Felsic Igneous Rocks Research Articles

Lithogeochemistry of Upper Precambrian terrigenous rocks of Belarus. Communication 2. Provenance, paleogeodynamics, paleogeography, paleoclimate

This publication completes the consideration of the lithogeochemical features study of a pilot collection of the Riphean and the Vendian sandstones, siltstones and mudstones (last one identified based on the interpretation results) of the Belarus. Data on the age of detrital zircon published in recent years suggest that the source rocks for the Upper Precambrian deposits of this region were the Osnitsk-Mikashevichi and the Trans-Scandinavian igneous belts, the Volyn-Brest large igneous province, rapakivi granites, as well as various associations of rocks of Sarmatia, the Danopolonian orogen and Svecofennides. The distribution of clastic rocks data points of our collection on provenance, paleogeodynamic, paleogeographic and paleoclimatic discriminant diagrams, which based mainly on the lithogeochemical composition, allows us to draw a number of conclusions. We are considering that the Riphean and the Vendian strata of the Belarus are composed mainly of intraplate granitoids erosion products, as well as various felsic igneous rocks of island-arc and syncollisional genesis. The part of mafic rocks erosion products among them generally does not exceed 30%. It`s noticeable mainly in rocks of the Volyn series (products of the Volyn-Brest large igneous province erosion), as well as in some samples of the Nizov, Selyavy and Kotlin formations (fragments of mafic rocks from other sources?). Source to sink transporting was carried out mainly by large rivers. Paleogeodynamic settings varied from quite active to quite passive. The paleoclimate in the Riphean was most likely arid/semiarid, and in the Vendian it was humid, from subtropical in the early (except for the Glussk Formation) to tropical in the Late Vendian. The research results also make it possible to show some work features for known paleoclimate reconstruction methods and techniques.

Early Neoproterozoic (Tonian) subduction-related magmatism and tectonothermal activity in Shetland and northern mainland Scotland: implications for the tectonic evolution of NE Laurentia and Rodinia reconstructions

The tectonic setting of Tonian orogenic events recorded in the present-day circum-North Atlantic region is uncertain. U–Pb zircon geochronology shows that the Yell Sound and Westings groups (Shetland) and metasedimentary rocks of the Naver Nappe (northern mainland Scotland) were deposited between c. 1050 and 960 Ma and intruded by mafic, intermediate and felsic igneous rocks at c. 965–950 Ma. Chemical discrimination diagrams and Hf and Nd isotope data together suggest that the protoliths of the mafic meta-igneous rocks were emplaced as relatively juvenile crustal contributions in an active plate margin. Zircon growth at c. 920 Ma within the Yell Sound Group correlates with high-grade metamorphism documented previously in Shetland. Further zircon growth and Pb loss at c. 470–460 Ma indicates overprinting during the Ordovician Grampian orogenic event. Similar age successions of Ellesmere Island, Svalbard and East Greenland also contain evidence for Tonian magmatism (some calc-alkaline), deformation and metamorphism. The new data favour Rodinia reconstructions that incorporate subduction-related magmatism and associated tectonism along the margin of NE Laurentia during the Tonian. The Yell Sound Group and correlative peri-Laurentian successions were intruded by subduction-related magmas and deformed and metamorphosed during development of the Valhalla exterior accretionary orogen, part of a more extensive peri-Rodinian subduction system.

MINERALOGICAL, PETROGRAPHIC, AND LITHOCHEMICAL FEATURES OF THE UPPER JURASSIC–LOWER CRETACEOUS SECTION OF THE NORDVIK PENINSULA (NORTH OF EASTERN SIBERIA)

Some intervals of the Jurassic–Cretaceous strata of the Anabar-Lena sedimentary basin have a certain oil and gas production potential, which can be realized in the synchronous offshore horizons of the adjacent territories of the Arctic shelf. Among the most representative objects in this regard are the outcrops of Upper Jurassic and Lower Cretaceous formations of the Nordvik Peninsula. The main data on the composition and structure of this section were obtained mainly at the beginning of the second half of last century. The results of complex mineralogical, petrographic, and lithochemical studies of the Urdyuk-Khaya and Paksa formations of Cape Urdyuk-Khaya of the Nordvik Peninsula presented in here enabled us to identify 10 boundaries for changing of sedimentation regimes of the paleobasin. It was found that the Urdyuk-Khaya Formation was formed mainly in the conditions of the shelf transition zone (moderate deep water) with low rates of terrigenous material intake, some depletion of oxygen in bottom waters, and a trend towards an increase in the depths of the basin. The basal stratum of the Paksa Formation was formed in offshore conditions with periodically occurring dioxic conditions in bottom waters and extremely low rates of terrigenous material intake. The overlying part of the formation was formed in various parts of offshore transition conditions with a gradual decrease in the depths of the basin, an increase in the oxygen content in the bottom layer of water, and the rate of terrigenous material intake. The main provenance area was igneous rocks of mafic, possibly intermediate composition. There was some influence of felsic igneous rocks, or ancient sedimentary rocks rich in quartz. The parent strata were subjected to moderate and severe chemical weathering, in a warm humid climate. The revealed features of the studied strata are similar to the characteristics of the same-age sediments of the lower reaches of the Anabar River, which determines their high correlation potential and allows us to judge the evolution of the western part of the Anabar-Lena basin.

