Continental Margin Environment Research Articles

Geochemistry of lower proterozoic greywackes from the Birim diamondiferous field, Ghana

The Birim Diamondiferous field from which most of the alluvial diamonds are recovered is underlain by Lower Proterozoic Birimian metasedimentary and associated mafic rocks. Bulk rock geochemistry was carried out on the sandstones from the metasedimentary rocks to constrain the tectonic setting during the early Proterozoic. Bulk rock geochemical studies on greywackes show that they are characterized by moderately low but variable SiO2 contents, generally high Al2O3 and ferromagnesian element (e.g., Fe, Mg, Cr, V, Co, Sc) contents, and K2O/Na20 commonly less than 1. The geochemical data indicate that the greywackes were deposited in an active continental margin environment. The detritus were mainly derived from the upper continental crust but with an admixture of mafic component. Recent studies have shown that some of the mafics/ultramafics are diamondiferous metakimberlites. The inferred tectonic (arc) setting makes the emplacement of the so-called Proterozoic diamondiferous metakimberlites unlikely, if we are to go by the Clifford's Rule. However, if these ultramafic rocks are really metakimberlites and the source of the diamonds, then the Clifford's Rule may not be applicable in the Ghanaian situation, and the Birim field may be one of the few exceptions to this general rule. This would, then, present a typical example of subduction zone related diamondiferous kimberlites. Keywords: Birim Diamondiferous field, Birimian, geochemistry, Ghana, tectornc settingAfrican Journal of Science and Technology Vol. 5(2) 2004: 9-18

Sr and Nd isotope systematics of Francistown plutonic rocks, Botswana: implications for Neoarchaean crustal evolution of the Zimbabwe craton

The Francistown plutonic rocks at the south-western margin of the Zimbabwe craton consist of three igneous suites: Sanukitoid, Tonalite–Trondhjemite–Granite (TTG) suites and High-K granites. The TTG suite is subdivided into High Aluminum-TTG (HA-TTG) and Low Aluminum-TTG (LA-TTG) sub-suites. Their Rb–Sr isotope systems were partially homogenized by post-crystallization thermo–tectonic events, in which hydrothermal solutions and migmatization played an important role. Therefore, the Rb–Sr isochron age of 2427±54 Ma can only be regarded as a lower limit to the Francistown plutonic rock age. The large errors in the Sm–Nd isochron dates of Francistown granitoids indicate that these dates are not really constrained. In this study we compared the rock types of Francistown and adjacent areas, adopting the precise U, Th–Pb single zircon SHRIMP ages from the Vumba area as references. For TTG and Sanukitoid suites, the age we adopted is ca. 2.7 Ga, which is close to their depleted-mantle Sm–Nd model ages (TDM). For High-K granites, the age adopted is ca. 2.65 Ga, which is also close to their Sm–Nd isochron age. The highest eNdt values of Sanukitoids and TTG are +2.1 and +2.3, respectively. The positive eNdt values and trace element geochemistry support partial melting of a depleted mantle and young oceanic crust for the genesis of Sanukitoid and the TTG suites respectively. The lowest eNdt values of Sanukitoids and TTGs are −1.0 and −1.1, respectively, indicating contamination by continental crust, up to 10 and 14%, respectively. The eNdt values of TTG decrease with decreasing Al2O3 and Sr contents and increasing Eu negative anomalies (Eu*–Eu), suggesting that the TTG magmas underwent a coupled fractionation crystallization and crustal contamination, and that the LA-TTG was the product of the fractionation and contamination of the HA-TTG sub-suite. In contrast, negative eNdt values for the High-K granites (from −0.4 to −3.5) indicate the involvement of LA-TTG and some materials from an old continental crust in their genesis. The products of partial melting of both oceanic and continental crusts at the south-western margin of the Zimbabwe craton occurred within a short time interval (from 2.7 to 2.65 Ga ago) suggesting that the Francistown plutonic rocks were formed in a active continental margin environment, where a young ocean plate (Limpopo oceanic plate) subducted underneath an old continental plate (Zimbabwe craton).

