The impact of exhumation onto fluid-mobile element budget and Rb-Sr isotope heterogeneity of the subducted eclogitic crust (Alag-Khadny, SW Mongolia)

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Sergei Yu. Skuzovatov, Anfisa V. Skoblenko, Adrien Vezinet, Anas A. Karimov, Tatsuki Tsujimori
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引用次数: 0

Abstract

Subduction-zone fluid–rock interactions have a direct impact onto elemental and isotopic homogeneity of progressively buried and exhumed crustal lithologies by providing an interface for local mass-transfer and enhancing metamorphic reactions. In order to assess the scales of fluid mobility, chemical and isotopic inheritance, as well as resulting degrees of isotopic heterogeneity in the exhumed high-pressure lithologies, we performed the detailed mineralogical, in-situ trace-element and Rb–Sr isotope studies, combined with P–T–X thermodynamic modelling of representative eclogites from the Alag Khadny accretionary complex (SW Mongolia). The eclogites (garnet + omphacite + phengite + rutile + quartz + retrograde amphibole and clinozoisite) display records of subduction-related burial to 540–625 °C and 1.7–2.1 GPa, with the enclosed phengite supposed to be in equilibrium at prograde-to-peak conditions. Trace-element signatures, including Cs/Rb (0.03–0.08) and Ba/Rb (7.1–13.8) ratios of phengite, are consistent with moderately to strongly altered protoliths of eclogites, which is supported by elevated δ18O values and in-situ Rb–Sr constraints on the initial (87Sr/86Sr)I ratios of phengite within 0.70549–0.70957. Partial backward rehydration (~ 0.5–1.0 wt% of H2O added) during decompression from 1.6 to 1.2 GPa produced amphibole- and clinozoisite-bearing assemblages, did not significantly affect LILE systematics and variable 87Rb/86Sr ratios of phengite. Limited Rb and Ba loss from phengite during recrystallization is suspected in the evidently deformed eclogites based on the LILE mineral–fluid and phengite–amphibole partitioning data. No exclusive evidence is found in amphibole for LILE-rich metasedimentary fluid with high (87Sr/86Sr)I released into eclogites. Instead, unradiogenic 87Sr/86Sr (0.70279–0.70301) of clinozoisite highlights metasomatic addition from the underlying mafic crust or dehydrated peridotitic mantle. Variable deformation-enhanced fluid-rock interaction during early exhumation was recorded by in-situ phengite Rb-Sr geochronology at 568 ± 9 Ma, which is considered a direct fluid flow snapshot and place a new minimum age constraint for the peak subduction burial. We argue that, except cases of apparent metasomatic origin of phengite, its (87Sr/86Sr)I ratios may be a sensitive tracer for the eclogite precursor alteration due to limited Sr mobility. Sample-scale Rb-Sr isotopic heterogeneities may be preserved in the orogenic eclogites due to multi-stage retrograde hydration and should be taken into account while interpreting the bulk-rock Sr isotope data.

Abstract Image

掘起对俯冲蚀变地壳(蒙古西南部阿拉格-哈德尼)流体流动元素预算和铷-锶同位素异质性的影响
俯冲带流体与岩石的相互作用为局部质量转移提供了一个界面,并加强了变质反应,从而对逐渐埋藏和揭露的地壳岩性的元素和同位素均质性产生了直接影响。为了评估流体流动、化学和同位素继承的规模,以及由此导致的出露高压岩性的同位素异质性程度,我们对来自 Alag Khadny 增生复合体(蒙古西南部)的代表性斜长岩进行了详细的矿物学、原位痕量元素和铷锶同位素研究,并结合 P-T-X 热力学建模。斜长岩(石榴石 + 奥辉石 + 黝帘石 + 金红石 + 石英 + 逆冲闪石和黝帘石)显示了与俯冲有关的埋藏记录,温度为 540-625 °C,压力为 1.7-2.1 GPa,其中包裹的黝帘石应该处于级进-峰值条件下的平衡状态。痕量元素特征,包括辉长岩的Cs/Rb(0.03-0.08)和Ba/Rb(7.1-13.8)比率,与中度至强烈蚀变的斜长岩原岩一致,这一点得到了δ18O值升高和辉长岩初始(87Sr/86Sr)I比率在0.70549-0.70957范围内的原位Rb-Sr约束的支持。在从 1.6 GPa 到 1.2 GPa 的减压过程中,部分反向再水化(添加约 0.5-1.0 wt% 的 H2O)产生了含闪石和黝帘石的集合体,但并未显著影响辉绿岩的 LILE 系统学和可变的 87Rb/86Sr 比率。根据LILE矿物流体和辉长岩-闪长岩分区数据,怀疑在明显变形的斜长岩中,辉长岩在再结晶过程中的Rb和Ba损失有限。在闪长岩中没有发现富含 LILE 的变质流体与高 (87Sr/86Sr)I 释放到斜长岩中的唯一证据。相反,黝帘石中的非辐射性 87Sr/86Sr (0.70279-0.70301) 凸显了来自下伏黑云母地壳或脱水橄榄岩地幔的变质流体。原位辉绿岩Rb-Sr地质年代学在568 ± 9 Ma记录了早期掘起过程中各种变形增强的流体-岩石相互作用,这被认为是流体流的直接快照,并为俯冲埋藏峰值提供了新的最小年龄约束。我们认为,除了辉绿岩明显的变质起源外,其(87Sr/86Sr)I比值可能是辉绿岩前驱蚀变的敏感示踪剂,因为锶的流动性有限。由于多级逆行水化作用,造山闪长岩中可能会保留样品尺度的铷锶同位素异质性,在解释块岩锶同位素数据时应将其考虑在内。
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来源期刊
Contributions to Mineralogy and Petrology
Contributions to Mineralogy and Petrology 地学-地球化学与地球物理
CiteScore
6.50
自引率
5.70%
发文量
94
审稿时长
1.7 months
期刊介绍: Contributions to Mineralogy and Petrology is an international journal that accepts high quality research papers in the fields of igneous and metamorphic petrology, geochemistry and mineralogy. Topics of interest include: major element, trace element and isotope geochemistry, geochronology, experimental petrology, igneous and metamorphic petrology, mineralogy, major and trace element mineral chemistry and thermodynamic modeling of petrologic and geochemical processes.
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