Zircon U–Pb and Hf isotope data integrated in this study for magmatic and metamorphic rocks fromthe Hida Belt,southwest Japan,lead to a new understanding of the evolution of the Cordilleran arc system along the ances...Zircon U–Pb and Hf isotope data integrated in this study for magmatic and metamorphic rocks fromthe Hida Belt,southwest Japan,lead to a new understanding of the evolution of the Cordilleran arc system along the ancestral margins of present-day Northeast Asia.Ion microprobe data for magmatic zircon domains from eight mafic tointermediate orthogneisses in the Tateyama and Tsunogawa areas yielded weighted mean 206Pb/238U ages spanning the entire Permian period(302–254 Ma).Under cathodoluminescence,primary magmatic growth zones in the zircon crystals were observed to be partially or completely replaced by inward-penetrating,irregularly curved featureless or weakly zoned secondary domains that mostly yielded U–Pb ages of 250–240 Ma and relatively high Th/U ratios(>0.2).These secondary domains are considered to have been formed by solid-state recrystallization during thermal overprints associated with intrusions of Hida granitoids.Available whole-rock geochemical and Sr–Nd isotope data as well as zircon age spectra corroborate that the Hida Belt comprises the Paleozoic–Mesozoic Cordilleran arc system built upon the margin of the North China Craton,together with the YeongnamMassif in southern Korea.The arcmagmatismalong this systemwas commenced in the Carboniferous and culminated in the Permian–Triassic transition period.Highly positiveεHf(t)values(>+12)of late Carboniferous to early Permian detrital zircons in the Hida paragneisses indicate that there was significant input from the depleted asthenospheric mantle and/or its crustal derivatives in the early stage of arc magmatism.On the other hand,near-chondriticεHf(t)values(+5 to−2)of magmatic zircons from late Permian Hida orthogneisses suggest a lithospheric mantle origin.Hf isotopic differences between magmatic zircon cores and the secondary rims observed in some orthogneiss samples clearly indicate that the zircons were chemically open to fluids or melts during thermal overprints.Resumed highly positive zirconεHf(t)values(>+9)shared by Early Jurassic granitoids in the Hida Belt and Yeongnam Massif may reflect reworking of the Paleozoic arc crust.展开更多
Northeast Asian continental margins contain the products of magma emplacement driven by prolonged subduction of the(paleo-)Pacific plate.As observed in many Cordilleran arcs,magmatic evolution in this area was punctua...Northeast Asian continental margins contain the products of magma emplacement driven by prolonged subduction of the(paleo-)Pacific plate.As observed in many Cordilleran arcs,magmatic evolution in this area was punctuated by high-volume pulses amid background periods.The present study investigates the early evolution of the Cretaceous magmatic flare-up using new and published geochronological,geochemical,and O-Hf isotope data from plutonic rocks in the southern Korean Peninsula.After a long(~50 m.y.)magmatic hiatus and the development of the Honam Shear Zone through flat-slab subduction,the Cretaceous flare-up began with the intrusion of monzonites,granodiorites,and granites in the inboard Gyeonggi Massif and the intervening Okcheon Belt.Compared to Jurassic granitoids formed during the former flare-up,Albian(~111 Ma)monzonites found in the Eopyeong area of the Okcheon Belt have distinctly higher zirconε_(Hf)(t)(-7.5±1.3)andδ^(18)O(7.78‰±0.25‰)values and lower wholerock La/Yb and Sr/Y ratios.The voluminous coeval granodiorite and granite plutons in the Gyeonggi Massif are further reduced in Sr/Y and to a lesser extent,in La/Yb,and have higher zirconε_(Hf)(t)values(-13 to-19)than the Precambrian basement(ca.-30).These chemical and isotopic features indicate that Early Cretaceous lithospheric thinning,most likely resulting from delamination of tectonically and magmatically overthickened lithospheric keel that was metasomatized during prior subduction episodes,and consequent asthenospheric upwelling played vital roles in igniting the magmatic flare-up.The O-Hf isotopic ranges of synmagmatic zircons from the Albian plutons and their Paleoproterozoic and Jurassic inheritance attest to the involvement of lithospheric mantle and crustal basement in magma generation during this decratonization event.Arc magmatism then migrated trenchward and culminated in the Late Cretaceous,yielding widespread granitoid rocks emplaced at shallow crustal levels.The early Late Cretaceous(94-85 Ma)granites now prevalent in Seoraksan-Woraksan-Sokrisan National Parks are highly silicic and display flat chondrite-normalized rare earth element patterns with deep Eu anomalies.Synmagmatic zircons in these granites mimic their host rock's chemistry.Delamination-related rejuvenation of crustal protoliths is indicated by zirconε_(Hf)(t)values of granites(-6 to-20)that are consistently higher than the Precambrian basement value.Concomitant core-to-rim variation in zircon O-Hf isotopic compositions reflects a typical sequence of crustal assimilation and fresh input into the magma chamber.展开更多
基金jointly supported by a Basic Research Project(20-3111-1:Geological survey in the Korean Peninsula and publication of the geological maps)of the Korea Institute of Geoscience and Mineral Resources,funded by the Ministry of Science and ICT,Korearesearch grants from the Korea Basic Science Institute(C070110,C030120).
