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Polyphase tectonic reworking of serpentinites and chlorite-tremolite-talc rocks (SW Spain) from the subduction forearc to intracontinental emplacement 蛇纹岩和绿泥石-透闪石-滑石岩(西班牙西南部)从俯冲弧前到陆内侵位的多期构造改造
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-12-14 DOI: 10.1111/jmg.12704
Benito Ábalos, Pablo Puelles, José Ignacio Gil Ibarguchi

The petrostructural and geochronological study of a poorly known ultramafic unit from SW Spain (Badajoz–Córdoba belt) combined with previous structural data permits disclosure of a history of metasomatism, tectono-metamorphism, reworking and isotopic resetting related to a poly-orogenic evolution in different geodynamic scenarios. The heterogeneous ultramafic unit studied contains antigorite-serpentinites and metasomatized ultramafic rocks (chlorite-talc schists, tremolite-talc-chlorite rocks and magnesio-hornblende-chlorite rocks). Mantle-wedge serpentinization was followed by Si and Al pre- to syn-metamorphic/tectonic metasomatism in a subduction realm. Petrofabrics of selected lithologies reveal variable syn-metamorphic crystal-plastic deformation and recrystallization (assisted by other mechanisms) under relative high pressure, concomitant with the conditions recorded by neighbouring tectonic units that were later intruded by Ordovician granites. The resultant ensemble was reworked and isotopically reset much later in an intracontinental ductile shear zone. Syn- to late-tectonic apatite from chlorite-talc schists provides an anchored Tera–Wassenburg isochron radiometric age of 342.8 ± 12.2 Ma that provides evidence for the decoupling between isotopic systems and microstructures. The results are discussed from a twofold perspective: with regard to the likely tectonic context of this ophiolite (the current analogue of the Mariana forearc) and with regard to regional geological implications.

通过对西班牙西南部一个鲜为人知的超镁铁质单元(Badajoz-Córdoba带)的岩石构造和地质年代学研究,结合以往的构造数据,揭示了在不同地球动力学情景下与多造山演化相关的交代、构造变质、改造和同位素重置史。研究的非均质超镁铁质单元包括反长岩-蛇纹岩和交代超镁铁质岩石(绿泥石-滑石片岩、透闪石-滑石-绿泥石岩石和镁-角闪石-绿泥石岩石)。地幔楔蛇纹岩作用之后是俯冲域中的Si和Al的前-同变质/构造交代作用。所选岩性的岩组在相对高压下显示出可变的同变质晶体塑性变形和再结晶(在其他机制的辅助下),并伴有随后被奥陶系花岗岩侵入的邻近构造单元所记录的条件。由此产生的整体在大陆内韧性剪切带中被重新加工和同位素重置。从绿泥石-滑石片岩中提取的顺-晚期构造磷灰石提供了一个锚定的terra - wassenburg等时辐射年龄为342.8±12.2 Ma,为同位素系统和微观结构之间的解耦提供了证据。结果从两个角度进行了讨论:关于这个蛇绿岩的可能构造背景(目前的马里亚纳前弧的类似物)和关于区域地质意义。
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引用次数: 1
Discrimination of thermodynamic and kinetic contributions to the heavy rare earth element patterns in metamorphic garnet 变质石榴石中重稀土元素模式的热力学和动力学贡献辨析
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-12-08 DOI: 10.1111/jmg.12703
Matthias Konrad-Schmolke, Ralf Halama, David Chew, Céline Heuzé, Jan De Hoog, Hana Ditterova

Variations of rare earth element (REE) concentrations in metamorphic garnet are an important source of information of geodynamic and geochemical processes in the deeper Earth. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified. Utilizing high-resolution trace element and μ-Raman mapping together with combined thermodynamic–geochemical–diffusion models, we demonstrate that the equilibrium and kinetic aspects of the REE uptake in metamorphic garnet can be discriminated by interpreting 2D trace element mapping in a single sample. The heavy (H) REE (Tb to Lu) zoning in the investigated garnet from a high-pressure blueschist comprises an inner part with an overall decrease from core to inner rim, followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. The broad overall shape of this pattern resembles those often observed in metamorphic garnet from different rock types and tectonic settings. Superimposed on this trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with at least six regularly spaced sets of peaks and troughs along the entire garnet radius. Comparison of the observed inclusion suite, the trace element maps and thermodynamic–geochemical models show that the inner part with decreasing HREE concentrations results from fractional garnet growth in an unchanged mineral assemblage, whereas the REE enrichment zone is caused by the breakdown of titanite. We suggest that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport-limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse-like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.

