Precambrian Sedimentary Rocks Research Articles

Coupling hydrogeochemistry and stable isotopes (δ2H, δ18O and δ13C) to identify factors affecting arsenic enrichment of surface water and groundwater in Precambrian sedimentary rocks, eastern salt range, Punjab, Pakistan.

The study area is a part of the Salt Range, where water quality is being deteriorated by natural and anthropogenic sources. This research integrates water quality assessment, arsenic enrichment, hydrogeochemical processes, groundwater recharge and carbon sources in aquifer. Total dissolved solid (TDS) contents in springs water, lake water and groundwater are in range of 681-847mg/L, 2460-5051mg/L and 513-7491mg/L, respectively. The higher concentrations of magnesium and calcium in water bodies next to sodium are because of carbonates, sulfates, halite and silicates dissolution. The average concentrations of ions in groundwater are in order of HCO3- > SO42- > Cl- > Na+ > Mg2+ > Ca2+ > K+ > NO3-, virtually analogous to springs water, but different from lake water, categorized as poor quality and unfit for drinking purposes. Based on major ions hydrochemistry, NaCl and mixed Ca-Mg-Cl type hydrochemical facies are associated with concentration of arsenic (4.2-39.5µg/L) in groundwater. Groundwater samples (70%) having arsenic concentration (11 ≤ As ≤ 39.5µg/L) exceeded from World Health Organization (WHO) guideline (As ≤ 10µg/L) in near neutral to slightly alkaline (6.7 ≤ pH ≤ 8.3), positive Eh(6 ≤ Eh ≤ 204mV), signifying its oxic condition. Eh-pH diagrams for arsenic and iron indicate that 80% of groundwater for arsenic and iron were in compartments of HAsO42- and Fe(OH)3, unveil oxic environment. Arsenic is moderately positive correlated with TDS, sodium, chloride, bicarbonate, nitrate, sulfate and weak negative with δ13CDIC in surface and groundwater, forecasting multiple sources of arsenic to aquifer. Stable isotopes of waters show recharge of groundwater from local rain and lake water. The lower δ13CDIC values of groundwater are modified by influx of CO2 produced during biological oxidation of soil natural organic matter.

Molar-Tooth Structure as a Window into the Deposition and Diagenesis of Precambrian Carbonate

Molar-tooth structure (MTS) is an unusual carbonate fabric that is composed of variously shaped cracks and voids filled with calcite microspar. Despite a century of study, MTS remains enigmatic because it juxtaposes void formation within a cohesive yet unlithified substrate with the penecontemporaneous precipitation and lithification of void-filling carbonate microspar. MTS is broadly restricted to shallow marine carbonate strata of the Mesoproterozoic and Neoproterozoic, suggesting a fundamental link between the formation of MTS and the biogeochemical evolution of marine environments. Despite uncertainties in the origin of MTS, molar-tooth (MT) microspar remains a popular target for geochemical analysis and the reconstruction of Precambrian marine chemistry. Here we review models for the formation of MTS and show how detailed petrographic analysis of MT microspar permits identification of a complex process of precipitation and diagenesis that must be considered when the microspar phase is used as a geochemical proxy. ▪ Molar-tooth fabric is an enigmatic structure in Precambrian sedimentary rocks that is composed of variously shaped cracks and voids filled with carbonate microspar. ▪ Time restriction of this fabric suggests a link between this unusual structure and the biogeochemical evolution of marine environments. ▪ Petrographic analysis of molar-tooth fabric provides insight into fundamental processes of crystallization.

Ediacaran-Cambrian microbialites of the Southern Amazon Craton: relation with the metazoan rise, sea-level changes, and global tectonics

Microbialites are the most abundant life evidence in Precambrian sedimentary rocks. They are produced by microbial interaction activity and sedimentary processes reflecting paleoenvironmental conditions. The Ediacaran-Cambrian carbonate and siliciclastic successions in the Southern Amazon Craton in Central Brazil, provide a key opportunity to understand how the metazoan life coexisted with the microbial communities. The spatial and temporal distribution of microbialites as well as morphological and paleoenvironmental changes have been assessed, reinterpreting previous works and including new data from the Araras-Alto Paraguai and Corumbá basins. The deposition was controlled by subsidence and sea-level changes that affected these basins, considered extensions of epicontinental seas during the Gondwana assembly. The stromatolites are restricted to coastal deposits and experienced thriving flourishment intervals after the Marinoan Glaciation (635 Ma). Post-glacial transgression was marked by microbial colonization in shallow platforms represented by stratiform and giant domical stromatolites in the Araras-Alto Paraguai Basin. The continuity of the transgression generated a moderately deep aragonite sea at about 622 Ma. A progressive sea-level fall caused the implantation of coastal environments under greenhouse conditions with tidal flat and sabkha settings colonized by centimetric-scale stromatolites. The sea retreat was accompanied by progressive uplift, causing a moderate inversion of the basin and erosion of the succession until ~560 Ma with the deposition of the last preserved tidal flat deposits with the occurrence of thrombolites. The subsiding Corumbá Basin was the site of microbially-induced deposition of carbonates in a shallow platform connected to an offshore setting with the proliferation of metazoan straddling the Ediacaran-Cambrian boundary. Microbial communities were restricted to lagoon deposits during the Lower Cambrian transgression in the Araras-Alto Paraguai Basin and the last phase refers to the sea retreat towards southeast, developing a fluvial system connected with the arid and evaporitic tidal flats colonized by microbialites that lasted until the upper Cambrian. Except for the post-glacial stromatolites, the columnar and domal microbialite indicate that the coastal settings dominated the Ediacaran-Cambrian transition. The preservation of microbialites in the post-glacial intervals can be associated with the Mg-Ca-CO3 oversaturation in dolomitic platforms. The rapid calcification and ability to resist the dissolution and replacement have increased the stromatolites’ preservation potential reported here, where its well-preserved occurrence in tidal flats and sabkha occurs due to intense early diagenetic silicification. The change from carbonate accumulation to siliciclastic-rich environments contributed to the demise of microbially-induced strata. In general, the scarce coexistence between coastal stromatolite and metazoan-bearing marine deposits makes it challenging to establish a competitive relationship between these organisms, as previously postulated.

