Repeated modification of lithospheric mantle in the eastern North China Craton: Constraints from SHRIMP zircon U-Pb dating of dunite xenoliths in western Shandong
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Large igneous province
<p>Figures S1–S6 and Tables S1–S7.</p>
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Geochemical and isotopic data provide insights into the origin and evolution of magmatism found at destructive plate margins. Tholeiitic magmas are dominant in the early stages of oceanic island-arc genesis and calc-alkalic magmas are most common in mature oceanic arcs and in continental arcs where they may range from basalt to rhyolite in composition, including voluminous intermediate (andesitic) rocks. Experiments suggest that calcalkalic mafic magmas are formed by melting of a hydrated mantle wedge and undergo low pressure fractional crystallization under near-H2O saturated conditions. Intermediate to felsic magmas are derived in a wide variety of ways, including the fractionation of a more mafic parent, mixing between mafic and felsic magmas (a process supported, in many cases, by field and textural evidence), crustal contamination, or partial melting of the crust. All these processes appear to take place, to some degree, in arc systems, although in any given arc system, one mechanism may predominate. Arc-related calc-alkalic and tholeiitic basalts typically show moderate degrees of light rare-earth-element (LREE) enrichment, and flat heavy rare-earth-element (HREE) profiles, indicating an origin in a shallow (spinel lherzolite) mantle. More evolved magmas exhibit Eu anomalies, consistent with low pressure plagioclase fractionation. Compared to within-plate settings, tholeiitic and calc-alkalic arc magmas have lower abundances in high-field-strength (HFS) elements, possibly because these elements are bound during the accessory phases in the mantle wedge, and are stable during partial melting. Compared to arc tholeiites, calc-alkalic magmas have higher abundances of incompatible large ion lithophile (LIL) elements reflecting enrichment in the mantle wedge source. This characteristic depletion in HFS, and enrichment in LIL, elements, in arc magmas is the basis for a variety of discrimination diagrams. These diagrams constrain processes operating in modern and ancient arc systems and include chondrite-normalized, MORB-normalized and mantle-normalized spidergrams, which are characterized by jagged patterns of trace-element abundances (in contrast to the relatively smooth patterns of within-plate suites). Some arc suites have depleted initial 143Nd/144Nd and lower initial 87Sr/86Sr than the bulk earth, and are similar to MORB. Other suites have enriched isotopic patterns consistent with the influence of subducted oceanic sediments on the composition of the magma. Samarium-Nd and Rb-Sr isotopic studies can be used to distinguish between felsic magmas derived from fractional crystallization of a more mafic parent (which would have similar values) and those derived from the melting of ancient crust. SOMMAIRE
Les donnees geochimiques et isotopiques fournissent des indications quant a l'origine et a l'evolution du magmatisme des marges de subduction des plaques tectoniques. Les magmas sont principalement tholeiitiques dans les premieres phases de formation des arcs insulaires oceaniques, alors qu'ils sont principalement calco-alcalins pendant les phases terminales des arcs insulaires oceaniques ainsi que dans les arcs insulaires continentaux, ou leur composition peut aller du basalte a la rhyolite, dont des volumes considerables de roches de composition inter-mediaire (andesitique). Des experiences permettent de penser que les magmas mafiques calco-alcalins sont formes par la fusion d'un biseau mantelique hydrate qui subit une cristallisation fractionnee a basse pression en des conditions de quasi-saturation en H2O. Les magmas de composition intermediaire a felsique resultent de mecanismes tres varies, dont le fractionnement d'une roche mere plus mafique, le melange de magmas felsiques et mafiques (mecanismes mis en evidence par des donnees de terrain et l'analyse texturale), la contamination crustale, ou la fusion partielle de la croute. Tous ces mecanismes semblent se produire, au moins partiellement, au sein d'arcs insulaires, mais l'un d'eux peut constituer le mecanisme predominant de quelque systeme d'arcs insulaires particulier. L'enrichissement modere typique des basaltes calco-alcalins et tholeiitiques d'arcs insulaires en elements legers des terres (LREE) rares