Fluid evolution during burial and Variscan deformation in the Lower Devonian rocks of the High-Ardenne slate belt (Belgium): sources and causes of high-salinity and C–O–H–N fluids

Fluid inclusions in quartz veins of the High-Ardenne slate belt have preserved remnants of prograde and retrograde metamorphic fluids. These fluids were examined by petrography, microthermometry and Raman analysis to define the chemical and spatial evolution of the fluids that circulated through the metamorphic area of the High-Ardenne slate belt. The earliest fluid type was a mixed aqueous/gaseous fluid (H₂O–NaCl–CO₂–(CH₄–N₂)) occurring in growth zones and as isolated fluid inclusions in both the epizonal and anchizonal part of the metamorphic area. In the central part of the metamorphic area (epizone), in addition to this mixed aqueous/gaseous fluid, primary and isolated fluid inclusions are also filled with a purely gaseous fluid (CO₂–N₂–CH₄). During the Variscan orogeny, the chemical composition of gaseous fluids circulating through the Lower Devonian rocks in the epizonal part of the slate belt, evolved from an earlier CO₂–CH₄–N₂ composition to a later composition enriched in N₂. Finally, a late, Variscan aqueous fluid system with a H₂O–NaCl composition migrated through the Lower Devonian rocks. This latest type of fluid can be observed in and outside the epizonal metamorphic part of the High-Ardenne slate belt. The chemical composition of the fluids throughout the metamorphic area, shows a direct correlation with the metamorphic grade of the host rock. In general, the proportion of non-polar species (i.e. CO₂, CH₄, N₂) with respect to water and the proportion of non-polar species other than CO₂ increase with increasing metamorphic grade within the slate belt. In addition to this spatial evolution of the fluids, the temporal evolution of the gaseous fluids is indicative for a gradual maturation due to metamorphism in the central part of the basin. In addition to the maturity of the metamorphic fluids, the salinity of the aqueous fluids also shows a link with the metamorphic grade of the host-rock. For the earliest and latest fluid inclusions in the anchizonal part of the High-Ardenne slate belt the salinity varies respectively between 0 and 3.5 eq.wt% NaCl and between 0 and 2.7 eq.wt% NaCl, while in the epizonal part the salinity varies between 0.6 and 17 eq.wt% NaCl and between 3 and 10.6 eq.wt% for the earliest and latest aqueous fluid inclusions, respectively. Although high salinity fluids are often attributed to the original sedimentary setting, the increasing salinity of the fluids that circulated through the Lower Devonian rocks in the High-Ardenne slate belt can be directly attributed to regional metamorphism. More specifically the salinity of the primary fluid inclusions is related to hydrolysis reactions of Cl-bearing minerals during prograde metamorphism, while the salinity of the secondary fluid inclusions is rather related to hydration reactions during retrograde metamorphism. The temporal and spatial distribution of the fluids in the High-Ardenne slate belt are indicative for a closed fluid flow system present in the Lower Devonian rocks during burial and Variscan deformation, where fluids were in thermal and chemical equilibrium with the host rock. Such a closed fluid flow system is confirmed by stable isotope study of the veins and their adjacent host rock for which uniform δ¹⁸0 values of both the veins and their host rock demonstrate a rock-buffered fluid flow system.     

RECENT STUDIES OF THE PALEOZOIC GROUP OF JAPAN

The history and research on the Paleozoic formations in Japan is summarized.

In the past the Japanese Paleozoic was dealt with as one system under the general name Chichibu Paleozoic, as the Chichibu system in the Kanto region was thought to represent all Paleozoic formations in Japan. However, since the discovery of Lower Carboniferous fossils by Ichiro Hayasaka in 1924, Upper Devonian fossils by Mitsuo Noda in 1934 and Gotlandian fossils by Yoshio Onuki in 1937, the distribution and stratigraphic sequence of the Japanese Paleozoic have become better known.

The Hida metamorphic rocks constitute the basement of Paleozoic sedimentary basins and are assigned to pre-Gotlandian age.

The Gotlandian system is found in regions of complex geologic structure. Its distribution, though sporadic, may bear an important significance in view of structural geology. It consists predominantly of fossiliferous limestone, associated with shale, slate, phyllite and tuff.

