High SiO2 Contents Research Articles

Geochemical Constraints on Petrogenesis and Tectonics of the Middle Devonian Granitic and Coeval Mafic Magmatism from the Tannuola Terrane (Northern Central Asian Orogenic Belt)

The Tannuola terrane, located in the northern Central Asian Orogenic Belt, comprises magmatic rocks, attributed to island-arc and collisional settings during the Early Cambrian to the Late Ordovician. However, zircon U-Pb age, geochemical, and Sr-Nd isotopic constraints demonstrate that there was a short episode of peralkaline A-type granite magmatism in the northeast border area of the Tannuola terrane. The obtained zircon U-Pb age of 387.7 ± 3.3 Ma indicates emplacement of the peralkaline A-type granitic rocks in the Middle Devonian (Eifelian–Givetian period boundary). Petrologically, these rocks are mainly composed of riebeckite granites and aplites, which are approximately synchronous with augite-rich dolerites. The granitic rocks are ferroan and calc-alkalic to alkali-calcic in composition. They are characterized by a high content of SiO2, total alkali, Zr, and total REE. Significant depletion of Ba, Sr, P, Ti, and Eu indicates fractionation of plagioclase and/or K-feldspar. The values of εNd(t) in riebeckite granites range from +5.61 to +6.55, and the calculated two-stage model age ranges between 610 and 520 Ma. Coeval dolerites on the chondrite-normalized REE pattern, (Th/Yb)pm–(Nb/Yb)pm, and Th/Yb–Nb/Yb diagrams show compositional affinity between E-MORB and OIB. They are rich in incompatible elements with high HFSE/LREE ratios (Nb/La > 1), indicating that the primary magma originated from the lithospheric mantle metasomatized by asthenosphere-derived melt. Based on these geochemical characteristics, it can be reasonably inferred that the peralkaline A-type granitic rocks, and the coeval mafic rocks, are anorogenic and were emplaced in an extensional tectonic environment despite slightly higher Y/Nb values, which might be caused by a crustal contamination effect. The geochemistry of mafic rocks suggests that the magma originated from the enriched mantle sources. The results of a zircon-saturation thermometer show high initial magma temperatures between 923 and 1184 °C, with an average of 1030 °C, indicating this rock association might be related to a mantle plume beneath the northern Central Asian Orogenic Belt.

Zircon U-Pb ages and Hf isotope characteristics of granitic rocks from the Shapinggou molybdenum deposit, Dabie Shan, China

The giant Shapinggou porphyry molybdenum deposit in Jinzhai county, Anhui province is located in eastern Dabie Shan, but its genesis, genetic type and tectonic setting are still debated. New zircon U − Pb age data presented in this contribution show that the granitic rocks at Shapinggou area have been episodically formed at ∼ 135 Ma, ∼127 Ma, ∼117 Ma and ∼ 113 Ma in Early Cretaceous, and can be classified into the early-stage (135–127 Ma) granodiorite and monzogranite plutons, and the late-stage (117–113 Ma) quartz-orthophyre stock and cryptoexplosive breccia pipes. Mo mineralization is closely associated with the late-stage shallow-seated granitic stock or pipes. Granitic rocks of both stages have high contents of SiO2, K2O + Na2O and Al2O3, low contents of TiO2, CaO and MgO, showing a metaluminous to peraluminous, high-K calc-alkaline to shoshonite affinity. They are generally featured by enrichments of LILE (Rb, U, Th) and LREE, and depletions of HFSE (Nb, Ta, Ti) and HREE, with high (La/Yb)N ratios. The early-stage granitic plutons show trace element geochemical signatures of partial melting of over-thickened continental crust (>50 km), whereas element geochemistry of the late-stage small-sized granitic intrusions suggests a shallow source depth (ca. 35 km) under post-collisional extension setting. The Hf isotope data indicate that the Shapinggou granitic rocks are originated from a crustal source predominated by the Kuanping Group mixed with other components such as the mantle. The southward subduction and partial melting of the southern North China Block (including Kuanping Group) beneath the Dabie Shan lead to the generation of the Shapinggou porphyry Mo system in the post-collisional extension setting. Hence the Shapinggou Mo system distinguishes for its crustal source and post-collisional tectonic setting from the porphyry Mo deposits of Climax- or Endako-types, and should belong to collision-type or Dabie-type porphyry Mo deposit.

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.

