Metamorphic Host Rocks Research Articles

Relationship between magnetic fabrics and deformation of the Miocene Pohorje intrusions and surrounding sediments (Eastern Alps)

The Miocene deformation history of magmatic and host metamorphic rocks and surrounding sediments was reconstructed by measuring meso- and microscale structures and anisotropy of magnetic susceptibility (AMS) data in order to constrain the structural evolution of the Pohorje pluton during the onset of lithospheric extension at the Eastern Alps–Pannonian Basin transition. Principal AMS axes, lineation and foliation are very similar to mesoscopic lineation and foliation data from the main intrusive body and from some dykes. Although contribution from syn-magmatic texture is possible, these structures were formed during the cooling of the pluton and associated subvolcanic dykes just shortly after the 18.64 Ma pluton intrusion. Dykes emplaced during progressively younger episodes reflect decreasing amount of ductile strain, while firstly mesoscopic foliation and lineation, and then the tectonic AMS signal gradually disappears. In the structurally highest N–S trending dacite dykes, the AMS fabric only reflects the magmatic flow. The Miocene sediments underwent the same, NE–SW to E–W extension as the magmatic and host metamorphic rocks as indicated by both AMS and fault-slip data. All these events occurred prior to ~ 15 Ma, i.e., during the main syn-rift extension of the Pannonian Basin and during the fastest exhumation of the Tauern and Rechnitz windows, both demonstrating considerable extension of diverse crustal segments of the Alpine nappe pile. After a counterclockwise rotation around ~ 15 Ma, the maximum stress axis changed to a SE–NW orientation, but it was only registered by brittle faulting. During this time, the overprinting of a syn-rift extensional AMS texture was not possible in the cooled or cemented magmatic, metamorphic and sedimentary rocks.

Petrology of Nickeliferous Mafic–Ultramafic Bodies of the Southeastern Aldan-Stanovoy Shield

Consideration is given to the nature of mafic–ultramafic bodies in the southeastern part of the Aldan-Stanovoi Shield using the Kun-Manie ore field as an example. These bodies played an important role in the formation of the Kun-Manie sulfide copper–nickel deposit as conduits of sulfide-bearing magmas. A detailed mineralogical and geochemical study with interval sampling was carried out on core samples from the drilling of two intrusive bodies, Treugolnik and Iken, using drill holes 21 and 25. The drill–core data present evidence for their origin from a single parent magma. The Treugolnik and Iken minor intrusions were emplaced in magmatic chambers at different depths during the cotectic crystallization of dark minerals from the primary picrite (Treugolnik body) and intermediate picrobasalt (Iken body) magmas. A comparative study of petrogeochemical and mineralogical data suggests that the Treugolnik intrusion formed as a closed system with magma emplaced in a single, relatively rapid injection and crystallized without losses; the Iken sill formed as an open system as a result of magma emplacement in several pulses with time intervals between injections. These magma bodies, together with the host metamorphic rocks, were involved in tectonic deformations after they crystallized.

Crystallization conditions and compositional variations of silicate and sulfide minerals in the Pb-Zn skarn deposits, Biga Peninsula, NW Turkey

