查看更多>>摘要：The Central Asian Orogenic Belt is sandwiched between the Siberian, East European (Baltica), Tarim, and North China cratons, which contain several Precambrian micro-continental blocks, such as the Kazakhstan-Yili-Central Tianshan blocks, the Junggar Block and the Amuria Block. The tectonic nature and positions of these micro continental blocks are unclear and disputed in many supercontinent reconstructions. In particular, the position of the Amuria Block in the eastern Central Asian Orogenic Belt, which is important for the reconstructions of the Precambrian Rodinia supercontinent, remains unknown because of the lack of late Palaeoproterozoic to Neoproterozoic geological and geochronological data. The Erguna Block, a part of the Amuria Block in NE China, has been considered as a constituent block of the Rodinia supercontinent. However, the late Palaeoproterozoic to Neoproterozoic tectonic evolution of the Erguna Block with respect to the assembly and break-up of Rodinia remains poorly understood. Here we present zircon U-Pb-Hf-O isotopic data and whole-rock elemental data of the newly discovered late Palaeoproterozoic and Neoproterozoic records in the northern Erguna Block. The gneissic granites have protolith and metamorphic ages of 1839 & PLUSMN; 24 and 497 & PLUSMN; 6 Ma, respectively, and relatively uniform two-stage Hf model ages (TDM1) of 2.67-1.26 Ga, indicating that the Erguna Block has a Palaeoproterozoic crystalline basement. The zircon U-Pb ages for the Neoproterozoic intrusive rocks define two stages of magmatism in early Neoproterozoic (Stage 1; i.e., ca. 946 Ma) and middle Neoproterozoic (Stage 2; i.e., 833-737 Ma), respectively, implying that the Erguna Block had switched to an intra-continental extensional setting (833-737 Ma) after the early Neoproterozoic (ca. 946 Ma) convergence-accretion. A comparison of data from the Erguna Block and its adjacent units reveals that the Erguna and Central Mongolia blocks were likely in a single crustal entity with the Tarim Craton associated with the Rodinia assembly at ca. 900 Ma. This composite block then detached from the Tarim Craton at ca. 800 Ma following the break-up of Rodinia, and drifted southwards to the vicinity of the Siberia Craton during the Cambrian.
查看更多>>摘要：The vast Tin Begane batholith (5000 km(2)) is located in the Laouni terrane, LATEA metacraton, Central Hoggar and emplaced in the middle crust (c. 4.2 kbar). High-K calc-alkaline, metaluminous and magnesian as a whole, it comprises two distinct groups: i/ a porphyritic amphibole-biotite monzogranite (598 +/- 3 Ma, U-Pb zircon) display a LREE enriched pattern and a negative Eu anomaly while ii/ a porphyritic biotite monzogranite (596 +/- 4, U-Pb zircon) and a fine-grained monzogranite (591 +/- 6 Ma, U-Pb zircon) show a highly fractionated REE patterns and slight Eu anomalies. Both groups enclosed numerous microgranular magmatic enclaves with the same contrasted chemical characteristics. In the center of the batholith, a basement sheet bounded by faults comprises migmatitic country rocks and leucogranites (595 +/- 6 Ma, U-Pb zircon) and a garnet-muscovite dyke swarm. Geochemical compositions of the Tin Begane magmatic rocks, including magmatic enclaves, and inherited zircons point to a Paleopmterozoic composite lower crustal metasedimentary source with a psammitic garnet-free component and a garnet-bearing greywacke melted at c. 900 degrees C with a minor basaltic mantle component. Leucogranites originated from the same sources but at a lower temperature (c. 780 degrees C) not far from the emplacement of the Tin Begane batholith. A detailed structural study shows that the Tin Begane batholith is post-collisional, syn-kinematic, bounded and controlled by large scale detachment shear zones, which restricted the flow by channeling the magma from depth to the middle crust and accommodated the magma emplacement. Late, when the batholith was largely crystallized, the subhorizontal magmatic fabric was progressively replaced by pronounced subvertical planar fabric when approaching the shear zones. The Tin Begane area shows a local but major high-temperature NNE-SSW (subvertical) extensional deformation, especially marked by a large NW-SE detachment, within the WNW-ESE regional compression (subhorizontal shortening) induced by the global transpressive system linked to the tectonic escape of the Tuareg terranes toward the north. This contrasted behavior is favored by the metacratonic character of LATEA.
