Fluid composition and evolution in coesite-bearing rocks (Dora-Maira massif,Western Alps): implications for element recycling during subduction

Fluid inclusions and F, Cl concentration of hydrous minerals were analysed in the coesite-pyrope quartzite, the interlayered jadeite quartzite and their country-rock gneiss from the Dora-Maira massif using a combination of microthermometry, Raman spectrometry, synchrotron X-ray microfiuorescence and electron microprobe analysis. Three populations of fluid inclusions were recognized texturally and can be related to distinct metamorphic stages. A low-salinity aqueous fluid occurs in the retrogressed country gneiss and as late secondary inclusions in jadeite quartzite and chloritized pyrope. An earlier secondary population is found in matrix quartz of the jadeite- and pyro-pe-quartzites. This population can be related to the early decompression and so to incipient breakdown of garnet into phlogopite-bearing assemblages. The inclusion fluid is highly saline (up to 84 wt% equivalent NaCl) and contains Na, Ca, Fe, Cu and Zn as major cations. In pyrope quartzite, additional K was found in these brines, which locally coexist with CO₂-rich inclusions. The oldest fluid inclusions are preserved in kyanite grains included in fresh pyrope and in pyrope itself. In pyrope, all inclusions have decrepitated and contain magnesite, an Mg-phosphate, sheet-silicate(s), a chloride and an opaque phase, with no fluid preser ved. In contrast, the kyanite inclusions in pyrope preserve primary H₂O-CO₂ low-salinity fluid inclusions, probably owing to the low compressibility of the kyanite inclusions and host garnet. In spite of in-situ re-equilibration, these inclusions can be interpreted as relics of the dehydration fluid that attended pyrope growth. These correlations between textural and chemical fluid inclusion data and metamorphic stages are consistent with the fluid composition calculated from the halogen content of different generations of phlogopite and biotite. The preservation of different fluid compositions, both in time and space, is evidence for local control and possibly origin of the fluids, in agreement with isotopic data. These results, in particular the absence of CO₂ in the jadeite quartzite, are best interpreted in terms of a fluid-melt system evolution. With increasing metamorphism, partitioning of H₂O, Na, Ca, Fe and heavy metals into melt (jadeite quartzite) and Mg, Na/K, F, CO₂ and P(?) into a residual aqueous fluid can account for depletion in Na, Ca and Fe of the pyrope quartzite. During the retrograde path, a _{H 2 O} rose as melt crystallized, generating the two populations of hypersaline and water-rich fluids that were highly reactive to pyrope. The process of fluid-melt interaction envisioned here coupled with models of melt extraction in subduction zones provides an attractive opportunity for the instantaneous ( < 1 Ma) and selective transport of elements between a downgoing slab and the overlying mantle wedge.     

Geochemistry of the volcanoplutonic association of the Shaw massif (East Antarctica): Composition,genesis, and geodynamic interpretation

This paper reports the first petrological and geochemical evidence for the Meso-Neoproterozoic metamorphic and metaintrusive rocks of the Shaw Mountain massif (Prince Charles Mountains, East Antarctica). It was shown that the orthogneisses (plagiogneisses) and metabasites of the massif were formed as constituents of a volcanoplutonic complex, which included a variety of igneous rocks of normal and subalkaline groups, from ultrabasic to silicic and was assigned to the volcanic tholeiite basalt-andesite-rhyolite, plutonic peridotite-gabbro, and late (?) calc-alkaline gabbro-diorite-plagiogranite associations. The distribution of major and compatible trace elements indicates the fractionation of the primary melts that produced the volcanoplutonic association of the Shaw massif. With respect to the distribution of REE and trace elements and some trace element ratios, the metabasic rocks of the Shaw massif correspond mainly to enriched and normal basalts of mid-ocean ridges, continental rifts, and ocean islands, which suggests a contribution from a plume mantle source. It was found that the region of the Shaw massif is a high-grade metamorphosed margin of the Fisher volcanoplutonic complex, a Mesoproterozoic structure of single geodynamic nature. This is supported by the spatial proximity of the Shaw and Fisher regions, the similar behaviors of most major elements and distribution patterns of trace elements and REE in comparable magmatic associations, and the similar ages of some plutonic associations. Based on the petrological and geochemical data, an alternative geotectonic model was proposed for this region. According to this model, the Fisher complex was formed in a setting of continental rifting coupled with the processes of mantle diapirism, which was subsequently changed by the compression stage. During rifting, the structure could experience significant opening accompanied by ultrabasic-basic tholeiitic magmatism with a significant contribution of mantle material. A subsequent inversion resulted in that the rift structure underwent considerable horizontal compression accompanied by calc-alkaline magmatism and the formation of narrow intracratonic fold zones. The cyclic character of rifting processes and superposition of young rift systems on older ones was also established in the Phanerozoic geotectonic history of the region of the Prince Charles Mountains.     

