Regional Tectonic Evolution Research Articles

Miocene to present oceanographic variability in the Scotia Sea and Antarctic ice sheets dynamics: Insight from revised seismic-stratigraphy following IODP Expedition 382

Scotia Sea and the Drake Passage is key towards understanding the development of modern oceanic circulation patterns and their implications for ice sheet growth and decay. The sedimentary record of the southern Scotia Sea basins documents the regional tectonic, oceanographic and climatic evolution since the Eocene. However, a lack of accurate age estimations has prevented the calibration of the reconstructed history. The upper sedimentary record of the Scotia Sea was scientifically drilled for the first time in 2019 during International Ocean Discovery Program (IODP) Expedition 382, recovering sediments down to ∼643 and 676 m below sea floor in the Dove and Pirie basins respectively. Here, we report newly acquired high resolution physical properties data and the first accurate age constraints for the seismic sequences of the upper sedimentary record of the Scotia Sea to the late Miocene. The drilled record contains four basin-wide reflectors – Reflector-c, -b, -a and -a' previously estimated to be ∼12.6 Ma, ∼6.4 Ma, ∼3.8 Ma and ∼2.6 Ma, respectively. By extrapolating our new Scotia Sea age model to previous morpho-structural and seismic-stratigraphic analyses of the wider region we found, however, that the four discontinuities drilled are much younger than previously thought. Reflector-c actually formed before 8.4 Ma, Reflector-b at ∼4.5/3.7 Ma, Reflector-a at ∼1.7 Ma, and Reflector-a' at ∼0.4 Ma. Our updated age model of these discontinuities has major implications for their correlation with regional tectonic, oceanographic and cryospheric events. According to our results, the outflow of Antarctic Bottom Water to northern latitudes controlled the Antarctic Circumpolar Current flow from late Miocene. Subsequent variability of the Antarctic ice sheets has influenced the oceanic circulation pattern linked to major global climatic changes during early Pliocene, Mid-Pleistocene and the Marine Isotope Stage 11.

Petrogenetic Constraints of Early Cenozoic Mafic Rocks in the Southwest Songliao Basin, NE China: Implications for the Genesis of Sandstone-Hosted Qianjiadian Uranium Deposits

The tectonic inversion of the Songliao Basin during the Cenozoic may have played an important role in controlling the development of sandstone-type uranium deposits. The widely distributed mafic intrusions in the host sandstones of the Qianjiadian U ore deposits provided new insights to constrain the regional tectonic evolution and the genesis of the U mineralization. In this study, zircon U-Pb dating, whole-rock geochemistry, Sr-Nd-Pb isotope analysis, and mineral chemical compositions were presented for the mafic rocks from the Qianjiadian area. The mafic rocks display low SiO2 (44.91–52.05 wt.%), high TFe2O3 contents (9.97–16.46 wt.%), variable MgO (4.59–15.87 wt.%), and moderate K2O + Na2O (3.19–6.52 wt.%), and can be subdivided into AB group (including basanites and alkali olivine basaltic rocks) and TB group (mainly tholeiitic basaltic rocks). They are characterized by homogenous isotopic compositions (εNd (t) = 3.47–5.89 and 87Sr/86Sr = 0.7032–0.7042) and relatively high radiogenic 206Pb/204Pb (18.13–18.34) and Nb/U ratios (23.0–45.6), similar to the nearby Shuangliao basalts, suggesting a common asthenospheric origin enriched with slab-derived components prior to melting. Zircon U-Pb and previous Ar-Ar dating show that the AB group formed earlier (51–47 Ma) than the TB group (42–40 Ma). Compared to the TB group, the AB group has higher TiO2, Na2O, K2O, P2O5, Ce, and HREE contents and Ta/Yb and Sr/Yb ratios, which may have resulted from variable depth of partial melting in association with lithospheric thinning. Combined with previous research, the Songliao Basin experienced: (1) Eocene (~50–40 Ma) lithospheric thinning and crustal extension during which mafic rocks intruded into the host sandstones of the Qianjiadian deposit, (2) a tectonic inversion from extension to tectonic uplift attributed to the subduction of the Pacific Plate occurring at ~40 Ma, and (3) Oligo–Miocene (~40–10 Ma) tectonic uplift, which is temporally associated with U mineralization. Finally, the close spatial relation between mafic intrusions and the U mineralization, dike-related secondary reduction, and secondary oxidation of the mafic rocks in the Qianjiadian area suggest that Eocene mafic rocks and their alteration halo in the Songliao Basin may have played a role as a reducing barrier for the U mineralization.

