Basic Volcanic Rocks Research Articles

Volcanism and Geochemistry of the Soil and Plant Cover in Kamchatka. Part 2. The Formation of the Elemental Composition of Volcanic Soils under Cold Humid Conditions

The Kamchatka volcanic soils have low concentrations of most chemical elements relative to their general abundances in continental soils and in European volcanic soils. The soils in different areas over the peninsula most typically involve higher bulk compositions of the elements that are typical of intermediate and basic volcanic rocks: Na, Ca, Mg, Cd, Mn, Co, and Cu, and persistently lower concentrations of the elements that are characteristic for acidic volcanic rocks: La, Ce, Pr, Nd, Nb, Hf, Tl, Rb, and Th. The diversity in the chemical composition of soils in the soil areas of Kamchatka as identified previously is controlled by the different conditions of volcanism that have existed in these areas in the past that also are observed at present.

Birkhin Volcanoplutonic Association, Ol’khon Region, Western Baikal Area: Petrological Criteria of Comagmatic Origin

This paper reports the results of thermodynamic modeling of the formation of the Birkhin volcanoplutonic association by means of geochemical thermometry. The obtained liquid line of descent for the magma chamber of the Birkhin massif indicates that the melt evolved into the field of subalkaline composition. The melts of the volcanics of the Tsagan-Zaba complex are identical to the melts in the magma chamber in both major components and REEs. It was concluded that the Birkhin and Tsagan-Zaba complexes are comagmatic. A scenario was proposed for the development of the Birkhin volcanoplutonic association involving multiple eruptions of volcanic rocks during its formation. It was shown that the intermediate rocks are not derivatives of basic magmas, but were produced by interaction of basic volcanic rocks with felsic intrusions cutting them.

Petrogenesis and metallogenic significance of multistage granites in Shimensi tungsten polymetallic deposit, Dahutang giant ore field, South China

The Shimensi tungsten polymetallic deposit, situated in the Dahutang ore field, South China, is one of the largest tungsten deposits in the world, with an estimated WO3 reserve of 0.74 million tons. Coarse-grained porphyritic biotite granite (CPBG), fine-grained porphyritic biotite granite (FPBG), fine-grained biotite granite (FBG) and biotite granite porphyry (BGP) are all ore-related, but their diagenetic relationships and contributions to W-Cu-Mo mineralization are still in dispute. LA-ICP-MS monazite U-Pb dating of the CPBG, FPBG, FBG and BGP yield emplacement ages of 147.9 ± 1.1 Ma, 146.4 ± 1.1 Ma, 138.6 ± 0.98 Ma and 142.8 ± 1.7 Ma, respectively. Whole-rock geochemical results indicate that the four granites should be classified as S-type granites, but BGP has distinct features transitional between S- and I-type granites. They were possibly generated by partial melting of upper crustal pelites and basic volcanic rocks with different proportion from the Neoproterozoic Shuangqiaoshan Group in the source. Proportional variation in the magmatic source (clay and basic basalts) induces the change of geochemical compositions of the Shimensi granites. Geochemical characteristics suggest that they were derived from two magma chambers (the CPBG, FPBG and FBG vs. the BGP) and experienced different evolutionary processes and different degree of magmatic differentiation during magmatic evolution. Chondrite-normalized REE patterns for the four granites display low total REE contents, variable and strongly enriched LREE relative to HREE and medium-strong negative Eu anomalies. They are enriched in Rb, Th, U, Ta and depleted in Ba, Nb, Sr, P, Ti. Biotites are iron-rich and aluminum-poor, and can be classified as ferro-biotite (CPBG, FPBG and FBG) and siderophyllite (BGP). The partial melting of tungsten-rich metasediments of the Shuangqiaoshan Group and high degree of fractional crystallization led to enrichment in tungsten in the magma suites. Oxygen fugacities of the CPBG and FPBG declined from early (most above the NNO buffers) to late stages of fractional crystallization (between the NNO and QFM buffers) because of the higher degree of magmatic differentiation in the late stages. In the early stages of fractionation, tungsten accumulated in the residual melts rather than partitioning into accessory minerals. In the late stages, lower oxygen fugacities and high fluorine contents promoted the removal of tungsten from the residual magma into reduced hydrothermal fluids. On the other hand, the FBG and BGP remained constant (above the NNO buffers) over the entire process of crystallization owning to the stable degree of magmatic differentiation, promoting retention of tungsten in the melt and resulting in low grade tungsten mineralization. Tungsten mineralization in the Shimensi deposit is greatly controlled by the redox states of the associated magma. The two porphyritic granites (the CPBG and FPBG) are most likely the main contributors of tungsten, while the FBG and BGP are mainly responsible for copper and molybdenum in the Shimensi deposit. Prolonged multiphase magmatism and prolonged W-Cu-Mo mineralization play important roles in the formation of Shimensi large tungsten polymetallic deposit.

