Lower Greenschist Facies Research Articles

Petrological and geochemical features of the Jingtieshan banded iron formation (BIF): A unique type of BIF from the Northern Qilian Orogenic Belt, NW China

The Jingtieshan banded iron formation (BIF) is located in the Northern Qilian Orogenic Belt (NQOB) in NW China. The BIFs are hosted in Mesoproterozoic Jingtieshan Group, a dominantly clastic-carbonate sedimentary formation, and was metamorphosed to lower greenschist facies. The Jingtieshan BIFs include oxide-, carbonate- and mixed carbonate–oxide facies, and consist of alternating iron-rich and silica-rich bands. The BIFs are composed essentially of specularite and jasper, with minor carbonate minerals and barite. The SiO2+Fe2O3 content is markedly high in the oxide facies BIF, followed by FeO, CO2 and Ba, with the other elements usually lower than 1%, suggesting that the original chemical sediments were composed of Fe, Si, CO32− and Ba. The positive correlation between Al2O3, TiO2 and Zr in the BIFs indicates that these chemical sediments incorporate minor detrital components. Oxide facies BIF shows low HFSE, low ∑REE and low Y/Ho. The Post Archean Australian Shale-normalized REE patterns for Jingtieshan BIFs are characterized slight LREE depletion, strong positive Eu anomalies and lack of significant negative Ce anomalies. Siderite in the carbonate- and mixed carbonate–oxide facies BIF shows negative δ13C values varying from −8.4‰ to −3.0‰, and δ18O values show a range of −16.6‰ to −11.7‰. The geochemical signatures and carbon–oxygen isotopes suggest origin from high-temperature hydrothermal fluids with weak seawater signature for the sediments of Jingtieshan BIFs. The absence of negative Ce anomalies and the high Fe3+/∑Fe ratios of the oxide facies BIF do not support ocean anoxia. In contrast to the three main types (Algoma-, Superior- and Rapitan-type) of global BIFs, the Jingtieshan BIFs represent a unique type with features similar to those of sedimentary-exhalative mineralization.

Backarc basin and ocean island basalts in the Narooma Accretionary Complex, Australia: setting, geochemistry and tectonics

The Cambrian–Ordovician Wagonga Group contains basalts at Melville Point and Barlings Beach, 20 km south of Batemans Bay, New South Wales. At Melville Point, the succession has basal altered basalts overlain by chert and interbedded siliceous mudstone of the Wagonga Group, in turn overlain by turbidites and chert of the Adaminaby Group with a latest Cambrian to earliest Ordovician age. By contrast, at Barlings Beach, basalt is associated with highly disrupted chert (tectonic mélange), various slivers of mudstone and turbidites, and turbidites of the Adaminaby Group. Immobile elements in the basalts show consistent patterns that allow the magmatic affinity and tectonic setting to be determined in spite of pervasive hydrothermal alteration and subsequent lower greenschist facies metamorphism that accompanied strong folding and multiple foliation development. The Melville Point basalts show Ti/V ratios transitional between arc and MORB and therefore may have formed in either a forearc or backarc basin setting. However, these rocks have higher Ti/V ratios, LREE, Th and Nb than found in forearc basalts and are therefore considered to have formed in a backarc basin setting. In contrast to Melville Point, most basalts at Barlings Beach have a geochemical signature distinctive of ocean island settings like those reported from elsewhere in the Wagonga Group. We believe these rocks developed in a Cambrian backarc basin setting. In the Early to Middle Ordovician, much of the ocean basin was inundated by quartzose turbidites followed by basin destruction with accretion/underplating at a Late Ordovician–early Silurian Benambran subduction zone and formation of the Narooma Accretionary Complex.

Metagabros de la isla Gran Roque (Venezuela). Geoquímica y petrografía

The Gran Roque Island, located in northern Venezuela, is part of the southern Caribbean islands where the basic basement is outcropping. This basement is composed of fine-grained metagabbros, with subophitic relict texture defined by the larger plagioclase laths that enclose the ferromagnesian minerals. The mineral assemblage of chlorite-actinolite-plagioclase-quartz indicates that the rock reached a low metamorphic grade green schist facies. The chemistry of the Gran Roque island metaggabros supports the petrographic identification and characterization. These rocks exhibit tholeiitic affinity and REE flat patterns, which could be correlated to magmas generated at oceanic ridges of N-MORB type

Evidence for Cool Extrusion of the North Indochina Block along the Ailao Shan Red River Shear Zone, a Diancang Shan Perspective