ainsi que le profil plat de leur contenu en elements lourds des terres rares (HREE) sont des indicateurs d'une origine mantelique peu profonde (iherzolithe a spinelle). Les magmas plus evolues affichent des anomalies en Eu, ce qui concorde avec un fraction-nement a basse temperature des plagioclases. Compares a ceux des contextes intra-plaques, les magmas tholeiitiques et calco-alcalins d'arcs insulaires affichent des contenus moindres en elements a grande intensite de champ, peut-etre parce que ces elements sont lies pendant les phases accessoires dans le biseau mantelique, et sont stables pendant la phase de fusion partielle. Compares aux tholeiites d'arcs insulaires, les magmas calco-alcalins ont des contenus plus eleves en elements lithophiles a grands champs ioniques incompatibles, ce qui est le reflet d'un enrichissement au sein du biseau mantelique source. Cet appauvrissement caracteristique en elements a grande intensite de champ (HFS) et cet enrichissement en elements lithophiles a grands champs ioniques (LIL) des magmas d'arcs insulaires forment la base d'une variete de diagrammes de discrimination. Ces diagrammes permettent de preciser les processus en jeu des systemes d'arcs insulaires modernes et anciens et incluent des diagrammes radiaux normalises pour les chondrites, pour les basaltes de dorsales oceaniques (MORB) et pour le manteau, lesquels son caracterises par des profils anguleux irreguliers des courbes de contenus en elements traces (en contraste avec les profils relativement reguliers des suites intra-cratoniques). Certaines suites d'arcs insulaires montrent des ratios initiaux 143Nd/144Nd appauvris et 87Sr/86Sr inferieurs a celui de la valeur planetaire actuelle, et qui sont semblables a celui des basaltes de dorsales oceaniques. D'autres suites ont des profils isotopiques enrichis, ce qui correspond a une influence de sediments oceaniques subductes sur la composition du magma. Les etudes samariumneodymium et rubidium-strontium peuvent etre utilisees pour differencier entre les magmas felsiques issus d'une cristallisation fractionnee d'une roche mere plus mafique (qui montrerait des valeurs similaires) et ceux provenant de la fusion d'une croute ancienne.
Igneous petrology
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The NE-trending Black Hills dolerite dykes make up a prominent swarm northeast of the Bushveld Igneous Complex in the Kaapvaal craton. Baddeleyite U-Pb dates of five dykes suggest emplacement ages between ca. 1.87 Ga and 1.85 Ga, with two samples yielding robust ages of 1852 ± 5 Ma and 1863 ± 7 Ma. The Black Hills swarm is thus largely coeval with the post-Waterberg dolerite sills (1.88-1.87 Ga) and basalts of the Soutpansberg Group in northern Kaapvaal as well as with the extensive Mashonaland sill complex (1.89-1.86 Ga) that is abundant across Zimbabwe and Botswana. Together, these intrusions and extrusions manifest a regional-scale extensional event that is common in both the Kaapvaal and Zimbabwe cratons. Additional, younger events common in both cratons are the ca. 1.1 Ga Umkondo and ca. 0.18 Ga Karoo large igneous provinces, suggesting that the Kaapvaal and Zimbabwe cratons have been nearest neighbours from at least 1.9 Ga to present time. In contrast, not a single common event older than 1.9 Ga has been recorded suggesting that the Kalahari craton was not formed until ca. 2.0 Ga.
Recent U-Pb dating has revealed the presence of older dykes, approximately 2.7 Ga in age (Johan Olsson, unpublished data), intermixed with the ca. 1.87– 1.85 Ga dykes of the Black Hills swarm. Geochemistry of 28 dykes of the Black Hills swarms and of 2 Mashonaland sills in Zimbabwe were analysed with respect to both major and trace elements. Geochemical data indicate that each generation of dykes can be petrogenetically related. There are no significant differences between the 2.7 Ga and ca. 1.87– 1.85 Ga dykes, but more so between more evolved and primitive dykes within each group. It is possible that primary melts were generated at relatively shallow (from the spinel stability field) mantle depths and that this primary melts subsequently experienced shallow crustal fractionation of, at least, plagioclase, some Mg-rich phase(s) and apatite. Relatively high concentrations of most incompatible elements suggest that the mantle source was more enriched than a normal MORB source. Any additional enrichment in large-ionic lithophile elements and negative Nb-Ta anomalies can be ascribed to contamination and/or partial melting of the Kaapvaal craton lithosphere.
