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The temporal evolution of subduction initiation in the Samail ophiolite: High-precision U–Pb zircon petrochronology of the metamorphic sole Samail蛇绿岩俯冲起始的时间演化:变质底的高精度U-Pb锆石年代学
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-03-10 DOI: 10.1111/jmg.12719
Matthew Rioux, Joshua M. Garber, Michael Searle, James L. Crowley, Sally Stevens, Mark Schmitz, Andrew Kylander-Clark, Kayla Leal, Tyler Ambrose, Andrew J. Smye

High-precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well-preserved and well-characterized ophiolite, we performed coupled zircon laser-ablation inductively coupled mass spectrometry trace element analyses and high-precision isotope dilution-thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman-United Arab Emirates). Zircon grains from amphibolite- to granulite-facies (0.8–1.3 GPa, ~700–900°C), garnet- and clinopyroxene-bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet-clinopyroxene amphibolite samples, Ti-in-zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti-in-zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic P–T–t path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet-free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon εHf (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high-grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate alo

蛇绿岩变质底的高精度定年可以为蛇绿岩的构造演化和俯冲带过程提供深入的认识。为了了解年轻的、保存完好的、具有良好特征的蛇绿岩的俯冲起始,我们对来自Samail蛇绿岩(阿曼-阿拉伯联合酋长国)变质底的25个样品进行了耦合锆石激光烧蚀电感耦合质谱痕量元素分析和高精度同位素稀释-热电离质谱U-Pb测年。角闪岩-麻粒岩相(0.8 ~ 1.3 GPa, ~700 ~ 900℃)的锆石颗粒,含石榴石-斜辉石角闪岩样品(n = 18)的重稀土元素斜率(HREE)呈系统递减趋势;Yb/Dy)随着Yb浓度的降低,反映了在石榴石生长过程中稀土元素的逐渐耗竭。对于一半的石榴石-斜辉石角闪岩样品,Ti - in -锆石温度升高,U-Pb年龄随着HREE斜率的减小而变年轻,这与锆石和石榴石在进变质过程中的耦合生长一致。在其余样品中,Ti - In -锆石温度随HREE斜率的减小没有明显变化,U-Pb和地球化学数据表明,锆石结晶沿变质P-T-t路径的渐近至峰或渐近至初始逆行部分。新资料将变质底最高品位岩石中前进式石榴石和锆石生长的时间定在96.698±0.094 ~ 95.161±0.064 Ma之间,而之前发表的地质年代学表明前进式变质作用在~104 Ma。鞋底较低品位(但仍为较高品位角闪岩相)部分的无石榴石角闪岩和白垩质角闪岩均富集稀土元素(Yb/Dy bbb5),比同一地点的较高品位岩石年轻~0.5 ~ 1.3 Myr,限制了高、低品位单元变质作用和并置的时间偏移。除一块角闪岩外,其余角闪岩的正锆石εHf(+6.5 ~ +14.6)与鞋底的洋玄武岩原岩一致。我们的新数据表明,前进单变质作用(96.7-95.2 Ma)早于上覆蛇绿岩壳(96.1-95.2 Ma)的发育和重叠。蛇绿岩北部单一变质作用开始与蛇绿岩岩浆活动开始之间的~600 ky偏移量比先前提出的~8 Ma短一个数量级,与自发俯冲起始或诱导俯冲起始的短暂初始逆冲期相一致。综上所述,底壳和蛇绿岩壳保存了前~1.5 Myr的俯冲记录。从西北(96.7 Ma)到东南(96.0-95.7 Ma)的高变质作用起始梯度可能记录了新生俯冲带的扩展和/或俯冲速率沿蛇绿岩长度的变化。
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引用次数: 2
Pressure–temperature paths of tectonic blocks in mélange: Recording thermal evolution of a subduction channel at an initial stage of subduction 朗格构造块体的压力-温度路径:记录俯冲初始阶段俯冲通道的热演化
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-03-09 DOI: 10.1111/jmg.12718
Toru Takeshita, Takeshi Imayama, Mizuho Ando, Yuto Kimura, Marie Python