Geochronology and tectonic significance of A‐type granite from Misho, NW Iran: Implications for the detachment of Cimmeria from Gondwana and the opening of Neo‐Tethys

The Cimmerian Terrane, part of which forms the Central Iranian Plateau, is one of several continental fragments produced during the early‐ to mid‐Palaeozoic breakup of the northern margin of Gondwana. Magmatic evidence for the final separation of the Terrane from Gondwana and formation of the intervening Neo‐Tethys Ocean is preserved in the Misho Mountains SW of Marand, NW Iran. A small (30 km2) body of subalkaline granite intruding Precambrian sedimentary rocks of the Kahar Formation has numerous chemical features that identify it as an A‐type granite, including high Ga/Al, K2O + Na2O, FeOt/(FeOt + MgO) and incompatible elements (LILE and HFSE), and low CaO and P2O5. Such features are consistent with the asthenospheric upwelling above a rising mantle plume. Zircon ages of 350.8 ± 3.0, 345.4 ± 3.2, and 343.6 ± 4.0 Ma measured on three samples of the Misho A‐type granite along with the same granites exposed in the western domain (Herris area, 306 Ma) reveal that the crystallization process occurred sequentially in different stages. The obtained ages record detachment of the Cimmerian Terrane from Gondwana and the initiation of Neo‐Tethys at least in the Early Carboniferous, significantly earlier than previous researchers have proposed.

Особенности формирования водопритоков к месторождению алмазов им. В. Гриба (Архангельская область)

Введение: На кимберлитовом месторождении им. В. Гриба принят поверхностный способ отработки, при котором осушению подлежат все водоносные породы над рабочим горизонтом. Фильтрация воды в отработанное пространство осуществляется по бортам карьера по всему периметру, что затрудняет ведение горных работ и может привести к негативным инженерно-геологическим явлениям в карьере (прорыву воды, оползням, обвалам). Основная цель исследований заключается в изучении процессов фильтрации подземных вод при осушении месторождения им. В. Гриба с использованием численного метода моделирования. Методика: Кимберлитовая трубка прорывает осадочные докембрийские отложения и перекрыта толщей среднекаменноугольных и четвертичных отложений. В районе исследований расположены многочисленные реки и озера, сооружено хвостохранилище. Определение источников формирования водопритоков в карьер осуществлялось с помощью разработанной численной геофильтрационной модели, на которой учитывались все элементы дренажной системы, активная взаимосвязь подземных и поверхностных вод, геолого-гидрогеологические особенности месторождения. Расчетная модель состоит из четырех слоев: четвертичный и олмугско-окуневский, урзугско-воереченский, падунский и мезенский комплексы. Фильтрационная задача решалась по программе MODFLOW лицензионного пакета GMS. Результаты и обсуждение: В процессе идентификации модели выявлен характер гидравлической связи водоносной толщи с речными и озерными долинами, определены участки питания и разгрузки водоносных пластов, фильтрационные потери из хвостохранилища Верхотинского ГОКа, установлены источники формирования водопритоков к дренажной системе карьера. Установлено, что водопритоки в карьер формируются за счет подземных вод и атмосферных осадков в зоне с активной гидравлической взаимосвязью подземных и поверхностных водных объектов. Заключение: С использованием численной модели определены основные источники формирования водопритоков в карьер: водоносные комплексы, получающие дополнительное питание из поверхностных водоемов и водотоков, установлена взаимосвязь изучаемого водоносного массива с долинами рек и озер, выявлены места питания и разгрузки исследуемой водоносной толщи, оценены инфильтрационные потери воды из хвостохранилища Верхотинского ГОКа.