Distribution of the Devonian system is limited. It consists of sandstone, shale and slate, accompanied by tuff and limestone lenses. Conglomerate of schistose pebbles is locally found. The system may be conformable with the underlying Gotlandian system.

The carboniferous system is widely distributed, usually associated with the Permian, and consists chiefly of sandstone, shale, slate and schalstein, but is locally represented by limestone facies. The Carboniferous is generally uncomformable with the Permian, although locally it may be conformable.

The Permian system has the widest distribution, amounting to nearly 80 percent of the known Paleozoic, hence its stratigraphic succession, fossil horizons and sedimentary facies are fairly well known. However, varied names now in use for the respective series require unification. The system consists of sandstone, shale, slate, schalstein, chert, often accompanied by limestone and conglomerate. Existence of conglomerate is one of the characteristics of the Permian system, although the constituent pebbles vary regionally and further study is needed to clarify the distribution, stratigraphic position and lithology of the conglomerates.

According to Hisakatsu Yabe, the name Chichibu system should be used only for the Carboniferous and Permian and a new name Kitakami system be used to denote the Gotlandian and Devonian.

Major orogenic movements of the Japanese Paleozoic are, 1) Akiyoshi orogenesis, 2) Setamai fold, 3) Shimizu fold, 4) Kesen fold and 5) Hida orogenesis. — Reiko Fusejima     

Anchimetamorphism of the Neoproterozoic and the Lower Paleozoic along the Profile of Yuanguping in Western Hunan Province, China

The Neoproterozoic and Lower Paleozoic along the profile of Yuanguping in western Hunan Province, China underwent anchimetamorphism. The illite crystallinity (IC) of the <2 μm fractions ranges from 0.23-0.34°△2θfor the Neoproterozoic to 0.23-0.35°△A2θ for the Lower Paleozoic (calibrated with the Kisch IC set, Kisch, 1991). This indicates that the metamorphic grade of the Neoproterozoic and Lower Paleozoic is the anchizone. The peak metamorphic temperature is estimated to be 290-210℃. This result does not agree with the greenschist or subgreenschist facies of the Banxi Group, nor with the lower-greenschist facies or sedimentary cover of the Sinian to Lower Paleozoic, as most previous researchers thought. The illite (K-mica) b0 values range from 0.9074 to 0.8963 (nm) for the Neoproterozoic and the Lower Paleozoic. Based on cumulative frequency curves of the illite (K-mica) b0, the peak metamorphic pressure of the Banxi Group was derived to be of a type that is slightly higher than that of the N. Ne     

Chlorite crystallinity: an empirical approach and correlation with illite crystallinity, coal rank and mineral facies as exemplified by Palaeozoic and Mesozoic rocks of northeast Hungary

Abstract Fairly strong (r= 0.75–0.85) positive linear correlations were found between crystallinity indices (peak widths) measured on the first two basal reflections of chlorite and those of illite–muscovite in <2-μm fractions of a representative shale–slate–phyllite series from Palaeozoic and Mesozoic formations of northeast Hungary. The metamorphic grade ranges from late or deep diagenesis through anchizone to epizone conditions. Chlorite crystallinity values measured on air-dried and ethylene-glycol-solvated samples suggest that the effects of expandable interlayers are negligable, especially in the higher grade (～temperature) part of the series. However, the greater scattering of crystallinity values for the chlorite 001 reflection compared to those of the 002 reflection may be related to the effects of minor amounts of interlayered and/or discrete smectite and/or vermiculite. With increasing metamorphic grade and advancing equilibrium recrystallization, the chlorite compositions in different samples become more homogenous. No correlation exists between crystallinity and changes in chlorite composition as estimated from the intensity ratios of basal reflections. Hence an increase of domain size and a decrease of lattice distortion with increasing grade (～temperature) may be decisive factors affecting chlorite crystallinity. Chlorite crystallinity can be applied as a reliable regional, statistical technique complementary with, or instead of, the illite crystallinity method. The illite and chlorite crystallinity scales used here are related to Kübler's epi-, anchi- and diagenetic zones and correlated with coal rank, conodont colour alteration and mineral facies data. As the effects of the detrital white mica can be observed even in the <2-μm fractions of anchizonal metapelites, the anchizone boundaries determined solely on the base of ‘fixed’illite crystallinity values may vary with amounts of detrital and newly formed muscovite–illite. Hence a complex approach utilizing more than one method for determination of grade is preferred for petrogenetic purposes, even if relationships between crystallinity scales, coal rank and mineral facies also vary strongly in different tectonic settings and lithologies.     