We investigated the tectonic evolution of amphibolite and blueschist tectonic blocks in the serpentinite- or pelite-matrix mélange, which are distributed at the highest structural level of the high-P/T type Kamuikotan metamorphic rocks in northern Japan. The tectonic blocks in this study area are divided into six rock types: garnet-epidote amphibolite, epidote amphibolite, amphibolite, plagioclase-poor amphibolite, epidote blueschist and glaucophane-bearing quartz schist. Based on phase equilibrium modelling, garnet-epidote amphibolite and epidote amphibolite experienced peak metamorphism at pressure and temperature conditions of 1.1–1.25 GPa and 550–590°C, and 0.8–1.3 GPa and 475–550°C, respectively, (at an apparent thermal gradient ranging between 13 to 17°C/km). By contrast, although the peak-metamorphic conditions for each one of amphibolite, plagioclase-poor amphibolite, and glaucophane-bearing quartz schist are not well constrained, they may have been originally metamorphosed at amphibolite to epidote-amphibolite facies at thermal gradients exceeding 20°C/km, inferred from the core composition of amphibole (edenite/magnesiohornblende/barroisite). The epidote blueschist experienced peak metamorphism at pressure and temperature conditions of 0.8–1.6 GPa and 360–520°C (most probably 0.8–0.85 GPa and 360–480°C). Although different types of tectonic blocks experienced a variety of peak metamorphism under different P/T conditions, all of them underwent epidote blueschist facies metamorphism at the peak or retrograde stage (as shown by the glaucophane rims of amphibole with different core compositions). The overall P–T paths appear counter-clockwise, which could be interpreted to reflect the cooling history of the subduction channel from the beginning to the steady state of subduction. The geothermal gradient could have changed from 15–17° to ~10°C/km over ~20–25 Myr, as estimated by previously reported radiometric ages. The protoliths to the tectonic blocks could have begun to subduct into the subduction channel at different times (where the thermal structure evolved with time), acquiring different prograde P–T paths. Subsequently, these tectonic blocks were juxtaposed at a certain depth and incorporated into the overlying serpentinite during the subduction stage. Finally, the serpentinite- or pelite-matrix mélange, including these tectonic blocks, were exhumed together with the coherent accretionary units as the former was emplaced over the latter.

摘要研究了位于日本北部高P/T型Kamuikotan变质岩最高构造层位的蛇纹岩或泥质岩基质岩中角闪岩和蓝片岩构造块体的构造演化。研究区构造块体划分为石榴石-绿帘石角闪岩、绿帘石角闪岩、角闪岩、斜长石-贫角闪岩、绿帘石蓝片岩和含蓝帘石石英片岩6种岩石类型。基于相平衡模型,石榴石—绿帘石角闪岩和绿帘石角闪岩分别在1.1 ~ 1.25 GPa和550 ~ 590℃、0.8 ~ 1.3 GPa和475 ~ 550℃的压力和温度条件下(表观热梯度范围为13 ~ 17℃/km)经历了变质峰。相比之下,虽然角闪岩、斜长石-贫角闪岩和含蓝闪石的石英片岩的峰值变质条件没有得到很好的限制,但从角闪岩的岩心组成(铁长岩/镁角闪石/钡辉石)推断,它们可能在温度梯度超过20℃/km时,从角闪岩到绿长岩-角闪岩相最初变质。绿帘石蓝片岩在0.8 ~ 1.6 GPa和360 ~ 520℃的压力和温度条件下(最有可能是0.8 ~ 0.85 GPa和360 ~ 480℃)发生了变质峰。虽然不同类型的构造块体在不同的P/T条件下经历了不同的峰值变质作用,但它们都在峰值或逆行阶段经历了绿帘岩蓝片岩相变质作用(如不同岩心组成的角闪洞的蓝绢岩边缘)。整体P-T路径呈逆时针方向,反映了俯冲通道从开始到稳定状态的冷却历史。根据先前报道的辐射年龄估计,地热梯度可能在~ 20-25 Myr范围内从15-17°变化到~10°C/km。构造块体原岩可能在不同时期(热构造随时间演化)开始向俯冲通道俯冲,形成不同的进积P-T路径。随后,这些构造块体在一定深度处并置,并在俯冲阶段与上覆蛇纹岩合并。最后,包括这些构造块体在内的蛇纹岩或泥岩基质与相贯的增生单元一起被发掘出来,因为前者位于后者之上。
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引用次数: 0
A rutile and titanite record of subduction fluids: Integrated oxygen isotope and trace element analyses in Franciscan high-pressure rocks 俯冲流体的金红石和钛矿记录:方济各高压岩石的综合氧同位素和微量元素分析
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-02-27 DOI: 10.1111/jmg.12717
F. Zeb Page, Craig D. Storey, EIMF

In situ oxygen analysis of garnet in eclogite and related rocks is increasingly being used to probe the composition of subduction fluids. However, in many cases, these samples contain textural signs of both fluid flow and retrograde metamorphism, some of which may take place outside the garnet stability field. In order to test the connection between polymetamorphism and fluid infiltration, rutile rimmed by titanite from high-grade tectonic blocks of the Franciscan Formation (California, USA) was analysed for oxygen isotope ratios and trace element concentrations. Zirconium concentrations in rutile yield temperatures of ~600°C for eclogite and hornblende eclogite from three well-studied localities (Junction School, Tiburon and Ward Creek). Rutile trace element concentrations are generally low and consistent with a mafic protolith. Titanite surrounding rutile has inherited much of its trace element content from rutile, and Zr-in-titanite temperatures are spuriously high. Titanite in rutile-free samples (blueschist and eclogite from Jenner beach) have similar compositions suggesting that they were formed at the expense of rutile as well. Oxygen isotope ratios from rutile and titanite in the same sample are fortuitously similar, indicating disequilibrium between these minerals, which formed at different times and temperatures but in equilibrium with the same oxygen reservoir. Rutile in blocks with garnets zoned in oxygen isotopes are generally in equilibrium with the rims rather than the cores. Slow oxygen diffusion in rutile and the low temperatures of formation require that rutile recrystallized after fluid interaction and before blueschist facies metamorphism. External fluid interaction of Franciscan eclogites took place near the peak of metamorphism.

