Low SiO2 Contents Research Articles

The role of zircon in hydrothermal heavy REE mineralisation: The case for unconformity-related ore deposits of north-west Australia

Zircon is hailed for its chemical and physical durability, but can undergo extensive chemical and structural modification due to radiation damage via the emission of alpha (α) particles, and subsequent low-temperature hydrothermal alteration. Here, we investigate Archean zircons from arkosic metasediments of the Browns Range Metamorphics (BRM) to evaluate their role in the formation of local unconformity-related heavy rare earth element (REE) ore deposits, within the Browns Range Dome, Western Australia. We determine that the heavy REE inventory of the BRM are primarily hosted in zircon, and that these zircons have a wide range of major element totals (77 to ~100 wt%, including low SiO2 and ZrO2 contents), and high and variable ‘non-formula’ components (U, Th, Y, REE, Nb, P, Al, Ca, Fe, Ti, F and OH− or H2O). Concentrations of Y + REE in some cases exceed 8 wt%. Extensive radiation damage (metamictisation) is confirmed by structural features including porous and amorphous domains, cavities, and voids. The lack of regional thermal events over an extended period likely prevented thermal annealing of these radiation-damaged zircons. Uptake of non-formula elements in metamict zircon, most likely during sedimentation in the late Archean, promoted further radiation damage, such that these grains remained highly susceptible to alteration by subsequent hydrothermal fluid circulation. We propose that the unconformity-related REE mineralisation was formed by saline fluids leaching Y + REE (and possibly P) from metamict zircon in the BRM, followed by ore mineral precipitation in fault zones near, and along the regional unconformity. More broadly, this model of ore formation may be relevant to other basin-hosted mineral systems, and could be used to guide exploration for unconformity-related REE deposits in Australia, and globally.

Timing of Transition from Proto- to Paleo-Tethys: Evidence from the Early Devonian Bimodal Volcanics in the North Qaidam Tectonic Belt, Northern Tibetan Plateau

The transition from the Proto- to the Paleo-Tethys is still a controversial issue. This study reports a new petrology, zircon U–Pb geochronology, and whole-rock geochemistry of volcanic rocks from the Maoniushan Formation in the Nankeke area, northern Qaidam (NQ) of the Tibetan Plateau, to provide new evidence for the transition from the Proto- to the Paleo-Tethys oceans. The volcanic suite consists mainly of rhyolitic crystal lithic tuff lavas and minor basalts. Zircon U–Pb data indicate that the bimodal volcanic rocks were formed during the Early Devonian (ca. 410–409 Ma). Geochemically, the basalts have low contents of SiO2 (48.92 wt.%–51.19 wt.%) and relatively high contents of MgO (8.94 wt.%–9.99 wt.%), TiO2 (1.05 wt.%–1.29 wt.%), K2O (2.35 wt.%–4.17 wt.%), and K2O/Na2O ratios (1.04–2.56), showing the characteristics of calc-alkaline basalts. Their rare earth element (REE) patterns and trace element spider diagrams are characterized by enrichments in LREEs (LREE/HREE = 18.31–21.34) and large ion lithophile elements (LILEs; Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; Nb, Ta, P, and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.82–0.86), which are similar to Etendeka continental flood basalts (CFB). These features suggest that the basalts were most likely derived from low degree (1%–5%) partial melting of the asthenospheric mantle, contaminated by small volumes of continental crust. In contrast, the felsic volcanics have high SiO2 (68.41 wt.%–77.12 wt.%), variable Al2O3 (9.56 wt.%–12.62 wt.%), low MgO, and A/CNK ratios mostly between 1.08 and 1.15, defining their peraluminous and medium-K calc-alkaline signatures. Their trace element signatures show enrichments of LREEs and LILEs (e.g., Rb, Th, U, K, and Pb), depletion of HFSEs (e.g., Nb, Ti, Ta, and P), and negative Eu anomalies (Eu/Eu* = 0.22–0.66). These features suggest that the felsic volcanics were derived from partial melting of the middle crust, without interaction with mantle melts. Considering all the previous data and geochemical features, the Maoniushan Formation volcanic rocks in NQ formed in a post-collisional extensional setting associated with asthenospheric mantle upwelling and delamination in the Early Devonian. Together with the regional data, this study proposed that the Proto-Tethys Ocean had closed and evolved to the continental subduction/collision orogeny stage during the Middle to Late Ordovician, evolved to the post-collisional extensional stage in the Early Devonian, and finally formed the Zongwulong Ocean (branches of the Paleo-Tethys Ocean) in the Late Carboniferous, forming the tectonic framework of the Paleo-Tethys Archipelagic Ocean in the northern margin of the Tibetan Plateau.

