Pub Date : 2021-07-31DOI: 10.4454/OFIOLITI.V46I2.542
R. Compagnoni, R. Cossio, D. Regis
The high-pressure (HP) continental Etirol-Levaz slice (ELS) is exposed on the right side of the Valtournenche (Aosta) at the contact between the overlying blueschist-facies Combin Zone and the underlying eclogite-facies Zermatt-Saas Zone. The ELS contains a layered metagabbro showing a complex evolution, which includes the pre-Alpine high-temperature (HT) recrystallization of the igneous mineralogies, and the polyphase Alpine metamorphism, initially at high-pressure (HP) quartz eclogite-facies, and greenschist-facies (GS) retrogression.In this paper, an unusual corundum-bearing ultramafic rocks associated with metagabbros are described. The studied sample is a Mg-chloritite with relict green spinel partly replaced by corundum, Mg-beltrandoite-2N3S (a new mineral of the hogbomite supergroup), Mg-chlorite and dolomite. The rock is crossed by mm- to cm-thick veins composed of coarse-grained corundum + Mg-chlorite + dolomite. P-T phase-diagram projections indicate that the corundum-bearing assemblages formed in the presence of a water-rich fluid (X(CO2) ≤ 0.04), during retrograde decompressional evolution. This stage follows the prograde HP-peak of the associated eclogites. The newly inferred prograde-to-retrograde P-T path suggests that the ELS and the underlying Zermatt-Saas Zone shared a common Alpine metamorphic evolution.The detailed study of the relict minerals preserved in the chloritite indicates its pre-Alpine protolith, a green spinel websterite, and its evolution, characterized by a high-T recrystallization of the original igneous asemblages. During the Alpine orogeny, the spinel websterite experienced metasomatic hydration that converted the original igneous rock into a chloritite.
{"title":"CORUNDUM-BEARING VEINS IN CHLORITITE FROM THE ETIROL-LEVAZ AUSTROALPINE CONTINENTAL SLICE (VALTOURNENCHE, AOSTA, ITALY)","authors":"R. Compagnoni, R. Cossio, D. Regis","doi":"10.4454/OFIOLITI.V46I2.542","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I2.542","url":null,"abstract":"The high-pressure (HP) continental Etirol-Levaz slice (ELS) is exposed on the right side of the Valtournenche (Aosta) at the contact between the overlying blueschist-facies Combin Zone and the underlying eclogite-facies Zermatt-Saas Zone. The ELS contains a layered metagabbro showing a complex evolution, which includes the pre-Alpine high-temperature (HT) recrystallization of the igneous mineralogies, and the polyphase Alpine metamorphism, initially at high-pressure (HP) quartz eclogite-facies, and greenschist-facies (GS) retrogression.In this paper, an unusual corundum-bearing ultramafic rocks associated with metagabbros are described. The studied sample is a Mg-chloritite with relict green spinel partly replaced by corundum, Mg-beltrandoite-2N3S (a new mineral of the hogbomite supergroup), Mg-chlorite and dolomite. The rock is crossed by mm- to cm-thick veins composed of coarse-grained corundum + Mg-chlorite + dolomite. P-T phase-diagram projections indicate that the corundum-bearing assemblages formed in the presence of a water-rich fluid (X(CO2) ≤ 0.04), during retrograde decompressional evolution. This stage follows the prograde HP-peak of the associated eclogites. The newly inferred prograde-to-retrograde P-T path suggests that the ELS and the underlying Zermatt-Saas Zone shared a common Alpine metamorphic evolution.The detailed study of the relict minerals preserved in the chloritite indicates its pre-Alpine protolith, a green spinel websterite, and its evolution, characterized by a high-T recrystallization of the original igneous asemblages. During the Alpine orogeny, the spinel websterite experienced metasomatic hydration that converted the original igneous rock into a chloritite.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":" ","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49387485","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-07-31DOI: 10.4454/OFIOLITI.V46I2.544
M. Simonetti, A. Langone, S. Corvò, M. Bonazzi
The direct observation and investigation of rift-related structures at the mesoscale is uncommon. Hence, detailed constraints on the evolution of the main faults and shear zones developed during crustal extension are not always available.The Ivrea-Verbano Zone, in the Italian Southern Alps, samples remnants of the former lower crust of the rifted margins surrounding the Alpine Tethys and therefore provides the opportunity to directly investigate rift-related tectonics. Here, several shear zones have been recognized and interpreted as related to Mesozoic rifting. However, even if there is a general agreement with this interpretation, the precise age of activity of many of those shear zones is not well constrained.In this paper we present a review of the Triassic-Jurassic geochronological and thermochronological data available for two sections of the Ivrea-Verbano Zone, the Strona di Omegna and Ossola valleys, where at least two extensional shear zones are exposed. Ductile deformation occurred under amphibolite-facies conditions and it was alternatively attributed to late Variscan deformation or to Triassic-Jurassic rifting-related tectonics.We discuss the available chronological data and the different interpretations provided for the shear zones considering also new geochronological studies on other lower crustal shear zones exposed in other sectors of the Ivrea-Verbano Zone. This review allows us to strengthen the more recent interpretations indicating that these shear zones are important tectonic structures related to Late Triassic-Jurassic deformation in the lower crust of the Adriatic margin.