Much debate surrounds the temporal-thermal-structural evolution of the >1000-km NW-SE-trending Ailao Shan–Red River (ASRR) shear zone. This mainly reflects the contradictory interpretations of the timing and P-T conditions for the occurrence of left-lateral shearing event due to a diverse data set of mineral ages. Our new microstructural, petrological, and geochronological data from the southern section of the Diancang Shan (DCS) block along the west wall of the ASRR support slow cooling from ∼300°C to ∼150°C from the Middle Eocene to the Late Miocene, accompanied by left-lateral shearing activity. This conclusion is based on the following: (1) The lower greenschist facies assemblages of biotite replaced by muscovite or chlorite, the albitized K-feldspar, and the stable association of K-feldspar + chlorite rather than muscovite + biotite are universally observed for the synkinematic Sn+1 and mylonitic S/C fabrics from both quartzofeldspathic and amphibolite domains. (2) New 40Ar/39Ar data for K-feldspar and muscovite of various structural domains such as the K-feldspar phenocrysts (∼20 to ∼40 Ma) from the early Triassic magmatic protolith (zircon U-Pb ages of 230–250 Ma), pre- to synkinematic muscovite (∼17 to ∼23 Ma) in mylonites, and K-feldspar (∼5 to ∼10 Ma) in postmylonitic cross-cutting veins produce an age spectrum that increases from ∼5 to ∼40 Ma under lower greenschist facies. Together these data indicate that the DCS block was slowly extruded along the ASRR shear zone through much of the Tertiary. These results contradict earlier interpretations of rapid cooling from synkinematic amphibolite metamorphic conditions caused by shear heating based in part on the assumption that all Miocene ages were cooling ages rather than crystallization ages.

Significant Zn–Pb–Cu remobilization of a syngenetic stratabound deposit during regional metamorphism: A case study in the giant Dongshengmiao deposit, northern China

The giant Dongshengmiao Zn–Pb–Cu deposit is located in the Langshan district, northern China. The ores are hosted within a Proterozoic rift sequence, which underwent lower greenschist facies metamorphism and shear deformation during development of Early Cretaceous intraplate orogenic belt. Northwest-dipping thrust faults, which share similar orientations and dip angles with the orebodies, are well developed in the mining area. Syngenetic stratabound sulfides were formed during the Proterozoic rifting event, but syngenetic ore textures have seldom been preserved except for some pretectonic fine-grained pyrite. Petrological observation, 39Ar/40Ar geochronology, combined with previous isotopic and fluid inclusion studies indicates that significant Zn–Pb–Cu remobilization took place as a result of thrust faulting associated with metamorphic devolatilization of ore-hosting rocks at ca. 136Ma, coeval with the intraplate orogeny and regional crustal shortening. Sulfides were redistributed in shear structures or along grain boundaries of ore-hosting carbonates, and Fe-rich carbonates were ideal sites for Zn–Pb–Cu precipitation.

Potential of pressure solution for strain localization in the Baccu Locci Shear Zone (Sardinia, Italy)

The mylonites of the Baccu Locci Shear Zone (BLSZ), Sardinia (Italy), were deformed during thrusting along a bottom-to-top strain gradient in lower greenschist facies. The microstructure of metavolcanic protoliths shows evidence for composite deformation accommodated by dislocation creep within strong quartz porphyroclasts, and pressure solution in the finer grained matrix. The evolution of mylonite is simulated in two sets of numerical experiments, assuming either a constant width of the deforming zone (model 1) or a narrowing shear zone (model 2). A 2–5 mm y−1 constant-external-velocity boundary condition is applied on the basis of geologic constraints. Inputs to the models are provided by inverting paleostress values obtained from quartz recrystallized grain-size paleopiezometry. Both models predict a significant stress drop across the shear zone. However, model 1 involves a dramatic decrease in strain rate towards the zone of apparent strain localization. In contrast, model 2 predicts an increase in strain rate with time (from 10−14 to 10−12 s−1), which is consistent with stabilization of the shear zone profile and localization of deformation near the hanging wall. Extrapolating these results to the general context of crust strength suggests that pressure-solution creep may be a critical process for strain softening and for the stabilization of deformation within shear zones.

Two-stage development of the Paparoa Metamorphic Core Complex, West Coast, South Island, New Zealand: Hot continental extension precedes sea-floor spreading by ∼25 m.y.

