Craton Margin Research Articles

Provenance and sedimentation age of the Proterozoic clastic succession of the Garhwal‐Kumaon Lesser Himalaya, NW‐India: Clues from U–Pb zircon and Sr–Nd isotopes

The paper presents data on the U–Pb chronology of detrital zircon grains and radiogenic isotopic composition (87Sr/86Sr, ɛNd) from the Proterozoic clastic successions of the Garhwal‐Kumaon Lesser Himalaya representing the extended northern Indian cratonic margin, NW India. The Proterozoic Lesser Himalayan Basin in Garhwal‐Kumaon Himalaya is divided into two sectors, namely, Inner Lesser Himalaya (ILH) and Outer Lesser Himalaya (OLH) by a tectonic boundary, namely the Tons Thrust (TT). Age distribution from inner and outer sectors of the Lesser Himalaya shows that the U–Pb chronology of most of these zircons provides Palaeoproterozoic (between 1.6 and 1.8 Ga) to Neoproterozoic (800 Ma) ages. The age data suggest sedimentation of the Rautgara Formation (Damtha Group) of ILH continued till the Neoproterozoic (~850 Ma), which was earlier regarded as ≤1.6 Ga. Tracking the detrital U–Pb zircon ages in the near adjacent cratonic parts point towards Aravalli Orogen as the major source region. Whole‐rock ƐNd(0) values for ILH rocks range from −37.6 to −14.6 and for OLH it ranges from −19.6 to −6.7. More negative ƐNd values along with dominance of Neoarchean‐Palaeoproterozoic ages in ILH indicates supply from more evolved protolith or recycled sources and less negative ƐNd values with major Neoproterozic zircon ages from OLH supports for less evolved source rock. The change from more negative ƐNd to less negative ƐNd values progressively upward the stratigraphy can be due to a shift in source with time. Both U–Pb zircon and ƐNd supports for a continuous sedimentation model, rule out the presence of ~500 Ma unconformity within the LH and argues for separate evolution of the Lesser Himalayan Basin on the trailing edge of the extended north Indian craton in “Columbia” configuration.

Provenance, Age, and Tectonic Settings of Rock Complexes (Transangarian Yenisey Ridge, East Siberia): Geochemical and Geochronological Evidence

The tectonic evolution of the Siberian Cratonic margins offers important clues for global paleogeographic reconstructions, particularly with regard to the complex geological history of Central Asia and Precambrian supercontinents Columbia/Nuna and Rodinia and its subsequent breakup with the opening of the Paleo-Asian Ocean. Here, we present an overview of geochemical, petrological, and geochronological data from a suite of various rocks to clarify the age, tectonic settings, and nature of their protolith, with an emphasis on understanding the tectonic history of the Yenisey Ridge fold-and-thrust belt at the western margin of the Siberian Craton. These pre-Grenville, Grenville, and post-Grenville episodes of regional crustal evolution are correlated with the synchronous successions and similar style of rocks along the Arctic margin of Nuna-Columbia and Rodinia and support the possible spatial proximity of Siberia and North Atlantic cratons (Laurentia and Baltica) over a long period ~1.4–0.55 Ga.

The role of edge-driven convection in the generation ofvolcanism – Part 2: Interaction with mantle plumes, applied to the Canary Islands

