The Finnmarkian Orogeny revisited: An isotopic investigation in eastern Finnmark, Arctic Norway

The Scandinavian Caledonides have been viewed as resulting from either a single Silurian (i.e. Scandian) event or from polycyclic orogenies involving several collisions on the margin of Baltica. Early studies of the Kalak Nappe Complex (KNC) in Finnmark, Arctic Norway, led to the hypothesis of an Early Cambrian-Early Ordovician (520-480 Ma) Finnmarkian Orogeny, though the nature of this tectonic event remains enigmatic. In this contribution we have employed in situ UV laser ablation Ar-Ar dating of fine-grained phyllite and schist from the eastern Caledonides of Arctic Norway to investigate the presence of pre-Scandian tectonometamorphic events. U-Th-Pb detrital zircon and whole rock Sm-Nd analyses have been used to test the regional stratigraphic correlations of these metasedimentary rocks. These results indicate that the Berlevåg Formation within the Tanafjord Nappe, previously assumed to be part of the KNC, was deposited after 1872 Ma and prior to a low temperature hydrothermal event at 555 ± 15 Ma. It has a likely provenance on the Baltica continent, lacks any Grenville-Sveconorwegian detrital zircons, and thus cannot be part of the KNC which contains abundant detritus in this age range. Instead the Berlevåg Formation is interpreted as part of the Laksefjord Nappe Complex, which structurally underlies the KNC. Laser-ablation argon-argon dating also shows that late Caledonian (i.e. Scandian) tectonometamorphism affected both the KNC and its immediate footwall at c. 425 ± 15 Ma. This is corroborated by a step-heating argon-argon muscovite age of 424 ± 3 Ma which is interpreted as dating cooling. However, within two samples from the KNC, an earlier (Middle-Late Cambrian) metamorphic event is also recorded. A biotite-grade schist yielded an Ar-Ar inverse isochron age of 506 ± 17 Ma from whole rock surfaces, in which the mineral domains are too fine-grained to date individually. An early generation of muscovite from a coarser-grained amphibolite-facies sample yielded an inverse isochron of 498 ± 13 Ma. Both isochron ages have atmospheric argon intercept values. Previous studies have documented similar Cambrian ages in the Caledonian nappes below the KNC. These results suggest correlative tectonometamorphic events in the eastern KNC and its footwall at c. 500 Ma. This Cambrian event may reflect the arrival of the Kalak Nappe Complex as a previously constructed exotic mobile belt onto the margin of Baltica. Combined with recent studies from the western Kalak Nappe Complex, the results do not support the traditional constraint on the Finnmarkian Orogeny sensu stricto. However they vindicate classic tectonic models involving a Cambrian accretion event.     

Plagioclase microfabrics in a ductile shear zone from the Jotun Nappe,Norway

Gabbroic and anorthositic rocks from the Jotun Nappe are transected by small ductile shear zones in which a high-grade paragenesis and a new foliation are formed. Most plagioclase grains show evidence of plastic deformation, and have recrystallized by subgrain rotation and ‘bulge’ processes to fine-grained mylonite. During these processes a plagioclase grain-shape fabric was destroyed, and with increasing strain a pre-existing plagioclase crystal fabric was successively modified, enhanced and finally obliterated. This could be evidence of superplastic flow. Inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that (021) ⊥ 〈a〉 could be an operative slip system, and planes close to (132) could be possible slip planes.     

Petrography and petrogenesis of nephelinized metagabbros from Finnmark,Northern Norway

Plagioclase in olivine metagabbros metasomatized by nepheline syenite magmas was replaced by intergrowths of nepheline and vesuvianite or salite together with smaller amounts of calcic amphibole. Olivine was simultaneously replaced by diopside. The survival of the gross features of protolith textures and clustering of line intersections in composition-volume diagrams suggest that nephelinization took place at approximately constant volume and oxygen content. Nephelinization involved the introduction of Na, K and Fe³⁺ in an infiltrating peralkaline fluid and depletion of Fe²⁺ and Fe^tot and was promoted by the primary mineral parageneses of the protoliths. Concurrent fenitization of quartzo-feldspathic gneiss without the generation of feld-spathoids shows that the processes of nephelinization and fenitization may result from the same fluid.     