Proterozoic mafic–ultramafic intrusions in the Arunta Region, central Australia: Part 1: Geological setting and mineral potential

Proterozoic mafic–ultramafic intrusions of the Arunta Region record a protracted period of magmatism during the evolution of this geologically complex and tectonically long-lived terrane in central Australia. New U–Pb zircon geochronology data highlight the episodic emplacement of the mafic–ultramafic systems. Five major events of dominantly tholeiitic mafic magmatism have been recognised at ∼1810 Ma, ∼1780 Ma, ∼1690 Ma, and ∼1635 Ma, and a much younger event of probably early Palaeozoic age. A sixth event at ∼1135 Ma has alkaline-ultramafic affinities. Chondrite- and mantle-normalised multi-element patterns of rocks with melt-like compositions have refined the correlations of the magmatic systems indicated by the geochronological framework. Continental margin environments (e.g. subduction or back-arc related) are generally indicated for the magmatic events at ∼1810 Ma, ∼1780 Ma, and ∼1635 Ma, whereas the mafic magmatism at ∼1690 Ma and in the early Palaeozoic is more characteristic of intracontinental settings (e.g. extensional rifts). The intrusions occur in proximity to major province-wide faults. Differential vertical movements along these faults provide the opportunity to examine geological processes at crustal depths ranging from ∼5 km to 25 km (2 kb to 8 kb). The intrusions form large homogeneous mafic granulite bodies, granulite bodies interfolded with felsic units, contaminated gabbroic sheets, stacked sequences of high-level doleritic sills, small pods and laterally extensive sheets of amphibolite, and rare plug-like ultramafic bodies. Metamorphic overprints range from granulite to sub-amphibolite facies, with a concentration of high-grade mafic granulite bodies in the central Arunta Region attributed to deeper levels of crust uplifted north of the Redbank Thrust. Mafic rocks in the intrusions of high-metamorphic grade are dominantly two-pyroxene mafic granulites with high clinopyroxene to orthopyroxene ratios and variable amounts of minor hornblende, biotite, quartz, and garnet. In contrast, gabbroic rocks in the lower-grade intrusions of the western Arunta Region contain more orthopyroxene, alkali feldspar, quartz, and Fe–Ti oxides. LREE-enrichment trends with decreasing 147Sm/ 144Nd ratios and initial ɛ Nd values (+1.5 to −4.7) indicate that felsic crustal contamination processes were particularly important during the evolution of the latter intrusions. Incompatible trace-element trends show that the Arunta intrusions fall into two major geochemical groups that for the first time highlight geographical differences in mineral prospectivity. 1. A S-rich group (∼1200 to 300 ppm S: Andrew Young Hills intrusion, Mount Hay Granulite, Mount Chapple Metamorphics) from the western and central Arunta regions that has potential for basal Ni–Cu–Co-sulphide associations. 2. A relatively S-poor (<300 ppm S), slightly more primitive group (Attutra Metagabbro, Mordor Complex) from the eastern Arunta Region that has greater potential for stratabound PGE-sulphide associations. Some potential also exists for structurally controlled hydrothermal deposits of Cu–Au ± PGEs ± Ag ± Pb spatially associated with mafic–ultramafic rocks (Riddock Amphibolite). The major challenges for finding massive Ni–Cu–Co sulphides in the lower parts of prospective S-saturated intrusions (e.g. Andrew Young Hills, Mount Hay Granulite) are to determine the pre-deformational geometries of the bodies and to locate favourable mineralised environments, such as embayments in basal contacts and feeder conduits, concealed by thin Cainozoic alluvial deposits.