The Biga Peninsula (NW Turkey) is an important sector of the Tethyan Metallogenic Belt. In addition to Au and Ag, this region hosts several Pb, Zn, Cu, Fe, and Mo deposits. Pb-Zn skarn mineralizations in the Biga Peninsula (Biga Pb-Zn skarn) are bounded by carbonate lenses in the metamorphic rocks and localized along the regional faults and at the contact between carbonate units and igneous-metamorphic rocks. The primary ore minerals are galena, sphalerite, chalcopyrite, pyrite, arsenopyrite, magnetite, and hematite. Ore-bearing altered rocks are composed primarily of calc-silicate alteration minerals, including both prograde (grossular-andradite garnets and johannsenite-hedenbergite pyroxenes) and retrograde (epidote, chlorite, quartz, and calcite) assemblages. The garnet composition is a solid solution with <96% andradite and 1.4% to 94.4% grossular content. Some garnet grains exhibit zonings because of fluctuations in the content of Al3+ and Fe3+. Mn/Fe ratios are high, and the outermost grains have a high MnO content, up to 3 wt%. The pyralspite content is <10 wt%. The proportions of johannsenite and hedenbergite in clinopyroxenes range from 0.35 to 94.69 mol% and 2.13 to 77.90 mol%, respectively. Clinopyroxenes have high Mn/Fe values, with Mn content up to 10 wt%. The epidote-group minerals with a high Al3+ and Fe3+ content belong to the epidote-clinozoisite series (Ep37-21Czo78-63Pie1-0). Mineral chemistry data indicate that Biga Pb-Zn skarn deposits and distal Pb-Zn skarn deposits throughout the world have similar characteristics. The abundance of graphite in the metamorphic host rocks and the andradite-hedenbergite content of the skarns imply that the studied mineralizations were formed under reducing conditions. The ranges of crystallization depth and pressure values calculated from the chemistry of garnet-clinopyroxene pairs are 1.3–1.6 km and 10–12 MPa, respectively. Because these values are less than those of plutons hosting the Biga Pb-Zn skarn deposits, the mineralizations likely formed in the distal zones of plutons.

Rare-earth accessories and secondary minerals in plagiogneisses from the Stankuvatske Li-deposite

The Stankuvatske Li deposit (SD) is situated at western flank of the Lypniazka structure (Ingul megablock of the Ukrainian Shield). Knowledge about REE content in host metamorphic rocks is based on the results of bulk chemical analysis, but their minerals have not been determined. For the first time rare-earth mineralization of the Stankuvatsky lithium deposit has been investigated in fine-grained gneiss with «augen» and schistose structure, porphiroblastic texture, formed as result of tectonical alteration. Our investigations were carried out using petrographic and microprobe analysis (EPMA). Mineralogically gneisses consist of quartz, plagioclase, zoizite, biotite, graphite, chlorite and abundant sulphides mainly represented by pyrite, arsenopyrite, sphalerite, molibdenite. Accessory minerals presented by titanite, apatite, monazite, zircon and coffinite. Gneisses have been subjected to deformation and hydrothermal — metasomatic alteration. An investigation of rock-forming and accessory minerals allows to revel low-temperature alterations of primary allanite by bastnäsiteand chlorite with formation of secondary bastnäsite-chlorite-coisite-pyrite association with «coronary» texture. The penetration of S, F, CO2, H2O enriched fluids were caused disintegration, partial redistribution and reprecipitation of rare earth elements. As result synhysite-chlorite-pyrite association was formed.

Minerály kontaminovaných granitových pegmatitů z lomu Pohled u Havlíčkova Brodu (moldanubikum), část I: oxidy, karbonáty, silikáty a fosfáty

In the Pohled quarry near Havlíčkův Brod town (central part of Czech Republic), granitic pegmatites form dikes or irregular bodies cementing breccia of host metamorphic rocks (paragneisses, amphibolites) belonging to the Monotonous (Ostrong) Group of the Moldanubicum of the Bohemian Massif. Pegmatites exhibit coarse-grained textures and very simple mineral composition, which, however, was in places strongly modified by superimposed hydrothermal alterations and locally also by crystallization of ore minerals. K-feldspar, plagioclase (An0-38), biotite (phlogopite), apatite (fluorapatite to hydroxylapatite), zircon, allanite-(Ce) and part of quartz undoubtedly originated during magmatic stage. Products of hydrothermal alterations include younger quartz, Fe-Mg chlorites (older clinochlore and younger chamosite with an admixture of Ca-smectite), prehnite, clinozoisite, amphibole (actinolite), titanite, calcite, and very probably also ilmenite (with up to 22.5 mol. % of pyrophanite), rutile, anatase and V-Cr-Fe-rich grossular with 13 - 25 mol.% of goldmanite and 12 - 24 mol.% uvarovite. The elevated contents of Mg, Ca, V and Cr found in some minerals are associated with a material contamination by the surrounding metamorphic rocks (especially amphibolites, serpentinites and perhaps also graphitic lithologies), which took place with varying intensity during both magmatic and hydrothermal stages of evolution of the studied pegmatites. Geochemically, they are poorly fractionated pegmatites, whose origin was probably connected with anatexis of the host Moldanubian metamorphic rocks, which was likely associated with emplacement of adjacent small body of the Pohled Granodiorite.

