HP/LT metamorphism in the Volparone Breccia (Northern Corsica, France): evidence for involvement of the Europe/Corsica continental margin in the Alpine subdution zone

The Alpine belt in Corsica (France) is characterized by the occurrence of stacked tectonic slices derived from the Corsica/Europe continental margin, which outcrop between two weakly or non‐metamorphic tectonic domains: the ‘autochthonous’ domain of the Hercynian basement to the west and the Balagne Nappe (ophiolitic unit belonging to the ‘Nappes supérieures’) to the east. These slices, including basement rocks (Permian granitoids and their Palaeozoic host rocks), Late Carboniferous–Permian volcano‐sedimentary deposits, coarse‐grained polymict breccias (Volparone Breccia) and Middle Eocene siliciclastic turbidite deposits, were affected by a polyphase deformation history of Alpine age, associated with a well‐developed metamorphic recrystallization. This study provides new quantitative data about the peak of metamorphism and the retrograde P–T path in the Alpine Corsica: the tectonic slices of Volparone Breccia from the Balagne region (previously regarded as unmetamorphosed) were affected by peak metamorphism characterized by the phengite + chlorite + quartz ± albite assemblage. Using the chlorite‐phengite local equilibria method, peak metamorphic P–T conditions coherent with the low‐grade blueschist facies are estimated as 0.60 ± 0.15 GPa and 325 ± 20 °C. Moreover, the retrograde P–T path, characterized by a decrease of pressure and temperature, is evidence of the first stage of the exhumation path from the peak metamorphic conditions to greenschist facies conditions (0.35 ± 0.06 GPa and 315 ± 20 °C). The occurrence of metamorphic peak at high‐pressure/low‐temperature (HP/LT) conditions is evidence of the fact that these tectonic slices, derived from the Corsica/Europe continental margin, were deformed and metamorphosed in the Alpine subduction zone during their underplating at ～20 km of depth into the accretionary wedge and were subsequently juxtaposed against the metamorphic and non‐metamorphic oceanic units during a complex exhumation history.     

Regional metamorphism and tectonic evolution of the Inner Mongolian suture zone

Abstract Regional metamorphism in central Inner Mongolia has occurred during four different periods: the middle Proterozoic, the early Palaeozoic, the middle Palaeozoic and the late Palaeozoic tectonic cycles. The middle Proterozoic and late Palaeozoic metamorphic events are associated with rifting and are characterized by low-pressure facies series. The early Palaeozoic metamorphism occurred in two stages: (1) subduction zone metamorphism resulted in paired metamorphic belts in the Ondor Sum ophiolite and Bainaimiao island arc complex; and (2) orogenic metamorphism occurred during the collision of an island arc with the continent. Two types of middle Palaeozoic metamorphism are represented: (1) subduction zone metamorphism, which affected the melange; and (2) orogenic metamorphism that resulted from continent–continent collision.     

Fault-related rocks: deciphering the structural–metamorphic evolution of an accretionary wedge in a collisional belt,NE Sicily

The Alpine chain exposed in the Western Mediterranean area represents a front several kilometres in width, dismembered by more recent tectonics and by opening of the Tyrrhenian Basin. In most exposures of this mountain belt, relics of older metamorphic rocks occur. The deformational sequence of events may be revealed by the recognition of metamorphic records associated with different structures. Within a tract of the Alpine front cropping out in the Peloritani Mountains (NE Sicily), we distinguished two metamorphic complexes characterized by different tectonometamorphic histories. Their present tectonic juxtaposition is a cataclastic thrust linked to the recent Africa-verging Sicilian–Maghrebian fold-and-thrust belt. The Lower Complex is characterized by Hercynian metamorphism (P > 0.2 GPa and T ≈ 350°C) exclusively. It essentially consists of very low-grade metapelites and metavolcanic rocks overlain by an unmetamorphosed sedimentary cover. The Upper Complex, comprising different tectonic slices, consists of medium- to high-grade Hercynian metamorphic rocks (P?=?0.3–0.8 GPa and T up to 650°C) with Alpine metamorphic overprint (T > 250°C) affecting also the Mesozoic–Cenozoic cover. Lithotypes, structures, and inferred P–T conditions of investigated rocks suggest the existence of an Alpine accretionary wedge during the Cretaceous deformational collision. Within the Upper Complex, a polyphase Palaeogene mylonitic horizon involving rocks belonging to different tectonic slices fully preserves the tectonometamorphic evolution. For this reason, we focused our attention on these sheared rocks in order to reconstruct the entire tectonic history of this geologically complex area. Our new basic model allows the complex structure of the nappe-pile edifice of the Peloritani Mountains to be simplified, casting new light on the tectonic evolution of this key sector of the southern Calabrian-Peloritani Orogen.     