Pantelleria Island as a centre of production for the Archaic Phoenician trade in basaltic millstones: New evidence recovered and sampled from a shipwreck off Gozo (Malta) and a terrestrial site at Cádiz (Spain)

New discoveries of flat Archaic querns recovered from the cargo of a Phoenician shipwreck at a depth of 110 m in Xlendi Bay (Gozo, Maltese Islands) dated to ca. 7th century BC and millstone fragments found at Cerro del Castillo (Cádiz, Spain) from Phoenician material between 7th and 6th century BC share the same petrographic texture and geochemical composition. The four samples consist of vesicle-rich grey basaltic lavas with a moderate porphyritic index and a microcrystalline intergranular groundmass. Major and trace elements distributions of the investigated artifacts highlight a provenance from Pantelleria Island, which is located in the Mediterranean Sea between Sicily and Cape Bon (Tunisia). This rules out other important widespread sources of basic volcanic rocks that were also used for millstones in ancient times, such as Etna volcano, Iblei Mountains and the Levant. Due to its barycentric position in the Sicily Channel, Pantelleria Island could therefore have represented one of the main supplying area of volcanic millstones for the Phoenicians. The provenance from Pantelleria was already documented for other volcanic millstones in Phoenician sites and Carthaginian settlements of Sicily (e.g. Motya and Entella) and Tunisia (e.g. Utica and Carthage) and those found in the shipwreck of El Sec (off the coast of Mallorca), dated to ca. 4th century BC. The new millstones from Pantelleria discovered in (i) an older shipwreck cargo (i.e. Xlendi Bay) and (ii) the most westerly Phoenician settlement of Gadir (nowadays Cadiz, ca. 1500 km far from Pantelleria) point to widespread Phoenician trade in Pantellerian basaltic rocks in the Archaic Period. Pantelleria Island can therefore be considered as a nodal point for trade in volcanic millstones, many centuries before the Hellenistic people and the Romans would largely exploit the basaltic lavas from Etna and Iblei Mountains for the same purpose.

Fluid Inclusion Study on the Moyoulete Sb-Cu Deposit, Xinjiang, China

The Moyoulete Sb-Cu deposit occurs in Devonian Kelan volcanic-sedimentary basin, which strikes NW-SE along the south margin of Altay Mountains, hosted in lower Devonian upper kangbutiebao formation. Sulfide quartz veins could be divided into two stages: early stage (Q1), charactered by white lenticular or veinlet quartz veins of copper mineralization occurring in metamorphic crystal tuff, metamorphic basic volcanic rock, or rhyolite porphyry; late stage (Q2), charactered by copper-bearing grey-white pyrite-quartz veins cutting the metamorphic rocks. The four types of fluid inclusions in the quartz veins can be identified: 1) CO2-rich fluid inclusions; 2) Water-rich fluid inclusions; 3) Carbonic fluid inclusions; 4) Aqueous fluid inclusions. CO2-rich fluid inclusions in Q1 have Tm, CO2 = -61.9∼ -57.9 °C and Th,CO2 = -10.0∼21.6 °C; the Th,totS’ are 358∼388 °C, while in Q2 have Tm,CO2 = -63.2∼ -60.2 °C and Th,CO2 = -20.4∼17.4°C; the Th,totS’ are 270∼325°C in the Moyoulete Sb-Cu deposit. The characteristics of CO2-rich fluid inclusions from the Moyoulete Sb-Cu deposit are similar to those from other Cu-Au mineralization localities. The genesis of vein Sb-Cu mineralization was connected with the orogeny-metamorphism hydrothermal fluid, resulting from the orogeny metamorphism of the Late Permian-Triassic in the south margin of the Altay Mountains.