Geological position of the Junggar terrane (Southern Kazakhstan) in the structure of Rodinia supercontinent: results of research of the late-precambrian metasedimentary complexes

The results of studying of the Precambrian metasedimentary sequences of the Junggar terrane located in South Kazakhstan are given in the article. In the structure of the Junggar terrane, we studied the rocks of the Sarychebyn group and Kosagash formation. Petrogeochemical data combined with the results of U‒Pb and Lu‒Hf isotope-geochronological studies of the detrital zircons showed that the Sarychebyn Group and the Kosagash formation represent a similar stratigraphic level that accumulation occurred in the Late Mesoproterozoic to Early Neoproterozoic (~1026‒~920 Ma). The main sources of the detrital zircon age populations were the Mesoproterozoic and Paleoproterozoic complexes. Among these complexes can be identified metabasites and metapelites of intermediate and high metamorphic grades, as well as felsic igneous rocks formed with the participation of various sources, can be distinguished. The Junggar terrane exhibits a close tectonic affinity with the Aktau-Mointy, Yili, Issyk-Kul, Chinese Central Tien-Shan, and the Northern Kazakhstan terranes in the Late Precambrian. They were probably located near the Sveconorwegian orogen in the western Baltica within the Rodinia supercontinent structure.

Geochemical behaviour of biotite during interaction with aqueous and brine fluids: Constraints from hydrothermal batch experiments

Biotite plays an important role in the geochemical cycle of Li, Rb, Cs, and Ba in the upper continental crust, as it is a significant carrier of Li and large-ion lithophile elements in felsic igneous rocks and high-grade detrital metasedimentary rocks. During its interaction with meteoric and hydrothermal fluids, biotite can be transformed into various types of clay minerals (mostly, interlayer-deficient biotite, vermiculites and smectites). These transformations can cause fractionation of the alkaline trace-element ratios Rb/Li, Cs/Li and Rb/Cs between biotite and its replacement products. This study examines the mineral transformations that occur when biotite interacts with aqueous and saline fluids and the poorly understood geochemical behaviour of the resulting phyllosilicates. For this purpose, we performed batch hydrothermal experiments of the interaction of biotite + quartz ± graphite with ultrapure H2O, and 2 M NaCl, 2 M CaCl2 and 1 M NaF brine fluids at 170 °C and 10 bar using Teflon bombs, and at 550 °C and 800 to 1400 bar using autoclave apparatus. At lower-T conditions, biotite was replaced by 2:1 trioctahedral clay minerals (interlayer-deficient biotite, smectite, vermiculite, and other phyllosilicate species with higher interlayer charge) and Fe oxy-hydroxide minerals by coupled dissolution-precipitation mechanisms. At higher-T conditions, these mechanisms caused the transformation of biotite into the mineral assemblages (quartz ± graphite): diopside + anorthite + titanite (CaCl2 brine experiments), albite + ilmenite + clay minerals (NaCl brine experiments), and cryolite + alkali feldspar with albite rimmed by K-feldspar + Fe-oxides (NaF brine experiments). Therefore, a significant reduction of the clay mineral stability in the presence of NaF and CaCl2 brine fluids is inferred. The biotite replacements by phyllosilicates were mostly controlled by the ion exchange of K+ by H+ (or its hydrate state H3O+), hydrated Na+ and Ca2+, and NaF in the interlayer site. Conservation of the total mass and the Si, Al and Mg abundances occurred in most experimental phyllosilicates. However, in the products of the low-T NaF brine ± graphite experiments, the total mass may have a gain of 5.3–11 % assuming Mg conservation. Sc, V, Nb and Ta abundances were also conserved, but a significant fractionation of the Rb/Li, Cs/Li, and Ba/Li ratios occurred in the experimental phyllosilicates. The experiments predict the generation of highly fractionated Rb/Li and Cs/Li phyllosilicates by replacement of biotite during interaction with aqueous fluids and, mostly, NaCl and NaF brine fluids at high-T and low-T conditions, respectively. This demonstrates a key role of biotite in the fractionation of Rb/Li, Cs/Li and Rb/Cs during the hydrothermal alteration of felsic igneous rocks. Conversely, a reversal in the mobility of Li with respect to Rb and Cs occurred in the phyllosilicate products when biotite interacted with NaCl or CaCl2 brine fluids at relatively low-T conditions. These experimental results highlight the key role of biotite-fluid interaction processes in controlling the budget of alkaline trace elements in the continental crust.