Origin of the giant Luziyuan Zn-Pb-Fe(-Cu) distal skarn deposit, Baoshan block, SE Tibet: Constraints from Pb–Sr isotopes, calcite C–O isotopes, trace elements and Sm–Nd dating

Abstract The giant Luziyuan Zn-Pb-Fe(-Cu) deposit is located in the southern Baoshan block of SE Tibet. The deposit hosts over 4.2 million tonnes (Mt) of contained Zn + Pb metal and 301 Mt Fe ore reserve. The stratiform-/vein-type orebodies are hosted along the interlayer fractures in the Upper Cambrian Shahechang Formation marble and skarn. Three alteration/mineralization stages have been identified: (I) pre-ore, (II) syn-ore (oxide-ore and sulfide-ore), and (III) post-ore. An increasing Fe(-Cu) and decreasing Zn-Pb trend is present from the shallow-level SW to deep-level NE Luziyuan. Lead isotope compositions are similar among all Luziyuan ore sulfide types, and show positive correlations plotted between the regional Early Cretaceous granites and local wallrocks in both uranogenic and thorogenic diagrams, suggesting a mixed Pb source between intermediate-felsic magmatic fluids and sedimentary wallrocks. This is similar to many coeval northern Baoshan skarn deposits, e.g., Jinchanghe and Hetaoping. Initial 87Sr/86Sr ratios of the pre-ore (0.715919–0.720102), syn-ore (0.713502–0.719259) and post-ore (0.714091–0.720304) minerals lie between those of coeval granitoids and skarn deposits in the region and the average continental crust. This further supports a mixed ore-forming material source, and is suggestive of distal-skarn mineralization type. Syn-ore calcite samples yielded an Early Cretaceous Sm–Nd isochron age (130 ± 15 Ma). Regional tectonic evolution suggests that the Luziyuan mineralization was likely formed under post-collisional extension after the Lhasa-Tengchong accretion onto the Eurasian margin along Baoshan-Sibumasu. Such an extension had most likely induced crustal anatexis and the formation of the ore-causative S-type granites.

The Origin Analysis of “reversal foreset” of Late Carboniferous MKT Formation in Central Block of Pre-caspian Basin

The configuration of “reversal foreset”, which is a common geologic sedimentary phenomenon in seismic exploration, has been fully understood on its mechanism. In central block of pre-caspian Basin, a series of NE “reversal foreset” configurations were found. This kind of reflection is completely opposite to the paleogeomorphology which is high in the west and low in the east. It is difficult to clarify the origin mechanism of the “reversal foreset” configuration without sufficient data and understanding of regional tectonic evolution. Interpretations provided by previous studies did not reach a consensus understanding of the tectonic and sedimentary structures in the area. In this paper, based on well and regional data, the previous interpretations are analyzed, and the idea is put forward that the “reversal foreset” configuration represents superimposed retrogradation bodies developed in the process of rapid water flooding, and its formation is controlled by three factors -- “weak supply of single source”, “rapid transgression (weak source supply)” and “low and gentle depositional slope”. This recognition helps understanding the sedimentary evolution characteristics of the Late Carboniferous MKT formation in central block of Pre-caspian basin and promotes the later exploration of lithologic reservoir.

The granite porphyry hidden in the Shuangjianzishan deposit, southern Great Xing’an Range, NE China: geochronology, isotope geochemistry and tectonic implications

AbstractThe Shuangjianzishan vein-type Ag-Pb-Zn deposit in the southern Great Xing’an Range (GXR), NE China, is hosted in the slate of the Lower Permian Dashizhai Formation intruded by granite porphyry. In this paper, U–Pb zircon ages and bulk-rock and isotope (Sr, Nd, Pb and Hf) compositions are reported to investigate the derivation, evolution and geodynamic setting of this granite porphyry. It is closely associated with Pb-Zn-Ag mineralization in the southern GXR and contains important geological information relating to regional tectonic evolution. Laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) zircon U–Pb dating yields an emplacement age of 131 ± 1 Ma for the granite porphyry. Bulk-rock analyses show that the Shuangjianzishan granite porphyry is characterized by high Si, Na and K contents but low Mg and Fe contents, and that the enrichment of Zr, Y and Ga suggests an A-type granite affinity. Most of the studied samples have relatively low 87Sr/86Sr values (0.70549–0.70558), with positive ϵNd(t) (0.71–0.88) and ϵHf(t) (4.9–6.9) values. The Sr–Nd isotope modelling results, in combination with the young TDM2 ages of Nd and Hf (850–864 and 668–778 Ma, respectively), reveal that the Shuangjianzishan granite porphyry may be derived from the melting of mantle-derived juvenile component, with minor lower crustal components; this finding is also supported by Pb isotopic compositions. Considering the widespread presence of granitoids with coeval volcanic rocks and regional geology data, we propose that the Shuangjianzishan granite porphyry formed in a post-orogenic extensional environment related to the upwelling of asthenospheric mantle following the closure of the Mongol–Okhotsk Ocean.