Assessment of radon risk areas in the Eastern Canary Islands using soil radon gas concentration and gas permeability of soils

The Basic Safety Standard (BSS) Directive 2013/59/EURATOM of the European Union (EU) has stated the need for member states to establish national action plans to mitigate their general population's long-term risks of exposure to radon gas. Maps of radon-prone areas provide a useful tool for the development of such plans. This paper presents the maps of radon-prone areas in the Eastern Canary Islands (Gran Canaria, Fuerteventura and Lanzarote) obtained from assessment of Geogenic Radon Potential (GRP) distribution in the territory. GRP constitutes a magnitude that is contingent on both radon activity concentration and gas permeability of soils. An extensive campaign covering all geological formations of the Eastern Canary Islands was undertaken to locally sample these parameters. Geostatistical analysis of the spatial distribution of radon concentration in soils, permeability and GRP was performed on each of the islands, and the relationship between these magnitudes and the characteristic geological formations of the volcanic islands was investigated. Areas dominated by basic volcanic and plutonic rocks (originated by both recent and ancient volcanism) exhibit relatively low levels of radon in soils, and with the exception of specific cases of very high permeability, these areas are not classified as prone to radon risk according to international criteria. Areas in which intermediate or acidic volcanic and plutonic rocks predominate are characterised by greater radon activity concentration in soils, rendering them radon-prone. Given these results, Lanzarote is classified as an island with low radon risk all over its surface; Fuerteventura presents low-medium risk; and Gran Canaria contains extensive areas in the centre and north where the risk is medium or high. This classification is consistent with the risk maps obtained by National and European agencies from indoor radon measurements conducted on these islands.

Ediacarenian–middle Cambrian basic volcanism of Wrangel island: age and geodynamic setting (Russian East Arctic region)

New results from the study of zircon U–Pb ages by SIMS and the geochemistry of basic volcanism of Wrangel Island’s central part are considered. Volcanites are represented by rhyolites and spilitized basalts. For a long time, the age of rocks was considered as early Carboniferous, since they are discordantly overlapped by late Carboniferous limestones (Kosko et al., 2003). Recently, the age of rhyolites is estimated at ~590-610 million years, their compositions are compared with A-type granites (Luchitskaya et al., 2017). For basic volcanites, there is a large spread of U-Pb SIMS ages of zircon grains. The age of crystallization corresponds to the Ediacaran-middle Cambrian time (~510-608 million years). The volcanic rocks’ compositions have high concentrations of TiO2 up to 3.4 wt. %. Chondrite-normalized rare earth element pattern exhibit pronounced enrichment of light REE respect to medium (La/Sm)n= 1.4–2.4 and heavy (La/Yb)n= 2.2-6.1 REE. primitive-mantle normalized spidergrams show negative Nb-Ta anomaly, significant enrichment with LILEs and most HFSE. The geochemical characteristics of the basic volcanic rocks of the Wrangel island indicate the similarity with the traps of to traps of the Noril’sk area in northen Central Siberia and intraplate environment. The geological observations and the obtained ages suggest that the basic volcanic rocks are comagmatic with rhyolites having the geochemical features of A-type granites. It is assumed that this bimodal association was formed within the continental rift zone. It is assumed that the rocks were formed in a one single rift system with the coeval orthogneisses and gabbroids (Amato et. al., 2014) of the Alaskan Seward Peninsula

РУДНО-МАГМАТИЧЕСКАЯ СИСТЕМА SUKUMALAND (ТАНЗАНИЯ)

The overwhelming part of the gold ore potential of the United Republic of Tanzania is confined to the systems of the Neoarchaean greenstone belts (GSB) of the metallogenic province of Lake Victoria in the northwestern part of the country. Based on the interpretation of space imagery, in the western part of the province the Sukumaland ore-magmatic system of concentric-zonal structure has been distinguished. It is formed by a group of arched greenstone belts and associated gold deposits. They form two arcuate branches: the internal and external branch, differing in their composition and structure. The internal arc is represented by the Rwamagaza and Kahama GSB with gold deposits of Tulawaka, Buckreef, Mawe-Meru, Tembo, and Bulyanhulu. The external arc is represented by Geita and Mabale-Buhungurica GSB with gold deposits of Ridge-8, Nyankanga, Geita, Matandani, Kukuluma, Nyanzaga, Kitongo, Golden Ridge, Nyakafuru, and Miabu. Their huge gold potential (over 50 million ounces of gold) stipulates for the urgency of the study of this gigantic structure. GSB of the inner arc (Rwamagaza and Kahama) are characterized by a predominance of basic volcanic rocks, and GSB of the external arc (Geita and Mabale-Buhungurica) – by sedimentary and volcanic-sedimentary formations, in particular, banded ironstone formations (BIF). Accordingly, gold deposits of the inner arc are mainly associated with basic volcanic rocks (Tulawaka, Buckreef, Mawe-Meru, Bulyanhulu, Tembo) while gold deposits of the external arc – with BIF (Geita, Nyankanga, Kukuluma, Matandani). The nuclear part of the ore-magmatic system is of special interest, although manifestations of volcanism are absent there and gold or other mineral deposits are unknown yet, but a tectonic pattern of its structural elements indicates its significant ore potential. In particular, the presence of numerous spatially close systems of radial-circular faults attests to the presence of weakened zones in the space above the core of a hypothetical magma chamber. Gold deposits of different geological and industrial types, yet undiscovered, as well as manifestations of diatreme magmatism may be associated with these weakened zones. In particular, one cannot exclude the possible presence of diamond-bearing volcanic pipes. It is assumed that the ore-magmatic system has originated and evolved in the course of a complex multi-stage ore-magmatic process under the influence of ascending flows of depth fluids due to the long-term functioning of a zone with anomalously heated mantle in that area. During further geological exploration it is recommended to take into account the predictable availability of a single integrated ore-magmatic system of longdevelopment and its structural features.