Timing of metamorphism in the Tauern Window, Eastern Alps: Rb-Sr ages and fabric formation

The Peripheral Schieferhülle of the Tauern Window of the Eastern Alps represents post-Hercynian Penninic cover sequences and preserves a record of metamorphism in the Alpine orogeny, without the inherited remnants of Hercynian events that are retained in basement rocks. The temperature-time-deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whose P-T evolution and structural setting are already well understood. The Eclogite Zone of the central Tauern formed from protoliths with Penninic cover affinities, and suffered early Alpine eclogite facies metamorphism before tectonic interposition between basement and cover. It then shared a common metamorphic history with these units, experiencing blueschist facies and subsequent greenschist facies conditions in the Alpine orogeny. The greenschist facies phase, associated with penetrative deformation in the cover and the influx of aqueous fluids, reset Sr isotopes in metasediments throughout the eclogite zone and cover schists, recording deformation and peak metamorphism at 28-30 Ma. The Peripheral Schieferhülle of the south-east Tauern Window yields Rb-Sr white mica ages which can be tied to the structural evolution of the metamorphic pile. Early prograde fabrics pre-date 31 Ma, and were reworked by the formation of the large north-east vergent Sonnblick fold structure at 28 Ma. Peak metamorphism post-dated this deformation, but by contrast to the equivalent levels in the central Tauern, peak metamorphic conditions did not lead to widespread homogenization of the Sr isotopes. Localized deformation continued into the cooling path until at least 23 Ma, partially or wholly resetting Sr white mica ages in some samples. These isotopic ages may be integrated with structural data in regional tectonic models, and may constrain changes in the style of crustal deformation and plate interaction. However, such interpretations must accommodate the demonstrable variation in thermal histories over small distances.     

Metamorphism of an ophiolitic tectonic mélange, northern California Klamath Mountains, USA

ABSTRACT Mineral assemblages in pelitic, mafic, calcareous and ultramafic rocks within a metamorphosed tectonic mélange indicate that the Marble Mountain terrane and adjacent Western Hayfork subterrane (northern California) underwent regional low- to medium-pressure amphibolite facies metamorphism. Metamorphic conditions estimated by comparison of observed assemblages with experimentally-determined reaction boundaries and by geothermometry constrain metamorphic temperatures between about 500° and 570°C. The occurrence of andalusite in regionally metamorphosed pelites indicates pressures below about 370 MPa. Metabasite amphibole compositions also suggest low to intermediate metamorphic pressures. Metaserpentinites containing the upper amphibolite facies assemblage (olivine + enstatite + anthophyllite) are found locally within the study area and have been reported previously by other workers elsewhere in the Marble Mountain terrane. These assemblages may reflect higher temperatures of recrystallization than assemblages in surrounding rocks and may represent vestiges of an earlier high-temperature metamorphic event undergone by the ultramafic rocks prior to incorporation in the mélange. Although the age of the low- to intermediate-pressure metamorphism is poorly constrained, cross-cutting plutons indicate that metamorphism must be older than about 162 Ma. Therefore this regional metamorphic event, which probably marks the accretion of these terranes to the North American continental margin, is older than the currently accepted 151–147 Ma age of the Nevadan event in the Klamath Mountains. The inferred low to intermediate pressures of metamorphism and the lithologies of the protoliths suggest a near-arc tectonic setting and refute a subduction zone model for this event.     

