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Chromium in minerals as tracer of the polycyclic evolution of eclogite and related metabasite from the Pohorje Mountains, Slovenian Eastern Alps 矿物中的铬是斯洛文尼亚东阿尔卑斯山波霍尔耶山脉埃克洛辉石及相关辉长岩多环演化的示踪剂
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-10-11 DOI: 10.1111/jmg.12746
Botao Li, Hans-Joachim Massonne

Significantly different peak pressure–temperature (P–T) conditions (18–26 kbar and 630–760°C versus 29–37 kbar and 750–940°C) have previously been published for eclogite and related metabasites from the south-eastern flank of the Pohorje Mountains in Slovenia. These rocks can show a bimodal distribution of chromium in the rock-forming minerals, particularly garnet, the role of which in their metamorphic evolution is unclear. Therefore, we studied an eclogite and a related rock with clinopyroxene containing only 17 mol% jadeite + acmite (sample 18Ca35a). KαCr intensity maps of garnet particularly in sample 18Ca35a show a sharp irregular boundary between the core (Gt1) and the mantle (Gt2). Gt1 of millimetre-sized garnet in this rock is nearly Cr-free and unzoned, whereas Gt2 is of different composition (0.22 wt.% Cr2O3) and slightly zoned. Nearly Cr-free amphibole, (clino)zoisite, kyanite and staurolite inclusions are present in Gt1. The matrix consists of garnet and Cr-bearing clinopyroxene, (clino)zoisite and amphibole. Thermodynamic modelling suggests peak P–T conditions of 22.5 ± 2 kbar at 710 ± 25°C (Gt1) and 23 ± 2 kbar at 700 ± 25°C (Gt2) in both samples. We interpret these findings to suggest that olivine- and hornblende-bearing gabbros with some chromite experienced early metamorphism in the eclogite facies, when Gt1 formed. The rock was subsequently exhumed and cooled leading to significant garnet corrosion. A second stage of metamorphism, recognized by mappable Cr contents in garnet, led to the growth of Gt2 and other Cr-bearing minerals at the expense of chromite relics, which survived stage I. The peak P–T conditions of stage II are compatible with those previously derived by same authors and support the view that probably no ultrahigh-pressure eclogite exists in the Pohorje Mountains. We relate the two metamorphic events to the Cretaceous and Palaeogene high-pressure events recently reported from micaschists of the Pohorje Mountains.

此前曾发表过斯洛文尼亚波霍尔耶山脉东南侧的埃克洛辉石和相关偏闪长岩的峰值压力-温度(P-T)条件(18-26 千巴和 630-760°C 与 29-37 千巴和 750-940°C)明显不同。这些岩石的成岩矿物(尤其是石榴石)中的铬呈双峰分布,而铬在其变质演化过程中的作用尚不清楚。因此,我们研究了一块斜长岩和一块相关的岩石,其中的黝帘石只含有 17 mol% 的翡翠+黝帘石(样品 18Ca35a)。特别是样品 18Ca35a 中石榴石的 KαCr 强度图显示,岩芯(Gt1)和地幔(Gt2)之间存在一个尖锐的不规则边界。该岩石中毫米大小石榴石的 Gt1 几乎不含铬且无分带,而 Gt2 的成分不同(0.22 wt.% Cr2O3)且略有分带。Gt1 中存在几乎不含铬的闪石、(clino)黝帘石、黝帘石和白云石包裹体。基质由石榴石和含铬的霞石、(clino)黝帘石和闪石组成。热力学模型表明,两个样品的峰值 P-T 条件分别为 710 ± 25°C 时 22.5 ± 2 千巴(Gt1)和 700 ± 25°C 时 23 ± 2 千巴(Gt2)。我们对这些发现的解释是,当Gt1形成时,含有橄榄石和角闪石的辉长岩和一些铬铁矿经历了夕闪岩面的早期变质作用。岩石随后被掘出并冷却,导致石榴石严重腐蚀。第二阶段的峰值 P-T 条件与同一作者之前得出的峰值 P-T 条件相吻合,支持了波霍尔耶山脉可能不存在超高压斜长岩的观点。我们将这两个变质事件与最近从波霍列山云母岩中报告的白垩纪和古近纪高压事件联系起来。
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引用次数: 0
Polymetamorphism during the Grenvillian Orogeny in SE Ontario: Results from trace element mapping, in situ geochronology, and diffusion geospeedometry 安大略东南部格伦维利造山运动期间的多变质作用:痕量元素绘图、原位地质年代学和扩散地质速度测量的结果
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-09-16 DOI: 10.1111/jmg.12745
F. Gaidies, T. Mccarron, A. D. Simpson, R. M. Easton, S. Glorie, B. Putlitz, K. Trebus
<p>The Flinton Group is a metasedimentary succession of the Grenville Province in SE Ontario, potentially allowing insight into the tectono-thermal evolution of continental crust during the Mesoproterozoic. At its Green Bay locality, Flinton Group metapelites of the staurolite zone contain abundant, post-kinematic garnet porphyroblasts. Whereas the larger garnet crystals are typically impinged, smaller crystals are isolated from each other, occasionally exhibiting elongated shapes with apparently trigonal morphology. Central sections of the garnet population of a representative sample reveal that garnet is composed of different compositional and microstructural domains. In the largest crystals of the population, garnet contains rectangular to rhombic domains, marked by sharp increases in the concentrations of Nb, V, Ti, and Cr. These domains are associated with irregularly shaped patches, characterized by spatially heterogenous enrichments of Ca and LREE, and depletions in the contents of P, Y, MREE, and HREE, accompanied by increased densities of comparatively coarse-grained quartz inclusions hosting apatite. Microstructural relationships indicate that these domains correspond to portions of garnet that pseudomorphed