Active Continental Margin Setting Research Articles

Petrogenesis of Permian Granodiorite and Diorite in Eastern Jilin Province and Its Constraints on the Late-Stage Evolution of the Paleo-Asian Ocean

The Solonker-Xar Moron-Changchun-Yanji Suture Zone is the result of the final closure of the Paleo-Asian Ocean (PAO). However, the closure time of the PAO in Northeast China remains controversial. The Hunchun area is located in the easternmost part of the Solonker-Xar Moron-Changchun-Yanji Suture Zone. Tectonism and magmatism in the Hunchun area can provide important information for understanding the late-stage evolution of the PAO. In this study, our zircon U-Pb ages show that the granodiorites and diorites in the Hunchun were formed at 282.3–251.4 Ma. This geochronological evidence suggests prolonged Permian magmatism in the Hunchun area. Whole-rock geochemistry, zircon trace, and Lu-Hf isotope data show that all the intrusive rocks are mainly calc-alkaline series to arc tholeiite series. Granodiorites are I-type granites formed by the partial melting of juvenile lower crust derived from the mantle. Diorites show similar characteristics to the sanukitic high-Mg diorite and are formed by the partial melting of the depleted mantle metasomatized by subduction sediments and/or slab-derived fluids. These results indicate that the Permian diorites and granodiorites in the Hunchun area formed in an active continental margin setting related to the subduction of the PAO plate. Significantly, sudden changes in the whole-rock Sr/Y and (La/Yb)N ratios and zircon εHf(t) values are observed in the Late Permian-Early Triassic igneous rocks in the eastern Central Asian Orogenic Belt (CAOB). This indicates that the final closure of the PAO in Northeast China likely occurred in the Late Permian-Early Triassic.

Geochemistry of siltstones in the North Qilian Zhongbao–Mayinggou formations (Late Ordovician–Early Silurian): implications for paleoclimate, paleoweathering, and tectonic setting

The Late Ordovician–Early Silurian period witnessed the Phanerozoic mass extinction, glacial events, and volcanic events. Paleoweathering indexes chemical index of alteration (CIA), chemical index of weathering (CIW), and plagioclase index of alteration (PIA) indicated that the source area weathering changed from weak to moderate to intense. CIA values in the upper Zhongbao formation ranged from 66.71% to 73.97%, indicating a drier and colder climate. Upward, the CIA values on the bottom of the Mayinggou formation returned to the high value quickly (from 73.86% to 81.31%), suggesting that the ice age ended, the climate became warmer and wetter, and the sea level rose. The Al2O3–(CaO*+Na2O)–K2O triangular plots, Hf-La/Th, and SiO2–Al2O3/TiO2 bivariate plots inferred that the source of the siltstones in the two formations is mostly from the felsic igneous rocks. The samples from the Zhongbao–Mayinggou formations have chondrite-normalized rare earth element (REE) patterns similar to that of the North Qilian volcanic arc rocks. Geochemical discrimination plots displayed that the sediments of the Zhongbao–Mayinggou formations came from the active continental margin setting.

Detrital mode and geochemical constraints on the provenance, paleoweathering and tectonic setting of the Surma sediments of Chittagong-Tripura fold belt, Tripura, India

The Chittagong Tripura Fold Belt (CTFB), the folded eastern part of the Bengal Basin is critical for understanding sediments origin and tectonic evolution linked to interactions among the Indian, Eurasian, and Burma plates. The upper Bhuban and Bokabil sediments (Surma Group) from Atharamura anticline, Tripura, India were analyzed using whole-rock geochemistry and petrography to assess their provenance, tectonics, weathering, and depositional milieu in this least explored area. Petrographic study indicates sub-lithic arenite to sub-arkosic composition, with moderate-high quartz, feldspar poor containing metamorphic and sedimentary lithic fragments (Bh Q88.3F3.0L8.6; Bk Q83.5F7.6L8.8), indicating significant recycled orogen source. Geochemical characteristics such as Eu/Eu* (0.6, 0.7), La/Sc (3.2, 2.4), Th/Sc (1.7, 1.4), Th/Co (1.9, 1.8), CIA/WIP (∼1–2), high Hf, Zr/Sc, low K2O/Al2O3 and enriched LREEs, depleted HREEs, and negative Eu anomalies, indicate a recycled provenance of felsic origin that underwent moderate to intense chemical weathering in active continental margin settings. Radiolarian chert, volcanic lithic fragments with Barail-like recycled sediments strongly suggests an additional detritus from uplifted Indo-Burman Ranges from the east. Elevated Th/U and Th/Rb ratios, sediment immaturity and poor sorting suggest first-cycled detritus. This study indicate that Tripura Neogene sediments are chiefly Himalayan-derived, with minor inputs from IBR and Meghalaya Plateau. Sediments were deposited under brackish near shore to shallow marine oxic basins to the west of the uplifted IBR during the Miocene. The study offers insights into regional geological processes, facilitating correlation of foreland Neogene basins and enhances understanding of the geological evolution of the CTFB.