The Sedimentary Evolution During Late Triassic‐Jurassic Period and the Hydrocarbon Exploration Direction in the Eastern Part of the Qiangtang Basin, Tibet

AbstractBased on the field outcrops surveyed, combined with recent published the regional tectonic evolution and geochronology data, we analyzed the lithologies and rock associations of strata, identified the sedimentary facies types, and discussed the distribution sedimentary facies and the hydrocarbon accumulation in the eastern Qiangtang basin during the Late Triassic – Jurassic. Marked by regional unconformities, there are two tectono‐stratigraphic units (from the Carnian to the Norian and from the Rhaetian to the Kimmeridgian, respectively) in the eastern part of Qiangtang basin. We systematically described the distribution range, thickness variation and lithological characteristics of different formations in the tectono‐stratigraphic units. The Late Triassic‐Jurassic is dominated by marine facies and marine ‐ continental transitional facies. The marine ‐ continental transitional facies include deltaic and tidal‐lagoon facies. Marine facies including gentle carbonate slope, evaporative platform, restricted platform, littoral, neritic, bathyal and abysmal facies. The Carnian stage is dominated by littoral–neritic–bathyal–abysmal facies in the north Qiangtang depression otherwise the littoral–neritic facies in the south Qiangtang depression. The early Norian stage is dominated by carbonate gentle slope‐mixed continental shelf facies. The late Norian, Bajocian, Callovian and Kimmeridgian stage are dominated by tidal flat‐delta facies in the north Qiangtang depression and littoral‐neritic facies in the south Qiangtang depression. The Bathonian and Oxfordian stage are dominated by evaporative platform‐ restricted platform‐ mixed continental shelf facies. The sedimentary facies formed zones from north to south and extended in an E–W direction. The Eastern Lower Uplift (ELU) played an important role in the division zones of sedimentary facies from north to south. During the Bathonian and Oxfordian, the ELU developed below the sea level and controlled the distribution of restricted platform, evaporative platform and platform margin. We analyzed 20 source rock samples from the upper Triassic‐Jurassic. The total organic carbon (TOC) value from Qoimaco, Buqu and Adula Formations. of late Triassic‐Jurassic in the eastern Qiangtang basin are ranges from 0.17∼0.33% (average 0.28%), 0.05∼0.25% (average 0.15%) and 10.32∼28.78% (average 19.33%), respectively. Obviously, the Adula Fm. developed good source rocks. The values of Tmax and S1+S2 in the Adula formation are 459–461°C (average 460°C) and 6.75‐28.55 mg/g (average 18.18mg/g), indicating that the Adula source rock has reached high‐over‐maturity stage. The bathyal, gentle slop and platform facie belts of the Upper Triassic can configurate the good hydrocarbon prospects in the northeastern area of the Qiangtang basin.

Using element geochemistry and detrital zircon geochronology to constrain provenance of the Eocene sediments in the deep-water area of the Pearl River Mouth Basin, South China Sea

• The Eocene parent rocks formed in an island arc-active continental margin setting. • These sediments were mainly derived constantly from local topographic highs. • The Panyu Low Uplift began to receive sediments of South China since the late Eocene. As the largest sag in the Zhu II Depression within the Pearl River Mouth Basin, the Baiyun Sag (BYS) lies in an area of interaction among the Eurasian Plate, Pacific Plate, and Indian Plate. The BYS mainly experienced the Paleogene rift stage and Neogene–Quaternary post-rift stage. Thick syn -rift sediments in the BYS have recorded the nature of provenance and provide an excellent record of regional tectonic evolution prior to the opening of the South China Sea. In this study, elemental geochemical analysis combined with published zircon detrital U-Pb dating results are applied to better understand the provenance change and tectonic setting of the BYS during the Eocene. In this respect, the chondrite normalized REE patterns of Eocene samples from six wells are highly similar to upper continental crust (UCC) and post-Archean Australian shale (PAAS), particularly with respect to LREE enrichment, significantly negative Eu anomalies, and slight Ce anomalies. Geochemical results confirm that the BYS Eocene sediments were derived from a continental island arc to an active continental margin. In addition, the rock types of sources and the relatively narrow detrital zircon ages show that Mesozoic intermediate-felsic igneous and metamorphic rocks derived from local topographic highs contributed to infilling most of the BYS. Furthermore, the results show that the Panyu Low Uplift began to accept large sediments transported by the paleo-Pearl River from the South China coast during the active syn -rift stage, prior to the opening of the South China Sea.

