Geochronology and paleomagnetism of Traissic-Jurassic tholeiitic magmatism in Brazil and implications for the Central Atrantic Magmatic Province.
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Occurrences of multistage dykes provide a critical opportunity to constrain the nature of geological setting in the Daqingshan region during the Late Neoarchean to Middle Palaeoproterozoic. In this article, we present the results of zircon U–Pb ages and whole‐rock geochemical data from mafic‐felsic dykes in the Hadamengou area in order to constrain their ages and tectonic significance. Based on their zircon U–Pb ages, these dykes can be divided into four types: 2.52 Ga metadiabase, 2.45 Ga K‐feldspar granite, 2.44 Ga granitic pegmatite, and 1.99 Ga metagabbro. Geochemically, the 2.52 Ga metadiabase rocks have low SiO 2 , high MgO, and Na 2 O/K 2 O ratios, with enrichment in LILE and depletion in HFSE, which suggests these rocks originated from melting of the lithosphere mantle components metasomatised by subducted slab‐derived fluids. The 2.45 Ga K‐feldspar granite and the 2.44 Ga granitic pegmatite are characterised by high SiO 2 and Na 2 O + K 2 O contents, low MgO, Ni and Cr contents, and high A/CNK ratios, similar to typical features of strongly peraluminous granites. Additionally, these rocks show a variation of Eu anomalies and strong depletion of Th, U, Nb, and Ta elements, which characteristics are considered as a feature of anatectic origin. The 1.99 Ga metagabbro rocks belong to calc‐alkaline rock series, and contain low SiO 2 contents, high MgO, Cr and Ni contents, negative Nb, Ta, and Ti anomalies, and high LILE abundances without Eu anomalies. Geological features of the metagabbro dykes indicate that they are likely derived from partial melting of the subduction‐related metasomatised lithospheric mantle. Combined with the evidence of mafic‐felsic dykes in the Hadamengou and magmatism in the adjacent area, a complex tectonic evolution history during Late Neoarchean to Middle Palaeoproterozoic is presented in the Daqingshan region, which is associated with the amalgamation of micro‐blocks within the North China Craton (NCC), regional extension after cratonisation and subduction of the old ocean between the Yinshan Block and the Ordos Block.
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This paper presents geochronological, geochemical, and zircon Hf–O isotope data for late Mesozoic intrusive rocks from the northeastern North China Craton (NCC), with the aim of constraining the late Mesozoic tectonic nature of the NE Asian continental margin. U–Pb zircon data indicate that the Late Mesozoic magmatism in the northeastern NCC can be subdivided into two stages: Late Jurassic (161 − 156 Ma) and Early Cretaceous (125 − 120 Ma). Late Jurassic magmatism consists mainly of monzogranites. These monzogranites display high Sr/Y ratios and the tetrad effect in their REE, respectively, and have negative εHf(t) values (−22.6 to −15.8). The former indicates that the primary magma was generated by partial melting of thickened NCC lower crust, the latter suggests that the monzogranites were crystallized from highly fractionated magma, with the primary magma derived from partial melting of lower continental crust. Combined with the spatial distribution and rock associations of the Late Jurassic granitoids, we conclude that the Late Jurassic magmatism in the eastern NCC formed in a compressional environment related to oblique subduction of the Paleo-Pacific Plate beneath the Eurasia. The Early Cretaceous magmatism consists mainly of granitoids and quartz diorites. The quartz diorites formed by mixing of melts derived from the mantle and lower crust. The coeval granitoids are classified as high-K calc-alkaline and metaluminous to weakly peraluminous series. Some of the granitoids are similar to A-type granites. The granitoid εHf(t) values and TDM2 range from −14.3 to −1.4 and 2089 to 1274 Ma, respectively. These values indicate that their primary magma was derived from partial melting of lower crustal material of the NCC, but with a contribution of mantle-derived material. We therefore conclude that Early Cretaceous magmatism in the northeastern NCC occurred in an extensional environment related to westward subduction of the Paleo-Pacific Plate beneath Eurasia.