Depositional age, provenance, and tectonic implications of Neoproterozoic sedimentary rocks in the Xiangshan area, South China

Detrital zircon U–Pb–Hf isotopes and whole‐rock geochemical compositions of exposed Neoproterozoic sedimentary rocks in the Xiangshan area are determined to discuss provenances and the boundary between the Yangtze and Cathaysia blocks. Zircon dating results reveal that the exposed Precambrian sedimentary rocks in the Xiangshan area were deposited in the Late Neoproterozoic era. The age spectra and whole‐rock geochemistry show that the detrital materials of the Shenshan Formation originated from the Yangtze Block and that the detritus of the Kuli and Shangshi formations came mainly from the Yangtze Block and partly from the Cathaysia Block. The source for the Neoproterozoic sedimentary rocks in the Xiangshan area may been intermediate rocks, mainly from a mixture of felsic and mafic rocks in the Yangtze Block. In combination with previous studies, we suggest that the Xiangshan area belonged to the Cathaysia Block before the amalgamation between the Cathaysia and Yangtze blocks and that the real boundary between the two blocks is located in the northern part of the Xiangshan area.

Geology and evolutionary stages of the Late Precambrian Hammamat sediments at Gebel Um Tawat, North Eastern Desert, Egypt

The Um Tawat Late Precambrian immature molasse-type Hammamat sedimentary rocks, located in the Northern part of the Eastern Desert of Egypt, were deposited at the late stage evolution of the Pan-African Orogeny. The Um Tawat molasse sediments are mainly derived from Dokhan volcanics and rest unconformably on these volcanics. They are intruded by the younger granites of Sala’at El-Belih granites and Gebel Gattar batholith. Hammamat sediments consist of repeated cycles of conglomerates, sandstones, siltstones, and minor mudstones deposited in an inter-mountainous basin by braided stream and alluvial fan regime constituting a stratigraphic section of about 500 m thick. Petrographically, the conglomerates are of polymictic type. Sandstone could be termed arkosic greywacke and shows ripple marks and in some places cross bedding. Siltstone is characterized by clear lamination and graded bedding. Mudstone is characterized by very fine grains of silt and clay size particles. The evolutionary stages of the study area is divided into five stages: (1) compressional regime resulted in the formation of the metavolcanics and metagabbro-diorite complex; (2) tension and relaxation crustal regime and uplift of the old basement forming G. Dokhan; (3) relaxation period, weathering, and erosion processes of the Dokhan volcanics formed the Hammamat molasse-type sediments; (4) tensional regime or weak rift-like processes led to the eruption of the younger granites and (5) regional compressional deformation subjected to the Hammamat sediments. This phase form G. Um Tawat and forming one regional open anticline fold whose fold axis is trending N47°E–S47°W and plunges 5° toward SW.

Unraveling the Mineralogical Complexity of Sediment Iron Speciation Using Sequential Extractions

AbstractIron speciation is one of the most widely applied proxies used to reconstruct oxygen levels and redox conditions in past aqueous environments. The iron speciation proxy estimates proportions of different reactive iron species in fine‐grained sedimentary rocks, which are mapped to redox conditions based on empirical calibrations from modern sediments. It is based on a standardized extraction technique of sequentially applying acetate, hydroxlamine‐HCl, dithionite, and oxalate solutions to a powdered sample in order to dissolve iron phases and quantify the amount of iron carried by carbonates, “easily reducible” oxyhydroxides, ferric iron (oxyhydr)oxides, and magnetite, respectively. Although tested on pure minerals and mixtures, assessments of whether this sequential extraction process accurately dissolves the targeted minerals in natural sediments and sedimentary rocks are lacking. In our study, residues from each sequential extraction step were analyzed using rock magnetic and X‐ray diffraction experiments to identify and quantify the iron‐bearing minerals that were dissolved. The dithionite extraction robustly removes the targeted mineralogy as magnetic data show it to solubilize nearly all of the goethite. However, magnetic quantification of magnetite was orders of magnitude less than the iron measured in the oxalate extraction; X‐ray diffraction data suggest that dissolution of iron‐bearing clays, specifically berthierine/chamosite, could explain this disparity. Our data compilation shows higher values of iron from the oxalate extraction in Precambrian sedimentary rock samples, suggesting a significant temporal shift in iron cycling. Recognition of heterogeneity in chemical extraction efficiency and targeting is vital for holistic multiproxy interpretation of past oxygen levels and communication between disciplines.

Widespread deposition of greenalite to form Banded Iron Formations before the Great Oxidation Event

Banded Iron Formations (BIFs) are Precambrian sedimentary rocks interpreted to have been precipitated from anoxic seawater prior to the first permanent rise in atmospheric oxygen in the Great Oxidation Event at ca. 2.45–2.32 Ga. BIFs hold the key to understanding the chemistry of the oceans and atmosphere, and how these interacted with microbial life, prior to and during the evolution of oxygenic photosynthesis. To unlock this information, it is essential to know how BIFs formed, that is, what minerals were precipitated, by what mechanisms and in which environments. BIFs are found in almost all depositional settings at times when input of clastic detritus was lacking, for example, submarine proximal volcanic environments, basin floor, slope, deep marine shelf and shallow shelf settings. High-resolution electron microscopy of finely laminated BIFs and ferruginous cherts that range in age from 3.45 to ca. 2.4 Ga and that preserve depositional features indicates that the original sediment was a very fine mud composed of nanoparticles of the iron-silicate greenalite. In places, the greenalite mud experienced very early silicification on the sea floor enabling preservation of the mineral and its depositional textures. Very fine grained siderite and hematite in laminated BIFs post-date dehydration of the early silica cement and are not primary minerals. Greenalite nanoparticles are found in BIFs from all depositional settings indicating a common origin, likely precipitation resulting from mixing of plume water from hydrothermal vents with ambient seawater. The nanoparticles were carried throughout the oceans and were deposited on the seafloor and on continental margins to form the primary sediments of BIFs.