变质石榴石中稀土元素(REE)浓度的变化是地球深部地球动力学和地球化学过程的重要信息来源。为了提取这些信息,必须区分和量化石榴石中REE吸收的热力学平衡和动力学贡献。利用高分辨率微量元素和μ拉曼图谱,结合热力学-地球化学-扩散模型,我们证明了变质石榴石中REE吸收的平衡和动力学方面可以通过解释单个样品中的2D微量元素图谱来区分。所研究的高压蓝片岩石榴石中的重(H)REE(Tb至Lu)分带包括一个内部,从核心到内缘总体减少,然后是一个HREE富集的同心区,并向最外缘急剧减少。石榴石核心的中心峰强度随REE原子序数的减小而减小。这种模式的总体形状类似于在不同岩石类型和构造环境的变质石榴石中经常观察到的形状。叠加在这一趋势之上的是HREE浓度的微小重复波动的同心模式,沿着整个石榴石半径至少有六组规则间隔的波峰和波谷。对观察到的包裹体套件、微量元素图谱和热力学-地球化学模型的比较表明,HREE浓度降低的内部是由未改变的矿物组合中的部分石榴石生长引起的,而REE富集区是由钛酸盐的分解引起的。我们认为,中心峰的宽度由相互连接的传输基质的整体渗透率和石榴石平衡的基质矿物的分数控制。叠加的REE波动是寄主岩石元素迁移特性变化的结果,标志着石榴石中从平衡REE吸收到迁移受限REE吸收的反复变化。这种波动的元素输运特性可以用脉冲状流体通量来最好地解释,这种流体通量有节奏地改变晶间输运矩阵的互连性。越来越多已发表的空间高分辨率REE分析表明,这种微量元素波动在变质石榴石中很常见,这表明脉冲流体通量导致的岩石渗透率反复变化是变质过程中的常见现象。
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引用次数: 5
Repeated metamorphism in the pelitic granulites of the Hidaka metamorphic belt, Hokkaido, Japan: Implications for the formation of the present-day trench-arc-basin system in NE Asia 日本北海道Hidaka变质带泥质麻粒岩的重复变质作用:对现今东北亚槽弧盆地系统形成的影响
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-11-23 DOI: 10.1111/jmg.12701
Jinrui Zhang, Wenliang Xu, Yu Dong, Zheng Ji, Chunjing Wei, Shuang Tang, Yibing Li, Kiyoaki Niida

The timing and mechanism of the tectonic transition from an active continental margin to a trench-arc-basin system in NE Asia are debated. In this study, we report the pressure–temperature–time (PTt) path of this transition based on petrographic observations, phase-equilibrium modelling, and U–Pb ages of zircon and rutile from pelitic granulites in the Hidaka metamorphic belt (Hokkaido, Japan). The granulites contain an early phase mineral assemblage of staurolite + sillimanite + biotite + plagioclase + quartz + rutile/ilmenite, a peak phase granulite assemblage of garnet + biotite + cordierite + plagioclase + quartz + rutile/ilmenite and a symplectic intergrowth of spinel + cordierite ± sillimanite within garnet porphyroblasts. Phase-equilibrium modelling indicates two phases of metamorphism with PT conditions, respectively, of ~6 kbar/620–670°C and ~6 kbar/850°C. A clockwise PT path was thus reconstructed for the granulites, showing a near-isobaric temperature increase to the peak conditions and a post-peak cooling. U–Pb dating of zircon and rutile in the granulites yielded two groupings of metamorphic ages at c. 37 Ma and 19 Ma, related to early phase amphibolite facies and late phase granulite facies metamorphism, respectively. The age of magmatism from the previous work at the NE Asian continental margin overlaps with these metamorphic ages, and the two phases of metamorphism in the pelitic granulites is attributed to discrete episodes of supra-subduction-zone magmatism (late Eocene, c. 37 Ma) and back-arc extension (early Miocene, 24–19 Ma). Consequently, we suggest that the Hidaka metamorphic belt has undergone two phases of metamorphism, which represent two pulsed and separated thermal events. Moreover, we relate the granulites facies metamorphism to the underplating of mafic magma and lithospheric thinning during the opening of the Japan Sea at 24–19 Ma, which is attributed to slab rollback and trench retreat processes in NE Asia.