The Paparoa Metamorphic Core Complex (PCC) developed in the mid-Cretaceous due to continental extension, which conditioned the crust for the eventual breakup of the Gondwana Pacific margin and formation of the Tasman Sea. The PCC has two detachment systems with opposite senses of shear: the top-to-the-NE Ohika Detachment in the north and the top-to-the-SW Pike Detachment in the south. Rb-Sr dating on mylonite shows that the Pike Detachment was active before 116.2 ± 5.9 Ma. It was the dominant detachment exhuming Cretaceous synextensional migmatites and was synchronous with the intrusion of the Buckland Granite, from which U-Pb zircon crystallization ages between 110.41 and 109.73 Ma were obtained. The ductile shear zone beneath the Pike Detachment records upper-amphibolite to lower-greenschist facies metamorphism and cataclastic deformation. Pronounced hydrothermal alteration at 108.91 ± 0.04 Ma is interpreted to be related to initial movement on the Ohika Detachment. The structural hinge separating top-to-the-SW from top-to-the-NE shearing has been located in the northern part of the PCC, also indicating that the Pike Detachment is the master detachment of the PCC. Fission-track data indicate a period of enhanced heat flow resulting in reset and partially reset apatite and zircon fission-track ages at ca. 75 Ma concurrent with the onset of sea-floor spreading in the Tasman Sea. Our data show that initial extension in the mid-Cretaceous proceeded under high-temperature conditions and preceded continental breakup by ∼ 25 m.y.

Geological Map of the Monte Grighini Variscan Complex (Sardinia, Italy)

The study area, belongs to the Nappe zone of the Sardinian Variscan chain at the NW termination of the Flumendosa antiform. The Monte Grighini Complex is a NW-SE trending metamorphic complex made up of three tectonic units and synkinematic magmatic intrusions that show a section of the deepest portion of the Nappe zone. The tectonic units stacked and folded under lower greenschist and upper amphibolite facies conditions, were finally juxtaposed by a kilometer-wide NW-SE trending dextral transtensive shear zone during the Late Variscan tectonics. The 1:25,000 scale geological map, cross sections and shear zone deformation map illustrate the tectonic and metamorphic setting of the area, resulting from the polyphasic Variscan collisional evolution including early nappe stacking and later strike slip and extensional tectonics coeval with a late Carboniferous magmatism. Deformation structures and metamorphic assemblages recorded by the tectonic units as well as synkinematic intrusions, provide constraints of fundamental importance for the reconstruction of southern Variscan chain tectono-metamorphic history.

Structural characterization by Raman hyperspectral mapping of organic carbon in the 3.46 billion-year-old Apex chert, Western Australia

The 3.46 billion years old Apex Chert, Pilbara Craton, Western Australia, is well-known for hosting the oldest, highly disputed microfossils on Earth. This rock has a complex history of thermal alteration that includes circulation of hydrothermal fluids, lower greenschist-facies regional metamorphism, and post-metamorphic weathering by meteoric fluids. Carbonaceous material occurs in the sedimentary stratiform part of the chert as well as the underlying intruding hydrothermal black chert veins. In order to identify a least-altered remnant of early life it is necessary to develop a method that enables rapid evaluation of CM structural order on a small spatial scale. Here we present the detailed characterization of CM in the Apex chert by Raman hyperspectral mapping. It is shown that this approach gives better estimates of average Raman band ratios than individual point analyses, and it is demonstrated that significant differences in structure exist between CM in the stratiform part of the Apex chert and CM in an underlying black chert vein. The large Raman map-based datasets also reveal that significant mixing took place between these two end members CM’s, indicating that the Apex chert has been thoroughly altered by hydrothermal fluid circulation. At the brecciated intersection between the stratiform chert and the intrusive hydrothermal chert vein very poorly ordered CM was found that is not in line with lower greenschist-facies regional metamorphism. It is speculated here that this CM represents an organic fraction that was introduced or thoroughly altered by late stage meteoric fluids. Alternatively, the ubiquitous presence of hematite in this sample caused a perturbation in the Raman spectra of the CM, leading to an artifact in the calculated Raman-based band ratios. Overall it can be concluded that the best preserved CM occurs in the stratiform parts of the Apex chert, while earlier discussions on organic microfossils in this rock often focused on parts of the chert that either represented the hydrothermal veins or the brecciated intersection between the veins and the stratiform part.