Emerald Deposits: A Review and Enhanced Classification

Although emerald deposits are relatively rare, they can be formed in several different, but specific geologic settings and the classification systems and models currently used to describe emerald precipitation and predict its occurrence are too restrictive, leading to confusion as to the exact mode of formation for some emerald deposits. Generally speaking, emerald is beryl with sufficient concentrations of the chromophores, chromium and vanadium, to result in green and sometimes bluish green or yellowish green crystals. The limiting factor in the formation of emerald is geological conditions resulting in an environment rich in both beryllium and chromium or vanadium. Historically, emerald deposits have been classified into three broad types. The first and most abundant deposit type, in terms of production, is the desilicated pegmatite related type that formed via the interaction of metasomatic fluids with beryllium-rich pegmatites, or similar granitic bodies, that intruded into chromium- or vanadium-rich rocks, such as ultramafic and volcanic rocks, or shales derived from those rocks. A second deposit type, accounting for most of the emerald of gem quality, is the sedimentary type, which generally involves the interaction, along faults and fractures, of upper level crustal brines rich in Be from evaporite interaction with shales and other Cr- and/or V-bearing sedimentary rocks. The third, and comparatively most rare, deposit type is the metamorphic-metasomatic deposit. In this deposit model, deeper crustal fluids circulate along faults or shear zones and interact with metamorphosed shales, carbonates, and ultramafic rocks, and Be and Cr (±V) may either be transported to the deposition site via the fluids or already be present in the host metamorphic rocks intersected by the faults or shear zones. All three emerald deposit models require some level of tectonic activity and often continued tectonic activity can result in the metamorphism of an existing sedimentary or magmatic type deposit. In the extreme, at deeper crustal levels, high-grade metamorphism can result in the partial melting of metamorphic rocks, blurring the distinction between metamorphic and magmatic deposit types. In the present paper, we propose an enhanced classification for emerald deposits based on the geological environment, i.e., magmatic or metamorphic; host-rocks type, i.e., mafic-ultramafic rocks, sedimentary rocks, and granitoids; degree of metamorphism; styles of minerlization, i.e., veins, pods, metasomatites, shear zone; type of fluids and their temperature, pressure, composition. The new classification accounts for multi-stage formation of the deposits and ages of formation, as well as probable remobilization of previous beryllium mineralization, such as pegmatite intrusions in mafic-ultramafic rocks. Such new considerations use the concept of genetic models based on studies employing chemical, geochemical, radiogenic, and stable isotope, and fluid and solid inclusion fingerprints. The emerald occurrences and deposits are classified into two main types: (Type I) Tectonic magmatic-related with sub-types hosted in: (IA) Mafic-ultramafic rocks (Brazil, Zambia, Russia, and others); (IB) Sedimentary rocks (China, Canada, Norway, Kazakhstan, Australia); (IC) Granitic rocks (Nigeria). (Type II) Tectonic metamorphic-related with sub-types hosted in: (IIA) Mafic-ultramafic rocks (Brazil, Austria); (IIB) Sedimentary rocks-black shale (Colombia, Canada, USA); (IIC) Metamorphic rocks (China, Afghanistan, USA); (IID) Metamorphosed and remobilized either type I deposits or hidden granitic intrusion-related (Austria, Egypt, Australia, Pakistan), and some unclassified deposits.

Chronology of the Kašperské Hory orogenic gold deposit, Bohemian Massif, Czech Republic

The Kasperske Hory deposit represents one of the economically most important hydrothermal gold deposits in the Bohemian Massif (Czech Republic). We present new Re–Os age determinations for sulfides complemented by Ar–Ar determinations on silicate minerals in order to provide temporal constraints on the deposit formation. Arsenopyrite and molybdenite formed during gold-bearing stages I–III yielded similar Re–Os ages between ~ 341 and 333 Ma, overlapping with the Ar–Ar cooling age of the metamorphic host rocks below ~ 500 °C. Continuous cooling of the ore system below 270 °C during stage III is reflected by Ar–Ar muscovite ages spanning from ~ 332 to 325 Ma representing the long interval of formation of gold-productive stage III. Thus, the Kasperske Hory gold deposit is younger than the gold deposits spatially associated with the Central Bohemian Plutonic Complex (~ 349–339 Ma) belonging to the Central Bohemian Metallogenetic Zone (e.g., Mokrsko-West, Jilove, Petrackova Hora). This may indicate a rather close relationship to the nearby Moldanubian Batholith while still suggesting an intimate link between mineralization and Variscan orogenesis and associated magmatism.