对东北亚从活动大陆边缘向沟弧盆地系统的构造转变的时间和机制进行了讨论。在本研究中,我们根据岩相观察、相平衡建模以及Hidaka变质带(日本北海道)泥质麻粒岩锆石和金红石的U–Pb年龄,报告了这种转变的压力-温度-时间(P–T–T)路径。麻粒岩包含一个早期的十字石矿物组合 + 硅线石 + 黑云母 + 斜长石 + 石英 + 金红石/钛铁矿&石榴石的峰相麻粒岩组合 + 黑云母 + 堇青石 + 斜长石 + 石英 + 金红石/钛铁矿与尖晶石的辛共生 + 堇青石 ± 石榴石斑岩中的硅线石。相平衡模型表明变质作用的两个阶段,P–T条件分别为~6 kbar/620–670°C和~6 kbar/850°C。因此,重建了麻粒岩的顺时针P–T路径,显示出接近等压的温度上升到峰值条件和峰后冷却。麻粒岩中锆石和金红石的U–Pb测年在c.37产生了两组变质年龄 马和19 Ma,分别与早期角闪岩相和晚期麻粒岩相变质作用有关。东北亚大陆边缘先前工作的岩浆作用年龄与这些变质年龄重叠,泥质麻粒岩的两个变质阶段归因于超俯冲带岩浆作用的离散事件(始新世晚期,c.37 Ma)和弧后伸展(早中新世,24-19 马)。因此,我们认为Hidaka变质带经历了两个变质阶段,代表了两个脉冲和分离的热事件。此外,我们将麻粒岩相变质作用与24-19日本海开放期间镁铁质岩浆的底侵作用和岩石圈变薄联系起来 Ma,这归因于东北亚的板块后退和海沟后退过程。
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引用次数: 2
Petrochronology of polygenetic white micas (Naxos, Greece) 多成因白色云母(希腊纳克索斯)的岩石年代学
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-11-22 DOI: 10.1111/jmg.12700
Igor M. Villa, Johannes Glodny, Alexandre Peillod, Alasdair Skelton, Uwe Ring

Naxos in the Greek Cyclades preserves a type example of polymetamorphism. The southern and northern parts of the island record different Tertiary P–T histories between Eocene and Miocene times, including a blueschist facies event, one or more amphibolite/greenschist facies overprint(s) and contact metamorphism. Age attributions for these events are inconsistent in the literature. Here, we propose a new approach that combines electron probe microanalyzer (EPMA) characterization of the white mica (WM) with 39Ar-40Ar–Rb-Sr multichronometry. Textural–petrographic–compositional observations reveal that the polygenetic WM consists of five different generations: pre-Eocene relicts, paragonite, high-Si phengite, low-Si phengite and muscovite. EPMA mapping of four WM samples, previously analysed by Rb-Sr, reveals major element compositions heterogeneous down to the μm scale. Each WM consists of chemically distinct generations, documenting submicron-scale retrogression of high-pressure (HP) phengite grains to muscovite. Four WM samples from a N-S traverse across the island were analysed by 39Ar-40Ar stepheating, comparing coarse and fine sieve size fractions to obtain overdetermined K-Ar systematics. Fine sieve fractions are richer in Cl than coarse ones. Linear arrays in Cl/K-age isotope correlation diagrams show two predominant WM generations (one Cl-poor at ca. 38 Ma and one Cl-rich at <20 Ma). A lower-grade sample from southern Naxos was less pervasively recrystallized, provides older ages and preserves at least three WM generations, including a relict WM with a pre-Palaeocene K-Ar age, consistent with the high Ar retentivity of WM in the absence of complete recrystallization. The age of the Cl-poor end-member WM approximates the age of the HP event, 38 Ma. Ar inheritance in Cretaceous mica relicts is heterogeneous at the single-grain scale. Comparing the degassing rates of the WM fractions rules out ‘multidomain’ diffusion. As no sample is monomineralic, the degassing rate of each polygenetic mica is instead controlled by the mass balanced sum of the unrelated rate constants of its constituent minerals. Given the commonness of zoned and composite micas, the approach detailed here is potentially useful for reconstructing polyphase metamorphic histories worldwide.

希腊基克拉泽斯中的纳克索斯保留了一个多聚同构的典型例子。岛的南部和北部记录了始新世和中新世之间不同的第三纪P–T历史,包括蓝片岩相事件、一个或多个角闪岩/绿片岩相叠加和接触变质作用。这些事件的年龄归属在文献中并不一致。在这里,我们提出了一种新的方法,将白云母(WM)的电子探针显微分析仪(EPMA)表征与39Ar‐40Ar–Rb‐Sr多时间测定相结合。纹理-岩相学-成分观察表明,多成因WM由五个不同的世代组成:前始新世残余岩、共生岩、高硅多硅土、低硅多硅石和白云母。先前由Rb‐Sr分析的四个WM样品的EPMA图谱显示,主要元素组成在μm范围内是不均匀的。每个WM由化学上不同的世代组成,记录了高压(HP)多硅土颗粒向白云母的亚微米级退化。通过39Ar‐40Ar步进加热分析了岛上N‐S导线的四个WM样品,比较了粗粒度和细粒度,以获得超定的K‐Ar系统。细筛级分比粗筛级分富含Cl。Cl/K年龄同位素相关图中的线性阵列显示了两个主要的WM世代(其中一个在ca。38 Ma和一个在<20时富含Cl 马)。来自南部Naxos的低品位样品的再结晶不太普遍,提供了较老的年龄,并保存了至少三代WM,包括一个具有前古新世K-Ar年龄的残余WM,这与WM在没有完全再结晶的情况下的高Ar保留率一致。Cl较差的最终成员WM的年龄接近HP事件的年龄,38 白垩纪云母残余物中的Ma。Ar继承在单颗粒尺度上是不均匀的。比较WM馏分的脱气速率排除了“多畴”扩散。由于没有样品是单矿物的,因此每个多成因云母的脱气速率由其组成矿物的不相关速率常数的质量平衡和控制。鉴于分区和复合云母的普遍性,这里详细介绍的方法可能有助于重建世界范围内的多相变质历史。
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引用次数: 2
Partial melting and reaction along deformation features in plagioclase 斜长石部分熔融及沿变形反应特征
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-11-22 DOI: 10.1111/jmg.12702
Sarah Incel, Marie Baïsset, Loïc Labrousse, Alexandre Schubnel

