An exceptionally well‐exposed part of the Flin Flon Greenstone Belt (Manitoba/Saskatchewan) is used to characterize the mineral assemblage evolution associated with prehnite–pumpellyite through amphibolite facies metamorphism of basalts. Data from these rocks are combined with a large literature data set to assess the ability of current thermodynamic models to reproduce natural patterns, evaluate the use of metabasic rocks at these grades to estimate pressure–temperature (P–T) conditions of metamorphism, and to comment on the metamorphic devolatilization that occurs. At Flin Flon, five major isograds (actinolite‐in, prehnite‐ and pumpellyite‐out, hornblende‐in, oligoclase‐in, and actinolite‐out) collectively represent passage from prehnite–pumpellyite to lower amphibolite facies conditions. The evolution in mineral assemblages occurs in two narrow (~1,000 m) zones: the prehnite–pumpellyite to greenschist facies (PP‐GS) transition and greenschist to amphibolite facies (GS‐AM) transition. Across the GS‐AM transition, significant increases in the hornblende and oligoclase proportions occur at the expense of actinolite, albite, chlorite, and titanite, whereas there is little change in the proportions of epidote. The majority of this mineral transformation occurs above the oligoclase‐in isograd within the hornblende–actinolite–oligoclase zone. Comparison with thermodynamic modelling results suggests data set 5 (DS5) of Holland and Powell (1998, Journal of Metamorphic Geology, 16(3):309–343) and associated activity–composition (a–x) models is generally successful in reproducing natural observations, whereas data set 6 (DS6) (Holland & Powell, 2011, Journal of Metamorphic Geology, 29(3):333–383) and associated a–x models fail to reproduce the observed mineral isograds and compositions. When the data from Flin Flon are combined with data from the literature, two main pressure‐sensitive facies series for metabasites are revealed, based on prograde passage below or above a hornblende–albite bathograd at ~3.3 kbar: a low‐pressure ‘actinolite–oligoclase type’ facies series, characterized by the appearance of oligoclase before hornblende, and a moderate‐ to high‐pressure ‘hornblende–albite type’ facies series, characterized by the appearance of hornblende before oligoclase. Concerning the PP‐GS transition, the mineral assemblage evolution in Flin Flon suggests it occurs over a small zone (<1,000 m), in which assemblages containing true transitional assemblages (prehnite and/or pumpellyite coexisting with actinolite) are rare. This contrasts with thermodynamic modelling, using either DS5 or DS6, which predicts a wide PP‐GS transition involving the progressive appearance of epidote and actinolite and disappearance of pumpellyite and prehnite. Patterns of mineral assemblages and thermodynamic modelling suggest a useful bathograd (‘CHEPPAQ bathograd’), separating prehnite–pumpellyite‐bearing assemblages at low pressures and pumpellyite–actinolite‐bearing assemblages at higher pressures, occurs at ~2.3 to 2.6 kbar. Observations from the Flin Flon sequence suggests devolatilization across the GS‐AM transition (average: ~1.8 wt% H₂O) occurs over a very narrow interval within the actinolite–hornblende–oligoclase zone, associated with the loss of >75% of the total chlorite. By contrast, modelling of the GS‐AM transition zone predicts more progressive dehydration of ~2 wt% H₂O over a >50°C interval. Observations from the field suggest devolatilization across the PP‐GS transition occurs over a very narrow interval given the rarity of transitional assemblages. Modelling suggests fluid release of 1.0–1.4 wt% resulting from prehnite breakdown over a ~10°C interval. This fluid may not be entirely lost from the rock package due to involvement in the hydration of igneous mineralogy across the PP‐GS transition as observed in the Flin Flon sequence.