The development of grain boundary orientation distribution in quartz

The fabric of high-angle boundaries in recrystallized quartz aggregates deformed under distinct strain and metamorphic conditions has been investigated in samples from iron formations recrystallized under lower greenschist to lower amphibolite facies conditions. Orientation of grain boundaries was measured using U-stage, while crystallographic fabric and misorientation between grains were measured using EBSD. In domains where grains were recrystallized under lower greenschist facies conditions, grain boundaries are sutured, composed of several small straight segments, whose traces can be parallel to traces of any crystallographic plane. In contrast, in domains of upper-greenschist–lower-amphibolite facies, grain boundaries are predominantly composed of long straight segments and belong to girdle of rhombohedral planes, while segments within girdle of basal or prism planes are rare. The longest segments (larger than 270 μm) are commonly within girdle of rhombohedral planes for the two grains that share a boundary surface, possibly representing Dauphine twin boundaries. Under higher temperature condition of the lower amphibolite facies (500–550 °C), boundary segments are longer and tend to represent coincident crystallographic orientations for the two grains that share a boundary.

The Paleoproterozoic Aripuana Zn-Pb-Ag (Au, Cu) Volcanogenic Massive Sulfide Deposit, Mato Grosso, Brazil: Geology, Geochemistry of Alteration, Carbon and Oxygen Isotope Modeling, and Implications for Genesis

Aripuana is a Paleoproterozoic (1.76-1.75 Ga), stratiform, volcanogenic Zn-Pb-Ag-(Au-Cu) massive sulfide deposit, has an estimated resource of 11.6 million metric tons (Mt) of ore, and is located on the south-southwest border of the Amazon craton (Brazil). Aripuana resides in a caldera composed of strongly fractionated, felsic metavolcanic rocks, transitional between calc-alkaline and tholeiitic compositions. The entire region was metamorphosed into lower greenschist facies (1.68-1.63 Ga) and the deposit was subsequently thermally metamorphosed and metasomatized (1.56-1.53 Ga). The mineralized bodies are lenticular and elongated. They are located at the base of a sequence of turbidites, which overlie dacitic and rhyolitic ignimbrites. The rocks of the footwall are silicified, sericitized, and chloritized, and the hanging-wall rocks are carbonate rhythmites and turbidites, hydrothermal marbles, carbonate breccias, and breccia-like rocks, carbonate + talc + tremolite + chlorite rock, talc-, tremolite-, and biotite-rich rocks, and carbonate- and fluorite-bearing cherts. The deposit was formed during at least four submarine-exhalative hydrothermal cycles in which siliceous marbles and marls represent the initial phase of each cycle, followed by the coprecipitation of pyrite + pyrrhotite + sphalerite + argentiferous galenas ± chalcopyrite, which were disseminated in clay-carbonate breccias or formed massive lenses. This phase was followed by fluoritic cherts and argillites, which were followed by carbonate and (fluorite?) rhythmites, and turbidites to close the cycle. The variation in total rare earth element (REE) contents and in the Eu and Ce anomalies indicates that the original sediments were mixtures of hydrothermal carbonate and smectitic clay and were deposited below a redoxcline. Isotopic analysis identified δ 18OSMOW values between 8 and 13‰ and δ 13CPDB values between −7 and −1‰. Modeling isotopic data showed that mineralizing fluids were distinct based on the proportion of magmatic fluid that mixed with the seawater. Carbonate rocks and sulfide minerals precipitated while hydrothermal activity and volcanism evolved from an exhalative to an exhalative-explosive character with temperatures increasing between 100° and 200°C. The regional metamorphism of the deposit formed carbonate + talc + tremolite + chlorite rocks, tremolite-, biotite-, and talc-rich rocks; it recrystallized the minerals, foliated and folded all of the rocks, and caused limited decarbonation at the sites where carbonate rocks contained silica and smectite. However, the original isotopic signatures were maintained. The thermal metamorphism erased the metamorphic foliation, and the metasomatism generated veinlets of chlorite with sulfides.

Field and Textural Relationship in Pelitic Schists and Gneisses from the area around Mangpu, Darjeeling District, West Bengal

Abstract The metasedimentary rocks of the area around Mangpu constitute a portion of the hinge zone of the northern limb of the major synform of Lower Darjeeling Himalaya. The rocks display evidences of multiple deformation and at least three major phases of deformation have been recognized. The time relations between the phases of deformation (D1, D2, D3) and metamorphic crystallization reveal a single major prograde metamorphic event that initiated with the D1 deformation and finally outlasted it. The earlier phase of this metamorphism is essentially regional syn-tectonic low-grade (greenschist facies) which may be designated (M1, early). This was followed by regional static metamorphism (M1, late) in the post-tectonic phase between D1 and D2 deformations (upper green schist and amphibolite facies). This M1 metamorphism is superposed by later retrogressive metamorphism (M2) during the D2 and D3 deformations (lower greenschist facies). Within the study area four isograds have been delineated by the first appearance of index minerals in the pelitic schists and gneiss which display Barrovian type of metamorphism.