HREE enrichment during magmatic evolution recorded by apatite: Implication for the ion-adsorption HREE mineralization in South China

Understanding the enrichment of heavy rare earth elements (HREE) and high HREE/LREE (light REE) ratios in the granites is critical to understand the ore-genesis of exclusive ion-adsorption HREE deposits in South China. In this study, we conducted geochronological, whole-rock and apatite geochemical, and isotopic studies on the Xinfeng pluton, of which only parts show high HREE/LREE ratios and are ore-related. Both the ore-related granite and ore-unrelated granite from the Xinfeng pluton are characterized by high SiO2 contents, the same Late Jurassic age (ca. 155 Ma), and enriched Nd isotopic compositions. However, the ore-related granite has lower TiO2, CaO, P2O5, Fe2O3, Ba, Sr, Eu, and LREE contents, lower La/Yb, Zr/Hf and Th/U ratios, higher Rb, Cs, Nb, Ta, Pb, and HREE contents, and higher Rb/Sr ratios than the ore-unrelated granite. These features indicate that the ore-unrelated and ore-related granite are cogenetic and their geochemical differences can be explained by magmatic differentiation. The primary igneous apatite from both the ore-unrelated granite and ore-related granite shows negligible hydrothermal alteration. Fractional crystallization of feldspar (∼20%) and allanite (< 0.3%) led to higher HREE contents and HREE/LREE ratios, lower Sr contents, and a more pronounced negative Eu anomaly in the apatite from the ore-related granite. This study highlights that the magma evolution via mineral fractional crystallization could achieve the relatively high HREE contents and HREE/LREE ratios in the granites, and favors the formation of exclusive ion-adsorption HREE deposits in South China.

Occurrences of Rare Earth Element (REE) Bearing Minerals in Migmatitic Gneiss and Granitoids of Chhotanagpur Granite Gneissic Complex, Bihar, India

Abstract This paper presents the geochemical characteristics, petrogenetic processes and source characteristics of Rare Earth Elements (REE) bearing migmatitic gneiss and granitoids near Tangeshwar village, Banka district, Bihar, in order to demarcate the potential zone of REE mineralization. These migmatitic gneiss and granitoids belongs to the Chhotanagpur granite gneissic complex (CGGC). The study of migmatitic gneiss and granitoids of CGGC suggest that they have high content of SiO2, low contents of Fe2O3 and MgO and SiO2 vs. Na2O and K2O shows positive correlation which suggest, they are dominated by sodic and alkali rich minerals and were derived from a felsic (granitic) source. Also LREE-LILE enrichment, HFSE depletion and calc-alkaline nature indicates that the magmatism is related to the subduction zone. Petrographic and EPMA studies of these migmatitic gneiss and granitoids confirms the presence of REE bearing mineral phases such as allanite, xenotime, monazite, and basnasite. They are generally present at the rim of biotite, as an inclusion in biotite and also along the grain boundaries and within the cracks of major silicate minerals. This suggest that the REE formation takes place at the deuteric stage of magma crystallisation followed by the formation of granite. Chemical analysis of bed rock samples for rare earth elements using ICPMS shows that the highest concentration is in migmatitic gneiss and granitoids, which varies from 47.85- 925.67 ppm and 47.85- 626.48 ppm respectively.

Transformasi Abu Vulkanik dan Limbah Seng menjadi Nanokomposit ZnO-SiO2 dan Aplikasinya untuk Degradasi Rhodamin B

Rhodamine B is a non-biodegradable organic pollutant that is difficult to decompose by microorganisms. If it is possible to be degraded it will take a long time. The semiconductor used is ZnO using SiO2 host material. The reason for using SiO2 is that apart from high efficiency, there are also abundant raw materials because the eruption of Mount Merapi occurs regularly every 4 years. The high content of SiO2 in volcanic ash can be utilized and processed into silica gel through the formation of potassium silicate resulting from the reaction between SiO2 in volcanic ash and potassium hydroxide. In addition to the presence of abundant SiO2, ZnO sources from lathe workshop waste are also easily obtained. The manufacture of ZnO/SiO2 nanocomposites was carried out using the sol – gel method because it is easy and has high effectiveness. ZnO/SiO2 nanocomposite was applied as an adsorbent to degrade Rhodamine B. This study aims to determine the character of ZnO/SiO2 nanocomposite and to determine the best conditions for optimal degradation. Synthesis of ZnO/SiO2 nanocomposite based on volcanic ash and zinc waste resulted in a composite size with a size range of 100-200 nm and a uniform circular shape. The results of the FTIR test show that SiO is at a wavelength of 993.34 and 1109.07 cm-1, while ZnO is at a wavelength of 443.63 cm-1. The XRD results of ZnO/SiO2 nanocomposite are known to peak at 2𝜃, namely 30.42o, 31.56o, and 44.40o. The application of dye degradation on UV irradiation, as well as looking for pH, time, and concentration gave the maximum Rhodamine B degradation value. The results of this study indicate that the highest efficiency in decreasing the concentration of Rhodamine B was obtained at pH 3 with a contact time of 60 minutes and a concentration of Rhodamine B of 10 ppm. The degradation efficiency of Rhodamine B obtained under these conditions is 95.8690%. Based on the concentration variation data processing using the Langmuir isotherm equation, it is known that the adsorption capacity of Rhodamine B is 97.3458521%.

