Palaeo-tectonic environment of the Precambrian basaltic mafic dykes on the application of chemical discriminants

Dykes of Chitaldurg and Newer Dolerite I are OFB and represent early attempts to rift and to generate oceanic crust. Shimoga dykes are CAB and belong to orogenic phase. Guntur dykes and Ananthapur-Chittoor dykes are OIB and these are related to ‘hot spot’. Newer Dolerite II is CONB and belongs to healing and filling of fractured crust. The Cuddapah dykes are transitional to all magma types and these represent a compound event of rifting and filling of fractured continental crust.     

Stratabound ore deposits related to synsedimentary tectonics: South-West Sardinia (Italy) and Sierra de Aracena (Spain); a comparison

Deposition of stratabound Zn-Pb-Ba ores in the Lower Cambrian carbonates in South-west Sardinia show a distinct relation with the prominent tensional tectonics in the area. A part of the mineral deposits are syngenetic (sedimentary-exhalative), other massive sulfides and/or barite are early diagenetic. Other occurrences (MVT) are from early to late-diagenetic. They are mostly confined to matrix- and cementbreccias which might be dependent on the Cambrian tectonics. The Precambrian-Lower Cambrian (?) carbonates occupying the core of the Aracena anticline, at the boundary between the provinces of Huelva and Badajoz (South-west Spain), are also an important metallotect for Zn-Pb-Ag-Ba mineralization. The ores at Aracena are stratabound (to stratiform) (Sedex) and occur as disseminations or layers in a volcano-sedimentary sequence with carbonate intercalations. Both the carbonate intervals and the ores show evidence of synsedimentary tectonics, like slumpings, slump breccias and neptunian dykes. Their deposition most likely occurred in a subtidal environment controlled by faults. Apparently, in South-west Sardinia and in the Sierra de Aracena, both sedimentary evolution and ore deposition were strictly related, and conditioned by several pulses of tensional tectonics.     

Fractional crystallization and magma mixing: evidence from porphyritic diorite-granodiorite dykes and mafic microgranular enclaves within the Zhoukoudian pluton,Beijing

The Zhoukoudian pluton in the North China craton is a circular granodiorite intrusion containing porphyritic diorite dykes (PDDs), porphyritic granodiorite dykes (PGDs) and abundant mafic microgranular enclaves (MMEs), which provide an excellent opportunity to study fractional crystallization and magma mixing. The PDDs and PGDs are located in the western part of the pluton with the PDDs intruded by the PGDs. The dykes have similar mineral assemblages although plagioclase in the PDDs has higher anorthite content than the PGDs. Linear relationships between the SiO₂ and most major and trace element contents, as well as a positive trend of initial ⁸⁷Sr/⁸⁶Sr ratios and a negative trend of epsilon Nd values with increasing SiO₂ contents for the dykes suggest that both types were formed by assimilation and fractional crystallization of a common parental magma. Major oxide mass balance and trace element Rayleigh fractionation modeling points to early separation of garnet (11 %), clinopyroxene (27 %), orthopyroxene (16 %), plagioclase (25 %), biotite (19 %), and apatite (2 %) and late fractionation of hornblende (25 %), plagioclase (46 %), biotite (25 %), apatite (1 %), and magnetite (3 %). Most MMEs occur within the transitional granodiorite of the Zhoukoudian pluton. Zoned MMEs, dyke-like MME swarms, local presence of concave margins, veins and enclaves of host granodiorite within some MMEs, and several MMEs surrounded by the biotite-rich granodiorite support their formation by multiple magma mixing events, which finally resulted in different whole-rock major oxides and compatible elements, but homogeneous mineral major oxides (except zoned plagioclase), whole-rock incompatible elements and Sr-Nd isotopes between the MMEs and their host granodiorite. We suggest that multiple magma mixing events might also cause complexly zoned plagioclase in the Zhoukoudian pluton. Relative calcic, irregular or patchy cores and dusty zoned mantles from the zoned plagioclase crystals and their relatively low anorthite content indicate multiple mixing events between mafic/intermediate and felsic magmas. The mafic/intermediate end-members could be represented by the diabase dykes and the PDDs. Therefore, the dykes and MMEs in the Zhoukoudian pluton are genetically linked.     