Sanukite（赞岐岩）的地球化学特征、成因及其地球动力学意义

赞岐岩 (サヌカイト ,sanukite)是指发现于日本四国北部的一种富Mg的火山岩 ,主要产于日本中新世 (11～14Ma)Setouchi火山岩带 ,是一种黑色玻璃质的火山岩。其化学成分以富Si质 (安山英安质 )、具很高的Mg# 值 (>0 .6 )、高的Cr、Ni丰度和K/Na值 (0 .33～ 0 .5 2 )为特征。赞岐岩的形成与菲律宾海板块年轻的热的岩石圈俯冲和四国盆地的张开有关 ,产于岛弧的弧前或弧后盆地环境。赞岐岩不只代表火山岩 ,也包括侵入岩。Shirey和Hanson(1984 )将该术语引入太古宙 ,将太古宙具上述赞岐岩特征 (Si过饱和、Mg# 高和Ni、Cr、LILE含量高 )的深成岩和火山岩称为sanukite岩套。赞岐岩与埃达克岩具有大体类似的地球化学特征 ,但前者更富Mg、Cr和Ni,表明赞岐岩可以直接由地幔岩部分熔融形成 ,而埃达克岩只能由玄武岩部分熔融形成。现代的赞岐岩很少见 ,而太古宙的赞岐岩比较常见 ,暗示太古宙导致板片熔融的消减的岩石圈本身或上地幔可能具有更高的温度。赞岐岩集中出现在晚太古代 (<3.0Ga) ,表明板块消减作用可能在 3.0Ga之后才成为一个重要的过程 ,晚太古代赞岐岩的出现可能标志着现代类型板块构造的开始     

北京白查A型花岗岩的地球化学特征及其成因与构造指示意义

位于北京昌平区的年龄为127Ma的白查岩体是一个晶洞过碱性花岗岩岩株,其侵位于燕山造山带西段的八达岭岩基中。白查岩体具有高硅、Zr、Ga、Pb、HFSE和REE含量,高Fe/Mg和Ga/Al比值,强烈亏损Ca、Sr、Eu,具有亏损放射性成因Pb同位素的特征。白查岩体属于Dall’Agnol和Oliveira (2007)定义的典型的还原型A型花岗岩,其地球化学特征与燕山带中生代晚期的后石湖山(山海关)和窟窿山A型花岗岩非常相似。白查花岗岩起源于华北克拉通下地壳镁铁质源岩在相对贫水、高温、低氧逸度条件下的低程度部分熔融。综合区域地质背景和该岩体的成岩机制判断,白查花岗岩的形成标志燕山西段造山崩塌阶段的到来。除浙江东南瑶坑过碱性花岗岩外,白查岩体是具有A1型花岗岩元素地球化学特征但并非形成于非造山大地构造背景的过碱性花岗岩的又一个实例。     