biotite, with the enrichments of Nb, V, Ti, and Cr outlining the original biotite shapes. The compositional patterns formed by Ca, P, Y, and REE indicate that apatite participated in the grain-fluid interactions that operated during the metasomatic replacement of biotite by garnet. The statistical analyses of the garnet number and size distributions confirm that garnet initially nucleated on biotite, controlled by the kinetics of attachment and detachment processes at the garnet/biotite interface, resulting in the typical impingement habit. In situ Lu–Hf garnet geochronology applied to garnet that did not pseudomorph biotite, and hence is enriched in HREE, points to a first metamorphic event at c. 1080 <math> <mo>±</mo></math> 31 Ma. Subsequent pseudomorphism of staurolite by white mica in a Al<sub>2</sub>O<sub>3</sub>- and FeO-mobile system resulted in the concomitant crystallization of a new garnet generation, forming overgrowths on the first garnet generation and nuclei in the fine-grained matrix. Garnet that nucleated during this event grew to isolated and elongated crystals with apparently trigonal morphology, aligned in a direction c. perpendicular to the rock matrix foliation. The open-system behaviour during this event limits the use of whole-rock-based geochronological and thermobarometrical applications. However, previously published in situ U–Pb ages of monazite included in the rims of the garnet crystals and in the rock matrix indicate that this event took place at c. 976 <math> <mo>±</mo></math> 4 Ma, likely associated with a period of increased hydrothermal activity late in the metamorphic history of the Grenvillian Orogeny. Diffusion geospeedometry calculations indicate that garnet grow
弗林顿组是安大略省东南部格勒维尔省的一个元古代演替,有可能让人们了解中新生代大陆地壳的构造-热演化过程。在格林湾矿区,闪长岩带的弗林顿组玄武岩含有丰富的后激变石榴石斑块。较大的石榴石晶体通常呈撞击状,而较小的晶体则彼此孤立,偶尔会呈现出明显呈三棱状的拉长形状。一个代表性样本中石榴石群的中心切面显示,石榴石由不同的成分和微结构域组成。在最大的晶体群中,石榴石包含矩形至菱形结构域,其特征是 Nb、V、Ti 和 Cr 的浓度急剧增加。这些域与形状不规则的斑块有关,其特征是钙和 LREE 在空间上异质富集,而 P、Y、MREE 和 HREE 的含量则减少,同时还伴随着寄生磷灰石的相对粗粒石英包裹体密度的增加。显微结构关系表明,这些区域对应于石榴石假象生物玢岩的部分,Nb、V、Ti 和 Cr 的富集勾勒出原始生物玢岩的形状。由 Ca、P、Y 和 REE 形成的成分模式表明,磷灰石参与了石榴石对生物橄榄石的元气置换过程中的晶粒-流体相互作用。对石榴石数量和尺寸分布的统计分析证实,石榴石最初成核于黑云母,受石榴石/黑云母界面附着和脱离过程动力学的控制,形成典型的撞击习性。原位 Lu-Hf 石榴石地质年代学应用于没有假象生物橄榄岩的石榴石,因此富含 HREE,这表明第一次变质事件发生在约 1080 ± 31 Ma。随后,白云母在 Al2O3- 和 FeO- 移动体系中对白云母进行假变质,导致新一代石榴石随之结晶,在第一代石榴石上形成过生层,并在细粒基质中形成晶核。在这一过程中成核的石榴石长成了孤立的细长晶体,具有明显的三棱柱形态,其排列方向与岩石基质折线垂直。这一事件中的开放系统行为限制了基于整块岩石的地质年代和温压计应用。不过,之前公布的石榴石晶体边缘和岩石基质中的独居石的原位 U-Pb 年龄表明,这一事件发生在约 976 ± 4 Ma,很可能与格勒维利造山带变质历史晚期热液活动增加的时期有关。扩散测地计算表明,在这一热液活动期间石榴石的生长持续了不到 6 Myr。
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引用次数: 0
Formation of low-pressure reaction textures during near-isothermal exhumation of hot orogenic crust (Bohemian Massif, Austria) 热造山运动地壳近等温剥蚀过程中低压反应纹理的形成(奥地利波希米亚山丘)
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-09-14 DOI: 10.1111/jmg.12744
Dominik Sorger, Christoph A. Hauzenberger, Fritz Finger, Manfred Linner, Etienne Skrzypek, Simon Schorn

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet–biotite–sillimanite–plagioclase–K-feldspar–quartz–granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of 0.9–1.1 GPa and 780–820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite–spinel and cordierite–quartz reaction textures at former garnet–sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching 0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite–spinel and cordierite–quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4–0.5 GPa and 750–800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP–UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at 340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of 335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

来自 Loosdorf 复合地层(奥地利东北部波希米亚山丘)的两种类型的铝质辉长岩含有粗粒花岗岩集合体和逆冲反应纹理,通过研究这些集合体和纹理,可以确定波希米亚山丘南部 Gföhl 单元的峰后历史。这两种类型都具有石榴石-生物陶瓷-菱镁矿-斜长石-K长石-石英-花岗岩熔体±黝帘石±钛铁矿±金红石的峰值组合,记录了等化学相平衡模型估计的峰值变质条件:0.9-1.1 GPa 和 780-820°C 。第一类样品(Ysper副玢岩)在石榴石周围形成了(i)堇青石冠状体,(ii)在原石榴石-矽线石界面形成了堇青石-尖晶石和堇青石-石英反应纹理。计算的化学势关系表明,这些纹理是在达到 ∼0.4 GPa 的峰后近等温减压过程中形成的。纹理的形成有两个步骤。最初,堇青石冠层在石榴石和矽线石之间生长。随着这些冠层的增厚,它们促进了局部成分域的发展,从而形成了堇青石-尖晶石和堇青石-石英共辉石。第二种样品类型(Pielach片麻岩)仅在石榴石斑岩周围表现出不连续的堇青石冠脉,但缺乏共辉石。那里的堇青石的形成也表明,在 0.4-0.5 GPa 和 750-800°C 的温度下发生了近等温减压。这种相对较热的减压路径可以用现在位于卢斯多夫复合地层下的大型 HP-UHT 花岗岩体的同期出露来解释。花岗岩和整个波希米亚山丘南部的区域变质作用的时间得到了很好的确定,其峰值在 ∼340 Ma。从洛斯多夫(Loosdorf)片麻岩样本中提取的独居石的年龄略小,为 ∼335 Ma。虽然这些年龄在误差范围内重叠,但它们被解释为反映了近等温减压和剥蚀作用,从而形成了所观察到的反应纹理。
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引用次数: 0
Origin of Erzgebirge ultrahigh-pressure garnetite: Formation from a basaltic protolith by serpentinization-assisted metasomatism? Erzgebirge超高压石榴石的成因:玄武岩原岩经蛇纹岩辅助交代作用形成?