在中尺度上对裂谷相关结构的直接观测和调查并不常见。因此，对地壳伸展过程中形成的主要断层和剪切带的演化的详细约束并不总是可用的。意大利南阿尔卑斯山的Ivrea Verbano带对阿尔卑斯特提斯周围裂谷边缘的前下地壳残余进行了采样，因此提供了直接研究裂谷相关构造的机会。在这里，几个剪切带已被确认并解释为与中生代裂谷作用有关。然而，即使人们普遍同意这一解释，许多剪切带的确切活动年龄也没有得到很好的限制。在本文中，我们回顾了Ivrea Verbano带的两个部分，Strona di Omegna和Ossola山谷的三叠纪-侏罗纪地质年代和热年代数据，其中至少暴露了两个伸展剪切带。韧性变形发生在角闪岩相条件下，可归因于晚华力西期变形或三叠纪-侏罗纪裂谷相关构造。我们讨论了可用的年代数据和对剪切带的不同解释，同时考虑到对Ivrea-Verbano带其他部分暴露的其他下地壳剪切带的新地质年代研究。这篇综述使我们能够加强最近的解释，表明这些剪切带是与亚得里亚海边缘下地壳晚三叠纪-侏罗纪变形有关的重要构造结构。
{"title":"TRIASSIC-JURASSIC RIFT-RELATED DEFORMATION AND TEMPERATURE-TIME EVOLUTION OF THE FOSSIL ADRIATIC MARGIN: A REVIEW FROM OSSOLA AND STRONA DI OMEGNA VALLEYS (IVREA-VERBANO ZONE)","authors":"M. Simonetti, A. Langone, S. Corvò, M. Bonazzi","doi":"10.4454/OFIOLITI.V46I2.544","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I2.544","url":null,"abstract":"The direct observation and investigation of rift-related structures at the mesoscale is uncommon. Hence, detailed constraints on the evolution of the main faults and shear zones developed during crustal extension are not always available.The Ivrea-Verbano Zone, in the Italian Southern Alps, samples remnants of the former lower crust of the rifted margins surrounding the Alpine Tethys and therefore provides the opportunity to directly investigate rift-related tectonics. Here, several shear zones have been recognized and interpreted as related to Mesozoic rifting. However, even if there is a general agreement with this interpretation, the precise age of activity of many of those shear zones is not well constrained.In this paper we present a review of the Triassic-Jurassic geochronological and thermochronological data available for two sections of the Ivrea-Verbano Zone, the Strona di Omegna and Ossola valleys, where at least two extensional shear zones are exposed. Ductile deformation occurred under amphibolite-facies conditions and it was alternatively attributed to late Variscan deformation or to Triassic-Jurassic rifting-related tectonics.We discuss the available chronological data and the different interpretations provided for the shear zones considering also new geochronological studies on other lower crustal shear zones exposed in other sectors of the Ivrea-Verbano Zone. This review allows us to strengthen the more recent interpretations indicating that these shear zones are important tectonic structures related to Late Triassic-Jurassic deformation in the lower crust of the Adriatic margin.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":" ","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42403660","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-07-31DOI: 10.4454/OFIOLITI.V46I2.543
L. Barale, C. Bertok, A. d’Atri, F. Piana, S. Bernasconi, G. Czuppon, L. Palcsu, A. Gerdes, D. Birgel, L. Martire
In the Maritime Alps (NW Italy - SE France), the Middle Triassic-Berriasian platform carbonates of the Provencal Domain are locally affected by an intense hydrothermal dolomitization. This dolomitization resulted from a large-scale hydrothermal circulation related to deep-rooted faults, and is indirect evidence of a significant earliest Cretaceous fault activity in this part of the Alpine Tethys European palaeomargin. New carbonate U-Pb dating and geochemical (stable isotope and noble gases on inclusion-hosted water, 87Sr/86Sr, clumped isotopes) data allowed a better understanding of the timing and mechanisms of the hydrothermal circulation. Hydrothermal fluids probably originated from seawater, which was involved in a deep circulation within the underlying crystalline basement, undergoing heating to more than 200°C and substantial compositional modification by prolonged interaction with basement rocks. Thin cement rims rich in carbonaceous material, locally alternated with hydrothermal dolomite cements, are interpreted as remnants of ephemeral microbial communities that could colonize the upper part of hydrothermal conduits during periods of reduced hot fluid flow and contemporaneous downward seawater infiltration.
