Intracontinental Setting Research Articles

Zyryanka Basin Composite Tectono-Sedimentary Element, northeastern Asia

The Zyryanka Composite Tectono-Sedimentary Element is located in NE Asia. It consists of a synonymous sedimentary basin that is one of the least studied onshore basins in NE Asia due to its remoteness and the scarcity of wells and geophysical data. The basin evolved through four episodes of subsidence and two episodes of contraction. The Late Jurassic–Late Cretaceous subsidence is inferred to be caused by tectonic loading during contraction events within the Verkhoyansk–Kolyma Orogen, which led to the deposition of up to 10 km of predominantly continental siliciclastic strata. In the Cenozoic, the basin evolved in a post-orogenic intracontinental setting and accumulated a 2–3 km-thick succession of continental siliciclastic strata. In the Oligocene–Mid-Miocene, the basin evolved in a contractional setting, caused by a convergence between the Eurasian and North American lithospheric plates, and underwent a contractional deformation in the Middle Miocene. We recognize four individual tectono-sedimentary elements that are related to different stages of the basin's evolution. The basin contains significant coal resources. It was also considered a potential prospective area for the formation of hydrocarbon accumulations, especially in its Upper Jurassic–Cretaceous strata; however, drilled wells did not encounter any oil- or gas-bearing reservoirs.

First identification of late Mesozoic intraplate magmatism in the Chinese north Tianshan: implications for the orogenic architecture and crustal evolution

The formation and dynamics of granitoids in an intra-continental setting are crucial for understanding the architecture and evolution of continental crust. Here, we report geochronological, geochemical and Sr–Nd–Hf isotopic data for newly discovered late Mesozoic granitic intrusions in the Tianshan belt, northwestern China. These granitoids are I-type granites derived from an igneous precursor and were emplaced at c . 145–132 Ma. They have positive ε Nd ( t ) values and young Nd model ages, together with relatively low Sr/Y ratios, indicating that they might have originated from partial melting of the juvenile lower crust. There is a prominent decoupling between zircon Hf and bulk-rock Nd isotopes, which may have resulted from the early crystallization of Ti-rich minerals. These granitic intrusions also display subduction-related geochemical characteristics, which are probably inherited from Paleozoic crustal sources that were metasomatized by subduction-related fluids. We conclude that these late Mesozoic granitoids were emplaced in an intra-continental setting, and were probably triggered by thermal relaxation owing to crustal shortening and thickening. These data further imply that the Tianshan changed into crustal reworking during the Mesozoic from its prominent crustal growth in the Paleozoic. Supplementary material: LA-ICP-MS zircon U–Pb dating results, whole-rock major and trace element compositions, whole-rock Sr–Nd isotopic data and zircon Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.7167890 Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change

Metasomatism of the continental crust and its impact on surface uplift: Insights from reactive‐transport modelling

AbstractHigh‐elevation, low‐relief continental plateaus are major topographic features and profoundly influence atmospheric circulation, sediment transport and storage, and biodiversity. Although orogenic surface‐uplift mechanisms for modern continental plateaus near known plate margins like Tibet are well‐characterized, they cannot account for examples in intracontinental settings like the Colorado Plateau. In contrast to canonical plate‐tectonic uplift mechanisms, broad‐scale hydration‐induced metasomatism of the lower crust has been suggested to reduce its density and increase its buoyancy sufficiently to contribute to isostatic uplift. However, the relationships between key petrophysical properties in these environments are not fully quantified, which limits application of this model. Here, we develop a series of petrological models that describe the petrological and topographic effects of fluid–rock interaction in non‐deforming continental crust of varying composition. We apply an open‐system petrological modelling framework that utilizes reactive‐transport calculations to determine the spatial and temporal scales over which mineralogic transformations take place compared with the magnitude of infiltration of aqueous fluids derived from devolatilization of subducting oceanic lithosphere. The buoyancy effect of hydration‐induced de‐densification is most significant for metabasic lower crust, intermediate for metapelitic crust, and minimal for granodioritic crust. We apply these results to a case study of the ~2 km‐high Colorado Plateau and demonstrate that under ideal conditions, hydration of its lower–middle crust by infiltrating aqueous fluids released by the Farallon slab during Cenozoic low‐angle subduction could have uplifted the plateau surface by a maximum of ~1 km over 16 Myr. However, realistically, although hydration likely has a measurable effect on surface tectonics, the uplift of orogenic plateaus is likely dominantly controlled by other factors, such as lithospheric delamination.