Detrital glaucophane in graywackes of the Rhenohercynian Harz mountains and the geodynamic implications

Detrital blue amphibole was found for the first time in two samples of the Famennian section of the South Harz-Selke Graywacke and the Tanne Graywacke (Middle Visean) of the Harz Mountains. Microprobe analyses reveal that the blue amphiboles represent glaucophane with Fe³⁺/(Fe³⁺+Al^VI)=0.22 molar ratio. The minimum pressure required for the formation of glaucophane of this composition is estimated to be approximately 8 kbar. The source area of the detrital glaucophane is assumed to be located between the Northern Phyllite Zone and the Mid-German Crystalline Rise, in areas which have been downfaulted (?subducted) during the Variscan orogeny. The age of blueschist-facies metamorphism in the source area must be of pre-Upper Devonian age. This metamorphic event is significantly older than the Lower Carboniferous high-pressure/low-temperature metamorphism documented in parts of the northern Phyllite zone. Hence, the convergent tectonics connected with blueschist-facies metamorphism is not restricted to the Lower Carboniferous, but can be traced back at least to the early Upper Devonian. These data are in accordance with a southerly directed underplating (?subduction) at the northern margin of the Saxothuringian zone active during at least from early Upper Devonian to Lower Carboniferous.     

Incipient metamorphism in the Lower Palaeozoic marginal basin of Wales

Abstract A diagenctic through anchizone to epizone transition is demonstrated in pelitic rocks of the Lower Palaeozoic marginal basin of Wales by examination of variations in phyllo-silicate mineralogy, illite crystallinity and b_o parameter of white micas. This transition represents a temperature range from ∼ 150°C to ∼ 400°C and the metamorphism is of a low-pressure facies series type, with a geothermal gradient of ∼ 40°Ckm^-1. Variations in grade can be correlated largely with the original basin and shelf form, suggesting a depth-related metamorphism. However, in areas closer to the site of Caledonian plate collision an increasingly syn-tectonic metamorphic event is apparent.
Correlation of pelite data with metabasite assemblages is variable, the most consistent relationship being between epizone crystallinity values andepidote-actinolite (greenschist facies) assemblages. Diagenetic clay mineral assemblages are found associated with prehnite-pumpellyite assemblages in metabasites and it is suggested that the latter represent non-buffered, and therefore non-diagnostic, assemblages.     

云南墨江镍金矿床主要控矿因素分析与研究

采用控矿因素分析研究，通过构造岩石地层及构造变形筛分探讨镍金矿床的成矿过程，认为本区金镍矿主要为地层及构造控矿。在矿床内，热（泉）水喷溢构造（成矿构造）有似环状粘土岩化带及热水同生沉积－交代－喷流构造岩石层序，脆－韧性剪切带（储矿构造）由近水平的纵张裂隙构造、左行共轭剪切裂隙及相对封闭的脆韧性剪切带组成。脆性张剪性X形微裂隙系统为富金矿脉的定位提供了构造空间。成矿演化过程为：①晚泥盆世热水同生沉积成岩成矿期形成含金黄铁矿硅质岩；②印支期逆冲推覆型脆韧性剪切构造成岩成矿期形成含金脆韧性剪切带；③燕山－喜马拉雅山期深源热流体叠加成矿期形成富金矿脉。指出3处成矿远景区，认为红土型金矿很可能是本区一种新的金矿类型，值得重视研究。     

Fluid-driven low-grade metamorphism in polydeformed rocks of Avalonia (Arisaig Group, Nova Scotia, Canada)