Kinematic analysis and analogue modelling of the Passeier- and Jaufen faults: implications for crustal indentation in the Eastern Alps

Crustal deformation in front of an indenter is often affected by the indenter’s geometry, rheology, and motion path. In this context, the kinematics of the Jaufen- and Passeier faults have been studied by carrying out paleostress analysis in combination with crustal-scale analogue modelling to infer (1) their relationship during indentation of the Adriatic plate and (2) their sensitivity in terms of fault kinematics to the geometry and motion path of Adria. The field study reveals mylonites along the Jaufen fault, which formed under lower greenschist facies conditions and is associated with top-to-the-west/northwest shear with a northern block down component. In addition, a brittle reactivation of the Jaufen shear zone under NNW–SSE to NW–SE compressional and ENE–WSW tensional stress conditions was deduced from paleostress analysis. The inferred shortening direction is consistent with fission track ages portraying Neogene exhumation of the Meran-Mauls basement south of the fault. Along the Passeier fault, deformation was only brittle to semi-ductile and paleostress tensors record that the fault was subjected to E–W extension along its northern segment varying into NW–SE compression and sinistral transpression along its southern segment. In the performed analogue experiments, a rigid, triangular shaped indenter was pushed into a sand pile resulting in the formation of a Passeier-like fault sprouting from the indenter’s tip. These kinds of north-trending tip faults formed in all experiments with shortening directions towards the NW, N, or NE. Consequently, we argue that the formation of the Passeier fault strongly corresponds to the outline of the Adriatic indenter and was only little affected by the indenter’s motion path due to induced strain partitioning in front of the different indenter segments. The associated fault kinematics along the Passeier fault including both E–W extension and NNW to NW shortening, however, is most consistent with a northward advancing Adriatic indenter.

Geochemistry and genesis of low-grade metasediment-hosted Zn–Pb–Ag mineralization, southern Proterozoic Curnamona Province, Australia

Sediment-hosted, Paleoproterozoic low-grade Zn–Pb–Ag and Cu–Au mineralization occurs at the Polygonum, Thunderdome, Hunters Dam, and Benagerie Ridge prospects under a thick sedimentary cover in the Mulyungarie Domain (MD) of the southern Curnamona Province (SCP), Australia. Host rocks and mineralization were metamorphosed to lower greenschist facies at Benagerie Ridge, upper greenschist facies at Hunters Dam, and lower to upper amphibolite facies at Polygonum and Thunderdome. The giant Paleoproterozoic Pb–Zn–Ag Broken Hill deposit, which was metamorphosed to the granulite facies, is hosted in the Broken Hill Domain (BHD). The general lithostratigraphy for each prospect is similar and can be subdivided into four main units. Unit 1, at the base of the stratigraphic section, hosts Cu–Au mineralization and is characterized by the presence of oxidized magnetic metapsammopelites. Unit 2 consists of sulfide-rich, pyritic carbonaceous metapelites, calcareous metashales/siltstones, Mn-bearing sideritic metacarbonates, metacalc-silicates, and carbonate and feldspar lenses and layers. Syn-sedimentary sulfide mineralization occurs as laminations of fine-grained pyrite and sphalerite and resembles that at the HYC deposit, Queensland. Differences in δ13C values (δ13CVPDB=−10.2 and −0.8‰) of calcite in unit 2 from the same area suggest variable amounts of original organic C. The δ13C values of carbonates in Broken Hill ore and those obtained here from the Esmeralda deposit near Broken Hill are extremely low (δ13C=−25 to −21‰) and likely resulted from volatilization and high temperature effects during high-grade metamorphism. Barren laminated carbonaceous metapelites in unit 3 constitute a marker unit throughout the SCP. Unit 4 at the top of the stratigraphic sequence consists of psammopelitic and laminated andalusite or chiastolite phyllites that contain garnet and gahnite, laminated garnetite, quartz garnetite, Mn-bearing banded iron formation, amphibolite, and stratabound hydrothermal Pb–Zn mineralization. This unit, which is present at the Polygonum and Thunderdome prospects, is stratigraphically equivalent to the upper Broken Hill Group, which hosts the Broken Hill deposit. Base metals occur in metacarbonates and metapelites and close to redox boundaries between units 1 and 2, and units 2 and 3. Geochemical indicators of stratabound Pb–Zn mineralization in the northern part of the SCP include whole-rock values of >4ppm Tl, >25ppm Cd, and >17ppm Se in units 2 and 4, (Mn+Fe+Mg)/(Mn+Fe+Mg+Ca) ratios>0.9 for carbonates, Mn/(Mn+Fe+Mg) ratios>0.6 for garnet in garnet–quartz rocks, and Mn/(Mn+Ca+Fe) ratios>0.3 for garnet in metacalc-silicate rocks.