Early Permian ammonoids from the Kaeng Krachan Group of the Phatthalung-Hat Yai area, southern peninsular Thailand

An Early Permian small ammonoid fauna consisting of Neocrimites sp., Agathiceras suessi Gemmellaro, A. girtyi Böse, Agathiceras? sp., and Miklukhoceras sp. was found in nodules of a fine sandstone bed exposed in the Phatthalung-Hat Yai area of southern peninsular Thailand. The ammonoid-bearing bed belongs stratigraphically to the uppermost part of the Kaeng Krachan Group, which is essentially a clastic-dominant, Late Carboniferous (?) to Early Permian stratigraphic unit, widely distributed in western and peninsular Thailand. This ammonoid fauna is considered to be of Bolorian (Kungurian) age and includes Agathiceras girtyi Böse, which is described for the first time from Thailand. The present discovery of Bolorian ammonoids suggests that the uppermost part of the Kaeng Krachan Group is slightly younger than previously considered and around the latest Early Permian. This further implies that the continental margin environment of the Sibumasu Block drastically changed at around Bolorian time from a cool, clastic-dominant shelf condition to a temperate to subtropical, carbonate platform due to rapid northward drift after middle Artinskian rifting.

Marine hydrogeology: recent accomplishments and future opportunities

Marine hydrogeology is a broad-ranging scientific discipline involving the exploration of fluid–rock interactions below the seafloor. Studies have been conducted at seafloor spreading centers, mid-plate locations, and in plate- and continental-margin environments. Although many seafloor locations are remote, there are aspects of marine systems that make them uniquely suited for hydrologic analysis. Newly developed tools and techniques, and the establishment of several multidisciplinary programs for oceanographic exploration, have helped to push marine hydrogeology forward over the last several decades. Most marine hydrogeologic work has focused on measurement or estimation of hydrogeologic properties within the shallow subsurface, but additional work has emphasized measurements of local and global fluxes, fluid source and sink terms, and quantitative links between hydrogeologic, chemical, tectonic, biological, and geophysical processes. In addition to summarizing selected results from a small number of case studies, this paper includes a description of several new experiments and programs that will provide outstanding opportunities to address fundamental hydrogeologic questions within the seafloor during the next 20–30 years.

Comparative geochemistry of cadmium, rhenium, uranium, and molybdenum in continental margin sediments

The concentrations of authigenic phases of Cd, Re, U, and Mo increase with depth in four 45-cm-long sediment box cores collected along the axis of the Laurentian Trough, Gulf of St. Lawrence. Average authigenic accumulation rates, estimated from element inventories, are similar to rates in other continental margin environments. Strong regional variations in sediment accumulation rate and sulfide concentration have little influence on the accumulation rates of Cd and Re. This suggests that slow precipitation kinetics controls the accumulation of Cd and Re in these sediments. The accumulation rate of authigenic U is more variable; it may be tied to the kinetics of microbially mediated U reduction and be controlled by the availability of reactive organic matter. Authigenic Mo is distinguished by a sharp subsurface concentration minimum, above which Mo cycles with manganese. Mo released to pore water upon reduction of Mn oxides diffuses downward and enriches the subsurface sediment. Mo accumulates most rapidly in the sediment with the highest sulfide content. Slow conversion of molybdate to thiomolybdate may explain the much slower Mo accumulation rate in the less sulfidic sediments. A component of authigenic Mo accumulates with pyrite in an approximately constant Mo:Fe ratio. The accumulation rate of pyrite and associated Mo is insensitive to AVS abundance. Pyrite formation may be limited by the reactivity of iron oxide minerals.

Rare earth element and Sr–Nd isotope geochemistry of primary apatite crystals in metabentonites from Dob’s Linn, southern Scotland

Synopsis New Sr and Nd isotope and rare earth element data obtained from primary apatite microphenocysts indicate that the Dob’s Linn metabentonites originated from a mildly depleted mantle source, probably generated in a continental margin or back-arc basin environment. There is a small but notable reduction in crustal contamination of the source magmas through two graptolite zones ( convolutus to sedwickii ) indicated by a shift to higher ɛ Nd values. This shift is consistent with a magma chamber being periodically refilled with mantle-derived melts.