Geochemical transformation of tuffaceous materials in tight sandstone and its significance for reservoir reconstruction: A case study from the Taiyuan and Shihezi Formation in Dingbei area, Ordos Basin

Combined with thin sections and electronic probe microanalyzer (EPMA) analysis, we studied the effect of the chemical composition of tuffaceous materials on the quality of sandstone reservoir in the Dingbei area. Analysis of 100 thin sections from five drilled wells indicates that the tuffaceous materials in sandstones from the Taiyuan and Shihezi Formation are common due to the volcanic eruptions. There are three types of tuffaceous materials in the sandstone: intermediate-basic type, intermediate-acid type, and mixed type. Seven thin sections were selected to analyze the chemical elements of tuffaceous materials by EPMA, with 3–8 test points on each thin section. The results indicate a clear decline in most major elements except for SiO2 and Al2O3. The elemental differentiation of tuffaceous sandstone is largely controlled by the activity of the diagenetic system. Under the slow-moving fluid, the active elements such as K, Na, Ca, Mg, and Fe are lost, and Si and Al are enriched. The SiO2 contents of EPMA results are up to 42%. Under the strong-flow fluid, Si and Al can be transferred to precipitate the kaolinites. In the closed diagenetic system, the contents of active elements such as Na and K are high. Dissolution or alteration of tuffaceous materials with different types under the acid fluid is one of the reasons for the generation of heterogeneous reservoirs. The intermediate-acid tuffaceous materials with high SiO2 content experience transformation without dissolution. The mixed tuffaceous materials are mainly transformed into kaolinite to form intercrystalline pores. Due to a large number of soluble components in the intermediate-basic tuffaceous materials, a large number of secondary pores are developed.

Early Eocene A-type (ferroan) rhyolites in southwestern Tibet: A far-field tectonic effect of the India–Eurasia collision

ABSTRACT Eocene igneous rocks along the Gangdese belt in southern Tibetan Plateau are important for understanding the India–Eurasia collision and Tibetan Plateau uplift. These magmatic rocks are widely considered to be related to roll-back or break-off of the Neotethyan slab during northward subduction. However, Eocene rhyolites in the northwestern Gangdese belt (SW Qiangtang margin) do not fit either the rollback or breakoff models. This paper investigates these Early Eocene lavas and compares them with contemporaneous igneous rocks along the Gangdese belt via detailed field observations, petrology, zircon geochronology, zircon Lu–Hf isotopes and whole-rock geochemistry. These felsic lavas form large outcrops in western Tibet and have two zircon U–Pb ages of 54.71 ± 0.14 Ma and 54.74 ± 0.27 Ma. They have high SiO2 and alkali contents, FeO*/MgO, and Ga/Al ratios as well as strongly negative Eu anomalies and slightly positive zircon εHf(t) values (+2.5 to +4.4). Both Ti-in-zircon and Zr saturation thermometers confirm high magmatic temperatures (~900°C). They are ferroan, A-type rhyolites. Furthermore, the distinctive low εHf(t) values imply that these rhyolites are likely partial melting products of the southwestern Qiangtang Terrane. The lithosphere in this region undergone long-term shortening and thickening since 80–95 Ma. Pre-thickened lithosphere may be a prerequisite for the Early Eocene delamination, and the simultaneous inducement is a far-field tectonic effect from the India–Eurasia collision. We suggest that: (1) India collided with Eurasia before 55 Ma and triggered delamination to form the Shiquanhe A-type rhyolites on the southwestern Qiangtang margin thereafter; and (2) tectonic stresses related to India–Eurasia collision during the Eocene were transmitted efficiently over large distances.