Geochemistry and Mineralogy of Upper Paleozoic Coal in the Renjiazhuang Mining District, Northwest Ordos Basin, China: Evidence for Sediment Sources, Depositional Environment, and Elemental Occurrence

This study aims to investigate the depositional environment, sediment sources, and elemental occurrence of Upper Paleozoic coal in the Renjiazhuang Mining District, Western Ordos Basin. Furthermore, SEM-EDX, optical microscope (OM), ICP-AES, ICP-MS, and AAS were used. Compared with hard coal of the world, M3 coals were enriched in Ga, Li, Zr, Be, Ta, Hf, Nb, Pb, and Th, M5 coals were enriched in Li (CC = 10.21), Ta (CC = 6.96), Nb (CC = 6.95), Be, Sc, Ga, Hf, Th, Pb, Zr, In, and REY, while M9 coals were enriched in Li (CC = 14.79), Ta (CC = 5.41), Ga, W, Hf, Nb, Zr, Pb, and Th. In addition, minerals were mainly composed of kaolinite, dolomite, pyrite, feldspar, calcite, and quartz, locally visible minor amounts of monazite, zircon, clausthalite, chalcopyrite, iron dolomite, albite, fluorite, siderite, galena, barite, boehmite, and rutile. In addition, maceral compositions of M3 coals and M9 coals were dominated by vitrinite (up to 78.50%), while M5 coals were the main inertite (up to 76.26%), and minor amounts of liptinite. REY distribution patterns of all samples exhibited light REY enrichment and negative Eu anomalies. The geochemistry of samples (TiO2 and Al2O3, Nb/Y and Zr × 0.0001/TiO2 ratios, and REY enrichment types) indicates that the sediment sources of samples originated from felsic igneous rocks. Indicator parameters (TPI, GI, VI, GWI, V/I, Sr/Ba, Th/U, and CeN/CeN*) suggest that these coals were formed in different paleopeat swamp environments: M3 coal was formed in a lower delta plain and terrestrial (lacustrine) facies with weak oxidation and reduction, and M5 coal was formed in a terrestrial and dry forest swamp environment with weak oxidation–oxidation, while M9 coal was formed in a seawater environment of humid forest swamps and the transition from the lower delta plain to continental sedimentation with weak oxidation and reduction. Statistical methods were used to study the elemental occurrence. Moreover, Li, Ta, Hf, Nb, Zr, Pb, and Th elements were associated with aluminosilicates, and Ga occurred as silicate.

Detrital clay mineral input reconstructed based on weathering records and its influence on organic matter enrichment: A case study of the Paleogene Shahejie Formation in the Dongpu Sag, Bohai Bay Basin

As major components of shales, clay minerals are important for investigating unconventional petroleum geology. However, there are few quantitative methods for investigating possible inputs of clay minerals during sedimentary processes. In this study, shales from the Shahejie Formation in the Dongpu Sag, Bohai Bay Basin, were chosen for investigating clay mineral inputs. The shales are abundant in a variety of clay minerals. The shales, uniformly derived from felsic igneous rocks, underwent mild K metasomatism. Thus, the detrital clay mineral input was primarily influenced by weathering intensity. Accordingly, representative samples subjected to varying weathering intensities were selected to identify detrital and diagenetic clay minerals. Kaolinite and chlorite are of detrital origin. Illite and illite/smectite mixed layers (I/S) have both detrital and diagenetic origins. The precursor minerals of diagenetic minerals can be identified based on their morphological and elemental characteristics. After quantifying the area proportions of various clay minerals, the detrital clay mineral compositions of selected samples were calculated. Further analysis indicated that the detrital illite content and the detrital chlorite content are closely related, while the detrital smectite content correlates well with the chemical index of alteration (CIA). Based on the preceding analysis, the fitting formulas were established to quantitatively characterize the input of detrital clay minerals. The shales from the north sag had a high input of detrital smectite, whereas the shales from the south sag had a high input of detrital illite. Consequently, the shales from the north sag contain more organic matter than those from the south sag.

Extension settings favour initial REE enrichment in the parent granites of ion-adsorption REE deposits: implication from Late-Permian to Triassic granites in South China

The mechanism of rare earth elements (REE) enrichment in granites is important to understand generation of ion-adsorption REE deposits. We investigated Late Permian to Triassic granites from South China to investigate these issues. The ore-related granites with high REE contents (192-467 ppm) intruded in Late Permian have high K 2 O and Zr+Ce+Nb+Y contents, high Ga/Al and K/Na ratios, and low CaO contents, which belong to aluminous A-type granite derived from melting of felsic igneous rocks at a high-temperature extensional setting. The ore-unrelated Triassic Pingtian granite with relatively low REE contents (81.7-127 ppm) is peraluminous with occurrence of muscovite, indicating a S-type affinity. Its high Rb/Sr and low CaO/Na 2 O ratios indicate magma origin from a metapelitic source related to the compressional setting due to the continental collision. In the extensional settings in South China, the high temperature condition promoting melting of REE-rich accessory minerals and F-rich conditions improving the solubility of REE in the melts could be the reasons for the initial REE enrichment in these granites. Given that parent rocks for ion-adsorption REE deposits are all generated in extensional settings, multiple tectonic stages and long-term extensional setting in South China contributed to generation of the specific ion-adsorption REE deposits in South China.