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-09-07 DOI: 10.1111/jmg.12742
Esther Schmädicke, Thomas M. Will
<p>Erzgebirge ultrahigh-pressure (UHP) garnet peridotite includes scarce layers of garnet pyroxenite, nodules of garnetite and, very rarely, of eclogite. Peridotite-hosted eclogite shows the same subalkali-basaltic bulk rock composition, mineral assemblage and peak conditions as gneiss-hosted eclogite present in the same UHP unit. Garnetite has considerably more Mg, moderately enhanced Ca and Fe and significantly lower contents of Na, Ti, P, K and Si than eclogite, whereas Al is very similar. In addition, the compatible trace elements (Ni, Co, Cr, V) are elevated and most incompatible elements (Zr, Hf, Y, Sr, Rb and rare Earth elements [REE]) are depleted in garnetite relative to eclogite. In contrast to other large ion lithophile elements (LILEs), Pb (+121%) and Ba (+83%) are strongly enriched. The REE patterns of garnetite are characterized by depletion of light and heavy REE and a medium REE hump indicative of metasomatism, features being absent in eclogite. An exceptional garnetite sample shows an REE distribution similar to that of eclogite. Garnetite is interpreted to have formed from the same, but metasomatically altered, igneous protolith as eclogite. Except for Ba and Pb, the chemical signature of garnetite is explained best by metasomatic changes of its basaltic protolith caused by serpentinization of the host peridotite. Garnetite is chemically similar to basaltic rodingite/metarodingite. Although rodingite is commonly more enriched in Ca, there are also examples with moderately enhanced Ca matching the composition of Erzgebirge garnetite. Limited Ca metasomatism is attributed to the preservation of Ca in peridotite during hydrous alteration. This can be explained by incomplete serpentinization favouring metastable survival of the original clinopyroxene. In this case, most Ca is retained in peridotite and not available for infiltration and metasomatism of the garnetite protolith. This inescapable consequence is supported by the fact that clinopyroxene is part of the garnet peridotite UHP assemblage, which would not be the case if Ca had been removed from the protolith prior to high-pressure metamorphism. The enrichment of compatible elements in garnetite is attributed to decomposition of peridotitic olivine (Ni, Co) and spinel (Cr, V) during serpentinization. Enrichment of Ba and Pb contrasts the behaviour of other LILEs and is ascribed to dehydration of the serpentinized peridotite (deserpentinization). This requires two separate stages of metasomatism: (1) intense chemical alteration of the basaltic garnetite precursor, together with serpentinization of peridotite at the ocean floor or during incipient subduction; and (2) prograde metamorphism and dehydration of serpentinite during continued subduction, thereby releasing Pb–Ba-rich fluids that reacted with associated metabasalt. Finally, subduction to >100 km and UHP metamorphism of all lithologies led to formation of garnetite, eclogite and garnet pyroxenite hosted by co-facial g
Erzgebirge超高压(UHP)石榴石橄榄岩包括稀少的石榴石-辉石岩层、石榴石结核,以及非常罕见的榴辉岩。与同一UHP单元中存在的片麻岩榴辉岩相比,以橄榄岩为主的榴辉岩显示出相同的亚碱性玄武岩块状岩石成分、矿物组合和峰值条件。与榴辉岩相比,石榴石具有相当多的Mg,适度增强的Ca和Fe,以及显著降低的Na、Ti、P、K和Si含量,而Al非常相似。此外,与榴辉岩相比,石榴石中的相容微量元素(Ni、Co、Cr、V)升高,大多数不相容元素(Zr、Hf、Y、Sr、Rb和稀土元素[REE])贫化。与其他大离子亲石元素(LILEs)相比,Pb(+121%)和Ba(+83%)强烈富集。石榴石的REE模式以轻、重REE贫化和中等REE峰为特征,表明交代作用,榴辉岩中没有这种特征。一个特殊的石榴石样品显示出与榴辉岩相似的REE分布。石榴石被解释为由与榴辉岩相同但交代蚀变的火成原岩形成。除Ba和Pb外,石榴石的化学特征最好通过寄主橄榄岩蛇纹石化引起的玄武岩原岩交代变化来解释。石榴石在化学性质上类似于玄武岩绿柱石/变质绿柱石。尽管绿柱石通常富含Ca,但也有与Erzgebirge石榴石成分相匹配的适度增强Ca的例子。有限的Ca交代作用归因于含水蚀变过程中橄榄岩中Ca的保存。这可以解释为不完全的蛇纹石化有利于原始斜辉石的亚稳生存。在这种情况下,大部分Ca保留在橄榄岩中,不可用于石榴石原岩的渗透和交代。斜辉石是石榴石-橄榄岩UHP组合的一部分,这一事实支持了这一不可避免的后果,如果在高压变质作用之前从原岩中去除了Ca,情况就不会如此。石榴石中相容元素的富集归因于蛇纹石化过程中橄榄岩-橄榄石(Ni,Co)和尖晶石(Cr,V)的分解。Ba和Pb的富集与其他LILE的行为形成对比,并归因于蛇纹石化橄榄岩的脱水(去萜化)。这需要两个独立的交代阶段:(1)玄武岩-石榴石前体的强烈化学蚀变,以及海底或初始俯冲期间橄榄岩的蛇纹石化;以及(2)在持续俯冲过程中,蛇纹岩的进变质作用和脱水作用,从而释放出与相关变质玄武岩反应的富含Pb–Ba的流体。最后,俯冲至>100 所有岩性的km和UHP变质作用导致形成石榴石、榴辉岩和石榴石辉石岩,由Erzgebirge中观察到的共面石榴石橄榄岩托管。
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引用次数: 0
Long-lived high-grade metamorphism in southern India: Constraints from charnockites and sapphirine-bearing semipelitic granulites from the Madurai Block 印度南部长寿命的高品位变质作用:来自马杜赖地块的charnockites和含蓝宝石的半泥质麻粒岩的限制
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-08-31 DOI: 10.1111/jmg.12743
Ashish Kumar Tiwari, Tapabrato Sarkar, Sourav Karmakar, Nilanjana Sorcar, Sneha Mukherjee