Origin and Geological Implications of Monzogranites and Rhyolitic Porphyries in the Wunugetu Porphyry Copper–Molybdenum Deposit, Northeast China: Evidence from Zircon U-Pb-Hf Isotopes and Whole-Rock Geochemistry

The Wunugetu deposit, a large-scale porphyry copper–molybdenum deposit, is located in the southern Erguna block. Its ore bodies are primarily found within monzogranites, granite porphyries, and biotite monzogranites. Additionally, the deposit contains late-stage intrusive dykes of rhyolitic porphyries. This study examined the deposit’s monzogranites and rhyolitic porphyries using lithogeochemistry, zircon U-Pb dating, and Hf isotopic analysis. The main findings include: (1) Zircon U-Pb dating showed that the monzogranites formed around 209.0 ± 1.0 Ma, whereas the rhyolitic porphyries in the northern portion formed around 170.49 ± 0.81 Ma, suggesting magmatic activity in the deposit spanned from the Late Triassic to the Middle Jurassic. (2) The monzogranites exhibited high silicon content (73.16–80.47 wt.%) and relatively low aluminum content (10.98–14.37 wt.%). They are enriched in alkalis (content: 3.42–10.10 wt.%) and deficient in magnesium and sodium, with aluminum saturation indices (A/CNK) ranging from 1.1 to 2.9. In addition, the monzogranites are enriched in large-ion lithophile elements (LILEs) such as Rb, K, and Ba and deficient in high-field-strength elements (HFSEs) like Nb, P, and Ti. (3) The monzogranites have low Zr + Nb + Ce + Y contents of (151.3–298.6 ppm) × 10−6 and 10,000 × Ga/Al ratios varying between 1.20 and 2.33, suggesting that they are characteristic of I-type granites. (4) Positive zircon εHf(t) values ranging from +0.3 to +7.6 in both rhyolitic porphyry and monzogranite samples, increasing with younger emplacement ages, imply that the deposit’s rocks originated from magmatic mixing between mantle-derived mafic magmas and remelts of the juvenile crust. Considering these results and the regional geological evolution, this study proposes that the Wunugetu deposit was formed in an active continental margin setting and was influenced by the Late Triassic–Middle Jurassic southeastward subduction of the Mongol-Okhotsk Ocean.

Petrography, geochemistry and detrital zircon U–Pb dating of the Pliocene‐Pleistocene Dupi Tila Formation from the Lalmai Anticline, Bengal Basin: Regional tectonic implications

The Bengal Basin is a well‐known foreland basin that archives the tectonic evolution of the Himalayan and Indo‐Burman Ranges (IBR). The Dupi Tila Formation is the youngest among the stratigraphic successions of the Bengal Basin, which witnessed the last phase of the Himalayan uplift. However, there is still controversy regarding sediment provenances since the Himalayan and IBR are in the late Neogene exhumation stage. In this study, the petrographical and geochemical compositions of the Dupi Tila sandstones from the Lalmai Anticline of the Bengal Basin are examined to infer the source rocks, tectonic settings, palaeoclimate and weathering intensity in provenance areas. The detrital modes and geochemical discriminations, as well as Chondrite‐normalized rare earth elements patterns with enrichment in light rare earth elements and nearly flat heavy rare earth elements and negative Eu anomalies (average Eu/Eu* = 0.77) indicate dominantly felsic source rocks. The geochemical data also suggest that these source rocks in provenance areas were influenced by weak to moderate chemical weathering (chemical index of alteration: 65.64–75.62) and multiple recycling under warm and humid to semi‐humid climatic conditions. The detrital zircon ages show three dominant peaks at ca. 100, 500 and 1000 Ma with minor 1700 Ma. The amount of young zircon grains (<120 Ma) is noticeably higher (~18%) in this Lalmai outcrop compared to the Dupi Tila samples from the Sylhet Trough and Chittagong‐Tripura Fold Belt. Results from integrated zircon geochronology and geochemistry indicate that the detritus are likely derived from a recycled orogen provenance related mainly to active continental margin setting with subordinate influence from passive setting. Therefore, the regionally extensive Himalayan Orogen appears to be the primary sediment source, with additional mixing from the IBR and recycled sediments originating from the Gangdese batholith of the Trans‐Himalaya.