Isotope provinces, mechanisms of generation and sources of the continental crust in the Central Asian mobile belt: geological and isotopic evidence

The available geological, geochronological and isotopic data on the felsic magmatic and related rocks from South Siberia, Transbaikalia and Mongolia are summarized to improve our understanding of the mechanisms and processes of the Phanerozoic crustal growth in the Central Asian mobile belt (CAMB). The following isotope provinces have been recognised: ‘Precambrian’ ( T DM=3.3–2.9 and 2.5–0.9 Ga) at the microcontinental blocks, ‘Caledonian’ ( T DM=1.1–0.55 Ga), ‘Hercynian’ ( T DM=0.8–0.5 Ma) and ‘Indosinian’ ( T DM=0.3 Ga) that coincide with coeval tectonic zones and formed at 570–475, 420–320 and 310–220 Ma. Continental crust of the microcontinents is underlain by, or intermixed with, ‘juvenile’ crust as evidenced by its isotopic heterogeneity. The continental crust of the Caledonian, Hercynian and Indosinian provinces is isotopically homogeneous and was produced from respective juvenile sources with addition of old crustal material in the island arcs or active continental margin environments. The crustal growth in the CAMB had episodic character and important crust-forming events took place in the Phanerozoic. Formation of the CAMB was connected with break up of the Rodinia supercontinent in consequence of creation of the South-Pacific hot superplume. Intraplate magmatism preceding and accompanying permanently other magmatic activity in the CAMB was caused by influence of the long-term South-Pacific plume or the Asian plume damping since the Devonian.

PETROGENESIS AND ORIGIN OF THE CHENAR GRANITOID STOCK, NW OF KERMAN, IRAN: EVIDENCE OF NEOTETHYS SUBDUCTION RELATED ARC MAGMATISM

The lower to middle Miocene Chenar granitoid stock, a part of Central Iranian volcanic belt, is intruded into Eocene volcano-sedimentary complex in northwestern Kerman in south Central Iran. The Chenar granitoid stock consists mainly of coarse to medium grain granodiorite and monzogranite with subordinate tonalite and syenogranite. Alkali granites as aplitic veins and quartz dioritic dykes frequently cut through the granitoid stock. Enclaves of various types and sizes (andesitic and microgranular mafic, few meters to several centimeters wide) also occur near the margin. The granitoid rocks represent convergent margin magmas enriched in large ionic lithophile elements (LILE) such as Rb, Ba, K, Ce and depleted in high field strength elements (HFSE) such as Y, Nb and Zr. The REE patterns have relatively smooth LREE enriched chondrite normalized REE profiles with (La/Yb)n between 4.28 to 14.66. A lack of Eu anomalies along with a continuous increase in the slope of LREE profile from tonalite to syenogranite exhibit a relatively low fractionated chondrite normalized pattern [(La/Yb)n] and also involvement of plagioclase in the melting processes. Small negative Eu anomaly in alkali granite is an indication of more differentiated rock type [(La/Yb)n=42.78] in Chenar granitoid stock. Geochemical data, various trace element discriminant diagrams, common igneous microgranitoid enclaves, and ocean ridge granite normalized multi-element diagrams indicate that the Chenar granitoid stock has characteristics of metaluminous to slightly peraluminous, calc-alkaline, I-type granite of a volcanic arc settings and is formed in an active continental margin environment. In this model, a NE-dipping subduction zone of Neotethys could have accounted for the arc volcanism of the Central Iran where felsic, Andean-type magmatic arc volcanics and plutonics were formed. As a result of subduction mafic arc magmas with significant fluids were produced following the dehydration of oceanic crust and partial melting of mantle wedge which in turn provoke partial melting of considerably metasomatised and enriched subcontinental lithosphere, leading to generation of siliceous magma. Its low-pressure crystal fractionation gave rise to generation of hydrous calc-alkaline magmas, represented in part by Chenar granitoid stock.

Diagenetic Ge-Si fractionation in continental margin environments: further evidence for a nonopal Ge sink

