We performed a combined study of He–Ar isotopes and element geochemistry for pyroxene megacrysts and mantle xenoliths from Cenozoic basalt in the Changle–Linqu area in western Shandong.The results are used to trace ...We performed a combined study of He–Ar isotopes and element geochemistry for pyroxene megacrysts and mantle xenoliths from Cenozoic basalt in the Changle–Linqu area in western Shandong.The results are used to trace the evolution of subcontinental lithospheric mantle after destruction of the North China Craton.The3He/4He ratios of pyroxene megacrysts,websterite,and dunites are 7.0–7.7 Ra,7.4 Ra,and 7.0–7.7 Ra,respectively.They are related to the origin of host basalts,and are slightly lower than that of mid-ocean ridge basalts(MORB).Their40Ar/36Ar ratios are much lower than that of MORB and close to that of air.The lherzolites and wehrlites represent the fragments of the newly accreted lithospheric mantle,and their REE and trace elements indicate that they experienced melt metasomatism and partial melting.The3He/4He ratios of their olivine are slightly lower than that of MORB,but3He/4He ratios of their clinopyroxene are low(2.3–7.1 Ra)and display an inverse correlation with(La/Yb)N.The40Ar/36Ar ratios of these clinopyroxene are much lower than that of MORB and close to the air ratio.Combining existing studies of petrology,Sr–Nd–Pb isotopes,O isotopes,trace elements,and Mg isotopes,we infer that the juvenile lithospheric mantle in the Changle–Linqu area was metasomatized by oceanic crust-derived melts,which transfer the supracrustal Ar isotope signatures to the mantle sources.The low4He abundance and low3He/4He ratios of clinopyroxene in the lherzolites and wehrlites are ascribed to metasomatism by crustal melts from the subducted Pacific plate.展开更多
The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subductio...The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subduction・related magmatic activities and subsequent exhumation processes occurred.Here,we report new low-temperature thermochronology of apatite and zircon data from the granitoid samples in the Yanji area.The exhumation rates of Tianfozhishan,Yanji area,were〜0.049 and〜0.073 mm/year,interpreted from the elevations and apatite and zircon fission track ages,respectively.The exhumation,integrated with the geological setting,suggested that the paleogeothermal gradient of the Tianfozhishan,even extending to the Yanji area,was possibly to be greater than 35℃/km in the Late Cretaceous.The thermal history modeling of the data indicates a basically similar pattern,but the various timing for different samples between the Oligocene-Early Miocene and the Middle Miocene in the Yanji area.We hence conclude that a fourstages of cooling,from〜6.7℃/Ma(during the Late Cretaceous),to〜0.8℃/Ma(during the Late Cretaceous to the Oligocene-Early Miocene),then to〜2-3℃/Ma with varied styles(between the Oligocene-Early Miocene and the Middle Miocene),and finally to<0.2℃/Ma(since the Middle Miocene),has taken place through the exhumation of the Yanji area.The maximum exhumation is>3 km under a reasonable paleogeothermal gradient(>35℃/km),speculated from the possible exhumation rate of Tianfozhishan.Combined with the tectonic setting,this exhumation,including two stages of pronounced tectonic uplift and denudation and two stages of weak exhumation driven by the low regional erosion rate,is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the Late Cretaceous.This study used more robust evidence to propose higher paleogeothermal gradient(>35℃/km),reflecting exhumation of>3 km in the Yanji area since the Late Cretaceous.展开更多
基金supported by the National Basic Research Program of China(2009CB825001)the National Natural Science Foundation of China(90714010)
文摘We performed a combined study of He–Ar isotopes and element geochemistry for pyroxene megacrysts and mantle xenoliths from Cenozoic basalt in the Changle–Linqu area in western Shandong.The results are used to trace the evolution of subcontinental lithospheric mantle after destruction of the North China Craton.The3He/4He ratios of pyroxene megacrysts,websterite,and dunites are 7.0–7.7 Ra,7.4 Ra,and 7.0–7.7 Ra,respectively.They are related to the origin of host basalts,and are slightly lower than that of mid-ocean ridge basalts(MORB).Their40Ar/36Ar ratios are much lower than that of MORB and close to that of air.The lherzolites and wehrlites represent the fragments of the newly accreted lithospheric mantle,and their REE and trace elements indicate that they experienced melt metasomatism and partial melting.The3He/4He ratios of their olivine are slightly lower than that of MORB,but3He/4He ratios of their clinopyroxene are low(2.3–7.1 Ra)and display an inverse correlation with(La/Yb)N.The40Ar/36Ar ratios of these clinopyroxene are much lower than that of MORB and close to the air ratio.Combining existing studies of petrology,Sr–Nd–Pb isotopes,O isotopes,trace elements,and Mg isotopes,we infer that the juvenile lithospheric mantle in the Changle–Linqu area was metasomatized by oceanic crust-derived melts,which transfer the supracrustal Ar isotope signatures to the mantle sources.The low4He abundance and low3He/4He ratios of clinopyroxene in the lherzolites and wehrlites are ascribed to metasomatism by crustal melts from the subducted Pacific plate.
基金supported by the DREAM project of MOST China (2016YFC0600406)the National Natural Science Foundation of China (Grant Nos. 41072158, 41372227)
文摘The Yanji area,northeastern China,a part of the orogenic collage between the North China Block in the south and the Jiamusi-Khanka Massifs in the northeast,is the most likely location where the Pacific Plate subduction・related magmatic activities and subsequent exhumation processes occurred.Here,we report new low-temperature thermochronology of apatite and zircon data from the granitoid samples in the Yanji area.The exhumation rates of Tianfozhishan,Yanji area,were〜0.049 and〜0.073 mm/year,interpreted from the elevations and apatite and zircon fission track ages,respectively.The exhumation,integrated with the geological setting,suggested that the paleogeothermal gradient of the Tianfozhishan,even extending to the Yanji area,was possibly to be greater than 35℃/km in the Late Cretaceous.The thermal history modeling of the data indicates a basically similar pattern,but the various timing for different samples between the Oligocene-Early Miocene and the Middle Miocene in the Yanji area.We hence conclude that a fourstages of cooling,from〜6.7℃/Ma(during the Late Cretaceous),to〜0.8℃/Ma(during the Late Cretaceous to the Oligocene-Early Miocene),then to〜2-3℃/Ma with varied styles(between the Oligocene-Early Miocene and the Middle Miocene),and finally to<0.2℃/Ma(since the Middle Miocene),has taken place through the exhumation of the Yanji area.The maximum exhumation is>3 km under a reasonable paleogeothermal gradient(>35℃/km),speculated from the possible exhumation rate of Tianfozhishan.Combined with the tectonic setting,this exhumation,including two stages of pronounced tectonic uplift and denudation and two stages of weak exhumation driven by the low regional erosion rate,is possibly related to the subduction of the Pacific Plate beneath the Eurasian Plate since the Late Cretaceous.This study used more robust evidence to propose higher paleogeothermal gradient(>35℃/km),reflecting exhumation of>3 km in the Yanji area since the Late Cretaceous.