Geochemistry, geochronology and fluid inclusion studies of the Dongji epithermal gold deposit in Fujian province, south‐east China

The Dongji epithermal Au deposit formed in the Early Cretaceous is located in the Dongkeng volcanic Basin of the South China Block. The orebodies of the deposit are mainly hosted in the Jurassic volcanic–subvolcanic rocks. Here, detailed zircon U–Pb geochronology, major‐trace elements, and fluid inclusion data of the Dongji deposit are reported. At least two epochs of magmatic activity were identified at Dongji, including the Late Jurassic rhyolitic crystal tuff lava (165.3 ± 1.8 Ma to 162.8 ± 2.6 Ma) and quartz porphyry (156.0 ± 1.9 Ma), and the Early Cretaceous dacitic crystal tuff lava (108.4 ± 5.7 Ma) and purplish‐red crystal tuff lava (98.1 ± 0.8 Ma). The Late Jurassic volcanic–subvolcanic rocks are characterized by high SiO2 content, enriched LILE (large ion lithophile element) elements, depleted HFSE (high field strength element) elements, and obvious P, Ti and Eu anomalies. These geochemical characteristics suggest that the volcanic–subvolcanic rocks were generated by partial melting of the lower crust and controlled by fractional crystallization of Ti‐bearing phases, apatite, and plagioclase. In comparison, the Early Cretaceous volcanic–subvolcanic rocks are characterized by high K contents, and arc‐like signatures (e.g., enriched Rb, Th, K, and depleted Nb, Ta, Ti), indicating a partial melting of crust–mantle interaction under an extensional tectonic setting. The ore‐bearing dacitic crystal tuff lava and the caprock of purplish‐red tuff lava constrain the timing of gold mineralization in the range of 108–98 Ma. The Au mineralization at Dongji is divided into the early stage of quartz‐pyrite association and the late main mineralization stage of quartz‐polymetallic sulfide association. The fluid inclusion microthermometry results show that the ore‐forming fluids at the early mineralization are characterized by enrichment of liquid‐rich two‐phase inclusions and CO2‐bearing aqueous inclusions. The fluids with moderate homogenization temperatures (292–382°C) and low salinities (5.4–10.0 wt% NaClequiv) were possibly exsolved from magmatic rocks. Nevertheless, fluid inclusions in the late‐stage contain liquid‐rich two‐phase inclusions and pure liquid phase inclusions. The homogenization temperatures (222–309°C) and salinities (2.4–8.3 wt% NaClequiv) of the late‐stage fluid inclusions are relatively lower than those of the early stage, probably indicating a fluid mixing of magmatic and meteoric fluids. Based on the above, we propose that fluid mixing is the key factor facilitating Au precipitation in the Dongji deposit, probably applicable to other Au deposits in the Dongkeng volcanic Basin.

Remote sensing techniques and geochemical constraints on the formation of the Wadi El-Hima mineralized granites, Egypt: new insights into the genesis and accumulation of garnets

The Wadi El-Hima Neoproterozoic I- and A-type granites in the Southern Eastern Desert of Egypt are rich in garnets (up to 30 vol%) and are cut by NW–SE strike-slip faults, as confirmed from structure lineament extraction maps. These mineralized granites and garnet mineralization zones can be successfully discriminated using remote sensing techniques. Spectral angle mapper and matched filtering techniques are highly effective for mapping garnet-rich zones and show that the highest garnet concentrations occur along the intrusive contact zone of NW–SE striking faults. El-Hima granites have high SiO2 (73.5–75.1 wt%), Al2O3 (13.4–15.3 wt%) and total alkali (6.7–8.7 wt%) contents, suggesting that they were sourced from peraluminous (A/CNK > 1) parental magmas. Garnet-bearing trondhjemites are metasomatic in origin and formed after I-type tonalite-granodiorites, which originated in a volcanic arc tectonic setting. Garnet-rich syenogranites and alkali-feldspar granites are both post-collisional A-type granites: the syenogranites formed from peraluminous magmas generated by partial melting of lower crustal tonalite and metasedimentary protoliths during lithospheric delamination, and the alkali-feldspar granites crystallized from highly fractionated, felsic and alkali-rich peraluminous magmas in the upper crust. Garnets in El-Hima mineralized granites occur in three forms: (1) subhedral disseminated crystals, (2) vein-type crystals, and (3) aggregated subhedral crystals, reflecting different mechanisms of accumulation. All are dominantly almandine in composition (Alm76Sps10 Prp7Grs6Adr1) and have high average concentrations of heavy rare earth elements (HREE) (ΣHREE = 1636 ppm), Y = (3394 ppm), Zn (325 ppm), Li (39.17 ppm) and Ga (34.94 ppm). Garnet REE patterns show strong negative Eu anomalies with HREE enriched relative to LREE, indicating a magmatic origin. These magmatic garnets are late-stage crystallization products of Al-rich hydrous magmas, and formed at low temperature (680–730 °C) and pressure (2.1–2.93 kbar) conditions in the upper continental crust. Peculiar garnet concentrations in syenogranites near and along contact zones with alkali feldspar granites are related to peraluminous parent hydrous magma compositions. These garnets formed by in situ crystallization from A-type granite melts, alongside accumulation of residual garnets left behind after partial melting of the host garnet-rich granites along the intrusive contact. Magmatic-fluid flow along the NW–SE striking fault of Najd system enhanced garnet accumulation in melts, which formed clots and veins of garnet.