Geochemistry and clay mineralogy of shales in the Sulu Orogenic Belt, East China: suggestion for continental weathering during an Early Cretaceous hyperthermal interval

ABSTRACT Understanding past weathering–climate feedback mechanisms under greenhouse conditions has significance for guiding the development of climate mitigation strategies. The continental paleoweathering state of Early Cretaceous shales in the Sulu Orogenic Belt of East China during a hyperthermal interval related to oceanic anoxic event 1a (OAE1a) was investigated via geochemical and clay mineralogical analyses of shale samples. The resulting diagrams of ternary 15Al2O3–Zr–300TiO2 discrimination, Th/Sc and Zr/Sc show that the geochemical composition was mainly controlled by source composition. This is supported by observations of low Zr contents, high indexes of compositional variability (ICV; > 1), and micropetrological characteristics. All samples showed narrow SiO2/Al2O3 and (Fe2O3 + MgO)/Al2O3 ranges that were within the predicted weathering trends of the ternary A-CN-K diagram, indicating that the shale geochemistry was not influenced by silicification and K-metasomatism. The chondrite-normalized rare-earth-element pattern suggests that all samples had the same felsic igneous parent rock. The geochemistry of whole rocks and their silicate fractions indicates that the shale had low chemical indexes of alteration (CIA) but high ICV values compared to post-Archean Australian Shale, indicating low sediment maturity and extremely weak chemical weathering in the source terrane and/or sedimentary basin under hot-dry conditions. Moreover, the clay minerals in the shale were mainly illite, suggesting low mineral weathering. Thus, during the OAE1a-related hyperthermal interval, chemical weathering was relatively weak in some mid-latitude arid inland areas. This may be mainly due to aridification decreasing water–rock reactions. Chemostratigraphic analysis suggests that the CIA, Ga/Rb, and Rb/Sr values of shales from the upper strata of the Yangjiazhuang and Shuinan formations were greater than those from lower strata. Moreover, the opposite trend in K2O/Al2O3 ratios was found. These indices suggest gradual increases in chemical weathering as the climate changed from hot-dry to warm-humid. In contrast, proxy indices of shale from the Zhifengzhuang Formation showed the opposite trend, suggesting a gradual decrease in chemical weathering as the climate changed from warm-humid to hot-dry. Considering the composition of the clay minerals, it is speculated that continental weathering was dependent on atmospheric humidity rather than temperature under the extreme greenhouse conditions, resulting in ineffective climate regulation by chemical weathering. This comprehensive study advances our understanding of the weathering–climate feedback mechanism under greenhouse regimes.

Sediment composition and U–Pb ages of detrital zircons in the Salina Cruz and Puerto Ángel beaches along the Gulf of Tehuantepec, Mexican Pacific

The Salina Cruz and Puerto Ángel beach areas in the Gulf of Tehuantepec, Mexican Pacific coast represent an important economic sector of the region. In this study, the mineralogy and geochemistry of bulk sediments, and geochronology of 400 detrital zircons recovered from the beach sediments were analysed to investigate their origin. The sediments are abundant in quartz, feldspar, ilmenite, cordierite, aragonite and anorthite. The chemical index of weathering revealed a moderate to intense weathering in the source area. The chondrite normalized REE patterns of bulk sediments are similar to the found in the Upper Continental Crust, suggesting the derivation of sediments from felsic igneous rocks. The REE patterns of zircons and the trace elemental ratios reveal a continental crust origin. Zircon U–Pb ages in the Salina Cruz beach were represented by Proterozoic (~545.1–1314.1 Ma; n = 170) and Cenozoic (~0.01–66 Ma; n = 20). The Puerto Ángel beach was abundant in Proterozoic zircon grains (~600.9–1171.4 Ma; n = 109) and followed by Mesozoic grains (~73.78–246.9 Ma; n = 40). The comparison of zircon U–Pb ages of this study with probable source rocks reveals that the Oaxaquia Terrane and Chiapas Massif Complex were the major contributors of Proterozoic zircons to the coastal areas. Similarly, the results indicate that the Cenozoic zircons were contributed by the Chiapas Massif Complex, coastal batholith and Cuicateco Terrane. The Mesozoic zircons are very few, derived from the nearby Xolapa Complex and the Chuacús Terrane.