The Lower Silurian??Lower Devonian Arisaig Group (Antigonish Highlands) in the Canadian Appalachians is a sequence of shallow marine strata deposited after the accretion of Avalonia to Baltica during the closure of the Iapetus Ocean. Deformation of the strata is widely attributed to the Devonian Acadian orogeny and produced shallowly plunging regional folds and a cleavage of varying penetrativity. Phyllosilicate minerals from the finest-grained rocks exhibit very low-grade (diagenetic-anchizone) metamorphic conditions. X-ray diffraction study reveals that the sampled rocks contain quartz, K-white mica, chlorite, and feldspars; illite?Csmectite and chlorite?Csmectite mixed-layers are common but Na?CK mica and kaolinite occur only in some samples. The identification of illite?Csmectite mixed-layers in diagenetic samples, with Kübler Index >0.50 ??°2?? and the highly heterogeneous b-cell dimension of the K-white micas are in agreement with the variable chemical composition of dioctahedral micas, which present low illitic substitution and variable phengitic content. The spatial variation in the above crystal-chemical parameters was plotted along a NW?CSE composite cross section across the regional folds. No correlation was found between the metamorphic conditions and either the stratigraphic depth or the strain values measured by phyllosilicates orientation analyses, as a function of the penetrativity of the cleavage. However, the metamorphic grade generally increases towards the Hollow Fault, and is highest in samples located within a 1?km corridor from the fault surface. Incipient cleavage is observed in the anchizonal samples located in the vicinity of the Hollow Fault and in some of the diagenetic samples, indicating cleavage development under low temperatures (<200?oC). These relationships, together with regional syntheses, suggest low-grade metamorphism post-dated regional folding and was coeval with Late Carboniferous dextral movement along the Hollow Fault. Fluid circulation associated with movement along this major fault may be the driving mechanism for the increasing metamorphism towards it.     

Domainal variations of U–Pb monazite ages and Rb–Sr whole-rock dates in polymetamorphic paragneisses (KTB Drill Core, Germany): influence of strain and deformation mechanisms on isotope systems

Polyphase metamorphic paragneisses from the drill core of the continental deep drilling project (KTB; NW Bohemian Massif) are characterized by peak pressures of about 8 kbar (medium‐P metamorphism) followed by strain accumulation at T >650 °C, initially by dislocation creep and subsequently by diffusion creep. U–Pb monazite ages and Rb–Sr whole‐rock data vary in the dm‐scale, indicating Ordovician and Mid‐Devonian metamorphic events. Such age variations are closely interconnected with dm‐scale domainal variations of microfabrics that indicate different predominant deformation mechanisms. U–Pb monazite age variations dependent on microfabric domains exceed grain‐size‐dependent age variations. In ‘mylonitic domains’ recording high magnitudes of plastic strain, dislocation creep and minor static annealing, monazite yields concordant and near concordant Lower Ordovician U–Pb ages, and the Rb–Sr whole‐rock system shows isotopic disequilibrium at an mm‐scale. In ‘mineral growth/mobilisate domains’, in which diffusive mass transfer was a major strain‐producing mechanism promoting diffusion creep of quartz and feldspar, and in which static recrystallization (annealing) reduced the internal free energy of the strained mineral aggregates, concordant U–Pb ages are Mid‐Devonian. Locally, in such domains, Rb–Sr dates among mm³‐sized whole‐rock slabs reflect post‐Ordovician resetting. In ‘transitional domains’, the U–Pb‐ages are discordant. We conclude that medium‐P metamorphism occurred at 484±2 Ma, and a second metamorphic event at 380–370 Ma (Mid‐Devonian) caused progressive strain in the rocks. Dislocation creep at high rates, even at high temperatures, does not reset the Rb–Sr whole‐rock system, while diffusion creep at low rates and stresses (i.e. low ε/D_eff ratios), static annealing and the presence of intergranular fluids locally assist resetting. At temperatures above 650 °C, diffusive Pb loss did not reset Ordovician U–Pb monazite ages, and in domains of overall high imposed strain rates and stresses, resetting was not assisted by dynamic recrystallization/crystal plasticity. However, during diffusion creep at low rates, Pb loss by dissolution and precipitation (‘recrystallization’) of monazite produces discordance and Devonian‐concordant U–Pb monazite ages. Hence, resetting of these isotope systems reflects neither changes of temperature nor, directly, the presence or absence of strain.     