Geological processes involving deformation and/or reactions are highly influenced by the rock grain size, especially if diffusion-controlled processes take place such as metamorphic reactions and diffusion creep. Although many processes, inducing grain-size reduction, are documented and understood at relatively high stresses and low temperatures (e.g., cataclasis) as well as at lower stress and higher temperature conditions (e.g., bulging and subgrain rotation), deformation twinning, a plastic deformation mechanism active in various minerals at lower temperatures, has been neglected as nucleation site for melting and reaction and thus as a cause for grain-size reduction so far. We conducted experiments on natural plagioclase-bearing aggregates at 2.5 to 3 GPa confining pressure and temperatures of 700°C to 950°C using two different deformation apparatus, a deformation multianvil apparatus (DDIA) and a Griggs press, as well as a piston-cylinder apparatus. Regardless of the apparatus type, we observe the breakdown of plagioclase into an eclogite-facies paragenesis, which is associated with partial melting in the high temperature domain of the eclogite facies. Partial melting mostly takes place along the grain and interphase boundaries. However, several melt patches or plagioclase decomposition products coincide with the occurrence of deformation twins and grain-scale microcracking in plagioclase indicating intracrystalline melting and reaction in addition to melting and reaction along grain and interphase boundaries. In the present study, we demonstrate how the interplay between brittle microcracking and plastic deformation twinning can cause intracrystalline melting and/or reaction, which has the potential to lower the effective grain size of plagioclase-rich rocks and thus impacts their reactivity and deformation behaviour.

涉及变形和/或反应的地质过程受到岩石粒度的高度影响,特别是如果发生变质反应和扩散蠕变等扩散控制过程。虽然在相对较高的应力和较低的温度条件下(例如,碎裂)以及在较低的应力和较高的温度条件下(例如,胀形和亚晶粒旋转)记录和理解了许多诱导晶粒尺寸减小的过程,但变形孪晶,一种在较低温度下活跃于各种矿物的塑性变形机制,作为熔化和反应的成核场所,因此作为晶粒尺寸减小的原因,迄今为止一直被忽视。我们使用两种不同的变形装置,变形多砧装置(DDIA)和格里格斯压力机,以及活塞-气缸装置,在2.5至3gpa围压和700°C至950°C的温度下,对天然斜长石骨料进行了实验。无论仪器类型如何,我们都观察到斜长石被分解成榴辉岩相共生,这与榴辉岩相高温域的部分熔融有关。部分熔融主要沿晶界和相界发生。然而,在斜长石中出现变形孪晶和晶粒级微裂的同时,也出现了一些熔体斑块或斜长石分解产物,这表明除了沿晶界和相界熔化和反应外,还发生了晶内熔化和反应。在本研究中,我们展示了脆性微裂和塑性变形孪晶之间的相互作用如何导致晶内熔化和/或反应,这有可能降低富含斜长石的岩石的有效晶粒尺寸,从而影响其反应性和变形行为。
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引用次数: 2
Preservation of mantle heterogeneities and serpentinization signature during antigorite dehydration: The example of the Bergell contact aureole 反Gorite脱水过程中地幔非均质性和蛇纹石化特征的保存:以Bergell接触Aurele为例
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-11-14 DOI: 10.1111/jmg.12699
Romain Lafay, Lukas P. Baumgartner, Adélie Delacour