{"title":"U-PB DATING AND GEOCHEMICAL CONSTRAINTS TO EARLY CRETACEOUS HYDROTHERMAL DOLOMITIZATION IN THE PROVENÇAL DOMAIN (MARITIME ALPS, NW ITALY - SE FRANCE)","authors":"L. Barale, C. Bertok, A. d’Atri, F. Piana, S. Bernasconi, G. Czuppon, L. Palcsu, A. Gerdes, D. Birgel, L. Martire","doi":"10.4454/OFIOLITI.V46I2.543","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I2.543","url":null,"abstract":"In the Maritime Alps (NW Italy - SE France), the Middle Triassic-Berriasian platform carbonates of the Provencal Domain are locally affected by an intense hydrothermal dolomitization. This dolomitization resulted from a large-scale hydrothermal circulation related to deep-rooted faults, and is indirect evidence of a significant earliest Cretaceous fault activity in this part of the Alpine Tethys European palaeomargin. New carbonate U-Pb dating and geochemical (stable isotope and noble gases on inclusion-hosted water, 87Sr/86Sr, clumped isotopes) data allowed a better understanding of the timing and mechanisms of the hydrothermal circulation. Hydrothermal fluids probably originated from seawater, which was involved in a deep circulation within the underlying crystalline basement, undergoing heating to more than 200°C and substantial compositional modification by prolonged interaction with basement rocks. Thin cement rims rich in carbonaceous material, locally alternated with hydrothermal dolomite cements, are interpreted as remnants of ephemeral microbial communities that could colonize the upper part of hydrothermal conduits during periods of reduced hot fluid flow and contemporaneous downward seawater infiltration.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":" ","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47081642","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-07-31DOI: 10.4454/OFIOLITI.V46I2.541
M. Beltrando
scale of observation. Here we report a case from the Balmuccia peridotite massif, where microstructural observations on the websteritic dykes reveal that they recorded a two-stage deformation history. Lattice Preferred Orientations (LPO) measured in the peridotitic host rock indicate that the oldest deformation was characterized by sinistral simple shear while the youngest by pure shear flow regimes. However, the distribution of folded, stretched and folded-then-stretched dykes at the outcrop scale follows a pattern concordant with large strain accommodated by pure shear only. On the other hand, the only indication of a more complex evolution is provided by the uneven dyke distribution, which is inferred to be inherited from an older deformation event. In order to define the orientation of the dykes prior to the last deformation event, a backward restoration is presented. Integrating micro- and meso-structural observations, the relative orientation of the flow plane for the first deformation event and the strain ellipse for the second has been assessed. This approach resulted in the determination of a two-stage evolution for the Balmuccia peridotite. This local evolution allows discussing wider speculations on the late Paleozoic tectonics. The Balmuccia massif, while being deformed in a flow regime characterized by sinistral simple shear, presumably during a late-Variscan lithospheric-scale extensional event, was intruded by synkinematic Al-augite websterites. As extension continued, in the Early Permian, the massif was deformed by nearly pure shear flow and large horizontal stretching could bring the Balmuccia massif to lower crustal depths.
{"title":"CAN THE SCALE OF OBSERVATION HIDE COMPLEXITIES IN THE DEFORMATION HISTORY OF A TERRANE? AN EXAMPLE FROM THE BALMUCCIA PERIDOTITE MASSIF, IVREA ZONE (NW ITALY)","authors":"M. Beltrando","doi":"10.4454/OFIOLITI.V46I2.541","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I2.541","url":null,"abstract":"scale of observation. Here we report a case from the Balmuccia peridotite massif, where microstructural observations on the websteritic dykes reveal that they recorded a two-stage deformation history. Lattice Preferred Orientations (LPO) measured in the peridotitic host rock indicate that the oldest deformation was characterized by sinistral simple shear while the youngest by pure shear flow regimes. However, the distribution of folded, stretched and folded-then-stretched dykes at the outcrop scale follows a pattern concordant with large strain accommodated by pure shear only. On the other hand, the only indication of a more complex evolution is provided by the uneven dyke distribution, which is inferred to be inherited from an older deformation event. In order to define the orientation of the dykes prior to the last deformation event, a backward restoration is presented. Integrating micro- and meso-structural observations, the relative orientation of the flow plane for the first deformation event and the strain ellipse for the second has been assessed. This approach resulted in the determination of a two-stage evolution for the Balmuccia peridotite. This local evolution allows discussing wider speculations on the late Paleozoic tectonics. The Balmuccia massif, while being deformed in a flow regime characterized by sinistral simple shear, presumably during a late-Variscan lithospheric-scale extensional event, was intruded by synkinematic Al-augite websterites. As extension continued, in the Early Permian, the massif was deformed by nearly pure shear flow and large horizontal stretching could bring the Balmuccia massif to lower crustal depths.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":" ","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42044833","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-07-31DOI: 10.4454/OFIOLITI.V46I2.546
G. Frasca, R. Compagnoni
Subduction plate margins have an intrinsically poor preservation potential for pre-subduction settings because most rocks sink and disappear into Earth’s mantle. However, a preserved pre-subduction setting has been reported in the Lower Penninic Units (Valaisan Basin) of the Italian Western Alps (Beltrando et al., 2012). Here, we review two main aspects of the Valaisan Basin cropping out in Italy: the nature of the syn-rift detrital sediments and the characteristics of the metabasalts in proximity to the continental crust metagranitoid. New fossil findings are reported, which confirm a Mesozoic age for the Valaisan rifting. Field and petrographic data from metabreccias and pillowed metabasalts near the fossil-rich high-pressure metasediments support the interpretation of the Valaisan Domain in the Breuil valley (Aosta) as a fossil ocean-continent transition zone.