ORIGIN AND DISTRIBUTION OF CLAY MINERALS IN THE MUDSTONES OF THE KOLOSH FORMATION INRAWANDOZ AREA, NORTHEASTERN IRAQ

The mudstone/ shale units constituted 60% of the Kolosh Formation of 170 m thickness, which were studied and the clay minerals were identified to conclude the origin and paleoclimate. The studied area is located near the suture of Zagros Thrust Ophiolite Zone in NE Iraq. Kolosh mudstones contain chlorite, vermiculite, illite, nontronite, serpentine with varieties of mixed-layer clays. These clay minerals are of detrital and diagenetic origin, which are controlled mainly by the source rocks, paleoclimatic conditions and the burial diagenesis. Some of chlorite, chlorite/ vermiculite mixed layer, nontronite and smectite refer to hydrothermal origin comes from submarine basaltic flows and the high-temperature pore waters.The distribution of the clay minerals in the stratigraphic section of the Kolosh Formation controlled by episodic thrusting, episodic volcanic eruptions and interbedded sedimentary rock units. These are controlled by the changes in the paleoclimatic conditions, chemical weathering and the diagenetic processes.The clay mineral assemblages refer to be derived mainly from Fe-Mg rich minerals with minor Si-Al rich silicate minerals, which are very common in the ophiolites and associated basic volcanic rocks. These rocks composed the major lithological units in the Zagros Thrust Belt of NE Iraq.

A global erodibility index to represent sediment production potential of different rock types

Erosion drives landscape evolution and is fundamental for understanding the interplay between climate and tectonics as well as land-ocean matter fluxes. Large scale erosion models are an important part of the suite of Earth system models. However, their representation of the sediment production potential of different rock types is very limited. A new global erodibility index is proposed herein. It depicts the relative erosion potential of different lithological classes in the Global Lithological Map (GLiM). The erodibility index was developed by correlating observational hillslope data against climatic and tectonic setting. Hillslope data were compared for different lithological classes in regions of similar uplift across a range of landscapes. An erodibility index was defined for each lithological class, based on the relative hillslope compared to that of acid plutonic rocks in the respective area, which are here used as base lithology. The erodibility indices of different lithological classes fall within three groups: low erodibility - 1.0 for acid plutonic rocks, metamorphic rocks, and carbonate sedimentary rocks, 1.1 for acid volcanic rocks, 1.2 for mixed sedimentary rocks; medium erodibility - 1.5 for basic plutonic rocks and siliciclastic rocks of all grain sizes, 1.4 for basic volcanic rocks; high erodibility – 3.2 for unconsolidated sediments. This erodibility index is the first estimate of sediment production potential of different rock types aimed at regional to global scale application. The erodibility index presented here can help improve our understanding of global erosion and landscape evolution. A new lithology erodibility layer, the global erodibility index dataset (GEroID), is produced and used as input in a global sediment flux model (WBMsed) to demonstrate a potential utilization of the new dataset. Using an updated lithological map paired with the new erodibility index yields improved regional model results compared to the original model input.