榴辉岩及相关岩石中石榴石的原位氧分析越来越多地用于探测俯冲流体的成分。然而,在许多情况下,这些样品含有流体流动和逆变质作用的结构迹象,其中一些可能发生在石榴石稳定区之外。为了测试多聚变质作用与流体渗透之间的联系,分析了方济各会组(美国加利福尼亚州)高级构造块体中由钛矿镶边的金红石的氧同位素比率和微量元素浓度。来自三个研究充分的地区(Junction School、Tiburon和Ward Creek)的榴辉岩和角闪榴辉岩的金红石中的锆浓度产生温度为~600°C。金红石微量元素浓度通常较低,与镁铁质原岩一致。金红石周围的钛矿继承了金红石的大部分微量元素含量,并且Zr-in-钛矿的温度高得离谱。无金红石样品(詹纳海滩的蓝片岩和榴辉岩)中的钛矿成分相似,表明它们也是以牺牲金红石为代价形成的。同一样品中金红石和钛矿的氧同位素比例偶然相似,表明这些矿物之间存在不平衡,它们在不同的时间和温度下形成,但与同一氧气库保持平衡。含氧同位素石榴石块体中的金红石通常与边缘而非核心处于平衡状态。氧气在金红石中的缓慢扩散和形成的低温要求金红石在流体相互作用后和蓝片岩相变质作用前再结晶。方济各榴辉岩的外部流体相互作用发生在变质作用高峰期附近。
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引用次数: 0
To be or not to be Alpine: New petrological constraints on the metamorphism of the Chenaillet Ophiolite (Western Alps) 是否为阿尔卑斯山:Chenaillet蛇绿岩(西阿尔卑斯)变质作用的新岩石学约束
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-02-26 DOI: 10.1111/jmg.12716
Alberto Corno, Chiara Groppo, Alessandro Borghi, Pietro Mosca, Marco Gattiglio

The Chenaillet Ophiolite represents a very well-preserved portion of Ligurian-Piedmont ocean in the Western Alps. It is formed from an oceanic lithospheric succession comprising exhumed mantle, various mafic intrusives (i.e., gabbro sensu lato), and a world-renowned sequence of pillow basalts. Apart from scarce breccias closely related to oceanic lithosphere, no sedimentary cover is exposed. Historically, the Chenaillet Ophiolite has been known for its very low temperature–low pressure Alpine metamorphism, ascribed to obduction processes. However, studies aimed at constraining the peak pressure–temperature (P–T) conditions of Alpine metamorphism are virtually lacking, the general focus having been so far on its high temperature metamorphism and geochemical features. In this paper, we investigate two kinds of rocks: gabbro and albitite/alkali syenite, whose petrographic features shed light on the complex metamorphic history of the Chenaillet Ophiolite. Detailed analyses of mineral assemblages, blastesis/deformation relationships, and mineral chemical data allow two metamorphic events to be distinguished: an earlier, high temperature event (already reported in the literature) and a second, later low temperature, high pressure event, recognized here for the first time. The low temperature, high pressure event is strikingly testified by the occurrence of lawsonite relicts in the gabbro and of interstitial omphacite in the albitite. Thermodynamic modelling (i.e., via isochemical phase diagrams) performed on a gabbro sample suggests for this unit a minimum of 9 kbar and 300°C and a maximum of 15 kbar and 450°C. Overlapping these P–T conditions with those inferred for the albitite based on the observed mineral assemblage allows the Alpine peak metamorphism to be constrained to 10–11 kbar and 340–360°C. These P–T conditions suggest a thickness of the overlying nappe stack of about 35–40 km, which is incompatible with obduction or burial processes, and instead consistent with subduction processes related to the Alpine orogeny. We argue that, opposite to the common belief that the Chenaillet Ophiolite escaped Alpine metamorphism, our new data strongly support the idea that it experienced low temperature-blueschist-facies metamorphism, whose evidence can still be tracked in those (few) rocks that better recorded and preserved it. This finding generates new challenging questions regarding both subduction and exhumation processes in complex orogens such as the Western Alps.