<p>The Granulite Terrane of Southern India is a collage of Mesoarchean–Neoproterozoic crustal blocks that underwent high-grade metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic–Cambrian. Here, we investigate the charnockites and associated sapphirine-bearing semipelitic granulites from the eastern part of the Madurai Block (MB). We present new petrographic, mineral chemistry, and geochronological data to constrain the <i>P</i>–<i>T</i>–<i>t</i> evolution of the block and unravel the timescale and source of heat for the ultrahigh-temperature metamorphism. Both the rock types contain coarse-grained porphyroblastic garnet and orthopyroxene, yielding peak <i>P</i>–<i>T</i> conditions of 950 ± 30°C at 10.5 ± 0.8 kbar and 970 ± 40°C at 10 ± 0.5 kbar for semipelite and charnockite, respectively, using conventional thermobarometry. Peak ultrahigh temperatures are further supported by high Al content in the orthopyroxene (8.78 wt% Al<sub>2</sub>O<sub>3</sub>) coexisting with garnet (<i>X</i><sub>Mg</sub>: up to 0.57) and feldspar thermometry of the mesoperthites and antiperthites in the semipelite, yielding 950–980°C at 10 kbar. Subsequent decompression has led to the formation of coronal orthopyroxene3 + plagioclase3 in the charnockite and symplectic orthopyroxene3 + cordierite ± sapphirine ± plagioclase3 in the semipelite, yielding <i>P</i>–<i>T</i> range of 950–850°C and 9.5–6.8 kbar for semipelites and 950–820°C and 8–6.5 kbar for charnockite. Based on the obtained <i>P</i>–<i>T</i> estimates, preserved reaction textures, and phase equilibria modelling in the MnNCKFMASHTO system, a clockwise <i>P</i>–<i>T</i> evolution with isothermal decompression followed by cooling is inferred for both the rock types.</p><p>Texturally constrained in situ monazite dating and rare earth element (REE) patterns show that the core of matrix monazite having low-Th, Y, and extreme heavy rare earth element (HREE) depletion, yielding weighted mean ages of 582 ± 12 and 590 ± 22 Ma for semipelite and charnockite, respectively, dates the prograde evolution. The mantle of the matrix monazite in semipelite and comparable rim in charnockite, having relative Th-enrichment compared to the core, yielding weighted mean ages of 552 ± 9 and 557 ± 13 Ma, respectively, dates extensive dissolution–reprecipitation from the melt at the peak stage. The relatively Th- and Y-rich and moderately HREE-depleted rim of matrix monazite in the semipelite, yielding weighted age of 516 ± 6 Ma, date initial garnet breakdown during post-peak melt crystallization. By contrast, compositionally homogenous HREE + Y-enriched monazite in the symplectite and retrograde monazites yielding weighted mean ages of 487 ± 47 Ma for semipelites and 508 ± 19 Ma for charnockites dates extensive garnet breakdown during final stages of melt crystallization and subsequent cooling. Our findings point to collision initiation at ~590 Ma, with the peak conditions attained
南印度麻粒岩地体是中太古宙-新元古代地壳块体的拼贴,这些块体经历了与新元古代-寒武纪晚期冈瓦纳超大陆最终组装相关的高变质作用。在这里,我们研究了马杜赖地块(MB)东部的charnockites和伴生的含蓝宝石的半长粒麻粒岩。我们提出了新的岩石学、矿物化学和地质年代学数据,以限制该地块的P-T-t演化,并揭示了超高温变质作用的时间尺度和热源。这两种岩石类型都含有粗粒斑绿石榴石和正辉石,使用常规热压测量法,半长辉石和绿辉石的峰值P-T条件分别为950±30°C和970±40°C,分别为10.5±0.8 kbar和10±0.5 kbar。高铝含量的正长辉石(8.78 wt% Al2O3)与石榴石(XMg:高达0.57)共存,半长辉石中中长辉石和反长辉石的长石测温进一步支持了峰值超高温,在10 kbar下产生950-980℃。随后的减压导致在沙砾岩中形成冠状正辉石e3 +斜长石3,在半长岩中形成辛状正辉石e3 +堇青石±蓝宝石±斜长石3,半长岩的P-T范围为950 ~ 850℃,9.5 ~ 6.8 kbar,沙砾岩的P-T范围为950 ~ 820℃,8 ~ 6.5 kbar。根据获得的P-T估计、保存的反应结构和MnNCKFMASHTO体系的相平衡模型,推断这两种岩石类型都是顺时针的P-T演化,先是等温减压,然后是冷却。结构约束的原位独居石定年和稀土元素(REE)模式表明,基质独居石的核心具有低Th、Y和极重稀土元素(HREE)的损失,半长粒石和沙砾石的加权平均年龄分别为582±12 Ma和590±22 Ma,属于渐进演化。半长岩中基质独居石的地幔和沙砾岩中类似的边缘相对于岩心具有相对的Th富集,加权平均年龄分别为552±9 Ma和557±13 Ma,表明在峰值阶段熔体发生了广泛的溶解-再沉淀。半长岩中相对富Th -和富Y -和中等贫HREE -的基质独居石边缘,产生的加权年龄为516±6 Ma,表明峰后熔融结晶过程中石榴石的初始分解。相比之下,正长石和逆行独居石中成分均匀的富ree + Y独居石的加权平均年龄为487±47 Ma,半长粒独居石的加权平均年龄为508±19 Ma,表明在熔融结晶的最后阶段和随后的冷却阶段,石榴石发生了广泛的分解。我们的研究结果表明,碰撞开始于~590 Ma,在~550 Ma达到峰值,随后在~ 510-490 Ma发生伸展塌陷,导致在持续的超高温(UHT)条件下快速挖掘下地壳岩石到中地壳水平,然后冷却以达到稳定的地热。我们的研究结果表明,MB中存在长期的热造山运动,其中UHT条件持续了至少40 MYr。UHT条件最有可能在长寿命热造山带的核心中通过放射性衰变和地幔热供应的传导加热的综合作用而达到,其中前者是主要驱动因素。