LA-ICP-MS zircon U–Pb ages, geochemistry, and genesis of Paleozoic granites from Biezhentao Mountain, Western Tianshan, Xinjiang

The geologic history of the Western Tianshan region is important for understanding the evolution of the Central Asian Orogenic Belt and accretionary orogenesis. Paleozoic and Mesozoic igneous rocks are present along the northern margin of the Yili Block; however, studies of Paleozoic subduction and collisional events have been relatively limited. Published geochronologies of Middle Devonian magmatic rocks in this region are also lacking. Therefore, this study analyzed the zircon U–Pb ages and major and trace elemental compositions of three granite types collected from Biezhentao Mountain (Wenquan County, Western Tianshan). The medium-grained diorite (384.1 ± 3.6 Ma) and diorite–porphyrite (382 ± 3.2 Ma) are silica-rich, weakly peraluminous, alkali-rich, enriched in large-ion lithophile elements, depleted in high field strength elements, and belong to the calc–alkaline series, with A-type granite characteristics. The monzogranite (423.3 ± 9.4 Ma) exhibits A-type granite characteristics and belongs to the peraluminous calc–alkaline series. The findings suggest that the granitoids recorded two stages of tectonomagmatism during the Middle Devonian–Late Silurian. The Middle Devonian medium-grained diorite and diorite–porphyrite formed in a back-arc extensional setting, whereas the Late Silurian monzogranite formed in an active continental margin setting during the subduction of the North Tianshan Ocean. These results provide insights into the tectonic and magmatic processes that occurred during the evolution of the Western Tianshan region and the formation and evolution of the Central Asian Orogenic Belt.

Early Paleozoic tectonic affinity of the Lesser Xing'an–Zhangguangcai Range, NE China: A continental arc along the western margin of the Jiamusi Block

The Lesser Xing'an–Zhangguangcai Range (LXZGCR) in NE China was traditionally interpreted to be the eastern part of the Songliao–Xilinhot Accretionary Terrane, separated from the Jiamusi–Khanka Block to the east by the Heilongjiang Complex. However, recent studies have shown that the Heilongjiang Complex formed in the late Paleozoic–early Mesozoic. Therefore, the tectonic affinity of the LXZGCR during the early Paleozoic remains unclear. To constrain this key issue, we present new zircon UPb ages, Hf isotopic compositions, and whole-rock geochemical data for early Paleozoic igneous rocks in the LXZGCR. The new ages, along with previously published data, suggest that the early Paleozoic magmatism in the LXZGCR can be divided into four stages: Middle–Late Cambrian (523–486 Ma), Early Ordovician (485–469 Ma), Late Ordovician (464–443 Ma), and Late Silurian (433–423 Ma). Middle–Late Cambrian igneous rocks are dominated by calc-alkaline I-type granitoids and high-Mg andesite–dacite–rhyolite association. Early Ordovician igneous rocks consist of a suite of calc-alkaline diorites, granodiorites, monzogranites, alkali-feldspar granites, and rhyolites, with I-type affinities. Late Ordovician igneous rocks consist mainly of I-type granitoids with minor A-type granitoids and rhyolites. Late Silurian igneous rocks are composed of calc-alkaline I-type granitoids with minor dacites. The early Paleozoic granitoids and felsic volcanic rocks originated from partial melting of dominantly Paleoproterozoic–Mesoproterozoic lower crust, whereas the Late Cambrian high-Mg andesite–dacite–rhyolite association was probably derived from partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. The rock associations and geochemical characteristics suggest that these early Paleozoic igneous rocks formed in active continental margin and back-arc extension settings. Comparing the early Paleozoic magmatism, zircon Hf isotopic compositions, Nd model ages, and Precambrian basement features of the LXZGCR and the Jiamusi Block suggest that the LXZGCR was part of the same tectonic unit as the Jiamusi Block during the early Paleozoic. We argue that the formation of the early Paleozoic igneous rocks in the LXZGCR was related to the eastward subduction of a branch ocean of the Paleo-Asian Ocean beneath the Jiamusi–Khanka Block.

