Precambrian Research最新文献

The Paleoproterozoic Tanami Group of the Granites—Tanami Orogen hosts a known endowment of > 20 Moz of gold mineralization across numerous deposits. Throughout the region, host lithologies impart a first-order control on the style, grade and endowment of mineralization. However, the genetic relationship and regional distribution of three regionally correlative formations (the Dead Bullock, Mount Charles and Stubbins formations) remains enigmatic. Here, we combine lithostratigraphic analysis, lithogeochemistry, detrital zircon geochronology and geophysical observations to propose a new evolutionary model for the Tanami Group. We find that the Tanami Group was deposited in a continental back-arc basin setting and developed in four stages. Package A marked the onset of the rifting of the continental Archean basement. Sedimentary rocks within Package A are dominantly felsic in composition and display a unimodal detrital zircon age population at ∼ 2530 Ma. Package B marks the onset of mafic volcanism; consequently, volcaniclastic and sedimentary rocks within Package B display a distinct mafic geochemical signature. Detritus shed from an Archean basement persisted throughout this period, and a unimodal detrital age component at ∼ 2530 Ma persists. Package C marked the waning stages of voluminous mafic volcanism across the Tanami Basin. Sedimentary volcaniclastic rocks within Package C display a mixed mafic and felsic composition. Detrital zircon age spectra record multiple peaks at ∼ 2500, 3000 and 3350 Ma, which may reflect erosion of older Archean basement domains or changes in paleo-drainage. Package D marked a significant change in the source of detritus entering the basin from dominantly Archean to Paleoproterozoic, with dominant detrital age peaks at ∼ 1860 Ma. Despite significant lithofacies variations, the presented model highlights multiple correlations between the Dead Bullock and Mount Charles formations. The Mount Charles Formation likely represents a volcanic source-proximal, high-energy depocenter. In contrast, the Dead Bullock Formation was deposited in a lower energy setting, distal to volcanic centers. These observations are important to mineral exploration. Notably, the Mount Charles Formation is distributed over a significantly greater area than previously thought, and the under-represented Hangingwall and Footwall sequences of this formation are interpreted to host significant gold mineralization at the Oberon and Groundrush deposits.

Impact cratering is an important geological process on Earth. This review summarizes the state of knowledge of the Precambrian (Hadean, Archean, and Proterozoic) impact record on Earth. After an early collision that may have led to the formation of the Moon, both Earth and Moon suffered intense post-accretionary bombardment at about 4.5 and 3.9 billion years ago. Evidence for a “late heavy bombardment” phase at about 3.85 Ga is currently debated because the lunar rock record might be biased, and no relevant impact record has yet been confirmed on Earth. Several 3.5 to 2.5 Ga old spherule layers in South Africa and Australia, and two large impact structures, Vredefort and Sudbury, at about 2 Ga, represent most of the oldest actual terrestrial impact record. The impact record for more than half of the geological history of the Earth is incomplete, and, as a result of the lack of old continental crust on Earth, there is also only limited evidence for impact processes during the first 2.5 billion years of Earth history. Some more (mostly badly dated) impact structures are (partly) preserved for the Proterozoic period, as are a couple of ejecta layers. Given that the rock record preserved on Earth is very restricted for this early time period, the limited impact record is not surprising, but as recent discoveries show, there is still room for more research and new findings.

The ca. 2.08 Ga Devarabanda mafic dyke swarm in the eastern Dharwar craton exhibits a spectacular radiating geometry spanning 167˚. This study focuses on unravelling the intra-swarm variability in geochemistry and emplacement systematics across its three distinct sub-swarms designated as SS-1, SS-2 and SS-3, trending N, NW, and NE, respectively. Through an integrated study that includes whole rock geochemistry, anisotropy of magnetic susceptibility (AMS), and available geochronology and gravity data, we identify shared differentiation trends across all the sub-swarms, suggesting a common genetic linkage. While crustal influence is evident, the SS-3 displays relatively primitive geochemical signatures, the other two sub-swarms signify more highly differentiated pulses, indicating multiple magma pulses in the ca. 2.08 Ga dyke swarm. AMS analyses reveal lateral magma flow for SS-3, while vertical injection feeds the other two sub-swarms. Based on this study, we propose the role of a mantle plume for the formation of the three sub-swarms; SS-3 formed from a shallow crustal magma chamber during the pre-rift phase, due to a central domal uplift. Subsequently, SS-1 and SS-2 were emplaced at a later rift phase, where SS-2 was emplaced along a pre-existing NW-trending rift, and SS-1 formed during active rifting and crustal thinning. Evidence supporting an older NW-trending rifting event in the region corresponding to the SS-2 exposures was observed from available gravity data. The presence of discrete crustal magma reservoirs, separate for each sub-swarm, surrounding the domal uplift, explains the intervening dyke-poor areas, and suggests the formation of an apical graben as a precursor to the Proterozoic Cuddapah basin.