Geochronology and petrogenesis of the Early Palaeozoic Fuxi magnesian granodiorite in southern Zhuguangshan, South China Block and its geodynamic significance

This study provides relevant data on the granodiorite in southern Zhuguangshan, South China Block (SCB), including mineralogical information, zircon U–Pb ages, zircon Hf isotope data, whole‐rock geochemical data, and Sr–Nd–Pb isotopic data. These data indicate that the granodiorite in southern Zhuguangshan crystallized at approximately 448.7 Ma. The hornblende and micas of the granodiorites are magnesio‐hornblende and Mg‐rich biotite, respectively. The granodiorite samples are characterized by relatively high SiO2 contents of 61.72–66.19 wt.%, K2O contents of 3.77–5.18 wt.%, Na2O contents of 2.42–3.16 wt.%, low FeOT contents of 3.93–5.79 wt.%, and relatively high MgO contents of 1.78–2.55 wt.%, with Mg# values (molar Mg/[Mg + Fe] × 100) of 43–50. The contents of rare earth elements (REEs) in the granodiorite specimens are 166–283 ppm, and the REEs show both a chondrite‐normalized REE pattern that inclines rightward and slightly negative Eu anomalies (0.62–0.90). The Fuxi granodiorite is rich in large‐ion lithophile elements (LILEs; Rb, Th, U, and K) but is depleted in high‐field‐strength elements (HFSEs; Nb, Ta, and Ti). All the samples have similar Sr–Nd–Pb and zircon Hf isotopic compositions and have a (87Sr/86Sr)i ratio of 0.707319–0.710888, a (143Nd/144Nd)i ratio of 0.511705–0.511760, a (206Pb/204Pb)t ratio of 17.918–18.459, a (207Pb/204Pb)t ratio of 15.711–15.813, and a (208Pb/204Pb)t ratio of 37.953–38.563. The εNd(t) and εHf(t) values range from −7.12 to −5.88 and from −9.81 to −4.45, respectively, with corresponding two‐stage Nd model ages of 1.66–1.76 Ga and two‐stage Hf model ages of 1.55–1.78 Ga. These findings indicate that the granodiorite is magnesian andesite and that the magmas are derived from a mixture of crustal and mantle materials. Based on these findings in combination with previous research results, it can be concluded that the mantle beneath the Cathaysia Block was moderately depleted during the Early Palaeozoic and that Early Palaeozoic magmatism may be related to Early Palaeozoic oceanic crust subduction.

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.

Petrogenesis of Eocene high-silica granites in the Maliaoshan area, northern Tibet: Implications for the Eocene magmatic flare-up in the Northern Qiangtang Block

The Eocene magmatic flare-up in the Northern Qiangtiang Block is a key event that was genetically related to the uplift of the Tibetan Plateau. This uplift has been considered to have an important impact on the global cooling and oceanic isotopic compositions. Here, we report zircon ages and bulk rock elemental and isotopic compositions for high-silica porphyritic granites in the Maliaoshan area of the Northern Qiangtang Block. Zircon U–Pb dating shows that the porphyritic granites were generated at ca. 41.02 Ma. The porphyritic granites are characterized by uniformly high SiO2 contents (74.3–75.0 wt%), high contents of total alkalis, metaluminous compositions, and low contents of TiO2, Fe2O3T, MgO, CaO, and P2O5. Moreover, the Maliaoshan porphyritic granites have Nd–Hf isotopic compositions similar to those of coeval metaluminous magmatic rocks in the Northern Qiangtang Block. These data together with geochemical modeling results suggest that the Maliaoshan porphyritic granites were formed by the fractional crystallization of 43.06% major minerals and 0.76% accessory minerals from the assumed starting composition (average Eocene metaluminous igneous rock). Based on our new data and Eocene magmatic flare-up event in the Northern Qiangtiang Block, we propose a new geodynamic model to explain such a magmatic flare-up event. The stress produced by the collision of India and Eurasia gradually transferred northward, prompting the Songpan–Garzê Block to subduct southward beneath the Northern Qiangtang Block at ca. 55 Ma, accompanied by minor magmatic activity. Subsequently, the break-off of the Songpan–Garzê Block at ca. 46 Ma resulted in the Eocene magmatic flare-up.