流体作用下石榴石溶解-沉淀蠕变过程——以红河-哀牢山剪切带内石榴夕线片麻岩研究为例

石榴夕线片麻岩是中、下地壳主要组成岩石之一,岩石内石榴石和夕线石的结晶学优选方位会显著影响地壳深部流变性质,因此探讨特征变质矿物的变形机制和主要受控因素对构造带深部演化过程有深远意义。本文选取红河-哀牢山韧性剪切带内石榴夕线片麻岩为研究对象,通过定向切片内显微构造、电子探针、X-ray成分扫描、电子背散射衍射（EBSD）和相平衡模拟综合研究,揭示出石榴石在溶解沉淀反应过程中具有明显的粒度敏感性,不同粒径石榴石表现出截然不同的长宽比、成分环带、包裹体排列方式和压力影发育情况。石榴石表面流体活动明显截切早期生长环带。EBSD分析揭示石榴石破碎颗粒以绕〈112〉轴机械旋转为主,溶解过程主要集中于颗粒表面和裂隙内高曲率位置。夕线石的EBSD结果表明基质内夕线石以绕〈010〉轴旋转为主,而流体作用明显区域夕线石则以（100）[001]滑移为主。岩石相平衡模拟限定岩石变质峰期P-T条件达高压麻粒岩相,退变过程中同剪切变形导致大量流体渗入而形成降温降压退变轨迹,由~9.5kbar、760℃演化至~6.0kbar、500~600℃,并在粗粒石榴石内保存早期进变质环带,剪切抬升过程中石榴石内普遍发育垂直剪切方向的裂隙,并在流体作用下进一步改造其形态。此研究揭示红河-哀牢山剪切带内除前人报道的石榴石高温韧性变形外,还存在大量中-上地壳层次同变质反应下的溶解-沉淀蠕变作用。因此,石榴石变质-变形的综合研究有助于揭示变质杂岩带挤压-剪切-伸展多阶段构造演化过程。

     

Modern pollen spectra from lake sediments in Finland and Finnmark, north Norway

Absolute (APF) and relative surface pollen spectra from 67 moderate-sized lakes in Finland and Finnmark are presented as pollen diagrams and isopoll maps and analysed by multidimensional scaling. The results are applicable to problems of regional vegetational history through comparisons of pollen spectra and the use of R-values. It is shown that relative spectra can discriminate the major vegetational regions. APFs give better resolution among northern forest regions and low APFs characterize tundra. Gradients of tree-pollen percentages in south and central Finland are related quantitatively to the major forest gradients, which are azonal, mainly related to soils, and affected but not obscured by human influence. Forest clearance is reflected in frequencies of various non-tree pollen types. Non-tree pollen and spores also provide vegetational and climatic information in the far north.     

Tide- and wave-dominated inshore and shoreline sequences from the late Precambrian, Finnmark, North Norway

The Late Precambrian Ekkerøy Formation of Finnmark, northern Norway represents a 40 m thick coarsening upward, prograding deltaic sequence. The upper 6 m of this formation consists of two small-scale fining upward sequences representing two contrasting shoreline environments. These two sequences have been the focus of a detailed facies analysis based on grain size, bed thickness and sedimentary structures. The study area comprises a series of horizontally bedded rocks which are continuously exposed for 3 km enabling individual facies and sedimentary structures be to traced-out laterally. The lower sequence exhibits a wide variety of tidal features including directional bimodality of both large-scale and small-scale cross-stratification, herringbone cross-stratification, various sandstone-mudstone alternations including lenticular and wavy bedding, and a general fining upward tendency. A detailed field analysis of the internal structures within the large-scale cross-stratified unit is presented and is interpreted as a sub-tidal channel complex. The associated sandstone-mudstone facies represents intertidal flats built up by tidal and storm-generated currents. The upper sequence displays a wide variety of wave-formed structures including wave-ripples and swash-lamination. Palaeocurrent data indicates a dominance of shoreward migrating megaripples and wave-ripple crests oriented approximately parallel to the inferred shoreline with a distinct longshore component. The sequence is interpreted as a beach-shoreface environment. In terms of sedimentary structures, palaeocurrent patterns and facies distributions, the sequence bears a remarkable similarity to the high-energy, non-barred shoreline of the modern Oregon coast U.S.A., described by Clifton, Hunter & Phillips (1971).     

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.     