查看更多>>摘要：The Mozaan Group represents the youngest unit of the c. 2.9 Ga Pongola Supergroup located along the southeastern margin of the Kaapvaal Craton. It comprises a ca. 4800 m thick succession of clastic sedimentary rocks intercalated by minor chemical and volcano-sedimentary rocks deposited in shallow marine to fluvial environments, and is stratigraphically correlated with the auriferous Witwatersrand Supergroup. This correlation, however, is speculative, in particular as systematic information about depositional ages and sediment provenances are absent. To address these problems, we present new combined sets of U-Pb ages, Hf isotopes, and shape parameters (width, length, aspect ratios and roundness) of > 700 detrital zircon grains from seven samples of the Mozaan type profile in the Hartland area. These data reveal a switch in provenance between the lower and upper Mozaan Group. Zircons in sandstones of the lower Mozaan Group (Sinqeni to Ntombe formations) were supplied from surrounding proto-Kaapvaal Craton, and those in upper Mozaan Group rocks (Delfkom to Ntanyana formations) predominately from a juvenile hinterland, comprising sources as far as the Pietersburg and/or Kimberley blocks, which became amalgamated to the proto-Kaapvaal Craton at 2.97-2.87 Ga. Significant overlap of zircon age spectra, Hf isotope data, and maximum depositional ages (2908 & nbsp;+/- 8 Ma to 2866 +/-& nbsp;7 Ma) suggest similar sources for upper Mozaan Group and Central Rand Group sediments of the Witwatersrand Basin. In contrast, sedimentary rocks of the West Rand Group have no counterparts in the Pongola Basin, except for the Orange Grove Formation, which shows good agreement with the Sinqeni Formation. The provenance switch indicated by the age-Hf isotope data is not identified by zircon shape parameters. These rather reflect differences in depositional environment (littoral, fluvial, volcanogenic), related to the duration and energy of sediment transport and reworking, as indicated by specific patterns in grain size vs. roundness diagrams.
查看更多>>摘要：The Earth-life system witnessed dramatic changes over the Neoproterozoic (1000-541 Ma). It saw supercontinent reconfigurations from the breakup of Rodinia to the assembly of Gondwana; climatically, the deepest global freeze in Earth history; geochemically, some of the most extreme perturbations on record; and biologically, the prominent rise of complex multicellular eukaryotes. However, the potential mechanisms that lie behind the convergence of such geological and biological revolutions remain elusive. The 27 articles in this special issue represent frontiers in disentangling the potential links between geological and biological changes in the Neoproterozoic and will inspire future interdisciplinary studies toward a better understanding of the Neoproterozoic Earth-life system.
Alfimova, Nadezhda A. A.Kuznetsov, Anton B. B.Klimova, Ekaterina, VBekker, Andrey...
12页
查看更多>>摘要：Precambrian paleoweathering profiles provide insight into environmental changes and mark stratigraphic boundaries. However, the well-preserved and exposed stratigraphic boundaries, not affected by tectonics, are rare for the Precambrian. We report sedimentological, geochemical and Sr, C and O isotopic data for the Paleoproterozoic weathering profile in Chapanshary Island on Lake Segozero, Central Karelia, the eastern part of the Fennoscandian Shield. The Chapanshary Island weathering crust contains a unique carbonate caliche, rarely developed on the Archean granite-gneiss basement. The crust laterally grades into carbonate-cemented boulder conglomerate, which is overlain by sedimentary carbonate. The chemical index of alteration (CIA; 60-67) and plagioclase index of alteration (PIA; 53-82) values for the weathering crust correspond to weathered felsic/mafic substrate and to illite and K-rich composition of the regolith. The delta O-18 values of carbonate cement and sedimentary carbonate are highly negative (-22.3 to -17.6 parts per thousand & nbsp; V-PDB). The Sr-87/Sr-86 values of Fe- and Mn-rich carbonate of the regolith, boulder conglomerate cement and sedimentary carbonate are 0.7133 to 0.7242 and are exceptionally high for marine Paleoproterozoic carbonate. The high delta C-13 values (+3.9 to + 5.2 parts per thousand V-PDB) of carbonate cement and sedimentary carbonate indicate deposition during the ca. 2.22-2.06 Ga Lomagundi-Jatuli carbon isotope excursion of the early Paleoproterozoic. The Chapanshary Island weathering profile developed in a terrestrial setting with arid to semi-arid climate ca. 2.2-2.1 Ga ago under atmosphere with high CO2 content. The overlying sedimentary carbonates were deposited in a shallow-water setting (ephemeral ponds and lakes) evolved in the inner part of Fennoscandia.