Major, minor, and trace element geochemistry as well as iron oxidation state and isotopes were investigated in serpentinites and olivine-talc fels present along a metamorphic gradient in the Bergell contact aureole (Central European Alps) to evaluate element mobility during serpentine. This aureole is an ideal target to study dehydration of mantle rocks due to the increase in temperature from greenschist facies conditions (350°C) to amphibolite facies conditions (750°C) at low pressures of 0.4 GPa. Petrography and geochemistry document several events of fluid–rock interaction and metamorphism. Serpentinization of the mantle rocks started on the ocean floor. Subsequent Alpine regional metamorphism led to the formation of antigorite-serpentinites containing olivine and diopside. These antigorite-serpentinites were transformed into olivine-talc fels in a large part of the contact-aureole. Bulk-rock major and trace element compositions maintain the geochemical signature of the precursor antigorite-serpentinites. No apparent changes are indeed observed despite the fact that major dehydration reactions occurred. In addition, changes neither in Fe3+/Fetot ratio nor in δ56Fe values were observed. Local composition variations of antigorite-serpentinites and olivine-talc fels reflect chemical heterogeneities related to protolith composition and serpentinization processes on the ocean floor prior to contact metamorphism. Hence, prograde dehydration reactions occurring during contact metamorphism did not induce substantial element mobility, change in redox state, or isotopic fractionation in these contact metamorphic rocks.

主要、次要和微量元素地球化学以及铁氧化态和同位素在沿着Bergell接触Aurele(中欧阿尔卑斯山)变质梯度存在的蛇纹岩和橄榄石滑石中进行了研究,以评估蛇纹岩期间的元素迁移率。由于温度在0.4 GPa的低压下从绿片岩相条件(350°C)上升到角闪岩相条件(750°C),该金黄色带是研究地幔脱水的理想目标。岩石学和地球化学记录了几个流体-岩石相互作用和变质作用的事件。地幔岩石的蛇形作用始于海底。随后的阿尔卑斯区域变质作用导致了含有橄榄石和透辉石的反gorite蛇纹岩的形成。这些反gorite蛇纹岩在接触Aurele的大部分转化为橄榄石滑石毡。大块岩石的主要元素和微量元素组成保持了前体反gorite蛇纹岩的地球化学特征。尽管发生了主要的脱水反应,但确实没有观察到明显的变化。此外,既没有观察到Fe3+/Fetot比值的变化,也没有观察到δ56Fe值的变化。反gorite蛇纹岩和橄榄石滑石毡的局部成分变化反映了与接触变质作用前的原岩成分和海底蛇纹石化过程有关的化学不均匀性。因此,在这些接触变质岩中,接触变质过程中发生的进变质脱水反应没有引起实质性的元素迁移、氧化还原状态的变化或同位素分馏。
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引用次数: 1
Trapped K-feldspar phenocrysts as a signature of melt migration pathways within active high-strain zones 作为活动高应变带内熔体迁移路径标志的捕获钾长石斑晶
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-11-02 DOI: 10.1111/jmg.12698
David Silva, Sandra Piazolo, Nathan R. Daczko

Melt migration through high-strain zones in the crust fundamentally influences their rheological behaviour and is important for the transfer of fluids to upper crustal regions. The inference of former melt-present deformation, based on field observations, may be hampered if the high-strain zone experience a low time-integrated melt flux or high melt volume expulsion during deformation. In these cases, typical macro-scale field evidence of former melt presence limits interpretations. In this contribution, we investigate igneous field evidence ranging from obvious to cryptic in the Gough Dam shear zone (central Australia), a 2- to 4-km-wide high-strain zone shown to have acted as a significant melt pathway during the Alice Springs Orogeny. Within bands of the high-strain zone, granitic lenses are easily discernible in the field and are inferred to have formed during melt present deformation. Related coarse K-feldspar is observed in biotite-rich (>75 vol%) schist (glimmerite) as either isolated grains, forming trails (sub)parallel to the main foliation, or in aggregates with subordinate quartz. Detailed characterization of the granitic lenses shows that pockets of phenocrysts may be entrained in the shear zone. If melt expulsion and melt-rock interaction is severe, isolated K-feldspar grains in glimmerite may form. These grains exhibit (i) partially preserved crystal faces; (ii) a lack of internal grain deformation; (iii) reaction textures preferentially formed along the main crystallographic axes showing dissolution of K-feldspar and precipitation of dominantly biotite; (iv) low-strain domains between multiple K-feldspar grains being inferred to enclose crystallized melt pockets, with some apparently isolated grains showing connectivity in three dimensions; and (v) a weak quartz and K-feldspar crystallographic preferred orientation. These observations suggest an igneous phenocrystic origin for the isolated K-feldspar grains hosted in glimmerite, which is consistent with the observed REE concentration patterns with positive Eu anomaly. We propose that the K-feldspar phenocrysts are early-formed crystals that were entrained into the glimmerite rocks as reactive melt migrated through the actively deformatting high-strain zone. Previously entrained K-feldspar phenocrysts were trapped during the collapse of the melt pathway when melt flux-related fluid pressure waned while confining pressure and tectonic stress were still significant. The active deformation facilitated expulsion or loss of the melt phase but retainment and trapping of phenocrysts. Hence, the presence of isolated or ‘trains’ of K-feldspar phenocrysts is a cryptic signature of syndeformational melt transfer. If melt transfer occurs in an open chemical system, phenocrysts will be entrained within the reaction product of melt-rock interaction. We suggest that these so-called trapped phenocrysts are a viable indicator of former syntectonic melt passage through rocks.