We present data for dissolved germanic and silicic acids from several settings: sediment pore water profiles collected from the Peru-Chile continental margin, fluxes measured with in situ benthic flux chambers and shipboard whole-core incubations, and water column profiles from the California continental margin. Collectively, these data show that Ge and Si are fractionated in these continental margin sedimentary environments during diagenesis with ∼50% of the Ge released by opal dissolution being sequestered within the sediments. The areal extent of this diagenetic fractionation covers station depths from ∼100 m to >1000 m. Sediments from these sites typically have high pore water Fe 2+ present in the upper ∼2 cm. At sites with low Fe 2+ concentrations in the upper pore waters, the Ge:Si benthic regeneration ratio indicates little or no fractionation during diatom dissolution. Consistent with the sedimentary fractionation, water column dissolved Ge:Si ratios along the continental margin are on average lower (0.66 μmol/mol) than the global average ratio (0.72 μmol/mol, Mortlock and Froelich, 1996). This lower “average” ratio is driven by two distinct ΔGe:ΔSi data trends having similar slopes but different intercepts. Data from the upper ∼150 m has a Ge:Si slope of 0.74 ± 0.04 μmol/mol (2σ) and an intercept of 0.5 ± 0.4; whereas below ∼550 m the slope is 0.70 ± 0.06 μmol/mol, but the intercept is −5.0 ± 8.0. Assuming that the sediments sampled here are representative of all reducing marine environments, an assumption requiring further testing, our calculations indicate that sequestration of Ge into a nonopal phase throughout the global ocean in the depth range 0.2–1 km is sufficient to balance the Ge budget. Thus, we tentatively conclude that sequestering of Ge in reducing continental margin sediments is the “missing” Ge sink.

Neoarchaean magmatism and metamorphism of the western granulites in the central domain of the Mozambique belt, Tanzania: U–Pb shrimp geochronology and PT estimates

U–Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet–biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U–Pb and Pb–Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50–100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet–biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800–850 °C at 12–14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east.

Geological Signature and Possible Position of the North China Block in the Supercontinent Rodinia

Recent studies suggest that the South China Block (SCB) was located at a central position in the Neoproterozoic supercontinent Rodinia between ca. 1000 Ma and 800 Ma, but fewer studies dealt with the North China Block (NCB) in the reconstruction of Rodinia. The NCB was suggested, in general, to be in a stable cratonic setting from ca. 1650 Ma to ca. 160-140 Ma. Our recent study suggests that there were two metamorphic-magmatic events at ∼1300-1000 Ma and ∼800-650 Ma in the NCB. Although these events were weak, they have been regarded as marking the assembly (Grenville-age orogenic event) and breakup (Neoproterozoic mafic dyke swarm and granitoid intrusive) of the supercontinent Rodinia. A possible Grenvillian ophiolite or convergent margin complex in the NCB exists only in the North Qinling end-Mesoproterozoic orogenic belt. The major meta-volcanic-sedimentary rocks in this complex belong to the Kuanping Group and are associated with gabbros and serpentinized peridotites. Collisional and post-collisional granites have been recognized along the North Qinling belt. Volcanic and sedimentary rocks along northern margin of the NCB indicate possible continent rift or continental margin environment. The main rock associations of ca. ∼800-650 Ma are mafic dyke swarms with e Nd(t) values of enriched mantle. Therefore, this paper suggests that the mafic dykes were possibly related to the breakup of the Rodinian supercontinent. The tectonostratigraphic constraints and palaeomagnetic reconstruction show that the Proterozoic NCB was different from the SCB, but somewhat similar to Siberia. The above features possibly suggest that the NCB, as part of the supercontinent Rodinia, was possibly located in a marginal location not directly connected to the SCB, but likely adjacent to the Siberian Craton.

Geocronologia e evolução crustal da área do depósito de Cu-Au Gameleira, Província Mineral de Carajás (Pará), Brasil

