Geochemistry and geodynamic implications of the mafic magmatic plumbing system of the ca. 1.98–1.97 Ga Jhansi Large Igneous Province in the northern Indian Shield
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Sill
Large igneous province
Baddeleyite
Indian Shield
To better constrain the age and duration of the magmatism associated with the Karoo large igneous province (LIP), we present new U-Pb ID-TIMS dates and εHf values from baddeleyite and zircon grains from Karoo basin mafic sills and from felsic samples from the Lebombo and Mwenezi monoclines, together with an 40Ar/39Ar age database of Karoo rocks that has been filtered for true plateau ages with >70% of 39Ar released and in which all 40Ar/39Ar ages were recalculated using the current best estimates for the decay constants. Zircon and baddeleyite ages from three Karoo basin sills range from 183.36 ± 0.17/0.27 to 183.06 ± 0.07/0.21 Ma, where the two uncertainties reflect the analytical error and the additional error associated with decay constant uncertainty. Zircon from the Mutandawhe pluton are dated to 176.84 ± 0.06/0.20 Ma, which represents the first high-precision U-Pb age of the late stage Karoo-LIP magmatism in the northern Lebombo-Mwenezi region. Initial hafnium isotopes are close to chondritic for the Karoo basin and central Lebombo samples (εHf from −2 to +3), but more negative for zircon grains from the Mutandawhe pluton (−11.3 ± 1.1, 2SD). In combination with previous studies and in agreement with the updated 40Ar/39Ar ages, we show that the sill complex that intruded the Karoo basin was short-lived at ~320 ± 180 ka and that it pre-dated the magmatism of the Ferrar-LIP by around 460 ka, whereas the entire Karoo-LIP was emplaced over a period of ca. 6.5 Ma. Based on high-precision U-Pb geochronology, Karoo-LIP magmatism occurred after 183.36 ± 0.17 Ma and therefore postdated the extinction pulses of the late Pliensbachian and likely the Pliensbachian-Toarcian boundary. However, we support previous conclusions that the start of the Karoo-LIP activity agrees with the onset of the Toarcian oceanic anoxic event and the early Toarcian warming, indicating that these environmental changes were likely a response to the magmatic activity of the Karoo-LIP.
Baddeleyite
Large igneous province
Sill
Geochronology
Felsic
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Reliable geochronological results gathered so far (n = 76) have considerably constrained the timing of the emplacement of the Karoo large igneous province (LIP). Yet strikingly missing from this dating effort is the huge southern sill complex cropping out in the >0.6 × 10 6 km 2 Main Karoo sedimentary basin. We present 16 new 40 Ar/ 39 Ar analyses carried out on fresh plagioclase and biotite separates from 15 sill samples collected along a N–S trend in the eastern part of the basin. The results show a large range of plateau and miniplateau ages (176.2 ± 1.3 to 183.8 ± 2.4 Ma), with most dates suggesting a ∼3 Ma (181–184 Ma) duration for the main sill events. The available age database allows correlation of the Karoo LIP emplacement with the Pliensbachian‐Toarcian second‐order biotic extinction, the global warming, and the Toarcian anoxic event (provided that adequate calibration between the 40 K and 238 U decay constant is made). The mass extinction and the isotopic excursions recorded at the base of the Toarcian appear to be synchronous with both the increase of magma emission of the Karoo LIP and the emplacement of the sills. The CO 2 and SO 2 derived from both volcanic emissions as well as carbon‐rich sedimentary layers intruded by sills might be the main culprits of the Pliensbachian‐Toarcian climate perturbations. We propose that the relatively low eruption rate of the Karoo LIP is one of the main reasons explaining why its impact on the biosphere is relatively low contrary to, e.g., the CAMP (Triassic‐Jurassic) and Siberia (Permo‐Triassic) LIPs.
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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.
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In recent years, there has been a significant improvement in our ability to determine the ages of extensive short-lived magmatic events (large igneous provinces) dominated by rocks of mafic (silica-poor) compositions. This has been aided by targeting the magmatic feeders to these large igneous provinces (e.g., dykes and sills), which often host trace amounts of datable zirconium minerals such as baddeleyite and zircon. These age determinations are of great importance for unravelling the geological history and paleogeography of our planet. The Kaapvaal Craton in southern Africa hosts a rich and pristine geological history with many geological units and events in need of better age constraints. One is the Neoarchean Ventersdorp Supergroup, with the Klipriviersberg Group, and underlying successions of the Meso- to Neoarchean Witwatersrand Supergroup, of which the latter is world-wide known for its association with gold findings. By extracting baddeleyite from intrusive mafic sills from the Witwatersrand sediments, we can obtain critical age constraints for these successions using U–Pb geochronology on baddeleyite. Here we show that feeders to the Klipriviersberg Group are ca. 2790 Ma using ID-TIMS and complimentary LA-ICP-MS on baddeleyite from two mafic sills. This age makes the Klipriviersberg Group magmatic event almost 80 Myr older than previously thought, and is in agreement with several indirect studies which have suggested a similar age in Ventersdorp Supergroup temporally equivalent basins. Our results imply that there is a significant time gap in the Ventersdorp Supergroup between the deposition of the Klipriviersberg Group and the Platberg Group, and is associated with craton-wide mafic and felsic magmatism. This conclusion implies that the deposition of the Witwatersrand Supergroup ceased before 2790 Ma, which is when the eruption of the Klipriviersberg Group basalts commenced. Our results define a new large igneous province on the Kaapvaal Craton, and redefine the timing of both the beginning of Ventersdorp and termination of Witwatersrand supergroups. This new timing indicates that world class gold-bearing conglomerates within the Witwatersrand succession were deposited before 2790 Ma.