Can vein-filling speleothems constrain the timing of deep-seated gravitational slope deformation? A case study from the Vinschgau (Italian Alps)

The steep, south facing slopes of the central Vinschgau (Val Venosta, South Tyrol, northern Italy) are characterized by deep-seated compound rockslides affecting 51 km2 and reaching beneath the Quaternary valley fill. Morphological features include double ridges, trenches, scarps, and counterscarps, whereby the extent of displacement rises from W to E. Near the toe of these slopes, abundant fractures are present whose orientation and spatial strongly suggest that they are related to the gravitational slope deformations. These fractures host carbonate speleothems which are associated with local tufa occurrences at the surface. Given that the metamorphic host rock is essentially carbonate-free, these carbonate deposits require extensive water-rock interactions to form. Modern springs along the toe of the slope are also mostly supersaturated with respect to calcite and locally also aragonite. The invariant chemical composition, very low tritium levels, and in particular the low oxygen isotope values indicate that this groundwater infiltrates at high elevation and is characterized by a long residence time. On the other hand, local infiltration in the lower reaches of the slope is very limited due to the semiarid climate and is incompatible with the measured groundwater stable isotope composition and the high degree of mineralization. We therefore propose a conceptual model which reconciles the deep-seated gravitational slope deformation with the occurrence of mineralized groundwater and associated carbonate precipitates in fractures near the toe of the slope. These deposits provide a means to place constraints on the timing of rock fracturing and hence the slope movements. U/Th dating of 34 samples from eight sites shows that speleothem deposition started 14.2 ka BP and occurred semi-continuously throughout the Holocene. Thus, gravitational displacements likely commenced with a lag of ~3–4 ka after the deglaciation of the valley, which is also consistent with the few available dates on the onset of other deep-seated slope deformations in the Alps.

New 40Ar/39Ar age constraints on cooling and unroofing history of the metamorphic host rocks (and igneous intrusion associates) from the Bulfat Complex (Bulfat area), NE-Iraq

The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~ 40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~ 80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. 40Ar/39Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78 ± 0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~ 400 °C/Ma and ~ 3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78 ± 0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).

Structural control, magmatic-hydrothermal evolution and formation of hornfels-hosted, intrusion-related gold deposits: Insight from the Thaghassa deposit in Eastern Anti-Atlas, Morocco

In the Moroccan Eastern Anti-Atlas, the Thaghassa intrusion-related gold deposit is hosted in hornfelsed metasedimentary rocks that lie adjacent to the Ikniwn granodiorite. Field studies reveal three tectono-magmatic stages controlling the formation of the deposit. i) The first stage refers to the top-to-the-south asymmetry and the syn-kinematic Ikniwn pluton emplacement controlled by a compressional or transpressional strain regime. ii) The second stage is characterized, from older to younger and further away from the intrusion, by: metatexite with leucocratic stromatic bands, aplo-pegmatite sills, intermediate veinlets composed of quartz, K-feldspar and muscovite, and then gold-bearing striped foliation-veins. All these features are assumed to have been emplaced during a large-scale ENE-WSW dextral shearing process that results from an ESE-WNW shortening direction during transtensive tectonics. We suggest that the progressive and continuous shearing was initiated since the aplo-pegmatite stage and achieved during the hydrothermal phase. The existence of intermediate veins characterized by quartz-rich core and apatite-muscovite-feldspar-rich rims demonstrates the progressive evolution from the magmatic to the hydrothermal stage and evidence for the persistence of the magmatic character, at least until the onset of the hydrothermal process. iii) The late stage developed large volcanic dyke swarm and brittle faulting.Zircon U-Pb LA-ICP-MS dating yields a Concordia age of 563.5 ± 6.3 Ma for the Ikniwn granodiorite intrusion. The fluid inclusions data besides the mineral thermometry indicate that two main types of fluids can been highlighted: i) a hot aquo-carbonic (H2O-NaCl-CO2) fluid with N2 and CH4, evolving from vapour-rich N2 and CH4 inclusions for the magmatic stage (∼550 °C) to CH4-CO2 biphased inclusions for the ongoing hydrothermal stage (∼450–300 °C) and ii) an always secondary low T (∼200 °C) saline aqueous type (probably NaCl) free from volatiles, with very variable salinity. Eventually, we highlight that in the hornfels-hosted IRG deposits, fluid sources may originate from both magmatic processes and devolatization of the metamorphic host rocks.