O depósito de Cu-Au Gameleira está hospedado nas rochas do Grupo Igarapé Pojuca, pertencente ao Supergrupo Itacaiúnas, Província Mineral de Carajás, SE do Cráton Amazônico. Esse grupo está representado principalmente por rochas metavulcânicas máficas (RMV), anfibolitos, biotita xistos, formações ferríferas e/ou hidrotermalitos, cortadas por rochas intrusivas máficas (RIM), bem como por granitos arqueanos (2,56 Ga, Granito Deformado Itacaiúnas) e paleoproterozóicos (1,87 - 1,58 Ga, Granito Pojuca e Leucogranito do Gameleira). Cristais de zircão de um saprolito (2615 ± 10 Ma e 2683 ± 7 Ma) e de uma amostra de RIM (2705 ± 2 Ma), mostraram ser contemporâneos aos dos gabros do depósito Águas Claras. Datações Pb-Pb em rocha total e calcopirita de RMV indicaram idades de 2245 ± 29 Ma e 2419 ± 12 Ma, respectivamente, enquanto lixiviados de calcopirita indicaram idades de 2217 ± 19 Ma e 2180 ± 84 Ma. Essas idades são interpretadas como rejuvenescimento parcial provocado pelas intrusões graníticas proterozóicas (1,58 e 1,87 Ga) ou pelas reativações tectônicas associadas aos Sistemas Transcorrentes Carajás e Cinzento, ou total, provocada pelas últimas. As idades-modelo T DM de 3,12 e 3,33 Ga para as RMV e RIM e os valores de µNd (t) de -0,89 a -3,26 sugerem contribuição continental de rochas mais antigas e magmas gerados possivelmente em um ambiente de rifte continental ou de margem continental ativa.

The origin of major talc deposits in the Eastern Desert of Egypt: relict fragments of a metamorphosed carbonate horizon?

Detailed geochemical and mineralogical investigation of four talc deposits in the Eastern Desert of Egypt (Atshan, Abu Gurdi, Darhib and Kashira) suggests that the deposits form a distinct lithological unit within the Shadli metavolcanic rocks. The talc crystallized from the replacement of siliceous carbonate beds locally intercalated with clastic sediments. Th/Yb vs. Ta/Yb ratios of the rocks suggest that the sediments and the host volcanic rocks formed in an active continental margin (ACM) environment. Thus, the talc deposits may represent relict fragments of an ancient, regionally extensive carbonate horizon within the arc-related metavolcanics. The talc-rich rocks, which contain relict carbonate, serpentinized olivine and tremolite, have low (<3 wt%) Al 2O 3, Cr, Ni (<20 ppm), Co and Sc (<15 ppm) concentrations, precluding mafic or felsic igneous protoliths. The deposits were locally affected by contact metamorphism, giving rise to pyroxene-hornfels and granulite facies assemblages, and by regional metamorphism which produced greenschist-amphibolite grade assemblages. Disseminated sulfides commonly occur in the talc-tremolite-rich rocks (having low Al 2O 3 concentrations), suggesting that the metals were probably present in the original carbonate beds, but were remobilized and reconcentrated during the various metamorphic events.

Early diagenetic processes in the muddy sediments of the Bay of Biscay

Abstract In order to understand the early diagenesis processes occurring in continental margin environment, modern sediments collected in six different sites from the Bay of Biscay have been studied. These sites can be separated into two groups. In the shallowest stations, where sediments are highly bioturbated, organic carbon levels are higher than 2%. In the deepest stations, sediments are much less bioturbated, and organic carbon levels are lower. In all sites, the vertical distribution of redox sensitive species can be explained by the well-established depth sequence of redox reactions, based on the bacterially mediated oxidation of organic matter. We have considered some alternative reaction pathways to explain the profiles of Fe, Mn, and N species. These reactions deal with the ammonia oxidation by manganese oxide, the aerobic denitrification and the oxidation of dissolved iron (II) by nitrate or Mn-oxides. Vertical flux calculations with a simple diffusion model indicate that these reactions could account for the reduction of all the Mn-oxides and the oxidation of all the upward diffusing Fe(II). They may also be responsible for a significant part of the total dinitrogen production. The relative importance of these pathways on early diagenetic processes and benthic fluxes has not been determined and must be examined with additional experimental works. Our study suggests, however, that the coupling between the benthic cycles of iron, manganese and nitrogen could strongly influence the carbon cycling at the ocean floor.