Diversity and origin of quartz cements in continental carbonates: Example from the Lower Cretaceous rift deposits of the South Atlantic margin

Silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis. The Lower Cretaceous rift deposits of the South Atlantic equatorial margin, which are intensively explored by petroleum companies, provide good examples of such silicifications in carbonates, exhibiting a wide diversity of petrographic habit of early to late quartz cements. In order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity, we integrated detailed petrography of diagenetic sequences and quartz habit with δ18Oquartz measurements (by SIMS) of individual cements observed in samples from the offshore and onshore basins of the West African margin. The petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length-fast or length-slow habit, in addition to laminated microquartz and micro- or mega-quartz forms. Amongst the isotopic analysis, the δ18Oquartz data show that length-slow cements are generally strongly enriched in 18O (δ18Oquartz ranging from 31 to 37‰ SMOW), whereas length-fast forms show less elevated values (<32‰ SMOW). The highest δ18Oquartz values for fibrous microquartz are interpreted to reflect precipitation from evaporated meteoric fluids at temperatures >25 °C and <100 °C. The alkalinity required to favor the precipitation of length-slow fibrous microquartz cements is probably related to fluid/rock interactions with underlying mantel-related or basic volcanic rocks. Such interactions would be in agreement with the recent geodynamic models of the South Atlantic passive margin. The length-fast fibrous microquartz associated with δ18Oquartz values ranging from 27 to 32‰ SMOW, probably reflect precipitation from moderately to non evaporated, fairly neutral to acid, fluids. The partial dissolution of carbonate cements prior to quartz cementations represents the signature of those acidic conditions. We therefore suggest that acidic pH was obtained through fluid/rock interactions with the intermediate to acid volcanic rocks encountered along the palaeohydrological pathway. Other quartz phases, such as the megaquartz cement, exhibit highly variable δ18Oquartz values ranging from 20 to 40‰. This variation may reflect significant variation in temperature conditions (between 100 and 200 °C) or changes in fluid δ18O at the small scale. For these populations of non-fibrous quartz cement, the very high δ18Oquartz values may reflect a contribution of fluids that have either suffered strong evaporation or strong water/rock interaction.

Zeolites filling cavities and amigdales of basic volcanic rocks in NW of Davazdah-Emam mountain, north Central Iran

Zeolites filling cavities and amigdales of basic volcanic rocks in NW of Davazdah-Emam mountain, north Central Iran

Petrogenesis of Mesozoic volcanic rocks in the Zha'ertai area, Inner Mongolia: Constraints from trace elements and Sr–Nd–Pb isotopes

The Mesozoic acid volcanic rocks of the Zha'ertai area, located in Inner Mongolia, were formed in the late Early Cretaceous and resulted in polymetallic deposits of gold, silver, and other precious metals with huge commercial value. The more developed acid volcanic rocks include subvolcanic and pyroclastic rocks and volcanic lava, with SiO2 of 73.18–83.82%, Na2O + K2O of 6.17–9.03%, and K2O/Na2O of 1.28–70.27%. These rocks are enriched in large‐ion lithophile elements and high‐field strength elements. They are also enriched in light rare earth elements (REEs), with light to heavy REE ratios of 2.91–14.30; light to heavy REE fractionation is significant, and there are obvious negative Eu anomalies (δEu = 0.02–0.25). The acid volcanic rocks have extremely low TiO2, MgO, P2O5, and TFe2O3 contents, exhibit significant enrichment in Th, U, and Pb, and are strongly depleted in Ba and Sr, showing the trace elements feature of A‐type rhyolite. The rocks have smaller (87Sr/86Sr)i, lower εNd (t), younger TDM2, lower (206Pb/204Pb)i,(207Pb/204Pb)i, and (208Pb/204Pb)i. The acid volcanic rocks differ considerably from the intermediate to basic volcanic rocks in geochemical composition, and there is a compositional hiatus present in the succession. This indicates that neither endmember of these Mesozoic bimodal volcanic rocks is cogenetic and that rhyolitic rocks might not be a product of basaltic magma crystallization differentiation. Instead, they may result from partial melting of crust‐derived rock, stemming from partial melting of rocks in the pre‐existing crust underplated by mantle‐derived magma. This suggests that the acid volcanic rocks were formed in a lithospheric extensional setting during the Early Cretaceous.

Geochronology and geochemistry of late Mesozoic volcanic rocks in the Wenkutu area, Great Xing'an Range, China