Late Palaeozoic tectonic evolution of the eastern Palaeo-Asian Ocean: new evidence from the early Permian arc magmatic suites in the Kulun region

AbstractLate Palaeozoic igneous rock associations in response to subduction, accretion, and final closure of the eastern Palaeo-Asian Ocean play a significant role in understanding the geodynamic evolution of the southeastern Central Asian Orogenic Belt. Previous studies have identified a Permian arc magmatic belt associated with the southward-dipping subduction of the eastern Palaeo-Asian Ocean along the Solonker–Changchun suture zone. The genetic mechanism and associated geodynamic settings are of great importance in deciphering the evolution of the eastern Palaeo-Asian Ocean. This paper presents zircon U–Pb–Hf isotope and whole-rock geochemical analyses for a suite of magmatic rocks including the early Permian diorite porphyrites (ca. 281.0 Ma), andesites (ca. 276 Ma) and rhyolites (ca. 275 Ma) in the Kulun region. The diorite porphyrites and andesites have high SiO2 and total alkali contents, and low MgO contents and Mg no. values, with enrichments in large ion lithophile elements and depletions in high-field-strength elements. These geochemical characteristics, together with low-Sr and high-Yb contents, a weak concave-upward shape of middle rare earth elements and negative Eu anomalies, suggest that these intermediate igneous rocks were generated by partial melting of amphibolitic lower crust at a crustal depth of 30–40 km. The rhyolites have heterogeneous isotopic compositions, with ϵHf(t) values and TDM2 ages of –20.8 to +0.5 and 3578∼1494 Ma, implying that they were likely derived from partial melting of a mixed source dominated by recycled ancient crust with minor juvenile crustal materials. The rhyolites show potassic affinity with relatively high K2O and very low Na2O, which was attributed to liquid immiscibility of felsic magma and subsequent limited fractional crystallization of plagioclase. The regional igneous associations, metamorphic events, and coeval sedimentary rocks along the Solonker–Changchun suture zone indicate that the early Permian igneous rocks were formed in an active continental arc environment in response to southward subduction of the eastern Palaeo-Asian Ocean.

Tridymite in a lacustrine mudstone in Gale Crater, Mars: Evidence for an explosive silicic eruption during the Hesperian

The unexpected detection of ∼16 wt.% monoclinic tridymite, a high-temperature silica polymorph, within an otherwise lacustrine mudstone in Gale crater, Mars raises significant questions about its formation and the extent of magmatic evolution on that planet. The rock sample, analyzed by the X-ray diffractometer onboard the Curiosity rover, also contained feldspar, cristobalite, and opaline silica (±Si-glass). Monoclinic tridymite is extremely rare on Earth, and has only been discovered in silicic volcanic environments, high-temperature impact settings, and extraterrestrial rocks. We review the most common formation pathways of natural tridymite and run thermodynamical models to investigate possible formation mechanisms. We consider the broader context of the sample to propose a formation and transport mechanism based on: (1) the mineralogical assemblage of the mudstone and rocks in the vicinity, (2) the composition of the mudstone layer, and (3) the overall geological context. Based on the large amount of tridymite, the high SiO2 and low Al2O3 concentration of the mudstone, and the low temperature context within distal lacustrine mudstone, we propose that an explosive eruption released Si-rich ashes, which were deposited into Gale crater's watershed as a tridymite-rich ashfall along with cristobalite, feldspar, Ti-oxide, and Si-rich glass, when Gale was still a lake (Hesperian). The dissolution of Si-rich glass and mineral sorting during transport would have concentrated tridymite, caused opaline silica precipitation, and relatively lowered the Al2O3 concentration. This scenario implies that explosive volcanism on Mars occurred during the Hesperian and might not be restricted to basaltic eruptions, revealing the complexity of Mars magmatism.

Paleoenvironment Evolutionary Characteristics of Niutitang Shale in Western Hubei, Middle Yangtze, China.

The black shale developed in the first section of the Niutitang formation (ϵ1n1) is one of the most important shale gas reservoirs in western Hubei, and its geological characteristics have been sufficiently studied by many predecessors. However, there are still three aspects that need further research: the origin of silicon, the discrimination of the euxinic sulfuretted and the anoxic ferruginous conditions, and the main controlling factors of organic matter enrichment. Based on geochemical data from well ZD1 located in the city of Yichang in western Hubei, first, the geochemical characteristics of ϵ1n1 are analyzed, then the provenance, depositional site, and paleoenvironment evolution are discussed, and finally, the main controlling factor of organic matter enrichment is revealed. The results show that ϵ1n1 can be divided into two units, organic-rich shales (ORS) and organic-lean shales (OLS), which have average total organic carbon contents of 4.21 and 0.84%, respectively. Additionally, the ORS is characterized by high contents of SiO2, U, V, Ni, Zn, and Cu and left-inclining types of rare earth element distribution curves. ϵ1n1 is located in a passive continental margin with a material source mainly from mixed felsic and mafic rocks. Compared with the OLS, the content of biological quartz is much greater, and the terrigenous input is less in the ORS. The paleoclimate is cold and humid with low salinity in the ORS, whereas it is hot and dry with high salinity in the OLS. ϵ1n1 is deposited in a semistagnant basin, and the ORS shows a relatively lower stagnant degree with euxinic to anoxic conditions and moderate to high paleoproductivity, while the OLS shows a high stagnant degree with suboxic to oxic conditions and lower paleoproductivity. The redox conditions are the main controlling factors affecting organic matter enrichment. The environmental evolution model with three stages shows that there is a good causal relationship between redox conditions, paleoproductivity, and sea level fluctuation. The black carbonaceous siliceous in the lower part of the ORS with a thickness of approximately 40 m is the most favorable layer, which will provide a theoretical basis for further shale gas exploration of ϵ1n1 in the western Hubei.

