Role of magma injection and mixing in the formation of chromitite in Archean anorthosites: Evidence from the Sittampundi Complex, southern India
Role of magma injection and mixing in the formation of chromitite in Archean anorthosites: Evidence from the Sittampundi Complex, southern India
复制标题
岩浆注入和混合在太古代斜长岩中铬铁矿形成中的作用：来自印度南部 Sittampundi 杂岩的证据
DOI：
10.1016/j.precamres.2020.105914
复制
发表时间：
2020-11
影响因子：
3.8
通讯作者：
Lai Chun-Kit
中科院分区：
文献类型：
--
作者：
He Hai-Long;Wang Yu-Quan;Bao Zhi-An;George P.M.;Veni S.;Sajeev K.;Guo Jing-Hui;Zhai Ming-Guo;Lai Chun-Kit
铬铁矿存在于许多太古代斜长岩杂岩中。印度南部的新太古代 Sittampundi 斜长岩杂岩体 (SAC) 主要由（富含角闪石）斜长岩、少量辉长岩、两块辉石麻粒岩以及许多与斜长石相关的铬铁矿层和透镜体组成。高镁斜辉石巨晶存在于铬铁矿中，但不存在于斜长石中。这些巨晶体被铬尖晶石和间隙角闪石扰动和/或腐蚀成碎片，但在很大程度上仍然是光学连续的。斜长岩中不存在斜方辉石，表明该矿物是在较早阶段在深层岩浆房中形成的，然后在进入现存岩浆房时部分重熔。根据结构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是粗粒的，通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) 和较低的总 REE 含量 (ΣREE = 2.90–4.42 ppm)。这意味着铬铁矿中的角闪石-1 是从斜长石中捕获的。同时，角闪石-2 是细粒的，并且作为铬尖晶石、斜长石和角闪石-1 之间的间隙填充物出现。 Amphibole-2 显示负到弱正 Eu 异常 (Eu/Eu* = 0.67–1.17)，并且 REE 总含量略有升高 (ΣREE = 5.42–14.20 ppm)。斜方辉石、角闪石-1 和斜长石的吸收结构，以及铬尖晶石和角闪石-2 之间的密切共生关系，表明铬尖晶石通过斜长石糊状物与补充的、更原始的岩浆的混合而饱和。角闪石-2 稀土元素模式表明，注入的岩浆部分腐蚀了先前存在的斜长石和角闪石-1，并与糊状物中的间隙熔体混合。补充岩浆中的 Cr2O3 和斜长石晶浆中的 SiO2 的添加可能将熔体推向铬尖晶石饱和状态并引发铬铁矿形成。铬尖晶石中高Al2O3含量可能是由斜长石重熔导致的，斜长石重熔增加了混合岩浆中Al3+的含量。世界范围内太古宙斜长岩的类似铬尖晶石成分表明，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。
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 Cr2O3from the replenished magma and SiO2from the anorthositic crystal mush had likely pushed the melt toward the Cr-spinel saturation and triggered chromitite formation. The high Al2O3content 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.
登录
查看更多内容
影响因子：
3
作者：
B. Ghosh;Ritam Konar
通讯作者：
B. Ghosh;Ritam Konar
影响因子：
3.8
作者：
Plavsa, Diana;Collins, Alan S.;Clark, Chris
通讯作者：
Clark, Chris
影响因子：
5
作者：
B. Murck;I. Campbell
通讯作者：
B. Murck;I. Campbell
影响因子：
3.8
作者：
A. Polat;A. Hofmann
通讯作者：
A. Polat;A. Hofmann
影响因子：
5.3
作者：
B. O’Driscoll;J. Day;J. Daly;R. J. Walker;W. McDonough
通讯作者：
B. O’Driscoll;J. Day;J. Daly;R. J. Walker;W. McDonough