This paper reports on the petrography, geochemistry, and geochronology of late Mesozoic volcanic rocks in the Wenkutu area of the northern Great Xing'an Range, North‐east China, and discusses the formation age, petrogenesis, and tectonic environment of the Manketouebo, Manitu, Baiyingaolao, and Meiletu formations. Results of Zircon U–Pb dating show that the volcanic rocks in the Wenkutu area were formed during the Early Cretaceous (Manketouebo Formation at 145 ± 1 Ma, Manitu Formation at 141.3 ± 1.7 Ma, Baiyingaolao Formation at 128.7 ± 1.1 Ma, and Meiletu Formation at 129 ± 1 Ma). The Manketouebo, Manitu, and Baiyingaolao formations are dominated by intermediate to acid volcanic rocks that belong to the high‐K calc‐alkaline series, with Eu/Eu* values of 0.38–0.84.They are enriched in light rare earth elements (LREE) and depleted in heavy rare earth elements (HREE). The Meiletu Formation is dominated by intermediate to basic volcanic rocks, belongs to the high‐K calc‐alkaline series, has Eu/Eu* values of 0.89–0.97, and is enriched in Ba, K, Th, U, La, Ce, Sr, Nd, and Hf but relatively depleted in Nb, Ta, Zr, P, and Ti, with no obvious fractionation of LREE and HREE. Results indicate that the magma of the Manketouebo, Manitu, and Baiyingaolao formations had a crustal origin, whereas the Meiletu Formation was sourced from the mantle and subsequently contaminated by crustal materials. The late Mesozoic volcanic rocks in the study area were formed during an orogenic stage, and the compressional environment was caused by subduction of the paleo‐Pacific Plate below the Asian continental plate.

Field occurrences and Nd isotopic characteristics of the meta-mafic-ultramafic rocks from the Caozhuang Complex, eastern Hebei: Implications for early Archean crustal evolution of the North China Craton

Crust-mantle differentiation and growth of early crust are critical issues in the field of solid earth science and the proper understanding of how Earth evolved during Eoarchean-Paleoarchean greatly relies on these issues. The Caozhuang Complex, one of the oldest rock units in the North China Craton (NCC), is located in eastern Hebei and composed of the Caozhuang supracrustal rocks and the Huangbaiyu orthogneisses. The Caozhuang supracrustal rocks are considered to be the oldest sedimentary sequences in the NCC, into which the Huangbaiyu grey gneisses with ∼3.3 Ga zircon ages intruded. In previous studies, the Caozhuang supracrustal rocks are represented by fuchsite-bearing quartzites with ∼3.8 Ga detrital zircons and metamorphosed mafic rocks (amphibolites) with ∼3.5 Ga single-stage Nd model age relative to depleted-mantle (TDM(Nd)).Here we present our new study on meta-mafic–ultramafic rocks of the Caozhuang supracrustal sequence in accordance with detailed large-scale field mapping, petrology, elemental and Sm-Nd isotopic analyses, and then discuss the possible crust-mantle differentiation events that could take place during Eoarchean and Paleoarchean in the Caozhuang area.The meta-mafic-ultramafic rocks can be subdivided into two groups based on field occurrence. Group One rocks are amphibolites and hornblendites that occur as layers or slabs only in sedimentary sequences, which are typically accompanied by banded iron formations (BIFs). By contrast, Group Two never coexists with BIFs, and they mainly occur as amphibolites and hornblendites lenses or deformed dykes and sills in supracrustal sequences and orthogneisses, with blastophitic textures preserved. Sm-Nd isotopic analyses show that rocks from Group One have a Sm-Nd isochron age of 3580 ± 98 Ma and εNd(t) of +0.5 to +1.3, corresponding to TDM(Nd) of ∼3.7–3.5 Ga; while Group Two rocks yield a Sm-Nd isochron age of 3179 ± 90 Ma and εNd(t) between +4.6 and +6.6, corresponding to TDM(Nd) of ∼3.3–3.1 Ga. Geochemical characteristics of the two groups, such as moderate Mg#, negative correlation between Fe2O3T and MgO, and positive correlation between Ni and MgO, suggest they both experienced fractional crystallization of ferromagnesian minerals (dominated by olivine and clinopyroxene) during emplacement of magma(s). And combined with the flat HREEs patterns, and N-MORB-like trace element spider diagrams and positive εNd(t) values, it could be indicated that they could be partial melting products of ancient depleted mantle in the spinel stability fields. Based on the consistent whole rock Sm-Nd isochron ages with TDM(Nd), we suggest that two episodes of crustal growth occurred during ∼3.7–3.5 Ga in late Eoarchean and ∼3.3–3.1 Ga in late Paleoarchean in the Caozhuang area. The depleted characteristics of Group One indicate lithospheric mantle in the Caozhuang area might have been depleted before ∼3.7–3.5 Ga. And the Sm-Nd isochron ages of Group One basic volcanic rocks constrained that Caozhuang supracrustal sequence may have deposited during ∼ 3.7–3.5 Ga. Basic dykes or sills with TDM(Nd) of ∼3.3–3.1 Ga and time-equivalent TTG gneisses represent a significant thermal event in late Paleoarchean. The meta-mafic-ultramafic rocks in the Caozhuang Complex are an important window to reveal and understand the early crustal evolution of the NCC, which probably has global significance.