Investigation on the regulation mechanism of Fe2O3 / SiO2 on ash melting and flow behavior under gasification conditions

ABSTRACT Stable flow and discharge of the slag in an entrained flow gasifier are the key factors to ensure the long-term stable operation of gasifier. In this work, the melting and flow behavior of coal ash with different Fe2O3/SiO2 ratios was studied in a reduced atmosphere. The results show that the viscosity of slag decreases significantly with the increase of Fe2O3/SiO2. And the critical viscosity-temperature (TCV) decreases from 1309°C to 1226°C. It is found that more than 90% of iron ions exist in the form of Fe2+ in liquid slag by chemical titration. There are more simple ion clusters with low bridge oxygen number (BO) with the increase of Fe2+ and lead to depolymerization of slag. Furthermore, the ash with high SiO2 content can be transformed into slag with high viscosity at a low melting rate by the analysis of the slag area curve and X-ray diffraction pattern, which is considered to be controlled by the “softening-melting” mechanism. The ash with high Fe2O3/SiO2 can be transformed rapidly into a low viscosity liquid at a short melting interval, which follows the “melting-dissolution” mechanism. And it is helpful to promote the formation of low-temperature eutectics with the increase in Fe2O3/SiO2 by thermodynamic calculation. Research on the melting mechanism of slag will provide theoretical support for the regulation of slag flow characteristics.

Geochemistry, Zircon U–Pb Ages, and Hf Isotopes of the Devonian Mafic and Acidic Dykes of the Jinba Gold Deposit, NW China: Petrogenesis and Tectonic Implications

The Jinba deposit is an orogenic gold deposit located in the Markakuli shear zone, in the southern part of the Altay orogenic belt, northwestern China. Several granite and diorite dykes are present in the area of the mine, with ore bodies occurring in the diorite dykes. However, the diagenetic age, genesis, and evolution of these magmas, and the tectonic setting of the dyke emplacement process, remain unclear. The present study is based on a detailed geological survey, zircon U–Pb dating, Lu–Hf isotopes, and geochemical analysis of the granitic and diorite dykes. The crystallization ages of the granitic and diorite dykes are 384.5 ± 1.2 Ma and 393.9 ± 3.5 Ma, respectively, which indicates formation in the Early to Middle Devonian. Zircon εHf(t) values of the granitic and diorite dykes are 1.43 to 5.2 and −4.47 to −1.18, respectively, with a corresponding two-stage model of depleted mantle (TDM2) ages of 1046 to 1285 Ma and 1242 to 2623 Ma, respectively. This indicates that the granitic and diorite dykes were formed by the mixing of mantle magma and crustal materials to varying degrees, and diorite dykes are more obviously contaminated by the lower crust. Geochemical analysis shows that the granite dykes have a high SiO2 (72.51%–74.87%) and moderate Al2O3 (12.88%–14.04%) content, a total alkali of (K2O + Na2O = 5.51%–6.44%), and aluminous (A/CNK = 1.0–1.02). Granite dykes are enriched in LREE and Th, U, and Pb, and depleted in P, Sr, and Nb elements, with clear Eu negative anomalies (δEu = 0.62~0.66). The SiO2 content of diorite dykes is 51.48%–53.71%, Al2O3 contents are high (14.70%–15.99%), K2O is 1.94%–2.54%, Na2O is 2.97%–3.96%, MgO contents are high (5.15%–6.46%), and TFe2O3 is (13.42%–15.13%), enriched Sr, U, Pb, deficient Th elements, rare earth fractionation is not obvious, and Eu anomaly is not obvious (δEu = 0.93~1.1). We conclude that the Early to Middle Devonian magmatism in the southern margin of Altay (which corresponds to the Jinba gold deposit) may have formed in an island arc-related subduction environment.