The problem of rock assimilation by Somma-Vesuvius magma

Mineralogical and chemical compositions of ejected carbonate inclusions of Vesuvian gaseous phase are compared with those of carbonate sediments of the Somma-Vesuvius area. The basement of Vesuvius mainly consists of Mesozoic limestones. Most of these are characterized by extremely low insoluble residues. In this area thick dolomite beds occur in the Triassic system only. A calcareous layer of approximately 100 to 150 meters thickness characterized by high Sr contents (0.19% Sr in the average) is to be found within the Triassic dolostones. Several carbonate ejecta also show high Sr contents (0.1% Sr) but chemical composition of some of these ejecta differs somewhat from that of the Triassic layer high in Sr. Contact with the volcanic volatile phase and melt has produced some alterations in the composition of many ejecta. Magnesian calcites are abundant and periclase, brucite, tremolite, phlogopite and magnesite were found in the ejecta examined. Silicon, manganese, iron, zinc and, to a smaller extent, potassium and copper have been most probably transported from the volcanic gas phase into ejected carbonate inclusions. Owing to metasomatic actions of the volcanic volatile components, rearrangement and alteration of Ca and Mg contents occurred in the carbonate minerals of several ejecta. Under the influence of volcanic pressure and temperature, magnesium content originally in dolomite might be transformed into the calcite structure. \(\frac{{{\text{MgO}}}}{{{\text{CaO + MgO}}}}\) molar ratios of several carbonate ejecta do not correspond with those found in sedimentary limestones and dolostones. It cannot be proved whether the Mg of the mixed calcite-dolomite ejecta has been partially introduced from the volatiles or lost from the carbonate phases.     

Two-dimensional modeling of the accumulation of carbonate deposits in marine environments

A program is developed for modeling the accumulation of deep-water, shallow-sea, reef and fragmentary carbonate deposits in marine environments. The formation of the Maldives carbonate platform is modeled using the developed program. The comparison of the modeling results and the geological cross-section demonstrates that the algorithms allow the construction of geological cross-sections in sufficient detail.     

Chemistry of peralkaline phonolite dykes from the Grønnedal-Íka area,South Greenland

The chemical variation observed in a suite of fifteen aphyric peralkaline phonolite dykes of mid-Gardar age from the vicinity of the Grønnedal-Íka alkaline complex in south-west Greenland is discussed. From relationships in the system Na₂O-K₂O-Al₂O₃-SiO₂ it is argued that the members of the series are related by the fractionation of feldspar approximating to Ab₅₅Or₄₀An₅ in composition, along with augite and lesser amounts of other ferromagnesian minerals. The bearing of these rocks on phase equilibria in the analogous natural system is discussed, and consideration is given to the possible origins of the initial peralkaline phonolite magma.     

Development of rodingite in basaltic rocks in serpentinites,East Liguria,Italy

Hydrogrossular replacement of plagioclase in basaltic rocks enclosed in serpentinite, and relationships between hydrogrossular, pumpellyite, vesuvianite are described. Rogingitic rocks are dominated by mixed layer chlorite-smectite, chlorite, pumpellyite, hydrogrossular and vesuvianite. In these rocks pumpellyite attains maximal Mg contents, and chemical analysis shows extreme removal of Na₂O, K₂O, TiO₂ and SiO₂ from the basalts, and increase in the Fe³⁺/Fe²⁺ ratio. Leaching during serpentinization by extremely alkaline solutions dominated by Ca-Mg(OH) may explain removal of components, but the oxidation may suggest that at an earlier stage dykes may have acted as input aquifers.     

Carbonatite nomenclature

Based upon the composition of carbonatites of the Fen Complex in Norway and the Alnö Complex in Sweden, the following nomenclature of carbonatitic rocks is suggested: Carbonatites (> 70 % carbonate minerals), sub-divided into the calcitic sövites and alvikites, the dolomitic to ankeritic rauhaugites and beforsites and the alkali-carbonatitic lengaite. Silico-carbonatites are rocks with 50–70 % carbonate minerals. They are subdivided into silico-sövites, silico-alvikites, silico-rauhaugite and silico-beforsite.     