新疆西天山尼勒克二叠纪火山岩的地球化学特征及构造意义

尼勒克二叠纪火山岩位于伊犁盆地北缘阿吾拉勒山西段,该地区发育一套完整的陆相二叠纪地层。本文对该区二叠纪中下统哈米斯特组和乌郎组火山岩进行了系统的岩相学、岩石地球化学和Sr-Nd同位素研究。尼勒克火山岩岩石类型多样,有玄武岩、安山岩、粗面岩、流纹岩等,地球化学特征复杂。上火山岩哈米斯特组火山岩具有典型的钾玄岩特征,高K₂O(2.81%~3.91%)、Sr(>1000×10^-6)、稀土(ΣREE>200×10^-6)和(La/Yb)_N比值(9.7~11.7),低Nb/La比值(大多<0.2)。下火山岩乌郞组可分为二组,一组具最低SiO₂,最高MgO,较低Sr(<500×10^-6)、稀土(50×10^-6<ΣREE<80×10^-6)和(La/Yb)_N比值(1.6~2.2),相对较高的Nb/La比值(>0.35),类似富集型洋中脊(E-MORB)特征,暗示其岩浆是浅部源区高程度部分熔融的产物;另一组火山岩有着相对较大的变化,总体特征介于哈米斯特组与第一组火山岩两者之间。尼勒克火山岩有高度亏损的Sr-Nd同位素组成,类似MORB,表明它们源自长期亏损的地幔源区。然而,火山岩高度亏损的Nb-Ta、分异且富集大离子亲石元素的特征暗示,地幔源区受到近期俯冲带流体的交代作用,并受部分熔融程度及结晶分异作用的控制,是不同批次岩浆作用在不同演化阶段的产物。尼勒克大规模玄武质火山岩的形成可能和塔里木二叠纪地幔柱活动有关。     

Geochemistry and geodynamic implications of the Triassic bimodal magmatism from Western Kunlun Orogen,northwest China

The Western Kunlun Orogen occupies a key tectonic position at the junction between the Tarim block and the Tethyan domain. However, the late Paleozoic to early Mesozoic, especially the middle to late Triassic tectonic evolution history of the Western Kunlun Orogen remains controversial. This study reports SHRIMP zircon U–Pb ages and geochemical as well as Sr–Nd–Hf isotopic data for middle to late Triassic Taer pluton in Western Kunlun Orogen, Northwest China. The Taer pluton shows a strong bimodal distribution of compositions, with the felsic rocks dominant and the mafic rocks subordinate. Zircon U–Pb dating reveals that the coexisting mafic and felsic rocks are coeval, both emplacing in a period between 234 and 225 Ma. Most of the studied rocks are potassium rich and can be classified into high-K calc-alkaline to shoshonitic series. They are also strongly enriched in LREE, LILE and depleted in HFSE with strong negative Ti and Nb anomalies, and characterized by enriched Sr–Nd–Hf isotopic signatures. Detailed geochemical and isotopic studies indicate that the Taer pluton was emplaced in a post-collisional extensional setting, with the mafic rocks derived from partial melting of the enriched continental lithospheric mantle in the spinel facies field, and the felsic rocks formed by anatexis of newly underplated basaltic rocks. The existence of middle to late Triassic post-collisional magmas in Western Kunlun region suggests that the final closure of Paleo-Tethys and the initial collision between the Western Kunlun and the Qiangtang terranes may have happened before ~234 Ma, most probably in late Permian, rather than in late Triassic or early Jurassic. In assistance with other geological evidences, such as the presence of early Triassic to late Triassic/early Jurassic S-type magmatism, terrestrial molasse depositions, regional unconformities, and strong deformation, we propose that the Western Kunlun Orogen may have undergone a long post-collisional intracontinental process from early Triassic to late Triassic/early Jurassic.     

Lower Palaeozoic volcanism of the Eastern Alps and its geodynamic implications

A geodynamic model for the lower Palaeozoic evolution of the Eastern Alps (upper Ordovician to middle Devonian; Gurktal Nappe, Styrian Greywacke Zone, Palaeozoic of Graz) is discussed, based on new field work and numerous geochemical analyses of volcanic rocks. It is suggested that in the upper Ordovician there existed on the one hand a passive continental margin or continental rift flooded by the sea, where mildly-alkaline basalts erupted (Magdalensberg-Series/Carinthia; Kaser-Series/Gurktaler Alpen). On the other hand, a convergent plate boundary in the form of an active continental margin or island arc or in the form of an orogen formed by continent-continent collision which can be recognized by the eruption of calc-alkaline volcanic rocks, probably also existed in the upper Ordovician (Blasseneck »Porphyroid«/Styrian Greywacke Zone; NockSeries/Gurktaler Alpen). In the lower Silurian a reorganization of the plate boundaries took place, resulting in the disintegration of the upper Ordovician collision zone along deep-reaching faults. Further extensional movements during the Silurian and Devonian led to the disintegration of a broad shelf platform and to the eruption of alkaline within-plate basalts. The extensional movements were responsible for the thinning of the continental crust and a general subsidence, which probably lasted until Lower Carboniferous time.