Possible retrogressive flow slide deposits from the Kongsfjord Formation: a Precambrian submarine fan, Finnmark, N. Norway

Three thinning and fining-upward turbidite sequences are described from the Precambrian Kongsfjord Formation, a 3.5 km thick flysch succession. Their thicknesses range between about 2 and 5 m. They show a progressive upward decrease in bed thickness, bulk mean grain size and the ratio of the higher to lower energy division of the Bouma sequence. In one case, however, there is an initial upward bed thickness and grain size increase, with an increase in the proportion of the higher energy division. The absence of structureless mud of the Bouma E division and the presence of wavy interfaces between beds, together with similar palaeo-currents within each sequence suggest that these sequences resulted from related depositional events. These sequences are interpreted as the deposits of retrogressive flow slides, as an alternative to the classic mechanism of channel fill after abandonment.     

Strain distribution within a km-scale,mid-crustal shear zone: The Kuckaus Mylonite Zone,Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D₁) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D₁ migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D₁ fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.     

He diffusion systematics in minerals: Evidence from synthetic monazite and zircon structure phosphates

Helium diffusivity was measured in synthetic rare-earth-element orthophosphates with systematically varying properties to evaluate potential controls on He transport in minerals. In the zircon structure phosphates (in this study, the phosphates of Tb, Dy, Ho, Er, Tm, Yb, and Lu as well as synthetic xenotime, YPO₄), He diffusion is strongly anisotropic. Transport apparently proceeds preferentially through channels aligned with the c-axis. The activation energy for diffusion is almost the same (122 ± 6 kJ/mol) in all members of this family, but there is a monotonic decrease in D_o with atomic number from TbPO₄ (∼10⁵ cm²/s) to LuPO₄ (∼10 cm²/s). The c-parallel channels become increasingly constricted in the same sequence, likely accounting for the systematically decreasing diffusivity. The He closure temperature (r = 1 cm, dT/dt = 10 °C/Myr) increases with atomic number from 44 °C for TbPO₄ to 88 °C for LuPO₄. Diffusion of radiogenic helium from natural zircon and xenotime is much slower than these synthetic analogs predict, suggesting that coupled substitution of REE and P for Zr and Si and/or radiation damage profoundly modify the energetics of interstitial He diffusion. In particular, α-recoil may play a key role by damaging the continuity and integrity of the channels.Monazite structure phosphates (here La, Ce, Pr, Nd, Sm, and Gd phosphate) are far more He retentive than those of the zircon structure. Activation energies increase smoothly with atomic number from LaPO₄ (183 kJ/mol) to NdPO₄ (224 kJ/mol) then decrease again to GdPO₄ (198 kJ/mol). D_o values mimic this pattern, spanning a range from ∼10⁻¹ cm²/s (GdPO₄) to 10⁴ cm²/s (NdPO₄). Nevertheless, He closure temperatures increase monotonically with atomic number, from 300 °C in LaPO₄ to 410 °C in GdPO₄. No evidence was obtained bearing on diffusion anisotropy, but the monazite structure lacks through-going channels so it is not expected. Diffusion parameters for radiogenic helium in natural monazite are similar to those obtained on the synthetic analogs.Ionic porosity is not the primary control on He diffusion in the orthophosphates. Within a given structure and with limited elemental substitution, ionic porosity and He closure temperature are negatively correlated, as predicted. However, differences between crystal structures are far more important than ion packing density: at comparable ionic porosity the monazite structure phosphates have He closure temperatures ∼300 °C higher than the xenotime structure phosphates. Modifications to the structures by radiation damage likely play a similarly significant role in controlling He diffusion.     

Isotopic constraints on fluid infiltration from an eclogite facies shear zone, Holsenøy, Norway