P-T conditions of earliest Late Cretaceous metamorphism in the Western Sonobari Complex,northwestern Mexico: tectonic implications

The Western Sonobari Complex in northwestern Mexico consists of metamorphosed rocks mostly derived from Palaeozoic (?) sedimentary and Mesozoic igneous protoliths. Rocks of this complex display amphibolite facies orogenic metamorphism, pervasive foliation, migmatization, and four folding phases. These features are ascribed to a contractional tectonic event with NNW–SSE shortening direction, which caused thrusting, thickening of the crust, and sinking of the lithological units. U–Pb geochronology of migmatitic leucosome bands indicates that peak metamorphic conditions were reached between ~93 and 89 Ma. Post-tectonic Late Cretaceous peraluminous aplite-pegmatite dikes transect the metamorphic foliation. Traditional thermobarometry in the metamorphic rocks yields average pressures and temperatures of 9.0–7.1 kbar and 745–663°C, typical of intermediate P/T Barrovian metamorphism. On the basis of its age and contractional character, the thickening event originating the metamorphism may be related to collision of the Alisitos island arc against crustal blocks of Mexico. Thermobarometric data of post-tectonic intrusives including Late Cretaceous granodiorite and Eocene gabbro indicate emplacement within an overthickened crust, while P-T conditions of post-tectonic dikes point towards an almost isothermal decompression path along the amphibolite facies field. Rock units of similar age and metamorphic character are discontinuously exposed from the Islas Marias offshore the Nayarit coast to the Peninsular Ranges batholith of Baja California, and even extend north into the Sierra Nevada batholith and the Sevier hinterland. This extensive belt of Barrovian metamorphic rocks thus provides a record of middle Cretaceous shortening and crustal thickening related to arc-continent collision followed by subduction resuming.     

The Arosa zone in Eastern Switzerland: oceanic, sedimentary burial, accretional and orogenic very low- to low grade patterns in a tectono-metamorphic mélange

In the area of Arosa?CDavos?CKlosters (Eastern Switzerland) the different tectonic elements of the Arosa zone mélange e.g. the Austroalpine fragments, the sedimentary cover of South Penninic ophiolite fragments, as well as the matrix (oceanic sediments and flysch rocks) show distinctively different metamorphic histories and also different climaxes (??peaks??) of Alpine metamorphism. This is shown by a wealth of Kübler-Index, vitrinite and bituminite reflectance measurements, and K-white mica b cell dimension data. At least six main metamorphic events can be recognized in the area of Arosa?CDavos?CKlosters: (1) A pre-orogenic event, typical for the Upper Austroalpine and for instance found in the sediments at the base of the Silvretta nappe but also in some tectonic fragments of the Arosa zone (Arosa zone mélange). (2) An epizonal oceanic metamorphism observed in the close vicinity of oceanic basement rocks units of the Arosa zone (South Penninic) is another pre-orogenic process. (3) A metamorphic overprint of the adjacent Lower Austroalpine nappes and structural fragments of the Lower Austroalpine in the Arosa zone. This metamorphic overprint is attributed to the orogenic metamorphic processes during the Late Cretaceous. (4) A thermal climax observed in the South Penninic sediments of the Arosa zone can be bracketed by the Austroalpine Late Cretaceous event (3) and the middle Tertiary event (5) in the Middle Penninic units and predates Oligocene extension of the ??Turba phase??. (6) North of Klosters, in the northern part of our study area, the entire tectonic pile from the North Penninic flysches to the Upper Austroalpine is strongly influenced by a late Tertiary high-grade diagenetic to low-anchizone event. In the Arosa zone mélange an individual orogenic metamorphic event is evidenced and gives a chance to resolve diagenetic?Cmetamorphic relations versus deformation. Six heating episodes in sedimentary rocks and seven deformation cycles can be distinguished. This is well explained by the propagation of the Alpine deformation front onto the foreland units. Flysches at the hanging wall of the mélange zone in the north of the study area (Walsertal zone) show data typical for low-grade diagenetic thermal conditions and are therefore sandwiched between higher metamorphic rock units and separated from theses units by a disconformity. The Arosa zone s.s., as defined in this paper, is characterised by metamorphic inversions in the hanging wall and at the footwall thrust, thus shows differences to the Walsertal zone in the north and to the Platta nappe in the south.     