Magmatic and sedimentological arguments for an Ediacaran active margin in the Bayankhongor Zone in western Mongolia, Central Asian Orogenic Belt

Geochronological and geochemical investigation of the magmatic and sedimentary rocks from the south-eastern tip of the Bayankhongor Zone (central Mongolia) constrains the Neoproterozoic evolution of the northern margin of the Baidrag Block. There, ultramafic to felsic igneous rocks of the Khan-Uul Massif intruded the volcano-sedimentary sequence of the Ulziit Gol Unit. U–Pb zircon ages show that both the plutonic and volcanic rocks were coeval products of the same Ediacaran (598–564 Ma) magmatism. Most of the samples have low contents of high field strength elements typical of arc-related magmas; however, some mafic rock samples display N-MORB-like chemistry. Magmatic rocks in the Khan-Uul Massif and Ulziit Gol Unit yielded exclusively positive initial εHf in zircon values (+4.9 to +13.8), implying derivation from depleted-mantle sources. Furthermore, heterogeneity of the whole-rock initial εNd values (–4.0 to +2.1) documents that fractional crystallization was accompanied by variable crustal contamination. The detrital zircon age patterns of the sandstone and tuffites in the Ulziit Gol Unit indicate that the sequence was filled dominantly by Ediacaran magmatic detritus derived from the Khan-Uul Massif and its volcanic equivalents, while the older cratonic material from the Baidrag basement represented only a subordinate component. Combined magmatic and sedimentary records imply that both the Khan-Uul Massif and the Ulziit Gol Unit may have formed in the same back-arc basin environment. This challenges the previous view that the entire Bayankhongor Zone was ophiolitic in nature. Distribution of coeval Ediacaran supra-subduction systems on the scale of the Mongolian Collage indicates the closure of multiple oceanic basins rimming the Siberian Continent. Following Rodinia break-up, this peri-Siberian realm was presumably formed by sub-parallel continental ribbons and oceanic basins. It is proposed that the amalgamation of these blocks and closure of intervening oceans reflected the Ediacaran advancing mode of the Palaeo-Pacific subduction.

Control of paleoclimate and paleoweathering on chromium contents in a non-ultramafic aquifer hosting high chromium groundwater.

Cr(VI) is a carcinogen with proven mutagenic and genotoxic effects. The effects of the depositional environment (e.g., paleoweathering, paleoclimate, and paleoredox condition) on Crenrichment in non-ultramafic aquifer solids are unclear. In this study, we presented the sedimentary characteristics of a borehole from a typical non-ultramafic aquifer with high Cr groundwater in Jingbian, central Ordos Basin, China. Chromium was enriched in the K1h sandstone aquifer, especially at depths of 400-500m, with the highest value of mass transport coefficient (τAl,Cr) up to 92.13% and τAl,Fe up to 33.5%. The provenance of aquifer Cr was predominantly intermediate and felsic igneous rocks with a mafic rock mixture. This mafic source was inferred from Cr-rich granodiorite and mafic/ultramafic rocks in the Yinshan (Daqingshan-Wulashan) Block, northern Ordos Basin. The Cr-rich aquifer in K1h was developed due to a moderate chemical index of alteration (CIA) (mean, 56.7) under relatively warm and humid paleoclimate, as evidenced by high CIA-temperature (CIA-Temp) (mean, 6.79°C) and paleoclimatic index values (mean, 0.40). Fe-Mn redox cycling in the oxic to suboxic environments contributed to aquifer Cr accumulation. Using path analysis, we identified that paleoclimate created favorable weathering conditions and enrichment of Fe contributed to the formation of high-Cr aquifers. The study reveals the formation of positive Cr anomalies in non-ultramafic aquifers, which is the potential source of groundwater Cr, and highlights the effects of depositional factors on Cr accumulation during aquifer deposition or early diagenesis. It can provide new insights into the natural processes of high-Cr sediments occurring in non-ultramafic aquifers.

Lateral redox variability in ca. 1.9 Ga marine environments indicated by organic carbon and nitrogen isotope compositions.