Petrogenesis of Jurassic granitic plutons in the Yanbian area, NE China: Implications for the subduction history of the Paleo-Pacific Plate

It is widely accepted that the subduction of the Paleo-Pacific Plate beneath the Eurasian continent began during the early Mesozoic; however, the details of the subduction process remain uncertain. This paper presents new zircon U–Pb–Hf and whole-rock geochemical data for Jurassic intrusive rocks from the Yanbian area of NE China to provide a better understanding of the subduction history of the Paleo-Pacific Plate. Zircon U–Pb dating results reveal that three stages of Jurassic magmatism are recorded in the Yanbian area, namely, Early Jurassic (188–179 Ma), Middle Jurassic (172–163 Ma), and Late Jurassic (158–156 Ma). Early Jurassic intrusive rocks from the Yanbian area are predominantly calc-alkaline gabbro, gabbro-diorite, quartz monzonite, granodiorite, and syenogranite. These geochemical features and rock associations are indicative of formation in an active continental margin setting related to the initial subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The Early Jurassic granitoids were formed mainly by the partial melting of heterogeneous lower crust. The Middle Jurassic granitoids are geochemically similar to adakites and have εHf(t) values of + 2.0 to + 10.1. These rocks are inferred to have been derived from magmas generated by the partial melting of thickened juvenile lower-crustal material. Their formation was related to the subduction of the Paleo-Pacific Plate. Late Jurassic adakites in the Yanbian area formed in a compressional tectonic setting. Their negative εHf(t) values (−17.2 to − 8.6) and TDM2 ages (2025–1552 Ma) indicate that the primary magma was derived from the partial melting of ancient continental crust. Combining our data with the distribution of Late Jurassic igneous rocks in the NE Asian continental margin and the northward migration of Jurassic accretionary complexes along the margin, we conclude that the subduction of the Paleo-Pacific Plate beneath the Eurasian continent during the Late Jurassic–earliest Early Cretaceous had a low angle of obliquity.

Tectonic setting and provenance analysis of the Yaojia Formation in the northeastern Songliao Basin, NE China: constraints from sandstone geochemistry, Hf isotopes, and zircon U–Pb chronology

The Songliao Basin is rich in uranium ores, and the Yaojia Formation, which is dominated by gray fine-grained sandstones, contains the main ore-bearing stratum. Rocks in the formation contain high SiO2, Al2O3, and total alkali, with enrichment in Rb, Th, U, K, and light rare earth elements but are depleted in high field strength elements, similar to upper crustal rocks. U–Pb dating of zircon grains from sandstones in the formation yielded four groups of ages, including 99–182, 202–245, 284–365, and 1800–1900 Ma. These ages combined with the Hf isotope composition, geochemical characteristics, and regional history suggest that the Yaojia Formation rocks are associated with passive and active continental margin settings. These clastic rocks originate principally from felsic rocks in the Zhangguangcailing–Xiao Xing'anling area, and these were deposited in oxic freshwater environments.

Age and Tectonic Setting of Layered Lead–Zinc Ore Bodies in the Xiaohongshilazi Deposit: Constraints from Geochronology and Geochemistry of the Volcanic Rocks in Central Jilin Province, NE China

The newly discovered Xiaohongshilazi deposit located in Panshi City, central Jilin Province, NE China, is a medium-scale Pb–Zn–(Ag) deposit. The Pb–Zn–(Ag) orebodies are divided into layered and vein-type orebodies, which have different ore geneses. The layered Pb–Zn orebodies are mainly hosted within and spatially controlled by the volcanic rocks. To constrain the age and tectonic setting of the layered Pb–Zn mineralization, we completed laser-ablation–ICP–MS zircon U–Pb dating and whole-rock major and trace element analyses of the ore-bearing volcanic rocks. The dacite samples were confirmed as belonging to the Daheshen Formation and were the main ore-bearing volcanic rocks for the layered orebodies. They yielded concordia U–Pb ages of 278.1 ± 1.8 Ma and 278.3 ± 1.8 Ma, respectively, indicating that the volcanic rocks from the Daheshen Formation and related layered Pb–Zn mineralization were formed in the early Permian. The andesite and rhyolite located above the layered orebodies yielded concordia U–Pb ages of 225.0 ± 1.1 Ma, 225.3 ± 1.5 Ma, and 224.7 ± 1.2 Ma, respectively; these substances are considered to be of the Sihetun Formation and were first reported in the area. The dacite samples associated with layered Pb–Zn mineralization were high in SiO2 (62.54–65.02 wt.%), enriched in LREEs and LILEs (e.g., Rb, Ba, and K), and showed depletion in HFSEs (e.g., P and Ti). It showed slightly negative Eu anomalies (δEu = 0.60–0.65) and negative Nb anomalies, with Th/Nb (1.12–1.21) and La/Nb (2.8–4.7) ratios, presenting subduction-related arc magma affinity formed in an active continental margin setting. In agreement with previous studies on zircon Hf isotopes (εHf (t) = +0.23~ +10.60) of the volcanic rocks from the Daheshen Formation, we infer that they were derived from the partial melting of the depleted lower crust. In conclusion, mineralization characteristics, geochronological data, geochemical features, and regional tectonic evolution suggest that two Pb–Zn–(Ag) mineralization stages from the Xiaohongshilazi deposit occurred: the layered VMS-type Pb–Zn mineralization associated with the marine volcanic rocks from the early Permian Daheshen Formation, which was induced by the subduction of the Paleo-Asian oceanic plate beneath the northern margin of the North China Craton, and the vein-type Pb–Zn–(Ag) mineralization caused by the subduction of the Paleo-Pacific Plate in the early Jurassic. Considering this, along with the mineralization characteristics of the same-type polymetallic deposits in this region, we propose that the early Permian marine volcanic rocks have great prospecting potential for the VMS-type Pb–Zn polymetallic deposits.