构造动力变质作用初论

本文提出了构造动力变质作用的基本概念。即在构造动力及其引发的动力热流共同作用下,岩石、矿物在构造运动中发生的形变、相变而呈现的变质作用。按构造动力变质作用的物理化学条件,动力变质岩的结构构造、动态重结晶和混合岩化作用、应力矿物共生组合关系及其空间分布等特征,将构造动力变质作用由弱到强划分为4个发展阶段和相应的4种基本变质类型——脆性、脆-塑性、塑性和动力热流变形变质,并论述了它们各自的基本特征。      

Garnet evolution in pre-Variscan pelitic rocks from the Lake Emosson area, Aiguilles Rouges Massif, Western Alps

Abstract The chemical evolution of garnets from pelitic rocks of probable Palaeozoic age corresponds to a complex metamorphic evolution of the host rocks.
Among the almandine-rich garnets (Alm_60–80), two main types of evolution can be distinguished. Early Mn-rich garnets coexisting with kyanite may be replaced by plagioclase and then, during a late stage, by biotite and/or sillimanite. The second type of evolution corresponds to an overgrowth of Mn-poor late-stage garnet on older Mn-rich garnets which corresponds to a thermal peak with sillimanite-type of metamorphism. This new garnet may appear either as an overgrowth with a strong discontinuity, or as small, new euhedral garnet or as skeletal garnet.
This chemical evolution of garnet corresponds to an early collisional stage of metamorphism (of high pressure type with high Mn values) of probable Ordovician age followed by uplift and a thermal peak (low Mn values) in Devonian times.     

U?Pb and 40Ar?39Ar geochronology of Palaeozoic units in the northern Apennines: determining protolith age and alpine evolution using the Calamita Schist and Ortano Porphyroid

In the northern Apennines, the Palaeozoic basement involved in the Late Oligocene–Middle Miocene nappe stack contains metamorphic units for which hypothetical ages have been assigned on the basis of lithological correlations with the Palaeozoic formations of the Variscan chain in Sardinia. This uncertainty concerning the age poses limitations to reconstructing the Palaeozoic stratigraphy, defining the Alpine and pre‐Alpine histories and correlations with other domains of the Variscan chain. We present the U Pb age of detrital zircon and the ⁴⁰Ar ³⁹Ar age of metamorphic muscovite for the Calamita Schist and Ortano Porphyroid, two metamorphic units of undetermined Palaeozoic age cropping out in the eastern Elba Island. The radioisotopic data allows us to: (i) define the Early Carboniferous and Middle Ordovician ages for the Calamita Schist and Ortano Porphyroid, respectively, as well as their derivation (flysch deposit and magmatic rocks); (ii) pose some constraints concerning their alpine tectonic and metamorphic histories. These new data generate a more precise reconstruction of the Palaeozoic sequence in the northern Apennines, and they document that the Palaeozoic basement involved in the alpine deformation underwent internal stacking with an inversion of the original sequence. Copyright © 2010 John Wiley & Sons, Ltd.     