Early Paleozoic orogeny in southern South America

In northwest Argentina, weakly metamorphic clastic and calcareous sedimentary rocks of latest Precambrian to Lower Cambrian age (Puncoviscana Formation and related units) contain an abundant ichnofauna of both chronostratigraphic and paleoenvironmental value. In the western and central Sierras Pampeanas, metasedimentary and metavolcanic rocks are considered to form part of the same geotectonic unit. This “Pampean orogenic cycle” includes geosynclinal sedimentation of latest Precambrian to Lower Cambrian age, as well as magmatism, metamorphism and deformation of Middle to Upper Cambrian age, documented by an angular unconformity below the Upper Cambrian to Devonian rocks of the “Famatinian orogenic cycle”. In some of the metamorphic rocks of the Pampean Cycle a pre-Ordovician folding is also distinguished from a later tectonic overprinting. Hence, the concept of a Pampean cycle differs from other concepts of late Precambrian orogenic cycles of South America which are only defined by radiometric ages. The Pampean orogenesis may be compared with the Ross orogenesis of the Transantarctic Mts., the Tyennan orogenesis of Australia and some of the deformation phases of the Damara orogen in Namibia.     

赣南某离子吸附型稀土矿床浅变质岩的矿化特征

近几年在赣南新元古代浅变质岩风化壳中发现了离子吸附型稀土矿床,对矿区及南岭科学深钻中的浅变质岩样品进行了岩石、矿物及地球化学特征研究.矿区内主要出露神山组和库里组（南华系）,前者以千枚岩为主,含少量片岩,后者以厚层变质沉凝灰岩和中厚层变质砂岩为主;南岭科学深钻1 165~1 170.77 m库里组（青白口系）以变质沉凝灰岩为主夹薄层凝灰质板岩.浅变质岩类中新生变质矿物有绢云母、绿泥石、磁铁矿、堇青石、白云母、黑云母等,稀土矿物有新奇钙铈矿、独居石、磷钇矿、水独居石等;稀土含量为162×10-6~723×10-6,富集轻稀土.新奇钙铈矿是矿体中离子相稀土的主要来源;赣南青白口纪-南华纪中厚层变质沉凝灰岩和变质凝灰岩风化壳具有较好的稀土成矿前景.      

赣南新元古代变质岩稀土矿物及其地球化学特征

近年来赣南地区首次报道了变质岩离子吸附型稀土矿床的发现,为离子吸附型稀土的找矿提供了新思路。赣南地区新元古代变质岩大面积分布,风化壳也广泛发育。文章对30件稀土元素含量(300×10~(-6))高的变质岩矿物样品进行了详细的电子探针分析,旨在查明赣南新元古代不同类型变质岩中的稀土矿物种类及特征,探讨其成因、对全岩稀土元素含量的贡献以及离子吸附型稀土元素的成矿潜力。研究表明,区域上变质岩可大致分为6类,分别是变质凝灰岩类、板岩类、千枚岩类、片岩类、变砂岩类和变粒岩类,不同类型变质岩的稀土矿物组合不同,除了普遍存在的、难风化的独居石、磷钇矿和锆石外,部分岩性中出现易风化的褐帘石、含稀土元素绿泥石和含稀土元素金红石,及表生的水磷酸盐和磷铝酸盐等矿物。这些富稀土矿物贡献了全岩中大部分稀土元素,且部分矿物成因与后期流体作用相关,为成矿提供了良好的条件。文章总结分析认为,赣南地区广泛分布的变质岩中,片岩类、变砂岩类和变质凝灰岩类均具有相对易风化的稀土矿物组合,尤其变质凝灰岩类和变砂岩类,能为离子吸附型稀土成矿提供充足的物质来源,具有可观的离子吸附型稀土成矿潜力。     

The farnacht formation along the south side of the clew bay fault zone,Western Ireland: Its chemistry and age of metamorphism