Geochemistry of Jutogh Metasediments, Lesser Himachal Himalaya, India, and their Implications in Source Area Weathering, Provenance, and Tectonic Setting during Paleoproterozoic Nuna Assembly

Abstract This paper reports the results of the geochemical study of the Jutogh metasedimentary rocks that occur as a tectonic window between the Lesser and the Higher Himalaya. The Jutogh Group of rocks are mostly mica-schists of different metamorphic grade. The weathering intensity parameters, such as chemical index of alteration (CIA), plagioclase index of alteration (PIA), and index of compositional variability (ICV), range from 81 to 65 (mean = 72), 97 to 68 (mean = 84), and 1.9 to 0.6 (mean = 1.2), respectively indicating low to moderate degrees of weathering. Transition element ratios [Ni/Co (6.98 to 2.88), and V/Ni (2.32 to 1.27)], and major and trace element geochemistry imply recycled, felsic to intermediate, Archean to post-Archean are the sources for Jutogh rocks. The tectonic discrimination diagram implies an active continental margin setting for the deposition of the Jutogh metasediments. The geochemical data from the Jutogh rocks, when combined with the S-type Paleoproterozoic granite magmatism, imply the existence of an active tectonic setting during the deposition of the Jutogh sediments at the northern margin of the Indian continent during Nuna assembly.

Palaeoweathering Conditions, Provenance, and Tectonic Setting of the Early Silurian Longmaxi Formation in the Upper Yangtze Region (Southern China): Evidence from Geochemistry

The Longmaxi-1 black shales in the Upper Yangtze region are a vital source rocks in southern China. This study investigates the palaeoweathering conditions, provenance, and tectonic setting of the Longmaxi-1 black shale from an elemental geochemical perspective, ultimately revealing its tectonic setting. The results showed that the Longmaxi-1 black shales in the sedimentary period have the characteristics of primary deposition. The Longmaxi-1 black shales were deposited as a primary deposition under a mainly humid climate. However, fluctuations in climatic conditions were recorded from the bottom to the upper part of the formation. The parent rock of the Longmaxi-1 black shales in the Upper Yangtze region is a mixture provenance, mainly composed of intermediate-acid volcanic lithologies (granite and granodiorite), followed by mature quartzite and basalt. Black shale deposition is related to the tectonic setting of active continental margin and island arc-continent collision. The Cathayian orogenic belts and the North Qiangling orogenic belt may have played a role in the genesis of the Longmaxi-1 black shales within the Upper Yangtze region. This study provides significant clues regarding the reconstruction of the palaeoclimatic and palaeogeographical conditions of the Upper Yangtze region during the Early Silurian period.

Palaeoenvironment of the Albian‐Cenomanian Mundeck Formation in the Douala Basin (SW, Cameroon): Evidence from facies analysis and geochemistry

The Lower Cretaceous syn‐rift deposits of the Douala Basin (Mundeck Formation), located in the equatorial West African margin were investigated in order to determine the palaeoenvironment and organic matter source, redox conditions, palaeoclimate, and tectonic settings, based on facies, organic and inorganic geochemical analyses. The sediments consist of pebble, coarse, medium‐ to fine‐grained sandstone, siltstone, and grey to black fossiliferous shales. The shales are characterized by an average total organic carbon (TOC) of 1.02%, fair to poor source generative potential with present‐day hydrogen index values, which range from 31 to 350 mg HC/g TOC suggesting that the shale contains Type III kerogens with mainly gas generative potential. The organic particles are dominated by phytoclasts (40% on average) and non‐fluorescent amorphous organic matter (34% on average). The palynomorphs represent about 26% on average with a large amount of pollen grain and a minor amount of dinoflagellate cysts and microforaminifera test linings. The inferred depositional environment of the studied sediments is an alluvial fan to fluvio‐lacustrine with shallow marine incursion. This depositional environment received the contribution of aquatic algae and microorganisms with a large amount of terrestrial organic matter, evidenced by the presence of some dinoflagellate cysts and microforaminifera test linings, and relatively higher amounts of SiO2, Al2O3, and phytoclasts. The presence of trace elements such as Ba, Sr, V, Ni, Co and Cr in the analysed samples and their ratio, suggest suboxic to anoxic conditions of deposition with low salinity. The warm to cold and dry climatic conditions prevailed during the deposition of the Mundeck deposits and sediments from active continental margin setting during the lower Cretaceous periods in the Douala Basin as confirmed by inorganic geochemical data. This study sheds light on the early stage of the evolution of the Douala Basin which could be integrated into the context of a mid‐Jurassic to lower Cretaceous regional evolution of the opening of a passive margin. This evolution is characterized by an alluvial fan, fluviatile, lacustrine, lagoon and marine depositional system. This depositional system is recognized as part of the petroleum system recognized within the syn‐rift deposits in the others basins along the West African margin, where giant hydrocarbons have been discovered.