Joint Inversion of the 3D P Wave Velocity Structure of the Crust and Upper Mantle under the Southeastern Margin of the Tibetan Plateau Using Regional Earthquake and Teleseismic Data

AbstractThe special seismic tectonic environment and frequent seismicity in the southeastern margin of the Qinghai–Tibet Plateau show that this area is an ideal location to study the present tectonic movement and background of strong earthquakes in mainland China and to predict future strong earthquake risk zones. Studies of the structural environment and physical characteristics of the deep structure in this area are helpful to explore deep dynamic effects and deformation field characteristics, to strengthen our understanding of the roles of anisotropy and tectonic deformation and to study the deep tectonic background of the seismic origin of the block's interior. In this paper, the three‐dimensional (3D) P‐wave velocity structure of the crust and upper mantle under the southeastern margin of the Qinghai–Tibet Plateau is obtained via observational data from 224 permanent seismic stations in the regional digital seismic network of Yunnan and Sichuan Provinces and from 356 mobile China seismic arrays in the southern section of the north–south seismic belt using a joint inversion method of the regional earthquake and teleseismic data. The results indicate that the spatial distribution of the P‐wave velocity anomalies in the shallow upper crust is closely related to the surface geological structure, terrain and lithology. Baoxing and Kangding, with their basic volcanic rocks and volcanic clastic rocks, present obvious high‐velocity anomalies. The Chengdu Basin shows low‐velocity anomalies associated with the Quaternary sediments. The Xichang Mesozoic Basin and the Butuo Basin are characterised by low‐velocity anomalies related to very thick sedimentary layers. The upper and middle crust beneath the Chuan–Dian and Songpan–Ganzi Blocks has apparent lateral heterogeneities, including low‐velocity zones of different sizes. There is a large range of low‐velocity layers in the Songpan–Ganzi Block and the sub–block northwest of Sichuan Province, showing that the middle and lower crust is relatively weak. The Sichuan Basin, which is located in the western margin of the Yangtze platform, shows high‐velocity characteristics. The results also reveal that there are continuous low‐velocity layer distributions in the middle and lower crust of the Daliangshan Block and that the distribution direction of the low‐velocity anomaly is nearly SN, which is consistent with the trend of the Daliangshan fault. The existence of the low‐velocity layer in the crust also provides a deep source for the deep dynamic deformation and seismic activity of the Daliangshan Block and its boundary faults. The results of the 3D P‐wave velocity structure show that an anomalous distribution of high‐density, strong‐magnetic and high‐wave velocity exists inside the crust in the Panxi region. This is likely related to late Paleozoic mantle plume activity that led to a large number of mafic and ultra‐mafic intrusions into the crust. In the crustal doming process, the massive intrusion of mantle‐derived material enhanced the mechanical strength of the crustal medium. The P‐wave velocity structure also revealed that the upper mantle contains a low‐velocity layer at a depth of 80–120 km in the Panxi region. The existence of deep faults in the Panxi region, which provide conditions for transporting mantle thermal material into the crust, is the deep tectonic background for the area's strong earthquake activity.

Geochemical characteristics of Oligocene-Miocene sediments from the deepwater area of the northern South China Sea and their provenance implications

Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene. Zircon ages of the pre-rift Eocene sequences are dominated by Yanshanian ages with various peak values (110–115 Ma for U1435 and L21; 150 Ma for H1), indicating local sediment supply from the pre-existing Mesozoic magmatic belt. For the Oligocene sediments in the northern part of the basin, the rare earth elements show different distribution characteristics, indicating sediment supply from the paleo-Zhujiang River (Pearl River), as also confirmed by the multimodal zircon age spectra of the Lower Oligocene strata in Well X28. By contrast, a positive Eu anomaly characterizes sediments from the western and southern parts of the basin, indicating potential provenances from intermediate to basic volcanic rock materials. The Baiyun Movement at the end of the Oligocene contributed to a large-scale subsidence in the deepwater area and also a northward retreat of continental shelf break, leading to deepening depositional environment in the basin. As a result, all the detrital zircon ages of the Upper Oligocene strata from Wells X28, L13, and L21 share a similar distribution, implying the possible control of a common source like the Zhujiang River. During the Miocene, whereas sediments in the northern area were mainly sourced from the Zhujiang River Delta, and those in the southern deepwater area continued to be affected by basic volcanic activities, the Dongsha Uplift could have contributed as the main source to the eastern area.