Chenaillet蛇绿岩代表了西阿尔卑斯利古里亚-皮埃蒙特海洋中保存非常完好的部分。它由海洋岩石圈序列形成,包括折返地幔、各种镁铁质侵入岩(即广义辉长岩)和世界著名的枕状玄武岩序列。除了与海洋岩石圈密切相关的稀少角砾岩外,没有沉积覆盖层暴露出来。从历史上看,Chenaillet蛇绿岩以其非常低温-低压的阿尔卑斯变质作用而闻名,这归因于造山过程。然而,实际上缺乏旨在限制阿尔卑斯变质作用的峰值压力-温度(P–T)条件的研究,到目前为止,普遍关注的是其高温变质作用和地球化学特征。本文研究了辉长岩和钠长石/碱性正长岩两种岩石,它们的岩石学特征揭示了Chenaillet蛇绿岩的复杂变质史。通过对矿物组合、爆破/变形关系和矿物化学数据的详细分析,可以区分两个变质事件:一个是早期的高温事件(已在文献中报道),另一个是后期的低温高压事件,这是本文首次认识到的。辉长岩中的洛氏体残余物和钠长岩中的间隙绿辉石的出现,有力地证明了低温高压事件。对辉长岩样品进行的热力学建模(即通过等化学相图)表明,该单元的最低温度为9kbar和300°C,最高温度为15 kbar和450°C。将这些P–T条件与根据观察到的矿物组合推断的钠长石条件重叠,可以将阿尔卑斯峰变质作用限制在10–11 kbar和340–360°C。这些P–T条件表明上覆推覆岩堆的厚度约为35–40 km,这与造山或埋藏过程不相容,而是与阿尔卑斯造山运动相关的俯冲过程一致。我们认为,与Chenaillet蛇绿岩逃脱阿尔卑斯变质作用的普遍看法相反,我们的新数据有力地支持了它经历了低温蓝片岩相变质作用的观点,其证据仍然可以在那些更好地记录和保存它的(少数)岩石中追踪。这一发现引发了关于西阿尔卑斯山等复杂造山带俯冲和折返过程的新的挑战性问题。
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引用次数: 3
Prograde zircon growth in migmatites 杂岩中渐进式锆石生长
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-02-23 DOI: 10.1111/jmg.12715
Chris Yakymchuk

Empirical studies of zircon in migmatites document features compatible with growth during heating at suprasolidus conditions. However, numerical modelling of zircon behaviour suggests that suprasolidus zircon is expected to grow only during cooling and melt crystallization. Here, phase equilibrium modelling coupled with mineral–melt Zr partitioning is used in an attempt to reconcile the observations from migmatites with the predictions of previous numerical models of zircon behaviour in anatectic systems. In general, an equilibrium-based model that includes Zr partitioning does not allow prograde suprasolidus zircon growth. However, melting of metapelites at temperatures just above the wet solidus may allow limited zircon growth because of the low solubility of zircon in melt coupled with a source of Zr from minor garnet and ilmenite breakdown. Preservation of this zircon requires entrapment in growing peritectic minerals during subsequent heating and further melting. Heating above muscovite exhaustion in metapelites is unlikely to grow zircon because of the progressive increase in zircon solubility as well as an increasing compatibility of Zr in the residual mineral assemblage. The modelled compatibility of Zr in the residue of a metabasite decreases during heating, but an increase in zircon solubility in melt counteracts this; prograde suprasolidus zircon growth in metabasites is unlikely. Infiltration of Zr-rich melt into a migmatite during open-system anatexis provides an additional potential mechanism for prograde suprasolidus zircon growth during high-temperature metamorphism.

混合岩中锆石的经验研究记录了在超固结条件下加热时与生长相容的特征。然而,锆石行为的数值模拟表明,超固结锆石预计仅在冷却和熔融结晶过程中生长。在这里,相平衡建模与矿物-熔体Zr分配相结合,试图将混合岩的观测结果与先前对深熔系统中锆石行为的数值模型的预测相协调。通常,包括Zr分配的基于平衡的模型不允许进行超固结锆石的生长。然而,由于锆石在熔体中的溶解度低,再加上少量石榴石和钛铁矿分解产生的Zr源,在刚好高于湿固相线的温度下熔融化镁石可能会限制锆石的生长。这种锆石的保存需要在随后的加热和进一步熔融过程中被包裹在生长中的包晶矿物中。由于锆石溶解度的逐渐增加以及Zr在残余矿物组合中的相容性的增加,在变质精英中加热超过白云母耗竭不太可能生长锆石。在加热过程中,Zr在偏硼酸盐残留物中的模型相容性降低,但锆石在熔体中溶解度的增加抵消了这一点;变质基岩中进行中的超固结锆石生长是不可能的。在开放系统锐钛矿过程中,富锆熔体渗透到混合岩中,为高温变质过程中前进的超固结锆石生长提供了额外的潜在机制。
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引用次数: 3
Carboniferous–Triassic tectonic and thermal evolution of the middle crust section of the Dervio–Olgiasca Zone (Southern Alps) 南阿尔卑斯德尔维奥-奥尔吉亚斯卡带石炭纪-三叠纪中地壳构造与热演化
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-02-23 DOI: 10.1111/jmg.12714
Christophe Real, Kathrin Fassmer, Rodolfo Carosi, Nikolaus Froitzheim, Daniela Rubatto, Chiara Groppo, Carsten Münker, Simona Ferrando