Gridino melange zone of the Belomorian eclogite province: Succession of tectonic events and structural position of mafic dyke swarms

Based on relationships between Paleoproterozoic mafic dykes, lithotectonic complexes, and tectonic structures of the Gridino Zone in the Belomorian eclogite province of the Fennoscandian Shield, deformations have been divided into groups differing in age and the succession of tectonic events has been reconstructed. The formation of Neoarchean eclogite-bearing melange was related to disintegration of large eclogite sheets in the course of near-horizontal ductile flow accompanied by syntectonic granitoid magmatism, multiple migmatization, and granulite-to amphibolite-facies metamorphism. The exotic blocks, including eclogites, were incorporated into TTG gneisses as sheets and lenses up to a few hundreds of meters in thickness and oriented conformably with gneissic banding. As a result of ductile flow, the lithotectonic complexes were transported at the level of discrete brittle-ductile deformations expressed as strike-slip faults and associated folds. Under conditions of a relatively rigid medium, individual structural elements underwent rotation approximately through 90° in plan view. Under the extension regime in the Early Paleoproterozoic, several swarms of mafic dykes were injected into the already cold framework rocks, as is evident from dyke morphology. The dykes crosscut all predated structures, included turned blocks, and are therefore important reference points for subdivision of Neoarchean and Paleoproterozoic processes. The Svecofennian postdyke tectonic activity was accompanied by local shearing and boudinage of metabasic rocks, development of quartz and pegmatite veins along tension cracks, disharmonic folding, and discrete retrograde metamorphism up to amphibolite-facies conditions. The postdyke deformations did not exert a substantial effect on the previously formed regional structure.     

Manganese deposits in northeastern European Russia and the Urals: Isotope geochemistry,genesis, and evolution of ore formation

Based on new data on the lithology, mineralogy, chemistry, and isotopic composition of manganese carbonate ores and rocks at the deposits and occurrences in the Novaya Zemlya Archipelago, the Pai-Khoi, and the Urals, as well as using data from the literature, the main Phanerozoic basins of manganese deposition have been established in the geological history of Laurasia, Pangea, and Siberian paleocontinents. The formation conditions of manganese ore gradually changed from hydrothermal-sedimentary in the Middle Paleozoic to sedimentary-diagenetic in Mesozoic and Cenozoic. The ore was also formed under catagenetic conditions. Carbon of oxidized organic matter plays a substantial role in the formation of manganese carbonates.     

A microseismic experiment in Abu Dhabi,United Arab Emirates: implications for carbonate reservoir monitoring

A microseismic experiment utilizing a single downhole array of eight 3-component receivers was conducted in an offshore oilfield in the emirate of Abu Dhabi in the United Arab Emirates. The Lower Cretaceous Thamama Group is the major carbonate reservoir in the field producing from six zones. Microseismic data was acquired in conjunction with gas injections that took place in the Thamama IV A and IV B reservoirs during 9 days of acquisition. The aim of the experiment was to monitor the microseismic activity arising in the carbonate reservoirs as a result of gas injection. A total of 103 microseismic events were detected from two of the receivers. For the majority of these events, there were no well-defined P wave arrivals, probably due to the presence of strong background noise and the very weak microseismic signals. The results from this experiment indicate that the detected events are probably related to the microseismic activities caused by the gas injection within the Thamama IV B reservoir. Therefore, downhole microseismic monitoring methods can potentially provide valuable information about the fracture systems within the carbonate reservoirs of Abu Dhabi's oilfields.     

Heavy metals in Xuzhou urban roadside topsoils (China): magnetic characterization and enrichment mechanism

Heavy metal contamination is prevalent in urban areas where soil represents a significant sink. Urban topsoil samples were collected and metal concentrations including Pb, Zn, Cu, Cd, Ni and Co were measured by inductively coupled plasma mass spectrometry. All analyzed metals showed elevated concentrations compared to local background concentrations with the exception of Co. Cadmium was the metal most enriched in the analyzed area, presenting enrichment factor, on average, of 16.5. Mineral magnetic concentration parameters (χlf and SIRM) exhibited significant relationships with Pb, Zn, Cu and Cd, respectively. Zinc, Co and Ni exhibited a substantial build-up in the finest particles (<28 μm). Enrichment was positively correlated with carbonate complexation constants (but not bulk solubility products) and suggests that the dominant mechanism controlling metal enrichment in these samples is a precipitation of carbonate surfaces that subsequently adsorb metals.     