Abstract. In the eastern Atlantic Ocean, several volcanic archipelagos are located close to the margin of the African continent. This configuration has inspired previous studies to suggest an important role of edge-driven convection (EDC) in the generation of intraplate magmatism. In a companion paper (Manjón-Cabeza Córdoba and Ballmer, 2021), we showed that EDC alone is insufficient to sustain magmatism of the magnitude required to match the volume of these islands. However, we also found that EDC readily develops near a step of lithospheric thickness, such as the oceanic–continental transition (“edge”) along the western African cratonic margin. In this work, we carry out 3D numerical models of mantle flow and melting to explore the possible interactions between EDC and mantle plumes. We find that the stem of a plume that rises close to a lithospheric edge is significantly deflected ocean-ward (i.e., away from the edge). The pancake of ponding hot material at the base of the lithosphere is also deflected by the EDC convection cell (either away or towards the edge). The amount of magmatism and plume deflection depends on the initial geometric configuration, i.e., the distance of the plume from the edge. Plume buoyancy flux and temperature also control the amount of magmatism, and influence the style and extent of plume–EDC interaction. Finally, comparison of model predictions with observations reveals that the Canary plume may be significantly affected and deflected by EDC, accounting for widespread and coeval volcanic activity. Our work shows that many of the peculiar characteristics of eastern Atlantic volcanism are compatible with mantle plume theory once the effects of EDC on plume flow are considered.

Age and origin of the West Kimberley lamproites, Western Australia

New high precision 40Ar/39Ar ages are presented which more tightly constrain the eruption ages of the West Kimberley lamproites of northwestern Western Australia. The data show a southward progression in ages of the lamproites over a distance of ∼135 km, from ∼22.5 Ma in the northernmost lamproites of the Ellendale Field to ∼19 Ma in the Noonkanbah Field in the south, excluding the Walgidee Hill lamproite. The large (∼490 ha) zoned Walgidee Hills lamproite at the northern edge of the Noonkanbah Field is the youngest of the West Kimberley lamproites and represents a later pulse of lamproite magmatism at 17.4 Ma, >1 Ma after the main lamproite magmatism.The southward progression of lamproite ages extends across a wide swath from the Proterozoic King Leopold Orogen at the southern margin of the Kimberley Craton to the Phanerozoic Fitzroy Trough in the Canning Basin. The magmatism does not appear to be the result of the migration of a mantle plume, but more likely reflects small degrees of partial melting of old, thick, metasomatised lithosphere extending from the craton margin beneath the basin, with melting triggered by regional uplift and/or small-scale mantle convection at the craton margin. Emplacement of the lamproites likely exploited ancient trans-lithospheric structures associated with the Proterozoic orogen at the craton margin and structures associated with the formation of the Canning Basin in the Phanerozoic.

Investigating assembly timing of Ediacaran Serra dos Órgãos batholith – Hints on prolonged magmatism at Ribeira belt, SE Brazil

The Pan-African/Brasiliano orogeny, result of the West-Gondwana assembly from the Neoproterozoic to Ordovician, had produced the Ribeira and Araçuaí belts at the edges of the São Francisco and Congo cratonic margins. In the central Ribeira belt, the docking of accretionary systems to the reworked continental margin of the São Francisco paleoplate culminated in a complex Ediacaran granite production event. Syn-to late-collisional I- and S-type granites and hybrid granitoids intruded the reworked margins and accreted continental masses, as well as the Tonian to Ediacaran juvenile magmatic arcs represented by the Serra da Prata and Rio Negro Complexes. The Serra dos Órgãos batholith is the main intrusion emplaced at the juvenile crust, forming an elongated NE-SW trending tabular body that crops out in the highland region of Rio de Janeiro State, Southeast Brazil. Geochemical and geochronological data in addition to field and petrological evidence was gathered to explore the assembly timing of the SOB granitic end-member. Zircon U–Pb SHRIMP and LA-ICP-MS geochronological data yield a prolonged emplacement timelapse for the studied granites and, along with published U–Pb ages of the SOB granodioritic end-member, enable the characterization of the Serra dos Órgãos as a composite batholith. The wide time interval of heat input inside intermediate crust is consistent with a prolonged, slow-cooling hot orogenic belt model for the Araçuaí-Ribeira Orogenic System.