A well-preserved remnant of the middle crust of the former Adriatic passive margin is exposed in the Southern Alps (Italy). The Dervio–Olgiasca Zone is located south of the Insubric Line along the northern part of Como Lake and, because of the lack of Alpine overprint, provides favourable conditions to investigate the pre-Alpine (rift-related) history. We reconstruct the P–T–t–d evolution of the Adria middle crust through petrological (petrography, mineral chemistry, thermobarometry and thermodynamic modelling) and geochronological (Lu/Hf in garnet and U–Pb in monazite) data from pegmatites and host micaschists. These data allow reconstruction of a complex tectono-thermal evolution of the future proximal Adriatic margin at the onset of Alpine rifting. The amphibolite-facies Carboniferous metamorphic basement (7.6–10 kbar and 610–660°C at 318–312 Ma) was affected by pervasive extensional deformation (5.1–7.6 kbar and 580–660°C) in the Middle- to Late-Permian (257.5 ± 3.8 Ma). Pegmatite intruded at 249.8 ± 1.1 Ma in an extensional phase that re-equilibrated rocks of the basement at 3.5–4.5 kbar and 560–600°C. During the Middle- to Late-Triassic (241–235 Ma), the basement experienced static thermal recrystallization (T = 689 ± 41°C and ~5.0 kbar). This Late-Anisian to Early-Carnian thermal event was simultaneous with the emersion of carbonate platforms, volcanism and ore deposition in the future proximal Adriatic margin. The subsequent cooling of the middle crust was synchronous with large-scale extensional detachments developed in the upper crust (e.g., the Lugano-Val Grande Fault), which controlled the formation of the Monte Generoso Basin. This study reveals that the local post-Carboniferous thinning and heating events recorded in the Adriatic middle crust were interconnected to other processes occurring at different crustal levels that were, in turn, induced by crustal stretching in the early stages of the Alpine rifting.

前亚得里亚海被动边缘的中地壳遗迹保存完好,暴露在南阿尔卑斯山(意大利)。Dervio–Olgiasca带位于科莫湖北部Insubric线以南,由于缺乏阿尔卑斯山套印,为研究前阿尔卑斯山(裂谷相关)历史提供了有利条件。我们通过伟晶岩和寄主云母的岩石学(岩石学、矿物化学、热气压测量和热力学建模)和地质年代(石榴石中的Lu/Hf和独居石中的U–Pb)数据重建了亚得里亚中地壳的P–T–T–d演化。这些数据可以重建阿尔卑斯裂谷作用开始时亚得里亚海近端边缘的复杂构造热演化。角闪岩相石炭系变质基底(7.6–10 kbar和610–660°C,318–312 Ma)受到二叠纪中期至晚期普遍伸展变形(5.1–7.6 kbar和580–660°C)的影响(257.5 ± 3.8Ma)。伟晶岩侵入249.8 ± 1.1 Ma,处于伸展阶段,在3.5–4.5 kbar和560–600°C下使基底岩石重新平衡。三叠纪中期至晚期(241–235 Ma),基底经历静态热再结晶(T = 689 ± 41°C和~5.0 kbar)。这一晚阿尼斯期至早卡尼期热事件与未来亚得里亚海近端边缘碳酸盐岩平台的出现、火山活动和矿床沉积同时发生。中地壳随后的冷却与上地壳(如卢加诺-瓦尔-格兰德断层)中形成的大规模伸展分离同步,后者控制了蒙特杰尼罗索盆地的形成。这项研究表明,亚得里亚海中地壳记录的局部后石炭世变薄和加热事件与不同地壳水平上发生的其他过程相互关联,而这些过程又是由阿尔卑斯裂谷早期的地壳拉伸引起的。
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引用次数: 5
Incompatibility between serpentinization and epidote formation in the lower oceanic crust: Evidence from the Oman Drilling Project 大洋下地壳蛇纹石化和绿帘石形成之间的不相容性:来自阿曼钻探项目的证据
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-02-04 DOI: 10.1111/jmg.12713
Toshio Nozaka, Yamato Tateishi