Carbonate-hosted siliceous crust type mineralization of Carnic Alps (Italy-Austria)

A carbonate-hosted stratabound siliceous crust type (SCT) mineralization (base metal sulphides, barite, fluorite) occurs over large areas of Carnic Alps and Karawanken in the Eastern Alps. It concerns a pervasively silicified lithological unit, up to some tens of metres thick, which caps the unconformity landscapes developed on epicontinental Devonian–Dinantian carbonates. The SCT mineralization is directly overlaid by different transgressive siliciclastic sediments, which range from Lower Carboniferous to Lower Permian. The presence of fragments of the SCT mineralization in the transgressive siliciclastic sediment bounds its whole lithological evolution within a short stratigraphic interval of Lower Carboniferous age. Selected features of the regional and lithostratigraphic setting are discussed. The chemical characterisation is based on the statistical evaluation of compositional data of 581 selected samples. Three significant groups of elements have been distinguished: (1) the hydrological and metasomatically active elements (Si, Ba, F), which show a strong negative correlation amongst themselves and characterise the silica-saturated aqueous solutions; (2) the terrigenous elements (Al 2O 3, K 2O, Fe tot, TiO 2, B, Be, Ce, La, Nb, V, Y, Zr), which suggest a continental margin environment for silica deposition; (3) the sulphide metals (Cu, Pb, Zn, Ni, Sb, As, Hg, Cd), which define the metalliferous signature of the SCT mineralization. Some consistent, but still debatable genetic aspects of the SCT mineralization are as follow: (1) silica may be supplied by illitization of clay-rich basinal sediments during their diagenesis. δ 18O of microcrystalline quartz ranges from +18.5‰ and +24.6‰ and is very similar to δ 18O of authigenic quartz deriving from diagenetic processes of illitization of clay-rich basinal sequences. (2) The diagenetic evolution of these sediments may trigger off the movement of silica-rich marine pore waters. δ 34S of barite ranges from +15.5‰ to +19.3‰ with an average of +17.7‰ and are in good agreement with δ 34S of sulphate in ocean waters of Upper Devonian–Lower Carboniferous age. (3) A convective hydrological system, connected with sinsedimentary transtensive tectonics, active in the Carnic Alps since the Frasnian, may be the transport mechanism of aqueous solutions. (4) A weak drop in pH in the dominant carbonate environment represents the conditions for silica precipitation. SCT mineralization, showing persistent, independent and distinct characters, occurs over large areas also in other sites of the Alpine belt and outside Italy and Austria. Therefore, it points to important markers for some sedimentary sequences as well as to a worldwide significant cyclic metallogenic event. It represents a new ore deposit-type within the carbonate-hosted mineralization.

Sedimentary Carbon, Sulfur, and Iron Relationships in Modern and Ancient Diagenetic Environments of the Eel River Basin (U.S.A.)

ABSTRACT Depositional and diagenetic controls on the distributions of carbon, sulfur, and iron (C-S-Fe) in modern sediments and upper Pleistocene mudrocks of the Eel River Basin (ERB), northern California continental margin, were investigated using a combination of geochemical, radioisotopic, and sedimentological methods. A mass balance based on down-core profiles of porewater and solid-phase constituents and diagenetic modeling suggests that only 12-30% of the pyrite-S produced via SO4-2 reduction during burial is retained in modern shelf and upper slope deposits of the ERB. Bioturbational reoxidation of initially reduced S is inferred to be the major control on S preservation, on the basis of an observed inverse relationship between pyrite-S retention and biological mixing intensity, estimated from profiles of excess 234Th. Importantly, these findings argue that massive depositional episodes on the shelf following floods of the Eel River have a negligible long-term impact on bioturbating macrofauna and the potential to affect geochemical properties of the sediments. Down-core profiles of reactive Fe3+ and Py-Fe(II) for the modern deposits suggest that highly reactive Fe phases are sulfidized well within 500-2000 years of burial, thereby limiting later pyritization, which could occur through sulfidation of less reactive phases. This result explains the low ( 0.4) degree of pyritization (DOP) values exhibited by both modern and ancient deposits of the ERB and lends support to the notion that pyritization in aerobic continental-margin sediments is largely associated with highly reactive detrital Fe oxides. Comparable mean C/S weight ratios for modern sediments (5.4 ± 3.3, 1) and mudrocks (6.9 ± 4.5) of the ERB suggest that the upper Pleistocene strata reflect a geochemical environment analogous to that of the modern margin. Specifically, the C-S-Fe signatures shared by the modern and ancient deposits are a consequence of similar detrital Fe mineralogies, initial organic-matter content (Corg 1%) and composition (C/N = 13 to 17, 13Corg = -22 to -25o/oo), burial rate, and importantly, bioturbation intensity. The findings of this study have important implications for the use of C-S-Fe signatures as indicators of diagenetic processes in dynamic, continental-margin environments.