熔体通过地壳高应变带的迁移从根本上影响其流变行为,并对流体向上地壳区域的转移至关重要。如果高应变区在变形过程中经历低时间积分熔体通量或高熔体体积排出,则基于现场观测的先前熔体存在变形的推断可能会受到阻碍。在这些情况下,先前熔体存在的典型宏观尺度现场证据限制了解释。在这篇文章中,我们研究了Gough Dam剪切带(澳大利亚中部)的火成岩场证据,从明显到隐蔽,这是一个2至4公里宽的高应变带,在Alice Springs造山运动中被证明是一个重要的熔体通道。在高应变带的带内,花岗岩透镜体在野外很容易辨认,并被推断是在熔体变形过程中形成的。在富含黑云母(>75 vol%)片岩(格列美脲),其为孤立颗粒,形成平行于主要叶理的轨迹(亚),或与次要石英聚集在一起。花岗岩透镜体的详细特征表明,剪切带中可能夹带斑晶。如果熔体排出和熔体-岩石相互作用严重,则可能会在格列美脲中形成孤立的钾长石颗粒。这些颗粒表现出(i)部分保留的晶面;(ii)内部晶粒没有变形;(iii)反应织构优先沿主结晶轴形成,显示钾长石的溶解和主要黑云母的沉淀;(iv)多个钾长石颗粒之间的低应变域被推断为包围结晶熔体袋,一些明显孤立的颗粒显示出三维连通性;和(v)弱石英和钾长石晶体的优选取向。这些观察结果表明,存在于格列美脲中的孤立钾长石颗粒是火成斑晶成因,这与观察到的具有正Eu异常的REE浓度模式一致。我们提出,钾长石斑晶是早期形成的晶体,当反应性熔体迁移通过主动变形的高应变带时,这些晶体被夹带到格列美脲岩石中。先前夹带的钾长石斑晶在熔体通道坍塌期间被捕获,此时熔体通量相关的流体压力减弱,而围压和构造应力仍然很大。主动变形促进了熔体相的排出或损失,但保留和捕获了斑晶。因此,孤立的或“一连串”的钾长石斑晶的存在是同变形熔体转移的一个隐蔽特征。如果熔体转移发生在开放的化学系统中,斑晶将被夹带在熔体-岩石相互作用的反应产物中。我们认为,这些所谓的捕获斑晶是前同构造熔体穿过岩石的可行指标。
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引用次数: 1
Geothermometric constraints on the thermal architecture, metamorphism, and exhumation of the Northern Range, Trinidad 地质测温对特立尼达北部山脉热构造、变质作用和剥露的限制
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-10-31 DOI: 10.1111/jmg.12697
Ivano Gennaro, John Weber, Alberto Vitale Brovarone, Jeanette Arkle, Xu Chu

The Northern Range of Trinidad is composed of Mesozoic passive margin sedimentary rocks that underwent ductile deformation and subgreenschist- to greenschist-facies metamorphism in the early Miocene. Previous studies suggested a two-stage formation of the Northern Range between the Caribbean and South American plates: an initial collision drove mountain building in the Miocene and subsequent strike-slip plate motion preferentially exhumed the western segment, producing a westward increase in the metamorphic thermal gradient. However, these studies were not able to resolve whether this gradient was discrete or continuous so the tectonic model awaits testing. In this study we use Raman spectroscopy on carbonaceous material (RSCM), an empirical geothermometer, to constrain peak temperatures across the Northern Range with a greater resolution than was available in previous studies. The RSCM temperatures show an abrupt increase from 337°C ± 10°C in the east to 442°C ± 16°C west of Chupara Point, where a range-cutting fault (Chupara Fault) had been inferred in previous geologic mapping campaigns. Thus, the discrete thermal discontinuity of ~100°C very likely represents the Chupara Fault. Our RSCM-derived peak metamorphic temperatures are 50°C to 100°C higher than those from previous estimates, requiring revision of tectonic models to account for deeper burial and greater exhumation. The peak metamorphic conditions determined here, and the deduced timing of faulting from published thermochronological data, are consistent with the two-stage tectonic model proposed in previous studies.