{"title":"PILLOWED METABASALTS AND METABRECCIAS FROM THE VALAISAN OCEAN-CONTINENT TRANSITION PRESERVED IN THE BREUIL VALLEY, AOSTA (ITALIAN WESTERN ALPS)","authors":"G. Frasca, R. Compagnoni","doi":"10.4454/OFIOLITI.V46I2.546","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I2.546","url":null,"abstract":"Subduction plate margins have an intrinsically poor preservation potential for pre-subduction settings because most rocks sink and disappear into Earth’s mantle. However, a preserved pre-subduction setting has been reported in the Lower Penninic Units (Valaisan Basin) of the Italian Western Alps (Beltrando et al., 2012). Here, we review two main aspects of the Valaisan Basin cropping out in Italy: the nature of the syn-rift detrital sediments and the characteristics of the metabasalts in proximity to the continental crust metagranitoid. New fossil findings are reported, which confirm a Mesozoic age for the Valaisan rifting. Field and petrographic data from metabreccias and pillowed metabasalts near the fossil-rich high-pressure metasediments support the interpretation of the Valaisan Domain in the Breuil valley (Aosta) as a fossil ocean-continent transition zone.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":" ","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44140340","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-07-31DOI: 10.4454/OFIOLITI.V46I2.547
Gilles de Broucker, Yves Siméon, G. Stampfli, D. Thiéblemont, P. Lach, M. Marthaler
Based on detailed field investigations and structural mapping in the Petit-Saint Bernard Pass area (French-Italian border), the Versoyen Complex is newly defined as an imbrication of four structural units. The first one is composed of the remnant of the Valaisan ocean floor (“basalt” and “black schist” formation) and the other three units -grouped under the name of “schistes a blocs”- contain blocks with different origin: oceanic, continental or a mix of both, embedded in a matrix of grey micaschists. Kinematic analysis highlights the compressional style of this complex with a well-developed stack of tectonic slivers. Rocks in the tectonic melange include Paleozoic green gneiss and grey-green micaschists blocks, together with Upper Paleozoic granitoid and layered gabbro, which display a typical orogenic calc-alkaline signature, comparable to that of coeval plutonic rocks of the Brianconnais continental basement attributed to a late Variscan back-arc environment. A new isotopic dating on a calc-alkaline layered gabbro block gave a U/Pb zircon age of 310 ± 4 Ma, in the range of other dated plutonic rocks such as the Punta Rossa and Aiguille du Clapet megablocks. These Paleozoic continental basement rocks of the Versoyen Complex were initially rift allochthons, but are now outcropping as a tectonic melange, with imbricated blocks and slivers within an Early Cretaceous grey micaschists matrix. Radiolarians sampled by Beltrando et al. (2012) within the grey micaschists have been attributed to the Late Jurassic - Early Cretaceous. Additional stratigraphic and geodynamic constraints allow us to restrict the deposition of the Versoyen sediments from the Aptian to Cenomanian times. The Versoyen Complex is unconformably overlain by the Valaisan Trilogy (Aroley-Marmontains-Saint Christophe) of Cretaceous age. The Turonian Aroley strata deposited on the Versoyen “schistes a blocs” units and the absence of any Aroley limestones within the underlying tectonic melange testify a clear stratigraphic unconformity, mapped regionally, between the Versoyen Complex and Valaisan Trilogy. Both are affected by a high-pressure metamorphism related to the Alpine collision around 40 Ma and trace of former metamorphism are nowhere observed suggesting that the former melange did not result in a significant tectonic thickening. We interpret the Versoyen Complex, as a pre-Alpine Cretaceous subduction-related accretionary prism, formed during the closure of the Valaisan Ocean. Valaisan rifting initiated during the Late Jurassic, as a consequence of an eastwards propagation of the North-Atlantic rifting towards the Alpine region. This propagation followed pre-existing Variscan structures along the Zone Houillere, explaining the present marked differences between Variscan basements on each side of the Valaisan suture (External Crystalline Massifs and Brianconnais basement). Due to the southward subduction of the Valaisan Ocean, this passive margin was later on deformed through com