Episodic evolution of intracontinental orogenic deformation: Insight from the southern Bogda fold-and-thrust belt of the Tianshan Mountains, NW China

The Mesozoic-Cenozoic Tianshan orogenic belt, a key area for studying intracontinental orogenic deformation, experienced multiple uplift episodes due to the far-field effects from a series of collision events along the southern margin of Eurasia. Focusing on the fold-and-thwrust belt at the southern margin of the Bogda Mountain, a branch of the eastern Tianshan orogenic belt, we integrated 2D seismic data, borehole and outcrop data to construct a balanced and sequentially restored cross-section across the central part of this belt. The activities and mechanical evolution of the major faults were analyzed by using the discrete-element modeling (DEM) method, offering new insights into the tectonic transformation of the Turpan-Hami Basin influenced by episodic uplifts of the Bogda Mountain in an intracontinental setting. The studied fold-and-thrust belt can be divided into two deformation systems: (i) the piedmont deformation system, comprising a southward basement thrust system and a thin-skinned thrust system on Jurassic detachment, and (ii) the thrust front belt, exhibiting an en-echelon pattern in the basin center and featured by fault-propagation folds along the lower Jurassic coal unit. Stratigraphic and reported thermochronological analyses pinpoint four key contractional deformation periods: Late Jurassic, Late Cretaceous, Paleocene-Pliocene, and Pleistocene-present. The DEM numerical simulation results indicate a critical conical Coulomb wedge model governed by the episodic activities of the southern Bogda fold-and-thrust belts from the Mesozoic to the Cenozoic. Surface processes (syntectonic erosion/deposition) significantly impacted differential activities between the orogenic belt and the basin's thrust structures. We propose that periodic arid climates in the Tianshan Mountains reduced erosion rates, promoting the expansion of the southern Bogda fold-and-thrust belts toward the basin and the widening of the orogenic belt.

Geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of the Late Jurassic granitoids from the Shuangyuangou deposit, South Qinling, China: Implications for petrogenesis and porphyry Cu metallogeny

The Shuangyuangou copper deposit is located in the southern segment of the Qinling orogenic belt in central China. The ore‐related intrusions include quartz diorite porphyry, granite porphyry and lamprophyre. In this study, we present whole‐rock geochemistry, zircon U–Pb geochronology, trace elements and Lu–Hf isotope data on the Shuangyuangou intrusions which provide critical constraints on the emplacement age, magma origin, petrogenesis and implications for porphyry Cu mineralization in this newly discovered deposit. Zircon U–Pb dating constrains the emplacement age of quartz diorite porphyry, lamprophyre and granite porphyry at 150–147 Ma. The quartz diorite porphyry and granite porphyry display high‐K calc–alkaline I‐type granite affiliation. The rocks show enrichment of LREE and LILE (Rb, Ba), depletion in HREE and HFSE (Nb, Ta, Ti) and absence of Eu anomaly. Zircon εHf(t) values of quartz diorite porphyry, lamprophyre and granite porphyry range from −7.5 to −1.7 with TDM2 ages of 1.3–1.6 Ga. Our data suggest that the parental magma was derived from partial melting of juvenile lower crust and metasomatic lithospheric mantle, with more involvement of mantle material for the lamprophyre. The zircon Eu/Eu* values of 0.55–0.78 suggest high magmatic water content which suppresses the fractionation of plagioclase. In addition, the ΔFMQ values calculated from zircon trace elements range from −0.90 to 2.96 which is a favourable factor for porphyry Cu mineralization at Shuangyuangou. The Late Jurassic Shuangyuangou intrusion and associated Cu mineralization were formed within an intracontinental setting after the collisional assembly of the Qinling orogenic belt.