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Zusammenfassung Für das Alt-Paläozoikum der Ostalpen (oberes Ordoviz bis Mittel-Devon; Gurktaler Decke, Steirische Grauwak-kenzone, Grazer Paläozoikum) wird ein geodynamisches Modell diskutiert, das sich auf neue Kartierergebmsse und zahlreiche geochemische Analysen von Vulkaniten stützt. Es wird angenommen, daß im oberen Ordovizium einerseits ein passiver Kontinentalrand oder ein vom Meer überflutetes kontinentales Dehnungsfeld bestand, in welchem schwachalkalische Intraplattenbasalte gefördert wurden (Magdalensberg-Serie/Kärnten; Kaser-Serie/Gurktaler Alpen). Andererseits existierte im oberen Ordovizium wahrscheinlich auch eine konvergierende Plattengrenze in Form eines aktiven Kontinentalrandes oder Inselbogens oder m Form eines durch Kontinent-Kontinent-Kollision entstandenen Orogens, welche sich durch die Förderung kalkalkalischer Vulkanite bemerkbar machte (Blasseneck-»Porphyroid«/Steirische Grauwackenzone; Nock-Serie/Gurktaler Alpen). Im unteren Silur kam es zu einer Reorganisation der Plattengrenzen, in deren Folge die oberordovizische Kollisionszone an tiefreichenden Störungen zerbrach. Weitere Dehnungsbewegungen führten im Silur und Devon zur Zerbrechung einer breiten Schelfplattform und zur Förderung von alkalischen Intraplatten-Basalten. Die Dehnungsbewegungen waren für eine Ausdünnung der kontinentalen Kruste und eine damit verbundene generelle Absenkung der Sedimentations-Becken verantwortlich, die wahrscheinlich bis zum Unterkarbon anhielt.

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Résumé L'auteur présente un modèle géodynamique relatif au Paléozoïque inférieur des Alpes orientales (Ordovicien supérieur à Dévonien moyen; nappe de Gurktal, Zone des Grauwackes de la Styrie, Paléozoïque de Graz). Ce modèle se base sur de nouvelles investigations de terrain et sur un grand nombre d'analyses géochimiques de roches volcaniques. Il propose l'existence pendant l'Ordovien supérieur:- d'une part d'une marge continentale passive ou d'une zone de distension continentale submergée accompagnée d'émission de basaltes intraplaques faiblement alcalins (série du Magdalensberg/Carinthie; Série du Kaser/Gurktaler Alpen);- d'autre part d'une limite de plaques convergentes qui revêt la forme d'une marge continentale active, ou d'un arc insulaire, ou d'un orogène formé par une collision continent-continent, comme en témoigne l'éruption de roches volcaniques calco-alcalines à l'Ordovicien supérieur (»Porphyroïd« du Blasseneck/Zone des Grauwackes de la Styrie; Série du Nock/Gurktaler Alpen). Pendant le Silurien inférieur, les limites de plaques sont réorganisées, entraînant la désintégration de la zone de collision ordovicienne le long de fractures profondes. De nouveaux mouvements distensifs au cours du Silurien et du Dévonien mènent à la désintégration d'une vaste plateforme continentale et à l'éruption de basaltes alcalins intraplaques. Les mouvements distensifs sont responsables d'un amincissement de la crôute continentale et d'une subsidence généralisée qui ont probablement duré jusqu'au Carbonifère inférieur.

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Three steps of serpentinization in an eclogitized oceanic serpentinization front (Lanzo Massif – Western Alps)

The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean–continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta‐ophicarbonates bordering the foliated serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P–T (estimated in eclogitized metagabbros at 2–2.5 GPa and 550–620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into serpentinite during alpine subduction and collision. Thus, the serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo‐Moho.     