Accretion and post-accretion metamorphism in subduction complex terranes of the New England Fold Belt, eastern Australia

Abstract Regional metamorphic rocks that form Late Palaeozoic subduction complexes in central Queensland, Australia, are products of two metamorphic episodes. Synaccretion metamorphism (M1) gave rise to prehnite-pumpellyite and greenschist facies rocks, whereas a subsequent episode (M2) at about 250 Ma formed upper greenschist to upper amphibolite facies rocks of both intermediate- and low-pressure type, probably in a compressive arc or back-arc setting. A similar pattern can be recognized for 1000 km along the New England Fold Belt, although at several localities, where higher grade rocks are exposed, metamorphism was essentially continuous over the M1-M2 interval, with a rapid rise in geothermal gradient at the end of accretion. Where out-stepping of tectonic elements has occurred at long-lived convergent margins elsewhere, similar overprinting of high- by lower-pressure facies series is anticipated, complicating the tectonic interpretation of metamorphism. The discrete character of metamorphic events may be blurred where conditions giving rise to a major episode of accretion and out-stepping are followed by the subduction of a major heat source.     

德国的厄尔士山脉——北冈瓦纳的俯冲部分:在变质逆冲单元中重现早古生代变质沉积层序的地球化学证据

作为波希米亚地块的一个主要变质地体，厄尔士山脉记录了风瓦纳古生代边缘的储冲部分。低绿片岩到变粒岩相变质作用的不同质单元的非钙质沉积物的地球化学研究支持了近来建立的逆冲模式，厄尔士山脉变质沉积物的地球化学识别和对比表明在变质逆冲单元中重现了早古生代变质沉积层序。这个新发现与变质流纹岩夹层的最新放射性测年结果相一致，放射性测年值为480Ma左右。此外，与图林根低级变质标准剖面的对比也支持了这种观点。图林根剖面具有较高成熟度的沉积物，代表从造山带过渡到被动边缘环境。还识别出至少在厄尔士山脉的三个变质单元中再现的三种不同的岩石类型的重要地球化学特征。通过使用平均值的简单对比和统计学方法，建立了这些单元的地球化学对比模式。变质岩组不同岩类的重要地球化学特征对地形分析和古构造背景建具有重要意义。前进变质单元中的岩性恢复及其不同的化学成分可用来研究在巴罗型变质作用中元素的活动性。统计对比显示，从绿片岩到角闪岩相Li逐渐减少，而Ca则少量富集，其它被研究的元素均是不活动的。     

Metamorphism of an Early Palaeozoic continental margin, western Baie Verte Peninsula, Newfoundland

Metamorphic mineral assemblages and textures from Early Palaeozoic continental margin rocks in north-western Newfoundland indicate that different structural levels have contrasting metamorphic histories. Rocks of the East Pond Metamorphic Suite, which represent the older, structurally lower level of the margin, experienced an early high-pressure–low-temperature stage of metamorphism (10–12 kbar minimum, 450–500°C) which produced eclogite in mafic dykes and phengite–garnet assemblages in pelites. This was overprinted by higher temperature–lower pressure amphibolite facies metamorphism (700–750°C, 7–9 kbar minimum) which produced complex symplectic textures in rocks of all compositions. Rocks of the Fleur de Lys Supergroup, which were deposited in the stratigraphically higher levels of the rifted margin, reached pressures of 7–8.5 kbar at about 450°C during the early stages of metamorphism, overprinted by assemblages which indicate maximum temperatures of 550–600°C at about 6.5 kbar. The metamorphic history of both units is interpreted to be the result of thermal relaxation following initial burial of a continental margin by overriding thrust sheets. Since there is no evidence that maximum pressures or temperatures within the Fleur de Lys Supergroup were ever as high as those reached in the East Pond Metamorphic Suite, these rocks may have followed parallel, 'nested' P–T–t paths, with the more deeply buried East Pond Metamorphic Suite subjected to greater thermal relaxation effects. Quantitative modelling of P–T–t paths is not possible with the present data, owing to both large uncertainties in P–T estimates, and in the time of metamorphism.     