The stepwise oxygenation of Earth's surficial environment is thought to have shaped the evolutionary history of life. Microfossil records and molecular clocks suggest eukaryotes appeared during the Paleoproterozoic, perhaps shortly after the Great Oxidation Episode at ca. 2.43 Ga. The mildly oxygenated atmosphere and surface oceans likely contributed to the early evolution of eukaryotes. However, the principal trigger for the eukaryote appearance and a potential factor for their delayed expansion (i.e., intermediate ocean redox conditions until the Neoproterozoic) remain poorly understood, largely owing to a lack of constraints on marine and terrestrial nutrient cycling. Here, we analyzed redox-sensitive element contents and organic carbon and nitrogen isotope compositions of relatively low metamorphic-grade (greenschist facies) black shales preserved in the Flin Flon Belt of central Canada to examine open-marine redox conditions and biological activity around the ca. 1.9 Ga Flin Flon oceanic island arc. The black shale samples were collected from the Reed Lake area in the eastern part of the Flin Flon Belt, and the depositional site was likely distal from the Archean cratons. The black shales have low Al/Ti ratios and are slightly depleted in light rare-earth elements relative to the post-Archean average shale, which is consistent with a limited contribution from felsic igneous rocks in Archean upper continental crust. Redox conditions have likely varied between suboxic and euxinic at the depositional site of the studied section, as suggested by variable U/Al and Mo/Al ratios. Organic carbon and nitrogen isotope compositions of the black shales are approximately -23‰ and +13.7‰, respectively, and these values are systematically higher than those of broadly coeval continental margin deposits (approximately -30‰ for δ13Corg and +5‰ for δ15Nbulk). These elevated values are indicative of high productivity that led to enhanced denitrification (i.e., a high denitrification rate relative to nitrogen influx at the depositional site). Similar geochemical patterns have also been observed in the modern Peruvian oxygen minimum zone where dissolved nitrogen compounds are actively lost from the reservoir via denitrification and anammox, but the large nitrate reservoir of the deep ocean prevents exhaustion of the surface nitrate pool. Nitrogen must have been widely bioavailable in the ca. 1.9 Ga oceans, and its supply to upwelling zones must have supported habitable environments for eukaryotes, even in the middle of oceans around island arcs.

Provenance of sediments and environmental risk assessment of heavy metals in the “Mis Amores” beach, Veracruz, Gulf of Mexico, Mexico

In this study, grain-size, mineralogy, and geochemistry of Mis Amores (MA) beach sediments, Tuxpan, Veracruz State, Gulf of Mexico are analyzed. The textural parameters reveal that the sediments are fine-grained and vary from well-sorted to very well-sorted nature. The SEM-EDS analysis reveal that the sediments are abundant in minerals such as quartz, alkali feldspars, zircon, ilmenite, and pyroxene. Geochemically, the sediments are classified as sub-arkose type. The Chondrite normalized rare earth elements (REE) pattern suggest that the source area is dominated by felsic and intermediate igneous rocks (Eu/Eu* = 0.90 - 1.19, number of samples n = 16). The provenance discrimination diagrams indicated that the MA sediments were derived by the weathering of felsic igneous rocks, probably from the Trans Mexican Volcanic Belt. The results of this study reveal that the Tuxpan River played an important role in delivering sediments to the MA beach area. The environmental indices suggest that the sediments are moderately contaminated by Zn and moderate to extremely contaminated by Cu and As. The Cu (> 84%) and Zn (> 82%) concentrations are predominantly associated with the exchangeable fraction, which are readily bioavailable. Cu, As, and Zn in the MA sediments were derived from the agricultural activities and waste water discharges from the sanitary network of the Tuxpan town and port.

Paleo-Sedimentary Environment and Formation Mechanism of the Organic-Rich Shale of the Permian Lucaogou Formation, Jimsar Sag, Junggar Basin, China

The Jimsar Sag is an important shale oil exploration target area in the Junggar Basin, northwestern China. The Permian Lucaogou Formation, with a thickness of 200–300 m, is the primary exploration target. High-frequency variation in lithology is a typical feature of the Lucaogou Formation, reflecting the fluctuation of the depositional environment and organic matter enrichment. The evolution of the depositional environment and accumulation mechanism of organic matter still need to be elucidated for the Lucaogou Formation. High-resolution sampling of the entire Lucaogou Formation was applied to a 248 m long core from Well JX in the Jimsar Sag to examine the depositional environment and organic matter enrichment. The findings unveiled that the Lucaogou Formation was deposited under a hot and arid climate, within the confines of a closed saline paleo-lake, where sediments endured an extended period of anoxic conditions, displayed periodic oscillations in paleo-temperature and paleo-salinity values over time, alongside a continuous rise in paleo-water depth. The predominant source lithology of the Lucaogou Formation is felsic igneous rock. Small-scale transgression and hydrothermal sedimentation occurred during the deposition of the Lucaogou Formation. The prevailing hot climate and enduring reducing environment fostered ideal circumstances for the enrichment of organic matter in the Lucaogou Formation. Due to different sedimentary environments and enrichment mechanisms, organic matter is enriched in two modes in the Lucaogou Formation.