根据小圣伯纳山口地区（法意边界）的详细实地调查和构造测绘，Versoyen杂岩被新定义为四个构造单元的叠瓦状。第一个单元由瓦莱桑海底的残余物（“玄武岩”和“黑片岩”地层）组成，其他三个单元以“片岩-块体”的名称分组，包含不同来源的块体：海洋、大陆或两者的混合物，嵌入灰色云片岩基质中。运动学分析突出了该杂岩的挤压样式，具有发育良好的构造碎屑堆。构造混杂岩中的岩石包括古生代绿色片麻岩和灰绿色云片岩块体，以及上古生界花岗质岩石和层状辉长岩，它们显示出典型的造山钙碱性特征，与归因于晚华力西期弧后环境的Brianconnais大陆基底的同时代深成岩体相当。在钙碱性层状辉长岩块体上进行的新同位素测年给出了310±4 Ma的U/Pb锆石年龄，该年龄在其他已测年的深成岩体范围内，如Punta Rossa和Aiguille du Clapet巨型块体。Versoyen杂岩的这些古生代大陆基底岩石最初是裂谷异地岩，但现在以构造混杂岩的形式露头，在早白垩世灰色云片岩基质中有叠瓦状块体和碎屑。Beltrando等人（2012）在灰色云片岩中采样的放射虫被认为是晚侏罗纪-早白垩纪。额外的地层和地球动力学限制使我们能够限制从阿普特阶到塞诺曼阶Versoyen沉积物的沉积。Versoyen杂岩不整合地被白垩纪时期的瓦莱桑三部曲（Aroley Marmontains Saint-Christophe）覆盖。沉积在Versoyen“片岩-块体”单元上的Turonian Aroley地层以及下伏构造混杂岩中不存在任何Aroley石灰岩，证明了VersoyenComplex和Valaisan Trilogy之间的明显地层不整合（区域映射）。两者都受到与大约40 Ma的阿尔卑斯碰撞有关的高压变质作用的影响，并且没有观察到以前变质作用的痕迹，这表明以前的混杂岩没有导致显著的构造增厚。我们将Versoyen杂岩解释为前阿尔卑斯-白垩纪俯冲相关的增生棱镜，形成于瓦莱桑洋闭合期间。瓦莱桑裂谷始于晚侏罗纪，是北大西洋裂谷向阿尔卑斯地区向东扩展的结果。这种传播遵循了Houillere带上预先存在的华力西断层构造，解释了瓦莱桑缝合线两侧华力西基底（外部结晶体和Brianconnais基底）之间目前的显著差异。由于瓦莱桑洋向南俯冲，这一被动边缘后来通过挤压剪切构造变形，形成了“片岩-块体”组构，并使以前的裂谷异地叠瓦。在第三纪阿尔卑斯山碰撞期间，Versoyen和Valaisan Trilogy单元被深度挤压不足，然后在Subbrianconnais单元下被挖掘出来，包括Petit Saint Bernard和Arguerey Liassic钙片岩，它们反过来又挤压不足Brianconnai Front。
{"title":"EARLY CRETACEOUS ACCRETIONARY COMPLEX OF THE VALAISAN OCEAN, WESTERN ALPS?","authors":"Gilles de Broucker, Yves Siméon, G. Stampfli, D. Thiéblemont, P. Lach, M. Marthaler","doi":"10.4454/OFIOLITI.V46I2.547","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I2.547","url":null,"abstract":"Based on detailed field investigations and structural mapping in the Petit-Saint Bernard Pass area (French-Italian border), the Versoyen Complex is newly defined as an imbrication of four structural units. The first one is composed of the remnant of the Valaisan ocean floor (“basalt” and “black schist” formation) and the other three units -grouped under the name of “schistes a blocs”- contain blocks with different origin: oceanic, continental or a mix of both, embedded in a matrix of grey micaschists. Kinematic analysis highlights the compressional style of this complex with a well-developed stack of tectonic slivers. Rocks in the tectonic melange include Paleozoic green gneiss and grey-green micaschists blocks, together with Upper Paleozoic granitoid and layered gabbro, which display a typical orogenic calc-alkaline signature, comparable to that of coeval plutonic rocks of the Brianconnais continental basement attributed to a late Variscan back-arc environment. A new isotopic dating on a calc-alkaline layered gabbro block gave a U/Pb zircon age of 310 ± 4 Ma, in the range of other dated plutonic rocks such as the Punta Rossa and Aiguille du Clapet megablocks. These Paleozoic continental basement rocks of the Versoyen Complex were initially rift allochthons, but are now outcropping as a tectonic melange, with imbricated blocks and slivers within an Early Cretaceous grey micaschists matrix. Radiolarians sampled by Beltrando et al. (2012) within the grey micaschists have been attributed to the Late Jurassic - Early Cretaceous. Additional stratigraphic and geodynamic constraints allow us to restrict the deposition of the Versoyen sediments from the Aptian to Cenomanian times. The Versoyen Complex is unconformably overlain by the Valaisan Trilogy (Aroley-Marmontains-Saint Christophe) of Cretaceous age. The Turonian Aroley strata deposited on the Versoyen “schistes a blocs” units and the absence of any Aroley limestones within the underlying tectonic melange testify a clear stratigraphic unconformity, mapped regionally, between the Versoyen Complex and Valaisan Trilogy. Both are affected by a high-pressure metamorphism related to the Alpine collision around 40 Ma and trace of former metamorphism are nowhere observed suggesting that the former melange did not result in a significant tectonic thickening. We interpret the Versoyen Complex, as a pre-Alpine Cretaceous subduction-related accretionary prism, formed during the closure of the Valaisan Ocean. Valaisan rifting initiated during the Late Jurassic, as a consequence of an eastwards propagation of the North-Atlantic rifting towards the Alpine region. This propagation followed pre-existing Variscan structures along the Zone Houillere, explaining the present marked differences between Variscan basements on each side of the Valaisan suture (External Crystalline Massifs and Brianconnais basement). Due to the southward subduction of the Valaisan Ocean, this passive margin was later on deformed through com","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":" ","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42097246","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-01-29DOI: 10.4454/OFIOLITI.V46I1.535