Granulite facies anorthosites on Holsenøy Island in the Bergen Arcs region of western Norway are transected by shear zones 0.1–100 m wide characterized by eclogite facies assemblages. Eclogite formation is related to influx of fluid along the shears at temperatures of c. 700d?C and pressures in excess of 1.7 GPa. Combined carbon and nitrogen stable isotope, ⁴⁰Ar/³⁶Ar, trace-element and petrological data have been used to determine the nature and distribution of fluids across the anorthosite-eclogite transition. A metre-wide drilled section traverses the eclogitic centre of the shear into undeformed granulite facies garnet-clinopyroxene anorthosite. Clinozoisite occurs along grain boundaries and microcracks in undeformed anorthosite up to 1 m from the centre of the shear and clinozoisite increases in abundance as the edge of the shear zone is approached. The eclogite-granulite transition, marked by the appearance of sodic pyroxene and loss of albite, occurs within the most highly sheared section of the traverse. The jadeite-in reaction coincides with increased paragonite activity in mica. The separation between paragonite and clinozoisite reaction fronts can be semiquantitatively modelled assuming advective fluid flow perpendicular to the shear zone. The inner section of the traverse (0.25 m wide) is marked by retrogressive replacement of omphacite by plagioclase + paragonite accompanied by veins of quartz-phengite-plagioclase. C-N-Ar characteristics of fluid inclusions in garnet show that fluids associated with precursor granulite, eclogite and retrogressed eclogite are isotopically distinct. The granulite-eclogite transition coincides with a marked change in CO₂ abundance and δ¹³C (<36ppm, δ¹³C=-2% in the granulite; <180 ppm, δ¹³C=-10% in the eclogite). The distribution of Ar indicates mixing between influxed fluid (⁴⁰Ar/³⁶Ar > 25 times 10³) and pre-existing Ar in the granulite (⁴⁰Ar/³⁶Ar < 8 times 10³). δ¹⁵N values decrease from +6% in the anorthosite to +3% within the eclogite shear. The central zone of retrogressed eclogite post-dates shearing and is characterised by substantial enrichment of Si, K, Ba and Rb. Fluids are CO₂-rich (δ¹³C ～ -5%) with variable N₂ and Ar abundances and isotopic compositions. Both Ar and H₂O have penetrated the underformed granulite fabric more than 0.5m beyond the granulite/eclogite transition during eclogite formation. Argon isotopes show a mixing profile consistent with diffusion through an interconnecting H₂O-rich fluid network. In contrast, a carbon-isotope front coincides with the deformation boundary layer, indicating that the underformed anorthosite was impervious to CO₂-rich fluids. This is consistent with the high dihedral angle of carbonic fluids, and may be interpreted in terms of evolving fluid compositions within the shear zone.     

New Isotopic data from davidites and sulfides in the bidjovagge gold-copper deposit,Finnmark, Northern Norway

Summary The Bidjovagge gold-copper deposit (69°17'N, 22°29'E) occurs in the north-south trending Proterozoic Kautokeino greenstone belt. The greenstone belt consists of shallow marine sediments and mafic to ultramafic volcanites, which are intruded by diabase sills and granitoids. The sequence can be correlated with the 1930 to 2200 Ma Kiruna greenstone belt. Svecokarelian intrusions took place in two phases between 1880–1890 Ma and between 1860-1870 Ma.The deposit occurs in the stratigraphically lower part of the belt along a north-south trending anticlinal structure on the border between greenschist and amphibolite grade metamorphism within the Baltic-Bothnia megashear zone. The deposit is related to shear zones and hosted by albitic felsite, a metasomatic rock formed along the contact with the diabase sills.The only ore minerals of economic significance are native gold and chalcopyrite. Other common ore minerals are pyrite and pyrrhotite. Marcasite, magnetite, ilmenite, hematite, tellurides, rutile, sphalerite, galena, davidite and pentlandite occur in accessory amounts. A total of 10 samples of davidite, 14 sulfides and 4 samples of albitic felsite were analyzed for their isotopic compositions. Our best estimate for the age of the davidites (1885 ± 18 Ma) is based on the upper intersection of a U/Pb concordia diagram and is identified as the time of davidite (and gold) mineralization. This age is confirmed by a Sm/Nd date of 1886 ± 88 Ma on the same material. At 1339 ± 8 Ma, a metamorphic or metasomatic event disturbed the U/Pb system in some of the davidite samples and completely reset the U/Pb ratios in the albitic felsites. The sulfide minerals, containing trace U and Pb, were strongly disturbed during galena formation at about 500–550 Ma but apparently were initially formed at the same time as, or shortly after, the davidite. Based on an estimated time of formation of galena during the development of the early Paleozoic peneplain in the area, the Pb/Pb data are interpreted as a two-stage isochron with an initial age of 1876 ± 15 Ma.The relation between the mineralization and shear zones, the correspondence between the age of the mineralization and the Svecokarelian orogeny and the initial ratios of the radiogenic isotopes of the ore forming solutions, all support a metamorphic/metasomatic origin for the gold and copper mineralization.

