Mesozoic intermediate-felsic magmatic rocks in the eastern North China Craton commonly show geochemical similarity to adakites.However,the lack of direct constraints from partial melting experiments at high pressures ...Mesozoic intermediate-felsic magmatic rocks in the eastern North China Craton commonly show geochemical similarity to adakites.However,the lack of direct constraints from partial melting experiments at high pressures and temperatures fuels a debate over the origin of these rocks.In this work,we performed partial melting experiments at 1.5 GPa and 800–950℃on amphibolite samples collected from the vicinity of the Mesozoic potassium-rich adakitic rocks in the Zhangjiakou area,northern margin of the North China Craton.The experimental melts range from granitic to granodioritic compositions,with SiO_(2)=56.4–72.6 wt.%,Al_(2)O_(3)=16.1–19.3 wt.%,FeO^(*)=2.4–9.6 wt.%,MgO=0.3–2.0 wt.%,CaO=0.6–3.8 wt.%,Na_(2)O=4.7–5.3 wt.%,and K_(2)O=2.6–3.9 wt.%,which are in the ranges of the surrounding Mesozoic potassium-rich adakitic rocks,except for the higher Al_(2)O_(3)contents and the data point at 1.5 GPa and 800℃.Trace element compositions of the melts measured by LA-ICP-MS are rich in Sr(849–1067 ppm)and light rare earth elements(LREEs)and poor in Y(<10.4 ppm)and Yb(<0.88 ppm),and have high Sr/Y(102–221)and(La/Yb)n(27–41)ratios and strongly fractionated rare earth element(REE)patterns,whereas no obvious negative Eu anomalies are observed.The geochemical characteristics show overall similarity to the Mesozoic potassium-rich adakitic rocks in the area,especially adakites with low Mg#,again except for the data point at 1.5 GPa and 800℃.The results suggest that partial melting of amphibolite can produce potassium-rich adakitic rocks with low Mg#in the eastern North China Craton under the experimental conditions of 1.5 GPa and 850–950℃.The experimental restites consist of hornblende(Hbl)+plagioclase(Pl)+garnet(Grt)±clinopyroxene(Cpx),a mineral assemblage significantly different from that of the nearby Hannuoba mafic granulite xenoliths which consist of Cpx+orthopyroxene(Opx)+Pl±Grt.Chemically,the experimental restites contain higher Al_(2)O_(3)but lower MgO and CaO than the Hannuoba mafic granulite xenoliths.We therefore argue that the Hannuoba mafic granulite xenoliths cannot represent the direct products of partial melting of the experimental amphibolite.展开更多
The origin of lowδ^(18)O signals in zircons from the Early Cretaceous A-type granites in eastern China has long been disputed.It is uncertain whether the^(18)O-depleted features were inherited from high-temperature h...The origin of lowδ^(18)O signals in zircons from the Early Cretaceous A-type granites in eastern China has long been disputed.It is uncertain whether the^(18)O-depleted features were inherited from high-temperature hydrothermal altered source rock or resulted from water-rock interaction after emplacement.In this paper,zircon oxygen isotopes in the~130 Ma Kulongshan A-type granites in the northern North China Craton are analyzed.The zircons could be subdivided into 5 types based on their luminescent intensity and internal structures in CL images.Theirδ^(18)O values also vary in different types and show negative correlation with U and Th contents and accompanying cumulativeα-decay doses,implying that theirδ^(18)O values may have been modified to various degrees by meteoric water-rock interaction after the accumulation of radiation damage.The idea is further confirmed by oxygen isotopic equilibrium calculation between co-existing mineral pairs.It is inferred that only the least-influenced zircons,with slightly elevatedδ^(18)O values than normal mantle,have preserved the magmatic oxygen isotopes.In combination with other evidences,it is proposed that the A-type granites are lower-crustal-derived,unnecessarily invoking a high-temperature hydrothermal altered source.The proposition is applicable to many other Cretaceous A-type granites that have similar zircon behaviors.展开更多
基金the National Natural Science Foundation of China(Grant Nos.41772043 and 41802043)the Chinese Academy of Sciences“Light of West China”Program(Dawei Fan,2017 and Jingui Xu,2019)+1 种基金the Youth Innovation Promotion Association CAS(Dawei Fan,2018434)the Innovation and Entrepreneurship Funding of High-Level Overseas Talents of Guizhou Province(Dawei Fan,[2019]10).