The tectonically isolated Farnacht Formation consists of calc-alkaline dacitic-andesitic lavas of volcanic arc affinity. It is situated immediately to the south of the Clew Bay Fault Zone (western continuation of the Highland Boundary Fault Zone of Scotland) in the northeast corner of the Lower Palaeozoic South Mayo Trough in northwest Ireland. It has been metamorphosed to biotite grade greenschist facies following the development of a pervasive, c-s composite muscovite, quartz, and feldspar schistosity. The Farnacht Formation may comprise a terrane that is directly unrelated to nearby Ordovician and Silurian rocks; its present position was fixed largely by Wenlock times. The age of the Farnacht Formation and the deformational event(s) that produced the schistosity are not known. ⁴⁰Ar/³⁹ Ar step heating from four specimens have dated the crystallization of biotite at from 422 ± 2 to 405 ± 14 Ma with a mean age of 413 Ma. These ages date either the post-D2, pre-D3 metamorphic peak, or a hornfelsing of the same structural age related to an unseen thermal source, and provide a minimum age for the end Silurian - early Devonian Caledonian tectonothermal activity in the northeast part of the South Mayo Trough.     

Very Low-Grade Metamorphism of Clastic Rocks from the Meso-Neoproterozoic and the Paleozoic along the Profile Yueyang-Linxiang in Northeastern Hunan Province and Its Geological Implications

This study uses illite crystallinity,chlorite crystallinity,illite polytypes,the b_0 cell-dimension of K-white mica,clay mineral assemblages and mineral geothermo-geobarometers to investigate the overprint of diagenesis and metamorphism on the Meso-Neoproterozoic and the Lower Paleozoic along the profile Yueyang-Linxiang in northeastern Hunan Province,China.Illite crystallinity Kbler index(KI) of the 2μm fractions ranges from 0.225 to 0.485°Δ2θ while chlorite crystallinity Arkai index(AI) ranges from 0.244 to 1.500°Δ2θ.This indicates that the Meso-Neoproterozoic and the Lower Paleozoic along the profile Yueyang-Linxiang were overprinted with diagenesis and anchi- to epimetamorphism.Peak metamorphic temperature is estimated with the IV site chlorite geothermometer roughly at 360℃.The b_0 cell dimension values of illites(K-mica) range from 0.9002 to 0.9054 nm and,on average,at 0.9030 nm for the Meso-Neoproterozoic.Based on cumulative frequency curves of illite(K-mica) b_0 cell dimension,the peak metamorphic pressure of the MesoNeoproterozoic along the profile Yueyang-Linxiang is derived of an intermediate pressure type.Most illites occur in the 2M_1 polytype and some of them in a mixture of 2M1+1M types especially those in the Paleozoic.This result partly agrees with the conclusion of the lower greenschist and greenschist facies of the Lengjiaxi and Banxi Groups.However,it is not agreed with the sedimentary cover from the Sinian to the Lower Paleozoic or from the Banxi Group to the Lower Paleozoic.Crustal thickening due to "collision" between the Yangtze and Cathaysia blocks led to an increase in the thickness of the Meso-Neoproterozoic to ca.14 km and resulted in a temperature increase in those rocks due to burial.The very low grade to low grade metamorphism overprinting the Meso-Neoproterozoic implies that the so called "Chiangnania or Jiangnan orogen" was no relative with the "Grenvillian orogeney;instead,it might be a continuous amalgamation product between the Yangtze and Cathaysia blocks.     

Mineralogical and geochemical alteration of low-grade metamorphic black slates due to oxidative weathering

Low-grade metamorphic black slates of Silurian and Lower Devonian ages (from the Thüringisches Schiefergebirge in Germany) were investigated to identify mineralogical and geochemical alterations that occur during the oxidative weathering black slates.The slates exhibit an intense total organic carbon decrease (>90 wt%) due to oxidative weathering. The organic matter in black slate consists mainly of non-pyrolysable material with only minor portions of pyrolysable material. In contrast to the non-pyrolysable organic matter, the concentration of pyrolysable organic matter is not decreased during weathering. Heating experiments have yielded information about a potential protection of pyrolysable organic matter against weathering, which is probably caused by a structural configuration or the intergrowth of organic matter and illite.The slates consist mainly of illite. Iron oxide (goethite) and phosphate encrustations were formed on slate surfaces as a result of oxidative and acid weathering of both iron sulfide and monazite. Kaolinite often occurs as an older encrustation beneath the iron oxide encrustations. Encrustations of iron oxide and phosphate are major sinks for trace elements, released during oxidative weathering.     