Geochemistry of the Adamello massif (northern Italy)

The Tertiary Adamello massif, outcropping over an area of more than 550 km² in the southern Alps (northern Italy) is composed mainly of granitoid rocks (granodiorite, tonalite, quartz diorite) with minor amounts of diorite and gabbro. The major and trace element composition of these rocks is comparable to calc-alkaline volcanic rocks of continental margins. The granitoid rocks display spatial and temporal variations in their composition, particularly in Na, P, Sr, La, Nb and Y contents and ⁸⁷Sr/⁸⁶Sr ratios. The variations were probably produced by concurrent contamination/wall-rock assimilation and fractional crystallization of high-alumina basaltic magma.     

Evolution of zircon deformation mechanisms in a shear zone (Lanzo massif,Western-Alps)

Magmatic zircons within two sheared gabbroic dykes from the peridotitic massif of Lanzo (Western-Alps, Italy) revealed evolution of deformation from crystal plasticity to rigid body rotation during shear zone evolution. This is the first time that multiple zircon grains have been analysed in a kinematic context in a shear zone. Zircon grains recorded crystal plastic deformation activating the commonly inferred <100>{001} and <001>{100} glide-systems to the newly identified <001>{110} glide-system. The exact selection of glide-system could be dependant of deformation conditions such as pressure, temperature, and strain rate. Moreover, the activation of one or several glide-systems within a single grain could be favoured by the primary orientation of the grains combined with a high strain rate. In these sheared gabbros, the deformation mechanisms evolve from plastic deformation at low strain rate conditions to increase strain, strain softening and localisation of deformation. The progressive shear zone development and the softening of the matrix relative to the zircon has lead to a switch from crystal-plasticity to rigid body rotation of zircon. The zircon grains rigid body rotation involved that their long axes became parallel to the lineation of the shear zone, causing reorientation and dispersion of the misorientation axes away from kinematic Y.     

Geochemistry of mafic dikes in the Adirondack mountains: implications for late Proterozoic continental rifting

Mafic dikes of late Proterozoic age which cut Grenvillian crust in the northeastern Adirondack Mountains are mostly mildly alkaline basalts except for a few tholeiitic examples. All dikes are high in Ti, P, K, Zr, Y, and LREE, and plot in within-plate fields on tectonic discriminant diagrams. The dikes are similar in composition to Hudson Highland dikes in southern New York and New Jersey and to the Bakersville dike swarm in the southern Appalachians. They differ from the Grenville dike swarm in Ontario and Quebec in being alkaline and having higher Ti and P contents. Mesozoic alkaline dikes in the same geographic area as the Proterozoic ones are strongly enriched in Ba, K, Rb and LREE, and approach lamprophyre in composition. The Proterozoic dikes have low La/Nb and La/Ta ratios, suggesting that subduction-modified mantle lithosphere was not substantially involved in their genesis. This contrasts with certain Mesozoic tholeiitic dikes, associated with the opening of the Atlantic, which show sharp negative Nb or Ta anomalies relative to La indicating they were derived from subduction-modified lithospheric mantle. The trace element chemistry suggests that the source for the Proterozoic dikes was trace element-enriched asthenosphere (OIB-like source), as postulated for certain basalts erupted in the East African Rift system, and in parts of the Basin and Range Province of the southwestern United States of America. Finally, the Proterozoic dikes are chemically similar to rift volcanics from the western Vermont Appalachians, and thus they are thought to represent magmatism associated with extension of the Grenvillian crust prior to opening of the Iapetus ocean.     

Variscan geodynamic evolution of the Carnic Alps (Austria/Italy)

The South-Alpine Carnic Alps are part of the southern flank of the European Variscides and display a continuous sedimentary record from Late Ordovician to Devonian times followed by Carboniferous S-directed nappe stacking and Late Carboniferous to Early Permian post-collisional collapse. The tectonometamorphic and sedimentary evolution of the Carnic Alps resembles a continuous process where pre- and syn-orogenic volcanism, syn-orogenic flysch sedimentation, deformation including nappe stacking, metamorphism and tectonic collapse shift in age from internal zones in the N towards external zones in the S. New structural, petrological and sedimentological data are presented concerning the tectonometamorphic history of the Carnic Alps. We distinguish three thrust sheets or tectonic nappes differing in their stratigraphic, sedimentological, deformational and metamorphic histories which were thrust over each other in Carboniferous times. Our data lead to a new geodynamic model showing an evolution from rifting or back-arc spreading in the Late Ordovician to the establishment of a mature passive continental margin in the Late Devonian/Early Carboniferous, flysch sedimentation in an active continental margin setting during the Visean/Namurian and finally collision during the Late Carboniferous between the northern margin of Gondwana and a microcontinent to the N.     