Geodynamic setting of the south‐east margin of Xing'an–Mongolian Orogenic Belt: Constraints from geochronology and geochemistry of the Permian volcanic rocks in Yanbian area, NE China

The south‐east margin of the Xing'an–Mongolian Orogenic Belt (SM‐XMOB) is a tectonic junction that is characterized by widespread Late Palaeozoic to Early Mesozoic igneous rocks. To determine the tectonic settings and provide new constraints on the regional tectonic evolution, petrological, geochronological, geochemical, and Hf isotope analyses have been conducted on the Permian volcanic rocks along the SM‐XMOB. Zircon U–Pb geochronological data indicate that these rocks were formed at 271.4 ± 3.2 Ma and 268.3 ± 3.2 Ma during the Early–Middle Permian. Geochemical results suggest that the volcanic rock samples have 61.5–66.4 wt% SiO2 and 0.98–1.71 wt% MgO, and belong to the calc‐alkaline series, suggesting an affinity with fractionated I‐type granite. They also exhibit positive zircon εHf (t) values ranging from + 8.6 to + 14.7, indicating that the primary magmas were dominantly derived from partial melting of Early Palaeozoic juvenile lower crustal. These volcanic rocks show an enrichment in large‐ion lithophile elements (LILEs; e.g., Rb, Th, and U) and depletion in high‐field‐strength elements (HFSEs; e.g., Nb, Ta, Ti, and P), similar to that from an active continental margin setting. We therefore propose that the widespread Permian volcanic rocks in the SM‐XMOB were closely associated with the southward subduction of the Palaeo‐Asian Ocean (PAO) Plate to the lithosphere beneath the XMOB, and that the PAO had not closed prior to the Middle Permian in the SM‐XMOB.

Geochemistry and Petrogenesis of Volcanic Rocks from the Continent-Ocean Transition Zone in Northern South China Sea and Their Tectonic Implications

Miocene-Pliocene (22–5 Myr) volcanism and associated seamounts are abundant in the continent-ocean transition (COT) zone in the margin of the north South China Sea (SCS). The petrogenesis of volcanic rocks from these seamounts and regional tectonic evolution of COT zone are poorly known. In this paper, we obtained whole-rock major and trace element compositions and Sr-Nd-Pb isotopic data for these volcanic rocks from the Puyuan and Beipo seamounts within COT zone, in northeastern SCS. Based on the geochemical analyses, the volcanic rocks are classified as alkaline ocean island basalts (OIB) and enriched mid-ocean ridge basalts (E-MORB). The OIBs from the Puyuan seamount are alkaline trachybasalts and tephrites that show enrichment of the light rare earth elements (LREE) relative to heavy rare earth elements (HREE) and more radiogenic Sr-Nd isotopic compositions, and have significant ‘Dupal isotopic anomaly’. In contrast, the E-MORBs from the Beipo seamount are tholeiitic basalts that have less enrichment in LREE and less radiogenic Sr-Nd isotopic compositions than the counterparts from the Puyuan seamount. Petrological and geochemical differences between the OIBs and MORBs from these two seamounts can be explained by different mantle sources and tectonic evolution stages of the COT zone. Syn-spreading OIB type basalts from the Puyuan seamount were derived from an isotopically ‘enriched’, and garnet facies-dominated pyroxenitic mantle transferred by the Hainan mantle plume. In contrast, post-spreading E-MORB type basalts from the Beipo seamount are considered to be derived from the melting of isotopically ‘depleted’ pyroxenite mantle triggered by lithosphere bending and subsequent post-rifting at the lower continental slope of the northern margin.