Lesser Himalayan sequences in Eastern Himalaya and their deformation: Implications for Paleoproterozoic tectonic activity along the northern margin of India

Substantial part of the northern margin of Indian plate is subducted beneath the Eurasian plate during the Caenozoic Himalayan orogeny, obscuring older tectonic events in the Lesser Himalaya known to host Proterozoic sedimentary successions and granitic bodies. Tectonostratigraphic units of the Proterozoic Lesser Himalayan sequence (LHS) of Eastern Himalaya, namely the Daling Group in Sikkim and the Bomdila Group in Arunachal Pradesh, provide clues to the nature and extent of Proterozoic passive margin sedimentation, their involvement in pre-Himalayan orogeny and implications for supercontinent reconstruction. The Daling Group, consisting of flaggy quartzite, meta-greywacke and metapelite with minor mafic dyke and sill, and the overlying Buxa Formation with stromatolitic carbonate-quartzite-slate, represent shallow marine, passive margin platformal association. Similar lithostratigraphy and broad depositional framework, and available geochronological data from intrusive granites in Eastern Himalaya indicate strikewise continuity of a shallow marine Paleoproterozoic platformal sequence up to Arunachal Pradesh through Bhutan. Multiple fold sets and tectonic foliations in LHS formed during partial or complete closure of the sea/ocean along the northern margin of Paleoproterozoic India. Such deformation fabrics are absent in the upper Palaeozoic–Mesozoic Gondwana formations in the Lesser Himalaya of Darjeeling-Sikkim indicating influence of older orogeny. Kinematic analysis based on microstructure, and garnet composition suggest Paleoproterozoic deformation and metamorphism of LHS to be distinct from those associated with the foreland propagating thrust systems of the Caenozoic Himalayan collisional belt. Two possibilities are argued here: (1) the low greenschist facies domain in the LHS enveloped the amphibolite to granulite facies domains, which were later tectonically severed; (2) the older deformation and metamorphism relate to a Pacific type accretionary orogen which affected the northern margin of greater India. Better understanding of geodynamic evolution of the northern margin of India in the Paleoproterozoic has additional bearing on more refined model of reconstruction of Columbia.

Major and trace-element composition and pressure–temperature evolution of rock-buffered fluids in low-grade accretionary-wedge metasediments, Central Alps

The chemical composition of fluid inclusions in quartz crystals from Alpine fissure veins was determined by combination of microthermometry, Raman spectroscopy, and LA-ICPMS analysis. The veins are hosted in carbonate-bearing, organic-rich, low-grade metamorphic metapelites of the Bundnerschiefer of the eastern Central Alps (Switzerland). This strongly deformed tectonic unit is interpreted as a partly subducted accretionary wedge, on the basis of widespread carpholite assemblages that were later overprinted by lower greenschist facies metamorphism. Veins and their host rocks from two locations were studied to compare several indicators for the conditions during metamorphism, including illite crystallinity, graphite thermometry, stability of mineral assemblages, chlorite thermometry, fluid inclusion solute thermometry, and fluid inclusion isochores. Fluid inclusions are aqueous two-phase with 3.7–4.0 wt% equivalent NaCl at Thusis and 1.6–1.7 wt% at Schiers. Reproducible concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, B, Al, Mn, Cu, Zn, Pb, As, Sb, Cl, Br, and S could be determined for 97 fluid inclusion assemblages. Fluid and mineral geothermometry consistently indicate temperatures of 320 ± 20 °C for the host rocks at Thusis and of 250 ± 30 °C at Schiers. Combining fluid inclusion isochores with independent geothermometers results in pressure estimates of 2.8–3.8 kbar for Thusis, and of 3.3–3.4 kbar for Schiers. Pressure–temperature estimates are confirmed by pseudosection modeling. Fluid compositions and petrological modeling consistently demonstrate that chemical fluid-rock equilibrium was attained during vein formation, indicating that the fluids originated locally by metamorphic dehydration during near-isothermal decompression in a rock-buffered system.