Petrographic and geochemical study of Precambrian and Paleozoic rocks in the western Anti-Atlas belt, Morocco: Facies, weathering, provenance and economic exploitation

Detrital and volcanic-detrital rocks from the Ifni Buttonhole and Lakhssas Plateau were analyzed to determine their provenance, compositional maturity, and alteration source. Geochemically, the sediments were classified as arkoses, lithic arenites, grauwackes, sandstones, lithic arenites, and Fe-rich sands, indicating low compositional and mineralogical maturity. A high average SiO2 concentration and low Al2O3 were consistent with a low abundance of shale and clay components. The geochemical signatures of the detrital and volcano-detrital (RDVD) rocks indicate that they have undergone a moderate to low degree of chemical alteration. The CIA study also suggests that the granitic, granodioritic rocks represent the source provenance which, during weathering and transport, supplied the detritus to the supra-crustal units. The major trace element data suggest that these rocks are largely derived from felsic igneous rocks, namely granitoids, with a minor contribution from intermediate sources. The carbonate rocks do not represent a wide variety of facies: dolomitic limestone, calcareous limestone, and calcaro-dolomitic chert. Calcitic and dolomitic samples show a linear increase in SiO2, regardless of their CaO/LOI ratio values, which remain relatively constant. The highest SiO2 contents are observed in the calc-dolomitic chert. Geochemical analysis of RDVD from the Ifni buttonhole determined their origin, maturity, and alteration. Major oxides decreased with higher silica content, indicating quartz control. Samples, formed under semi-arid conditions, show maturity under stable deposition. They suggest a felsic, recycled source, with moderate alteration and zircon enrichment during recycling. In the study area, limestones and dolomites serve as materials applicable in the building sector, suitable for all types of concrete. The Taliwine Formation harbors Lower Cambrian dolomites and limestones, ideal for mosaic aggregates. Described as variable in color, compact, homogeneous, very hard, and resistant to alteration, the plutonic rocks form prominent peaks. They exhibit both subalkaline characteristics in granitoids and an alkaline trend in dolerite dykes. Most samples display minimal alteration, indicating the reliability of their major element compositions for geochemical analyses. These granitoids constitute valuable deposits for ornamental and building rock purposes.

Geochemistry of Pelites at Chengmen Area: Implications for Sediments Resource and Tectonic Environment.

The pelite samples were collected from drillholes from the Chengmen area of the Fuzhou Basin of Fujian, China to reveal the sediments resource and tectonic environment of these pelites via an analysis of its lithology and geochemistry. The rare earth element distribution pattern of the pelites exhibited a high degree of fractionation between light and heavy rare earth elements, moderate negative Eu anomalies, and positive Ce anomalies, which are consistent with the rare earth element distribution pattern of Minjiang sediments and Fujian soils. Moreover, the variations in trace elements are also generally consistent, indicating a common provenance. The extremely high chemical index of alteration (CIA) and index of chemical variability (ICV) values of the pelites suggest that the source area experienced intense weathering, indicating a subtropical hot and humid climate environment in the source area and the Fuzhou Basin at that time. The source rock attribute discrimination diagrams show that the main source of pelites is felsic igneous rocks. The ratios of REDOX parameters show that during the sedimentary period, the water body in the study area was predominantly in an oxidizing environment. Furthermore, the tectonic background diagrams reveal that the source area underwent geological tectonic evolution processes of active and passive continental margins, marking the transition from the continental margin above the subduction of the ancient Pacific plate to the continental margin extension after subduction cessation.

Early Cretaceous fayalite-, ferrosilite-, and biotite-bearing rhyolitic porphyries in the Baishuizhai area, South China: Formation by fractional crystallization in the shallow crust

Fayalite- and ferrosilite-bearing felsic igneous rocks are a distinctive category of A-type granitic magmas, and the origins of these rocks have proved controversial. In addition, the shallow-crust high-silica rhyolitic magmas responsible for such rocks are considered to have higher viscosities than their plutonic equivalents, which casts doubt on whether fractional crystallization could have occurred during the evolution of these high-silica melts. Here, we report a study of earliest Early Cretaceous (145–142 Ma) fayalite- and ferrosilite-bearing rhyolitic porphyries (FFBRPs) and closely associated biotite-bearing rhyolitic porphyries (BBRPs) from the Baishuizhai area, Guangzhou, Guangdong Province, South China. The Baishuizhai suite provides an excellent opportunity to establish the magmatic origins and evolutionary processes responsible for the FFBRPs. Rocks from the Baishuizhai suite have SiO2 contents of 73.2–77.7 wt%, Na2O + K2O contents of 7.83–9.42 wt%, and Fe2O3T contents of 1.52–2.97 wt%. They have A-type granite features including mineral assemblages (euhedral fayalite and ferrosilite in addition to anhedral amphibole and biotite) and geochemical characteristics [e.g., high 10,000 × Ga/Al values (3.7–4.7), high FeOT/MgO ratios (13.0–34.8), and high Zr (245–514 ppm) and Nb (56.4–96.5 ppm) contents]. The rocks have slightly enriched in situ plagioclase Sr [(87Sr/86Sr)i = 0.7070–0.7076] and whole-rock Nd [εNd(t) = −2.3 to −2.0] compositions and depleted zircon Hf isotopic compositions [εHf(t) = −0.9 to +7.0] relative to chondrite, as well as higher zircon δ18O values (6.3‰–7.8‰) and intermediate in situ plagioclase Pb isotopic compositions [(206Pb/204Pb)i = 18.368–18.727, (207Pb/204Pb)i = 15.488–15.673, and (208Pb/204Pb)i = 38.049–38.801] compared with depleted mantle. We infer that the primary magmas of this rock suite were formed by mixing of mantle-derived magmas with subordinate (20%–50%) metasedimentary-rock-derived magmas. Fayalite and ferrosilite in the FFBRPs were most likely crystallized from dry, hot, and reduced magma in a lower (<16.5 km depth) chamber. The lower magma gradually evolved during ascent to a water-enriched magma in an intermediate chamber at a shallow depth (3–4 km), forming the FFBRPs (elevation: 129–168 m). Finally, the residual melts migrated upward and resided in an upper (<3 km depth) chamber, forming the BBRPs (elevation: 196–824 m). According to the established temporal–spatial distribution of Early Cretaceous A-type granites and adakitic rocks in South China, we suggest that the Paleo-Pacific plate underwent diachronous rollback, starting during the earliest Early Cretaceous. This rollback enhanced the degree of crust–mantle interaction in the Guangzhou area.