苏鲁变质带北部的岩石构造单元及结晶块体推覆构造

虽然苏鲁变质带北部的超高压变质岩石的类型及其特征与大别山超高压带相似,但是要划出与大别山相对应的岩石构造单元是困难的。详细的区域地质、岩石学、同位素年代学及地球化学的研究已将超高压带的西界大致圈定在牟平断裂至米山断裂的范围内。特别需要提出的是,在荣成超高压变质岩石分布区的南北两侧,出露有3 种不同成因的麻粒岩,即榴辉岩化的麻粒岩;由超高压变质岩经高压升温变质作用生成的麻粒岩;以及未经过榴辉岩相变质的麻粒岩。它们有规律地成带分布。这些麻粒岩带以及在荣成地区出露的未经过麻粒岩相叠加变质的超高压变质带,各自都有着完全不同的变质历史,并且都以深大韧性剪切带为其边界。由此笔者将苏鲁变质带北部由南向北划分为海阳所变质地块(榴辉岩化扬子陆块基底变质单元)、荣成变质地块(超高压变质单元)、威海变质地块(麻粒岩相叠加变质的超高压构造岩片)、昆嵛山边界杂岩带。这些来源于地壳深层的结晶块体是超高压带形成和演化的产物,在后来的进一步碰撞挤压中,这些来源于深层而就位于中上地壳水平的结晶块体,有可能发生了与薄皮构造机制(thin-skin thrust)类似的构造过程。它们有如一系列的推覆体挤压叠置,使华北和华南陆块最后挤压在一起。     

Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps

Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross-section in the central Lepontine Alps in the Nufenen Pass area, Switzerland. Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone. The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther-mobarometers for the maximum post-D2 por-phyroblast stage are from North to South: 500° C at 5-6 kbar and 600° C at 7-8 kbar. Detailed thermobarometry of garnet por-phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P-T-path, post-D2 rims grew with increasing temperature but decreasing pressure, and finally post-D3 garnet formed along a retrograde P-T-path. It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post-D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel. Much of the observed metamorphic pattern can be explained as the result of a significant post-D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got-thard massif with its cover. Whereas, in the Pennine metasediments, post-D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure. The interpretation given here for the Nufenen Pass area may also apply to the Luk-manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic-Penninic contact zone.     

Significance and Petrochemistry of Upper Mantle–mantle Transition Zone Metaperidotites and Oceanic Crust Metagabbros within the Kazdağı Metaophiolite: Implications for the Geological Evolution of the Izmir–Ankara–Erzincan Ocean,Northwestern Turkey

The Kazda?? metaophiolite crops out in the Kazda?? (Ida) Mountains in the Biga Peninsula in northwestern Turkey. It is in stratigraphic contact with the high–grade metamorphic rocks of the Kazda?? Massif. Metaophiolitic and high–grade metamorphic rocks are tectonically overlain by low–grade metamorphic units of the Permo‐Triassic Karakaya Complex of the Sakarya Zone. Late Oligocene‐Early Miocene granites intruded these tectonic units (Okay and Sat?r, 2000; Duru et al. 2012). In the Kazda?? metaophiolitic sequence, upper mantle peridotites are represented by metaharzburgite and metadunite, whereas the mantle transition zone metaperidotites are composed of metadunite, metapyroxenite and minor plagioclase‐bearing metalherzolite. The upper part of the metadunites in the mantle transition zone show intercalation with metagabbros. Gabbros of oceanic crust experienced amphibolite facies metamorphism and are transformed into amphibolite, garnet amphibolite and migmatitic gabbros. The metagabbros and amphibolites display MORB‐ and IAT‐like geochemical features. The Kazda?? metaophiolite is conformably overline by basal conglomerates and hemi‐pelagic carbonate rocks continuing upward into forearc‐type flysch–like detrital sedimentary rocks interspersed with mafic volcanic intervals. These cover units underwent high–grade metamorphism into gneisses, migmatites, amphibolites and marbles in a compressional regime during the Alpine orogeny. New U–Pb zircon data from the metagabbros show two crystallization peaks at ～52 Ma and ～73 Ma. This has implications for the age of subduction of the Izmir–Ankara–Erzincan Ocean, generally assumed to be northward under the Sakarya Zone. During the Triassic to Middle Eocene, progressive overthrusting of the Sakarya Zone via a N–S compresional regime created by the Alpine orogeny onto subduction–accretion‐ and forearc‐units resulted in high–grade metamorphic conditions in the Biga Peninsula.     