Crustal development of the Hida belt, Japan: Evidence from Nd–Sr isotopic and chemical characteristics of igneous and metamorphic rocks

Chemical and Sr–Nd isotopic investigations were performed on the igneous and metamorphic rocks in the Hida belt (central Japan) to clarify their ages and possible sources, and to provide constraints on the crustal evolution of the eastern margin of the Asian continent. Igneous rocks of three main stages in the Hida belt, represented by the late Silurian amphibolites (first stage), the middle Carboniferous mafic dikes (second stage) and the late Triassic to early Jurassic granite-gabbro plutons (third stage), show alkaline to calc-alkaline chemical affinity. Major and trace element characteristics show that the three stages of igneous rocks were emplaced in a continental margin or continental arc environment. The mafic igneous rock suites have relatively high ε Nd (−0.5 to +5.5) and low 87Sr/ 86Sr (0.7049 to 0.7061) values, indicating their origin from upper mantle sources, though the third stage granitoids contain recycled crustal components. The Hida gneisses of clastic origin show two groups of model ages ( T DM): middle Proterozoic (1.4–2.2 Ga), and late Proterozoic to early Paleozoic (0.55–0.74 Ga). Consistent with geological, geochemical and radiometric age data are that the sedimentation and basaltic volcanism occurred in early to middle Paleozoic, and the protoliths with middle Proterozoic model ages are detritals from the Proterozoic continental provenances. A synthesis of available geological and geochemical data for the basement metamorphic and igneous rocks in eastern Asia suggests two possibilities for the Paleozoic to early Mesozoic history of the Hida belt: (1) the Hida belt was formed at the eastern margin of the North China Block with later evolution as a suture zone between the North and South China blocks; or (2) it evolved as a part of the East-Central Asian Orogenic Belt (ECAOB) to the north.

OPHIOLITES AND ISLAND ARCS IN THE LATE PROTEROZOIC NUBIAN SHIELD

The late Proterozoic ophiolites of the Nubian shield can be classified into two groups. One is mainly found in shallow structural positions thrust over presumably older continental crust. The other group predominantly occurs in steep structural orientations within suture zones separating arc terranes. These structural differences reflect the classification of the first group as Tethyan-type and of the second group as Cordilleran-type. The Cordilleran-type ophiolites characterize the ensimatic realm of the Arabian-Nubian shield, whereas the occurrences of the Tethyan-type ophiolites are restricted to the presumed continental margin environment of the Archaean to mid Proterozoic Nile craton. Using the obduction ages of the ophiolites as time markers of ocean closure, the time span of the subduction-related igneous activity and thus the life time of the distinct arc terranes can be estimated to be about 150 Ma.

Geochemistry of Lower Cretaceous sediments, Inner Zone of Southwest Japan. Constraints on provenance and tectonic environment.

A geochemical study was carried out on Lower Cretaceous sedimentary rocks of the Wakino Subgroup, Kenseki Formation, and Sasayama Group, distributed in the Inner Zone of Southwest Japan. The chemical characteristics of the Lower Cretaceous sediments indicate that these rocks are immature first-order sediments derived from igneous and/or meta-igneous rocks of predominantly felsic composition. The sediments from the Kenseki Formation and the Sasayama Group, however, show high Cr and Ni abundances, suggesting a significant contribution of detritus from ultramafic rocks. Weathering at the source areas was moderate. The high Th/U ratios (mostly > 3.8), negative Eu anomalies (Eu/Eu∗ between 0.67 and 0.93) and Th/Sc ratios (mostly between 0.5 and 1) of the Lower Cretaceous sediments suggest their derivation dominantly from an old upper crust with minor amounts of young arc-derived detritus. The major, trace and rare earth element compositions imply that deposition took place in an active continental margin environment. The small amounts of young arc-derived material in the sediments support the inference by other workers that arc magmatism was not so prominent in Southwest Japan during the early Cretaceous.