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Neue Isotopendaten von Davidit und Sulfiden aus der Bidjovagge Gold-Lagerstätte, Finnmark, N.-Norwegen

Zusammenfassung Die Bidjovagge Gold-Kupfer-Lagerstätte (69°17'N, 22°29'E) liegt in dem N-S streichenden proterozoischen Kautokeino Grüngesteinsgürtel. Er besteht aus marinen Flachwassersedimenten, sowie mafischen und ultramafischen Vulkaniten, die von Diabas-Sills und von Granitoiden intrudiert werden. Diese Gesteinsabfolge kann mit dem 1930–2200 Ma alten Kiruna Grüngesteinsgürtel verglichen werden. Sveko-karelische Intrusionen erfolgten zweiphasig, zwischen 1880–1890 Ma und zwischen 1860 und 1870 Ma.Die Lagerstätte liegt in den stratigraphisch tieferen Anteilen diese Grüngesteinsgürtel entlang einer N-S streichenden Antiklinalstruktur im Übergangsbereich zwischen der Grünschiefer und Amphibolitfazies innerhalb der baltisch-bothnischen Mega-Scherzone.Sie ist an Scherzonen und an Albit-Felsite, einem metasomatisch am Kontakt mit den Diabas-Sills gebildeten Gestein, gebunden.Gediegen Gold und Kupferkies sind die wirtschaftlich wichtigsten Erzminerale. Pyrit und Magnetkies sind häufig. Markasit, Magnetit, Ilmenit, Hämatit, Telluride, Rutil, Zinkblende, Bleiglanz, Davidit und Pentlandit kommen akzessorisch vor. 10 Proben von Davidit, 14 Sulfide und 4 Proben von den Albit-Felsiten wurden auf ihre Isotopenzusammensetzung hin untersucht.Der obere Schnittpukt im U/Pb Concordiadiagramm ergab die beste Altersabschätzung für die Davidite (1885 ± 18 Ma). Dieses Alter wird als Zeitpunkt der Davidit (und Gold)-Mineralisation betrachtet. Ein Sm/Nd-Alter von 1886 ± 88 Ma bestätigt dieses. Ein metamorphes oder metasomatisches Ereignis, datiert mit 1339 ± 8 Ma störte das U/Pb System in einigen Daviditproben und führte zu einer vollständigen Reequilibrierung der U/Pb Verhältnisse in den Albit-Felsiten. Die Isotopenverhältnisse der Sulfide, die Spuren von U und Pb enthalten, wurden während der Bildung von Bleiglanz um etwa 500–550 Ma ebenfalls massiv gestört; die Sulfide wurden aber offensichtlich gleichzeitig, oder aber kurz nach Davidit, gebildet. Basierend auf einer Abschätzung des Bleiglanz-Bildungsalters im Zuge der Entwicklung einer altpaläozoischen Peneplain in diesem Gebiet, werden die Pb/Pb Daten als Zweistufen-Isochrone mit einem Ursprungsalter von 1876 ± 15 Ma interpretiert.Die Bindung der Mineralisation an Scherzonen, die Übereinstimmung des Mineralisationsalters mit dem der Sveco-karelischen Orogenese und die Isotopeninitiale der erzbildenden Lösungen belegen einen metamorph/metasomatischen Ursprung der Gold-und Kupfervererzungen.

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With 9 Figures     

Fluid entrapment and reequilibration during subduction and exhumation: A case study from the high-grade Nestos shear zone, Central Rhodope, Greece