Lithostratigraphy, Origin, and Geodynamic Setting of Iron Formations and Host Rocks of the Anyouzok Region, Congo Craton, Southwestern Cameroon

In Cameroon, most of the iron formation occurrences reported are found within the Nyong and Ntem Complexes. The Anyouzok iron deposit is located in the Nyong Complex greenstone belts, which represent the NW margin of this Congo craton. The main lithological units comprise the iron formations (IFs) unit, consisting of banded IFs (BIFs) and sheared BIFs (SBIFs), and the associated metavolcanic rocks unit consisting of mafic granulite, garnet amphibolite, and biotite gneiss. Within the Anyouzok area, BIFs are rare, while SBIFs are ubiquitous. This study reports the petrography, mineralogy, and whole rock geochemistry of IFs and interbedded metavolcanic rocks of the Anyouzok iron deposit. The abundance of cavities, higher Fe contents (49.60–55.20 wt%), and strong Eu anomalies (Eu/Eu* = 2.14–3.17) within the SBIFs compared to the BIFs suggest that SBIFs were upgraded through post-depositional hydrothermal alteration activities. REE signatures indicate the contribution of both seawater and hydrothermal fluids during BIFs precipitation. Mafic granulite and garnet amphibolite protoliths were derived from the partial melting of a metasomatized spinel lherzolite depleted mantle source. The overall compositional variations of the Anyouzok IFs and interbedded metavolcanic rocks endorse an Algoma-type formation deposited in the back-arc basin under suboxic to anoxic conditions.

Ferruginious-aluminous metapelites of the North Yenisei Ridge: Formation paleosettings, nature and age of protolith

Research subject. Fe- and Al-rich metapelites of the Teya, Garevka and Angara complexes of the North Yenisei Ridge (western margin of the Siberian craton) were studied. Research methods. Recently-obtained geochronologic (SHRIMP II U-Pb zircon dating) and geochemical data on the distribution of major and trace elements were used to reproduce the composition and age of the protolith, along with the paleofacies formation conditions. Results. The ferruginous-aluminous metapelites of the North Yenisei Ridge consitute redeposited and metamorphosed products of Precambrian weathering crusts predominantly of the kaolinite rather than lateritic type, as was previously thought. The chemical weathering of rocks in the Early Proterozoic on the Yenisei Ridge did not reach the deep stage of lateritization with the formation of zones of final decomposition of aluminosilicates; however, this process was limited to the formation of weathering products of predominantly kaolinite-illite-montmorillonite-quartz composition. Conclusions. The petro- and geochemical characteristics of the studied metapelites are mainly due to the features of sedimentation during the formation of the protolith, which could have been formed due to the erosion of the Lower Proterozoic microgneisses of the Siberian craton with ages in the range of 1962-2043 Ma with the involvement of granitoid and volcanic admixture of mafic rocks into the erosion area and subsequent accumulation in marginal continental shallow basins under the conditions of a humid climate and a calm tectonic regime. These results are consistent with the data of lithofacies analysis and geodynamic reconstructions of the Precambrian evolution of geological complexes in the North Yenisei Ridge.

Imaging the Whole‐Lithosphere Architecture of a Mineral System—Geophysical Signatures of the Sources and Pathways of Ore‐Forming Fluids

AbstractMineral systems can be thought of as a combination of several critical elements, including the whole‐lithosphere architecture, favorable geodynamic/tectonic events, and fertility. Because they are driven by processes across various scales, exploration benefits from a scale‐integrated approach. There are open questions regarding the source of ore‐forming fluids, the depth of genesis, and their transportation through the upper crust to discrete emplacement locations. In this study, we investigate an Au–Cu metal belt located at the margin of an Archean‐Paleoproterozoic microcontinent. We explore the geophysical signatures by analyzing three‐dimensional models of the electrical resistivity and shear‐wave velocity throughout the lithosphere. Directly beneath the metal belt, narrow, vertical, finger‐like low‐resistivity features are imaged within the resistive upper‐middle crust and are connected to a large low‐resistivity zone in the lower crust. A broad low‐resistivity zone is imaged in the lithospheric mantle, which is well aligned with a zone of low shear‐wave velocity, examined with a correlation analysis. In the upper‐middle crust, the resistivity signatures give evidence for ancient pathways of fluids, constrained by a structure along a tectonic boundary. In the lower lithosphere, the resistivity and velocity signatures are interpreted to represent a fossil fluid source region. We propose that these signatures were caused by a combination of factors related to refertilization and metasomatism of the lithospheric mantle by long‐lived subduction at the craton margin, possibly including iron enrichment, F‐rich phlogopite, and metallic sulfides. The whole‐lithosphere architecture controls the genesis, evolution, and transport of ore‐forming fluids and thus the development of the mineral system.