The granite-greenstone belt is the main component of Archean crust, and is critical for resolving the disputes on the tectonic evolution and dominant regime of the early Earth. The North China Craton (NCC) is one of the oldest cratons in the world, and is composed of several continental micro-blocks. The greenstone belts, welding the micro-blocks, were generally considered to be continental or arc-continental collision belts during the amalgamation of the micro-blocks in the late Neoarchean. Thus, the structural pattern of these greenstone belts can provide essential clues for the crustal deformation style during early Precambrian orogeny. The western Shandong granite-greenstone belt (WSGB) located in the eastern NCC was considered as the collision belt of the Jiaoliao and Qianhuai Blocks. There exists a series of ductile shear zones with strike-slip kinematics, including Qixingtai, Nanliu, Yanlingguan, Xintai, Dongling–Huacun, Mengshan, Fengyang–Liangqiu, Qingyi and Baiyan. This paper conducts detailed geometric, kinematic and deformation geochronology studies, and reveal that the mylonitic foliation of the ductile shear zones in the WSGB strikes NW280–350° with angles of 60–80°. The mineral stretching lineation gently plunges to NE or SW at 2–25°. Kinematic indicators show sinistral strike-slip shearing sense. The deformation temperature can be divided into two ranges: 550–600 ℃ and 400–500 ℃, implying that the ductile shear zones in the WSGB was ought to experience early-stage high-temperature and late-stage low-temperature deformation. The deformation age is ∼ 2.58–2.47 Ga constrained by zircon LA-ICP-MS U–Pb and ⁴⁰Ar–³⁹Ar dating, and this age range includes early-stage high-temperature deformation. Structural restoration through balanced profiles and stereographic projection shows that the initial orientation of NW–NWW-trending ductile shear zones was dipping to the SW, and still were featured by strike-slip kinematics. Syn-kinematic arc-affinity magmatism and sedimentation demonstrate that the NW–NWW-trending ductile shear zones in the WSGB were caused by SE-ward plate convergence, which also led to the NW–SE-trending movement of different blocks along the strike of ductile shear zones. The WSGB represents an active continental margin in the southwestern Jiaoliao Block with intense arc-affinity magmatism and back-arc sedimentation, post-collision crustal-derived granites, traspressive shearing deformation, also indicating that the NCC was dominated by modern-style plate tectonism in the late Neoarchean.

Revealing the thermal evolution history of ultrahigh-temperature metamorphism (UHT) could help shed light on the genesis and evolution of the orogenic crust. However, it is generally difficult to constrain the duration of metamorphism, especially the heating stage due to the complex behavior of the datable accessory minerals (e.g., zircon and monazite). The Khondalite Belt of the North China Craton records Paleoproterozoic UHT metamorphic event which was previously constrained to be ca. 1920 Ma by using the weighted mean age of zircon U–Pb dating results, however, zircon could grow during both prograde and retrograde periods. Thus, the age of ca. 1920 Ma may be an oversimplified explanation and there could be a complex thermal evolution. In this study, combined with zircon U–Pb dating and Ti-in-zircon thermometry, the duration of the UHT metamorphism in the eastern Khondalite Belt was constrained to be 60–70 Myr with two short periods of decompression-heating (both lasting for ∼20 Myr) intervened by a period of cooling process (lasting for ∼30 Myr). This finding further expands our knowledge that there was a heating–cooling–heating cycle rather than a continuously prolonged cooling process in a long-lived UHT metamorphism. Our results show that the UHT metamorphism in the eastern Khondalite Belt requires two stages of lithosphere extension, which were possibly related to shallow slab breakoff and post-collisional lithospheric delamination, respectively. It further indicates that Paleoproterozoic orogenesis, although dominated by subduction of rheologically weak slab, is comparable to the formation of modern Himalaya orogens.

The Mesoarchaean Pongola Supergroup of South Africa hosts the oldest succession of stromatolitic carbonates deposited in an intracontinental setting. In this study, carbonates within two (tectono-) stratigraphic units in the Buffalo River inlier were investigated. Stratiform stromatolite and wave-ripple-laminated dolostone facies are the most common and are interpreted to have formed in a tide-dominated shallow-marine environment. Dolostones typically contain a large amount of siliciclastic detritus, and small domal stromatolites are commonly found scattered in subtidal sandstone facies. The carbonates experienced deformation and greenschist facies metamorphic conditions at peak temperatures of ∼ 450 ± 50 °C. Analyses for major, trace and rare earth elements (REEs) along with carbon, oxygen and strontium isotopes have been undertaken on the best-preserved samples. Their REE + Y distribution patterns reflect their marine origin, with positive La_SN, Gd_SN and Y_SN anomalies, super-chondritic Y/Ho ratios, and depleted light REEs relative to heavy REEs. The δ¹³C_VPDB and δ¹⁸O_VPDB values least affected by diagenetic and metamorphic overprints are 2.2 ‰ and −16.1 ‰, respectively. The least radiogenic ⁸⁷Sr/⁸⁶Sr ratio is 0.704 and reflects isotopic exchange with a siliciclastic component. Despite their alteration, the carbonates of the Buffalo River inlier provide additional constraints on microbial carbonate deposition on the Earth oldest preserved craton. They can be traced laterally for several tens of kilometres and point to environmental conditions suitable for the deposition and preservation of marine carbonate rocks 3.0 Ga ago.