A fluid inclusion study on metamorphic minerals of successive growth stages was performed on highly deformed paragneisses from the Nestos Shear Zone at Xanthi (Central Rhodope), in which microdiamonds provide unequivocal evidence for ultrahigh-pressure (UHP) metamorphism. The correlation of fluid inclusion density isochores and fluid inclusion reequilibration textures with geothermobarometric data and the relative chronology of micro- and macro-scale deformation stages allow a better understanding of both the fluid and metamorphic evolution along the P–T–d path. Textural evidence for subduction towards the NE is recorded by the orientation of intragranular NE-oriented fluid inclusion planes and the presence of single, annular fluid inclusion decrepitation textures. These textures occur within quartz “foam” structures enclosed in an earlier generation of garnets with prolate geometries and rarely within recrystallized matrix quartz, and reequilibrated both in composition and density during later stages of exhumation. No fluid inclusions pertaining to the postulated ultrahigh-pressure stage for microdiamond-bearing garnet–kyanite–gneisses have yet been found. The prolate shape of garnets developed during the earliest stages of exhumation that is recorded structurally by (L  S) tectonites, which subsequently accommodated progressive ductile SW shearing and folding up to shallow crustal levels. The majority of matrix kyanite and a later generation of garnet were formed during SW-directed shear under plane-strain conditions. Fluid inclusions entrapped in quartz during this stage of deformation underwent density loss and transformed to almost pure CO₂ inclusions by preferential loss of H₂O. Those inclusions armoured within garnet retained their primary 3-phase H₂O–CO₂ compositions. Reequilibration of fluid inclusions in quartz aggregates is most likely the result of recrystallization along with stress-induced, preferential H₂O leakage along dislocations and planar lattice defects which results in the predominance of CO₂ inclusions with supercritical densities. Carbonic fluid inclusions from adjacent kyanite–corundum-bearing pegmatoids and, the presence of shear-plane-parallel fluid inclusion planes within late quartz boudin structures consisting of pure CO₂-fluid inclusions with negative crystal shapes, bear witness of the latest stage of deformation by NE-directed extensional shear.This study shows that the textures of early fluid inclusions that formed already during the prograde metamorphic path can be preserved and used to derive information about the kinematics of subduction that is difficult to obtain from other sources. The textures of early inclusions, together with later generations of unaltered primary and secondary inclusions in metamorphic index minerals that can be linked to specific deformation stages and even P–T conditions, are a welcome supplement for the reconstruction of a rather detailed P–T–d path.     

Thermobarometric modelling of zircon and monazite growth in melt-bearing systems: examples using model metapelitic and metapsammitic granulites

U–Pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the Earth. Despite the emphasis placed on zircon and monazite ages, the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy. In addition, few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample. Such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner, as per this study. The data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock. Here experimental data for zircon and monazite saturation in melt-bearing rocks are integrated with phase diagram calculations. The calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure, temperature and silicate mineral assemblages in high-grade, melt-bearing, metasedimentary rocks. Several key results emerge from this modelling: first, that in aluminous metapelitic rocks (i.e. garnet + cordierite + sillimanite assemblages), zircon ages are older than monazite ages in the same rock; second, that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures; and third, that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature(s) at which zircon and monazite grow rather than differences in closure temperature systematics. Using our methodology the cooling rate of granulites from the Reynolds Range, central Australia, have been constrained at ∼4 °C Myr⁻¹. This study serves as a first-pass template on which further research in applying the technique to a field study can be based.     

Identification and implications of fight hydrocarbon fluid inclusions from the Proterozoic Bidjovagge gold-copper deposit, Finnmark, Norway

Carbonic fluid inclusions were observed in quartz-bearing veins at the Proterozoic Bidjovagge AuCu deposit within the Kautokeino greenstone belt in Norway, where mineralization occurred in oxidation zones of graphitic schists. A primary fluid inclusion zonation was observed with C0₂-rich fluid inclusions in the structural footwall of the deposit, and CH₄-rich inclusions within the ore zone in the oxidation zone. Microthermometry of the primary hydrocarbon inclusions revealed 2 groups; (1) a group which homogenized between −125°C and the critical temperature of CH₄ (−82.1°C), which indicated the presence of pure CH₄, and (2) a group which homogenized between the critical temperature of CH₄ and −42°C, which indicated the presence CH₄ and higher hydrocarbons (HHC). Raman microprobe analyses of the first group confirmed the presence of CH₄. The second inclusion group were fluorescent, and Raman spectra clearly displayed CH₄,C₂H₆, and rarer C₃H₈ peaks. A typical feature of the Raman spectra were elevated baselines at the hydrocarbon peaks. Carbon peaks were also usually detected in each inclusion by Raman analysis. Bulk gas chromatography analyses of samples containing the first group (CH₄) indicated the presence of CH₄ and low concentrations of C₂H₆ and C₃H₈. Gas chromatography analyses of samples containing the second group (CH₄ and higher hydrocarbons) confirmed the presence of CH₄, and higher hydrocarbons such as C₂H₆ and C₃H₈ and also butanes. Based on the spacial zonation of hydrocarbons and the estimated PT conditions of 300 to 375°C and 2 to 4 kbars, the authors suggest an abiotic origin for the hydrocarbons. It is suggested that the hydrothermal fluids oxidized the graphitic schist, precipitated Cu and Au and formed light gas hydrocarbons.     