The Composition and Depositional Environments of Mesoarchean Iron Formations of the West Rand Group of the Witwatersrand Supergroup, South Africa

This paper documents the sedimentological setting, mineralogy, and geochemistry of several iron formation units interbedded with siliciclastic strata of the Mesoarchean Witwatersrand Supergroup, well known for its world-class conglomerate-hosted Au-U deposits. Four major iron formation beds, with associated magnetic mudstones, are present in two distinctly different lithostratigraphic associations, namely shale- and diamictiteassociated iron formation. The shale association is represented by the Water Tower and Contorted Bed iron formations in the Parktown Formation of the Hospital Hill Subgroup in the lower part of the succession and the diamictite association by the Promise and Silverfield iron formations in the overlying Government Subgroup. The iron formation units have been subjected to lower greenschist facies metamorphism. Oxide (magnetite and limited hematite), carbonate, and silicate facies iron formations are recognized. The iron formations typically overlie major transgressive flooding surfaces in the succession and, in turn, form the base of progradational coarsening-upward increments of sedimentation comprising magnetic mudstone, nonmagnetic shale, and interbedded siltstone-quartzite. The upward transition from iron formation into magnetic mudstone is accompanied by a change in mineralogical composition from hematite-magnetite iron formation at the base in the most distal setting through magnetite-siderite- and siderite-facies iron formation in the transition zone to magnetic mudstone. The siderite with associated ankerite displays highly depleted δ13C values, suggesting crystallization via iron respiration in presence of organic carbon. The iron formations display positive postArchean Australian shale-normalized Eu and Y anomalies with depletion in light rare-earth elements relative to heavy rare-earth elements, indicating precipitation from marine water with a high-temperature hydrothermal component. Integration of sedimentological, petrographic, and geochemical results indicates that the shale-associated iron formation was deposited during the peak of transgression, when reduced iron-rich hydrothermal waters entered the Witwatersrand Basin over a limited vertical extent due to neutral buoyancy, with the top of the plume occurring below the photic zone. It is suggested that chemolithoautotrophic iron-oxidizing bacteria, which would have been able to exploit the difference in chemistry between the iron-enriched plume water and ambient ocean water to fuel metabolic activity in the presence of limited free molecular oxygen, were responsible for precipitation of initial ferric iron oxyhydroxides. The vertical facies associations in the iron formations most likely developed in response to the limited vertical extent of the hydrothermal plume, with (from distal to proximal) hematite preserved where the base of the plume was not in contact with the basin floor, magnetite where the plume water was in contact with bottom sediment, iron-rich carbonates where organic carbon input was high, iron-rich alumosilicates where siliciclastic input became significant in more proximal settings, and iron-poor sediment above the top of the plume. Diamictite-associated iron formations in the Witwatersrand are inferred to have been deposited in a fashion similar to the shale-associated iron formations, with the exception that major transgressions and hydrothermal plume invasion were preceded by glacial ice cover. The climate warming and increased volcanic activity required could have been related to increased tectonic activity inferred for the Witwatersrand Supergroup during deposition of the glacially associated iron formations.

Kinematics of the Guerrero terrane accretion in the Sierra de Guanajuato, central Mexico: new insights for the structural evolution of arc–continent collisional zones

The Guerrero terrane comprises Middle Jurassic–Early Cretaceous arc successions that were accreted to the North American craton in the late Early Cretaceous, producing closure of the Arperos oceanic basin and the formation of an approximately 100 km-wide fold–thrust belt. Such a suture is key to investigating the structural evolution related to Guerrero terrane accretion and, in general, to arc–continent collisional zones. The Sierra de Guanajuato is an exposure of the Guerrero terrane suture belt and consists of a complex tectonic pile that formed through at least three major shortening phases: D1SG, D2SG, and D3SG (SG, Sierra de Guanajuato). During the D1SG and D2SG phases, the Upper Jurassic–Lower Cretaceous successions of the Arperos Basin piled up, forming a doubly vergent imbricate fan of thrust sheets that accommodated substantial NE–SW shortening. Mylonite microtextures, as well as syntectonic minerals, indicate that the D1SG and D2SG deformation events took place under low greenschist-facies metamorphic conditions. We relate these deformation phases to the progressive NE migration of the Guerrero terrane, which triggered the collapse and closure of the Arperos Basin. During D3SG, the El Paxtle arc assemblage of the Guerrero terrane was tectonically emplaced onto the previously deformed successions of the Arperos Basin. However, D3SG structures indicate that during this deformational stage, the main shortening direction was oriented NW–SE and that contraction was accommodated mostly by SE-vergent ductile thrusts formed under low greenschist-facies metamorphic conditions. We suggest that the top-to-the-SE emplacement of the El Paxtle assemblage may be a result of the tectonic escape of the arc produced by the continuous NE impingement of the Guerrero terrane during its collisional addition to the Mexican mainland.