{"title":"Long-lived high-grade metamorphism in southern India: Constraints from charnockites and sapphirine-bearing semipelitic granulites from the Madurai Block","authors":"Ashish Kumar Tiwari,&nbsp;Tapabrato Sarkar,&nbsp;Sourav Karmakar,&nbsp;Nilanjana Sorcar,&nbsp;Sneha Mukherjee","doi":"10.1111/jmg.12743","DOIUrl":"10.1111/jmg.12743","url":null,"abstract":"&lt;p&gt;The Granulite Terrane of Southern India is a collage of Mesoarchean–Neoproterozoic crustal blocks that underwent high-grade metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic–Cambrian. Here, we investigate the charnockites and associated sapphirine-bearing semipelitic granulites from the eastern part of the Madurai Block (MB). We present new petrographic, mineral chemistry, and geochronological data to constrain the &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt;–&lt;i&gt;t&lt;/i&gt; evolution of the block and unravel the timescale and source of heat for the ultrahigh-temperature metamorphism. Both the rock types contain coarse-grained porphyroblastic garnet and orthopyroxene, yielding peak &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; conditions of 950 ± 30°C at 10.5 ± 0.8 kbar and 970 ± 40°C at 10 ± 0.5 kbar for semipelite and charnockite, respectively, using conventional thermobarometry. Peak ultrahigh temperatures are further supported by high Al content in the orthopyroxene (8.78 wt% Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;) coexisting with garnet (&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;Mg&lt;/sub&gt;: up to 0.57) and feldspar thermometry of the mesoperthites and antiperthites in the semipelite, yielding 950–980°C at 10 kbar. Subsequent decompression has led to the formation of coronal orthopyroxene3 + plagioclase3 in the charnockite and symplectic orthopyroxene3 + cordierite ± sapphirine ± plagioclase3 in the semipelite, yielding &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; range of 950–850°C and 9.5–6.8 kbar for semipelites and 950–820°C and 8–6.5 kbar for charnockite. Based on the obtained &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; estimates, preserved reaction textures, and phase equilibria modelling in the MnNCKFMASHTO system, a clockwise &lt;i&gt;P&lt;/i&gt;–&lt;i&gt;T&lt;/i&gt; evolution with isothermal decompression followed by cooling is inferred for both the rock types.&lt;/p&gt;&lt;p&gt;Texturally constrained in situ monazite dating and rare earth element (REE) patterns show that the core of matrix monazite having low-Th, Y, and extreme heavy rare earth element (HREE) depletion, yielding weighted mean ages of 582 ± 12 and 590 ± 22 Ma for semipelite and charnockite, respectively, dates the prograde evolution. The mantle of the matrix monazite in semipelite and comparable rim in charnockite, having relative Th-enrichment compared to the core, yielding weighted mean ages of 552 ± 9 and 557 ± 13 Ma, respectively, dates extensive dissolution–reprecipitation from the melt at the peak stage. The relatively Th- and Y-rich and moderately HREE-depleted rim of matrix monazite in the semipelite, yielding weighted age of 516 ± 6 Ma, date initial garnet breakdown during post-peak melt crystallization. By contrast, compositionally homogenous HREE + Y-enriched monazite in the symplectite and retrograde monazites yielding weighted mean ages of 487 ± 47 Ma for semipelites and 508 ± 19 Ma for charnockites dates extensive garnet breakdown during final stages of melt crystallization and subsequent cooling. Our findings point to collision initiation at ~590 Ma, with the peak conditions attained","PeriodicalId":16472,"journal":{"name":"Journal of Metamorphic Geology","volume":"41 9","pages":"1261-1297"},"PeriodicalIF":3.4,"publicationDate":"2023-08-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48583028","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
引用次数: 0
Petrochronologic constraints on inverted metamorphism, terrane accretion, thrust stacking, and ductile flow in the Gneiss Dome belt, northern Appalachian orogen 阿巴拉契亚造山带北部片麻岩丘带逆变质作用、地体增生、逆冲堆积和韧性流动的岩石年代学约束
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-08-22 DOI: 10.1111/jmg.12741
Ian W. Hillenbrand, Michael L. Williams, Emily M. Peterman, Michael J. Jercinovic, Craig W. Dietsch

Gneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U-Th-Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P-T-t) paths for each unit in the gneiss dome belt tectono-stratigraphy. The structurally lowest units, Laurentia-derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana-derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high-pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome-mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P-T-t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn-collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn-convergent orogen parallel ductile flow dramatically modified earlier accretion-related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn-collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.