Late Paleozoic and early Mesozoic granitoids in the northwestern Bureya Massif, Central Asian Orogenic Belt: Timing and tectonic significance

ABSTRACT The Bureya Massif is an important massif in the eastern Central Asian Orogenic Belt (CAOB), and contains voluminous late Palaeozoic–early Mesozoic granitoids that are ideal for investigating the tectonic evolution of the eastern CAOB. Here we report whole-rock geochemical, and zircon U–Pb geochronological and Lu–Hf isotope data for several granite intrusions in the northwestern Bureya Massif, with the aim of constraining the timing of magmatism and late Palaeozoic–early Mesozoic tectonism in the eastern CAOB. The zircon U–Pb dating results indicate that granitoids with ages of 306 ± 4, 243 ± 3, 191 ± 2, and 189 ± 4 Ma occur in the Bureya Massif. These, and previously published data, reveal that at least four stages of magmatism occurred in the northwestern part of the massif: ~306 Ma, ~243 Ma, 219–212 Ma, and ~201-189 Ma. The late Carboniferous (306 ± 4 Ma) gneissic monzogranites have an S and A-type granite affinity, and were derived with significant contribution of the mantle component in a syn- to post-collisional setting between the Bureya and Xing’an massifs. The Middle Triassic (243 ± 3 Ma) porphyritic I-type monzogranites, Early Jurassic (191 ± 2 Ma) porphyritic S-type syenogranites, and Early Jurassic (189 ± 4 Ma) gneissic S-type monzogranites have a supra-subduction zone geochemical signature. These granitoids formed in an active continental margin setting associated with southward subduction of the Mongol–Okhotsk oceanic plate.

Late Neoarchean plate subduction in Western North China Craton: Evidence from ca. 2.51 Ga to 2.46 Ga basement rocks in Northern Ordos Basin

The most poorly exposed and least understood in the North China Craton (NCC) lies largely hidden beneath the Ordos Basin, severely limiting the cognition of the Precambrian tectonic evolution in the western NCC and the tectonic division. Diverse orthogneiss samples collected from four boreholes drilled into the basement of the north side of Hangjinqi Fault Zone (HFZ) in the North Ordos Basin (NOB) are investigated by integrating zircon U-Pb dating, Hf-Nd isotopes, and whole-rock geochemical analyses. The TTG gneiss, quartz-monzonite gneiss, granodiorite gneiss, and amphibolite were emplaced at ca. 2.51–2.46 Ga, of which the amphibolite also recorded the metamorphic age of ca. 1.95 Ga. The TTG gneisses display low MgO, Mg#, Cr, and Ni, high HREE and Rb/Sr, enrichment of LILEs and depletion of Nb, Ta, and Ti, and positive εNd(t) (2.04–4.06) and εHf(t) (3.04–7.59) values, suggesting that they were mainly originated from partial melting of the tectonically thickened mafic island-arc crust. In contrast, the quartz monzonite gneisses and granodiorite gneisses show the geochemical features of Archean sanukitoids with higher MgO, Mg#, Cr, and Ni, and similar εNd(t) (1.34–3.82) and εHf(t) values (3.94–7.32), suggesting that they were derived from the partial melting of the mantle metasomatized by fluids and melts of the subduction. The amphibolites exhibit a mixed characteristic of E-MORB and arc-like geochemical affinities resembling the Mariana back-arc basin, which was derived from the partial melting of spinel lherzolite. The arc and arc-back lithological assemblages suggest that the north margin of the NOB might be in a geodynamic transition period from a compressional regime correlated with plate subduction to an extensional regime correlated with slab rollback during ca. 2.51–2.46 Ga. Combined with previous analytical results, the northern margin of the NOB was most likely in a long-term active continental margin setting during 2.51–1.95 Ga and involved in the collision event between the Yinshan Block and Ordos Block at ca. 1.95 Ga.