It is a general tendency that epidote, which is a typical greenschist facies mineral, is scarce in the lower oceanic crust, in spite of the widespread occurrence of the other minerals indicative of similar temperature conditions such as chlorite, actinolite, prehnite and serpentine. To find the cause of this, we carried out petrological analyses of lower crustal rocks of the Oman ophiolite sampled by the Oman Drilling Project of the International Continental Scientific Drilling Program (ICDP). Petrographic observations revealed the tendency, as expected, that the amount of epidote formed by static alteration of plagioclase decreases with depth. Because mineral assemblages indicative of a wide range of temperature conditions from amphibolite to subgreenschist facies occur throughout the cores without systematic variations of abundance, the decrease of epidote amount cannot be explained by the difference of temperature condition of alteration. Petrographic observations also revealed that epidote is absent or rare in rocks containing serpentinized olivine in contrast to prehnite showing a close association with serpentinization of olivine. In an exceptional sample containing both epidote and serpentinized olivine, epidote occurs with chlorite that cuts or replaces plagioclase, mantles adjacent olivine and is connected with chlorite + lizardite veins cutting mesh-forming serpentine veins. The distribution and mode of occurrence of epidote suggest decoupling of its formation with the main stage of serpentinization. Serpentine veins cutting olivine to form mesh texture are typically lizardite with magnetite ribbons at vein centres and have compositions of lizardite–cronstedtite solid solution at vein margins or in magnetite-free veins, suggesting a chemical condition with low silica and low oxygen potentials at an early stage of serpentinization. Thermodynamic modelling for olivine and plagioclase alteration at greenschist facies conditions indicates that silica potential for plagioclase alteration to form prehnite + chlorite and epidote + chlorite could be higher than for olivine serpentinization. On the other hand, oxygen potential for the prehnite + chlorite formation is lower than for the epidote + chlorite formation and is comparable with that for olivine serpentinization. From the observations and analyses, it is concluded that epidote formation is inhibited by olivine serpentinization, which maintains a reducing condition for alteration in the lower oceanic crust.

绿帘石是一种典型的绿片岩相矿物,尽管其他矿物普遍存在,表明温度条件相似,如绿泥石、阳起石、史前岩和蛇纹岩,但绿帘石在下洋地壳中普遍稀少。为了找出原因,我们对国际大陆科学钻探计划(ICDP)阿曼钻探项目采样的阿曼蛇绿岩下地壳岩石进行了岩石学分析。岩石学观察显示,正如预期的那样,斜长石静态蚀变形成的绿帘石数量随着深度的增加而减少。由于指示从角闪岩到亚绿片岩相的广泛温度条件的矿物组合出现在整个岩芯中,而丰度没有系统变化,因此绿帘石数量的减少不能用蚀变温度条件的差异来解释。岩石学观察还表明,在含有蛇纹石化橄榄石的岩石中,绿帘石不存在或罕见,而与橄榄石的蛇纹石化密切相关的史前岩则相反。在同时含有绿帘石和蛇纹石化橄榄石的特殊样品中,绿帘石与绿泥石一起出现,绿泥石切割或取代斜长石,覆盖相邻的橄榄石,并与绿泥石相连 + 橄榄岩脉切割网状蛇纹。绿帘石的分布和赋存方式表明其形成与蛇纹石化的主要阶段脱钩。切割橄榄石形成网状结构的蛇形矿脉通常是在矿脉中心具有磁铁矿带的丽锌矿,并且在矿脉边缘或无磁铁矿矿脉中具有丽锌矿-铬铁矿固溶体的成分,这表明在蛇纹石化早期存在低硅和低氧势的化学条件。绿片岩相条件下橄榄石和斜长石蚀变的热力学模型表明,二氧化硅有可能使斜长石蚀改形成预热器 + 绿泥石和绿帘石 + 绿泥石可能高于橄榄石蛇纹石化。另一方面,预热器的氧势 + 绿泥石的形成低于绿帘石 + 绿泥石形成,可与橄榄石蛇纹石化相媲美。根据观测和分析,绿帘石的形成受到橄榄石蛇纹石化的抑制,这为下洋地壳的蚀变保持了还原条件。
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引用次数: 0
Kinetics and pulses of zircon growth in migmatites beneath a volcanic arc: An example from the high-T Ryoke Complex, southwest Japan 火山弧下混合岩中锆石生长的动力学和脉冲:以日本西南部高T Ryoke杂岩为例
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-01-20 DOI: 10.1111/jmg.12711
Kazuhiro Miyazaki, Takeshi Ikeda, Hideki Iwano, Takafumi Hirata, Tohru Danhara