Adaminaby Group west of Batemans Bay: Deformation and metamorphism of the Narooma accretionary complex,NSW

This study provides new structural data that show that the Adaminaby Group is part of the Narooma accretionary complex and has been overprinted by HT/LP metamorphism associated with Middle Devonian Moruya Suite intrusions. The grade of metamorphism based on Kübler Indices is the same in the Wagonga and Adaminaby Groups at Batemans Bay inferring that these rocks were involved in the same accretionary event. White micas in slates of the Adaminaby Group record apparent K–Ar ages of 384.6 ± 7.9 Ma and 395.8 ± 8.1 Ma. These ages are believed to represent the age of Middle to Upper Devonian Buckenbowra Granodiorite. Kübler Index values indicate lower epizonal (greenschist facies) metamorphic conditions and are not influenced by heating in metamorphic aureoles of the plutons. All b cell lattice parameter values are characteristic of intermediate pressure facies conditions although they are lower in the metamorphic aureole of the Buckenbowra Granodiorite than in the country rock, defining two areas with dissimilar baric conditions. East of the Buckenbowra Granodiorite, b cell lattice parameter values outside the contact aureole (x = 9.033 Å; n = 8) indicate P = 4 kb, and assuming a temperature of 300°C, infer a depth of burial of approximately 15 km for these rocks with a geothermal gradient of 20°C/km. In the metamorphic aureole of the Buckenbowra Granodiorite, b cell lattice parameter values (x = 9.021 Å; n = 41) indicate P = 3.1 kb inferring exhumation of the Adaminaby Group rocks to a depth of approximately 11 km prior to intrusion. A geothermal gradient of 36°C/km operated in the aureole during intrusion. An extensional back-arc environment prevailed in the Adaminaby Group during the Middle to Upper Devonian.     

U–Pb evidence for a polyorogenic evolution of the HP–HT units of the NW Iberian Massif

. A isotope dilution thermal ionisation mass spectrometry U-Pb geochronological study was carried out on the high-pressure and high-temperature units (HP-HT units) overlying the oceanic suture in the Allochthonous Complexes of the NW Iberian Variscan Belt. The rocks investigated are seven granulite- to eclogite-facies paragneisses and one leucosome within mafic high-pressure granulites in the Ordenes and Cabo Ortegal Complexes of NW Spain. U-Pb dating of zircon, monazite, titanite and rutile reveal the presence of a pervasive Early Ordovician metamorphic event at ca. 500-480 Ma and a later Early Devonian event at ca. 400-380 Ma. The U-Pb ages, in conjunction with petrological and structural data, indicate that the high-pressure event recorded by these rocks is Early Ordovician in age. Monazite ages in the paragneisses suggest that peak metamorphic conditions were reached at ca. 500-485 Ma. Subsequently, the rock ensemble underwent exhumation accompanied by partial melting and zircon growth at ca. 485-470 Ma. Melting of mafic granulites was coeval with this latter episode as indicated by zircon crystallisation age in the leucosomes dated at ca. 486 Ma. Based on these data and on the general features of magmatism and metamorphic evolution, it is proposed that this process took place at a convergent plate boundary within a peri-Gondwanan oceanic domain. Monazite, titanite and rutile data in some of the samples studied show evidence of a second metamorphic episode that took place between ca. 400 and 380 Ma (with a peak at ca. 390-385 Ma). This Early Devonian event, at variance with the previous one, was not pervasive, but, rather, was localised in areas of intense Variscan tectonothermal reworking. It is claimed that this later metamorphic event was recorded by the U-Pb system in areas where monazite and titanite growth was enhanced by fluid circulation in highly strained rocks (Variscan shear zones). According to previous structural studies and Ar-Ar dating of fabrics, this Early Devonian episode took place as the HP-HT units were deformed and thrusted upon the ophiolitic units in the early stages of the Variscan collision.     

吉林省泥盆系层序的新划分

<正> 吉林省泥盆系露头比较零星,地层资料也比较少。1966年李东津等在永吉县黄榆屯西王家街发现泥盆纪珊瑚,长春地质学院等命名为王家街组。分布在磐石县呼兰镇至黑石镇一带的变质岩系呼兰群经郭鸿俊的研究被划为志留—泥盆系,但至今仍有争议。笔者通过珊瑚化石的研究,肯定了原定为志留系的二道沟灰岩为早泥盆世早期地层,又在王家     

Weathering of Fruchtschiefer building stones: mineral dissolution or rock disaggregation?