F. Rotondo, P. Tartarotti, S. Guerini, G. Porta, N. Campomenosi
A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T = 550-630°C, P = 1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e., serpentinite and carbonatebearingmetabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ18OVSMOW and δ13CVPDB values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg-H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different
在阿尔卑斯西北部泽尔马特-萨斯高压带，在米塞林湖蛇绿岩的变质绿岩(蛇纹岩和变质碳酸盐)基底和变质沉积层(混沌岩和钙片岩)之间形成交代层(MH)。宏观和微观结构分析，结合岩石学和地球化学对MH及其周围岩性的调查，揭示了一个多相的爆破-变形历史，这导致了蛇纹岩基底-沉积界面的复杂结构的形成和矿物化学蚀变。从早阿尔卑斯俯冲到HP-LT变质峰(T = 550-630°C, P = 1.8-2.5 GPa)，脱水、脱碳和碳酸作用相互作用，在MH原岩(即蛇纹岩和含碳酸盐的变质砾岩)的富碳酸盐和富蛇纹岩域中形成了独特的、普遍存在的角闪石(透闪石/放光石)替代。这种特征的角闪孔交代作用在与变质碳酸盐接触的方向更为明显，MH内白云岩的平均δ18OVSMOW和δ13CVPDB值(分别为+14.4‰和+0.7‰)介于变质碳酸盐和钙片岩之间。这些结果表明，脱水板的富mg - h2o流体、脱碳释放的CO2以及富sio2流体在混合的钙质岩中演化，主要沿变质岩基底/变质沉积层界面循环，在此形成了混合变质流体的优先通道。这些发现强调并证实了收敛系统中交代岩石的研究对于理解不同成岩作用的行为是至关重要的
{"title":"METASOMATIC HORIZON SEALING SERPENTINITE-METASEDIMENTS PAIR IN THE ZERMATT-SAAS METAOPHIOLITE (NORTHWESTERN ALPS): RECORD OF A CHANNEL FOR FOCUSSED FLUID FLOW DURING SUBDUCTION","authors":"F. Rotondo, P. Tartarotti, S. Guerini, G. Porta, N. Campomenosi","doi":"10.4454/OFIOLITI.V46I1.535","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I1.535","url":null,"abstract":"A metasomatic horizon (MH) occurs between the metaophiolite (serpentinite and metaophicarbonates) basement and metasedimentary sequence (chaotic rocks and calcschists) of the Lake Miserin Ophiolite, in the high pressure Zermatt-Saas Zone of the Northwestern Alps. Macro- and microstructural analyses combined with petrological and geochemical investigations of the MH and surrounding lithologies unravelled a polyphase blastesis-deformation history, which led to the formation of a complex fabric and minero-chemical alteration of the serpentinite basement-metasediments interface. Dehydration, decarbonation and carbonation interplayed from early Alpine subduction up to HP-LT metamorphic peak (T = 550-630°C, P = 1.8-2.5 GPa), to produce a distinctive, pervasive amphibole (tremolite/actinolite) replacement both in carbonate-rich and serpentinite-rich domains pertaining to the MH protoliths, i.e., serpentinite and carbonatebearingmetabreccia of the chaotic rock unit. This characteristic amphibole metasomatism is more pronounced toward the contact with the metaophicarbonates, and the average δ18OVSMOW and δ13CVPDB values of dolomite within the MH (+14.4‰ and +0.7‰ respectively) lie between those of the metaophicarbonates and of calcschist. These results suggest that Mg-H2O-rich fluids from the dehydrating slab, CO2 released by decarbonation and SiO2-rich fluids evolved in calcschists mixed together and circulated mostly along the metaophiolite basement/metasediments interface, where the MH developed and recorded a preferential channel for mixed metamorphic fluid flow. These findings highlight and confirm that the study of metasomatic rocks in convergent systems is crucial to comprehend the behaviour of different","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":"46 1","pages":"1-25"},"PeriodicalIF":1.3,"publicationDate":"2021-01-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47266295","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-01-29DOI: 10.4454/OFIOLITI.V46I1.539
S. Skolotnev, A. Sanfilippo, A. Peyve, Y. Nestola, S. Sokolov, L. Petracchini, K. Dobrolyubova, V. Basch, Alexei N. Pertsev, C. Ferrando, A. Ivanenko, C. Sani, A. Razumovskiy, F. Muccini, A. Bich, C. Palmiotto, Y. V. Brusilovsky, E. Bonatti, K. N. Sholukhov, M. Cuffaro, I. Veklich, V. N. Dobrolyubov, M. Ligi
The Charlie Gibbs offsetting by ~ 340 km the Mid Atlantic Ridge (MAR) axis at 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced from the south by the Azores and from the north by the Iceland mantle plumes, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ’70 and ’80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~ 40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54,552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. This work contains preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system.