Upper mantle melt caused by a subducted slab in the Indian-Eurasian continental subduction zone

A low-velocity layer atop the mantle transition zone has been extensively observed worldwide. In subduction zones, this layer is widely explained as partial melting triggered by slab subduction on a regional or global scale. However, direct observational evidence is still absent, and the response of the layer to slab subduction is not well known. Here, we image the seismic velocity around the mantle transition zone by matching synthetic and observed triplicated seismic P and sP waveforms in the Indian–Eurasian continental subduction zone. Our observations reveal a laterally varied low-velocity layer atop the mantle transition zone beneath the Hindu Kush, where a subducted slab extends to the mantle transition zone. The geometric morphology of the low-velocity layer documents that it is a partially molten layer induced by the subducted slab on a regional scale. Interestingly, our observations also support that the layer has a low viscosity. The decreased viscosity may facilitate slab motion in the deep mantle, contributing to slab stretching, tearing and break-off and its resultant rare recurring large intermediate-depth earthquakes in an intracontinental setting.

Mercury isotopic compositions of iron oxide‑copper‑gold (IOCG) hydrothermal systems: Deep Hg cycling in intracontinental settings

Mercury isotopes display unique mass-independent fractionation (MIF), typically expressed as Δ199Hg, which relates to photochemical reactions occurring at Earth's surface. Pronounced Hg-MIF signals have been observed in various hydrothermal systems in both convergent margins and intracontinental settings, highlighting the recycling of Hg from marine or terrestrial reservoirs into shallow continental hydrothermal systems. However, the geochemical fate of Hg in deep continental environments of intracontinental settings remains poorly understood. Iron oxide‑copper‑gold (IOCG) hydrothermal systems typically involve the circulation of fluids from both deep magmatism and continental basins, providing an opportunity to investigate this issue. Here, we present the Hg isotopic compositions of ore minerals from five representative IOCG deposits in the Kangdian region, South China. All the studied ore samples display large mass-dependent fractionation with δ202Hg of −3.23 to 1.06‰, but a relatively narrow range of Δ199Hg of −0.17 to 0.11‰. These isotopic signatures support the hypothesis of binary mixing of Hg from two sources: (1) a magmatic endmember (∼two thirds) derived from the lithospheric mantle with low δ202Hg (−3 to −1‰) and near-zero Δ199Hg, and (2) a crustal source (∼one third) originating from the basement rocks with relatively higher δ202Hg (−1 to 1‰) and negative Δ199Hg (−0.2 to 0‰). Compared to the shallow Pb-Zn-Au-Sb hydrothermal systems which formed in intracontinental basins through the circulation of fluids in the upper crust, the Hg components (and by analogy, the ore metals) in the Kangdian IOCG system are dominated by a lithospheric mantle source. Our study sheds light on deep Hg cycling and confirms the Hg isotope tracer as a tool for revealing metal sources in hydrothermal ore systems.

Crust-Mantle Interaction Controls the Formation of High-Mg Adakitic Rocks: Evidence from Early Cretaceous Intrusive Complexes in Luxi Terrane, North China Craton

AbstractHigh-Mg adakite rocks preserve crucial information about the crust-mantle interactions during the magma evolution. The Luxi Terrane, southeastern North China Craton, stores a set of Early Cretaceous high-Mg adakite rocks; nevertheless, their petrogenesis remains controversial. In this study, we present new whole-rock geochemistry, zircon U-Pb-Hf isotopes in the Tiezhai, Jinxingtou, and Sanshanyu complexes which are composed of gabbroic diorite, diorites, syenites, and monzonites. Field observations and zircon U-Pb dating indicate that all of the rock units crystallized contemporaneously at ca. 125–120 Ma. They are characterized by high Al2O3 and Sr contents, and low MgO, Y, Yb, and heavy rare earth elements contents, coupled with high Sr/Y values (42–163), showing adakitic affinities. The magma mixing process is supported by the following ample evidence: (1) the disequilibrium mineral textures and mafic enclaves; (2) high Mg# values (37–69, Mean = 58); and (3) widely zircons εHf(t) values (−25.6 to +7.8). The signature geochemical characteristics support that the adakites were generated by magma mixing of ancient crust-derived melts and relatively mafic melts from metasomatized mantle source. In combined with regional geology, the Early Cretaceous high-Mg adakites in Luxi Terrane represent the magmatic response of intensive crust-mantle interaction caused by the underplating of voluminous mantle-derived magma in an extension intracontinental setting.