Geochemical studies and petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm (trending N-S to NNW-SSE) from eastern Dharwar craton,India: implications for Paleoproterozoic large igneous provinces and supercraton superia

The Archean eastern Dharwar craton is transacted by at least four major Proterozoic mafic dyke swarms. We present geochemical data for the ~2.21–2.22 Ga N-S to NNW-SSE trending Kunigal mafic dyke swarm of the eastern Dharwar craton to address its petrogenesis and formation of large igneous province as well as spatial link to supercontinent history. It has a strike span of about 200 km; one dyke of this swarm runs ~300 km along the western margin of the Closepet granite. Texture and mineral compositions classify them as dolerite and olivine dolerite. They show compositions of high-iron tholeiites, high-magnesian tholeiites or picrites. Geochemical characteristics of the sampled dykes suggest their co-genetic nature and show variation from primitive (Mg#; as high as ~76) to evolved (differentiated) nature. Although geochemical characteristics indicate possibility of minor crustal contamination, they show their derivation from an uncontaminated mantle melt. These mafic dykes are probably evolved from a sub-alkaline basaltic magma generated by ~20 % batch melting of a depleted lherzolite mantle source and about 15–30 % olivine fractionation. Paleoproterozoic (~2.21–2.22 Ga) mafic magmatism is recognized globally as dyke swarms or gabbroic sill complexes in the Superior, Slave, North Atlantic, Fennoscandian and Pilbara cratons. Possible Paleoproterozoic Dharwar–Superior–North-Atlantic–Slave correlations are constrained with implications for the configuration of supercraton Superia.     

Constraints on metasomatized mantle under Central South America: evidence from Jurassic alkaline lamprophyre dykes from the Eastern Cordillera, NM Argentina

In the Río Grande Valley, NW Argentina, several porphyritic panidiomorphic, ocelli-bearing dykes and sills intrude the Neoproterozoic to lower Paleozoic basement of the Eastern Cordillera. New petrographical and geochemical data permit us to classify these rocks as ocellar-analcime monchiquites, a feldspar-free variety of alkaline lamprophyre composed of Ti-rich-diopside/augite, Ti-rich biotite/phlogopite, forsteritic olivine, titanian-pargasite and analcime, with abundant ocelli filled with analcime/carbonate. In terms of geochemical compositions they are characterized by LILE and LREE enrichment and lack of Nb-Ta and Eu anomalies. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd initial ratios range between 0.70377 to 0.70781 and 0.512506 and 0.512716 respectively, and T_DM model ages vary between 0.25–0.64 Ga. A K-Ar age of 163?±?9 Ma suggests that these rocks are related to the pre-rifting stage of the Mesozoic-Cenozoic continental Salta Rift in NW Argentina. Partial melting of a heterogeneous enriched metasomatized lithospheric mantle, magma mixing and fractionation are envisaged to explain the petrographic, geochemical and isotope characteristics of these magmas.     

A general account of Karoo vulcanicity in southern Africa

Volcanic rocks of late Triassic to early Cretaceous age which today cover more than 140 000 km² of the southern African sub-continent are discussed. The rocks occur as scattered outliers and represent small eroded remnants of an originally more extensive volcanic province. The rocks are best preserved in three main areas of southern Africa, in Namibia, in central South Africa, Transkei and Lesotho and in the southeastern areas of the sub-continent including South Africa, Swaziland, Mozambique and Zimbabwe. A history of previous work done on Karoo volcanism is presented and accounts of the geological relationships of the main, and some subsidiary, areas as well as events preceding the major phase of extrusive vulcanism are discussed. Relationships between dolerite dykes and basaltic lavas are noted. Volcano-stratigraphic and geochemical mapping have been used to subdivide the volcanic successions found in the different areas, and recently adopted nomenclature is presented. Considerable more variability has been recognised in the volcanic succession than was previously recognised, the flood basalt pile being more complex than was originally thought. Rocks from the central Karoo areas are primarily of basaltic composition whereas those from the western and eastern marginal areas include basic, and felsic types. Age relationships of the volcanics based on recent studies are outlined and geochemical variations within the entire province discussed. The classification system adopted for the Karoo volcanics is described briefly and chemical relationships of extrusive and intrusive rocks are included. In concluding some new ideas concerning the overall development of the Karoo volcanic province and related fragmentation of Gondwanaland are presented.     