Triassic Appinite from the Qinling Orogen (Central China): Hydrous Melting of Depleted Mantle Wedge in Post-Collision Stage

Mantle wedge melting and the formation of hydrous mafic melts in collision orogenic belts have great significance for crustal melting and the related granitic magmatism, which can provide key messages about the crustal–mantle interaction in the convergent margin. This paper reported Late-Triassic appinite (212 ± 2 Ma), which is closely associated with Late-Triassic granites. The large euhedral hornblende crystals in the appinite indicate a water-rich feature. This appinite displays low SiO2 (46.55% to 50.44%) contents, high MgO (7.62 to 10.76%) and Cr and Ni contents, with high Mg# values of 61 to 75. It also displays insignificant Eu anomalies (Eu*/Eu = 0.91 to 0.93), high Sr (596 to 676 ppm) contents and moderate Sr/Y (34 to 40) ratios; these features are similar to those of Sanukite formed by the hydrous melting of the mantle wedge. Its depleted zircon Lu-Hf isotopic composition (εHf(t) = +0.97 to +18.21) indicates a depleted mantle source. Zircons in the appinite display extremely high Ti-in zircon temperatures (>1000 °C) and high oxygen fugacity, indicating a high-temperature hydrous condition. In combination with its typical arc-like trace element geochemistry (depletion in Nb, Ta and Ti), it is proposed that this appinite represents hydrous mafic melts that derived from the melting of the depleted sub-arc mantle wedge. The occurrence of this appinite has great significance for the further understanding of Triassic granitic magmatism and potential magmatic metal ore deposits in the Qinling orogenic belt.

Reactivation of metal-fertilized lower continental crust: Origin of intrusion-related Asiha gold deposit in Eastern Kunlun orogenic Belt, China

Gold deposits in the East Kunlun Orogen Belt (EKOB) are generally identified as orogenic gold deposits. However, recently, a new magmatic-hydrothermal model has been proposed to explain the genesis of these deposits that show intimate spatial and temporal relationships with abundant Triassic granitic intrusions, implying an intrusion-related gold system. The petrogenesis of these granites and their relation to gold mineralization are still unclear, owing to the lack of direct evidence of the connection between gold mineralization and granitic rocks. The gold-bearing Asiha intrusions (GBAI) from the Gouli area of the eastern segment of the EKOB are not only contemporary and located with mineralization but also have abnormally high gold content and sulfide accessory mineral. The GBAI are mainly composed of quartz diorite and granite porphyry. An integrated investigation involving mineral composition, zircon U-Pb dating and Hf isotopes, whole-rock elemental (including Au) and Sr-Nd-Pb isotopic geochemistry of the GBAI, as well as the in-situ Pb isotopic geochemistry of pyrites from the Asiha gold deposit, was conducted to constrain magmatic sources and petrogenesis of the GBAI, and gold mineralization mechanism. Zircon U-Pb dating reveals that the Asiha quartz diorite was formed at 230 Ma. The mineralized granite porphyries with high SiO2 contents (71.7–74.3 wt%) are peraluminous high-K calc-alkalic rocks, while the unmineralized quartz diorite with low SiO2 contents (58.1–60.0 wt%) are metaluminous high-K calc-alkalic rocks. Both rocks are potassic, strongly enriched in LILEs and LREEs with high LILE/HFSE ratios, depleted in Nb, Ta, Ba and Sr, and have weakly negative Eu anomalies and high zircon saturation temperature. Petrographic and geochemical features suggest that the quartz diorite is typical of I-type, while the granite porphyry is highly fractionated I-type granite. Their variable concentrations of major and trace elements are due to fractional crystallization of plagioclase, hornblende, and accessory minerals (apatite, zircon and monazite). Notably, the Asiha intrusions all contain high Au contents (Au = 0.02–0.05 ppm) compared to the primitive mantle estimated from chondrite values (0.88 ppb), indicating the existence of gold enrichment. All these granites are characterized by limited ranges of initial 87Sr/86Sr (0.7087–0.7133), εNd(t) (−6.0 to −5.0), 206Pb/204Pb (18.1–19.0),207Pb/204Pb (15.6–15.7), and 208Pb/204Pb (38.2–38.9), as well as zircon εHf(t) with values of −4.4 to −1.7. Their lead isotopes are consistent with that of pyrite in the ores (206Pb/204Pb = 18.41–18.45, 207Pb/204Pb = 15.61–15.65, 208Pb/204Pb = 38.47–38.56). Based on our results, we suggest that the GBAI were originally generated by partially melting of a previously Au-fertilized juvenile lower crust, which had been formed by metasomatized SCLM-derived alkaline basaltic underplating, and underwent AFC processes during emplacement. Subsequently, the post-magmatic fluids concentrated and precipitated Au to form the Au-bearing quartz veins.