We present an analysis of kinetics and pulses of zircon growth in migmatites formed at middle to lower crustal depths beneath a volcanic arc. Migmatites in high-T metamorphic complexes at active continental margins, such as in the Ryoke Complex of southwest Japan, are thought to have been produced beneath volcanic arcs. Thermal models suggest that melt advection supplies the heat to form such high-T complexes. We found that zircons in the migmatites of the Ryoke Complex grew in multiple discrete stages by rapid diffusion-controlled growth. The individual growth pulses can be distinguished using a Gaussian mixture model when the duration of each growth pulse is shorter than 1σ of the analytical error of the zircon age dating, and where the interval between each growth pulse is larger than 2σ of them. This method allows extraction of the growth pulses even when the zircon exhibits incomplete textural evidence for multiple stages of growth. Application of the method to the Ryoke Complex revealed three and four pulses of zircon growth with 3–10 Myr intervals for two migmatite samples respectively in the Mikawa area and three pulses with 2–4 Myr intervals from one migmatite sample in the Yanai area. The detected zircon growth pulses are consistent with previously reported pulses of plutonic activity in the two areas, with the exception of the oldest growth pulse in the Mikawa area. Therefore, the growth pulses are interpreted to be the result of thermal pulses because of melt flux events at lower to middle crustal levels. The inferred intervals of pulsed melt fluxes are similar to those of individual caldera formation in coeval caldera clusters at the surface.

我们对火山弧下地壳中下部形成的混合岩中锆石生长的动力学和脉冲进行了分析。活跃大陆边缘的高T变质杂岩中的混合岩,如日本西南部的Ryoke杂岩,被认为是在火山弧下产生的。热模型表明,熔体平流提供了形成这种高T复合体的热量。我们发现,Ryoke杂岩混合岩中的锆石通过快速扩散控制生长,以多个离散阶段生长。当每个生长脉冲的持续时间小于锆石年龄测定分析误差的1σ,并且每个生长脉冲之间的间隔大于其中的2σ时,可以使用高斯混合模型来区分单个生长脉冲。即使锆石在多个生长阶段表现出不完整的结构证据,这种方法也可以提取生长脉冲。该方法在Ryoke杂岩中的应用表明,三川地区的两个混合岩样品分别有三个和四个锆石生长脉冲,间隔为3-10 Myr,而柳井地区的一个混合岩样本有三个锆石生长周期,间隔为2-4 Myr。检测到的锆石生长脉冲与之前报道的两个地区的深成活动脉冲一致,但三川地区最古老的生长脉冲除外。因此,生长脉冲被解释为热脉冲的结果,因为地壳中下层的熔体通量事件。推断出的脉冲熔体通量的间隔与表面同时代破火山口群中单个破火山口形成的间隔相似。
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引用次数: 1
Repeated Caledonian burial and ultrafast cooling and exhumation of high-pressure granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft, Western Gneiss Region, Norway 挪威西部片麻岩地区Fjørtoft岛bl<s:1> hope推覆体加里东期反复埋藏、超快冷却和高压麻粒岩相岩石的发掘
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-01-13 DOI: 10.1111/jmg.12710
Penglei Liu, Hans-Joachim Massonne

The allochthonous Blåhø Nappe in the Nordøyane ultra high pressure (UHP) domain, Western Gneiss Region in Norway, acts as a window to examine geological processes occurring in continent–continent collisional zones, but many aspects regarding its tectonometamorphic evolution remain debated and elusive. In this contribution, an integrated study including major- and trace-element zoning in garnet, phase equilibrium modelling and the simulation of cation diffusion in garnet was conducted on two high-pressure (HP) granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft. The results shed new light on the complex geodynamic processes that act in continent–continent collisional zones and finally shape collisional orogens. Phengite, biotite, amphibole, zoisite-allanite and low-Zr rutile enclosed in garnet likely attest to a prograde eclogite facies metamorphism for the studied rocks. Pressure–temperature (P–T) conditions of ~1.5–1.6 GPa and 615–670°C were retrieved for this stage. An extensive re-equilibration under peak HP granulite facies conditions of ~1.5 GPa and 925 ± 50°C followed. Subsequently, the rocks were cooled and reburied to eclogite facies conditions of ~1.8–1.9 GPa and 805–825°C. This was followed by a final stage of decompression and cooling to amphibolite facies conditions of ~650–780°C and 0.5–1.0 GPa. Cooling and exhumation rates of >400°C/Ma and >75 km/Ma, respectively, indicating an ultrafast temperature and pressure decrease are estimated for this stage from simulations of cation diffusion in garnet. The anticlockwise P–T path obtained here is relatively complete and compatible with a repeated burial history during the Caledonian orogeny but not with UHP conditions proposed for the Blåhø Nappe. Our model proposes that the rocks later forming the Blåhø Nappe were buried to lower crustal depths of approximately 55 km equating to a geothermal gradient of ~13°C/km during the early Caledonian orogeny. Subsequent heating of these rocks to HP granulite facies conditions was likely driven by slab break-off and hot mantle upwelling. Baltica underthrusting during the Scandian continent–continent collision cooled and transported the Blåhø Nappe to greater depths. The obtained cooling and exhumation rates indicate ultrafast exhumation, presumably in an exhumation channel.