Fe(II)-carbonate precipitation kinetics and the chemistry of anoxic ferruginous seawater

The chemical evolution of anoxic seawater is chronicled in part by the abundance of sedimentary Fe minerals. Our view of the chemical state of the oceans throughout much of Earth's history, therefore, hinges on understanding the behaviour of Fe2+ in seawater and sedimentary pore water. However, the formation pathways and precipitation rates of a number of sedimentary Fe(II)-minerals are poorly understood. For example, because siderite is widely distributed in Precambrian rocks, it is frequently assumed that seawater Fe2+ concentrations would have been poised at siderite equilibrium. Yet a variety of studies indicate that siderite precipitation would have been strongly controlled by kinetic processes rather than equilibrium behaviour, prompting a reconsideration of the Precambrian Fe cycle and those element cycles to which it was connected. To address this issue, we experimentally investigated spontaneous (or homogeneous) precipitation of Fe(II)-carbonate across a range of pH and pCO2 conditions, and in the presence/absence of SiO2(aq). Our data show that the formation of siderite directly from seawater is initiated by the formation of precursor amorphous Fe carbonate (AFC). The precursor, once formed, rapidly transforms into either siderite or chukanovite (Fe2CO3(OH)2; metastable with respect to siderite), dependent on the availability of carbonate ions. Precipitation rates estimated from our experiments reveal a strong exponential dependence on the saturation state of the solution, consistent with theoretical predictions. This relationship, in turn, delineates a critical supersaturation threshold for homogeneous Fe(II)-carbonate precipitation from anoxic and ferruginous water. Incorporation of our experimental data with constraints on the ocean–atmosphere carbon system shows that significant accumulations of siderite in Precambrian sedimentary rocks require an origin from fluids that were distinct from seawater in equilibrium with the ocean–atmosphere inorganic carbon reservoir. In addition, these data show that the maximum Fe2+ concentration in Precambrian oceans was controlled by Fe(II)-silicate precipitation and not by Fe(II)-carbonate, consistent with the sedimentary record of Fe-rich Precambrian rocks. More broadly, these results contribute to a quantitative framework that underpins the utility of Fe(II)-carbonate as a sensitive palaeo-environmental proxy for ferruginous systems where CO2 may have been present.

Tracing the fate of steroids through a hypersaline microbial mat (Kiritimati, Kiribati/Central Pacific).

Eukaryotic steranes are typically absent or occur in very low concentrations in Precambrian sedimentary rocks. However, it is as yet unclear whether this may reflect low source inputs or a preservational bias. For instance, it has been proposed that eukaryotic lipids were profoundly degraded in benthic microbial mats that were ubiquitous prior to the advent of vertical bioturbation in the Cambrian ("mat-seal effect"). It is therefore important to test the microbial turnover and degradation of eukaryotic steroids in real-world microbial mats. Here we assessed steroid inventories in different layers of a microbial mat from a hypersaline lake on Kiritimati (Central Pacific). Various eukaryote-derived C27 -C30 steroids were detected in all mat layers. These compounds most likely entered the mat system as unsaturated sterols from the water column or the topmost mat, and were progressively altered during burial in the deeper, anoxic mat layers over c. 103 years. This is reflected by increasing proportions of saturated sterols and sterenes, as well as the presence of thiosteranes in certain horizons. Sterol alteration can partly be assigned to microbial transformation but is also due to chemical reactions promoted by the reducing environment in the deeper mat layers. Notably, however, compounds with a sterane skeleton were similarly abundant in all mat layers and their absolute concentrations did not show any systematic decrease. The observed decrease of steroid/hopanoid ratios with depth may thus rather indicate a progressive "dilution" by lipids derived from heterotrophic bacteria. Further, pyrolysis revealed that steroids, in contrast to hopanoids, were not sequestered into non-extractable organic matter. This may lead to a preservational bias against steroids during later stages of burial. Taken together, steroid preservation in the microbial mat is not only controlled by heterotrophic degradation, but rather reflects a complex interplay of taphonomic processes.

Age and geochemistry of the Neoproterozoic granitoids in the the Songnen – Zhangguangcai Range Massif, NE China: Petrogenesis and tectonic implications

