首页> 外文期刊>Journal of Metamorphic Geology >Metamorphic evolution of medium-temperature ultra-high pressure (MT-UHP) eclogites from the South Dabie orogen, Central China: An insight from phase equilibria modelling
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Metamorphic evolution of medium-temperature ultra-high pressure (MT-UHP) eclogites from the South Dabie orogen, Central China: An insight from phase equilibria modelling

机译:华南大别造山带中温超高压榴辉岩的变质演化:相平衡模型的启示

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摘要

Medium-temperature ultrahigh pressure (MT-UHP) eclogites from the south Dabie orogen, as represented by samples from the Jinheqiao, Shuanghe and Bixiling areas, consist of garnet, omphacite, phengite, epidote, hornblendic amphibole, quartz/coesite and rutile with or without kyanite and talc. Garnet is mostly anhedral and unzoned, but a few porphyroblasts are weakly zoned with core-mantle increasing grossular (X_(gr)) and decreasing pyrope (X_(py)) contents. Garnet compositions are closely correlated with the bulk compositions. For instance, the X_(py) and X_(gr) contents are positively correlated with the bulk MgO and CaO contents. Phengite is occasionally zoned with core-rim deceasing Si content, and phengite grains as inclusions in garnet show higher Si than in the matrix, suggesting differently resetting during post-peak stages. The maximum Si contents are mostly 3.60-3.63?p.f.u. for the three areas. Pseudosections calculated using THERMOCALC suggest that the MT-UHP eclogites should have a peak assemblageof garnet?+?omphacite?+?lawsonite?+?phengite?+?coesite in most rocks of higher MgO content. In this assemblage, the X_(py) in garnet mostly depends on bulk compositions, whereas the X_(gr) in garnet and the Si contents in phengite regularly increase, respectively, as temperature and as pressure rise, and thus, can provide robust thermobarometric constraints. Using the X_(gr) and Si isopleths in pseudosections, the peak P-T conditions were estimated to be 40?kbar/730?°C for the Jinheqiao, 41?kbar/726?°C for the Shuanghe, and 37-52?kbar and 700-830?°C for the Bixiling eclogites. Some eclogites with higher FeO are predicted to have a peak assemblage of garnet?+?omphacite?+?coesite?±?phengite without lawsonite, where the garnet and phengite compositions highly depend on bulk compositions and generally cannot give available thermobarometric constraints. Decompression of the eclogites with lawsonite in the peak stage is inferred to be accompanied with cooling and involves two stages: an early-stage decompression is dominated by lawsonite dehydration, resulting in increase in the mode of anhydrous minerals, or further eclogitization, and formation of epidote porphyroblasts and kyanite-bearing quartz veins in eclogite. As lawsonite dehydration can facilitate evolution of assemblages under fluid-present conditions, it is difficult to recover real peak P-T conditions for UHP eclogites with lawsonite. This may be a reason why the P-T conditions estimated for eclogites using thermobarometers are mostly lower than those estimated for the coherent ultramafic rocks, and lower than those suggested from the inclusion assemblages in zircon from marble. A late-stage decompression is dominated by formation of hornblendic amphibole and plagioclase with fluid infiltration. The lawsonite-absent MT-UHP eclogites have only experienced a decompression metamorphism corresponding to the later stage and generally lack the epidote overprinting.
机译:大别造山带南部的中温超高压榴辉岩,以金河桥,双河和碧西岭地区的样品为代表,由石榴石,绿辉石,锂辉石,山石,角闪闪的角闪石,石英/柯石英和金红石组成,或没有蓝晶石和滑石粉。石榴石大部分为无角状和无带状,但少数成卟啉细胞呈弱带状,核心幔层增加了总体含量(X_(gr)),并减少了焦岩(X_(py))含量。石榴石成分与散装成分紧密相关。例如,X_(py)和X_(gr)的含量与大量MgO和CaO的含量呈正相关。变质岩偶尔会随着硅的边缘边缘Si含量的降低而分布,石榴石中的夹杂物变质岩中的变质硅比基体中的硅含量更高,这表明在峰后阶段的复位不同。最大Si含量大部分为3.60-3.63p.f.u。对于三个领域。使用THERMOCALC计算出的假剖面表明,在大多数MgO含量较高的岩石中,MT-UHP榴辉岩应具有石榴石,+方铁矿,+钙钠石,+方铅矿,+堇青石的峰组合。在此组合中,石榴石中的X_(py)主要取决于散装成分,而石榴石中的X_(gr)和硅藻土中的Si含量分别随温度和压力升高而规则地增加,因此可以提供可靠的热压法约束。使用假截面的X_(gr)和Si等值线,金河桥的PT峰值条件估计为40?kbar / 730?C,双河的最高PT条件为41?kbar / 726?C,而37-52?bar碧溪岭榴石的温度为700-830°C。某些FeO较高的榴辉岩的石榴石+杂方铁矿+堇青石+±方铅矿的石榴石组合的峰集最高,而榴石和石榴石的组成很大程度上取决于松散成分,通常不能给出可用的热压约束。据推测,在高峰期用钙钠铝石对榴辉岩减压伴随着冷却,并涉及两个阶段:早期减压以钙钠铝石脱水为主,导致无水矿物形态增加,或进一步凝结,并形成榴辉岩中的枝晶成岩细胞和含蓝晶石的石英脉。由于钠钙钠石的脱水可以促进在流体存在的条件下组合的演化,因此很难恢复含有钠钙钠石的UHP榴辉岩的真实P-T峰值。这可能是为什么使用热压计估算的榴辉岩的P-T条件大多低于相干超镁铁质岩石估算的P-T条件,并且低于大理石锆石中夹杂物组合所建议的P-T条件的原因。后期减压主要是形成角闪闪的角闪石和斜长石,并伴有流体浸润。缺少缺钙钠铁石的MT-UHP榴辉岩仅经历了与后期相对应的减压变质作用,并且通常没有附子的叠印。

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