Genesis of Mesozoic high-Mg dioritic rocks from the eastern North China Craton: Implications for the evolution of continental lithosphere

Abstract Pre-Cenozoic high-Mg andesites (HMAs) are mostly present in continental interiors, but their genetic relationship with continental lithosphere evolution remains unclear because of uncertainties of their mantle source, magmatic processes, and physicochemical conditions of formation. Early Cretaceous high-Mg dioritic rocks (HMDs, analogs of HMAs) of the Jinling complex in the Luxi area are typical intra-plate intrusions of the eastern North China Craton (NCC) and can be subdivided into two groups (Group-I and -II) on the basis of their petrographic and geochemical features. Group-I HMDs show low SiO2 contents (52.47–56.10 wt%) and Sr/Y (34.5–39.6) and (La/Yb)N (10.3–13.6) ratios but high contents of MgO (7.86–9.13 wt%), Y (18.3–20.3 ppm), Yb (1.43–1.47 ppm), and compatible elements (Cr = 407–585 ppm; Ni = 117–216 ppm), classifying as sanukitic rocks. Group-II HMDs are characterized by high SiO2 contents (63.81–64.87 wt%) and Sr/Y (47.1–63.4) and (La/Yb)N (16.1–17.5) ratios with low MgO (2.90–3.08 wt%), Y (0.88–1.04 ppm), Yb (0.88–1.04 ppm), and compatible elements (Cr = 201–213 ppm; Ni = 55–57 ppm) contents, belonging to adakitic rocks. Group-I and Group-II HMDs of the Jinling complex are closely related in spatial and temporal distribution, and all have enriched Sr-Nd isotopic compositions and arc-like trace element patterns with abundant hydrous minerals. Therefore, the Jinling HMDs should share a common source of ancient sub-continental lithospheric mantle that was metasomatized by aqueous fluids derived from the subducted Paleo-Pacific slab. The Jinling HMDs were not formed from interaction between slab-derived melts and mantle-wedge peridotites but were instead derived from partial melting of hydrous mantle peridotites in the continental interior of the eastern NCC. The distinctly different petrography, geochemistry, and mineralogy of the two groups of rocks resulted mainly from differing magmatic processes at crustal depths. Thus, Pre-Cenozoic intra-plate HMAs/HMDs are genetically distinct from Cenozoic HMAs that were mostly present in arc settings and generally represent juvenile crust growth. In a way, Archean tonalitic-trondhjemiticgranodioritic rocks (TTG) and sanukitoids, geochemically similar to HMAs/HMDs, could also be derived from interaction between slab-derived melts and mantle-wedge peridotites in arc settings or partial melting of hydrous mantle peridotites in continental interiors, and thus might not always be related with continental crustal growth and the onset of plate subduction.

Late Palaeozoic final subduction records of the southwestern Paleo-Asian Ocean revealed by ca. 286–283 Ma basaltic rocks in the West Junggar, NW China

ABSTRACT The late Early Permian basaltic rocks in the West Junggar of northwest China provide a critical geological record for revealing the termination of the Late Palaeozoic final-stage subduction history of the southwestern Palaeo-Asian Ocean. With the aim of providing constraints on the late stage of the Late Palaeozoic geodynamic evolution of the southwestern Altaids, an integrated study of detailed field observation, geochronology and geochemistry has been carried out on the Early Permian basaltic rocks from the Hala’alate Mountain of the southern West Junggar. Zircon LA-ICP-MS U-Pb dating of four basaltic andesites suggests that they were erupted during the late Early Permian with ages of ca. 286–283 Ma, coeval with the neighbouring mafic diabase stock and gabbroic dyke swarms. The basaltic rocks have low SiO2 and moderate MgO contents, with low (87Sr/86Sr)i ratios (0.70397–0.70447) and high positive εNd(t) values (+5.94–+7.19), and also show remarkable enrichment in large ion lithophile elements (Ba, U, K, Pb, Sr) and prominent depletion in high field strength elements (Nb, Ta, Zr, Hf, Ti), indicating that they were originated from the partial melting of a garnet-dominant with minor spinel lherzolitic depleted mantle source previously metasomatized by slab-derived fluids and subducted sediments. These basaltic rocks were most likely formed in a fore-arc extensional setting in response to the slab retreat of the Junggar oceanic slab during the mid-late stage of Early Permian. Combined with previously published regional achievements, our new results support that the down-going of the remnant oceanic slab and the resultant arc magmatic activity of the southern West Junggar probably continued until the ca. 283 Ma, and the closure of the West Junggar branch of the Palaeo-Asian Ocean should be later than the late Early Permian.

