Growth of corundum in metamorphosed anorthosites and related basic-ultra-basic rocks is an exceptional feature, and its origin remains elusive. We describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from two localities, Manavadi (MvAm) and Ayyarmalai (AyAm), contain anorthite lenses (An90–99) with euhedral to elliptical outline set in a finergrained matrix of calcic plagioclase (An85–90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic megacrysts, and the matrix are both recrystallized under static condition presumably during the regional high pressure (HP) metamorphism (~800 °C, 8–11 kbar) at ~2.45 Ga. Corundum occurs in the core of some of the recrystallized anorthite lenses (An95–99) in two modes: (1) Dominantly, it forms aggregates with magnetite (with rare inclusion of hercynite; in MvAm) or spinel (and occasionally hematite-ilmenite; in AyAm). The aggregates cut across the polygonal grain boundaries of the anorthite and contain inclusions of anorthite. (2) Corundum also occurs along the grain boundaries or at the triple junctions of the polygonal anorthite grains, where it forms euhedral tabular grains, sieved with inclusions of anorthite or forms skeletal rims around the recrystallized anorthite, such that it seems to be intergrown with anorthite. Combined petrological data and computed phase relations are consistent with growth of corundum in an open system during regional metamorphism in the presence of intergranular fluids. Two mechanisms are proposed to explain the formation of the corundum in the amphibolites: (1) corundum + magnetite/spinel aggregates formed dominantly by oxy-exsolution of pre-existing Al-Fe-Mg-(Ti)-spinel. This pre-existing spinel may be primary magmatic inclusions within the anorthite phenocrysts or could have formed due to reaction of primary magmatic inclusions of olivine with the host anorthite. Pseudosections of fO2-nH2OT-P in the CaO–FeO–MgO–Al2O3–SiO2–H2O (CFMASH) system indicate that fO2 and H2O strongly influence the formation of corundum + amphibole from the initial magmatic assemblage of anorthite (phenocrysts) + spinel ± olivine (inclusions). (2) The corundum with anorthite presumably formed through desilification and decalcification of anorthite, as is indicated by computed phase relations in isobaric-isothermal chemical potential diagrams (mSiO2-mCaO) in parts of the CASH system. Growth of corundum in this mode is augmented by high activity of anorthite in plagioclase, high pressure, and low-to-medium temperature of metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultra-basic rocks in general, which should provide new insight into lower crustal processes like high-pressure metamorphism.

中文翻译：

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印度南部花岗岩地块斜长闪岩中斜长岩巨晶中刚玉的起源

刚玉在变质斜长岩和相关基性-超基性岩石中的生长是一个特殊的特征，其起源仍然难以捉摸。我们描述了来自印度南部花岗岩地体 (GTSI) 约 2.5 Ga 古老基底的斜长闪岩中刚玉的发生并提供了解释。来自两个地方 Manavadi (MvAm) 和 Ayyarmalai (AyAm) 的研究角闪石包含钙长石透镜 (An90-99)，在钙质斜长石 (An85-90) 和铝质角闪石（细长闪长石 - ）。被解释为原生岩浆巨晶的透镜和基质都在静态条件下重结晶，推测是在约 2.45 Ga 的区域高压 (HP) 变质作用（~800 °C，8-11 kbar）期间。刚玉以两种模式出现在一些重结晶钙长石透镜 (An95-99) 的核心中： (1) 主要是与磁铁矿（很少包含铁长岩；在 MvAm 中）或尖晶石（偶尔赤铁矿-钛铁矿）形成聚集体；在 AyAm）。聚集体穿过钙长石的多边形晶界并包含钙长石内含物。(2) 刚玉也出现在晶界或多边形钙长石晶粒的三联结处，形成自形片状晶粒，筛分夹杂钙长石或在重结晶的钙长石周围形成骨架边缘，似乎与钙长石共生。钙长石。组合的岩石学数据和计算出的相位关系与在粒间流体存在的区域变质过程中开放系统中刚玉的生长一致。提出了两种机制来解释角闪石中刚玉的形成：(1) 刚玉 + 磁铁矿/尖晶石聚集体主要由预先存在的 Al-Fe-Mg-(Ti)-尖晶石的氧溶出形成。这种预先存在的尖晶石可能是钙长石斑晶内的原生岩浆包裹体，也可能是由于橄榄石的原生岩浆包裹体与主体钙长石反应而形成的。CaO-FeO-MgO-Al2O3-SiO2-H2O (CFMASH) 系统中 fO2-nH2OT-P 的伪截面表明 fO2 和 H2O 强烈影响刚玉 + 角闪石从钙长石（斑晶）+尖晶石的初始岩浆组合形成橄榄石（夹杂物）。(2) 含有钙长石的刚玉，推测是钙长石脱硅脱钙形成的，正如 CASH 系统部分中等压-等温化学势图 (mSiO2-mCaO) 中计算的相关系所示。斜长石中钙长石的高活性、高压和中低温变质作用促进了刚玉在这种模式下的生长。因此，这项研究为斜长闪岩和一般基性-超基性岩石中刚玉的起源提供了一种新的可行机制，这应该为高压变质作用等下地壳过程提供新的见解。