方辉石巨晶，但在斜长岩中不存在。这些巨晶被铬尖晶石和间质闪石扰乱和/或腐蚀成碎片，但在很大程度上仍然是光学连续的。斜长岩中没有斜方辉石表明该矿物在较早的阶段形成于深部岩浆房，然后在进入现存的岩浆房时部分重熔。基于纹理和微量元素特征识别两种类型的角闪石。铬铁矿中的角闪石 1 为粗粒状，常被铬尖晶石和角闪石 2 包围，沿角闪石 1 边缘形成吸收结构。铬铁矿和斜长岩中角闪石 1 的球粒陨石归一化 REE 模式具有类似的正 Eu* 异常 (1.53–3.04) 和低总 REE 含量 (ΣREE = 2.90–4.42 ppm)。这意味着铬铁矿中的角闪石 1 是从斜长岩中捕获的。同时，amphibole-2 是细粒度的，作为 Cr-尖晶石、斜长石和 amphibole-1 之间的间隙填充物。Amphibole-2 显示出负到弱正的 Eu 异常（Eu/Eu* = 0.67–1.17），并且总 REE 含量略有升高（ΣREE = 5.42–14.20 ppm）。斜方辉石、角闪石-1和斜长石的再吸收结构，以及Cr-尖晶石和角闪石-2之间的密切共生关系，表明铬尖晶石​​是通过斜长岩浆与补充的、更原始的岩浆混合而饱和的。Amphibole-2 REE 模式表明注入的岩浆部分腐蚀了先前存在的斜长石和amphibole-1，并与糊状物中的间隙熔体混合。来自补充岩浆的 Cr2O3 和来自斜长岩晶体糊状物的 SiO2 的添加可能将熔体推向 Cr-尖晶石饱和并引发铬铁矿的形成。Cr-尖晶石的高Al2O3含量可能是由斜长石重熔导致的，这增加了混合岩浆中Al3+的含量。世界范围内太古代斜长岩的可比 Cr-尖晶石成分表明，它们的铬铁矿可能是通过类似的岩浆注入和混合过程形成的。