Conditions of Rock Formation and Magma Sources of the Late Cenozoic Udokan Volcanic Plateau

The Udokan volcanic plateau (UVP), like other areas of the Late Cenozoic volcanic province of Central Asia, formed in the interval from the Middle Miocene to the Pleistocene. Its products have high alkalinity and vary in composition from alkaline picrobasalts and basanites to alkaline trachytes. Compositional changes were controlled by two differentiation trends, which corresponded to different conditions of the original magmas generation. Rocks with low SiO2 contents (45 wt. %) were formed from melts of low melting degrees that arose under conditions of elevated pressures and temperatures. Formation of rocks corresponding to the composition range 45–61 wt. %. % SiO2 was associated with the differentiation of basalt melts, which arose at shallower depths and at lower temperatures. The geochemical characteristics of the UVP basaltoids make them similar to OIB-type basalts. They are also close in Sr, Nd, and Pb isotopic composition, corresponding to the parameters of a moderately depleted mantle, which is close to the composition of oceanic basalt sources corresponding to the mantle of deep mantle plumes. The corresponding mantle component is present in the sources of other volcanic regions of the Late Cenozoic intraplate volcanic province of Central Asia, which indicates the involvement of the lower mantle plume in the formation of these regions.

The earliest Neoproterozoic Nb-enriched mafic magmatism indicates subduction tectonics in the southwestern Yangtze Block, South China

To better reveal the early Neoproterozoic tectonic process of South China with respect to the assembly of the Rodinia supercontinent, an integrated dataset of geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotopic compositions for diabases that intruded the Kunyang Group in the southwestern Yangtze Block is herein presented. Zircon U-Pb dating gives four weighted mean 206Pb/238U ages of 946 ± 4 Ma, 948 ± 6 Ma, 939 ± 4 Ma, and 918 ± 7 Ma, suggesting mafic magmatism in the earliest Neoproterozoic. The diabases possess Nb = 12.3–30.5 ppm, Nb/U = 19.99–37.06, Nb/La = 0.78–1.05, (Nb/Th)PM = 0.44–0.80, and (Nb/La)PM = 0.75–1.02, geochemically resembling those of typical Nb-enriched basalts. They are tholeiitic in composition with low SiO2 (49.49–51.95 wt%), high TiO2 (2.03–4.65 wt%), MgO (2.50–6.14 wt%), and Fe2O3T (15.38–18.84 wt%) contents. They are enriched in light rare earth elements (LREEs) with weak negative Eu anomalies (Eu/Eu* = 0.75–0.96) and show E-MORB-like characteristics with slight to moderate Nb-Ta depletion. The samples show zircon εHf(t) values ranging from + 5.8 to + 10.3 and whole-rock εNd(t) values from + 0.5 to + 0.9. These elemental and isotopic characteristics suggest that the 0.95–0.92 Ga diabases may have derived from an E-MORB-like magma source with the injection of metasomatized slab melts. Integrating previous studies with our results, we propose that the southwestern Yangtze Block evolved from an intracontinental rift to a passive continental margin in the late Mesoproterozoic, followed by the development of an earliest Neoproterozoic continental arc-back-arc basin system analogous to the Northern Okinawa Trough. The distinct tectonic processes between the southern and northern parts of the Yangtze Block at ca. 1.0–0.9 Ga favour the hypothesis that the final cratonization of the present-day Yangtze Block occurred slightly later than ∼ 0.9 Ga in response to the Rodinia supercontinent assembly.

Effects of Different Coals for Co-Combustion with Palm Oil Waste on Slagging and Fouling Aspects

ABSTRACT Implementing a co-combustion technology in coal-fired plants using biomass waste is a promising option to alleviate environmental pollution, reduce fuel costs, and minimize waste. The considerable potential of palm oil waste is believed to be able to substitute coal as a raw material in a coal-fired power plant. However, since each coal has different compositions and biomass generally has a high alkali content, the propensity of slagging and fouling during co-combustion needs to be clarified. This research evaluates the co-combustion ash characteristics of different types of coal, which is co-combusted with 25% palm oil biomass waste. The combustion test employs a drop tube furnace, where the produced ash composition and its morphology are investigated using scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction. The results show that the ash melting temperature of the three types of coal mixed with biomass is reduced, although insignificant. Bituminous coal has a low potential for slagging and fouling since SiO2 and Al2O3 dominate the coal ash composition. In addition, lignite coal has low SiO2 and Al2O3 contents but high alkali contents (K and Na). As confirmed by SEM-EDS analysis, probe observations also show that lignite coal co-combustion with empty fruit bunch and frond waste has higher slagging and fouling risks.

Petrogenesis of Early Cretaceous alkaline basalts in the West Qinling: Constraints from olivine chemistry