片麻岩圆顶是许多造山带的组成部分,通常为深入地壳提供构造窗口。阿巴拉契亚造山带新英格兰段的片麻岩圆顶在经典上与底辟作用和褶皱干涉有关,但也提出了涉及韧性流的替代模型。我们用U‐Th‐Pb独居石、异长岩、锆石和钛矿岩石年表以及主元素和微量元素热气压测量法对新英格兰西部片麻岩穹隆带的这些模型进行了评估。这些数据限制了片麻岩穹隆带构造地层学中每个单元的不同压力-温度-时间(P‐T‐T)路径。结构最低的单元,Waterbury穹隆的Laurentia衍生的混合岩片麻岩,记录了两个变质阶段(455-435和400-370 Ma),峰值阿卡迪亚变质条件为约1.0–1.2 GPa,温度为750–780°C,温度为391 ±7至386 ±4 Ma。下一个结构更高的单元,冈瓦纳大陆衍生的泰恩山组,记录了Taconic(峰值条件:0.6 GPa,600°C,441 ±4 Ma)和阿卡迪亚(峰值:0.8–1.0 GPa,在377时为650°C ±4 Ma)变质作用。上覆的Collinsville地层产生473 ±5Ma结晶年龄和436时650°C变质条件的证据 ±4 Ma和1.2–1.0 GPa,温度为750–775°C,温度为397 ±4至385 ±6 Ma。Collinsville组结构较高的Sweetheart Mountain段仅产出Acadian锆石、独居石和异长岩时代,以及380–375年左右高压麻粒岩相变质作用(1.8 GPa,815°C)的证据 Ma。圆顶覆盖层的盖层岩石海峡片岩在386年记录了约1 GPa、700°C的峰值条件 ±6至380 ±4 Ma。石榴石分解为独居石和/或异长岩发生在375–360和345–330左右的所有单元中 马。阿卡迪亚峰变质岩压力从结构最低单元到最高单元(从1.0到1.8GPa)有系统地增加。这种反向变质岩序列与底辟和褶皱干涉模型不兼容,后者预测结构最低水平的最高压力。基于P‐T‐T和结构数据,我们更喜欢一个首先涉及大约380 Ma逆冲叠加,随后是同碰撞造山带的平行伸展、韧性流动和380至365年之间的穹隆上升 马,大约345–330年石榴石击穿 马被解释为反映了阿卡迪亚造山高原崩塌期间的进一步剥露。这些结果突出了将岩石学约束与来自多个计时器的成对地球化学和地质年代数据相结合的力量,以测试结构和构造模型,并表明同收敛造山带平行韧性流极大地改变了新英格兰早期的吸积相关结构。此外,片麻岩穹隆带记录了同碰撞厚地壳环境中的片麻岩穹隆发育,提供了现代喜马拉雅山脉和帕米尔山脉可能发生的中下部地壳过程的一个古老例子。
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引用次数: 1
A wealth of P–T–t information from metasediments in the HP–UHP terrane of the Pohorje Mountains, Slovenia, elucidates the evolution of the Eastern Alps 斯洛文尼亚波霍杰山脉HP‐UHP地体变质沉积物中丰富的P‐T‐T信息阐明了东阿尔卑斯山脉的演化
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-07-26 DOI: 10.1111/jmg.12740
Botao Li, Hans-Joachim Massonne, Xiaoping Yuan

Contrasting views exist in regard of the evolution of metamorphic rocks in the southeastern Pohorje Mountains (Mts), located in the southeastern Eastern Alps. Major debated points are whether micaschists have experienced ultrahigh-pressure metamorphism in the Late Cretaceous (Eo-Alpine) and whether they were continuously exhumed or experienced a multiple subduction–exhumation process from that time on. Therefore, we studied micaschist sample 18Slo39 with two generations of garnet and phengitic muscovite from this area. Our detailed study of this rock included petrographic observations, chemical analyses of minerals with the electron microprobe, pseudosection modelling, conventional geothermometry, and monazite in-situ U-Th-Pb dating using laser-ablation inductively coupled plasma (ICP) mass spectrometry. The following results were obtained: The studied micaschist was subject to a peak pressure of 1.31 ± 0.14 GPa at 603 ± 26°C in Eo-Alpine times: 90.62 ± 2.78 (2σ) Ma (Stage I). Contact metamorphism at pressure–temperature conditions of 0.66 ± 0.10 GPa and 577 ± 23°C was induced by the intrusion of the Pohorje pluton (Stage III). We determined an early Miocene age of 18.33 ± 0.43 (2σ) Ma for this intrusion. Based on this study and the previously reported data for a micaschist (16Slo12) taken in the vicinity of sample 18Slo39, a geodynamic model is proposed for the region of the Pohorje Mts considering Eo-Alpine subduction of oceanic crust and European continental crust, of which the micaschist was part of. Another high-pressure event in the Eocene (Stage II) was the result of intracontinental subduction because of transpression by the Periadriatic fault system that separates the Eastern Alps from the Southern Alps. This type of subduction gave rise to magma generation and ascent to form the Pohorje pluton, which caused contact metamorphism in its vicinity.