Geochronology, geochemistry, and Sr–Nd isotopes of Permian dike swarms in the Beishan orogenic belt, southernmost Central Asian Orogenic Belt: Petrogenesis and implications for geodynamic setting

ABSTRACT Permian mafic dikes crop out widely within Beishan area in the southernmost Central Asian Orogenic Belt (CAOB). These mantle-derived products provide important clues for elucidating the genesis and geodynamic setting of Permian magmatism. The mafic dikes within the Beishan area display zircons U–Pb ages of 281 ± 4 Ma. Characterized by low SiO2 (44.88–47.73 wt.%) and high Mg# values (62.2–68.6), the type 1 dikes are relatively depleted in light rare earth elements (LREEs; (La/Yb)n = 0.72–1.43) but enriched in large ion lithophile elements (LILEs). They show high positive εNd(t) values (6.6–8.5) with a range of (87Sr/86Sr) i (0.703477–0.705789) ratios. In contrast, the type 2 dikes have higher SiO2 content (47.47–51.13 wt.%) and lower Mg# (40.9–63.0). They are enriched in LREEs ((La/Yb)n = 1.79–5.02) and LILEs, and have lower εNd(t) values (3.8–7.4) with a variable range of (87Sr/86Sr) i (0.702873–0.706949) ratios. Thus, the type 1 dikes resulted from lower degrees of fractional crystallization and contamination than the type 2 dikes. Considering the LILEs and Pb enrichments, high field strength elements depletions, and high Ba/Th (86–460) ratios, the mantle source was metasomatized by subduction-related fluids. These dikes should be derived from high degrees (>20%) of partial melting of a Sr–Nd isotope-depleted, phlogopite-bearing spinel lherzolite lithospheric mantle according to their high Rb/Sr (0.04–0.24), low Ba/Rb (0.9–3.3), low La/Sm, and constant Sm/Yb ratios in addition to Sr–Nd depletion. Combined with the coeval evidence of regional geology and novel tectonic discriminate diagrams, we suggest that the mafic dikes in Beishan area formed in arc-related setting rather than intracontinental setting during the Permian. The metasomatized lithospheric mantle by subduction-related fluids were heated by upwelling asthenosphere mantle triggered by the slab roll-back, resulting in the formation of the parental magma of the mafic dikes.

Paleo‐Pacific Plate Subduction and Basement Mobilization Triggered Large‐Scale Formation of Mesozoic Gold Deposits in the Northern Margin of the North China Craton

AbstractThe northern margin of the North China Craton (NNCC) hosts Mesozoic gold deposits of significant economic importance. The metal sources of these gold systems have long been debated with mainly magmatic‐hydrothermal versus metamorphic fluid models. Mercury (Hg) isotopes, which undergo unique mass‐independent fractionation, can provide important insights into the source of metals in gold deposits due to the close association between Hg and Au in such systems. Here, we investigated the Hg isotopic composition of six gold deposits of the Middle Jurassic to Early Cretaceous age and potential source rocks in the NNCC. Variable ∆199Hg values were observed in bulk ore and pyrite samples (−0.28‰ to 0.34‰, n = 51) and coeval granites (−0.21‰ to 0.13‰; n = 25). The negative ∆199Hg values of bulk ore and pyrite samples mostly agree with that observed for metamorphic basement rocks (−0.37‰ to 0.11‰; n = 32). The positive ∆199Hg values of bulk ore and pyrite samples agree with those reported in marine sediments (0‰–0.3‰). So the data suggest binary mixing of ocean‐recycled Hg originated from the subducted oceanic slab, and terrestrial‐recycled Hg from the Precambrian basement in Mesozoic gold deposits. The contribution of Hg (and Au, by analogy) from both reservoirs varies depending on active continental arc versus intracontinental setting and is ultimately controlled by translithospheric heat flow driven by paleo‐Pacific plate subduction.

Derivation of Lamproites and Kimberlites from a Common Evolving Source in the Convective Mantle: the Case for Southern African ‘Transitional Kimberlites’