Olivine crystals in alkaline basalts preserve important information about the Oxygen fugacity and thermal condition of the mantle‐derived magma. This paper presents new olivine mineral chemistry and whole‐rock geochemistry for Early‐Cretaceous alkali basalts in the Ganjia area, northeastern margin of the Tibetan Plateau. The olivine phenocrysts of Ganjia basalts have low Ni (74–2261 ppm) and high Ca contents (1217–4443 ppm), similar to the olivines in MORB, suggesting that they were crystallized from peridotite‐derived basaltic magma. Al‐in‐olivine/spinel thermometer results indicate crystallization temperatures of olivine around 1202 ± 46°C, corresponding to a mantle potential temperature of 1279 ± 47°C, with low oxygen fugacity of FMQ −1 to FMQ 0, suggesting a reduced mantle source. Whole‐rock geochemical compositions of the Ganjia basalts were characterized by low SiO2 contents (45.39%–48.70 wt%), high alkali contents (3.88–5.11 wt%), positive Nb and Ta anomalies, and enrichment in LREEs, HFSEs and LILEs, similar to those of OIB rocks, with high ratios of (Dy/Yb)N (1.38–1.54) and (La/Yb)N (5.37–7.42) ratios, indicating garnet residue. The above results suggest that the Early Cretaceous intraplate alkaline basalts in West Qinling were formed by a low degree of partial melting of peridotite and that no high‐temperature mantle plume existed. The Qinling Orogenic Belt may have been affected by the subduction of the Palaeo‐Pacific. We propose that the Ganjia basalts originated from the interaction between the upwelling asthenosphere and a lithospheric source.

On the origin of vesuvianite-rich rodingites from the Western Carpathians, Slovakia

Rodingite represents a particular metasomatic rock type that occurs as dykes or lenses in association with serpentinized ultramafic rocks and typically consists of Ca-rich, often hydrated silicate minerals. Rodingites have been recognized as a source of information on fluid compositions and their circulation in both ocean floor and subduction environments. Yet, the nature of the protoliths, the chemistry and the origin of fluids, the details of fluid-rock interaction, and the metamorphic context remain often obscure. This work investigates some of these questions in the case of rodingites from the Western Carpathians through petrological, mineralogical and geochemical studies. Rodingites are here associated in the field with serpentinized tectonically dismembered bodies of ultramafic rocks embeded into Carboniferous metasediments. They mostly consist of vesuvianite, diopside, and hydrated garnet with minor titanite, chlorite, epidote, calcite and FeTi oxides. Three successive hydrothermal events were recognized: 1) formation of diopside, vesuvianite and garnet, 2) formation of a second generation of vesuvianite and garnet together with titanite and minor chlorite, and 3) formation of a mineral assemblage consisting of epidote, chlorite and calcite. The first two events represent a pervasive and intense rodingitization process, whereas the third one represents late, channeled, fluid circulation under greenschist facies conditions. The presence of FeTi oxides influences the mineralogy and the whole rock chemistry of the investigated rodingites. Rutile and ilmenite are commonly associated with titanite and garnet, and whole rock analyses can reach up to 9.2 wt% TiO2. Vesuvianite and garnet also incorporate a significant amount of titanium. The lack of primary minerals, the mineral composition as well as high CaO and low SiO2 contents demonstrate high extent of Ca-metasomatism. The high modal amount of vesuvianite and the immobility of Al during metasomatic processes suggest that the protolith was relatively Al-rich. Taking into account the REE patterns of whole-rock and minerals, the high TiO2 content and the local Fe-Ti-rich accumulations, we suggest that the protoliths were more or less Fe-Ti-rich gabbroic rocks initially occurring in association with the serpentinized ultramafic body.

Synthesis and characterization of acrylic acid-based SiO2 nanocomposite hydrogels

Acrylic acid (AA) nanocomposite hydrogels (NCH) containing different kinds of SiO2 nanoparticles were prepared by way of free radical polymerization. The polymerization conditions were optimized considering the initiator and monomer concentrations in the mixture, as well as the reaction temperature and time. SiO2 nanoparticles were used in original (neutral), NH2 modified (hydrophilic) and perlite forms. Nanocomposite hydrogels that contain NH2 modified nano-SiO2 were found to present swelling values higher than those of the original acrylic acid nanocomposite hydrogel and the nanocomposite hydrogel that contain original SiO2. AA-perlite nanocomposite hydrogels were used to achieve the best thermal stability, although the perlite-doped hydrogels exhibit less swelling due to the large particle size of perlite and its lower SiO2 content. FESEM and mapping analyses were conducted to verify the incorporation of SiO2 nanoparticles into the hydrogel structure. Amorphous structures were determined by an XRD analysis. Thermal stability of hydrogels was measured by TG-DTA. The thermal stabilities of NH2-modified SiO2- and perlite-doped nanocomposite hydrogels were found to be higher than those of plain hydrogel and original SiO2-doped nanocomposite hydrogels.