Origin and metamorphism of ultrabasic rocks associated with a subducted continental margin, Naxos (Cyclades, Greece)

Meta-peridotites outcropping at different structural levels within the Alpine metamorphic complex of the Cycladic island of Naxos were studied to re-examine their metamorphic evolution and possible tectonic mechanisms for emplacement of mantle material into the continental crust. The continental margin section exposed on Naxos, consisting of pre-Alpine basement and c. 7 km thick Mesozoic platform cover, has undergone intense metamorphism of Alpine age, comprising an Eocene (M1) blueschist event strongly overprinted by a Miocene Barrovian-type event (M2). Structural concordance with the country rocks and metasomatic zonation at the contact with the felsic host rocks indicate that the meta-peridotites have experienced the M2 metamorphism. This conclusion is supported by the similarity between metamorphic temperatures of the ultrabasic rocks and those of the host rocks. Maximum temperatures of 730–760 °C were calculated for the upper-amphibolite facies meta-peridotites (Fo–En–Hbl–Chl–Spl), associated with sillimanite gneisses and migmatites. Relict phases in ultrabasics of different structural levels indicate two distinct pre-M2 histories: whereas the cover-associated horizons have been affected by low-grade serpentinization prior to metamorphism, the basement- associated meta-peridotites show no signs of serpentinization and instead preserve some of their original mantle assemblage. The geochemical affinities of the two groups are also different. The basement-associated meta-peridotites retain their original composition indicating derivation by fractional partial melting of primitive lherzolite, whereas serpentinization has led to almost complete Ca-loss in the second group. The cover-associated ultrabasics are interpreted as remnants of an ophiolite sequence obducted on the adjacent continental shelf early in the Alpine orogenesis. In contrast, the basement-associated meta-peridotites were tectonically interleaved with the Naxos section at great depth during the Alpine collision and high P/T metamorphism. Their emplacement at the base of the orogenic wedge is inferred to have involved isobaric cooling from temperatures of c. 1050 °C within the spinel lherzolite field to eclogite facies temperatures of c. 600 °C.     

Tectonic Units and Their Fundamental Characteristics on the Northern Margin of the Alxa Block

The northern margin of the Alxa block is the junction of a tectonic units. Four first-order tectonic units are distinguished: 1. the Yagan structural zone characteristic of an immature island arc; 2. the Zhusileng-Hangwula structural zone, which was a passive continental margin in the Early Palaeozoic and was transformed into an active continental margin in the Late Palaeozoic;3. the Shalazha structural zone characteristic of a mature island arc; 4. the Nuru-Langshan structural zone, which was a Proterozoic orogenic belt and later evolved into an extensional transtional crust in the Palaeozoic. The above-mentioned tectonic units differ remarkably in sedimentary formations, magmatic rock associations, metamorphism and geochemistry and are bounded by faults between one another.     