The Songnen–Zhangguangcai Range Massif (SZRM) is located in the eastern Central Asian Orogenic Belt and crops out over an extensive part of NE China. The massif was originally thought to contain numerous Precambrian terranes (e.g., Xingdong, Dongfengshan, Yimianpo and Zhangguangcailing groups). However, more recent zircon U–Pb dating indicates that the majority of these so‐called Precambrian sedimentary and igneous rocks actually formed during either the Paleozoic or Mesozoic and contain only minor Precambrian components (Wang et al., 2014). The presence of Neoproterozoic and Paleoproterozoic detrital zircons with magmatic origins from and Paleozoic units of the SZRM indicating that this area occurs Proterozoic magmatism (Wang et al., 2014), whereas no Proterozoic magmastism has been found. Recently, Pei et al. (2007) reported the ca. 1800 Ma magmastism, as evidenced by the data of exploration drillholes in the southern Songliao basin. However, an alternative view is that the basement within the SZRM is predominantly Phanerozoic, as evidenced by the presence of Paleozoic fossils and comparatively rare geochronological data (Guo and Liu, 1985; Wu et al., 2011), meaning that the ca. 1800 Ma rocks in this area may be a tectonically emplaced slice of the North China Craton (Zhang et al., 2005). All of this means that the age and nature of the SZRM basement, and whether this area records Neoproterozoic magmatism, remain unclear.This study presents new geochronological, whole‐rock geochemical, and zircon Hf isotopic data for early Proterozoic granitoids within the eastern margin of the SZRM of NE China. These data provide insights into the Neoproterozoic tectonic setting of the SZRM and the links between this magmatism and the evolution of the Rodinia supercontinent.The zircon U–Pb dating indicates that the Neoproterozoic magmatism within the SZRM can be subdivided into two stages: (1) a ∼917–911 Ma suite of syenogranites and monzogranites, and (2) an ∼841 Ma suite of granodiorites. The 917–911 Ma granitoids contain high concentrations of SiO2 (67.89–71.18 wt.%), K2O (4.24–6.91 wt.%), and Al2O3 (14.89–16.14 wt.%), and low concentrations of TFe2O3 (1.63–3.70 wt.%) and MgO (0.53–0.88 wt.%). They are enriched in the light rare earth elements (LREE) and the light ion lithophile elements (LILE), are depleted in the heavy REE (HREE) and the heavy field strength elements (HFSE; e.g., Nb, Ta, and Ti), and have slightly positive Eu anomalies, indicating they are geochemically similar to high‐K adakitic rocks. They have zircon εHf (t) values and TDM2 ages from −4.4 to +1.5 and from 1915 Ma to 1592 Ma, respectively, suggesting they were derived from a primary magma generated by the partial melting of ancient thickened lower crustal material. In comparison, the 841 Ma granodiorites contain relatively low concentrations of Al2O3 (14.50–14.58 wt.%) and K2O (3.27–3.29 wt.%), relatively high concentrations of TFe2O3 (3.78–3.81 wt.%) and the HREE, have negative Eu anomalies, and have zircon εHf (t) values and TDM2 ages from −4.7 to +1.0 and from 1875 to 1559 Ma, respectively. These granodiorites formed from a primary magma generated by the partial melting of ancient crustal material. The ∼917–911 Ma magmatism within the SZRM is inferred to have formed in an orogenic setting, whereas the ∼841 Ma magmatism formed in an anorogenic setting related to either a post‐orogenic tectonic event or the onset of Neoproterozoic continental rifting. It is proposed that the microcontinental massifs within the SZRM formed during or following the final stage of assembly of Rodinia before rifting away from the Tarim Craton in response to Rodinia breakup.

Widespread Neoarchean (~ 2.7–2.6 Ga) magmatism of the Yangtze craton, South China, as revealed by modern river detrital zircons

The Kongling Terrane, which is the Archean nucleus of the Yangtze craton, preserves Paleoarchean-Proterozoic rocks as old as 3.45Ga. However, the dominant stage of formation of this Archean terrane remains unclear. In this paper, U-Pb and Lu-Hf isotopes of detrital zircons from two rivers and one stream in the northern part of the Kongling Terrane were studied by LA-ICP-MS and LA-MC-ICP-MS, respectively. These zircons show complicated internal structures in cathodoluminescence images, but the majority of them have linear or oscillatory zoning patterns, indicating magmatic origins. In general, the detrital zircons from these three local rivers show similar U-Pb age distributions. Together, they yield age peaks at 3.3–3.1Ga (5%), 3.0–2.8Ga (18%), 2.7–2.6Ga (30%), 2.6–2.2Ga (15%), 2.0–1.9Ga (27%), 1.9–1.7Ga (1%), 1.7–1.5Ga (2%), and 1.0–0.8Ga (1%). This age distribution implies that the North Kongling Terrane formed primarily during the Meso- to Neoarchean (3.0–2.6Ga). Lu-Hf isotopic data reveal predominant subchondritic to chondritic εHf(t) values (−11.3 to 0) for the Mesoarchean zircons, indicating the dominant role of crustal reworking during this period. In comparison, most Neoarchean zircons exhibit near chondritic to suprachondritic εHf(t) values (up to +7.4), suggesting juvenile crustal additions. The 2.0–1.9Ga zircons have similar initial Hf isotopic compositions as those of the 2.7–2.6Ga zircons, suggesting a metamorphic recrystallization origin. This observation further implies that the proportion of Neoarchean ages might be underestimated. Previous studies have reported several 2.7–2.6Ga Archean outcrops, minor detrital zircons in Precambrian sedimentary rocks and xenocrystal zircons in volcanic rocks from the Yangtze craton. Combined with our data, we propose that the 2.7–2.6Ga magmatism may have played an important role in forming the Kongling Terrane and have widely affected other parts of the Yangtze craton as well, similar to many other Archean cratons worldwide. The ~2.7–2.6Ga may correspond to the initial stabilization of the Archean nucleus of the Yangtze craton. Thereafter, this craton shows comparable 2.5–2.2Ga age records to the recently proposed Nunavutia supercraton, which could provide further clues to the early tectonic process of the Yangtze craton.