{"title":"SEAFLOOR SPREADING AND TECTONICS AT THE CHARLIE GIBBS TRANSFORM SYSTEM (52-53ºN, MID ATLANTIC RIDGE): PRELIMINARY RESULTS FROM R/V A. N. STRAKHOV EXPEDITION S50","authors":"S. Skolotnev, A. Sanfilippo, A. Peyve, Y. Nestola, S. Sokolov, L. Petracchini, K. Dobrolyubova, V. Basch, Alexei N. Pertsev, C. Ferrando, A. Ivanenko, C. Sani, A. Razumovskiy, F. Muccini, A. Bich, C. Palmiotto, Y. V. Brusilovsky, E. Bonatti, K. N. Sholukhov, M. Cuffaro, I. Veklich, V. N. Dobrolyubov, M. Ligi","doi":"10.4454/OFIOLITI.V46I1.539","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I1.539","url":null,"abstract":"The Charlie Gibbs offsetting by ~ 340 km the Mid Atlantic Ridge (MAR) axis at 52°-53° N is one of the main transform systems of the North Atlantic. Located between long mid-ocean ridge segments influenced from the south by the Azores and from the north by the Iceland mantle plumes, this transform system has been active since the early phases of North Atlantic rifting. Object of several surveys in the ’70 and ’80, Charlie Gibbs received great attention for its unique structure characterized by two long-lived right-lateral transform faults linked by a short ~ 40 km-long intra-transform spreading centre (ITR) with parallel fracture zone valleys extending continuously towards the continental margins. In October 2020 expedition S50 of the R/V A.N. Strakhov surveyed an area of 54,552 km2 covering the entire Charlie Gibbs transform system and the adjacent MAR spreading segments. We collected new bathymetric, magnetic and high-resolution single channel seismic data, along with basaltic, gabbroic and mantle rocks from 21 dredges. This work contains preliminary data from cruise S50 and discusses the large-scale architecture of this unique, long-lived transform system.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":" ","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-01-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45411524","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-01-29DOI: 10.4454/OFIOLITI.V46I1.538
A. Eslami, G. Borghini, A. Montanini, G. Grieco, C. Marchesi
The Cheshmeh-Bid ophiolitic massif in the Khajeh-Jamali district (Southern Iran) is dominated by harzburgite-dunite tectonites locally intruded by orthopyroxenite dikes. These latter are composed of dominant coarse orthopyroxene with minor olivine, Cr-spinel, clinopyroxene and amphibole. Estimated equilibrium temperatures for Mg-hornblende and edenitic amphibole reveal a late stage magmatic origin. The Cheshmeh-Bid orthopyroxenites are characterized by very low Al2O3, CaO, Na2O and TiO2 abundances coupled to relatively high MgO and SiO2 contents. They display U-shaped REE patterns, selective LILE enrichment and positive Pb and Sr anomalies. The host harzburgites are highly refractory mantle residues resulting from fluid-assisted melting. Field observations and mineral assemblages suggest that the pyroxenites formed by melt injection along fractures within rather cold ambient harzburgites and chromitites at moderate pressure (P > 1 GPa). Based on bulk-rock compositions and mineral chemistry, we infer that the Cheshmeh-Bid orthopyroxenites originated from the intrusion and crystallization of hydrous Si-rich, low-Ca melts with a boninite signature in a suprasubduction environment. Fine-grained neoblastic domains developed in the pyroxenites in response to subsolidus ductile deformation and recrystallization, which were most likely related to the exhumation of the Cheshmeh-Bid ophiolite massif.