The Bocchigliero Palaeozoic sequence in the context of the Calabrian–Peloritan Hercynian Range (Italy)

The Calabrian–Peloritan Hercynian Range includes three weakly metamorphosed Palaeozoic sequences cropping out in north-eastern Sila (Bocchigliero sequence), southern Sila, Serre and Aspromonte (Stilo sequence), and in the Peloritan Mountains (Peloritan sequence). The work reported here considers the Bocchigliero sequence and comprises part of a geological, petrological and geochemical research programme on the Palaeozoic evolution of the Calabrian–Peloritan Arc. The Bocchigliero sequence constitutes the lower tectonic unit of the Hercynian Caiabrian–Peloritan Range and is overthrusted by the metamorphic Mandatoriccio Unit. The Bocchigliero sequence is a terrigenous–carbonate–volcanic association, is affected byclow grade metamorphism, contains Cambro-Ordovician fossils and extends in age from the Cambrian to the Devonian. The terrigenous material is represented by meta-arenites and metapelites (Cambrian–Devonian); the volcanics include metatuffites (Cambrian and Ordovician), metabasalts (Cambro-Ordovician), metaandesites and metarhyolites (Ordovician and Siluro-Devonian); limestone beds are present in the Devonian. It is believed that the Palaeozoic Bocchigliero basin formed in the Cambrian on a continental crust in which the rocks constituting today's Mandatoriccio Unit were located at 3–8 km depth. The crustal thinning in the Cambro-Ordovician led to fracturing and upwelling of alkaline within-plate basaltic magmas, whereas in the Ordovician the thinning took place under conditions of higher plasticity. In this latter period an increase in temperature resulting from mantle upwelling produced crustal partial melts of andesite and rhyolite composition. In addition, this thermal uprise was responsible for regional metamorphism characterized by low pressures and by the absence of penetrative deformation. The effects of this metamorphism are well developed in the rocks of the Mandatoriccio Unit. In the Silurian and Devonian, progressive closing of the basin took place. The Palaeozoic sequence was then subjected to Variscan low pressure–low temperature metamorphism and Alpine deformation.     

Eclogite facies relics and a multistage breakdown in metabasites of the KTB pilot hole,NE Bavaria: implications for the Variscan tectonometamorphic evolution of the NW Bohemian Massif

Complex reaction textures in coronitic metagabbros and retrograded eclogites of the KTB pilot and an adjacent drilling provide evidence for a multistage metamorphic history in the Variscan basement of the NW Bohemian Massif. The eclogites show complete metamorphic recrystallization leaving no textural or mineral relics of their igneous precursors. In contrast, textural relics of the igneous protolith are still preserved in the metagabbros where the metamorphic overprint under high pressure conditions achieved only partial replacement of the initial assemblage plagioclase + augite + amphibole (+olivine or orthopyroxene?) + ilmenite to form the eclogite facies assemblage garnet + omphacite + kyanite + zoisite + quartz+rutile. The garnets in the metagabbros occur in the typical ‘necklace’ fashion at the borders between the original plagioclase and mafic phase domains. In the same rocks, omphacite formed by a topotactic reaction mechanism replacing igneous augite as well as in smaller grains at the margins of the texturally igneous clinopyroxene where it occurs without fixed orientation with respect to the relict phase. Both eclogites and metagabbros show a partial breakdown under high pressure granulite (transitional to high pressure amphibolite) facies conditions during which omphacite broke down to vermicular symplectites of diopside + plagioclase. A later pervasive medium pressure metamorphism under amphibolite facies conditions led to the development of assemblages dominated by hornblende + plagioclase+titanite: phases prevailing in the overwhelming majority of the surrounding metabasites. Subsequent vein-associated retrogression produced minerals typical of the greenschist to zeolite facies. All metamorphic stages may be represented in a single thin section but although the overall reaction sequence is apparent, the obvious disequilibrium in the rocks makes the use of conventional geothermobarometry difficult. However, calculations made by assuming an approach to domainal equilibrium show that both the eclogite facies and early breakdown occurred above 10 kb. As the metamorphic unit hosting these particular metabasites is generally characterized by pressures below 10 kb these results have important implications for understanding the tectonometamorphic evolution of the region. The relationship between the studied rocks and other units in the NW Bohemian Massif exhibiting a multistage metamorphic evolution is discussed and possible tectonic models evaluated.