Chronology And Geochemistry Of Late Mesozoic Volcanic Rocks In Eastern Jilin And Heilongjiang Provinces

Detailed studies of chronology, petrology and geochemistry are carried out for the volcanic rocks from Suifenhe, Jingouling, and Tuntianying formations in eastern Jilin and Heilongjiang provinces in order to reveal their formation time and tectonic setting as well as the lithospheric evolution of northeast China in the late Mesozoic.1 Rock associations and geochemistry of late Mesozoic volcanic rocks in eastern Jilin and Heilongjiang provincesLate Mesozoic volcanic rocks in eastern Jilin and Heilongjiang provinces are mainly composed of basalt, basaltic andesite, andesite and dacite besides some basaltic trachyandesite and trachydacite from Tuntianying formation.They have SiO2 contents of 47.69%～65.47%, Al2O3 contents of 15.55%～18.76%, and are characterized by enrichment in magnesium (Mg#=45～53) and sodium (Na2O/K2O=1.49～3.63). Their K60 values range from 0.85 to 2.57. Plots of SiO₂ against other major oxides show broadly negative correlations with TiO₂, Fe₂O₃, MgO, CaO, and P₂O₅, and positive correlations with Na₂O and K2O, while Al₂O₃ increase firstly and then decrease with the increase of SiO₂ contents. They belong to subalkaline series and show a calc-alkaline evolutionary trend, most of them are plotted within (medium-K) calc-alkaline series in SiO₂ vs. K₂O diagram. Their total rare earth element contents (ΣREE) are between 94.17ppm and 142.32ppm. Their (La/Yb)_N ratios andδ_Eu values range from 3.20 to 18.90 and from 0.87 to 1.04, respectively. With the increase of SiO₂ contents, the light rare earth element (LREE) abundances increase, whereas heavy rare earth element (HREE) abundances decrease. On the chondrite-normalized REE diagram, the late Mesozoic volcanic rocks from eastern Jilin and Heilongjiang provinces are characterized by enrichment in LREEs, depletion in HREEs, and minor negative Eu anomalies or no Eu anomalies.On the primitive mantle-normalized spider diagram, all the samples display enrichment in large ion lithophile elements (LILEs, e.g., Rb, Ba, Th, U and K ), and LREEs, depletion in high field strength elements (HFSEs, e.g., Nb, Ta, Ti), which is similar to Andean arc volcanic rocks.Late Mesozoic volcanic rocks in eastern Jilin and Heilongjiang provinces have ⁸⁷Sr/⁸⁶Sr ratios of 0.704249～0.706130 and ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512596～0.512815, while their initial ⁸⁷Sr/⁸⁶Sr ratios (ISr) andεNd(t) values are from 0.7041 to 0.7057 and from 0.40 to 4.11, respectively. Their SiO₂ contents show broadly positive correlations with ⁸⁷Sr/⁸⁶Sr ratios and ISr values, and negative correlations with ¹⁴³Nd/¹⁴⁴Nd ratios andε_Nd(t) values.2 Chronology of late Mesozoic volcanic rocks in eastern Jilin and Heilongjiang provincesLA-ICPMS zircon U-Pb dating results indicate that volcanic rocks from Suifenhe formation, Jingouling formation, and Tuntianying formation formed at 105.5～93.2Ma, 106.6±2.1Ma, 97.3±1.4Ma, and 88.2±1.4Ma, respectively, i.e., from the late stage of Early Cretaceous to Late Cretaceous. This magmatic activity is consisitent with the time of alkaline basaltic magmatism occurred in the intracontinent of eastern China such as Qingshankou formation basalts from the Songliao basin, Fuxin basalts in western Liaoning Province, Daxizhuang basalts in eastern Shandong Province, and Qujiatun basalts in eastern Liaoning Province. 3 Magmatic evolution and magmatic source of late Mesozoic volcanic rocks in eastern Jilin and Heilongjing provincesThe existences of phenocrysts such as olivine, pyroxene, amphibole, and plagioclase in the late Mesozoic volcanic rocks suggest these minerals experience fractional crystallization during magmatic evolution, which is also proved by the linear correlation between SiO₂ and other major oxides and trace elements. The linear relationships between ISr, andε_Nd(t) values and SiO₂ contents imply that the contamination of crustal materials might be involved in magmatic evolution. Therefore, mineral fractional crystallization and assimilation of crustal materials (AFC) are the dominant mechanism to control magmatic evolution.All the samples possess low ISr values, postiveεNd(t) values, and young TDM , implying a juvenile depleted magmatic source. Geochemical characteristics of major elements and trace elements suggest that depletion of HFSEs such as Nb and Ta, can not be explained by residue of minerals enriched in HFSEs in magmatic source or by simple contaminations of crustal materials. In the Rb/Y vs. Nb/Y diagram and Nb/Zr vs. Th/Zr diagram, these samples exhibit variation of fluid-related metasomatism. Therefore, it is suggested that the parental magma could be derived from partial melting of the mantle wedge metasomatized by subduction-related fluids.4 Tectonic setting of late Mesozoic volcanic rocks in eastern Jilin and Heilongjing provincesThe basalt-basaltic andesite-andesite-dacite association for the late Mesozoic volcanic rocks in eastern Jilin and Heilongjing is similar to those from active continental margins (e.g., Andes), suggesting that they were formed in active continental margin setting.The REE and trace element geochemical data for the late Mesozoic volcanic rocks display the geochemical characteristics of arc volcanic rocks (e.g., Ba/Nb vs. La/Nb diagram and Th-Hf/3-Ta ternary diagram). Taken together with rock association, it is proposed that the late Mesozoic volcanic rocks from eastern Jilin and Heilongjiang provinces were formed under active continental margin setting.Late Mesozoic magmatic activity in eastern Jilin and Heilongjiang provinces is coeval to the change of subduction angle of the Izanagi Plate beneath Eurasia continent (from NNW to near NW) and its subsequent NW subduction. Therefore, its genesis should be related to the subduction of Izanagi Plate, i.e., its oblique subduction against the Eurasia continent led to the formation of late Mesozoic volcanic rocks in eastern Jilin and Heilongjiang province with affinities of active continental margin.