Late Archean–Paleoproterozoic plate tectonics along the northern margin of the North China craton

AbstractThe North China craton of central Asia is a natural laboratory for investigating early Earth tectonic processes including subduction and large-scale horizontal crustal motions. However, it remains unclear how and when the North China craton formed from the amalgamation of several blocks and orogens including the Archean Western and Eastern blocks, the late Archean Central Orogenic Belt, and the Paleoproterozoic Northern Margin orogen. In this study, we integrated new and existing geological field observations, zircon and baddeleyite U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-C-O isotope analyses along the northern margin of the North China craton to improve our understanding of its late Archean–Paleoproterozoic tectonic evolution. Observations show the Wuchuan ultramafic-mafic complex of the Northern Margin orogen contains a mixture of mylonite gneiss, serpentinized ultramafic blocks, and amphibolite pyroxenite xenoliths. Geochronology and geochemistry results suggest that the ca. 2.55–2.5 Ga development of the Wuchuan ultramafic-mafic complex was associated with subduction, which was followed by ca. 2.39 emplacement of lamprophyre dikes during continental rifting. Subsequent Paleoproterozoic subduction magmatism, continental collision, and post-orogenic rifting between the North China craton and southern margin of the Siberian craton are evidenced by ca. 2.0 Ga arc granitoids, ca. 1.87–1.85 Ga A-type, garnet-bearing granitoids and mafic dikes, and ca. 1.86 carbonatite. Ca. 2.0 Ga collision and ca. 1.87–1.85 Ga rifting were coincident with orogen-normal left-slip shear. Our findings show that the ca. 1.9 Ga North China-Siberia collision was a key event during the assembly of the Columbia supercontinent that occurred via modern plate-tectonic processes.

Incipient collision of the Rae and Slave cratons at ca. 1.95 Ga

AbstractIncipient collision between the Rae and Slave cratons represents a critical initial stage in the final assembly of the composite Laurentian craton. However, the timing and kinematics of this event remain poorly constrained. Here, we integrate phase equilibria modeling and Lu-Hf garnet and U-Pb monazite and zircon petrochronology to precisely date the timing of peak metamorphism along the Great Slave Lake shear zone, the central segment of the western Rae cratonic margin that was the locus of terminal Rae-Slave collision. Zircon and monazite U-Pb ages broadly constrain prograde-to-peak metamorphism to 1933–1913 Ma, whereas the timing of upperamphibolite facies peak metamorphism (0.8–1.0 GPa and 750–800 °C) is best constrained by two garnet Lu-Hf ages of 1931 ± 12 and 1917 ± 6 Ma. Our results highlight the inherent strengths and limitations of garnet, monazite, and zircon petrochronology in the context of dating Paleoproterozoic-aged metamorphism. Based on the timing of crustal thickening constrained by Lu-Hf garnet geochronology, we posit that the Slave craton first collided with the western Rae margin at ca. 1950 Ma. Taken together with prior constraints on the timing of metamorphism elsewhere along the western Rae margin, we propose incipient collision was followed by the northwards zippering of the margins and the eventual ~500 km of dextral translation of the Slave craton along the Great Slave Lake shear zone.