特立尼达北部山脉由中生代被动边缘沉积岩组成,在中新世早期经历了韧性变形和亚绿片岩-绿片岩相变质作用。先前的研究表明,在加勒比海板块和南美板块之间,北部山脉的形成分为两个阶段:中新世,最初的碰撞推动了造山运动,随后的走滑板块运动优先挖掘了西部板块,导致变质热梯度向西增加。然而,这些研究无法确定这种梯度是离散的还是连续的,因此构造模型有待检验。在这项研究中,我们使用了碳质材料的拉曼光谱(RSCM),一种经验地温计,以比以前的研究更高的分辨率约束北部山脉的峰值温度。RSCM温度从东部的337°C±10°C突然增加到丘帕拉点以西的442°C±16°C,在以前的地质测绘活动中推断出了一个范围切割断层(丘帕拉断层)。因此,~100°C的离散热不连续很可能代表丘帕拉断层。我们的RSCM得出的峰值变质温度比以前的估计高50°C到100°C,这需要修改构造模型以考虑更深的埋藏和更大的挖掘。本文确定的变质峰条件,以及根据已发表的热年代学资料推断出的断裂时间,与前人提出的两阶段构造模型一致。
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引用次数: 0
Alpine tectono-metamorphic evolution of the Corsica basement 科西嘉岛基底的阿尔卑斯构造变质演化
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-10-23 DOI: 10.1111/jmg.12696
Federico Rossetti, William Cavazza, Gianfranco Di Vincenzo, Federico Lucci, Thomas Theye

The Alpine orogenic edifice of Corsica (northern Tyrrhenian Sea) offers the possibility to investigate the mode through which continental crust responds to the propagation of regional shortening at convergent plate margins. The geology of Corsica has been traditionally described as separating domains affected by the Alpine tectonism (Alpine Corsica) from those that did not experience the Alpine tectono-metamorphic overprint (Hercynian Corsica), but recent studies show that most of Hercynian Corsica was thermally reset in post-Eocene times, questioning this paradigm. The continental units formed at the expenses of the stretched continental margin of the European plate and consist of Hercynian granitoid basement rocks and cover sequences (Permian volcaniclastics and Mesozoic sedimentary successions). By integrating meso- and microstructural investigations with metamorphic thermobarometry and 40Ar–39Ar geochronology along three E–W trending structural transects running across the basement section exposed below the Alpine orogenic wedge, we document middle-late Eocene (c. 50–33 Ma) westward-verging syn-metamorphic (low-grade blueschist facies) thick-skinned, basement-involved thrusting. Significantly, crustal shortening in the continental basement predated of c. 15–10 Ma the subduction zone metamorphism in the oceanic-derived Schistes Lustrés Complex. When the P–T–t–deformation history as reconstructed from the Corsica basement is integrated with the regional scenario of the Alpine–Apennine orogeny, a tectonic reconstruction is proposed, which frames the Alpine orogeny in Corsica within the Apennine–Maghrebian subduction system in the retroside (retrowedge) of the Apennine orogenic wedge.

科西嘉岛(第勒尼安海北部)的阿尔卑斯造山大厦提供了研究大陆地壳对会聚板块边缘区域缩短传播的响应模式的可能性。科西嘉的地质传统上被描述为受阿尔卑斯构造作用(阿尔卑斯科西嘉)影响的区域与未经历阿尔卑斯构造变质叠加作用的区域(海西西科西嘉)的分离区域,但最近的研究表明,海西西科西嘉的大部分地区在后始新世被热重置,这对这种模式提出了质疑。大陆单元是在欧洲板块伸展大陆边缘的基础上形成的,由海西期花岗岩基底岩和盖层(二叠纪火山碎屑岩和中生代沉积序列)组成。通过结合中观和微观构造研究、变质热气压测量和40Ar-39Ar地质年代学,沿着三个东西向的构造样带,穿过暴露在阿尔卑斯造山带楔下的基底剖面,我们记录了始新世中晚期(约50-33 Ma)向西辐合的同变质(低品位蓝片岩相)厚皮、基底相关的逆冲作用。值得注意的是,大陆基底地壳缩短早于约15-10 Ma的洋源片岩- lustr杂岩的俯冲带变质作用。将科西嘉基底重建的p - t - t变形历史与阿尔卑斯-亚平宁造山运动的区域情景相结合,提出了将科西嘉的阿尔卑斯造山运动置于亚平宁造山楔逆侧(反楔)的亚平宁-马格里布俯冲体系内的构造重建。
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引用次数: 2
Gabbroic eclogites formed during rapid and cold subduction of the Paleo-Tethys oceanic lithosphere in the Changning–Menglian orogenic belt, southeastern Tibetan plateau 青藏高原东南部长宁—孟连造山带古特提斯洋岩石圈快速冷俯冲过程中形成的辉长岩榴辉岩
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2022-10-18 DOI: 10.1111/jmg.12694
Huining Wang, Fulai Liu, Zaibo Sun, Lei Ji, Jia Cai, Jianjiang Zhu, Kun Zhou, Wei Wang