Alkali granite-syenite-carbonatite association in Munnar Kerala,India; implications for rifting,alkaline magmatism and liquid immiscibility

Occurrence of carbonatite is reported from the Munnar area, Kerala, where an alkali granite-syenite-carbonatite association is seen emplaced along the intersection zone of the Attur and Kerala fault-lineaments. The carbonatites are of two varieties, a calcite-rich sovite and a very coarse grained, calcite and dolomite bearing alvikite. Higher levels of SiO₂, Al₂O₃ and CaO are characteristic of these as compared to the composition of typical carbonatites. The transition element levels are high whereas the incompatible elements show lower values. The low Sr values, lower amount of apatite and absence of rare metal minerals preclude a primary carbonatite magma. The associated syenite and alkali granite have higher K₂O, K₂O/Na₂O, K/Rb, K/Ba and transition element levels. Petrochemical features suggest the rock association to be a result of separation of an immiscible fraction of less viscous carbonate liquid during cooling and ascent from a more viscous polymerized alkali silicate phase. The pre-requisites for melt equilibration and liquid immiscibility were achieved through volatile degassing related to crustal warping and rifting. The unique alkaline association of Munnar, which shows spatial relationships with deep-seated faults as well as a probable triple-point junction, is suggested to be a signature of late Precambrian alkaline magmatism which manifested in the Indian shield as a precursor to the rifting of the continental margin.     

Rare earth element geochemistry of the Fen alkaline complex,Norway

Determination of rare earth element (REE) abundances in rocks of the Fen complex has shown that within rocks of the first magmatic series REE abundances increase in the order urtiteFen magmas are discussed and it is considered that parental magmas had relatively high La/Yb ratios (40–60). Utilizing petrological evidence from other alkaline complexes coupled with experimental studies it is considered that the parental magma was a carbonated nephelinite produced by limited (<10%) partial melting of the mantle. All the Fen rocks are placed in a petrogentic scheme in which a carbonated nephelinite magma undergoes liquid immiscibility, differentiation and volatile transport.     

Serpentinization at Burro Mountain,California

The Burro Mountain ultramafic complex, Monterey County, California, consists of dunites and peridotites which are partially or wholly serpentinized. Primary minerals in both rock types are olivine, enstatite, diopside, and picotite which upon alteration yield chrysotile, lizardite, brucite, magnetite, talc, tremolite, and carbonate. Electron microprobe analyses show that enstatite, En_85.8 to En_90.8, alters to “bastite” composed only of lizardite (5.0–12.0 weight percent FeO), whereas olivine, Fo_90.8 to Fo_91.6, forms lizardite+chrysotile+brucite with or without magnetite. The chrysotile ranges from 3.0 to 5.0 weight percent FeO, the brucite from 16.0 to 43.0 weight percent FeO. As Serpentinization proceeds, the alteration products are enriched in FeO relative to MgO. Serpentinization probably originates in a changing \(P_{O_2 }\)-T environment by two different reactions:

1. (a)
   Olivine+enstatite+H₂O+O₂?Mg, Fe⁺² chrysotile+Mg, Fe⁺³, Fe⁺² lizardite with or without magnetite.     
   
   

Polycyclic aromatic hydrocarbons in surface sediments of Binacional Itaipu Lake,Brazil: characteristics,sources and toxicity evaluation

Itaipu Lake, which includes the Itaipu hydroelectric power plant, is one of the largest dams in the world and has a strong relationship with its surroundings. The flooded area has multiple uses such as navigation, recreation, water abstraction for industrial, urban and agricultural irrigation. The lake is located at the frontier between Brazil, Argentina and Paraguay. In this study, superficial sediments collected from nine sampling sites were analysed for grain size, organic matter and 16 priority polycyclic aromatic hydrocarbons (PAHs) using high-performance liquid chromatography (HPLC) with fluorescence detector. The total concentration of PAHs in the dry sediment ranged from 35.21 to 685.37 µg kg^?1. Diagnostic ratios showed that the possible source of PAHs in the Itaipu Lake could be pyrolitic and petrogenic. The potential toxicity of sediment of PAHs varied from not detected to 127.70 µg g^?1, suggesting that some adverse ecological effects would arise due to PAHs in these sediments.