俯冲带深部流体:来自超高压变质锆石晶内结构和包裹物的证据

利用阴极发光和激光拉曼研究了大别山俯冲带超高压基性和长英质变质岩中的锆石。超高压变质增生锆石环带或新生锆石颗粒从不同角度反映了俯冲带地幔深度流体作用信息。其一,在超高压变质锆石生长域中存在微米级流体包裹体。由于其十分细小,组成往往难以测定。但是在变基性岩中检测到富CO2的负晶形包裹体。其二,在超高压锆石生长域中较普遍地发现富含挥发份的多硅白云母和磷灰石包裹体,其与典型的超高压变质矿物———柯石英稳定共生。副矿物研究表明,即使是在新鲜的弱退变质的榴辉岩中,富含挥发份的矿物相———磷灰石仍是含量较高的矿物,说明在俯冲带的深部(>100km),流体仍可以稳定地保存在高挥发份的矿物晶体格架中。其三,超高压变质增生锆石环带的边界呈蚕食状或港湾状,指示流体参与了锆石的变质生长过程,即通过流体与锆石之间的相互作用导致原有锆石的溶解和新生锆石的沉淀结晶。考虑到如此高的锆溶解度,认为在地幔俯冲深度,流体的成分复杂,并不是简单的富水体系,其化学组成与深熔融体相似,且赋存状态应为含水融体相。     

Sedimentation and water escape structures in some late Precambrian shallow marine sandstones from Finnmark, North Norway

Water escape structures are abundant in the Grønnes Formation, a tectonically undeformed, late Precambrian shallow marine sandstone deposit in North Norway. Trough cross-bedded sandstones of the current-dominated shallow marine environment were frequently liquefied, presumably due to recurring seismic shocks. Subsequent dewatering resulted in deformation of the cross-bedding and the formation of convolute lamination. A three-fold upward vertical sequence developed where liquefaction occurred below the sediment-water interface: convoluted bed → passively deformed bed → undeformed bed. The passively deformed bed resulted from differential subsidence of a relatively plastic bed above a liquefied bed. It is characterized by anticlinal ridges and sand volcanoes at the sites of vertical sediment extrusion, and synclinal troughs at the sites of lateral sediment movement. Liquefaction may have been induced by either tectonic (earthquake shocks) or non-tectonic (storm-induced microseisms) trigger mechanisms, or a combination of both. The restriction of such a high frequency of water escape structures to deposits immediately above a gentle regional unconformity lends support for a tectonic trigger mechanism.     

Reactive monazite and robust zircon growth in diatexites and leucogranites from a hot,slowly cooled orogen: implications for the Palaeoproterozoic tectonic evolution of the central Fennoscandian Shield,Sweden

Monazite in melt-producing, poly-metamorphic terranes can grow, dissolve or reprecipitate at different stages during orogenic evolution particularly in hot, slowly cooling orogens such as the Svecofennian. Owing to the high heat flow in such orogens, small variations in pressure, temperature or deformation intensity may promote a mineral reaction. Monazite in diatexites and leucogranites from two Svecofennian domains yields older, coeval and younger U–Pb SIMS and EMP ages than zircon from the same rock. As zircon precipitated during the melt-bearing stage, its U–Pb ages reflect the timing of peak metamorphism, which is associated with partial melting and leucogranite formation. In one of the domains, the Granite and Diatexite Belt, zircon ages range between 1.87 and 1.86 Ga, whereas monazite yields two distinct double peaks at 1.87–1.86 and 1.82–1.80 Ga. The younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late-orogenic shear zones. Magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to Th/U ratios and preferential growth sites of zircon, reflecting the transition from melt to melt crystallisation of the magma. In the adjacent Ljusdal Domain, peak metamorphism in amphibolite facies occurred at 1.83–1.82 Ga as given by both zircon and monazite chronology. Pre-partial melting, 1.85 Ga contact metamorphic monazite is preserved, in spite of the high-grade overprint. By combining structural analysis, petrography and monazite and zircon geochronology, a metamorphic terrane boundary has been identified. It is concluded that the boundary formed by crustal shortening accommodated by major thrusting.     