Geology, mineralization and calcite‐rich potassic alteration at the Humpa Leu East (HLE) porphyry Cu‐Au prospect, Hu'u district, Sumbawa Island, Indonesia

AbstractThe Humpa Leu East (HLE) prospect is one of the newly discovered porphyry Cu‐Au prospects in the Hu'u district, Sumbawa Island, Indonesia. The HLE prospect was formed by calc‐alkaline magmatic activity in an active continental margin setting. The prospect is typical calc‐alkaline porphyry Cu‐Au mineralization related to multiphase diorite and quartz diorite porphyry intrusions, which are hosted by andesitic crystalline tuff, volcanic breccia, and andesite lava. Hydrothermal alteration recognized at the surface includes potassic, propylitic, advanced argillic, intermediate argillic, and argillic alteration. Two styles of Cu‐Au mineralization were identified in the HLE prospect, that is, quartz‐sulfide veins and sulfide dissemination, formed in the early, intermediate, and late stages. The early stage is associated with M (magnetite ± bornite ± chalcopyrite), A (quartz + magnetite), and AB (quartz + magnetite + chalcopyrite ± pyrite) veins. These veins were mainly formed in the potassic alteration zone. The intermediate stage is characterized by B (quartz + chalcopyrite + pyrite) and C (chalcopyrite ± pyrite) veins and mainly associated with the chlorite‐sericite and sericite alteration zones. The late stage is mainly associated with D (calcite + gypsum + quartz + pyrite ± chalcopyrite ± sphalerite ± galena) veins with sericite‐chlorite alteration halo. Petrography and electron microprobe analyses indicate that calcite mainly replaced Ca‐rich plagioclase. Fluid inclusion petrography and Raman spectroscopy revealed that monophase vapor inclusions, as well as two‐phase (V + L) and multiphase (V + S + L) fluid inclusions contain CO2 gas. The potassic alteration with significant amounts of calcite is indicative of CO2‐rich fluids, which is uncommon in other porphyry Cu‐Au deposits. On the basis of textural and fluid inclusion analyses, calcite formed by a reaction between Ca‐rich plagioclase and CO2‐rich hydrothermal fluids. The CO2 in the hydrothermal fluids of the HLE prospect was likely derived from the magma.

Tectonic setting, provenance, depositional, and paleo-climatic conditions of the late quaternary subcrop sediments of the southeastern coastal region of the Bengal basin

This is a systematic attempt to depict the genetic evolution of the Late Quaternary sediments of the southeastern (SE) coastal region of the Bengal basin regarding paleotectonic settings, sedimentation, provenance, paleo-climatic conditions, weathering condition and age. The study has considered multiple attributes such as, lithology/lithofacies, sedimentary features/records, major oxides, clay minerals, foraminifera, and radiocarbon dating. The lithological characters along with associated clay minerals confirmed that a Pleistocene paleosol horizon (over-bank deposits) of warm-humid nature is commonly encountered immediately on top of the sub-crop bed-rock in the area overlain by Holocene fluvio-marine sediments of the same nature. The lithofacies, foraminiferal assemblages, and sedimentary structures of the analyzed samples suggest that the Holocene sediments have been presumably deposited in a fluvio-marine condition after the Last Glacial Maximum (LGM) due to the transgression of the sea. Geochemically, the sediments are classified as Fe-rich shale, shale, and wake and primarily intermediate to felsic orogen provenance. These are possibly derived from intense weathered sources from the upheaval of Himalayan ranges of both active continental margin and Island Arc paleotectonic setting. The plot of the Index of Compositional Variability versus the Chemical Index of Alteration indicates that the sediments seemingly experienced intense weathering associated with warm and humid climatic conditions. The sedimentation rates of the area vary from place to place and layer to layer due to the complex delta-building process. The reconstructed Relative Sea Level Curve reveals that presumably, the sea level has reached its current position after the LGM. The deduction possibly will facilitate the (1) reconstruction of Late Quaternary coastal evolution after LGM, (2) support for future urbanization, land use plans, etc., and (3) also be helpful for international researchers to understand the possible sources of sediment input in the area from the complex interplay of the Indian-, Eurasian- and Myanmar-plates.