Comments on “Critical assessment of geochronological data from the Carajás mineral Province, Brazil: Implications for metallogeny and tectonic evolution”

Despite representing one of the largest cratons on Earth, the early geological evolution of the Amazonia Craton remains poorly known due to relatively poor exposure and because younger metamorphic and tectonic events have obscured initial information. In this study, we investigated the sedimentary archives of the Carajás Basin to unravel the early geological evolution of the southeastern Amazonia Craton. The Carajás Basin contains sedimentary rocks that were deposited throughout a long period spanning more than one billion years from the Mesoarchean to the Paleoproterozoic. The oldest archives preserved in this basin consist of a few ca. 3.6 Ga detrital zircon grains showing that the geological roots of the Amazonia Craton were already formed by the Eoarchean. During the Paleoarchean or the early Mesoarchean (<3.1 Ga), the Carajás Basin was large and rigid enough to sustain the formation and preservation of the Rio Novo Group greenstone belt. Later, during the Neoarchean, at ca. 2.7 Ga, the southeastern Amazonia Craton witnessed the emplacement of the Parauapebas Large Igneous Province (LIP) that probably covered a large part of the craton and was associated with the deposition of some of the world largest iron formations. The emplacement of this LIP immediately preceded a period of continental extension that formed a rift infilled first by iron formations followed by terrigenous sediments. This major change of sedimentary regime might have been controlled by the regional tectonic evolution of the Amazonia Craton and its emergence above sea-level. During the Paleoproterozoic, at ca. 2.1 Ga, the Rio Fresco Group, consisting of terrigenous sediments from the interior of the Amazonia Craton, was deposited in the Carajás Basin. At that time, the Amazonian lithosphere could have either underwent thermal subsidence forming a large intracratonic basin or could have been deformed by long wavelength flexures that induced the formation of basins and swells throughout the craton under the influence of the growing Transamazonian mountain belt.

Petrogenesis of the Late Mesozoic gabbros in the eastern Jiangnan Orogen, South China: Characteristics of the lithospheric mantle

As we know, mantle‐derived mafic rocks can be used to effectively trace the nature of the mantle source and constrain the regional tectonic setting and evolution. Recently, a Late Mesozoic mafic intrusive body (Huangmao pluton) has been found in the eastern Jiangnan Orogen (EJO). In this study, we investigate this pluton using petrography, whole‐rock geochemistry, Sr–Nd–Pb isotopes, and geochronology. The Huangmao pluton is primarily composed of a fine‐grained gabbro‐diabase, which is characterized by consistent SiO2, MgO, Fe2O3T, and TiO2 contents. These gabbros exhibit continental crust‐like trace element signatures, i.e., enriched in large‐ion lithophile elements (LILE), Pb, and light rare earth elements (LREE), but depleted in high‐field‐strength elements (HFSE). In addition, the Huangmao gabbros have moderate initial 87Sr/86Sr ratio (0.7065–0.7080) and slightly negative to weakly positive εNd(t) value (−2.06 to +0.84). Finally, U–Pb zircon dating confirms that the Huangmao pluton was emplaced in the Late Jurassic (152.6 ± 3.1 Ma). Through an integrated geochemical analysis, these Late Mesozoic gabbros were genetically originated from an enriched lithospheric mantle source, which formed by a metasomatic reaction between mantle peridotite and sediment‐derived felsic melt. Consequently, the lithologies of the metasomatite were predominantly pyroxenites with a small amount of pyroxene‐rich peridotites. Considering the large distance between the modern trench and the EJO, the flat‐slab subduction and rollback model of the palaeo‐Pacific Plate is the most reasonable interpretation of the geodynamic setting that resulted in Late Mesozoic magmatism in the EJO.

Tectonic Characteristics of the East Sichuan Basin and Regional Tectonic Evolution Characteristics of Regional Tectonic Belt

Most sedimentary basins in China have undergone multi-stage tectonic evolution. This paper takes the eastern Sichuan region as the research object and relies on modern geological research methods to systematically study the basin’s dynamic characteristics and tectonic evolution process in the eastern Sichuan region. The results show that the tectonic squeezing of SW and NW from the Daba Mountain orogenic belt and the Jiangnan-Xuefeng Mountain orogenic belt, respectively, make eastern Sichuan a complex structural interference zone that is both combined and compounded. The eastern Sichuan NE and NW structural combinations The type clearly records the structural transformation effect of the two. The history of tectonic evolution since the Middle Paleozoic in the eastern Sichuan Basin is divided into five phases: the deposition of source rocks of the Wufeng Formation-Longmaxi Formation in the Caledonian Period and the parallel unconformity contact with the overlying strata caused by the end of Silurian tectonic compression; Haixi The tectonic transformation of the Early-Indochina period is mainly manifested by the inheritance of the Caledonian period, and the interaction between the Devonian-Early Carboniferous strata and the early Indosinian marine facies and terrestrial sediments is lacking, and the fold deformation in the region is weak. In the early period, a series of NE-directional faults formed mainly in the eastern Sichuan region during the NNW-SSE compression. In the late Yanshan-Early Himalayan period, the NNW-SSE ~ NWW-SEE compression became stronger than in the early Yanshan period, and the eastern NE NE-boundary faults formed. In the late Himalayan period, NE-SW was squeezed. The Wujiang fault system at the border of the southeast of Sichuan was mainly formed during this period, and the southern strata were obviously transformed by this period.