Structural and Petrologic Studies of Bumaji Area, Southeastern Nigeria

The study area is part of the Basement Complex of Southeastern Nigeria. The major rock units in the area comprises of intrusives (pegmatites, dolerites and charnockites), and the host metamorphic rocks are; quartzites, migmatites, schists, and biotite gneiss. Structural evidence shows that the rocks are deformed and were intruded by the magmatic rocks. The metamorphic rocks are strongly deformed in N-S to NE-SW direction. The charnockites is composed of quartz, 32%, plagioclase, 25%, K-feldspar, 20%, hornblende, 15% and other accessory minerals. The dolerite consists of olivine, 30%, biotite, 17% and hornblende, 16%. It shows ophitic texture. Schist and migmatite indicatess faults, fractures, mineral lineations and planar features. Variation in structural trend and mineralogical composition of the metamorphic rocks is due to the series of deformational activities; metamorphism and magmatism, which has affected the rocks during geologic time. The small scale geological structures which were used to interpret large scale structures, are of the size a field outcrop scale of 1cm to 10km of the district geological features.

Geochemical modeling of orogenic gold deposit using PCANN hybrid method in the Alut, Kurdistan province, Iran

In this paper stream sediments based geochemical exploration program with the aim of delineating potentially promising areas by a comprehensive stepwise optimization approach from univariate statistics, PCA, ANN, and fusion method PCANN were under taken for an orogenic gold deposit located in the Alut, Kurdistan province, NW of Iran. At first the data were preprocessed and then PCA were applied to determine the maximum variability directions of elements in the area. Subsequently the artificial neural network (ANN) was used for quick estimation of elemental concentration, as well as discriminating anomalous populations and intelligent determination of internal structure among the data. However, both the methods revealed constraints for modeling. To overcome the deficiency and shortcoming of each individual method a new methodology is presented by integration of both “PCA & ANN” referred as PCANN method. For integrating purpose, the detected PCs pertinent to ore mineralization selected and intruded to neural network structure, as a result different MLPs with various algorithms and structures were produced. The resulting PCANN maps suggest that the gold mineralization and its pathfinder elements (Au, Mo, W, Bi, Sb, Cu, Pb, Ag & As) are associated with metamorphic host rocks intruded by granite bodies in the Alut area. In addition, more concealed and distinct Au anomalies with higher intensity were detected, confirming the privileges of the method in evaluating susceptibility of the area in delineating new hidden potential zones. The proposed method demonstrates simpler network architecture, easy computational implementation, faster training speed, as well as no need to consider any primary assumption about the behavior of data and their probability distribution type, with more satisfactory predicting performance for generating gold potential map of the area. Comparing the results of three methods (PCA, ANN and PCANN), representing the higher efficiency and more reliability of PCANN with lesser training time, simple structure, and correlate components while avoiding the duplicate entry of data to network. This study also suggests that in many similar cases integrated methods have capability to fix bugs more effectively and successfully in exploration programs.