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan plateau separates Gondwana- from Eurasia-derived continental blocks and marks the main suture of the Paleo-Tethys, as evidenced by a variety of oceanic basalt-derived eclogites. However, it is uncertain whether the belt contains high-pressure rocks derived from gabbro, which is a key component of oceanic ophiolite. Here, we present a study of newly discovered gabbroic eclogites from the CMOB. These eclogites preserve relic gabbroic crystals (diopside, bytownite/anorthite and ilmenite) that survived metamorphism and occur in the form of inclusions within porphyroblasts. The eclogites have positive εNd(t) values of +1 to +8 and have an affinity to N-MORB, with positive Eu anomalies and no depletions in high-field-strength elements (e.g. Nb, Ta, Zr and Hf). Cumulate gabbros generated in a mid-ocean ridge setting are possible protoliths for the studied samples. The eclogite facies mineralogy is defined by the assemblage of garnet + omphacite + kyanite + talc + phengite + rutile, which was followed by the post-kinematic crystallization of winchite and clinozoisite, and a later symplectite assemblage (diopside + sodic plagioclase + calcic amphibole + clinozoisite). Phase equilibrium modelling, average P–T thermobarometry and conventional mineral geothermobarometry constrain the P–T conditions for the peak-stage and initial post-peak-stage metamorphism and symplectite formation to 25.6–27.1 kbar/595–637°C, 15.3–17.9 kbar/563–605°C and 5.5–7.3 kbar/470–500°C, respectively, consistent with a subduction depth of 75–85 km. Metamorphic zircons yielded a Triassic mean U–Pb age of 223.7 ± 2.9 Ma, which is interpreted to record the early-stage decompressive overprinting. The similar paragenetic sequences, mineral evolution, peak P–T conditions and P–T–t paths for the gabbro- and basalt-derived eclogites in the CMOB indicate that these rocks formed in the Paleo-Tethys subduction regime. The lack of deformation, and the cold and rapid subduction history, contributed to the local preservation of gabbroic minerals and igneous textures under high-pressure conditions in the studied rocks. The gabbroic eclogites provide insights into the detailed metamorphic evolution during the burial–exhumation cycle of ophiolites in the Paleo-Tethyan regime.

青藏高原东南部的长宁-孟连造山带(CMOB)将冈瓦纳与欧亚大陆块体分离开来,标志着古特提斯的主缝合线,各种洋玄武岩衍生的榴辉岩证明了这一点。然而,该带是否含有来自辉长岩的高压岩石尚不确定,辉长岩是海洋蛇绿岩的重要组成部分。在这里,我们对新发现的CMOB辉长岩进行了研究。这些榴辉岩保存了残留的辉长岩晶体(透辉石、长石/钙长石和钛铁矿),这些晶体在变质作用中幸存下来,以包裹体的形式出现在卟绿母岩中。榴辉岩的εNd(t)值为+1 ~ +8,与N - MORB有亲和关系,Eu呈正异常,且高场强元素(如Nb、Ta、Zr和Hf)没有耗尽。在洋中脊环境中形成的堆积辉长岩可能是研究样品的原岩。榴辉岩相矿物学定义为石榴石+辉长石+蓝晶石+滑石+辉长石+金红石组合,其次是盘辉石和斜沸石的运动后结晶,后来是复晶组合(透辉石+钠质斜长石+钙质角闪石+斜沸石)。相平衡模拟、平均P-T热气压测量和常规矿物地温压测量将峰期和峰期后初始变质作用和正长岩形成的P-T条件分别限制在25.6 ~ 27.1 kbar/595 ~ 637°C、15.3 ~ 17.9 kbar/563 ~ 605°C和5.5 ~ 7.3 kbar/470 ~ 500°C,与俯冲深度75 ~ 85 km一致。变质锆石产生U-Pb三叠纪平均年龄为223.7±2.9,这是解释为记录早期必经阶段得到套印。CMOB辉长岩和玄武岩榴辉岩相似的共生序列、矿物演化、峰值P-T条件和P-T - t路径表明,这些岩石形成于古特提斯俯冲体系。由于缺乏变形和寒冷快速的俯冲历史,研究岩石在高压条件下局部保存了辉长岩矿物和火成岩结构。辉长岩提供了古特提斯时期蛇绿岩埋藏-挖掘周期变质演化的详细信息。
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引用次数: 1
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Journal of Metamorphic Geology
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