Abstract ‘Transitional kimberlite’ is a collective term previously used to classify rocks occurring in southern Africa that show bulk rock geochemical and Sr–Nd isotope features intermediate between (cratonic) lamproites and kimberlites. However, it is now well established that detailed petrographic and mineral chemical criteria represent a more robust approach towards the classification of kimberlites, lamproites and related rocks. Here, we re-assess the classification of southern African ‘transitional kimberlites’ by combining new petrographic observations and mineral compositional results for samples from six localities (Leicester, Frank Smith, Wimbledon, Melton Wold, Droogfontein, and Silvery Home) straddling the southwestern margin of the Kaapvaal Craton. These new data indicate that Leicester and Frank Smith are archetypal kimberlites, whereas Wimbledon, Melton Wold, Droogfontein, and Silvery Home represent bona fide olivine lamproites. We combine the mineral chemical results with new (Wimbledon) and existing bulk rock trace element and Nd–Hf isotope compositions, and emplacement ages, to assess whether the previously documented trends in Nd–Hf isotope vs time for these ‘transitional kimberlites’ constrain their petrological evolution. Modal groundmass mineralogy, bulk rock K/La and chromite compositions, the latter being a proxy for primitive melt composition, are linearly correlated with emplacement age and initial Nd–Hf isotope compositions. These observations suggest derivation of both older lamproites (181–115 Ma) and younger kimberlites (114–93 Ma), from a common evolving source. The temporal evolution of Nd–Hf isotope compositions in these rocks converge to values typical of archetypal Cretaceous kimberlites elsewhere in the Kaapvaal Craton, but are clearly different from the isotopic compositions of on-craton Kaapvaal lamproites (previously known as orangeites). This observation distinguishes the petrogenesis of the Wimbledon, Melton Wold, Droogfontein, and Silvery Home lamproites from those of ‘typical’ Kaapvaal lamproites. We hypothesize that progressive consumption of enriched and hence fertile K-bearing components in a common sub-lithospheric (i.e. convective mantle) source beneath the southwestern margin of the Kaapvaal Craton might represent a plausible scenario to explain the temporal evolution of petrographic and geochemical traits of the examined lamproites and kimberlites. A source in the lithospheric mantle is considered at odds with the contrasting location of the current localities as they occur both off- and on-craton. Migration of the African plate between 180 and 90 Ma over a relatively stationary convective mantle (plume?) source is not compatible with the spatial–temporal distribution of ‘transitional kimberlites’. Instead, we invoke viscous coupling between an upper asthenospheric source and the lithosphere to reconcile a single evolving source with the geographic and age distribution of these rocks. This work supports the hypothesis that olivine lamproites occurring in intra-continental settings share similar genetic features with kimberlites.

Potassium isotope fractionation during silicate-carbonatite melt immiscibility and phlogopite fractional crystallization

Abstract Potassium (K) isotopes have been used as a tracer of K recycling in the Earth, but K isotope fractionation during magma evolution is poorly constrained. Here, we present K isotope data for a magmatic suite of alkaline silicate-carbonatite affinity. The suite was formed from liquid-liquid immiscibility and subsequent phlogopite fractionation. The K isotopic signatures of different rock types are in the following order: alkaline silicate lavas (δ41K = –0.424 to 0.090‰) > carbonatitic silicate lavas (δ41K = –0.640 to –0.035‰) > carbonatites (δ41K = –0.858 and –0.258‰). Phlogopite phenocrysts in the silicate lavas are isotopically lighter (δ41K = –0.628 to –0.534‰) than the lavas in which they occur (Δ41KPhlogopite-whole rock = –0.502 to –0.109‰). Correlations between δ41K values and chemical proxies of melt immiscibility and phlogopite fractionation indicate that K isotopes are significantly fractionated by both processes at a ~0.6‰ magnitude. Such K isotope variation overlaps the range of δ41K in arc lavas. Compilations of literature data further confirm the critical roles of melt immiscibility and phlogopite fractionation in K isotope variations of high-K lavas (K2O >1 wt%) from post-collision orogenic and intra-continental settings. In comparison, basaltic arc lavas are depleted in K2O (mostly <1 wt%) and lack evidence of significant phlogopite fractionation. The K isotope variations of arc lavas are mainly controlled by their mantle sources, which were metasomatized by melt or fluid released from the subducting slab. Therefore, K recycling and K isotope variation are controlled by distinct mechanisms in different tectonic settings.