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Baddeleyite
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Dike
Baltic Shield
Large igneous province
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Mass extinction events are short-lived and characterized by catastrophic biosphere collapse and subsequent reorganization. Their abrupt nature necessitates a similarly short-lived trigger, and large igneous province magmatism is often implicated. However, large igneous provinces are long-lived compared to mass extinctions. Therefore, if large igneous provinces are an effective trigger, a subinterval of magmatism must be responsible for driving deleterious environmental effects. The onset of Earth's most severe extinction, the end-Permian, coincided with an abrupt change in the emplacement style of the contemporaneous Siberian Traps large igneous province, from dominantly flood lavas to sill intrusions. Here we identify the initial emplacement pulse of laterally extensive sills as the critical deadly interval. Heat from these sills exposed untapped volatile-fertile sediments to contact metamorphism, likely liberating the massive greenhouse gas volumes needed to drive extinction. These observations suggest that large igneous provinces characterized by sill complexes are more likely to trigger catastrophic global environmental change than their flood basalt- and/or dike-dominated counterparts.Although the mass end-Permian extinction is linked to large igneous provinces, its trigger remains unclear. Here, the authors propose that the abrupt change from flood lavas to sills resulted in the heating of sediments and led to the release of large-scale greenhouse gases to drive the end-Permian extinction.
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Flood basalt
Large igneous province
Permian–Triassic extinction event
Extinction (optical mineralogy)
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A new U–Pb baddeleyite age of 1970 ± 3 Ma for the Unoi dolerite sill of the Onega structure of Karelia craton matches other 1.98–1.95 Ga units across the Kola craton (Pechenga) and widely separated parts of the Karelian craton, including the Lake Onega sill area and a extensive NW-trending dolerite dyke swarm. Herein these coeval units are combined into the Pechenga–Onega Large Igneous Province. The sills in the Lake Onega area exhibit similar geochemical patterns, although the Unoi dolerite sills appear less contaminated and less differentiated than the Pudozhgora intrusion, Gabnev sill and Koikary-Svatnavolok and Palieyeozero sills but are similar to other doleritic sills in the northern part of the Onega structure. New AMS data from sills are consistent with emplacement along the same NW–SE trend as the dykes, consistent with the dykes acting as their feeder system. Paleomagnetic data obtained on 1.98–1.95 Ga magmatic rocks result in a new robust paleopole for the Karelian craton and accentuate its variable paleolatitude and paleoorientation during the Paleoproterozoic.
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Baddeleyite
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Swaging
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Large volumes of mafic igneous rocks are commonly emplaced during Large Igneous Province (LIP) eruptions, and these mafic rocks are often contemporaneous with periods of environmental disturbances, such as global ocean anoxia, and as a result, mass extinctions. The Paraná-Etendeka LIP is no exception, and has been previously correlated with, and interpreted to be the cause of the Valanginian oceanic anoxic event (OAE), a small global environmental disturbance. Here, we present new U-Pb ID-TIMS baddeleyite ages from high-Ti-Sr mafic intrusive rocks from the Paraná LIP, in Brazil. While these data are potentially complicated by the presence of Pb-loss and inheritance, it is possible to interpret geologically meaningful ages from them. The first high-precision age is reported for the type-locality of the Florianópolis Dyke Swarm, in North Santa Catarina Island, which yields an age of 132.53 ± 0.40/0.40/0.42 Ma. We also report an age of 132.07 ± 0.27/0.30/0.33 Ma for a dolerite sill, which intrudes organic-rich sedimentary rocks of the Paraná Basin. The emplacement of high-Ti-Sr magmas at ca. 132 Ma suggests that there was up to 2 Myr of intrusive magmatism in the southern part of the Paraná LIP. Further investigation on the mafic intrusive magmatism from the Paraná LIP through robust high-precision geochronology is required to elucidate the proposed linkage more clearly with environmental changes during the Early Cretaceous.
Baddeleyite
Large igneous province
Sill
Geochronology
Flood basalt
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Baddeleyite
Large igneous province
Dharwar Craton
Sill
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