文摘Mesozoic intermediate-felsic magmatic rocks in the eastern North China Craton commonly show geochemical similarity to adakites.However,the lack of direct constraints from partial melting experiments at high pressures and temperatures fuels a debate over the origin of these rocks.In this work,we performed partial melting experiments at 1.5 GPa and 800–950℃on amphibolite samples collected from the vicinity of the Mesozoic potassium-rich adakitic rocks in the Zhangjiakou area,northern margin of the North China Craton.The experimental melts range from granitic to granodioritic compositions,with SiO_(2)=56.4–72.6 wt.%,Al_(2)O_(3)=16.1–19.3 wt.%,FeO^(*)=2.4–9.6 wt.%,MgO=0.3–2.0 wt.%,CaO=0.6–3.8 wt.%,Na_(2)O=4.7–5.3 wt.%,and K_(2)O=2.6–3.9 wt.%,which are in the ranges of the surrounding Mesozoic potassium-rich adakitic rocks,except for the higher Al_(2)O_(3)contents and the data point at 1.5 GPa and 800℃.Trace element compositions of the melts measured by LA-ICP-MS are rich in Sr(849–1067 ppm)and light rare earth elements(LREEs)and poor in Y(<10.4 ppm)and Yb(<0.88 ppm),and have high Sr/Y(102–221)and(La/Yb)n(27–41)ratios and strongly fractionated rare earth element(REE)patterns,whereas no obvious negative Eu anomalies are observed.The geochemical characteristics show overall similarity to the Mesozoic potassium-rich adakitic rocks in the area,especially adakites with low Mg#,again except for the data point at 1.5 GPa and 800℃.The results suggest that partial melting of amphibolite can produce potassium-rich adakitic rocks with low Mg#in the eastern North China Craton under the experimental conditions of 1.5 GPa and 850–950℃.The experimental restites consist of hornblende(Hbl)+plagioclase(Pl)+garnet(Grt)±clinopyroxene(Cpx),a mineral assemblage significantly different from that of the nearby Hannuoba mafic granulite xenoliths which consist of Cpx+orthopyroxene(Opx)+Pl±Grt.Chemically,the experimental restites contain higher Al_(2)O_(3)but lower MgO and CaO than the Hannuoba mafic granulite xenoliths.We therefore argue that the Hannuoba mafic granulite xenoliths cannot represent the direct products of partial melting of the experimental amphibolite.
基金supported by the Key Program of the Bureau of International Cooperation,Chinese Academy of Sciences(132A11KYSB20180042)the Key Project of Frontier Science of Chinese Academy of Sciences(QYZDY-SSW-DQC017)the National Natural Science Foundation of China(41890832,41890830,and41890834)。
基金supported by the National Natural Science Foundation of China(No.41973034)the Postdoctoral Initiation Fund of Northwestern University(No.208521)。
文摘The origin of lowδ^(18)O signals in zircons from the Early Cretaceous A-type granites in eastern China has long been disputed.It is uncertain whether the^(18)O-depleted features were inherited from high-temperature hydrothermal altered source rock or resulted from water-rock interaction after emplacement.In this paper,zircon oxygen isotopes in the~130 Ma Kulongshan A-type granites in the northern North China Craton are analyzed.The zircons could be subdivided into 5 types based on their luminescent intensity and internal structures in CL images.Theirδ^(18)O values also vary in different types and show negative correlation with U and Th contents and accompanying cumulativeα-decay doses,implying that theirδ^(18)O values may have been modified to various degrees by meteoric water-rock interaction after the accumulation of radiation damage.The idea is further confirmed by oxygen isotopic equilibrium calculation between co-existing mineral pairs.It is inferred that only the least-influenced zircons,with slightly elevatedδ^(18)O values than normal mantle,have preserved the magmatic oxygen isotopes.In combination with other evidences,it is proposed that the A-type granites are lower-crustal-derived,unnecessarily invoking a high-temperature hydrothermal altered source.The proposition is applicable to many other Cretaceous A-type granites that have similar zircon behaviors.