Paleoproterozoic and Mesoproterozoic A-type granites occur on many cratons and possess important information for our understanding of the formation and breakup history of the supercontinent Columbia. It was argued previously, that Proterozoic A-type granites (∼1.8–1.5 Ga), exposed along the southern margin of the North China Craton formed in a post-collisional or anorogenic setting related to the final amalgamation of the North China Craton or to the breakup of Columbia. In the present study, we report zircon ages and geochemical data of the Maping A-type granite. This intrusion consists of a quartz monzonite porphyry and a granite porphyry that, based on U-Pb age dating, formed at ∼ 1.65 and ∼ 1.60 Ga, respectively. The granites show high contents in alkaline and high field strength elements, high Ga/Al and Fe/Mg ratios. The quartz monzonite porphyry of the early stage is metaluminous and relatively depleted in Hf-Nd isotopic compositions (whole-rock initial ε_Nd values −5.4 to −4.5; zircon initial ε_Hf values −8.5 to −1.5), while granite porphyry of the late stage is peraluminous and has lower initial ε_Nd and ε_Hf values (−6.4 to −5.9; −9.6 to −3.3). Inherited zircon grains in the Maping intrusion have relatively high initial ε_Hf values (>-0.7), likely originating from juvenile crust. The Pb isotopes of the quartz monzonite porphyry show characteristics similar to the lower crust, whereas the granite porphyry has more radiogenic Pb isotopic compositions. Apatite grains from the quartz monzonite porphyry have initial ⁸⁷Sr/⁸⁶Sr ratios of ∼ 0.7109–0.7133 and those from the granite porphyry have noticeably high initial ⁸⁷Sr/⁸⁶Sr ratios of ∼ 0.7862–0.8812. These isotopic characteristics imply the presence of various crustal rocks underneath the North China Craton with variable isotopic compositions. From the early to late magmatic stages, the Sr/Y ratios of granitic rocks decrease from 1.6 – 7.6 to 0.2–0.4, while the estimated magma temperatures, calculated from whole-rock compositions, slightly increase from approximately 900 °C to about 930 °C. These concurrent changes in temperature and chemical composition reflect ongoing asthenospheric mantle upwelling during crustal extension. This suggests a transition in the southern margin of the North China Craton from a post-collisional setting before 1.65 Ga to an anorogenic setting by 1.6 Ga.

The origin of Earth’s felsic continental crust remains a mystery. The generation of felsic continental crust requires a two-stage partial melting from original mantle sources. There are two hypotheses for the continental crust generation in the early Earth. One is the subduction-related magmatism, e.g., island arcs, which produces intermediate to felsic magma that constitutes the early buoyant continental crust. The other is the magmatism induced by the mantle plume that creates thick basaltic crust and finally the continental crust. However, there is controversy about the origin of plate tectonics, which is an obstacle for simply applying the subduction-induced model in the early Earth. On the other hand, the efficiency of mantle plume-induced continental crust growth remains unknown. In this study, we develop a new numerical model, integrating the petrological-thermo-mechanical model with melt migration and crystallization, to evaluate the efficiency of continental crust production by mantle plumes in Earth’s history. Our results indicate that mantle plumes are considerably more effective for continental crust generation in the hot early Earth than that in the present Earth. The contribution of plume-induced continental crust growth may be greatly promoted by the possible high frequency of mantle plume generation in the early Earth than the present.

The special calcite bodies (CBs) exposed within the ultrahigh temperature (UHT) granulite facies terrain in the Balangoda area of Sri Lanka preserve shreds of evidence for the generation of crust-derived carbonatite in the crust. The CBs are exclusively made up of massive calcites and appear as concordant bands extending tens of meters or as meso-scale isolated pockets hosting the massive dolomitic marble band. Various sizes of mafic and calc-silicate enclaves occur as rotated or tilted structures within the CBs. The contact between the CBs and the host marble is texturally and mineralogically gradational, while the contact between the enclaves and the CBs is sharp. The large-ion lithophile elements and rare earth element contents of the CBs show enrichment compared to the host marble, while depletion compared to typical carbonatites. Furthermore, the Sr content and C-O isotope values in CBs differ from those found in known carbonatites, hydrothermal carbonates, or metasomatic carbonates. We suggested that the crustal anatexis of marble should be hypothesized as the possible mechanism for the origin of the CBs. Microtexural evidence of the calcite grains shows indications of the melting of the host marble. The release of CO₂-rich fluids from the collision and thrusting of HC over VC, or related metamorphic events, likely lowered the solidus of carbonates, triggering crustal anatexis of marble during UHT granulite facies metamorphism. The generated low viscous carbonate melt may have moved rapidly, resulting in a low degree of mixing of silicates and fragmentation and dislocation of enclaves. The results of the present study reflect the existence of anatexis of carbonates under extreme crustal conditions and provide a better understanding of the sources, migration paths and reservoirs of the carbon recycling processes.