Abiologic silicon isotope fractionation between aqueous Si and Fe(III)–Si gel in simulated Archean seawater: Implications for Si isotope records in Precambrian sedimentary rocks

Precambrian Si-rich sedimentary rocks, including cherts and banded iron formations (BIFs), record a >7‰ spread in 30Si/28Si ratios (δ30Si values), yet interpretation of this large variability has been hindered by the paucity of data on Si isotope exchange kinetics and equilibrium fractionation factors in systems that are pertinent to Precambrian marine conditions. Using the three-isotope method and an enriched 29Si tracer, a series of experiments were conducted to constrain Si isotope exchange kinetics and fractionation factors between amorphous Fe(III)–Si gel, a likely precursor to Precambrian jaspers and BIFs, and aqueous Si in artificial Archean seawater under anoxic conditions. Experiments were conducted at room temperature, and in the presence and absence of aqueous Fe(II) (Fe(II)aq).Results of this study demonstrate that Si solubility is significantly lower for Fe–Si gel than that of amorphous Si, indicating that seawater Si concentrations in the Precambrian may have been lower than previous estimates. The experiments reached ∼70–90% Si isotope exchange after a period of 53–126days, and the highest extents of exchange were obtained where Fe(II)aq was present, suggesting that Fe(II)–Fe(III) electron-transfer and atom-exchange reactions catalyze Si isotope exchange through breakage of Fe–Si bonds. All experiments except one showed little change in the instantaneous solid–aqueous Si isotope fractionation factor with time, allowing extraction of equilibrium Si isotope fractionation factors through extrapolation to 100% isotope exchange. The equilibrium 30Si/28Si fractionation between Fe(III)–Si gel and aqueous Si (Δ30Sigel–aqueous) is −2.30±0.25‰ (2σ) in the absence of Fe(II)aq. In the case where Fe(II)aq was present, which resulted in addition of ∼10% Fe(II) in the final solid, creating a mixed Fe(II)–Fe(III) Si gel, the equilibrium fractionation between Fe(II)–Fe(III)–Si gel and aqueous Si (Δ30Sigel–aqueous) is −3.23±0.37‰ (2σ). Equilibrium Si isotope fractionation for Fe–Si gel systems is significantly larger in magnitude than estimates of a near-zero solid–aqueous fractionation factor between pure Si gel and aqueous Si, indicating a major influence of Fe atoms on Si–O bonds, and hence the isotopic properties, of Fe–Si gel. Larger Si isotope fractionation in the Fe(II)-bearing systems may be caused by incorporation of Fe(II) into the solid structure, which may further weaken Fe–Si bonds and thus change the Si isotope fractionation factor. The relatively large Si isotope fractionation for Fe–Si gel, relative to pure Si gel, provides a new explanation for the observed contrast in δ30Si values in the Precambrian BIFs and cherts, as well as an explanation for the relatively negative δ30Si values in BIFs, in contrast to previous proposals that the more negative δ30Si values in BIFs reflect hydrothermal sources of Si or sorption to Fe oxides/hydroxides.

Experimental fluorine liberation from Precambrian granites and Carboniferous-Permian sedimentary rocks associated with crystalline and sedimentary aquifers, Paraná Basin, southeastern Brazil

Experimental fluorine liberation from Precambrian granites and Carboniferous-Permian sedimentary rocks associated with crystalline and sedimentary aquifers, Paraná Basin, southeastern Brazil

Chapter 23 A brief synthesis of Indian Precambrian basins: classification and genesis of basin-fills

Abstract The geology, inferred evolution and classification according to widely accepted schemes of 22 basins from the Indian Precambrian record on the Arravali–Bundelkhand, Singhbhum, Bastar and Dharwar cratons are discussed in this volume. Although their classification is biased owing to all depositories having continental lithospheric substrates, most of the basins reflect divergent plate motion and thus lithospheric stretching and cooling. Convergent plate motion and concomitant lithospheric flexure owing to loading are postulated for the Kurnool Basin and the Eastern Dharwar Craton supracrustals. Transcurrent plate motion is interpreted for the Bhima and Kaladgi–Badami basins. The Cuddapah Basin suggests a complex polyhistory influenced by both mantle circulation/dynamic topography and loading-related flexure of the lithosphere. Mantle circulation and dynamic topography may have played a role in the evolution of the Dhalbhum and Dalma–Chandil basins. Examination of possible time trends indicates that pre- c. 2.0 Ga basins were mostly continental rifts, followed by some intracratonic basins and lesser rift-sag and back-arc basins; post- c. 2.0 Ga basins exhibit a much larger range of basin types. While this Memoir offers a broad sample of the application of plate-tectonic principles to the Precambrian basins of the large Indian shield, it also underscores that Phanerozoic-style plate tectonics and basin evolution histories are widely identified within the Precambrian sedimentary rock record. Analogously, the case studies in this book support the essential similarity between the features observed within the Precambrian basins of India and the norms that describe Phanerozoic successions in terms of sequence stratigraphic architecture. The Indian Precambrian basin-fill record shows that all types of sequence, systems tract and sequence stratigraphic surface that are known from the Phanerozoic record also occur within Precambrian successions. Differences between the stratigraphic architecture of Precambrian and Phanerozoic basin-fill successions can be ascribed to variable rates and intensities of the controls on accommodation and sediment supply, the changes inherent in the evolution of the hydrosphere–atmosphere system and related physical processes, and the evolution of the biosphere system and associated biogenic processes.