查看更多>>摘要：Paleoproterozoic plagiogranites were identified in the Gaoliduntai area of the Jiao-Liao-Ji Belt (JLJB), North China Craton (NCC). We conducted a geochronological, geochemical, isotopic, and mineral chemical investigation of the plagiogranites to constrain their petrogenesis and tectonic implications. The Gaoliduntai plagiogranites yielded zircon U-Pb ages of 1802 +/- 10, 1828 +/- 18, and 1853 +/- 11 Ma, indicating emplacement at the late Paleoproterozoic. Geochemically, They are characterized by high SiO2 and Na2O, and low K2O and TiO2 contents, with low total rare earth element (REE) concentrations. They also exhibit enrichment in light REEs and large-ion lithophile elements (e.g., Cs, Rb and Sr), and depletion in heavy REEs and high-field-strength elements (e.g., Nb and Ta). The plagiogranites show varying initial Sr-87/Sr-86 ratios (0.7012-0.7053), whole-rock epsilon Nd(t) values (-8.23-+1.15) and apatite epsilon(Nd)(t) values (-4.78-+3.30), with whole-rock( 206)Pb/Pb-204(t) (14.223-16.733), Pb-207/Pb-204(t) (15.310-15.644) and( 208)Pb/Pb-204(t) (35.310-35.998) values, indicating a depleted mantle source modified with an enrichment component. Zircons in the plagiogranites have positive epsilon(Hf)(t) values (+1.0 to +6.4) and T-DM2(Hf) ages of 2408-2078 Ma. Zircons delta O-18 compositions (2.1%o to 4.5 parts per thousand & nbsp;) are lower than the zircons from normal mantle but show similar features to typical plagiogranites. Furthermore, the plagioclases in these plagiogranites are albite (Ab = 95-99). These features lead us to propose that the Gaoliduntai plagiogranites formed by partial melting of high-temperature mafic oceanic crust, with contributions from delaminated crustal components. We suggest that the Paleoproterozoic Gaoliduntai plagiogranites provide evidence of the involvement of ancient oceanic crust in the tectonic evolution of the JLJB. Based on regional geological features and previous studies, we propose that the JLJB was an arc-continent collision belt during the Paleoproterozoic, and that the Gaoliduntai plagiogranites formed in an extensional tectonic setting.
查看更多>>摘要：The Precambrian basement of the Lhasa terrane provides important information for understanding the formation and evolution of the Tibetan Plateau. However, due to the paucity of exposure, the properties of the Precambrian basement of the Lhasa terrane remain poorly known. Here we report zircon U-Pb ages, bulk-rock geochemical and zircon Hf isotopic data on the orthogneisses in the Milin area of the southeastern Lhasa terrane, southern Tibet. Inherited magmatic zircon cores of these gneisses yield protolith crystalline ages of 1556-1517 Ma, whereas the zircon rims give metamorphic ages of 1197-1192 Ma, both could be local provenance for the Mesoproterozoic detrital zircons in the Neoprotemzoic-Paleozoic strata in the Lhasa terrane. Geochemical data indicate that the protoliths of the Mesoproterozoic rocks are continental arc-like granitoids with bulk-rock epsilon(Hf)(t) values of -3.7 to +0.7 (T-DM2 = 1.99-2.34 Ga) and zircon core epsilon(Hf)(t) values of +1.8 to +8.6 (T-DM2 = 1.71-2.17 Ga), suggesting their most likely derivation from partial melting of Paleoproterozoic crustal materials. We propose that the Mesoproterozoic gneisses in the Milin area represent the Precambrian metamorphic basement of the Lhasa terrane, likely related to the growth of Columbia supercontinent. Local exposure of the Precambrian metamorphic basement may be caused by crustal rapid uplift and erosion in the late Cenozoic and their inhomogeneous distribution.
Sotiriou, PaulPolat, AliWindley, Brian F.Kusky, Timothy...