The El Gigante Metamorphic Complex: A missing block with a Grenville-age basement in the Western Sierras Pampeanas, Argentina

The southwestern paleo-margin of Gondwana is interpreted as an accretionary margin that was active from the late Neoproterozoic to the late Paleozoic. The basement of this paleo-margin is widely exposed in the central-western part of Argentina (Sierras Pampeanas area), where a protracted evolution from the Mesoproterozoic to the late Paleozoic is recorded. Part of this evolution is preserved in the El Gigante Metamorphic Complex (Sierra de El Gigante; Western Sierras Pampeanas), a small rotated block within the Valle Fértil Lineament fault zone that separates the Western and Eastern Sierras Pampeanas. The complex is composed of medium-grade meta-siliciclastic and meta-carbonate rocks and medium- to high-grade meta-igneous rocks, affected by tight to isoclinal folds and a pervasive east–west foliation resulting from the Famatinian orogeny (broadly late Cambrian to early Devonian). Later events include localized ductile shear zones. The isotopic and geochronological data from El Gigante Metamorphic Complex reveal at least three distinct lithological assemblages: (1) metamorphosed felsic igneous rocks of ca. 1.11 Ga, i.e., late Mesoproterozoic, (2) a Neoproterozoic metasedimentary succession composed of quartzites and mica-schists, (3) possibly mid- to late-Cambrian marble and graphite-schist. The Grenvillian assemblage (1) was part of a large reworked Paleoproterozoic continental block called MARA (acronym of Maz-Arequipa-Río Apa) and was the basement over which the Neoproterozoic and Cambrian sedimentary successions were deposited. Based on U-Pb zircon ages, lithological similarities and Sr-isotope data, the three lithological assemblages of the El Gigante Metamorphic Complex can be correlated with similar ones in the nearby geologically better-known Sierra de Pie de Palo. The Neoproterozoic and the Cambrian metasedimentary successions are respectively equivalent to the Difunta Correa Metasedimentary Sequence and the Nikizanga-Caucete Groups, which are recognized throughout the Sierras Pampeanas, east and west of the Valle Fértil Lineament.

Provenance and tectonic setting of greywacke and siltstone of the Nampala gold deposit, Southern Mali

The petrographical and geochemical analyses of greywacke and siltstone samples from the Nampala gold deposit of the Paleoproterozoic Birimian supergroup, Southern Mali, have been carried out to make inferences on the provenance, tectonic settings, and weathering history of the sediments. Petrographical investigations revealed that greywackes can be classified as feldspathic wackes, with immature to moderately mature sediments, with the siltstones being immature. Geochemically, greywackes show features that resemble typical wacke sedimentary rocks, whereas the siltstones are arkose to litharenite. Overall, Nampala sedimentary rocks experienced low intensity of paleoweathering with &lt;70 CIA and &lt;90 CIW values and have not undergone significant sediment recycling having &gt;1 CIV values. Their quartz and feldspar contents, together with the ratio of Al2O3/TiO2 and La/Sc, suggest the derivation of sediments from felsic igneous rocks, with a possible minor contribution from mafic proto-source rock. The petrographic and geochemical features of the greywackes and siltstones are consistent with siliciclastic rocks formed in an active continental margin, similar to other sedimentary rocks of the Paleoproterozoic Birimian terrane. These findings put positive constraints on the subduction-accretion geodynamic model proposed for the evolution of the Birimian terrane of the West African Craton.