Carbonatite formation in continental settings via high pressure – high temperature liquid immiscibility

The goal of this study is to compare compositions of high temperature silicate-carbonatite immiscible melts, known from melt inclusions and experiments, to compositions of silica-undersaturated volcanic rocks from continental settings, in order to improve understanding of the formation of calcite carbonatite rocks worldwide. Melt inclusions in this study are abundant in perovskites crystals from magnetite-perovskite cumulates sampled at the Kerimasi volcano in the East African Rift System. The temperature of complete dissolution of daughter minerals in the melt inclusions and the high CO2−content of the silicate melt (5.4–9.8 wt%) support early formation of the rock and entrapment of melts at high temperatures (∼1100 °C) and pressures (≥1 GPa). Heated-quenched melt inclusions indicate the presence of immiscible mafic-melilitite and Ca-Na-K-carbonatite melts together with a fluid phase at entrapment. Melilitite melts are low in SiO2 (29.4–33.5 wt%), moderate in MgO (4.0–5.9 wt%) but high in CaO (16.3–24.4 wt%), FeOT (9.0–13.5 wt%), Na2O (6.8–12.7 wt%) and K2O (1.8–3.2 wt%). Coexisting carbonatite melts also show high CaO-content (28.4–39.0 wt%), along with moderate-to-high Na2O (8.2–20.2 wt%) + K2O (4.1–6.6 wt%) compositions (total alkalis ranges between 13.1 and 24.3 wt%).We compared the studied silicate melts in the inclusions with a global dataset of 146 continental melilitite and 640 nephelinite compositions (GEOROC database). We argue that the studied calcite-saturated melilitite melts formed in a continental rift setting and were able to exsolve carbonatite melts that crystallized voluminous calcite carbonatite rocks during their evolution. In contrast, magnesian melilitite and nephelinite volcanic rocks from intracontinental settings are compositionally far away from any immiscibility field at feasible pressures and were only able to unmix carbonatite melts during late-stage evolution, leaving little opportunity for calcite crystallization. CaO- and alkali-rich, but extremely SiO2-undersaturated melilitite melts play a key role in early silicate-carbonatite immiscibility, can only be preserved in melt inclusions and cannot be represented by volcanic rocks. The parental melts of the studied melilitite-carbonatite melts probably formed via low-degree partial melting of modally metasomatized continental lithosphere.

Trace element partitioning in silica-undersaturated alkaline magmatic systems

Alkaline magmatism is an important chemical end-member of magmatic activity that typically occurs in response to small volume melting of asthenospheric- and/or lithospheric mantle material in intra-continental settings. Understanding trace element partitioning and phase equilibria during alkaline magmatism can therefore provide constraints on intra-continental geodynamic settings. However, the partitioning of trace elements between alkaline melts and their dominant equilibrium mineral phases remains poorly constrained. Feldspathoids in particular have received limited attention with regards to their trace element contents, hampering our ability to interpret geochemical trends in alkaline magmatic systems. In this study, we performed a series of 1 atmosphere experiments in a gas-mixing furnace using a variety of highly alkaline (Na2O + K2O = 4.15–14.97 wt%) and silica-undersaturated (SiO2 = 36.73–45.96 wt%) lava compositions from Nyiragongo, Democratic Republic of Congo, in order to investigate the partitioning behaviour of trace elements in minerals from alkaline magmas. Experimental runs were performed with oxygen fugacity buffered at both QFM (quartz-fayalite-magnetite equilibrium) and QFM + 1 and cover a range of geologically-relevant temperatures (1025–1200 °C). The quenched products of these experiments contained leucite, nepheline, melilite, clinopyroxene, olivine, and rhönite crystals, of which glass-crystal pairs were analysed for rare earth elements, large-ion lithophile elements, and high-field-strength elements. Leucite and nepheline host considerable quantities of large-ion lithophile elements but take up negligible amounts of more highly charged cations. Åkermanitic melilite readily incorporates mono- to trivalent cations with a preference for light over heavy rare earth elements, but incorporates only select divalent cations. Rhönite and clinopyroxene have analogous partitioning behaviours, with a strong preference for heavy over light rare earth elements. Fractionation modelling using the reported partitioning behaviours reproduces the 2021 eruption products of Nyiragongo, with 48% fractionation from an olivine-melilitic parental melt composition. Crystallization of trace-element poor feldspathoid amplifies pre-existing high LREE/MREE ratios of the parental magma and progressively increase trace element abundances for all but monovalent cations.