CO2‐Rich Rejuvenated Stage Lavas on Hawaiian Islands

AbstractSedimentary carbonates are sent to the deep mantle if they are not completely destroyed at subduction zones, and subducted carbonates may contribute to plume volcanism. To better constrain the role of recycled carbonates in Hawaiian volcanism, we report high‐precision olivine and whole‐rock geochemical compositions of shield and rejuvenated stage lavas from Kauai, Oahu and Maui Islands. The studied rejuvenated stage whole‐rocks have low SiO2 and high CaO concentrations, and are depleted in high field strength elements, such as, Nb and Zr, consistent with a role of carbonated melt. Rejuvenated stage olivines have Ni abundance lower than and CaO and MnO contents similar to those of shield stage at a given Fo. The calculated partition coefficients of Ca () and Mn () between olivine and shield melts are consistent with those of a dry melt system. However, the low and for rejuvenated lavas can only be explained by a volatile‐rich melt system. Based on the observed and in rejuvenated lavas, and considering the effect of H2O, our modeling calculation shows that rejuvenated primary magmas contain up to ∼10 wt% CO2, indicating a CO2‐rich mantle source. Using olivine‐spinel aluminum exchange thermometry, we show that the rejuvenated primary magma temperatures are similar to those of shield stage. We posit that the CO2‐rich rejuvenated stage lavas with lower olivine Ni than those of shield stage lavas were originated from melting of carbonated peridotites in the plume. The carbonated melts that metasomatized the peridotites were likely derived from subducted ancient carbonate‐bearing lithospheric mantle.

Paleoenvironment of Mesoproterozoic Gaoyuzhuang and Wumishan Formations, North China: New Insights from Geochemistry and Carbon and Oxygen Isotopes of Dolostones

The Mesoproterozoic Gaoyuzhuang and Wumishan Formations are major geothermal reservoirs in the Hebei Province, North China. Compared to the exploration of geothermal resources and heat-controlling structures, carbon and oxygen isotopic records of the two formations are limited. Here, we present integrated field, petrological, geochemical, carbon, and oxygen isotopic data of carbonate rocks from the Gaoyuzhuang and Wumishan Formations. The Wumishan Formation is characterized by higher CaO and MgO contents and lower SiO2 contents than the Gaoyuzhuang Formation, indicating that the source of the Wumishan Formation likely contains less terrigenous clastic materials. The two formations have low total rare earth element contents, similar to marine carbonate rocks. They show different Eu and Ce anomalies, Al/(Al + Fe + Mn) and Fe/Ti ratios, and (Co + Ni + Cu) contents. They generally show similar carbon isotopic compositions, whereas the carbonate rocks of the middle-upper Gaoyuzhuang Formation show lower δ18O values than the samples from the Wumishan and lower Gaoyuzhuang Formations. These data suggest that the two formations have experienced different transgressive–regressive cycles and that their sedimentary environments varied and were unstable in different sedimentary periods. The middle-upper Gaoyuzhuang Formation was likely affected by hydrothermal fluids, whereas the Wumishan Formation is composed of normal seawater deposits. Integrated evidence reveals that both of the Mesoproterozoic Gaoyuzhuang and Wumishan Formations were deposited in rift environments caused by the breakup of the Columbia/Nuna supercontinent.

Petrogenesis of late Permian to Middle Triassic magmatic rocks on northern Hainan Island, South China: Implications for crust–mantle interaction and the tectonic evolution of the Paleo-Tethys

Geochronological, geochemical, and Sr–Nd–Hf isotope data are reported for the late Permian to Middle Triassic Juding gabbros, the Xinxing gabbroic–dioritic rocks, and granites and mafic enclaves from the northern part of Hainan Island, South China. Our data were used to constrain the petrogenesis of these rocks, the crust–mantle interactions involved, and the implications for the tectonic evolution of the area. Zircon U–Pb dating gave crystallization ages of ca. 235 Ma for the Juding gabbros, ca. 252 Ma for the Xinxing gabbroic–dioritic rocks, and 253–238 Ma for the granites and mafic enclaves. The Juding gabbros are characterized by weak negative Nb, Ta, and Ti anomalies, low initial 87Sr/86Sr ratios (0.704603–0.704630), and high εNd(t) values (+3.7 to +3.8). The Juding gabbros are inferred to have been derived by partial melting of depleted asthenospheric mantle, with these melts mixing with melts produced from enriched lithospheric mantle. The Xinxing gabbroic–dioritic rocks have relatively intermediate compositions, with initial 87Sr/86Sr ratios of 0.706600–0.708664, εNd(t) values of −6.3 to −5.1, and εHf(t) values of −3.3 to −0.6. The Xinxing gabbroic-dioritic rocks probably derived by partial melting of enriched lithospheric mantle that was modified by subduction-related melts and/or fluids. The granites from the northern part of Hainan Island have extremely high SiO2, low MgO and Ni contents, with initial 87Sr/86Sr ratios of 0.704794–0.711158, εNd(t) values of −8.5 to −6.6, and εHf(t) values of −15.0 to −1.2. They were probably derived by partial melting of a variety of ancient crustal sources with some addition of mantle material. The mafic enclaves in these granites have relatively low SiO2, high MgO and Ni contents, with initial 87Sr/86Sr ratio of 0.707882, εNd(t) value of −6.4, and εHf(t) values of −9.8 to −3.9. They seem to represent the remnants of a mafic component derived from enriched mantle. The gabbroic–dioritic rocks and gabbros formed in response to the subduction and break off of the Paleo-Tethyan oceanic slab. The late Permian to Middle Triassic magmatic rocks from the northern part of Hainan Island record complex multistage crust–mantle interactions and lithosphere–asthenosphere interaction, probably related to subduction during closure of the Paleo-Tethys Ocean and the consequential collision of the South China and Indochina blocks.

