Tectono-magmatic evolution and metallogenesis in the eastern Tethyan orogens
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Adakite
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We report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite,the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have εHf(t) values of magmatic 139–147 Ma zircons from −8.1 to −25.6, with two-stage model ages (tDM2) of 1.71–2.67 Ga, whereas εHf(t) values of inherited 582–844 Ma zircons range from 5.4 to −9.5, with tDM2 of 1.39–2.22 Ma, younger than tDM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, εHf(t) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the tDM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J3–K1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.
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Lile
Underplating
Continental arc
Adakite
Igneous differentiation
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In this study, we present new geochronological and petrogenetic data for the Triassic granitoids of the East Kunlun Orogenic Belt (EKOB), in order to constrain their precise ages, petrogenesis, and tectonic settings. LA-ICP-MS zircon U-Pb data indicate that the Triassic granitoids were emplaced in two stages: (a) Middle Triassic (247-240 Ma), represented by a suite of porphyritic granites and granodiorites; and (b) Late Triassic (234-227 Ma), forming an intrusive rock association of K-feldspar granites, granodiorites, and porphyritic granites. Geochemical analyses and mineral associations suggest that all the Triassic granitoids belong to I-type granites but have different origins. The Middle Triassic granitoids have high SiO2, low to moderate Mg-# values (25-37 for XSG porphyritic granite; 45-47 for DB granodiorite), low Sr/Y ratios (2.2-4.6 for XSG porphyritic granite; 17.8-20.8 for DB granodiorite), and relatively restricted zircon epsilon Hf(t) values (+2.4 - +4.6 for the ca. 247 Ma porphyritic granite, and - 8.0 to -1.5 for the ca. 240 Ma granodiorite), indicating that they were dominantly generated from partial melting of different crust sources (either juvenile or ancient) in a normal lower crust level. In contrast, the Late Triassic granitoids have high SiO2, K2O, and Y contents, low MgO and HREE contents, and variable zircon epsilon Hf(t) values (from negative to positive, -4.9 to +3.3), implying a strong crustal-mantle interaction that occurred during the Late Triassic, and this stage of granitoids were derived from a complex magma source possibly a mixture of mantle-derived and ancient crustal-derived materials. By combining these new data with the previous data, we conclude that the two stages of Triassic granitoids were emplaced in an active continental margin setting and a post-collisional extension setting, respectively. Moreover, this study suggests a tectonic shift of the Palaeo-Tethys Ocean in the EKOB from subduction during the Middle Triassic to a post-collision during the Late Triassic.
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Early Triassic
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Subduction of the Paleo-Pacific Plate plays a key role in the Early Jurassic magmatism evolution in the Yanbian area, Northeast China. In this paper, zircon U–Pb ages, whole-rock geochemical, and Sr–Nd–Hf isotopic compositions are presented. The Early Jurassic gabbros, diorite enclaves, granodiorites, and granites ages range from 183 to 189 Ma. They are characterized by enrichment in large-ion lithophile elements and depletion in high-field strength elements. The gabbros are classified as low- and medium-K tholeiitic, granodiorites and granites are high-K calc-alkaline I-type, and diorite enclaves are calc-alkaline transitional series. The gabbros have zircon εHf(t) values of 6.81–9.89, whole-rock 87 Sr/ 86 Sr (i) = 0.7043–0.7044, and εNd(t) from 2.72 to 2.80. The diorite enclaves have zircons with εHf(t) values of 8.26–12.80, whole-rock 87 Sr/ 86 Sr (i) = 0.7051, and εNd(t) from 0.96 to 1.09. The granodiorites and granites have zircon εHf(t) values of 7.59–12.87, whole-rock 87 Sr/ 86 Sr (i) = 0.7042–0.7066, and εNd(t) from 2.33 to 2.61. These data indicate that gabbroic magmas were derived from partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. The basaltic magma underplated and heated the juvenile crust, whereas the granodiorites and granites might be the product of fractional crystallization of a mixture of basaltic and felsic magmas or derived from partial melting of the juvenile lower crust. Our data constrain the petrogenesis of these Early Jurassic intrusive rocks which are attributed to subduction of the Paleo-Pacific Plate beneath Eurasia continent in northeastern China.
Fractional crystallization (geology)
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Felsic
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In this paper, we present new U-Pb zircon ages and major and trace elements for the Late Ordovician monzonitic granite rocks in Xiaoxilin area in the Eastern lesser khingan of Northeast China to elucidate the westward subduction of the Oceanic plate in Late Ordovician. U-Pb zircon dating results demonstrate that the monzonitic granites formed at 451±5Ma, belonging to the Late Ordovician. These monzonitic granites are characterized have SiO2 of 66.45%~72.93%, Na2O of 2.41%~3.93%, K2O of 3.09%~5.58%, CaO of 2.02%∼3.76%, ALK of 6.56%∼7.99% and A/CNK=0.97~1.02, belonging to the high-K calc-alkaline series. The monzonitic granites is characterized by the enrichment of LILEs (e.g., Rb and K) and incompatible elements (e.g., Th and U), and depletion of HFSEs (e.g., Nb, Ta, Sr, P and Ti), as well as very lesser Eu anomalies (Eu/Eu*=0.49~1.06). The rock falls into the zone of Pre-Plate collision with the characteristics of active continental margin before the collision. Combine the mineral assemblage and geochemical characteristics, it is believed that the monzonitic granite in Xiaoxilin area was formed in the environment of oceanic crust subduction, which indicates that there was an westward subduction oceanic plate between Jiamusi Massif and Songnen-Zhangguangcailing Massif. The melt/fluid precipitated from the subducting oceanic plate confessed the overlying mantle wedge and caused the partial melting of it, which resulted in the arc magmatism. The Jiamusi Massif and Songnen-Zhangguangcailing Massif had not yet joined in the Late Ordovician.
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This paper presents major element, trace element, and new zircon Hf isotopic data for the Early Mesozoic intrusive rocks in the south Hunchun, Yanbian area, Northeast China. These data are used to constrain the petrogenesis of these intrusive rocks and their implications for the Phanerozoic continental growth of the Central Asian Orogenic Belt (CAOB). Combining geology, geochronology, and whole-rock geochemistry, we identify three distinct episodes of magmatism as Early–Middle Triassic (249–237 Ma), Late Triassic (224–206 Ma), and Early Jurassic (200–187 Ma). The Early–Middle Triassic (249–237 Ma) adakitic tonalite and granodiorite were produced by the partial melting of subducted oceanic slabs, and the melts were contaminated by mantle peridotite during their ascent, whereas the coeval non-adakitic diorite and monzogranite were most likely derived from partial melting of crustal material. The remarkably high zircon Hf isotopic signature (εHf(t) = + 9.4 – +18.9), the enrichment in large-ion lithophile element and light rare earth elements, and the depletion in high field strength element suggest that these 224 Ma gabbros were derived from the partial melting of depleted mantle modified by subduction-related fluids. The 212 Ma monzogranite was most likely derived from juvenile material mixed with old crustal material as evidenced by their high SiO2, low MgO, and low Cr concentrations and variable εHf(t) values (–4.6 to +10.0). Except for the 197 Ma tonalites with affinity to the high silica adakites, the overall geochemical evolution of Early Jurassic (200–187 Ma) rocks was consistent with fractional crystallization from quartz diorite, granodiorite, and monzogranite to syenogranite. Both the Early Jurassic syn-subduction lateral continental growth by accretion of arc complexes and the Late Triassic post-collisional vertical continental growth by accretion of mantle-derived material played an important role in the Phanerozoic continental growth of the CAOB.
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