Late Mesozoic Nb-rich basaltic andesites and high-Mg adakitic volcanic rocks from the Hailar-Tamtsag Basin,northeast China,provide important insights into the recycling processes of crustal materials and their role in...Late Mesozoic Nb-rich basaltic andesites and high-Mg adakitic volcanic rocks from the Hailar-Tamtsag Basin,northeast China,provide important insights into the recycling processes of crustal materials and their role in late Mesozoic lithospheric thinning.The Late Jurassic Nb-rich basaltic andesites(154±4 Ma)are enriched in large-ion lithophile and light rare earth elements,slightly depleted in high-field-strength elements,and have high TiO_(2),P_(2)O_(5),and Nb contents,and(Nb/Th)PM and Nb/U ratios,which together with the relatively depleted Sr-Nd-Hf isotopic compositions indicate a derivation from a mantle wedge metasomatized by hydrous melts from subducted oceanic crust.The Early Cretaceous high-Mg adakitic volcanic rocks(129-117 Ma)are characterized by low Y and heavy rare earth element contents,and high Sr contents and Sr/Y ratios,similar to those of rocks derived from partial melting of an eclogitic source.They also have high Rb/Sr,K_(2)O/Na_(2)O,and Mg#values,and high MgO,Cr,and Ni contents.These geochemical features suggest that the adakitic lavas were derived from partial melting of delaminated lower continental crust,followed by interaction of the resulting melts with mantle material during their ascent.Our data,along with available geological,paleomagnetic,and geophysical evidence,lead us to propose that recycling of Paleo-Pacific oceanic crustal materials into the upper mantle due to flat-slab subduction and rollback of the Paleo-Pacific Plate during the late Mesozoic likely provided the precondition for lithospheric thinning in northeast China,with consequent lithospheric delamination causing recycling of continental crustal materials and further lithospheric thinning.展开更多
The Tianhuashan Basin is one of the most important volcanic basins in the northern Wuyi,southeastern China,comprising two successive volcanic units,the Daguding Formation and the overlying Ehuling Formation,along with...The Tianhuashan Basin is one of the most important volcanic basins in the northern Wuyi,southeastern China,comprising two successive volcanic units,the Daguding Formation and the overlying Ehuling Formation,along with several small associated igneous intrusions.The Lengshuikeng super-large-scale Ag-Pb-Zn deposit,which is closely related to these volcanic-intrusive rocks,is located in the northwestern part of the basin.In order to understand the basin evolution and magmatism,we determined LA-ICP-MS U-Pb zircon ages for the volcanic successions and associated intrusive rocks.U-Pb zircon dating of volcanic units yielded precise ages of 144±1 Ma for crystal tuff in the lower member of the Daguding Formation,142±1 Ma for andesite within the upper member of the Daguding Formation,140±1 Ma for tuffite of the first(i.e.,lowermost) member of the Ehuling Formation,and 137±1 Ma for rhyolitic ignimbrite within the third volcano-stratigraphic member of the Ehuling Formation.Three types of intrusive igneous rocks(quartz syenite porphyry,K-feldspar granite porphyry,and rhyolite porphyry) yielded precise weighted mean 206 Pb/238 U ages of 144±1,140±1,and 140±1 Ma,respectively,suggesting that these intrusions along with the aforementioned volcanics were all emplaced during the Early Cretaceous.In addition,the weighted mean 206 Pb/238 U ages determined on zircon from two samples of a granite porphyry intrusion,which hosts ore mineralization of the Lengshuikeng Ag-Pb-Zn deposit,are 158±1 and 157±1 Ma,indicating emplacement in the Late Jurassic.These new geochronological results for igneous rocks of the Tianhuashan Basin constrain the timing of volcanic and plutonic activity in the basin,and have important implications for our understanding the tectonic history of the region,and for identifying metallogenic types and the timing of ore deposition of the Lengshuikeng deposit.展开更多
基金supported by the National Natural Science Foundation of China(Grants 41888101 and 92062216).
文摘Late Mesozoic Nb-rich basaltic andesites and high-Mg adakitic volcanic rocks from the Hailar-Tamtsag Basin,northeast China,provide important insights into the recycling processes of crustal materials and their role in late Mesozoic lithospheric thinning.The Late Jurassic Nb-rich basaltic andesites(154±4 Ma)are enriched in large-ion lithophile and light rare earth elements,slightly depleted in high-field-strength elements,and have high TiO_(2),P_(2)O_(5),and Nb contents,and(Nb/Th)PM and Nb/U ratios,which together with the relatively depleted Sr-Nd-Hf isotopic compositions indicate a derivation from a mantle wedge metasomatized by hydrous melts from subducted oceanic crust.The Early Cretaceous high-Mg adakitic volcanic rocks(129-117 Ma)are characterized by low Y and heavy rare earth element contents,and high Sr contents and Sr/Y ratios,similar to those of rocks derived from partial melting of an eclogitic source.They also have high Rb/Sr,K_(2)O/Na_(2)O,and Mg#values,and high MgO,Cr,and Ni contents.These geochemical features suggest that the adakitic lavas were derived from partial melting of delaminated lower continental crust,followed by interaction of the resulting melts with mantle material during their ascent.Our data,along with available geological,paleomagnetic,and geophysical evidence,lead us to propose that recycling of Paleo-Pacific oceanic crustal materials into the upper mantle due to flat-slab subduction and rollback of the Paleo-Pacific Plate during the late Mesozoic likely provided the precondition for lithospheric thinning in northeast China,with consequent lithospheric delamination causing recycling of continental crustal materials and further lithospheric thinning.
基金supported by the National Natural Science Foundation of China (Grant No. 40930419)Special Research Funding for the Public Benefit Sponsored by MLR (GrantNo. 200911007)
文摘The Tianhuashan Basin is one of the most important volcanic basins in the northern Wuyi,southeastern China,comprising two successive volcanic units,the Daguding Formation and the overlying Ehuling Formation,along with several small associated igneous intrusions.The Lengshuikeng super-large-scale Ag-Pb-Zn deposit,which is closely related to these volcanic-intrusive rocks,is located in the northwestern part of the basin.In order to understand the basin evolution and magmatism,we determined LA-ICP-MS U-Pb zircon ages for the volcanic successions and associated intrusive rocks.U-Pb zircon dating of volcanic units yielded precise ages of 144±1 Ma for crystal tuff in the lower member of the Daguding Formation,142±1 Ma for andesite within the upper member of the Daguding Formation,140±1 Ma for tuffite of the first(i.e.,lowermost) member of the Ehuling Formation,and 137±1 Ma for rhyolitic ignimbrite within the third volcano-stratigraphic member of the Ehuling Formation.Three types of intrusive igneous rocks(quartz syenite porphyry,K-feldspar granite porphyry,and rhyolite porphyry) yielded precise weighted mean 206 Pb/238 U ages of 144±1,140±1,and 140±1 Ma,respectively,suggesting that these intrusions along with the aforementioned volcanics were all emplaced during the Early Cretaceous.In addition,the weighted mean 206 Pb/238 U ages determined on zircon from two samples of a granite porphyry intrusion,which hosts ore mineralization of the Lengshuikeng Ag-Pb-Zn deposit,are 158±1 and 157±1 Ma,indicating emplacement in the Late Jurassic.These new geochronological results for igneous rocks of the Tianhuashan Basin constrain the timing of volcanic and plutonic activity in the basin,and have important implications for our understanding the tectonic history of the region,and for identifying metallogenic types and the timing of ore deposition of the Lengshuikeng deposit.