Early Paleozoic Ocean Plate Stratigraphy of the Beishan Orogenic Zone, NW China: Implications for Regional Tectonic Evolution

AbstractThe Beishan orogenic zone is a key area to understand evolution of the Central Asian Orogenic Belt that is an accretionary factory well‐enough preserved in the Paleozoic. In early Paleozoic, the tectonic mélange zone containing the coherent unit and mélange unit is triggered by the complicated accretionary process of the Beishan area. The early Paleozoic tectonic evolution of the Beishan orogenic zone is investigated in this study using sedimentology and stratigraphic correlations of the lowe Paleozoic deposits. From the Cambrian to the middle Ordovician, this region was characterized by geographically extensive, flat‐bedded siliceous mudstone, indicating the existence of a large ocean basin. The oceanic plate entered the convergence phase in terms of the Wilson Circle during the Middle Ordovician, when numerous magmatic arcs formed along two opposite sides of the ocean. The magmatic arcs became the widest during the Silurian, suggesting that the Hongliuhe‐Niujuanzi‐Xichangjing Ocean (HNX; a southern branch of the Paleo Asian Ocean) was reduced to a small residual ocean in the central Beishan region by that time, and probably lasted till the Carboniferous or later by newly published data.

RETRACTED: Detrital zircon age distribution characteristics of Jurassic sandstone from the Southern Yili basin, China

The Central Asia orogenic belt contains a wide range of structural elements, including micro continent, back arc system, ocean island/plateau, ophiolite and subduction accretion complex. But its final closing time has been controversial. Based on the magmatic age of the surrounding orogenic belt, the source of this set of clastic sediments is determined, which provides new important evidence for the evolution of the South Tianshan orogenic belt. The results show that the Jurassic detrital zircons from the study area were mainly derived from magmatic zircons and are deposited in a proximal source. The detrital zircon age of the Lower Jurassic Badaowan and Sangonghe Formation are concentrated in 290–260 Ma, and in 350–290 Ma and 460–390 Ma, respectively. The detrital zircon age of the Middle Jurassic Xishanyao Formation concentrates in 370–320 Ma and 450–390 Ma. There are very few zircons from the Precambrian period. These ages are consistent with the timing, indicating these clastic sediments were mainly originated from the southern margin of the Yili - Middle Tianshan Block. The Late Permian - Middle Triassic detrital zircons almost do not exist, implying that there were no contemporary magmatism related to collision or post-collision in the South Tianshan district, its complex evolution and orogenic stage are still a challenging topic. In the ancient active plate margin, the sedimentary records in the pre-arc basin can provide more information about the magmatic arc and basin-orogen coupling than the present exposed arc itself. The rhyolite, trachyte, and trachyandesite of the Dahalajunshan Formation were widely developed in the Yili - Middle Tianshan Block during the Early Carboniferous. During the formation of Wulang Formation in the Early Permian, a large number of rhyolite were developed. The age data of 75 detrital zircons were obtained from the sandstone (J1s-5) of the Sangonghe Formation, of which 74 zircons have a concordance degree of over 90%, and their age data also fall on the harmonic curv. In addition, the age of the youngest zircons increased gradually from Early to Middle Jurassic, indicating that the sediments in this period had the feature of uncovering. Our study provides a good reference for the analysis of provenance and regional tectonic evolution.

Rock and age relationships within the Talkeetna forearc accretionary complex in the Nelchina area, southern Alaska

Subduction zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. We report the results of field mapping and petrographic, geochemical, and geochronological analyses of the McHugh Complex accretionary prism mélange in south-central Alaska that was recently exposed due to retreat of the Nelchina Glacier. Our new mapping and analyses of the mélange, as well as adjacent Talkeetna arc intrusives, suggests that the previously mapped trace of the Border Ranges fault should shift northward in this location. Detailed petrographic analysis places this mélange exposure with the Potter Creek assemblage of the McHugh Complex. Blocks of pillow lavas within the mélange have both mid-ocean ridge basalt and intra-plate geochemical affinities, attesting to the complex relations of subduction-zone inputs in an alternating erosive–accretionary margin. A new zircon U–Pb age and geochemical analyses of a set of felsic dikes that cross-cut the accretionary sequence provide constraints on the regional tectonic evolution, including near-trench plutonism associated with the migration of a subducting spreading ridge along the southern Alaska margin during the Paleocene–Eocene. The McHugh section and cross-cutting dikes in this location are pervasively hydrothermally altered, which we attribute to elevated temperatures related to ridge subduction. Late-stage motion along the Border Ranges fault system, which is also recorded in the area, may also have contributed to the widespread alteration. Our data indicate that the Talkeetna volcanic arc and associated accretionary prism sediments were in their current configuration by 55 Ma.