Post-collisional tectonics in the Northern Cottian Alps (Italian Western Alps)

Field mapping and structural analysis have allowed us to characterise the fault geometry and the post-metamorphic tectonics of an area located in the Northern Cottian Alps (inner Western Alps). Two main faulting stages were distinguished here. The first (Oligocene?-Early Miocene) is related to the development of an E–W-striking left-normal shear zone. This shear zone is interpreted as an antithetical of two regional, N–S right-lateral structures: the Col del Lis-Trana Deformation Zone (LTZ) and the Colle delle Finestre Deformation Zone (CFZ). The second faulting stage (post-Early Miocene) is related mainly to the development of N–S normal faults, coeval with the extensional reactivation of the LTZ and the CFZ. We discuss this kinematic evolution in the framework of the geodynamic evolution of the Western Alps.     

Age and geochemistry of contrasting peridotite types in the Dabie UHP belt,eastern China: Petrogenetic and geodynamic implications

The Dabie ultrahigh-pressure (UHP) metamorphic belt, central China, contains two contrasting types of ultramafic–mafic complex. The Bixiling peridotite in the southern Dabie terrane contains abundant garnet (21–32 vol.%) and thus has high CaO +Al₂O₃ (9.94–15.3 wt.%). The peridotite also has high REE contents with flat REE patterns, high contents of S and other incompatible trace elements, together with low-Mg^# olivine and pyroxene and low Ni and PGE contents. Zircons from this peridotite mostly have low Th/U ratios, interpreted to reflect a metamorphic origin, and give dominantly Triassic ages (ca. 210 Ma). Other zircons with high Th/U ratios give upper intercept ages of 745 Ma. Most zircons have positive ?_Hf (+- 3.6 ～ +- 8.1) values with depleted-mantle model ages (T_DM) of 0.6–1.0 Ga (mean 0.8 Ga) and crustal model ages (T_crust) of 0.8–1.4 Ga (mean 1.1 Ga). We interpret that the Bixiling complex was formed as cumulates in a Neoproterozoic asthenosphere-derived magma chamber in the continental crust, and was later carried to garnet-stable depths (ca. ～ 4.4 GPa) during the subduction of the Yangtze Craton in the Triassic. In contrast, the Raobazhai peridotite in the northern Dabie terrane was metamorphosed at lower P/T conditions (i.e. 15 Kb and 1000 °C). All zircon grains from the peridotite yield Triassic ages (ca. 212 Ma) and have negative ?_Hf values (? 16.6 ～ ? 3.2), Mesoproterozoic model ages (T_DM = 1.0–1.5 Ga) and Paleoproterozoic crustal model ages (T_crust = 1.5–2.3 Ga). The peridotite is enriched in LREE ((La/Yb)_n ≈ 3.5), has high-Mg^# olivine and pyroxene, high Ni and PGEs but low Pd/Ir (mean 3.0). It represents a highly refractory residue of partial melting (up to 18%) of primitive mantle, and is similar to the cratonic mantle xenoliths in Phanerozoic igneous rocks from the eastern North China Craton (e.g. Mengyin and Hebi areas). Negative Ce, Eu and HFSE anomalies in the peridotites suggest that their protolith was derived from the shallow part of the mantle wedge (e.g. plagioclase-stable field) of the North China Craton, and was pulled to a deeper level (e.g. spinel-stable field) during the subduction of the Yangtze Craton. The mantle wedge, like peridotitic xenoliths in the Jurassic Xinyang diatremes at the southern edge of the North China Craton, was metasomatised by fluids/melts released from the subducted continental crust. The fragments of this modified mantle wedge were incorporated into the Yangtze crust during its subduction.     