中文翻译：

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从芙蓉岩-球墨石岩到闪石岩相的变质岩矿物组合和相平衡，以弗林·弗隆绿岩带（曼尼托巴省）为例

Flin Flon绿岩带（马尼托巴/萨斯喀彻温省）的一个特别暴露的部分用于通过玄武岩的闪石岩相变质来表征与葡萄石-铅锌矿相关的矿物组合演化。这些岩石中的数据与大量文献数据集相结合，以评估当前热力学模型重现自然形态的能力，评估在这些等级下的变质岩的使用以估算压力-温度（PT））的变质条件，并评论发生的变质脱挥发分。在Flin Flon，五种主要的等梯度物（放线石，入黑闪石和pumpellyite出，角闪石入，寡石蜡入和放线石出）共同代表了从hn石-球墨石到较低的角闪岩相状态。矿物组合的演化发生在两个狭窄的区域（约1,000 m）：绿铅矿-铅锌矿到绿片岩相（PP-GS）的过渡和绿片岩到角闪石相（GS-AM）的过渡。在整个GS-AM过渡过程中，角闪石和寡糖酶的比例显着增加，但以阳起石，钠长石，绿泥石和钛铁矿为代价，而附子的比例几乎没有变化。这种矿物转化的大部分发生在角闪石-阳起石-寡糖酶带内的寡糖-等腰线以上。与热力学建模结果的比较表明，Holland and Powell（1998，Journal of Metamorphic Geology，16（3）：309–343）和相关的活动-组成（a–x）模型通常可以成功地再现自然观测，而数据集6（DS6）（Holland＆Powell，2011，Journal of Metamorphic地质，29（3）：333–383）和相关的a–x模型无法再现观察到的矿物等价物和组成。当将Flin Flon的数据与文献数据相结合时，基于在约3.3 kbar的角闪石-重晶石辉绿岩之上或之下的前进通道，揭示了两个主要的变质岩压敏相系列：低压“阳起石-寡角型类型的相系列，其特征是在角闪石之前出现了寡糖酶；中高压的“角闪类-阿尔比特型”相系列的特征是在寡糖酶之前出现了角闪石。关于PP-GS过渡，Flin Flon中的矿物组合演化表明它发生在一个很小的区域（<1,000 m），在该组合中包含真正的过渡组合（黑榴石和/或水铝榴石与阳起石共存​​）是罕见的。这与使用DS5或DS6进行热力学建模形成对比，后者使用PP或GS预测了广泛的PP-GS过渡，涉及到附子和阳起石的逐渐出现，以及绿水榴石和葡萄石的消失。矿物组合的模式和热力学模型表明，有用的盐卤（“ CHEPPAQ bathograd”）可以分离低压下的锂辉石-斜晶石-辉石组合，以及较高压力下的钠长石-斜纹岩-辉石组合，分离范围约为2.3至2.6 kbar。Flin Flon序列的观察结果表明，在GS-AM过渡过程中会发生脱挥发分（平均：〜1.8 wt％H 矿物组合的模式和热力学模型表明，有用的盐卤（“ CHEPPAQ bathograd”）可以分离低压下的锂辉石-斜晶石-辉石组合，以及较高压力下的钠长石-斜纹岩-辉石组合，分离范围约为2.3至2.6 kbar。Flin Flon序列的观察结果表明，在GS-AM过渡过程中会发生脱挥发分（平均：〜1.8 wt％H 矿物组合的模式和热力学模型表明，有用的盐卤（“ CHEPPAQ bathograd”）可以分离低压下的锂辉石-斜晶石-辉石组合，以及较高压力下的钠长石-斜纹岩-辉石组合，分离范围约为2.3至2.6 kbar。Flin Flon序列的观察结果表明，在GS-AM过渡过程中会发生脱挥发分（平均：〜1.8 wt％H₂ O）发生在阳起石-角闪石-寡糖酶带的非常狭窄的时间段内，损失了总绿泥石的> 75％。相比之下，对GS-AM过渡带的建模预测在大于50°C的时间间隔内〜2 wt％H ₂ O会进一步脱水。实地观察表明，鉴于过渡组合的稀有性，PP-GS过渡过程中的脱挥发分发生在非常窄的时间间隔内。模型表明，在约10°C的时间间隔内，由于亚硝酸盐分解而导致的流体释放量为1.0–1.4 wt％。正如在Flin Flon序列中观察到的那样，由于穿越PP-GS过渡过程中火成矿物的水化作用，这种流体可能不会从岩石包裹中完全流失。