Baddeleyite
Large igneous province
Sill
Geochronology
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Large igneous provinces are exceptional intraplate igneous events throughout Earth's history. Their significance and potential global impact are related to the total volume of magma intruded and released during these geologically brief events (peak eruptions are often within 1–5 m.y. in duration) where millions to tens of millions of cubic kilometers of magma are produced. In some cases, at least 1% of Earth's surface has been directly covered in volcanic rock, being equivalent to the size of small continents with comparable crustal thicknesses. Large igneous provinces thus represent important, albeit episodic, periods of new crust addition. However, most magmatism is basaltic, so that contributions to crustal growth will not always be picked up in zircon geochronology studies, which better trace major episodes of extension-related silicic magmatism and the silicic large igneous provinces. Much headway has been made in our understanding of these anomalous igneous events over the past 25 yr, driving many new ideas and models. (1) The global spatial and temporal distribution of large igneous provinces has a long-term average of one event approximately every 20 m.y., but there is a clear clustering of events at times of supercontinent breakup, and they are thus an integral part of the Wilson cycle and are becoming an increasingly important tool in reconnecting dispersed continental fragments. (2) Their compositional diversity in part reflects their crustal setting, such as ocean basins and continental interiors and margins, where, in the latter setting, large igneous province magmatism can be dominated by silicic products. (3) Mineral and energy resources, with major platinum group elements (PGEs) and precious metal resources, are hosted in these provinces, as well as magmatism impacting on the hydrocarbon potential of volcanic basins and rifted margins through enhancing source-rock maturation, providing fluid migration pathways, and initiating trap formation. (4) Biospheric, hydrospheric, and atmospheric impacts of large igneous provinces are now widely regarded as key trigger mechanisms for mass extinctions, although the exact kill mechanism(s) are still being resolved. (5) Their role in mantle geodynamics and thermal evolution of Earth as large igneous provinces potentially record the transport of material from the lower mantle or core-mantle boundary to the Earth's surface and are a fundamental component in whole mantle convection models. (6) Recognition of large igneous provinces on the inner planets, with their planetary antiquity and lack of plate tectonics and erosional processes, means that the very earliest record of large igneous province events during planetary evolution may be better preserved there than on Earth.
Large igneous province
Silicic
Flood basalt
Supercontinent
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Trap (plumbing)
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During the early Cenozoic, igneous activity of varying intensity was widespread along the NE Atlantic margin. Igneous features are well imaged on 3D seismic data on the eastern margin of the Rockall Trough. Magmatism was both extrusive and intrusive with
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Igneous petrology
Continental Margin
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Zircon from spatially and temporally distinct igneous rock units across the Gawler Craton, Australia, show subtle differences in trace and rare earth element ratios. These igneous suites range in composition from granite, rhyolite to gabbro and originate from the ca 2450 Ma Sleaford Complex, the ca 1850 Ma Donington Suite, the ca 1633–1608 Ma St Peter Suite, the ca 1595–1587 Ma Gawler Range Volcanics and the ca 1595–1575 Ma Hiltaba Suite granites. The geochemical characterisation of zircon is carried out on relatively unaltered samples in order to establish primary signatures and to understand the processes controlling the geochemical signatures of the igneous suites. A range of zircon morphologies and internal textures that are preserved in these igneous suites infer the geological history of the host rock, including metamorphism and multiple episodes of zircon saturation suggesting a complex magma evolution. Despite differences in whole-rock geochemistry between igneous suites, zircon largely overlaps geochemically. However, subtle differences in Th/U, U/Hf and (Nb + Ta)/P ratios are evident between the Donington Suite, Gawler Range Volcanics and Hiltaba Suite. This is likely due to the tectonic setting, composition of the crystallising magma and timing of zircon crystallisation in the magma evolution. The subtle geochemical differences in zircon show the potential of using these ratios to aid in provenance studies with large populations of zircon that are of similar age. However, as the dataset is limited and focused only on unaltered samples, further work needs to be undertaken to look at the altered counterparts of these igneous suites.KEY POINTSZircon recovered from unaltered samples of spatially and temporally distinct igneous suites in the Gawler Craton is used here.Th/U, U/Hf and (Nb + Ta)/P ratios of zircon can discriminate the Donington Suite, Gawler Range Volcanics and Hiltaba Suite.Zircon morphologies, internal textures and geochemical characteristics are a value aid to provenance studies.
Geochronology
Large igneous province
Petrogenesis
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