Geochemistry, mineralogy and depositional environment of black shales of the Duwi Formation, Qusseir area, Red Sea coast, Egypt

This study focuses on the geochemistry of redox-sensitive elements of marine black shales at the Qusseir area of the Duwi Formation to infer their depositional environment. The black shales from the uppermost member of the Duwi Formation in the Qusseir area is conformably overlain by the Qusseir variegated shale and underlain by the Dakhla shale. Detailed mineralogical and geochemical characteristics were examined using several techniques, including X-ray diffractometry, scanning electron microscopy and X-ray fluorescence. Mineralogically, black shales in the Duwi Formation are composed mainly of montmorillonite, kaolinite, calcite, gypsum, quartz and pyrite. They are detrital and authigenic in origin. They are most probably derived from basic volcanic rocks by intensive chemical weathering under the prevalence of semiarid conditions. The redox-sensitive trace metals suggest anoxic sulfidic conditions that favoured the accumulation of these metals in the Qusseir area of the Duwi Formation.

Effect of thermal damage on mineralogical and strength properties of basic volcanic rocks exposed to high temperatures

In this study, the engineering behaviors of rocks exposed to temperatures ranging from 200 to 1000 °C were investigated and changes in the geomechanical properties, as well as related mineralogical changes, were examined. To reduce the effect of rapid heating and prevent the formation of new thermo-mechanical cracks, a heating rate of 10 °C/min was used. In addition, a heating time of 2 h was applied for each temperature degree to observe the thermal heating effect. After heating, the samples were left to cool to room temperature, both dry or immersed in water. The strength of the samples gradually decreased from 128 to 25 MPa with increasing temperature. The strength value decreases by about 70% for the samples cooled in air and by about 80% when cooled in water. The strength decrease was greater for the water-cooled samples because of new microcracks and glass material formed during sudden cooling. The effect of different temperature degrees on rock samples was evaluated separately and approximately 80% of total strength decrease occurred in the 600–700 °C range. Also, new mineral formation and melting of the existent minerals were not observed, although the structural properties were significantly changed at these temperatures. At 800 °C and above, some secondary minerals (e.g., melilite and augite) were formed. Partial melting was seen at temperatures higher than 1000 °C. In the study, a predictive model as a function of the strength for determination of thermal damage on rocks was also suggested. Some equations characterized by a correlation coefficient of 96% were suggested, based on the curves of thermal damage, to predict the uniaxial compressive strength for the designed temperature.

Petrogenesis of the Fe–P–REE mineralized Shangzhuang ultramafic intrusion in the Lajishan tectonic belt, South Qilian Belt: Implications for mantle metasomatism and tectonic setting

The Shangzhuang ultramafic intrusion in the Lajishan tectonic belt, South Qilian Belt, is mainly composed of serpentinite, biotite clinopyroxenite, apatite clinopyroxenite, and hornblendite. Zircon grains selected from biotite clinopyroxenite have distinctly oscillatory and sector zoning and yield a weighted average 206Pb/238U age of 492.6 ± 4.1 Ma, which can be considered as the crystallization age of the Shangzhuang intrusion. Initial 206Pb/204Pb and 87Sr/86Sr ratios of the intrusion suggest an enriched mantle I source origin. Rocks from the intrusion have Rb/Cs from 16 to 29 and Nb/U from 0.2 to 11.7, different from mid‐ocean ridge basalt and ocean island basalt from the convecting mantle, suggesting a subcontinental lithospheric mantle source. In comparison with Sr–Nd isotopes of the regional 456‐ to 441‐Ma mafic intrusions and basic volcanic rocks, a gradually decreasing trend of εNd(t) values from +0.90 via −0.86 to −2.57 from 493 to 441 Ma argued that the subcontinental lithospheric mantle source of the Shangzhuang intrusion was progressively enriched by mantle metasomatism in the Early Palaeozoic. The elevated (Tb/Yb)N ratios of the Shangzhuang intrusion and positive correlations between light rare earth elements (La and Nd) and P2O5 of metamorphosed basalts from the regional Hualong Group imply that the source region was an apatite‐bearing garnet‐facies peridotite as a result of modal metasomatism. In general, the Shangzhuang intrusion was formed due to the northward subduction of the Proto‐Tethys Ocean underneath the Qilian Block during late Cambrian. Together with the ~491‐Ma regional ophiolites, we suggest that the Early Palaeozoic magmatism in the Lajishan belt was formed in a subduction zone, rather than intracontinental rifting or mantle plume settings.