Albian–Cenomanian granitoid magmatism in Eastern and Central Tibet as a result of diachronous, continental collision induced slab tear propagation

AbstractA discrete belt of Albian–Cenomanian granitoid plutons occurs in the Lhasa and Qiangtang terranes in the Central (CTP) and Eastern Tibetan Plateau (ETP) and represents a major magmatic pulse in the plateau’s crustal evolution during the Cretaceous. The geochemistry, petrogenesis, and magmatic development of these granitoids are different from those of magmatic arc granitoids along the southern edges of the Lhasa and Qiangtang terranes, indicating different heat and melt sources and tectonic setting of their formation. We present here new mineral, whole-rock and isotope geochemistry, and zircon U-Pb age data from the Xiasai pluton in the ETP and discuss its geochemical-petrological characteristics and magmatic development in comparison to the other Cretaceous plutons in the ETP and CTP, and within the tectonic framework of the Mesotethyan geology of Tibet. Zircons from the Xiasai and other plutons in the ETP have yielded U-Pb ages ranging from 106 Ma to 93 Ma in comparison to 115 Ma and 100.3 Ma zircons from the Southern Qiangtang Terrane (SQT) and 113.4 Ma and 109 Ma zircons from the Northern Lhasa Terrane (NLT) farther west. The Cretaceous granitoids in the ETP and CTP range in composition from granite, K-feldspar granite to monzogranite and biotite monzogranite, representing highly fractionated I-type granites with relatively high SiO2 and K2O contents, variable (Na2O + K2O)/CaO and FeOT/MgO ratios, and (Zr + Nb + Ce + Y) abundances. They display significant negative Eu anomalies (Eu/*Eu) = 0.04–0.12) and strong depletions in Sr and Ba, and are strongly enriched in large ion lithophile elements but depleted in high field strength elements. Their εHf(t) values correspond to wide ranging Hf isotope crustal model ages (TDMC) of 0.3–1.9 Ga, and their Sr-Nd isotopic signatures show elevated (87Sr/86Sr)i ratios (0.7034–0.7105) and negative εNd(t) values of –8.8 to –4.9. These high whole-rock (87Sr/86Sr)i ratios and relatively high Th/Nb and Th/Yb ratios indicate incorporation of melts derived from partial melting of subducted sediments into the melt evolution of these granitoids that involved partial melting of the subduction-metasomatized lithospheric mantle and the mafic- to intermediate-composition continental crust. The extant zircon crystallization ages of the granitoid intrusions in the CTP and ETP show eastward younging of their emplacement from 115 Ma to 93 Ma, suggesting an apparent eastward migration of the heat source through time. A diachronous collision of the NLT with the SQT during 145–120 Ma and the subsequent slab breakoff induced, eastward propagated slab tear and asthenospheric upwelling produced the hybrid melts of the Albian–Cenomanian granitoids and their emplacement in a discrete, narrow magmatic belt in the CTP and ETP.

Tectonics and sedimentology of accretionary and collisional orogens

The Eocene magmatic flare-up in the Northern Qiangtiang Block is a key event that was genetically related to the uplift of the Tibetan Plateau. This uplift has been considered to have an important impact on the global cooling and oceanic isotopic compositions. Here, we report zircon ages and bulk rock elemental and isotopic compositions for high-silica porphyritic granites in the Maliaoshan area of the Northern Qiangtang Block. Zircon U–Pb dating shows that the porphyritic granites were generated at ca. 41.02 Ma. The porphyritic granites are characterized by uniformly high SiO2 contents (74.3–75.0 wt%), high contents of total alkalis, metaluminous compositions, and low contents of TiO2, Fe2O3T, MgO, CaO, and P2O5. Moreover, the Maliaoshan porphyritic granites have Nd–Hf isotopic compositions similar to those of coeval metaluminous magmatic rocks in the Northern Qiangtang Block. These data together with geochemical modeling results suggest that the Maliaoshan porphyritic granites were formed by the fractional crystallization of 43.06% major minerals and 0.76% accessory minerals from the assumed starting composition (average Eocene metaluminous igneous rock). Based on our new data and Eocene magmatic flare-up event in the Northern Qiangtiang Block, we propose a new geodynamic model to explain such a magmatic flare-up event. The stress produced by the collision of India and Eurasia gradually transferred northward, prompting the Songpan–Garzê Block to subduct southward beneath the Northern Qiangtang Block at ca. 55 Ma, accompanied by minor magmatic activity. Subsequently, the break-off of the Songpan–Garzê Block at ca. 46 Ma resulted in the Eocene magmatic flare-up.