Prospectivity mapping of carbonatite-associated iron oxide deposits using an integration process of ASTER and Sentinel-2A multispectral data

ABSTRACT Carbonatites and associated rocks are commonly known to host precious commodities, including rare earth elements (REE), niobium, and phosphates. They form ring-shaped complexes frequently found as small occurrences in hard-to-reach areas, making their exploration a challenging task using conventional methods. Remote sensing has been increasingly regarded as a time- and cost-saving method to map and identify new potential zones, meeting the growing need for mineral resources. In this study, we use multispectral data by combining information from ASTER and Sentinel 2A sources to delineate and map carbonatite-associated REE- and Nb-enriched iron oxide deposits. The Moroccan Oulad Dlim massif, at the NW margin of the west African craton, where carbonatite occurrences are reported, was selected as a study area. A fuzzy gamma approach was adopted to combine information from several image processing techniques, including mineral indices, principal component analysis (PCA) and minimum noise fraction (MNF). A concentration-area (C-A) fractal model is applied together with the prediction-area (P-A) plot for objective anomalies classification and prediction ability assessment for each fuzzy integrated map. Subsequently, a prospective map that predicts and defines potential areas was produced through the integration of Sentinel- and ASTER-derived evidential maps. Validation was carried out via field surveys coupled with in-situ portable X-Ray Fluorescence analysis and laboratory mineralogical characterization. The Results were consistent with field data, reaching a prediction rate of 90%. They reveal the importance of the investigated structures as potential sites for REE, Nb, and phosphates. These outcomes are encouraging and provide evidence that data fusion (ASTER-Sentinel 2A) is an effective and helpful approach for geological exploration programmes.

Cambrian/Ordovician boundary as a milestone in the sedimentation history of the southern Siberian craton: Evidence from U-Pb dating of detrital zircons

The paper presents petrographic, geochemical, and new U-Pb zircon (LA-ICP-MS) geochronological data on the Upper Lena Fm. sediments which make part of the Cambrian sedimentary cover of the Siberian craton. The composition of lithic fragments in these clastic sediments, including mafic and felsic volcanics and granitoids, along with the correlation of major element ratios and the values of trace-element ratios point to predominant igneous (mainly felsic) rocks in the provenance. The U-Pb ages of detrital zircons from the sampled Upper Lena Fm. sandstones are mostly Early Paleozoic (∼500 Ma), with very few Archean-Paleoproterozoic determinations and quite many Neoproterozoic zircons. Ediacaran and Earliest Cambrian clastic sediments bearing Early Precambrian and Neoproterozoic detrital zircons, as well as Neoproterozoic and Early Paleozoic complexes of the Central Asian Orogenic Belt, could provide material for the Upper Lena clastic deposition. The geochronological evidence shows that the Upper Lena Fm. sandstones were deposited over vast territories of the southern Siberian craton in the Late Cambrian, in a single sedimentary basin, and originated from the same provenance. The Upper Lena deposition event in the Late Cambrian, immediately before the Ordovician period, makes a key milestone in the geological history of Siberia: the end of a large-scale Early Cambrian transgression and the beginning of gradual uplift of the southern craton margin, at least till the Carboniferous, which was associated with the formation of the Central Asian Orogenic Belt, a major tectonic unit of Asia.

Preservation of basement architecture in zircon Hf isotope maps

AbstractZircon Hf isotopic mapping has been proposed to image large‐scale lithospheric architecture. Here we present zircon Hf isotope maps of the Proterozoic Albany–Fraser Orogen (AFO) in Western Australia, which records a protracted geological history in both extensional and compressional tectonic regimes. The spatial variation in Hf isotope composition of AFO granites is nearly orthogonal to the dominant crustal fabric. Although the 176Hf/177Hf compositions are too juvenile to represent direct melts of the Archaean basement, the variability in 176Hf/177Hf in these AFO magmatic rocks closely mimics the structure of the adjoining Yilgarn Craton. This orogen‐parallel heterogeneity is best illustrated during earliest Palaeoproterozoic magmatic events, which are characterised by low Sr/Y ratios, consistent with tectonic models involving relatively uniform extension along the craton margin during the Palaeoproterozoic. These results demonstrate that regional‐scale zircon Hf variability does not always image coeval lithospheric architecture, but may help illuminate deep basement structure in regions with complex deformation histories.