Timeframe of hydrocarbon migration in the Paleogene Shahejie Formation in the Dongying depression, Bohai Bay Basin (northeastern China) based on fluid inclusions and oil geochemistry

The Dongying depression is an important oil-producing area in northeastern China, where the Paleogene Shahejie Formation is the main oil-bearing feature. Although petroleum migration and accumulation in the Dongying depression have been extensively studied, the charging history is still unclear. This paper defines the timeframe of petroleum migration on the basis of a comprehensive investigation of fluid inclusions coupled with basin modeling. According to the petrography and microthermometry of the fluid inclusions, types of fluid inclusion with low-peak-range distributions of the homogenization temperature (Th) (85–95 °C for the Niuzhuang sag and 90–105 °C for the Lijin sag) and high-peak-range Th distributions (105–120 °C for the Niuzhuang sag and 115–130 °C for the Lijin sag) were identified. By recovering the burial and thermal histories of Niuzhuang and Lijin sags with pressure–volume–temperature–composition simulation and one-dimensional basin modeling, it can be inferred that the trapping pressures of two types of hydrocarbon inclusions were higher than the corresponding paleopressure. (42.4 and 42.0 MPa, respectively), indicating rapid hydrocarbon charging resulting from abnormally high pressure. Two hydrocarbon charging events associated with the regional tectonic evolution of the Dongying depression, namely, continuous deposition and tectonics in the Paleogene Dongying Formation (Ed) and rapid deposition and tectonics in the Paleogene Minghuazhen Formation (Em), were inferred. The direction of oil charging was traced from the center to the margin in the north, and from the center to the central fault in the southeastern part of the depression in general, as indicated by the molecular maturity parameters, which roughly coincided with the abnormal pressure.

Characterizing subseismic faults from SK-2 drilling core (2900–4200 m): Implication for reservoir transmissibility and regional tectonic evolution

Undiscernible faults on seismic reflection profiles are referred to as subseismic faults. Although most subseismic faults are undetected, they play a significant role in understanding regional tectonic evolution and can influence the flow of oil and gas. The Songliao Basin in northeast China is a typical Meso-Cenozoic continental petroliferous basin characterized by stable sedimentation, rift-depression dual structure, and large-scale oil and gas production. However, the characteristics of subseismic faults and their effect on petroleum resources remain not well understood. We have examined findings from the SK-2 east borehole located in the Songliao Basin, which is the deepest (7018 m deep below the ground surface) continental scientific drilling borehole in East Asia. We identified 46 subseismic faults at 2900–4200 m depths based on the observations of core-scanning images, macro- and microstructures, and well-logging data. Macro- and microstructural analyses indicate that most of the subseismic faults in the borehole indicate normal slip. These observations suggest that these subseismic faults may form in response to regional extension in the Shahezi (K1sh) period. The cross-cutting relationships among several groups of sheared fault planes or elongated veins filled in the fractures likely reflect multistage faulting. The subseismic faults are considered to be related to the nearby larger scale faulting as interpreted on the seismic profile. The spatial correlation between the observed subseismic faults and elevated hydrocarbon concentrations documented by borehole mud gas logging suggests that the subseismic faults might have controlled gas migration in the study area.

Re-Os Geochronology, Whole-Rock and Radiogenic Isotope Geochemistry of the Wulandele Porphyry Molybdenum Deposit in Inner Mongolia, China, and Their Geological Significance

