Abstract The formation of incipient charnockite is significant for understanding the deep crustal process, which, however, remains under broad controversy. Patches of incipient charnockite occur ubiquitously in garnet-biotite gneiss in southern India. Here, we present new textural observations, geochemical analyses and phase equilibria modelling on the incipient charnockite and its garnet-biotite gneiss host from the Irinrayam quarry in the Trivandrum Block, with the aim to provide some new insights into the petrogenesis of incipient charnockite. Both rock types have similar bulk compositions which are characterized by high SiO2 (c. 70 wt%) and K2O (5.5–6.2 wt%), and low K/Rb (c. 150). Both rock types share similar peak mineral assemblage of garnet, K-feldspar, plagioclase, quartz, ilmenite and biotite except for the presence of orthopyroxene in the incipient charnockite. Phase equilibria modelling and thermobarometers reveal that these rocks record similar peak P–T conditions (870–900 °C, 6.2–7.2 kbar) followed by extensive cooling. Nonetheless, temperature versus mole H2O (T–M(H2O)) pseudosections show that orthopyroxene-bearing mineral assemblage is stable at lower M(H2O) than that without orthopyroxene. The post-peak cooling stage is characterized by fine-grained secondary garnet + quartz ± biotite ± feldspar symplectite in the incipient charnockite, but by simple biotite + quartz symplectite in the garnet-biotite gneiss. The garnet-bearing symplectite overgrows around peak ferromagnesian minerals in the incipient charnockite, corresponding to the increase of garnet proportion with the consumption of anatectic melt in the phase equilibria modelling. Garnet in the symplectite is extremely depleted in heavy rare earth elements (HREEs), which might be inherited from the anatectic melt and other reactants. These phase equilibria modelling and garnet REE results suggest that the garnet-bearing symplectite is a melt-related retrograde texture. The appearance of anhydrous garnet during cooling indicates that the system of the incipient charnockite, especially the anatectic melt itself, is more anhydrous than that of the garnet-biotite gneiss. Simple process of fluid-absent melting and melt loss was likely insufficient to dehydrate the melt itself, and we propose that the infiltration of low-water-activity fluid is likely a significant process to reduce the water activity of the whole petrogenetic system, leading to the incipient charnockite formation in this area.

中文翻译：

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印度南部特里凡得琅地块的早期炭黑形成：来自熔体相关反应结构和相平衡建模的证据

摘要 早期黄铜矿的形成对于理解地壳深部过程具有重要意义，但目前仍存在广泛争议。在印度南部的石榴石-黑云母片麻岩中随处可见一片片初期的炭黑。在这里，我们对来自特里凡得琅区块 Irinrayam 采石场的初期炭黑岩及其石榴石-黑云母片麻岩主体进行了新的结构观察、地球化学分析和相平衡建模，旨在为初期炭黑岩的岩石成因提供一些新的见解。两种岩石类型都具有相似的整体成分，其特点是高 SiO2（约 70 wt%）和 K2O（5.5-6.2 wt%）以及低 K/Rb（约 150）。两种岩石类型的石榴石、钾长石、斜长石、石英、钛铁矿和黑云母，除了初期的charnockite 中存在斜方辉石。相平衡模型和温度气压计显示，这些岩石记录了类似的峰值 P-T 条件（870-900 °C，6.2-7.2 kbar），然后是大量冷却。尽管如此，温度与摩尔 H2O (T-M(H2O)) 的假截面表明，含斜方辉石的矿物组合在较低的 M(H2O) 下比没有斜方辉石的矿物组合稳定。峰后冷却阶段的特征是初生黑云母中细粒次生石榴石+石英±黑云母±长石辛长石，而石榴石-黑云母片麻岩中为单纯黑云母+石英辛长石。含石榴石的辛辉石在初期的charnockite中的峰值铁镁矿物周围过度生长，对应于相平衡模型中石榴石比例随着深熔熔体消耗的增加。辛辉石中的石榴石极其缺乏重稀土元素 (HREE)，这可能是从深熔熔体和其他反应物继承而来的。这些相平衡模型和石榴石 REE 结果表明，含石榴石的辛辉石是与熔体相关的逆行结构。冷却过程中无水石榴石的出现表明，初期的charnockite系统，特别是深熔熔体本身，比石榴石-黑云母片麻岩更无水。无流体熔化和熔体损失的简单过程可能不足以使熔体本身脱水，