Hydrous Juvenile Lower Crust at the Western Yangtze Craton Margin as the Main Source of the Beiya Porphyry‐skarn Au Deposit

AbstractThe Beiya porphyry‐skarn Au deposit is one of the largest gold deposits in China, temporally and spatially associated with Eocene intrusions in a post‐collisional setting in western Yunnan, China. In this study, we report new whole‐rock geochemistry, Sr‐Nd isotope, zircon U‐Pb geochronology and in situ zircon Hf‐O isotopes of quartz‐monzonite and biotite‐monzonite porphyries from the Beiya deposit. The porphyry‐skarn mineralization at the Beiya deposit is mainly associated with the quartz monzonite porphyry (35.8 ± 0.6 Ma), while the biotite‐monzonite porphyry (34.3 ± 0.5 Ma) represents a post‐mineralization intrusion crosscutting the main orebodies and the quartz‐monzonite porphyry. Both intrusions have high‐K and adakitic composition and are characterized by high Sr/Y ratios, high SiO2 and Al2O3 concentrations (SiO2 = 69.80–73.86 wt%; Al2O3 = 14.11–15.19 wt%), and low MgO, Cr, and Ni concentrations (MgO = 0.2–1.0 wt%; Cr = 1.76–11.13 ppm; Ni = 2.52–11.72 ppm). Their Sr‐Nd isotope compositions (87Sr/86Sr = 0.7066–0.7077; εNd(t) = –5.3 to –1.5) are consistent with the lower crustal‐derived amphibolite xenoliths (87Sr/86Sr = 0.7060–0.7100; εNd(t) = –10.0 to 0.0), indicating that they might be derived from a thickened juvenile lower crust beneath the Yangtze Craton. The biotite‐monzonite porphyry has lower zircon δ18O values of +5.3‰ to +6.8‰ and higher εHf(t) values of –2.3 to +5.5 than those of the quartz‐monzonite porphyry with δ18O values of +7.1‰ to +8.2‰ and εHf(t) values of –3.8 to +1.5, implying that they were derived from different parts of the lower crust. High Ba/La and Pb/Ce ratios suggest that the quartz‐monzonite porphyry is derived from a volatiles‐rich reservoir. Relatively higher La/Yb, Sm/Yb and Dy/Yb ratios of the biotite‐monzonite porphyry indicate residual garnet in the source, indicating a deeper source than that of the quartz‐monzonite porphyry. The hydrous components should be represented by the amphibole‐rich lithologies, which has relatively shallower depth than that of the garnet‐bearing mafic thickened lower crust. Our data suggest that the mineralized quartz‐monzonite porphyry at the Beiya deposit is derived from partial melting of amphibole‐rich lithologies in the upper part of the thickened juvenile lower crust beneath the Yangtze Craton, while the post‐mineralization biotite‐monzonite porphyry is derived from the basal, and volatiles‐poor, part of the juvenile lower crust.

The record of plume-arc interaction in the Southern São Francisco Craton – Insights from the Pitangui greenstone belt

Greenstone belts are important windows to the geodynamic evolution of the early Earth. Comprehensive and continuous stratigraphic columns of greenstone belt sections are rarely preserved in South America due to polyphasic deformation, metamorphism and recycling at the cratonic margins and thick regolith layers mainly yielded by heavy weathering. Some recent studies in rocks intersected by drill cores have been pivotal to unveil the stratigraphy of Archean and Paleoproterozoic belts in covered greenstone belts in the São Francisco and Amazonian cratons. This work investigates drill cores from the Pitangui Greenstone belt (PGB), in the Southern São Francisco Craton (SSFC). Rapid localized tectonic transformations are revealed by lithogeochemistry. Two distinct lithochemical associations occur spatially intercalated. The first one is the komatiite-tholeiite association that presents plume signature (low LREE/HREE and no negative HFSE anomalies), with associated low Al2O3/TiO2 and elevated (Gd/Yb)N indicative of a garnet-bearing source, which was also slightly depleted, as suggested by the subchondritic Nb/Ta ratios. In contrast the arc-basalts, andesites and graywackes association displays high LREE/HREE and negative Nb and Ti signatures, which may indicate a subduction-like environment or of crustal contamination. Important part of the subduction-like package are the Nb-enriched basaltic andesites and related volcaniclastic rocks. The presence of this lithotype indicates that metasomatism of the mantle by slab-derived melts was probably an important mechanism in the PGB. It is argued that the assemblages mark a transient stratigraphy from a thick mafic-ultramafic domain generated in an oceanic-plateau to an overlying volcaniclastic-sedimentary dominated package likely generated in an arc-like environment at 2.77 Ga. The intercalation of plume and arc products in the PGB is interpreted as an example of plume-arc interaction, which seems to have been more common in the Archean, as evidenced by numerous occurrences interpreted as such worldwide.