Raman spectroscopy as a screening tool for ancient life detection on Mars.

The search for sp(2)-bonded carbonaceous material is one of the major life detection strategies of the astrobiological exploration programmes of National Aeronautics and Space Administration and European Space Agency (ESA). The ESA ExoMars rover scheduled for launch in 2018 will include a Raman spectrometer with the goal of detecting sp(2)-bonded carbonaceous material as potential evidence of ancient life. However, sp(2)-bonded carbonaceous material will yield the same Raman spectra of well-developed G and D bands whether they are synthesized biologically or non-biologically. Therefore, the origin and source of sp(2)-bonded carbonaceous material cannot be elucidated by Raman spectroscopy alone. Here, we report the combined approach of Raman spectroscopy and gas chromatography-mass spectrometry biomarker analysis to Precambrian sedimentary rocks, which taken together, provides a promising new methodology for readily detecting and rapidly screening samples for immature organic material amenable to successful biomarker analysis.

Flower-like apatite recording microbial processes through deep geological time and its implication to the search for mineral records of life on Mars

The biological cycling of phosphorus on Earth could be as early as the origin of life in early Archean. However, because of the low abundance and fine particle size, phosphate related to microbial ecophysiological activities in early sedimentary rocks, especially those deposited before the Great Oxidation Event (GOE, ca. 2.45-2.32 Ma), is still poorly addressed. It is not until recently that certain petrographic and mineralogical features of apatite in the early Precambrian sedimentary rocks were found related to microbial activities. In this study, we report high-resolution electron microscopic investigations on apatite from the Neoarchean to early Paleoproterozoic banded iron formations (BIFs), Mesoproterozoic to Lower Cambrian black shale and phosphorites, and Pliocene sediments. Apatite in BIFs occurs as 4-8 mm radial flowers with “petals” made of apatite rods. Their mineralogical and petrologic features are highly similar to those in the younger sedimentary rocks in which biomass have been confirmed to play an important role in the mineralization of phosphate. We suggest that these sedimentary rocks or sediments have experienced similar phosphogenetic processes mediated by biomass that led to the mineralization of phosphorus. The formation and preservation of phosphate (apatite) with conspicuous recognizable features in association with biological activities from Late Archean to Pliocene implies its universal significance in recording microbial processes through deep geological evolution. With mild dynamic processes, the martian (sub)surface has better preservation conditions than Earth, and the micro-structure of phosphate formed in environments mediated by microorganisms could be recognized by high-resolution observations on the surface of Mars or returned samples, if microbial life ever developed on Mars.

Geochronology, elemental and Nd–Hf isotopic geochemistry of Devonian A-type granites in central Jiangxi, South China: Constraints on petrogenesis and post-collisional extension of the Wuyi–Yunkai orogeny

Abstract Details on processes of the early-middle Paleozoic Wuyi–Yunkai orogeny in South China remain poorly defined. Most Silurian–Devonian granites in South China are S-type or I-type granites, which are suggested to be petrogenetically related to the Wuyi–Yunkai orogeny. This paper firstly reported a systematic study on two Devonian A-type granites in the central Jiangxi Province. LA-ICP-MS zircon U–Pb dating results imply that the Huitong and Epo granites were emplaced at about 415 Ma. Both of the two granites have the petrographic and geochemical characteristics of A-type granites. Interstitial biotites occur along the boundary of euhedral plagioclase and quartz and they formed later than plagioclase and quartz. It implies that the primary magma could have been anhydrous. Biotites from the two granites are Fe-rich and have high Fe 2 + /(Fe 2 + + Mg 2 + ) ratios (0.60–0.74). The magmatic temperatures estimated from zircon saturation thermometer are 802–920 °C for the two granites, higher than common I-type and S-type granites. The two granites show high contents of total alkalis (Na 2 O + K 2 O = 6.96–9.39 wt.%), high field strength elements (e.g. Zr = 181–437 ppm, Y = 22.1–39.7 ppm, Nb = 18.6–30.3 ppm and Zr + Nb + Ce + Y = 324–555 ppm), rare earth elements (total REE = 155–312 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 2.50–3.44). These geochemical characteristics are similar to those of A-type granites. The Huitong and Epo granites have relatively low e Nd (t) values of − 10.4 to − 7.7, and low zircon e Hf (t) values (peak value of − 8.0). Whole-rock two stage Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.78 Ga to 2.00 Ga. According to these data, we suggest that the two granites might have derived from partial melting of pre-Cambrian sedimentary rocks which had been granulitized during an earlier thermal event. The two granites contain abundant contemporaneous mafic microgranular enclaves (MMEs). The MMEs display igneous textures and contain acicular apatites, suggesting quenching of mafic magmas that have co-mingled with the host granites. The MMEs might have derived from the enriched lithospheric mantle source. This study on the Devonian Huitong and Epo A-type granites, together with previous studies on Paleozoic granites in South China, indicated that the Wuyi–Yunkai orogeny in South China had changed from syn-collisional crustal thickening to post-collisional extension at least from 415 Ma.

