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首页> 外文期刊>Contributions to Mineralogy and Petrology >The stability and origin of sodicgedrite in ultrahigh-temperature Mg-Al granulites: a case study front the Gondwana suture in southern India
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The stability and origin of sodicgedrite in ultrahigh-temperature Mg-Al granulites: a case study front the Gondwana suture in southern India

机译：超高温Mg-Al粒料中方晶石的稳定性和成因：以印度南部冈瓦纳缝合线为例

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Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone System (PCSZ) within the Gondwana suture zone in southern India contain sodicgedrite as one of the prograde to peak phases, stable during T=900-990 deg ultrahigh-temperature metamorphism. Gedrite in these samples is Mg-rich (Mg/[Fe+Mg]=X_Mg=0.69-0.80) and shows wide variation in Na_2O content (1.4-2.3 wt. percent, Na~A=0.33-0.61 pfu). Gedrite adjacent to kyanite pseudomorph is in part mantled by garnet and cordierite. The gedrite proximal to garnet shows anincrease in Na~A and A1~IV from the core (Na~A=0.40-0.51 pfu, A1~IV=1.6-1.9 pfu) to the rim (Na~A=0.49-0.61 pfu, A1~IV=2.0-2.2 pfu), suggesting the progress of the following dehydration reaction: Ged+Ky->Na-Ged+Grt+Crd+H_2O. This reaction suggests that, as the reactants broke down during the prograde stage, the remaining gedrite became enriched in Na to form sodicgedrite, which is regarded as a unique feature of high grade rocks with Mg-Al-rich and K-Si-poor bulk chemistry. We carried out high-P-T experimental studies on natural sodicgedrite and the results indicate that gedrite and melt are stable phases at 12 kbar and l,000 deg. However, the product gedrite is Na-poor with only<0.13 wt. percent Na_2O (Na~A=0.015-0.034 pfu). In contrast, the matrix glass contains up to 8.5 wt.percent Na_2O, suggesting that, with the progressive melting of the starting material, Na was partitioned into the melt rather than gedrite. The results therefore imply that the occurrence of sodicgedrite in the UHT rocks of the PCSZ is probably due to the low H_2O activity during peak P-T conditions that restricted extensive partial melting in these rocks, leaving Na partitioned into the solid phase (gedrite). The occurrence of abundant primary CO_2-rich fluid inclusions in this rock, which possibly infiltrated along the colli sional suture during the final amalgamation of the Gondwana supercontinent, strengthens the inference of low water activity.
机译：来自印度南部冈瓦纳缝合带内Palghat-Cauvery剪切带系统（PCSZ）的富含Mg-Al的岩石含有方铅矿作为推进到峰相的一种，在T = 900-990度超高温变质过程中稳定。这些样品中的石英晶富含Mg（Mg / [Fe + Mg] = X_Mg = 0.69-0.80），并且Na_2O含量变化很大（1.4-2.3 wt。％，Na〜A = 0.33-0.61 pfu）。邻近蓝晶石假晶的辉石部分被石榴石和堇青石覆盖。石榴石附近的胶凝体显示Na〜A和A1〜IV从核心（Na〜A = 0.40-0.51 pfu，A1〜IV = 1.6-1.9 pfu）到边缘（Na〜A = 0.49-0.61 pfu， A1〜IV = 2.0-2.2 pfu），表明以下脱水反应的进程：Ged + Ky-> Na-Ged + Grt + Crd + H_2O。该反应表明，随着反应物在预分解阶段分解，剩余的胶凝体富集到Na中形成方晶石，这被认为是富含镁铝和贫钾硅的高品位岩石的独特特征。化学。我们对天然方铅矿进行了高P-T实验研究，结果表明，在12 kbar和1000 deg时，铁氧体和熔体是稳定相。但是，产物铁氧体是贫钠的，仅＜0.13wt。％。百分比的Na_2O（Na〜A = 0.015-0.034 pfu）。相反，基体玻璃含有高达8.5重量％的Na 2 O，这表明随着原料的逐渐熔融，Na被分配到熔体而不是银矿中。因此，结果表明，在PCSZ的UHT岩石中发生了方晶晶，可能是由于在P-T峰值条件下H_2O活性低，限制了这些岩石中的广泛部分熔融，而使Na划分为固相（银晶）。冈瓦纳超大陆的最终合并过程中，该岩石中大量富含CO_2的富集流体包裹体的出现可能会沿着缝线渗入，从而增强了低水活度的推论。

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