The Wulandele molybdenum deposit is a porphyry-type Mo deposit in the Dalaimiao area of northern Inner Mongolia, China. Molybdenite Re-Os dating yields a model age of 134.8 ± 1.9 Ma, with the fine-grained monzogranite most closely related to the mineralization. The lithogeochemical data show that the monzogranite is weakly peraluminous, high-K calc-alkaline series, with reduced to slightly oxidized, highly fractionated I-type granite characteristics. The relatively low initial 87Sr/86Sr (range from 0.705347 to 0.705771), weakly negative εNd(t) (range from −2.0 to −1.3), and crust-mantle mixing of Pb isotopes suggest that the monzogranite originated from the partial melting of mafic juvenile lower continental crust derived from the depleted mantle, with a minor component of ancient continental crust. Combined with the regional tectonic evolution, we argue that the partial melting, then injection, of the monzogranite melt was probably triggered by collapse or delamination of the thickened lithosphere, which was mainly in response to the post-orogenic extensional setting of the Mongol–Okhotsk belt; this is possibly coupled with a back-arc extension related to Paleo-Pacific plate subduction. The extensively fractional crystallization of the monzogranite melt is the crucial enrichment process, resulting in magmatic hydrothermal Mo mineralization in the Wulandele deposit, and the Cretaceous granitoids are generally favorable to form Mo mineralization in the Dalaimiao area.

Geochronology, geochemistry, and Hf isotopic compositions of Triassic igneous rocks in the easternmost segment of the East Kunlun Orogenic Belt, NW China: implications for magmatism and tectonic evolution

ABSTRACT The voluminous Triassic igneous rocks in the East Kunlun Orogenic Belt (EKOB) are important for investigations of the tectonic evolution of the Paleo-Tethys Ocean. This study focused on the geochronology and petrogenesis of igneous rocks in the Elashan area with the aim of constraining the regional tectonic evolution. The 238 Ma syenogranites are characterized by high SiO2 contents and marked depletions in Sr and Eu. We conclude that they are highly fractionated I-type granites derived from partial melting of Mesoproterozoic lower crust with significant fractional crystallization. Comparing with contemporaneous granites and mafic rocks, the 236 Ma quartz monzodiorites have moderate MgO (1.5–1.6 wt.%) and SiO2 (61.0–61.9 wt.%) contents and εHf(t) values (–1.2 to +1.9). They were probably generated by the mixing of Mesoproterozoic lower-crust-derived and enriched lithospheric mantle-derived melts. The 230 Ma appinites have high MgO contents (3.8–6.1 wt.%) and enriched Hf isotopic compositions, and were likely derived from partial melting of lithospheric mantle that had been metasomatized by subduction fluids. We infer that these rocks were emplaced in syn-collisional slab break-off and the later initial post-collisional extensional setting between the BayanHar-Songpan Ganze and East Kunlun terranes. The basement composition, magmatism, and tectonic evolution of the Elashan area are similar to those of the eastern EKOB.

The genesis of felsic magmatism during the closure of the Northeastern Paleo-Tethys Ocean: Evidence from the Heri batholith in West Qinling, China

To better understand the origin of voluminous silicic rocks in a convergent continental margin, we conducted an integrated study in which we have obtained geochronological, mineralogical, and isotopic (including whole-rock Sr–Nd–Pb, in-situ zircon Hf) data of the Heri batholith in West Qinling on the NE Tibetan Plateau. The batholith is composed of metaluminous to weakly peraluminous granodiorites (235–233 Ma) and porphyritic granodiorites (230–223 Ma) with an I-type affinity. Both lithologies share similar major, trace elemental and SrNd isotopic compositions. Detailed elemental data demonstrate that these granodioritic rocks underwent fractional crystallization of hornblende and apatite, with plagioclase (i.e. sieve-textured plagioclase cores) accumulation to some extent. Except for porphyritic granodiorites, the Pb isotopes for other analyzed samples are characterized by high radiogenicity and uniformity ((206Pb/204Pb)t: 17.263–18.472, (206Pb/204Pb)t: 15.571–15.591, and (206Pb/204Pb)t: 38.032–38.304), together with limited variations in initial Sr ((87Sr/86Sr)t: 0.707251–0.708103) and Nd (εNd(t) = −7.1 to −6.3) isotopes with two-stage model ages (TDM2) of 1.58–1.52 Ga. These factors collectively point to a derivation from the Mesoproterozoic basement rocks at the lower crustal level, or a comprehensive mixing of different-age components that generated an average crustal residence age. The SrNd isotopic compositions of the porphyritic granodiorites are strikingly similar to those of granodiorites. Compared with the experimental melt compositions of amphibolites, the Heri granitoids are probably derived from an amphibolitic source under fluid-absent conditions due to the incongruent breakdown of amphibole and biotite. Based on the temporal–spatial distribution of granitic intrusions in West Qinling and the regional tectonic evolution, our interpretation is that the Heri batholith was formed during the initial collision between the North China Craton (NCC) and the South China Craton (SCC), which was accompanied by the closure of the Paleotethyan Ocean. Considering both previously published data and our new data, we propose that the Heri granitoids were mainly generated by the partial melting of lower crustal amphibolites, with minor mantle-derived melts.