Succession of Laramide Magmatic and Magmatic-Hydrothermal Events in the Patagonia Mountains, Santa Cruz County, Arizona

This investigation of the space-time progression of magmatism and hydrothermal activity in the Patagonia Mountains of southern Arizona is based on field and paragenetic relationships, and on U-Pb and 40 Ar/ 39 Ar geochronology of igneous and hydrothermal minerals. The Patagonia Mountains consist of Precambrian, Paleozoic, and Mesozoic sedimentary, granitic, and volcanic rocks, Laramide volcanic rocks, and a core of Laramide intrusions that comprise the Patagonia Mountains batholith. Laramide igneous rocks and adjacent Paleozoic and Mesozoic rocks contain significant porphyry Cu-Mo deposits, Mo-Cu breccia pipes, Ag replacement deposits, and numerous other Cu-Pb-Zn-Ag replacement and vein deposits. Ages of igneous and hydrothermal minerals from 20 U-Pb and 52 40 Ar/ 39 Ar determinations define four magmatic and magmatic-hydrothermal events that formed the batholith and altered parts of it and adjacent rocks; cumulatively the events span at least 16 m.y., from ~74 to 58 Ma. The oldest event of this succession includes the 74 Ma Washington Camp stock and spatially associated Cu-Pb-Zn-Ag replacement deposits in Paleozoic carbonate rocks of the Washington Camp-Duquesne district. Eruption of 73 to 68 Ma volcanic rocks in the northern part of the range was the next youngest event, which coincides temporally with replacement and vein deposits in Paleozoic carbonate rocks at the Flux mine (~71 Ma). An event at 65 to 62 Ma is marked by emplacement of small-volume quartz monzonite, granodiorite, and diorite intrusions, formation of the Ventura breccia deposit in Jurassic granite at 65 to 64 Ma, and formation of other Pb-Zn-Ag-Cu replacement and vein deposits (~62 Ma; Blue Nose and Morning Glory). The Red Mountain porphyry Cu-Mo system is hosted by ~62 Ma granodiorite and Laramide volcanic rocks (73–68 Ma) at the northern end of the batholith. It includes a deep, chalcopyrite-bornite resource (~60 Ma) that is associated with potassic and sericitic alteration and a near-surface chalcocite-enargite resource (60 Ma) that is associated with advanced, supergene-enriched argillic alteration. The youngest event includes the Sunnyside porphyry Cu-Mo system and a Cu-Mo breccia deposit at Red Hill (Four Metals mine), both of which formed in large-volume quartz monzonite, granodiorite, quartz monzonite porphyry, and quartz feldspar porphyry (~61–59 Ma). Similar to the Red Mountain system, the Sunnyside system consists of a deep chalcopyrite resource that occurs in ~60 to 59 Ma quartz feldspar porphyry, and a near-surface, slightly younger (~59–58 Ma) enargite-chalcocite-tennantite resource that occurs in quartz feldspar porphyry, quartz monzonite porphyry, and Mesozoic rocks. The Red Hill Cu-Mo breccia deposit is hosted by large-volume quartz monzonite, granodiorite, and quartz monzonite porphyry (~61–59 Ma). Discrepancies between field and paragenetic relationships and some analytic ages at Sunnyside and Red Hill preclude precise dating of mineralization stages, and may reflect disturbance of isotope systems by multiple, co-spatial to juxtaposed intrusive and hydrothermal events, and/or by unrecognized intrusions. Numerous vein and replacement deposits at the northern end of the batholith, including the Hardshell Ag resource and the Three R supergene chalcocite resource, are distal deposits of the Sunnyside and Red Mountain systems. Small, ~61 to 59 Ma Cu-Mo deposits in large-volume intrusions in the southern part of the batholith consist of hydrothermal quartz, biotite, K-feldspar, muscovite, chalcopyrite, and molybdenite. The age span of magmatic and magmatic-hydrothermal events in the Patagonia Mountains, minimally 16 m.y., is comparable to that of certain other magmatic-hydrothermal successions that contain porphyry Cu-Mo systems. Magmatic-hydrothermal events of the Wasatch-Oquirrh igneous trend, Utah, and the Boulder batholith, Montana, both span ~17 m.y. and include the Bingham and Butte porphyry Cu-Mo, vein and replacement deposits, respectively. Plutons and mineral deposits in the Pima district, Arizona, which includes the porphyry Cu-Mo deposits at Sierrita-Esperanza, Mission-Pima-San Xavier North, and Twin Buttes, formed over an interval of ~14 m.y. The diversity of igneous and hydrothermal products likely reflects evolutionary processes occurring at multiple sites in the lithosphere and at different time scales from >10 m.y. to less than the geochronologic precision currently achievable.