31页
查看更多>>摘要：Temporal variations in the incompatible trace element geochemistry of volcanic rocks in Archean greenstone belts have major implications for the style of tectonics that operated in the early Earth, and if and when plate tectonic processes occurred in the Archean, which are still subjects of substantial debate. Comparing the geochemistry of Archean volcanic rocks with that of Phanerozoic arc volcanic rocks has the potential to shed light on these questions. Geochemical data from 8,249 Eoarchean to Neoarchean volcanic rocks and 20,099 Phanerozoic arc volcanic rocks were compiled from the literature to address the above questions through the application of temporal incompatible trace element ratio variations, N-MORB-normalised trace element diagrams, and tectonic setting discrimination diagrams. The sampled rocks range in composition from ultramafic through basaltic and andesitic to dacitic/rhyolitic. Most of the incorporated Archean volcanic rocks were deemed to have been unaffected by significant alteration or crustal contamination in the literature and, therefore, to reflect their provenance, a feature that was corroborated by this study. Most of these Archean volcanic rocks plot in the plate margin, oceanic arc and continental arc fields in classification and tectonic setting discrimination diagrams, with the remainder plotting in the alkaline arc, mid-ocean ridge and oceanic island fields. Comparison between N-MORB-normalised trace element diagrams of volcanic rocks from well-studied Archean greenstone belts in Greenland, Canada, South Africa, China, Australia, India, Brazil and Finland and volcanic rocks from well-studied modern arcs demonstrates that their trace element patterns are remarkably similar. This indicates that the former formed in arc-related settings by modern-style plate tectonic processes that operated throughout the Archean. The Pb and Nb anomalies of most Archean volcanic rocks are fully consistent with an arc-related setting. The temporal variations in the incompatible trace element ratios of Archean volcanic rocks, coupled with their lithological associations, demonstrate that intra-oceanic arc magmatism was prominent in the Eoarchean before a shift in these ratios in the Paleoarchean signified the beginning of Andean-style continental arc magmatism between 3500 and 3200 Ma. Modern-style plate tectonic processes were a far more important contributor to Archean crustal growth and evolution than sagduction-driven vertical tectonic processes.
查看更多>>摘要：Mid-Neoproterozoic low-delta O-18 metamafic rocks from the Leeuwin Complex, southwestern Australia, are reported for the first time. Sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb dating of these upper amphibolite- to granulite-facies mafic rocks yields igneous protolith ages of 674-660 Ma. The metamafic rocks are generally classified as subalkaline tholeiitic rocks with an ocean island basalt (OIB) affinity. They have low Mg# values (22-50) and Cr (0.19-105 ppm) and Ni (0.62-115 ppm) contents, with whole-rock epsilon(Nd)(t) values of 1.4 to + 1.5 and zircon epsilon(Hf)(t) values of - 0.3 to + 3.5. Using these data in combination with the incompatible trace element characteristics, it is inferred that the protoliths of the rocks were derived from low-degree partial melting of relatively depleted asthenospheric mantle in a continental rift environment, and the magmas underwent some crustal contamination and fractional crystallization of mafic minerals. Zircon cores from the metamafic rocks yield delta O-18 values of 0.89 to 4.10 parts per thousand, which are lower than normal mantle values (5.3 +/- 0.3 parts per thousand). These cores preserve oscillatory zoning or banding in cathodoluminescence images, and individual samples have concordant ages and preserve a narrow range of delta O-18 values, suggesting that the low-delta O-18 signatures are of primary magmatic origin. It is inferred that these low-delta O-18 metamafic rocks were generated by contamination by low-delta O-18 felsic crustal wall rocks and interaction of the magma with surface water at shallow depths in an extensional regime during the mid-Neoproterozoic.
查看更多>>摘要：Late Neoproterozoic-Early Paleozoic basement in Iran mainly comprises granites and mafic rocks locally affected by high-grade metamorphism. In this contribution, we collate whole rock chemical data for more than 600 samples; Sr-Nd isotope data for 200 samples; and zircon U-Pb ages of > 2500 grains. On the basis of zircon U-Pb ages, Iranian basement spans 570-530 Ma. The mafic rocks are tholeiitic and the felsic (granitic) rocks show affinities for predominantly I-and S-type granite with minor A-type granite. Sr-Nd isotope ratios show two discrete sources for the basement rocks. The mafic rocks yield younger Nd model ages (T-DM < 1.0 Ga) and lack old zircon grains (mainly less than 800 Ma) confirming a juvenile mantle source. Conversely, the felsic rocks have older T-DM ages (2.5 to 1.6 Ga) similar to inherited zircon U-238-(206)pb ages (2.5 Ga to 650 Ma), which reflects recycling of old (Late Archean-Early Proterozoic) continental crust in the generation of felsic melts in the Late Neoproterozoic. Based in part on a recent study of Late Neoprotemzoic Gondwana evolution, long-lived Proto-Tethys subduction beneath northern Gondwana (Hun terranes) may have caused: (1) Proto-Tethys slab avalanche in the lower mantle; (2) subsequent triggering of mantle plume emplacement; and (3) the calving off of a ribbon of Hun terranes from northern Gondwana. That Iran was far from the Cadomian orogeny is supported by the fact that 'Cadomian' terranes of Iran show evidence of having formed in an extensional tectonic regime as opposed to that of a subduction zone as for Cadomian basement in Europe. Hence, the common-use of Cadomian basement to explain Ediacaran-Cambrian magmatism in Iran is a misinterpretation. This misconception stems largely from an erroneous correlation of Iran with North America and Europe, as opposed to correlation with Central and Eastern Asia such as microcontinents in the Tibetan Plateau.
NSTL
Elsevier