Loparite-(Ce) from the Khibiny Alkaline Pluton, Kola Peninsula, Russia

Data on the occurrence, morphology, anatomy, composition, and formation conditions of loparite-(Ce) in the Khibiny alkaline pluton are given. Loparite-(Ce), (Na,Ce,Sr)(Ce,Th)(Ti,Nb)2O6, resulted from metasomatic alteration and assimilation of metamorphic host rocks at the contact with foyaite as well as foyaite on the contact with foidolite. This alteration was the highest in pegmatite, and albitite developed there. A decrease in temperature resulted in enrichment of the perovskite and tausonite endmembers in loparite-(Ce) owing to a decrease in the loparite and lueshite endmembers. La and Ce sharply predominate among rare earth elements in the composition of loparite-(Ce).

The Traversella Mining Site as Piemonte Geosite

The Traversella mining area, nearly 60 km North of Torino, was, together with Brosso, a very important mining district for iron exploitation. The Traversella ore deposit was exploited from late Medieval Age up to the middle XX century. It is a typical skarn deposit at the contact between a dioritic body and preexisting metamorphic host rocks, which represents the only exploited skarn-type mineralization in the Alps. The iron ore mineral, exploited from different orebodies, was primarily magnetite. After the beginning of XX century, the extraction involved also pyrite and chalcopyrite, used mainly for the production of sulfuric acid. The mine, after some interruptions and re-openings, was officially closed in 1971, due to the high exploitation costs and the competition of the foreign iron ore deposits. The present research aims to emphasize the extraordinary importance of this mining site both from a scientific and a historical point of view, by also examining the exploitation and ore processing methods and the amount of production during the last three centuries, and highlighting how these activities contributed to the industrial development of the surrounding area and of the whole Piemonte Region. It aims also at illustrating the sociological and environmental impact of mining activities at regional level, highlighting the importance of the site from a geotouristic point of view, through the cultural exploitation of the mining site remains, the development and upgrade of the already existing mining museum, and the organization of geotouristic itineraries.

Timing the tectonic mingling of ultramafic rocks and metasediments in the southern section of the coastal accretionary complex of central Chile

ABSTRACTUranium-lead ages are reported for zircons from ultramafic bodies and metamorphic host rocks of the Western Series that outcrop at La Cabaña, in the southern section of the coastal accretionary complex of central Chile. Metasedimentary mica schists hosting the ultramafic bodies contain a main detrital zircon population of Devonian age (365–380 Ma) clustering around ~368 Ma, differing significantly from neighbouring areas where Devonian zircons are scarce. Zircons from the metasomatic reaction zones (albitites and chloritites), formed during the emplacement and alteration of the ultramafic bodies, are mainly Ordovician (~478 Ma) and lack Devonian zircons, resembling a typical detrital zircon pattern from other locations in the Western Series. Zircons from the chloritite reaction zone of the Lavanderos serpentinite, the easternmost ultramafic body in La Cabaña, are in textural equilibrium with metamorphic ilmenite. Some of these zircons yield an average age of 283.4 ± 7 Ma (n = 6) which is identical, within error, to a previously reported K-Ar fuchsite cooling age of 282 ± 6 Ma from the reaction zone. Most zircons extracted from chromitite boulders have euhedral oscillatory-zoned growth patterns with a similar range of ages than those reported for the Western Series (324–1090 Ma; n = 12), except for two zircons with cloudy appearance and high U/Th ratios which yielded an average age of 285.5 ± 7 Ma. The presence of Early Permian zircons (~280–290 Ma) in all studied rocks suggests remobilization of Zr, possibly triggered by metasomatic fluids released during the disequilibrium reaction associated with the tectonic emplacement of the ultramafic rocks into the metasedimentary rock. Simultaneously with the formation of metasomatic zircons, Palaeozoic and Mesoproterozoic zircons from the metasedimentary rocks were mechanically incorporated into the ultramafic rocks, thus providing a record of the timing of crustal emplacement of the ultramafic rocks into the accretionary complex.