文摘Zircon U–Pb and Hf isotope data integrated in this study for magmatic and metamorphic rocks fromthe Hida Belt,southwest Japan,lead to a new understanding of the evolution of the Cordilleran arc system along the ancestral margins of present-day Northeast Asia.Ion microprobe data for magmatic zircon domains from eight mafic tointermediate orthogneisses in the Tateyama and Tsunogawa areas yielded weighted mean 206Pb/238U ages spanning the entire Permian period(302–254 Ma).Under cathodoluminescence,primary magmatic growth zones in the zircon crystals were observed to be partially or completely replaced by inward-penetrating,irregularly curved featureless or weakly zoned secondary domains that mostly yielded U–Pb ages of 250–240 Ma and relatively high Th/U ratios(>0.2).These secondary domains are considered to have been formed by solid-state recrystallization during thermal overprints associated with intrusions of Hida granitoids.Available whole-rock geochemical and Sr–Nd isotope data as well as zircon age spectra corroborate that the Hida Belt comprises the Paleozoic–Mesozoic Cordilleran arc system built upon the margin of the North China Craton,together with the YeongnamMassif in southern Korea.The arcmagmatismalong this systemwas commenced in the Carboniferous and culminated in the Permian–Triassic transition period.Highly positiveεHf(t)values(>+12)of late Carboniferous to early Permian detrital zircons in the Hida paragneisses indicate that there was significant input from the depleted asthenospheric mantle and/or its crustal derivatives in the early stage of arc magmatism.On the other hand,near-chondriticεHf(t)values(+5 to−2)of magmatic zircons from late Permian Hida orthogneisses suggest a lithospheric mantle origin.Hf isotopic differences between magmatic zircon cores and the secondary rims observed in some orthogneiss samples clearly indicate that the zircons were chemically open to fluids or melts during thermal overprints.Resumed highly positive zirconεHf(t)values(>+9)shared by Early Jurassic granitoids in the Hida Belt and Yeongnam Massif may reflect reworking of the Paleozoic arc crust.
基金the National Research Foundation of Korea(NRF)grant funded by the Korea government(MSIT)(2021R1 A2C1003363)research grants from the Korea Basic Science Institute(C280100 and C230120).
文摘Northeast Asian continental margins contain the products of magma emplacement driven by prolonged subduction of the(paleo-)Pacific plate.As observed in many Cordilleran arcs,magmatic evolution in this area was punctuated by high-volume pulses amid background periods.The present study investigates the early evolution of the Cretaceous magmatic flare-up using new and published geochronological,geochemical,and O-Hf isotope data from plutonic rocks in the southern Korean Peninsula.After a long(~50 m.y.)magmatic hiatus and the development of the Honam Shear Zone through flat-slab subduction,the Cretaceous flare-up began with the intrusion of monzonites,granodiorites,and granites in the inboard Gyeonggi Massif and the intervening Okcheon Belt.Compared to Jurassic granitoids formed during the former flare-up,Albian(~111 Ma)monzonites found in the Eopyeong area of the Okcheon Belt have distinctly higher zirconε_(Hf)(t)(-7.5±1.3)andδ^(18)O(7.78‰±0.25‰)values and lower wholerock La/Yb and Sr/Y ratios.The voluminous coeval granodiorite and granite plutons in the Gyeonggi Massif are further reduced in Sr/Y and to a lesser extent,in La/Yb,and have higher zirconε_(Hf)(t)values(-13 to-19)than the Precambrian basement(ca.-30).These chemical and isotopic features indicate that Early Cretaceous lithospheric thinning,most likely resulting from delamination of tectonically and magmatically overthickened lithospheric keel that was metasomatized during prior subduction episodes,and consequent asthenospheric upwelling played vital roles in igniting the magmatic flare-up.The O-Hf isotopic ranges of synmagmatic zircons from the Albian plutons and their Paleoproterozoic and Jurassic inheritance attest to the involvement of lithospheric mantle and crustal basement in magma generation during this decratonization event.Arc magmatism then migrated trenchward and culminated in the Late Cretaceous,yielding widespread granitoid rocks emplaced at shallow crustal levels.The early Late Cretaceous(94-85 Ma)granites now prevalent in Seoraksan-Woraksan-Sokrisan National Parks are highly silicic and display flat chondrite-normalized rare earth element patterns with deep Eu anomalies.Synmagmatic zircons in these granites mimic their host rock's chemistry.Delamination-related rejuvenation of crustal protoliths is indicated by zirconε_(Hf)(t)values of granites(-6 to-20)that are consistently higher than the Precambrian basement value.Concomitant core-to-rim variation in zircon O-Hf isotopic compositions reflects a typical sequence of crustal assimilation and fresh input into the magma chamber.