P–T conditions for the eclogitic re-equilibration of the metaophiolites from Val d'Ala di Lanzo (internal Piemontese zone, Western Alps)

Abstract A study has been made of the high-pressure early-Alpine re-equilibration in the eclogites and metasedimentary cover of the Val d'Ala di Lanzo ophiolite. All of the main high-pressure minerals have been analysed and their compositions used to determine re-equilibration temperatures. The minimum conditions proposed ( P = 1.3 GPa, T = 450–460°C) are also indicated by the presence of a jadeite+quartz-bearing metagranite.
The temperatures are compared with those reported for similar eclogites from the Voltri Group, the Aosta Valley and the Valais. Comparison of recalculated temperatures shows that the temperature (and probably the pressure) of the eclogitic re-equilibration increased in the Aosta Valley and the Valais, in keeping with what has been observed in the internal Penninic basement of the Gran Paradiso and Monte Rosa crystalline massifs.     

Textures and c-axis orientations of deformed quartz crystals from porphyric dikes of the Alpine »Root Zone« (Western Alps)

Deformation textures and c-axis preferred orientations of quartz phenocrysts from porphyric dikes of the Alpine »Root Zone« have been formed under conditions of greenschist to low amphibolite facies. It is shown that the deformation textures as shear planes and kink bands, boudins, grain boundary sutures, subgrains, coarse and fine recrystallized grains, deformation lamellae and fractures are developed in a chronological sequence during first increasing and later decreasing temperatures.The c-axis orientations are chiefly influenced by kinking during the early stage of metamorphism and later by recrystallization but not by the initial orientations of the c-axes. The shape of crossed-girdles is related to the type of strain.At the beginning of polygonization during increasing temperatures basal-a glide as a dominant glide mechanism is replaced by prism-c glide and vice versa during decreasing temperatures. The average angular velocity of c-axes is 0.4 per 1 % flattening.

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Zusammenfassung In Quarz-Einsprenglingen aus porphyrischen Gängen der alpinen »Wurzelzone« sind während der Lepontin-Metamorphose unter Bedingungen der Grünschiefer- bis unteren Amphibolitfazies Deformationstexturen gebildet und die C-Achsen geregelt worden. Bei aufsteigender und wieder sinkender Temperatur entwickeln sich nacheinander Scherflächen und Knickbänder, Boudins, Grenzflächensuturen, Subkörner, grobe und feine Rekristallisate, Deformationslamellen und Brüche.Die C-Achsen-Regelungen werden vor allem durch Verknickung im frühen Metamorphosestadium und später durch Rekristallisation geprägt, jedoch nicht durch die Ausgangslage der C-Achsen. Die Form von Kreuzgürtelregelungen steht zur Art des Strains in Beziehung.Bei aufsteigender Temperatur wechselt mit beginnender Polygonisation der dominierende Deformationsmechanismus von Basis/a/-Gleitung zu Prismen/c/-Gleitung und wieder umgekehrt bei sinkender Temperatur. Die durchschnittliche Drehgeschwindigkeit der Quarz-C-Achsen beträgt 0.4 pro 1% Einengung.

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Résumé Dans les dykes porphyriques de la Zone des racines des Alpes, des textures de déformation et des orientations préférentielles des axes c dans les phénocristaux de quartz ont été engendrées dans des conditions qui vont du facies des schistes verts à celui des amphibolites de faible degré. Au cours de la montée, puis de la descente de la température se sont développés successivement: plans de cisaillement, kinkbands, boudins, sutures intergranulaires, sous-grains, recristallisations grossières et fines, lamelles de déformation et fractures.L'orientation des axes c est influencée principalement par le kinking pendant les premiers stades du métamorphisme et par la recristallisation dans la suite, mais pas par la position initiale des axes. La forme des fabriques en »ceintures croisées« dépend du type de déformation.Au début de la polygonisation, lors de la montée de la température, le mécanisme dominant passe d'un glissement basai a à un glissement prismatique c. L'inverse se produit lors de la baisse de température. La vitesse angulaire moyenne des axes c est de 0,4 par 1 % d'aplatissement.

