A Geochemical Study Of Mesozoic Intermediate To Mafic Dykes In The Jiaodong Area

A number of major geological events occurred in Eastern China since Phanerozoic Eon,such as the destruction of North China Craton,subduction/collision between the Yangtze plate and the North China Craton and subduction of paleo Pacific plate.At the same time,the magmatic rocks are wildly distributed in the eastern China,especially in the Mesozoic era.The source region,diagenesis and geochemical dynamics of these magmatic rocks provide important constraints on better understanding the properties and evolution of the Mesozoic lithospheric mantle in Eastern China.The Jiaodong Peninsula is located in the eastern part of China's central orogenic belt(Qinling-Dabie-Sulu orogenic belt).It is near the Bohai Bay in the east,North China Craton in the north,Sulu orogenic belt in the south.Jiaodong Peninsula is a natural laboratory to study the composition and evolvement of deep lithosphere mantle.In this paper,we study the petrography,zircon U-Pb age,whole rock major and trace elements and Sr-Nd-Pb isotopic composition of the intermediate to mafic dykes in Linglong and Rushan area of Jiaodong Peninsula,respectively.Combined with the existing data for Mesozoic mantle-derived intrusions and volcanic in Shandong Province,we further discussed the characteristics of the Mesozoic lithosphere mantle in the Jiaodong Peninsula.Linglong region is located in the northern part of the Jiaodong Peninsula,which belongs to the northern part of the North China Craton.The intermediate to mafic dykes are mainly composed of diabase,lamprophyre and diorite porphyrite.The LA-ICPMS zircon U-Pb dating results give the Early Cretaceous ages which range from 121.3 ±3.8?123.5 ± 4.6 Ma for these dykes.They have lower Si02(44.53?55.05 wt.%)and higher MgO(6.77?13.26%,Mg#= 64?73)content.They have high K2O(1.5?4.4 wt.%)content and high K2O/Na2O(0.60-1.82),high Cr(383?818 ppm),Ni(135?328 ppm)content.They mainly fall in the range of gabbro in the total alkaline vs.SiO2(TAS)diagram.The Rushan region is located in the southern part of the Jiaodong Peninsula,which belongs to the part of the continental collision orogenic belt.The intermediate to mafic dykes are mainly composed of diorite porphyry,and the LA-ICPMS zircon U-Pb dating results show that the formation age is 107.6 ± 1.6?118.2 ± 3.3 Ma.Compared to Linglong,the samples from the Rushan region have more variable SiO2(45.54?60.66 wt.%)and MgO contents(3.15?12.73 wt.%,Mg#= 58?76).They have high K2O(1.3?4.4 wt.%)content with high K2O/Na2O(0.47?1.71),high Cr(108?1176 ppm),Ni(18.5?313 ppm)content.They mainly fall in the range of diorite in the total alkaline vs.SiO2(TAS)diagram.In the trace elements and rare earth elements distribution diagram,the intermediate to mafic dykes from both Linglong and Rushan region are rich in large ion lithophile elements(LILE),light rare earth elements(LREE),depleted in high field strength elements(HFSE),heavy rare earth elements(HREE)showing a "arc-like" trace element distribution characteristic.Dykes from Linglong region have high initial 87Sr/86Sr ratio(0.7092?0.7112),the low ?Nd(t)value(-12.3??19.6)and initial 206Pb/204Pb(17.23?17.53),207Pb/204Pb(15.50?15.57),208Pb/204Pb(37.77?38.18)respectively.Compared to Linglong,dykes from Rushan region have a slightly lower initial B7Sr/86Sr ratio(0.7075?0.7094),low radioactivity ?Nd(t)(-10.9?-15.7)and lower initial 206Pb/204Pb(16.74?17.32),207Pb/204Pb(15.36?5.46),208Pb/204Pb(36.89?37.63).These geochemical characteristics indicate that these intermediate to mafic dykes in the Jiaodong Peninsula are derived from the enriched mantle and the lower TiO2 and(Fe2O3)T content indicate that the lithospheric mantle rather than asthenosphere is the possible source region.In the Sr-Nd-Pb isotopic diagram,the dykes are closer to the lithospheric mantle of the North China Craton and away from the Yangtze craton.The initial 87Sr/86Sr ratio and the initial ?Nd(t)keep unchanged with the varying Mg#indicating crustal contamination is not significant in the ascending process and the mantle source of these dykes are the lithospheric mantle beneath the North China Craton which have been metasomatized by fluids.The Nb/U and Ce/Pb ratios of these dykes suggest the contribution of the upper crustal material in the mantle source and the two-stage model age of Nd are consistent with that of the metamorphic rocks in Sulu orogenic belt indicating the mantle source of the intermediate to mafic dykes is metasomatised by the crustal materials of the subducted Yangtze plate.The geochemical characteristics of the intermediate to mafic dykes in this study are close to that of the Yinan Gabbro,Fangcheng basalt and volcanic rocks in Jiaolai basins near the Sulu orogenic belt,while is significantly different from that of the mafic intrutions in the Jinan and Zouping region.The former has the characteristics of EM2-type enriched mantle while the latter shows the characteristics of EM1-type enriched mantle.The subduction of the Yangtze plate is the main factor for the difference.Jiaodong Peninsula,which is adjacent to the orogenic belt,is affected greatly by the subduction of the Yangtze plate while the inner part of North China Craton.which is far from the orogenic belt,is less influenced by the subduction.From the perspective of time,the magmatism is happened during the early Cretaceous,about 90 Ma later than the subduction/collision of the Yangtze plate,which suggests that there are obvious age diversity between the orogenic process and the partial melting process of the lithosphere mantle.The mantle-derived igneous rocks in the Shandong Province can be divided into two age groups within 185?177 Ma(asthenosphere-derived)and 144?101 Ma(lithosphere-derived)for the western Shandong Province,134?102 Ma(lithosphere-derived)and 95?73 Ma(asthenosphere-derived)for the Jiaodong Peninsula.This age progression is consistent with the rolling-back of the subducted Paleo-Pacific plate in the Early Cretaceous.With the subduction and rolling-back of paleo-Pacific plate,the eastern China changed from compression to extension regime.The decompression is the main mechanism of the partial melting for the formation of the Early Cretaceous mantle-derived magma in Shandong Province.