Новые данные о составе пермских терригенных отложений Омолонского массива (Северо-Восток России): значение для палеогеографии и геодинамических реконструкций

The article discusses the results of studying the material composition of Permian terrigenous deposits in the Dzhigdali, Gizhiga, and Khivach formations in the southeastern part of the Omolon massif. The obtained data interpretation is based on the petrographic research and widely used petrochemical and discriminant paleogeodynamic diagrams. The lithochemical study showed that the deposits had accumulated under conditions of a moderately warm climate with some cooling episodes in the active continental margin settings. Clastic material entered the sedimentary basin from several provenance areas: the Permian Okhotsk-Taigonos volcanic arc, the Precambrian basement of the Omolon massif, and volcanic rock fields of the Kedon series.

The role of geochemical and coal petrographic analyses to determine the depositional sets of Visean coal and shale sequence in Um Thora-Abu Hish area, Central Sinai, Egypt

The Visean time at the Sinai Peninsula is characterized by the deposition of two conformable formations of a siliciclastic (Abu-Thora) resting upon a calcareous (Um-Bogma). The Abu-Thora Formation (Fm) is informally divided into three rock units that comprise a lower and upper sandstone sandwiched with a middle coal and shale sequence. A combination of paleoenvironmental indices and ratios from petrographic and geochemical analyses of the middle sequence succeed to build up the depositional paleoenvironment and may apply to any other coal and shale sequence, especially in coal mine regions. Geochemical analysis reveals that the studied shale strata are not homogenous and can be subdivided into four units from the base to the top. The lower shale unit at the base is 4m in thickness and composed of kaolinite-rich shale (10–77%; nacrite mineral). The second is the middle shale unit of 5.10m in thickness composed of Fe-shale (Fe2O3; 2.04–16.45 wt%). The third is the coal unit of 0.55m in thickness comprising coal seam, and the fourth is the upper shale unit of 6m in thickness composed of wacke (SiO2; 71.63–72.83 wt%). X-ray diffraction (XRD), x-ray fluorescence (XRF) plus mineralogical analyses are applied on the first, second, and fourth units of almost pure shale, whilst the third unit of almost pure coal is subjected to a petrographic analysis instead to illustrate the depositional site and type of vegetation where coal was formed. The palaeoclimatology based on parameters of CIW, C-value, Rb/Sr, and Sr/Cu refers to the deposition of shale initiated at a warm-humid and then to a hot semi-arid climate. The precursor rock of the studied shale is a basaltic and intermediate igneous rock. The shale is of mature components subjected to intense weathering processes based on CIA and ICV of 91.71 and 0.34, respectively. Furthermore, discriminant functions F-1 and F-2 reveal the shale components came also from quartzose sediments that promoted a multi-source origin that was gathered from both preexisting mafic and sedimentary provenances. The accumulation and deposition took place at a marine anoxic shale-carbonate of the active continental margin and oceanic island arc tectonic setting. Maceral composition of the coal unit reveals a depositional setting of a coal-forming mire system that occurred at a paralic aquatic margin setting involving a dominated reed herbaceous vegetation in a hydrological water-logged of the limnic marsh.

Zircon U–Pb Age and Geochemistry of Ore-Hosting Rocks from the Liuhe Orefield of the Jiapigou Gold Ore Belt, NE China: Magmatism and Tectonic Implications

Liuhe gold orefield is being newly explored in the southeast part of the Jiapigou gold ore belt, and occurs in the Neoarchean basement composed of trondhjemite–tonalite–granodiorite (TTG). Zircon U–Pb data suggest that the ore-hosting magma emplacement in the Liuhe orefield mainly took place in two epochs: late Neoarchean to early Paleoproterozoic (ca. 2500 Ma) and early Jurassic of the Mesozoic era (ca. 170 Ma). The TTG rocks show higher A12O3 (12.58 to 15.71%) and Na2O/K2O ratios (1.16 to 2.9), and lower MgO (0.93 to 2.73%) and Mg# values, with positive Eu anomaly and low Y and Yb content, and high Sr/Y (22.3–79.6), and the plot in the adakite field in the Sr/Y-Y discriminant diagram belongs to the modern island-arc adakite rocks. Samples in this study are plotted in the pre-plate collision area in the R1-R2 discrimination diagram, and fall into the VAG and VAG + Syn-COLG field in the Rb-Y + Nb and Nb-Y diagram, respectively, indicating that the magmatism is related to plate subduction. The ore-bearing TTGs of the late Neoarchean to early Paleoproterozoic deposits were derived from the partial melting of mafic lower crustal caused by the underplating of basaltic magma on the island-arc or active continental margin before plate collision. The magmatism of the Dajiagou deposit occurred in active continental margin setting associated with the westward subduction of the paleo-Pacific plate beneath Eurasian Plate during the early Jurassic of Mesozoic period.