关于位于东阿尔卑斯东南部的波荷杰山脉东南部变质岩的演化,存在着截然不同的观点。目前争论的主要问题是,晚白垩世(古阿尔卑斯)云母岩屑是否经历了超高压变质作用,以及从那时起,云母岩屑是连续被挖掘出来的,还是经历了多次俯冲-挖掘过程。因此,我们对该地区的云母岩18Slo39样品进行了两代石榴石和白云母的研究。我们对该岩石进行了详细的研究,包括岩石学观察、电子探针矿物化学分析、伪剖面建模、常规地热测量,以及利用激光烧蚀电感耦合等离子体(ICP)质谱法对独居石进行原位U-Th-Pb测年。结果表明:研究的云母岩在603±26℃(90.62±2.78 (2σ) Ma)的始高寒期(90.62±2.78 (2σ) Ma)经历了1.31±0.14 GPa的峰值压力,在0.66±0.10 GPa和577±23℃的压力-温度条件下发生了接触变质作用(第三阶段),确定了该侵入岩的早中新世年龄为18.33±0.43 (2σ) Ma。在此基础上,结合已有报道的18Slo39样品附近的云母岩(16Slo12)资料,提出了一个考虑洋壳和欧洲大陆地壳的ew - alpine俯冲作用的Pohorje Mts地区地球动力学模型,其中云母岩是其中的一部分。始新世(第二阶段)的另一个高压事件是由于分隔东阿尔卑斯和南阿尔卑斯的外亚得里亚海断裂系统的挤压作用造成的陆内俯冲。这种俯冲作用引起岩浆生成和上升,形成了波荷杰岩体,并在其附近引起了接触变质作用。
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引用次数: 1
40Ar/39Ar dates controlled by white mica deformation and strain localization: Insights from comparing in situ laser ablation and single-grain fusion techniques 由白云母变形和应变局部化控制的40Ar/39Ar日期:通过比较原位激光烧蚀和单晶粒融合技术的见解
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-07-26 DOI: 10.1111/jmg.12739
Christopher J. Barnes, David A. Schneider, Jarosław Majka, Alfredo Camacho, Michał Bukała, Adam Włodek

In situ laser ablation and single-grain fusion 40Ar/39Ar geochronological techniques were directly compared using white mica from nine metasedimentary rocks from the Vaimok Lens of the Seve Nappe Complex (SNC) in the Scandinavian Caledonides. Seven of the rocks are from the eclogite-bearing Grapesvare nappe within the lens that is defined by D2 structures (S2 and F2), which were formed during exhumation following late Cambrian/Early Ordovician ultra-high pressure metamorphism. Two other rocks were obtained from ‘Scandian’ shear zones that delimit the nappes within the lens. The shear zones were active during terminal collision of Baltica and Laurentia in the Silurian to Devonian. The rocks exhibit variable deformation intensities and degrees of strain localization, expressed in particular by white mica. The in situ laser ablation and single-grain fusion 40Ar/39Ar dates both span from the late Cambrian to Middle Devonian. Results of both techniques generally show decreasing dates with increasing bulk deformation intensity and successive structural generations (i.e., D2 then Scandian structures). Furthermore, several discrepancies are evident when comparing the results of the two techniques for the same rocks, indicating the 40Ar/39Ar dates are not solely governed by bulk deformation intensities and structural generations. Instead, the discrepancies demonstrate the additional influence of white mica strain localization, which is illuminated by the different analytical volumes of the techniques. Thus, the 40Ar/39Ar datasets are altogether deciphered as a function of bulk deformation intensity and degree of strain localization that affected the overall white mica volume. The former controls the gross 40Ar loss from the overall volume and the latter dictates the variability of 40Ar loss within the volume. Exploiting the interplay of these two phenomena for the Vaimok Lens rocks with in situ laser ablation allows for the broad span of 40Ar/39Ar dates to be contextualized into a sequence of tectonic events: (1) cooling at 474 ± 3 Ma, (2) pre-collision deformation at 447 ± 2 Ma and (3) activation of crustal-scale shear zones in the SNC related to continental collision at 431 ± 3 Ma and 411 ± 3 Ma.

利用斯堪的纳维亚Caledonides中Seve推覆杂岩(SNC) Vaimok Lens的9块变质沉积岩中的白色云母,对原位激光消融和单粒熔融40Ar/39Ar年代学技术进行了直接比较。其中7块岩石来自透镜体内含榴辉岩的Grapesvare推覆体,属于D2构造(S2和F2),是在晚寒武世/早奥陶世超高压变质作用下出土形成的。另外两块岩石来自“加拿大”剪切带,该剪切带划分了透镜体内的推覆体。在志留纪至泥盆纪波罗的海与劳伦提亚碰撞末期,剪切带活跃。岩石表现出不同的变形强度和应变局部化程度,特别是白云母。原位激光烧蚀和单粒熔融40Ar/39Ar的时间跨度均为晚寒武世至中泥盆世。这两种技术的结果通常表明,随着体变形强度的增加和连续的结构世代(即D2然后是Scandian结构),日期逐渐减少。此外,当比较两种技术对同一块岩石的结果时,一些差异是明显的,这表明40Ar/39Ar日期不仅仅是由体积变形强度和构造世代决定的。相反,这些差异表明了白云母菌株定位的额外影响,这是由不同的分析量的技术所阐明的。因此,40Ar/39Ar数据集被全部解读为影响整体白色云母体积的体变形强度和应变局部化程度的函数。前者控制总体体积的总40Ar损失,后者决定体积内40Ar损失的可变性。利用原位激光烧蚀法对Vaimok Lens岩石进行这两种现象的相互作用,可以将40Ar/39Ar的大跨度日期背景化到一系列构造事件中:(1)474±3 Ma的冷却,(2)447±2 Ma的碰撞前变形,以及(3)SNC中与431±3 Ma和411±3 Ma的大陆碰撞相关的地壳尺度剪切带的激活。
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引用次数: 0
Pressure–temperature evolution of the basement and cover sequences on Ios, Greece: Evidence for subduction of the Hercynian basement 希腊伊俄斯地区基底和盖层的压力-温度演化:海西期基底俯冲的证据
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-06-27 DOI: 10.1111/jmg.12738
Oliver M. Wolfe, Frank S. Spear, Jay B. Thomas, Eric M. Hasegawa, Grant T. Libby, John T. Cheney

High-pressure rocks from the island of Ios in the Greek Cyclades were examined to resolve the P–T conditions reached during subduction of the two distinct lithotectonic units that are separated by the South Cycladic Shear Zone (SCSZ)—the footwall complex composed of Hercynian basement gneisses, schists and amphibolites, and the hangingwall complex composed of blueschists and eclogites. A combination of elastic tensor quartz inclusion in garnet (QuiG) barometry and Zr-in-rutile (ZiR) trace element thermometry was used to constrain minimum garnet growth conditions. Garnet from the hangingwall (blueschist) unit record formation pressures that range from 1.5 to 1.9 GPa and garnet from the footwall basement complex record garnet formation pressures of 1.65–2.05 GPa. ZiR thermometry on rutile inclusions within garnet establishes the minimum temperature for garnet formation to be ~480–500°C. That is, there is no evidence in the QuiG and ZiR results that the rocks of the blueschist hangingwall and basement experienced different metamorphic histories during subduction. This is the first reported observation of blueschist facies metamorphism in the Hercynian basement complex. A model is proposed in which initial subduction occurred along a relatively shallow P–T trajectory of ~11°C/km and then transitioned to a steeper, nearly isothermal trajectory at a depth of ~45 km reaching similar peak metamorphic conditions of ~500–525°C at 2.0 GPa for all samples. Such a change in the subduction path could be accomplished by either an increase in the rate of subduction or an increase in the angle of the subduction zone. The present juxtaposition of samples with contrasting mineral assemblages and garnet growth histories is interpreted to have arisen from differences in bulk compositions and variations in the preservation of high-pressure prograde mineral assemblages during exhumation. The existence of similar P–T conditions and prograde paths in the two units does not require that the rocks were all metamorphosed at the same time and that the SCSZ experienced little movement. Rather, it is suggested that the two units experienced prograde and peak metamorphism at different times and were subsequently juxtaposed along the SCSZ.