Late Paleoarchean to Neoarchean sedimentation on the Singhbhum Craton, eastern India: a synthesis

Subaerial rise of Earth's first continents took place 3.3–3.2 Ga years ago. This had significant influence on the ocean, atmosphere, and biosphere. Two important sedimentological consequences of continental emersion were (1) development of terrestrial (alluvial fan–fluvial–aeolian) depositional systems and (2) formation of shallow-marine sedimentary basins near the continental margin. Late Paleoarchean–Neoarchean terrestrial deposits, including development of an extensive paleosol around 3.2 Ga, and Meso–Neoarchean shallow-marine deposits have been reported from the Singhbhum Craton, eastern India from several localities. However, lateral and vertical changes in sedimentary facies associations and stratigraphic correlation between these terrestrial to shallow-marine sedimentary deposits have not yet been investigated. In this paper, we have reviewed the sedimentological characteristics of Late Paleoarchean to Neoarchean (3.2–2.8 Ga) stratigraphic successions of the Singhbhum Craton and have discussed sea level change and mode of stratigraphic sequence building. Our analysis shows that Singhbhum granitoid complex was emplaced around 3.3 Ga in an intracontinental setting, resulting in high continental freeboard conditions that triggered terrestrial (alluvial fan–braided fluvial) sedimentation (Dhosrapahar Formation). Continental sedimentation continued until ca. 3.0 Ga. In the eastern (Simlipal Group), western (Birtola Formation), and southwestern (the Mankarchua, Mahagiri, and Keonjhar formations) parts, shallow-marine sedimentation occurred after 3.0 Ga as a result of transgression. However, in the northern part (Dhanjori Formation), continental sedimentation prevailed throughout the Archean. These Late Paleoarchean to Neoarchean terrestrial and near-coastal successions of Singhbhum Craton should be the prime targets for intense geobiological research.

GEOCHEMISTRY AND AGE OF MAFIC DYKES OF THE Be-DEPOSIT SNEZNOE: LATE PALEOZOIC MANTLE PLUME MAGMATISM EVIDENCE IN EASTERN SAYAN

Within the East Sayan rift zone of the Late Paleozoic Barguzin salic large igneous province (SLIP), along with alkaline-peralkaline granitoids with ore mineralization, mafic subvolcanic bodies (gabbroids, mafic dykes) are common. One of the typomorphic mafic – peralkaline granitoid association is the Snezhnoye beryllium deposit. Using the U–Pb LA–ICP–MS method, mafic dykes of the Snezhnoye beryllium deposit were dated. The age of zircons from the diabase (dolerite) dyke was 301 ± 6 Ma, and from the microgabbro one, 297 ± 2 Ma. The obtained dates are close to the age of fluorite-beryl mineralization (306 Ma) and peralkaline granites with Ta-Nb mineralization attributed to the Ognite complex (311–295 Ma). The geochemical features of mafic dykes suggest the generation of mafic magmas from an asthenospheric mantle source, which, taking into account the intracontinental setting, testifies in favor of the plume-mantle nature of mafic – alkaline-peralkaline granitoid magmatism of the East Sayan rift (rare-metal) zone.

Mantle contribution to the generation of the giant Jinduicheng porphyry Mo deposit, Central China: New insights from combined in-situ element and isotope compositions of zircon and apatite

Identifying crust–mantle interaction is important for understanding the genesis of granite-hosted mineral deposits in post-collisional or intracontinental settings. We have used combined zircon-apatite trace element compositions, zircon HfO isotopes and apatite SrNd isotopes, which were all acquired by in-situ techniques, to evaluate the mantle contribution to the formation of a giant porphyry Mo deposit, the Jinduicheng deposit in the East Qinling–Dabie orogenic belt, Central China. Zircons from the host granites yielded a UPb age of ∼139 Ma, suggesting a post-collisional setting for the ore-related intrusion. Most magmatic zircons from the granites have Ti-in-zircon temperatures lower than 830 °C, moderate Zr/Hf ratios (25–55), and mantle-like δ18O values (peaking at ∼5.6‰), similar to typical I-type granites worldwide. The magmatic zircons have variable εHf(t) values (−26.7 to −7.2) and Ti-in-zircon temperatures (646–894 °C, peaking at ∼740 °C). The coexisting igneous apatites have initial 87Sr/86Sr ratios of 0.70694–0.71027, and variable εNd(t) values (−16.0 to −9.3), Sr contents (132–663 ppm), and Eu/Eu* values (0.12–0.47). The Sr–Nd–Hf isotopes of the apatites and zircons, along with the positive correlations between apatite Sr contents, Eu/Eu* values, and εNd(t) values, a positive correlation between Ti-in-zircon temperatures and zircon εHf(t) values, as well as mixing modeling using SrNd isotopes, suggest that at least two end-member melts were involved in the formation of the host granites: one is the ancient lower crust-derived melt, and the other is the metasomatized lithospheric mantle-derived melt, which have relatively enriched and depleted Sr–Nd–Hf isotopic compositions, respectively. We suggest that the post-collisional lithospheric thinning, asthenospheric upwelling and underplating of the mantle-derived mafic magmas may have triggered the crustal melting, magma mixing, and generation of the host granites of the Jinduicheng deposit. We postulate that, besides the heat, the metasomatized lithospheric mantle-derived magmas likely supplied additional volatiles (e.g., Cl, S), facilitating the porphyry Mo formation. Our study highlights that, combined in-situ element and isotope compositions of accessory minerals can provide multidimensional insights into petrogenetic tracing, and enhance the genetic interpretations of granites and related mineral deposits.