{"title":"PETROLOGICAL CONSTRAINTS ON THE ORIGIN OF PYROXENITE DYKES IN THE LITHOSPHERIC MANTLE OF THE CHESHMEH-BID OPHIOLITIC MASSIF, SOUTHERN IRAN","authors":"A. Eslami, G. Borghini, A. Montanini, G. Grieco, C. Marchesi","doi":"10.4454/OFIOLITI.V46I1.538","DOIUrl":"https://doi.org/10.4454/OFIOLITI.V46I1.538","url":null,"abstract":"The Cheshmeh-Bid ophiolitic massif in the Khajeh-Jamali district (Southern Iran) is dominated by harzburgite-dunite tectonites locally intruded by orthopyroxenite dikes. These latter are composed of dominant coarse orthopyroxene with minor olivine, Cr-spinel, clinopyroxene and amphibole. Estimated equilibrium temperatures for Mg-hornblende and edenitic amphibole reveal a late stage magmatic origin. The Cheshmeh-Bid orthopyroxenites are characterized by very low Al2O3, CaO, Na2O and TiO2 abundances coupled to relatively high MgO and SiO2 contents. They display U-shaped REE patterns, selective LILE enrichment and positive Pb and Sr anomalies. The host harzburgites are highly refractory mantle residues resulting from fluid-assisted melting. Field observations and mineral assemblages suggest that the pyroxenites formed by melt injection along fractures within rather cold ambient harzburgites and chromitites at moderate pressure (P > 1 GPa). Based on bulk-rock compositions and mineral chemistry, we infer that the Cheshmeh-Bid orthopyroxenites originated from the intrusion and crystallization of hydrous Si-rich, low-Ca melts with a boninite signature in a suprasubduction environment. Fine-grained neoblastic domains developed in the pyroxenites in response to subsolidus ductile deformation and recrystallization, which were most likely related to the exhumation of the Cheshmeh-Bid ophiolite massif.","PeriodicalId":54690,"journal":{"name":"Ofioliti","volume":"46 1","pages":""},"PeriodicalIF":1.3,"publicationDate":"2021-01-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41946621","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2021-01-29DOI: 10.4454/OFIOLITI.V46I1.536
Heider Al Humadi, M. Väisänen, A. Ismail, M. Lehtonen, B. Johanson
The Mawat ophiolite is a fragment from the Neo-Tethyan oceanic lithosphere and is considered as the largest and best-exposed ophiolite within the Zagros Suture Zone, northeast Iraq. Felsic dykes are encountered in three locations in the Mawat ophiolite. Here, we use field work, petrology and geochemical data to investigate the magma sources of the felsic dykes. The felsic dykes are classified into two types: oceanic plagiogranites in the western (Mirza and Ismail, 2007) and leucogranites in the central (Mohammad et al., 2014) Mawat ophiolite, here called tonalites and granites, respectively. Both types, mixed in various proportions occur in the eastern felsic dykes. The eastern tonalites are weakly peraluminous to metaluminous and are low in K2O (0.13-0.24 wt%) and TiO2 (0.01-0.07 wt%) and high in Na2O (6.63-11.02 wt%). The eastern granites are strongly peraluminous, moderate to high in K2O (1.16-6.57 wt%) and Na2O (2.83-6.47 wt%), and very low in TiO2 (0.03-0.07 wt%). The western tonalites are similar to adakites and are interpreted to have crystallized from melts of subducted oceanic crust interacting with the mantle. The eastern tonalites underwent hornblende and plagioclase fractionation in shallow-level magma chambers modifying their original compositions. The granites are interpreted to derive from melting of psammitic sediments on top of the subducted slab. Parental melts of the tonalites and granites were partially mixed in shallow magma chambers in eastern Mawat.
Mawat蛇绿岩是新特提斯洋岩石圈的碎片，被认为是伊拉克东北部扎格罗斯缝合带内最大、暴露最好的蛇绿岩。在Mawat蛇绿岩的三个位置发现了长英质岩脉。在这里，我们利用野外工作、岩石学和地球化学数据来研究长英质岩脉的岩浆来源。长英质岩脉分为两种类型：西部的大洋斜长花岗岩（Mirza和Ismail，2007）和中部的浅色花岗岩（Mohammad et al.，2014）Mawat蛇绿岩，这里分别称为英云闪长岩和花岗岩。这两种类型以不同比例混合出现在东部长英质岩脉中。东部英云闪长岩为弱过发光至金属发光，K2O（0.13-0.24 wt%）和TiO2（0.01-0.07 wt%）含量低，Na2O含量高（6.63-11.02 wt%）。东部花岗岩为强过铝质，K2O（1.16-6.57 wt%）和Na2O（2.83-6.47 wt%）含量中等至较高，TiO2含量极低（0.03-0.07 wt%）。西部英云闪长岩类似于埃达克岩，被解释为是由与地幔相互作用的俯冲洋壳熔体结晶而成。东部英云闪长岩在浅层岩浆室内经历了角闪石和斜长石的分馏，改变了它们的原始成分。花岗岩被解释为来自俯冲板顶部的砂屑沉积物的熔融。英云闪长岩和花岗岩的母熔体在Mawat东部的浅层岩浆室内部分混合。
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