Crust–mantle interaction recorded by Early Cretaceous volcanic rocks within the southern margin of the North China Craton

This study presents new geochronological and geochemical data for Early Cretaceous volcanic rocks in the southern margin of the North China Craton (NCC), to discuss the crust–mantle interaction. The studied rocks include pyroxene andesites from Daying Formation, hornblende andesites and andesites from Jiudian Formation, and rhyolites from a hitherto unnamed Formation. These rocks formed in Early Cretaceous (138–120 Ma), with enrichment in light rare earth elements (REE), depletion in heavy REE and arc-like trace elements characteristics. Pyroxene andesites show low SiO2 contents and enriched Sr–Nd–Pb–Hf isotopic compositions, with orthopyroxene phenocryst and Paleoproterozoic (2320–1829 Ma) inherited zircons, suggesting that they originated from lithospheric mantle after metasomatism with NCC lower crustal materials. Hornblende andesites have low SiO2 contents and high Mg# (Mg# = 100 Mg/(Mg + Fe2+)) values, indicating a lithospheric-mantle origin. Considering the distinct whole-rock Sr isotopic compositions we divide them into two groups. Among them, the low (87Sr/86Sr)i andesites possess amount inherited Neoarchean to Neoproterozoic (2548–845 Ma) zircons, indicating the origin of lithospheric mantle with addition of Yangtze Craton (YC) and NCC materials. In comparison, the high (87Sr/86Sr)i andesites, with abundant Neoarchean–Paleozoic inherited zircons (3499–261 Ma), are formed by partial melting of lithospheric mantle with incorporation of NCC supracrustal rocks and YC materials. Rhyolites have extremely high SiO2 (77.63–82.52 wt.%) and low total Fe2O3, Cr, Ni contents and Mg# values, combined with ancient inherited zircon ages (2316 and 2251 Ma), suggesting an origin of NCC lower continental crust. Considering the presence of resorption texture of quartz phenocryst, we propose a petrogenetic model of ‘crystal mushes’ for rhyolites prior to their eruption. These constraints record the intense crust–mantle interaction in the southern margin of the NCC. Given the regional data and spatial distribution of Early Cretaceous rocks within NCC, we believe that the formation of these rocks is related to the contemporaneous far-field effect of the Paleo-Pacific Plate.

Si decline and diatom evolution: Insights from physiological experiments

In today’s oceans, diatoms are abundant and diverse primary producers distinguished by their silica shells. Although molecular clocks suggest that diatoms arose as much as 250 million years ago (Ma), the earliest known diatom fossils date from 190 Ma, leading to the suggestion that early diatoms were at best lightly silicified. By the Cretaceous Period, large circular (in cross section) diatoms with highly silicified frustules thrived in surface oceans, only later to be joined by species with elongated and thinner frustules, as well as lower SiO2 content. Decreased Si availability in surface oceans has been proposed as a principal driver of diatom evolution. Here, we investigate this through physiological experiments assessing the functional acclimation response of diatoms to reconstructed paleo-seawater. Four diatom species, differing in size and shape, were acclimated to reconstructed paleoenvironments mimicking Mesozoic/Cenozoic concentrations of nutrients in the presence of different Si regimes. When exposed to 500 µM Si, all populations, save for that of Conticribra weissflogii, became more highly silicified; the higher Si content per cell at 500 µM Si coincided with slower growth in small-sized cells. All species except C. weissflogii also showed lower photosynthetic efficiency as well as greater cell volume in comparison with diatoms acclimated to 205 or 25 µM Si. Average cell stoichiometry correlates with cell shape, but not size; pennates, in particular Phaeodactylum tricornutum, showed an acclimatory response to Si regimes, modulating Si use efficiency (the lower the external Si concentrations, the higher the C and N quotas per Si).Experimental data suggest that in the densely silicified and bigger C. weissflogii grown at higher Si, diffusion of silicic acid across membranes made a larger contribution to Si uptake, saving energy which could be reallocated into growth. In contrast, for less highly silicified and smaller species, high energy costs of Si homeostasis needed to prevent the overaccumulation of intracellular Si limited growth. While our experimental species reacted individualistically to changing silica availability, with distinct levels of plasticity, selective pressure associated with the temporal decline in Si availability may well have favored elongated shapes. Modern, less silicified species are unable to exploit high Si concentrations.