Contributions of juvenile lower crust and mantle components to porphyry Cu deposits in an intracontinental setting: evidence from late Mesozoic porphyry Cu deposits in the South Qinling Orogenic Belt, Central China

Contributions of juvenile lower crust and mantle components to porphyry Cu deposits in an intracontinental setting: evidence from late Mesozoic porphyry Cu deposits in the South Qinling Orogenic Belt, Central China

Intracontinental deformation of the Tianshan Orogen in response to India-Asia collision

How the continental lithosphere deforms far away from plate boundaries has been long debated. The Tianshan is a type-example of ongoing lithospheric deformation in an intracontinental setting. It formed during the Paleozoic accretion of the Altaids and was rejuvenated in the Cenozoic, which might be a far-field response to the India-Asia collision. Here we present seismic images of the lithosphere across the central Tianshan, which were constructed from receiver functions and Rayleigh wave dispersions along a N–S-trending linear seismic array. We observe an extensively deformed lithosphere in the Tianshan with inherited, structurally controlled brittle deformation in the shallow crust and plastic deformation near the Moho. We find that earlier multiple accretionary structures were preserved in the crust, which was deformed by pure-shear shortening in the south and thick-skinned tectonics in the north but was limitedly underthrusted by surrounding blocks. A balanced cross-section of Moho discontinuities supports the concept that intracontinental deformation in the Tianshan intensified synchronously with the direct contact between the underthrusting Indian slab and the Tarim Craton in the Late Miocene (~10 Ma). These findings provide a robust and unified seismic model for the Tianshan Orogen, and confirm that effective delivery of the India-Asia collision stress induced the rejuvenation of this intracontinental orogen.

Numerical study on the style of delamination

Delamination of the lower crust or lithospheric mantle is one explanation for the surface uplift observed in areas of mountain building. This process describes the removal of the lower part of the tectonic plate and can occur in various ways. Different styles of delamination typically have in common that the upper material (e.g., lowermost crust or lithospheric mantle) is denser than the underlying material (e.g., asthenosphere) and therefore sinks. It has been proposed that the higher density can be caused by the formation of eclogite. In this study we apply a thermomechanical model featuring a density increase within the lithosphere by a phase transition. The model setup is designed to investigate surface uplift and mountain building in an intracontinental setting. Specifically, the model is arranged to closely resemble central Mongolia. The models give insights into the dynamically evolving flow field with respect to the style of removal, therefore the general outcome is also applicable to other orogenic regions. In addition to a systematic study on the phase transition, we also investigate the influence of convergent motion and of the rheology of the crust. Our results reveal that for the absence of a dense (eclogite) layer, delamination initially occurs as a stationary Rayleigh-Taylor instability which appears as a late and short-lived event. In comparison, for a strong density contrast an early, long-lived peeling-off removal style with a stationary slab results. The subsequent asthenospheric upwelling causes further peeling-off events for all density contrasts. For this removal style a retreating slab is observed that occasionally breaks off giving way to a periodic behaviour. The findings confirm that a strong convergence and low viscosity of the crust promote delamination. In addition, the asthenospheric upwelling yields a wide and flat surface uplift. Such dome-like features are observed to be more pronounced for high density contrasts (i.e., strong eclogitisation).