本文研究了希腊基克拉迪群岛伊俄斯岛的高压岩石,以解决由南基克拉迪剪切带(SCSZ)分隔的两个不同的岩石构造单元——由海西基底片麻岩、片岩和角闪岩组成的下盘杂岩和由蓝片岩和榴辉岩组成的上盘杂岩在俯冲过程中所达到的P-T条件。采用弹性张量石英包体在石榴石(QuiG)气压测量和Zr - in -金红石(ZiR)微量元素测温相结合的方法来约束石榴石的最小生长条件。上盘(蓝片岩)单元石榴石记录的地层压力范围为1.5 ~ 1.9 GPa,下盘基底杂岩石榴石记录的地层压力范围为1.65 ~ 2.05 GPa。对石榴石中金红石包裹体的ZiR测温确定了石榴石形成的最低温度为~ 480-500℃。即在QuiG和ZiR结果中没有证据表明蓝片岩上盘和基底岩石在俯冲过程中经历了不同的变质史。这是海西期基底杂岩中首次观察到蓝片岩相变质作用。提出了一个模型,在该模型中,初始俯冲发生在~11°C/km的相对较浅的P-T轨迹上,然后在~45 km的深度过渡到更陡峭的近等温轨迹,在2.0 GPa下达到~ 500-525°C的类似峰值变质条件。这种俯冲路径的改变可以通过增加俯冲速率或增加俯冲带的角度来实现。目前对比矿物组合和石榴石生长历史的样品被解释为是由于体积成分的差异和挖掘过程中高压递进矿物组合保存的变化。两个单元具有相似的P-T条件和进动路径,并不要求岩石全部在同一时间变质,也不要求构造段运动少。相反,这两个单元在不同的时期经历了进变质和高峰变质作用,随后沿着南海并置。
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引用次数: 1
P–T–t conditions of Early Palaeozoic low-P high-T granulite facies metamorphism in the southern Truong Son Belt, Central Vietnam 越南中部张松带南部早古生代低磷高钾麻粒岩相变质作用的P - T - T条件
IF 3.4 2区 地球科学 Q1 GEOLOGY Pub Date : 2023-06-22 DOI: 10.1111/jmg.12737
Nam Nguyen Duc, Ching-Hua Lo, Tadashi Usuki, Yoshiyuki Iizuka, Pham Binh

High-grade metamorphic rocks are widely exposed along the SE–NW- to E–W-trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high-grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse-grained granulite mineral assemblages, defining a tight clockwise P–T path with near-isobaric heating to a near ultrahigh-temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse-grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K-feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post-peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium-pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine-grained kyanite-bearing mineral associations. Petrographic observations indicate that these fine-grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post-collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny.

高品位变质岩广泛暴露在越南中部Truong Son带的SE–NW至E–W走向剪切带上,但在该地区进行的岩石学研究很少。在此,我们报道了在Truong Son带最南端的Dai-Loc剪切带上出现的糜棱岩化麻粒岩。对两个麻粒岩样品的演化进行了详细的岩相分析、地球化学和P–T–T估计,以阐明这些高级变质岩的形成过程。结果表明,这些岩石经历了两个不同的变质旋回。第一个循环(M1)以粗粒麻粒岩矿物组合为特征,定义了一条紧顺时针P–T路径,在低压下近等压加热至近超高温峰值,然后冷却。前进矿物组合(M1a)由堇青石包裹体指示 + 硅线石 + 黑云母 + 石英 + 尖晶石 ± 粗粒石榴石、斜方辉石和堇青石中的斜长石。石榴石的矿物组合 + 斜方辉石 + 堇青石 + 斜长石 + 钾长石 + 钛铁矿 + 熔化 ± 黑云母(M1b)定义了5.3–6.3 kbar和850–920°C的峰值P–T条件。峰后冷却(M1c)以石英的形成为标志 + 黑云母在石榴石和斜方辉石周围呈同向分布。第二个旋回涉及中压角闪岩相变质作用(M2),其特征是细粒蓝晶石矿物组合的区域发育。岩石学观察表明,这些细粒组合是在糜棱岩化过程中形成的。锆石U–Pb定年显示,麻粒岩相变质作用的时间似乎与430–410碰撞后花岗质岩石的侵入同时发生 麻粒岩相变质作用和地壳熔融可能是由早古生代板块断裂引发的软流圈地幔上升流所驱动的。考虑到以往的构造和地质年代研究,第二次变质事件可能发生在三叠纪-印支造山运动期间。
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引用次数: 0
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Journal of Metamorphic Geology
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