Study of the Effect of Fluxing Ability of Flux Ores on Minimizing of Copper Losses with Slags during Copper Concentrate Smelting

The article describes the effect of the fluxing ability (FA) of ores used as a flux on slag formation and copper losses. In Kazakhstan, at the Balkhash copper smelting plant (BCSP), currently used fluxes have a very low SiO2 content—about 62%—whereas the Al2O3 content is 12%. Its fluxing ability (FA) was estimated, and it was shown that currently used silica fluxes have an exceedingly low FA. Only half of the fluxes participate in the slag formation. To obtain slags with a low magnetite concentration, a considerable surplus of flux must be added, which will result in a sharp reduction in its melt temperature, increased slag output, and therefore copper losses. The slag structure was studied by means of scanning electron microscopy and electron probe microanalysis (EPMA). To determine one of the primary causes of flux ores’ low FA, it is necessary to use the microstructural pattern of experimental samples.

Late Permian rift-related volcanic rocks from the Zhongba Terrane, Southern Tibet: implications for the opening of the Neotethys

ABSTRACT The questions of how and when the Neotethys Ocean opened are crucial for understanding the evolution of the Tethys Realm and the break-up of Gondwana. However, the evolution of the Neotethys remains controversial. Herein, we report the zircon U–Pb ages, whole-rock major as well as trace elements, and Sr–Nd isotopic data for the basaltic volcanic rocks from the Zhongba Terrane in the western Yalung–Zangbo Suture Zone. The results demonstrate that the Saga basalts were extruded during the Late Permian (257–258 Ma). All samples are characterized based on the low SiO2 content (44.03–50.33 wt%) and belong to the high-K calc-alkaline series. Furthermore, they exhibit light rare earth element enrichment ((La/Yb)N = 12.7–26.7); slightly positive Eu anomaly(Eu/Eu* = 1.01–1.14); prominent Nb and Ta positive anomalies; slightly negative Zr, Hf, and Ti anomalies and exhibit high ratios of Ta/Hf (0.93–1.21), Zr/Y (8.68–10.63), and Nb/Zr (0.39–0.45). The (87Sr/86Sr)i ratio varied from 0.7050 to 0.7063, with relatively constant and depleted εNd(t) values ranging from 0.52 to 1.08. The rocks were probably sourced from a low-degree partial melting of the asthenospheric mantle in a rift environment and had undergone the process of fractional crystallization. These characteristics indicate that the Saga basalts differ from the rift-related magmatic rocks produced by the Panjal Traps. Consequently, combined with the recently reported detailed fieldwork, the Zhongba Terrane disintegrated from the northern margin of Gondwana during the Late Permian, promoting the opening of the Neotethys Ocean, which is most likely caused by the northward subduction of the Paleotethys Ocean.

Age, petrogenesis, and tectonic setting of granitic gneiss and amphibolite from the Weizigou gold deposit, Heilongjiang Province, NE China: Constraints from geochronology and geochemistry

The Weizigou gold deposit is located in the western Jiamusi Massif, Northeast China. Gold mineralization is hosted in the amphibolite, which intruded the granitic gneiss. Although the deposit shows similarities to iron-oxide–copper–gold deposits, the detailed ore-forming process remains uncertain. To determine the formation age, petrogenesis, and tectonic setting of the granitic gneiss and amphibolite, LA–ICP–MS zircon, titanite, and monazite UPb dating, whole-rock major- and trace-element analyses, and LA–ICP–MS in situ zircon Hf isotope analyses were conducted on samples from these rocks. The granitic gneiss yielded two age populations of 951–882 Ma, and ca. 500 Ma, with a monazite UPb concordia age of 501.5 ± 5.1 Ma. The amphibolite yielded a crystallization age of 292 Ma, consistent with the results for magmatic titanite UPb dating, and a metamorphic age of 272–258 Ma. The granitic gneiss contains typical aluminum-rich minerals, such as garnet and muscovite, mean SiO2 = 73.31 wt%, and molar ratio Al2O3/(CaO + K2O + Na2O) values of 1.02–1.07, indicating an S-type granite protolith. The amphibolite belongs to the tholeiitic basalt series and has low SiO2 and high MnO contents. These results, together with εHf(t) values and two-stage model ages ranging from −9.5 to 2.3 and − 0.3 to 5.7, and from 2010 to 1659 Ma and from 1331 to 947 Ma, respectively, allow us to infer that the parental magmas of the granitic gneiss and amphibolite were derived from the partial melting of Paleoproterozoic lower crust and the partial melting of metasomatized depleted mantle, respectively. The granitic gneiss is characterized by positive Th and Hf anomalies, and negative Nb, Ta, Sr and Ti anomalies, whereas the amphibolite is enriched in K, Rb, and depleted in Ba, Nb, Ti, and Zr. These geochemical features suggest that the S-type granite was formed in an active continental margin during the Neoproterozoic and underwent granulite-facies metamorphism during the early Paleozoic. The protolith of the amphibolite was gabbro that formed in an extensional setting (e.g., a backarc basin) associated with westward subduction of the Paleo-Pacific oceanic plate beneath the eastern Jiamusi Massif during the early Permian. The gold mineralization can most likely be attributed to contact metasomatic metamorphism of gabbro during the middle–late Permian.