P-T-X conditions, origin, and evolution of Cu-bearing fluids of the shear zone-hosted Huogeqi Cu–(Pb–Zn–Fe) deposit, northern China

Huogeqi is a shear zone-hosted epigenetic deposit within the greenschist-amphibolite facies of the Mesoproterozoic Langshan Group in the Langshan district on the northern margin of the North China Craton (NCC). Copper mineralization in the Huogeqi deposit was formed in two stages: a main-stage controlled by the shear zone and characterized by brittle-ductile ore-forming structures and a lower greenschist facies mineral assemblage, and a late stage characterized by open space-filling textures and low-temperature minerals. Based on microthermometric and Raman microprobe analysis, the main-stage Cu-bearing fluid was mesothermal, low-salinity and H2O–CH4-dominant, and was generated by an interaction between a deep-crustal metamorphic fluid and graphite-bearing host rocks. This interaction resulted in a more CH4-rich fluid, which was more amenable to be immiscible. We showed that immiscibility of the H2O–CH4 fluid occurred due to temperature decrease, prior to the main-stage Cu mineralization; Cu was finally precipitated from the resultant H2O-rich aqueous fluid. Main-stage Cu mineralization temperature was obtained using various methods: 310–370°C by intersection of isochors of coexistent CH4 and aqueous inclusions; 364±41°C on average by pressure correction of the homogenization temperatures of aqueous inclusions; and 362±26°C using the chlorite geothermometer. Pressure during Cu-deposition fluctuated between lithostatic and hydrostatic at depths of ca. 10–12km, but it seemingly had no effect on the mineralization process. The late-stage Cu-bearing fluid was a low temperature, low salinity, H2O of meteoric origin.

Recognizing Propylitic Alteration Associated with Porphyry Cu-Mo Deposits in Lower Greenschist Facies Metamorphic Terrain of the Collahuasi District, Northern Chile--Implications of Petrographic and Carbon Isotope Relationships

The Collahuasi district is a porphyry Cu-Mo subprovince located on the Andean high plateau of northern Chile. Late Eocene-early Oligocene mineralization in the district involved hydrothermal alteration of the upper Paleozoic to Lower Triassic Collahuasi Group granodiorites, diorites, andesites, dacites, and rhyolites, which had previously experienced regional lower greenschist facies metamorphism as well as uneconomic porphyry-style mineralization and associated sericitic alteration. Alteration mapping represents an effective technique for prospecting for hydrothermal deposits, but, in low-grade metamorphic terrains, recognition of propylitic assemblages that represent the boundary of the mineralizing system is hampered by their resemblance to metamorphic mineral assemblages. Our results from Collahuasi indicate that petrographic criteria can distinguish propylitic alteration from the effects of lower greenschist facies metamorphism, but distinctions vary with rock type. Mafic, intermediate, and felsic rocks with varying degrees of propylitic alteration can be distinguished from background metamorphism mainly by the presence of calcite. However, propylitic alteration can also be distinguished by high whole-rock δ 13C values and carbon contents (−19.1 to −1.4‰ and up to 0.79 wt %, respectively) that result from the incorporation of magmatic CO2 during the mineralizing events. Lower δ 13C values (−32.7 to −13.1‰) and carbon contents (<0.09 wt %) were recorded in other types of hydrothermal alteration (intermediate argillic, sericitic, and potassic alteration) and associated with background metamorphism. They reflect a dominantly 13C depleted carbon component that remains in the rocks after extensive CO2 degassing during emplacement, alteration by hydrothermal fluids, or contamination by organic carbon. Carbon and oxygen isotope compositions of calcite from propylitic alteration, which range from −12 to +0.3‰ and 6.6 to 25.9‰, respectively, are consistent with a predominantly magmatic origin for the carbonates, although subsequent resetting of the oxygen isotopes by local ground water and minor contributions of organic carbon are recorded in some samples. Spatial variation in δ 13C values and carbon contents of outcrop samples from the Collahuasi district delineate zones of high magmatic CO2 component that correspond exclusively to propylitic alteration surrounding economic deposits.