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Permo-Carboniferous felsic magmatic rocks are widely exposed on the northern half of the south-eastern Central Asian Orogenic Belt. However, their origin and spatiotemporal distribution are unknown. Herein, zircon U-Pb-Hf isotopic data, major and trace element compositions of the whole-rock, and Sr-Nd isotopic data derived for felsic volcanic rocks and granites of the Chaganaobao and Baiyinaobao areas in central Inner Mongolia are combined with the data from previous studies. The integration of these results reveals that the Uliastai Continental Margin, Erenhot-Hegenshan Ophiolite Belt, and Northern Orogenic Belt have coeval magmatic lull or trough at c. 307 Ma, which overlap with the closure timing of the Hegenshan Ocean. The Latest Carboniferous−Early Permian (c. 303−295 Ma) felsic volcanic rocks in the Chaganaobao area of the Uliastai Continental Margin comprise dacite, trachydacite, and rhyolite, while demonstrating positive whole-rock εNd(t) (+2.9 to +4.7) and zircon εHf(t) values (+7.65 to +13.78). Early Permian granites of the Baiyinaobao area located within the Northern Orogenic Belt show positive εHf(t) values (+ 2.70 to +11.33) and low TFe2O3 + MgO + TiO2 contents. The geochemical composition reveals that those felsic magmatic rocks mainly originate from the partial melting of juvenile crustal components with subordinate input of mantle-derived melts. Those c. 305−275 Ma felsic magmatic rocks from the north of the south-eastern Central Asian Orogenic Belt exhibit affinities of post-collision type granites in geochemical nature. Therefore, the flare-up of the Early Permian magmatism surrounding the Erenhot-Hegenshan Ophiolite Belt could be related to the extension after the final closure of the Hegenshan Ocean.
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Abstract New integrated geochemical studies are reported for Jurassic granites of the Xingcheng area in the northeastern North China Craton. U–Pb zircon data indicate that the Huashan and Taili monzogranites were emplaced during the Early (189 ± 2 Ma) and Late (155 ± 1 Ma) Jurassic, respectively. They are typical of high‐K calc‐alkaline series rocks and I‐type granites, according to our whole‐rock geochemical researches. Both Early and Late Jurassic monzogranites show adakitic rock characteristics because of their high Sr contents (221–347 ppm) and Sr/Y ratios (28.7–37.5), and low Y contents (7.83 –14.7 ppm). The Early Jurassic monzogranite samples have an ( 87 Sr/ 86 Sr) i ratio of 0.7046, ∊ Nd ( t ) values of –11.62 to –11.51, and ∊ Hf ( t ) values of –13.6 to –6.4, whereas the Late Jurassic monzogranites have higher ( 87 Sr/ 86 Sr) i ratios of 0.7069–0.7071 and lower ∊ Nd ( t ) (–20.65 to –20.46) and ∊ Hf ( t ) (–27.6 to –20.0) values. We suggest that the Early Jurassic adakitic rocks were derived from partial melting of thickened lower crust contaminated with mantle‐derived materials, related to subduction of the Paleo‐Pacific Plate. The Late Jurassic adakitic rocks were derived from partial melting of thickened lower crust in an extensional tectonic setting associated with an active continental margin.
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The early Mesozoic marked an important transition from collisional orogeny to post‐orogenic extension at the northern margin of the North China Craton (NCC). In this study, we undertook zircon U‐Pb dating and whole‐rock major‐ and trace‐element geochemical analyses of early Mesozoic granitic rocks in the Chifeng area to establish their geochronological framework, petrogenesis, and implications for the tectonic evolution of the eastern Central Asia Orogenic Belt (CAOB). Zircon U‐Pb dating results show that these rocks were emplaced in three stages during the Triassic: (1) syenogranites during 250–248 Ma, (2) granodiorites during 244–243 Ma, and (3) monzogranites and granodiorites during 232–230 Ma. These Triassic granitoids belong to the high‐K calc‐alkaline series and are evolved I‐type granites. They have high SiO 2 and low MgO contents with enrichments in light rare‐earth elements, Zr, Hf, Rb, Th, and U, and depletions in Ba, Nb, Ta, Sr, and Eu. These geochemical data indicate that the granitoids were derived from partial melting of a lower‐crustal source under relatively low‐pressure conditions and subsequently underwent extensive fractional crystallization. Considering both the geochemical data and regional geological information, we propose that the 250–248 Ma syenogranites were emplaced in an extensional environment linked to slab break‐off after closure of the Paleo‐Asian Ocean (PAO) along the Solonker‐Xra Moron‐Changchun suture zone. The 244–243 Ma granodiorites were formed in a compressional orogenic setting during collision between the Erguna‐Xing'an‐Songliao composite block and the NCC. The 232–230 Ma granodiorites and monzogranites were emplaced during the transition from compressional orogeny to post‐orogenic extension. Overall, the early Mesozoic tectonic evolution of the Chifeng area can be divided into three main stages: (1) closure of the Paleo‐Asian Ocean and extension related to slab break‐off during the Early Triassic; (2) continuous collisional compression during the Middle Triassic after closure of the PAO; and (3) post‐orogenic extension during the Late Triassic, most probably due to lithospheric delamination after amalgamation of the Erguna‐Xing'an‐Songliao composite block and the NCC.
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