Carbon dioxide generation and drawdown during active orogenesis of siliciclastic rocks in the Southern Alps, New Zealand

Collisional mountain building influences the global carbon cycle through release of CO2 liberated by metamorphic reactions and promoting mechanical erosion that in turn increases chemical weathering and drawdown of atmospheric CO2. The Southern Alps is a carbonate-poor, siliciclastic mountain belt associated with the active Australian Pacific plate boundary. On-going, rapid tectonic uplift, metamorphism and hydrothermal activity are mobilising carbon. Here we use carbon isotope measurements of hot spring fluids and gases, metamorphic host rocks, and carbonate veins to establish a metamorphic carbon budget.We identify three major sources for CO2 within the Southern Alps: (1) the oxidation of graphite; (2) consumption of calcite by metamorphic reactions at the greenschist–amphibolite facies boundary, and (3) the dissolution of groundmass and vein-hosted calcite. There is only a minor component of mantle CO2 arising on the Alpine Fault. Hot springs have molar HCO3−/Ca2+ ∼9, which is substantially higher than produced by the dissolution of calcite indicating that deeper metamorphic processes must dominate.The total CO2 flux to the near surface environment in the high uplift region of the Southern Alps is estimated to be ∼6.4 × 108 mol/yr. Approximately 87% of this CO2 is sourced from coupled graphite oxidation (25%) and disseminated calcite decarbonation (62%) reactions during prograde metamorphism. Dissolution of calcite and mantle-derived CO2 contribute ∼10% and ∼3% respectively. In carbonate-rich orogens CO2 production is dominated by metamorphic decarbonation of limestones. The CO2 flux to the atmosphere from degassing of hot springs in the Southern Alps is 1.9 to 3.2 × 108 mol/yr, which is 30–50% of the flux to the near surface environment. By contrast, the drawdown of CO2 through surficial chemical weathering ranges between 2.7 and 20 × 109 mol/yr, at least an order of magnitude greater than the CO2 flux to the atmosphere from this orogenic belt. Thus, siliciclastic mountain belts like the Southern Alps are net sinks for atmospheric CO2, in contrast to orogens involving abundant carbonate rocks, such as the Himalaya, that are net CO2 sources.

Petrochemistry of Khunrang intrusive complex, southeast of Kerman, Iran: Implications for magmatic evolution of Sanandaj-Sirjan zone in the Mesozoic time

A noticeable characteristic of the Sanandaj-Sirjan zone (SSZ) of Iran is the presence of extensive mafic to felsic intrusive igneous rocks in the host metamorphic rocks of Palaeozoic and Mesozoic eras. A similar composition is evident in batholithic size Khunrang Instrusive Complex (KIC) of southern SSZ. The rocks that make the KIC complex are mostly leucocratic microdiorite, quartz diorite, tonalite, granodiorite, granite and subordinate mesocratic to melanocratic pyroxene hornblende-gabbro and microgabbro. Field evidence and geochemical data suggest that the felsic rocks are not the products of fractional crystallization from a mafic phase. Using various discrimination and normalized multi-element diagrams suggest that mafic rocks, of tholeiitic to calc-alkaline affinities, were formed in an island arc or continental arc setting, from a metasomatized lithospheric mantle, above the stability field of garnet in a subduction zone environment. The felsic rocks, calc-alkaline and metaluminous in nature, have I-type granite characteristics. Their relative enrichment in large ion lithophile elements (LILEs) such as Ba, Cs and K and depletion in high field strength elements (HFSEs) such as Nb, Ta and Ti, is a signature of their development in an arc related environment in an active continental margin, similar to KIC mafic rocks. Geochemical characteristics suggest that the KIC felsic rocks were formed by partial melting of metabasic rocks of lower crust in response to underplating of mantle-derived basaltic magmas in an active continental margin as a result of Neo-Tethys oceanic crust subduction beneath the Central Iranian microcontinent in Mesozoic time.

S-type granite generation and emplacement during a regional switch from extensional to contractional deformation (Central Iberian Zone, Iberian autochthonous domain, Variscan Orogeny)