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Deformation and metamorphism in the Bard area of the Sesia Lanzo Zone, Western Alps, during subduction and uplift

The structure, microstructure and petrology of a small area close to the village of Bard in Val d'Aosta (Italy) has been studied in detail. The area lies across the contact between the Gneiss Minuti (GM) and the Eclogitic Micaschist (EMS) Complexes of the Lower element of the Sesia portion of the Sesia-Lanzo Zone (Western Alps). Both complexes have undergone high-pressure metamorphism, but the metamorphic assemblages indicate a sudden increase in pressure in going across the contact from the GM to the EMS. Therefore, we interpret the contact as a thrust dividing the lower element of the Sesia into two sub-elements. This interpretation is supported by structural evidence.
The early Alpine (90-70 Ma) metamorphic history is best preserved in the EMS and is one of increasing pressure associated with thrusting. The maximum P/T recorded in the EMS is >1500 MPa (>15kbar) and 550°C and in the GM is < 1500-1300 MPa (< 15-13 kbar) and 500-550°C. We suggest that the rocks were probably in an active Benioff zone during this time.
From then on the histories of the GM and EMS are the same. Deformation continued and the thrust and thrust slices were folded during decreasing pressure. We interpret the first postthrusting deformation in terms of uplift associated with continued shortening of the crust and underplating after the Benioff zone had become inactive and a new Benioff zone had developed further to the north-west.
A still later deformation and the Lepontine metamorphism (38 Ma) are related to continued uplift. Much of this deformation is characterized by structures indicative of vertical shortening and lateral spreading as the mountains rose above the general level of the surface.     

On the age of the Vedrette di Ries (Rieserferner) massif and its geodynamic significance

The Rb/Sr whole rock method was used to carry out radiometric analyses on about forty samples from the Vedrette di Ries massif. The samples were arranged in five sample groups. The age of this massif was found to be Middle Oligocene, and the best-defined isochron (30±3 m. y.) was obtained with a group of nine samples from the Upper Anterselva Valley. The values of the initial Sr isotopic ratio range between 0.709 and 0.727. This fact suggests that these melts were the result of the anatexis of different crustal materials, with some contributions from components with low (⁸⁷Sr/⁸⁶Sr)i values. A genetic link is proposed between these Oligocene melts and the Tauern metamorphism.

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Zusammenfassung Für eine radiometrische Gesamtgesteinsanalyse von etwa vierzig Proben des Rieserferner-Massivs, hat man sich der Rb/Sr Methode bedient.Die Proben wurden in fünf Gruppen zusammengestellt.Daraus ergibt sich, daß das Alter dieses Massivs Mittel-oligozän ist.Die beste Isochrone (30±3 M. Jahre) wurde mit einer Gruppe von neun Proben, die aus dem oberen Antholzertal ergaben, erreicht.Die Werte der initialen Sr⁸⁷/Sr⁸⁶-Isotopenverhältnisse sind zwischen 0,709 und 0,727.Das regt zum Denken an, daß diese Schmelze das Ergebnis von Anatexis von verschiedenen Krustengesteinen mit der Hilfe von Komponenten mit niedrigen Werten (⁸⁷Sr/⁸⁶Sr); waren.Zwischen diesen Oligozänschmelzen und der Tauernmetamorphose wird eine genetische Bindung vorgeschlagen.

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Résumé Pour analyser radiométriquement environ quarante échantillons du massif de Riesenferner (Vedrette di Ries) on a employé la méthode Rb/Sr sur roche totale. Les échantillons ont été réunis en cinq groupes. Il en résulte que l'âge de ce massif remonte à l'Oligocène moyen. L'isochrone la meilleure (30±3 m.y.) a été obtenue avec un groupe de neuf échantillons provenant de la haute vallée d'Antholz (Anterselva). Les valeurs des rapports initiaux (⁸⁷Sr/⁸⁶Sr) se placent entre 0,709 et 0,727: Cela suggère que ces liquides ont été le résultat d'une anatexie portant sur différents matériaux de la croûte, avec contribution de composants à basses valeurs (⁸⁷Sr/⁸⁶Sr). On propose un lien génétique entre ces magmas de l'Oligocène et le métamorphisme des Tauern.

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