Long-term deformation and fluid-enhanced mass transport in a Variscan peridotite shear zone in the Ivrea Zone,northern Italy: a microtextural,petrological and geochemical study of a reactivated shear zone

The microtextural, petrological and geochemical study of a ductile shear zone in the phlogopite peridotite of Finero/Ivrea Zone (northern Italy) reveals the long-term deformation of this zone. The zone is divided into a protomylonitic and an ultramylonitic part. Both parts reflect different periods of deformation, although the orientation of the mineral lineations does not change. In the coarse-grained part (period 1) the deformation started under granulite facies conditions (about 775°C). Olivine, ortho- and clinopyroxene and phlogopite recrystallized dynamically. In the ultramylonitic part relics of the granulite facies event and evidence for a continuous or two-stage deformation history under amphibolite facies (minimum 640°C) to upper greenschist facies conditions (maximum 520°C) are preserved (period 2). Amphibolite facies conditions are indicated by olivine recrystallization, the monoclinization of orthopyroxene porphyroclasts and the recrystallization and chemical changes of clinopyroxene. The greenschist facies final stage of period 2 is characterized by decreased X _{CO 2} and the syntectonic formation of antigorite, tremolite and phlogopite at the expense of recrystallized and porphyroclastic olivine and pyroxene. Between both deformation periods a short break in deformation continuity is probable. Continuous deformation or reactivation in shear zones of the Ivrea Zone has not been described so far. During the granulite facies shearing, small amounts of channelized fluid flow led to a slight mass transfer. The shear zone shows a moderate enrichment of Na, Ba, Cu, Cs, H₂O and CO₂ and a small loss of P. The mass balance of the ultramylonite indicates a significant increase in mass transport. A mass gain can be inferred for H, Na, K, Ba, Al, Ti, P, S, Cs, Sr, Rb, C, Zn, Zr, S, Sc, light rare earth elements, Nb, Cl and Au. The zone is depleted in Ca, Cu, Co, F and Ni. Si, Mg, Cr, Mn, Y, Nb and V are constant within analytical error. Deformation and fluid infiltration led to a change in volume which increases during the granulite facies event by 5.7% and during the lower temperature phase by 3.3%. The calculated fluid to rock ratios by standard equations results in unrealistically high values. For the interpretation of highly deformed rocks with drastic grain size reduction it is therefore necessary to consider the enhanced diffusion, which is mainly controlled by the increased grain boundary surface.     

Residence time of S-type anatectic magmas beneath the Neogene Volcanic Province of SE Spain: a zircon and monazite SHRIMP study

Zircon and monazite from three restitic enclaves and one host dacite have been dated by ion microprobe (SHRIMP), with the aim of characterising their Miocene history and defining the timing relationships between crustal melting and eruption in the high-K calc-alkaline volcanics of the Neogene Volcanic Province of SE Spain. The studied samples are from the volcanic edifices of El Joyazo (Cerro del Hoyazo) and Mazarrón. Zircons in the enclaves are characterized by a thin euhedral rim overgrowing a detrital core. The core-rim boundary is marked by tiny glass inclusions of S-type granitic composition, which attest to the growth of zircon rims during a crustal melting event. At El Joyazo, where lavas erupted at 6.3 Ma (Zeck and Williams 2002), U-Pb ages of zircon overgrowths define an age of anatexis of 9.63±0.26 Ma (95% c.l.), in agreement with monazite ages of 9.74±0.21 Ma (95% c.l.). At Mazarrón, the age of anatexis provided by monazite at 9.13±0.18 Ma (95% c.l.) overlaps with that of melt-precipitated zircon in the host dacite, dated at 9.06±0.53 Ma (95% c.l.). These results indicate that after partial melting, the enclaves and the syngenetic S-type melts resided at depth for >3 m.y. at El Joyazo. Compared with the results from Mazarrón, the long residence time obtained at El Joyazo is probably due to the greater depth of melting (c. 25 km vs. c. 15 km). At such depth, corresponding to the Miocene palaeo-Moho, the more ductile regime of the crust is likely to have favoured magma ponding. The thermal anomaly beneath the Neogene Volcanic Province, which generated the S-type crustal melts, is today visible from geophysical data and can be traced back to the Lower Miocene. As a consequence, residence times longer than determined in this work may be expected.Editorial responsibility: J. Hoefs