Building stones manufactured from contact metamorphic slates (Fruchtschiefer slate) from Theuma (Sachsen, Germany) were investigated for mineralogical alterations as well as for changes in porosity and surface roughness due to weathering. After weathering periods of several years to decades, the originally dark gray-colored slates show pale spots of several centimeters in size at the surface of building stones. The dark-colored and light-colored sections of the slate show no differences in mineralogy. Surface weathering did not result in newly precipitated minerals. It was also found that the observed differences in color are not caused by variations in sedimentary organic carbon concentration or in sulfide/sulfate concentrations. Obtained results instead indicate that dark surface sections may show a thin cover of recent organic matter (OM), e.g., living OM, soot, dirt, etc. Microscopic investigations suggested that this cover was exfoliated at light-colored surface sections. The observed disaggregation of the upper 2 mm of the building block material results in an increase in porosity. Porosity of black (unweathered) slate is <2 vol.%. Due to weathering, the slate’s pores with diameters >1 μm show a significant increase in frequency compared to the original pore size distribution. Porosity of weathered rock volumes increased to approx. 8 vol.%. Discolored surface sections show a higher surface roughness (root-mean-square roughness, Rq ~ 1 μm) compared to dark-colored slate surfaces (Rq ~ 200 nm), both data are for cleavage planes. Preferentially, the discolored surface sections are located close to the edges of cut stones. This and the alteration in porosity, pore size, and surface roughness indicate that color changes of the slate are largely influenced by rock disaggregation proceeding from the edges into the center rather than by mineral dissolution/precipitation processes.     

Metamorphic inheritance of Rheic passive margin evolution and its early‐Variscan overprint in the Teplá‐Barrandian Unit,Bohemian Massif

Rift‐related regional metamorphism of passive margins is usually difficult to observe on the surface, mainly due to its strong metamorphic overprint during the subsequent orogenic processes that cause its exposure. However, recognition of such a pre‐orogenic evolution is achievable by careful characterization of the polyphase tectono‐metamorphic record of the orogenic upper plate. A multidisciplinary approach, involving metamorphic petrology, P–T modelling, structural geology and in situ U‐Pb monazite geochronology using laser‐ablation split‐stream inductively coupled plasma mass spectrometry, was applied to unravel the polyphase tectono‐metamorphic record of metapelites at the western margin of the Teplá‐Barrandian domain in the Bohemian Massif. The study resulted in discovery of three tectono‐metamorphic events. The oldest event M1 is LP–HT regional metamorphism with a geothermal gradient between 30 and 50 °C km^?1, peak temperatures up to 650 °C and of Cambro‐Ordovician age (c. 485 Ma). The M1 event was followed by M2‐D2, which is characterized by a Barrovian sequence of minerals from biotite to kyanite and a geothermal gradient of 20–25 °C km^?1. D2‐M2 is associated with a vertical fabric S2 and was dated as Devonian (c. 375 Ma). Finally, the vertical fabric S2 was overprinted by a D3‐M3 event that formed sillimanite to chlorite bearing gently inclined fabric S3 also of Devonian age. The high geothermal gradient of the M1 event can be explained as the result of an extensional, rift‐related tectonic setting. In addition, restoration of the deep architecture and polarity of the extended domain before the Devonian history – together with the supracrustal sedimentary and magmatic record – lead us to propose a model for formation of an Ordovician passive continental margin. The subsequent Devonian evolution is interpreted as horizontal shortening of the passive margin at the beginning of Variscan convergence, followed by detachment‐accommodated exhumation of lower‐crustal rocks. Both Devonian shortening and detachment occurred in the upper plate of a Devonian subduction zone. The tectonic evolution presented in this article modifies previous models of the tectonic history of the western margin of the Teplá‐Barrandian domain, and also put constraints on the evolution of the southern margin of the Rheic ocean from the passive margin formation to the early phases of Variscan orogeny.