Zircon grains extracted from S-type granites of the Meda-Escalhao-Penedono Massif (Central Iberian Zone, Variscan Orogen) constrain the timing of emplacement and provide information about potential magma sources. Simple and composite zircon grains from three samples of S-type granite were analyzed by LA-ICP-MS. New U–Pb data indicate that granites crystallized in the Bashkirian (318.7 ± 4.8 Ma) overlapping the proposed age range of ca. 321–317 Ma of the nearby S-type granitic rocks of the Carrazeda de Anciaes, Lamego and Ucanha-Vilar massifs. The timing of emplacement of such S-type granites seems to coincide with the waning stages of activity of a D2 extensional shear zone (i.e. Pinhel shear zone) developed in metamorphic conditions that reached partial melting and anatexis (ca. 321–317 Ma). Dykes of two-mica granites (resembling diatexite migmatite) are concordant and discordant to the compositional layering and S2 (main) foliation of the high-grade metamorphic rocks of the Pinhel shear zone. Much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid-state deformation. Field relationships suggest contemporaneity between the ca. 319–317 Ma old magmatism of the study area and the switch from late D2 extensional deformation to early D3 contractional deformation. Inherited zircon cores are well preserved in these late D2-early D3 S-type granite plutons. U–Pb ages of inherited zircon cores range from ca. 2576 to ca. 421 Ma. The spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the Ediacaran to Silurian metasedimentary and metaigneous rocks of the Iberian autochthonous and parautochthonous domains. This is evidence of a genetic relationship between S-type granites and the host metamorphic rocks. There is no substantial evidence for the addition of mantle-derived material in the genesis of these late D2–early D3 S-type granitic rocks. The eNd arrays of heterogeneous crustal anatectic melts may be just inherited from the source, probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures. The generation of the Bashkirian S-type granites has been dominated by continental crust recycling, rather than the addition of new material from mantle sources.

Boron Contamination in the West of Lake Urmia, NW Iran, Caused by Hydrothermal Activities

The environment of hot springs located in the western part of Lake Urmia and north of Urmia City, NW Iran, is contaminated with boron. Water analysis performed over a period of four years show maximum boron concentrations of 451 mg/l. An area of 250 km2 is heavily polluted. The tectonically strongly disturbed metamorphic and sedimentary rocks in this region provide good pathways for the circulation of fluids. The hydrothermal system consists mainly of Na-(SO4)-HCO3. The occurrence of boron can be attributed to the interaction of magmatic fluids, related to young volcanic activities in this area. They seem to be the result of an intensive fluid-rock interaction with the metamorphic host-rock, e.g. tourmaline rich gneiss. Hot fluids in the depth of the study area migrate through the fractured rocks and may leach boron during their path to the surface.

Hydrothermal flake graphite mineralisation in Paleoproterozoic rocks of south-east Greenland

Flake graphite mineralisation is hosted in the Kuummiut Terrane of the Paleoproterozoic Nagssugtoqidian Orogen, south-east Greenland. Eclogite-facies peak-metamorphic assemblages record temperatures of 640–830 °C and pressures of 22–25 kbar, and are retrogressed in the high-pressure amphibolite-facies during ca. 1870–1820 Ma. Graphite occurs as lenses along cleavage planes in breccia and as garnet-quartz-graphite veins in various metamorphic host rocks in the Tasiilaq area at Auppaluttoq, Kangikajik, and Nuuk-Ilinnera. Graphite contents reach >30 vol% in 0.2–4 × 20 m wide semi-massive mineralisation (Auppaluttoq, Kangikajik). Supergene alteration formed 1- to 2-m-thick and up to a 2.5 × 2.5 km wide loose limonitic gravel containing graphite flakes in places. The flake size ranges from 1 to 6 mm in diameter with an average of ~3 mm. Liberation efficiency is at minimum 60%. Hydrothermal fluids at ~600 °C, transporting carbon as CO2 and CH4, formed the mineralisation commonly hosted by shear zones, which acted as pathways for the mineralising fluids. The hydrothermal alteration assemblage is quartz-biotite-grunerite-edenite-pargasite-K-feldspar-titanite. The δ13C values of graphite, varying from −30 to −18‰ PDB, indicate that the carbon was derived from organic matter most likely from metasedimentary sources. Devolatilisation of marble may have contributed a minor amount of carbon by fluid mixing. Precipitation of graphite involved retrograde hydration reactions, depleting the fluid in H2O and causing graphite saturation. Although the high-grade mineralisation is small, it represents an excellent example of hydrothermal mineralisation in an eclogite-facies terrane during retrograde exhumation.