Paleoproterozoic collision tectonics in West Africa: Insights into the geodynamics of continental growth

In order to decipher the collision processes between the Eburnean (Paleoproterozoic) accretionary orogen and its flanking Archean craton, photointerpretation was combined with the extraction of structural data from geological quadrangle maps to draw the structural pattern of the southern West African craton. This pattern reveals multi-scale transpressive anastomosing shear zone networks affecting both the Eburnean orogen and its Archean buttress that absorbed protracted E-W convergence as juvenile crust was still being accreted. Collision involved oblique transcurrent shearing of the craton margin that led to the asymmetrical indentation and escape of the accretionary orogen. As part of the collisional deformation pattern, the craton margin developed a narrow orogenic system recording intense transpressional strain and high pressure-high temperature metamorphism. Lithospheric mantle delamination at the craton margin explains focused heat advection and extreme thermal softening of the crust, enabling steep narrow transpressional collisional orogeny at the rear of accretionary orogens. Delamination also provides a mechanism for the growth of accretionary orogens at the expense of their flanking cratons.

Upper mantle anisotropy and deformation beneath the western Mongolian Plateau revealed by SKS splitting

The Mongolian Plateau, as an important geological unit of the Central Asian Orogenic Belt, has experienced extensive deformations in multiple periods since the Proterozoic. In this study, we measured SKS splitting from 114 portable stations to reveal the complicated deformation of the upper mantle under western Mongolia. The predominant NW-SE-oriented fast polarization directions (FPDs) are consistent with lithospheric deformation and asthenospheric flow to the first order. However, the FPDs of the stations close to the Siberian Craton are more parallel to the cratonic margin than those departing from the craton. It indicates that the shape of the rigid Siberian Craton may determine the extent of the present lithospheric deformation and cause the deflection of the asthenospheric flow. Large SKS splitting delay times are distributed in the Hovsgol rift and Gobi-Altai range, implying superimposed past and present lithospheric anisotropy, and the asthenospheric anisotropy as well under the Hovsgol rift. Small delay times in the Hangai Dome indicate the asthenospheric upwelling which causes the uplift of the dome.

Genesis of orogenic gold systems in the Daduhe belt: Evidence of long-lived fertile mantle lithosphere as a source of diverse metallogeny on the western margin of the Yangtze Craton, China