挪威西部片麻岩区Nordøyane超高压(UHP)域的异地BlåhøNappe是研究大陆-大陆碰撞带中发生的地质过程的窗口,但关于其构造-变质演化的许多方面仍存在争议和难以捉摸。在这项贡献中,对Fjørtoft岛BlåhøNappe的两种高压(HP)麻粒岩相岩石进行了综合研究,包括石榴石中的主元素和微量元素分区、相平衡建模和石榴石中阳离子扩散的模拟。这些结果为作用于大陆-大陆碰撞带并最终形成碰撞造山带的复杂地球动力学过程提供了新的线索。石榴石中包裹的黄铜矿、黑云母、角闪石、帘石-尿囊石和低锆金红石可能证明了所研究岩石的进变质榴辉岩相变质作用。压力-温度(P–T)条件约为1.5–1.6 该阶段的GPa和615–670°C。在约1.5的HP麻粒岩相峰值条件下的广泛再平衡 GPa和925 ± 随后为50°C。随后,岩石被冷却并重新埋葬至约1.8–1.9的榴辉岩相条件 GPa和805–825°C。随后是减压和冷却的最后阶段,达到约650–780°C和0.5–1.0的角闪岩相条件 GPa。>400°C/Ma和>75的冷却率和剥露率 根据石榴石中阳离子扩散的模拟,估计了这一阶段的温度和压力的快速下降。这里获得的逆时针P–T路径相对完整,与喀里多尼亚造山运动期间的重复埋藏历史相兼容,但不符合BlåhøNappe提出的超高压条件。我们的模型提出,后来形成BlåhøNappe的岩石被埋藏在大约55的较低地壳深度 km,相当于早期喀里多尼亚造山运动期间约13°C/km的地热梯度。这些岩石随后加热到HP麻粒岩相条件可能是由板块断裂和热地幔上升流驱动的。斯堪的纳维亚大陆-大陆碰撞期间的Baltica欠冲冷却并将BlåhøNappe输送到更深处。所获得的冷却和挖掘速率表明,可能是在挖掘通道中进行了超快的挖掘。
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引用次数: 0
Pattern and source of unusually high-temperature metamorphism in an Eocene forearc recorded by the Pacific Rim Terrane, British Columbia, Canada 加拿大不列颠哥伦比亚省环太平洋阶地记录的始新世弧前异常高温变质作用的模式和来源
IF 3.4 2区 地球科学 Q1 Earth and Planetary Sciences Pub Date : 2023-01-10 DOI: 10.1111/jmg.12709
Alexander C. Geen, Dante Canil

The Pacific Rim Terrane is of forearc affinity and one of the most recent crustal elements accreted to the North American Cordillera in western Canada. Two units, the Leech River Complex and Pandora Peak Unit, within the terrane were subject to high-temperature, medium-pressure metamorphism. Biotite, garnet and staurolite isograds occur concentrically in the Leech River Complex, centred on the Leech River shear zone at its southern boundary. A local thermal overprint in the Pandora Peak Unit is characterized by replacement of prehnite-pumpellyite and lawsonite-bearing assemblages with muscovite + chlorite. Pseudosection models (Perple_X), and thermometry using garnet-biotite Fe-Mg exchange and Raman spectroscopy of carbonaceous material (RSCM) show a thermal gradient at ~3.8 kbar from ~230°C in the north to ~600°C in the south. Isotherms are continuous across the Leech River–Pandora Peak boundary. The small-volume, interfoliated intrusions of Eocene age occurring throughout the terrane show no spatial relation to the isotherms. Elevated forearc metamorphism is due to the subcretion at ~51 Ma of nascent oceanic crust (and related spreading ridge or hotspot) of the underlying Siletz-Crescent terrane along the south-bounding Leech River shear zone. Our re-evaluation of the metamorphic history requires revision of the role of magmatism as a source of heat transport in forearc metamorphism and the tectonic assembly in this setting.

环太平洋地系具有弧前亲缘性,是加拿大西部北美洲科迪勒拉最近形成的地壳元素之一。两个单元,Leech河复合体和Pandora峰单元,在地块内受到高温、中压变质作用。黑云母、石榴石和橄榄石等梯度在水蛭河杂岩中集中分布,以水蛭河剪切带为中心。潘多拉峰单元的局部热套印的特点是用白云母+绿泥石取代了含钙白云石-泵白云石和含钙褐煤的组合。伪截面模型(Perple_X)、采用石榴石-黑云母Fe - Mg交换和碳质材料拉曼光谱(RSCM)的测温结果显示,从北部~230°C到南部~600°C的温度梯度为~3.8 kbar。等温线在利奇河和潘多拉峰的边界上是连续的。始新世的小体积夹层侵入体与等温线没有空间关系。抬升的前弧变质作用是由于~51 Ma的新生洋壳(及其相关的伸展脊或热点)沿南界Leech河剪切带下伏Siletz - Crescent地体的亚地壳作用。我们对变质史的重新评价需要修正岩浆活动在弧前变质作用中作为热输送源的作用以及在此背景下的构造组合。
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引用次数: 1
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Journal of Metamorphic Geology
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