The poorly documented Daduhe gold belt on the western margin of the Yangtze Craton, an important metallogenic belt in southwest China, is characterized by typical orogenic gold systems against a highly anomalous geodynamic setting. A synthesis of regional and deposit geology, mineralization ages, and S-O isotopic compositions of 33 gold deposits from the Danba-Kangding-Shimian-Mianning area of the belt reveals regional metallogenic patterns, establishes a consistent genetic model, and demonstrates a source equivalent to other diverse mineral systems on the western margin of the Yangtze Craton. The Daduhe orogenic gold deposits are controlled by the lithosphere-scale Xianshuihe and Anninghe strike-slip faults in the northern and southern segments of the belt, respectively. The alteration and ore mineral assemblages of the Daduhe orogenic gold deposits define a wide range of hypozonal, mesozonal, and epizonal sub-types. They formed during two episodes: 1) Early Jurassic gold mineralization related to closure of the Paleo-Tethys Ocean, with a large hypozonal gold deposit on the margin of the Danba dome in the northern segment; 2) Cenozoic gold mineralization associated with continental collision on the Tibetan Plateau, with small mesozonal and epizonal gold deposits in shear zones in migmatite basement and at contacts with cover sequences in both the northern and southern segments. At the belt and deposit scales, the ore-controlling structures for both mineralization episodes are NNE-trending second-order extensional faults or their intersections with the dominant NW-trending compressional faults. The three deposit sub-types have overlapping δ34S of 0 ∼ 10‰, δ18Ofluid of 6 ∼ 11‰, and low He-Ar isotope ratios, all consistent with derivation of ore fluids from metasomatized mantle lithosphere. Importantly, the Daduhe orogenic gold deposits have similar S-O-He-Ar isotope compositions to the giant IOCG, Cu-Ni-PGE, Fe-V-Ti, anomalous porphyry Cu-Mo-Au, and carbonatite-related REE deposits in the Panxi region on the shared margin of the western Yangtze Craton. This suggests that these diverse mineral systems have a common source of ore fluids and metals derived from fertile mantle lithosphere during different geodynamic and metallogenic events. Such a widespread hydrous metal-rich deep source with lithosphere-scale faults as ore fluid pathways, and a depth continuum of orogenic gold deposits in the Daduhe belt suggest a high prospectivity and a higher future gold endowment for the belt. This study adds to an increasing awareness of the diversity of mineral systems on specific craton margins globally.

Geothermal Model of the Fore‐Yenisey Sedimentary Basin—Transitional Structure between the Ancient Siberian Platform and the Young West Siberian Plate

AbstractResults of the investigation into the geothermal regime of the Fore‐Yenisey sedimentary basin, formed during the collision and subsidence of the Kas‐Turukhan microcontinent and the western margin of the Siberian craton in the late Neoproterozoic and early Paleozoic are reported. It was established that the structural and geothermal conditions of the upper Precambrian–Paleozoic sections are similar to those in the western regions of the Siberian platform and are characterized by rather low geothermal gradients (12.5–25.5 °C/km). In the western parts of the basin, formation temperatures in the uppermost pre‐Jurassic sediments are 50°C–85°C, decreasing eastward to 30°C–55°C. For the first time, the detailed geothermal model of the basin sedimentary fill was developed. This model allows predictions of the geothermal conditions of Earth's interior.

GEOCHEMICAL FEATURES OF DIKE AILLIKITES AND ALKALINE ROCKS OF THE BOLSHETAGNINSKY MASSIF (URIK-IYA GRABEN, EAST SAYAN REGION)

The ICP-MS determinations have been made on microelement content of ~650–640 Ma ultramafic lamprophyre-aillikite dikes and alkaline silicate rocks and carbonatites from the Bolshetagninsky ijolite-syenite massif, spatially overlapped in the northern Urik-Iya graben on the southwestern margin of the Siberian craton. There have been identified two types of spectra of microelement distribution, typical of the Bolshetagninsky massif aillikites and alkaline silicate rocks, respectively; both types of spectra demonstrate significant (more than two orders of magnitude) enrichment in most incompatible elements relative to the primitive mantle. Aillikites have tilt-left distribution spectra of rare elements with Rb, К , Pb, Sr-P, Zr-Hf, ±U minimums and less-pronounced Y minimum. Multi-element spectra of ijolites-melteigites are characterized by Th, Та , Pb, Hf, ±Zr, ±Ti minimums which is also reflected in nepheline syenite and carbonatite spectra. The Bolshetagninsky massif aillikites and alkaline rocks differ also in Nb/Ta, Zr/Hf, Th/Nb, Th/U ratios. Geochemical differences imply that the parental melts of dike aillikites and alkaline rocks are derived from different mantle sources.