Chronology And Geochemistry Of Late Paleozoic Volcanic Rocks In Eastern And Southeastern Margins Of The Jiamusi Massif

Detailed studies on chronology, petrology and geochemistry are carried out for the volcanic rocks from the Late Devonian Laotudingzi Formation (Fm.), the Late Carboniferous-Early Permian (C2-P1) Zhenzishan Fm., the Late Permian Erlongshan Fm., the Late Triassic Haojiatun Fm., the Early Cretaceous Dongshan Fm. and the Late Permian Yanggang Fm in eastern and sortheastern margins of the Jiamusi Massif in order to reveal their formation time and tectonic implications as well as the tectonic evolution history of the Central Asian Orogenic Belt (CAOB).1 Chronology of late Paleozoic volcanic rocks in eastern and southeastern margins of the Jiamusi MassifLA-ICP-MS zircon U-Pb dating results indicate that the Late Paleozoic volcanic rocks in the study area can be further subdivided into two stages, i.e., the Early Permian (peak age of 288 Ma) and the Middle Permian (peak age of 268 Ma). The former includes the previously believed Haojiatun Fm.(T2), Dongshan Fm.(K1), Erlongshan Fm.(P3), with the ages of 286～291Ma, 293±2Ma and 288±2Ma, respectively. The latter includes the previously believed Dongshan Fm.(K1), Zhenzishan Fm.(C2-P1) and Yanggang Fm.(P3) with the ages of 264～268Ma, 263±5Ma and 263±2Ma, respectively. 2 Rock association and geochemistry of late Paleozoic volcanic rocks in eastern and southeastern margins of the Jiamusi Massif2.1 The Early Permian volcanic rocksThe Early Permian volcanic rocks in the study area are mainly composed of basalt, basaltic andesite, minor dacite and rhyolite.They have SiO2 contents of 46.48%~54.56%, Al2O3 contents of 16.36% ~20.28%, and are characterized by enrichment in magnesium (Mg#=40~59) and sodium (Na2O/K2O=1.26~4.25). Plots of SiO2 against other major oxides show broadly negative correlations with TiO2, TFeO, MgO, CaO, and P2O5, and positive correlations with Na2O and Al2O3, while Na2O 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 108ppm and 167μg/g. Their (La/Yb)N ratios andδEu values range from 5.6 to 10.6 and from 0.95 to 1.09, respectively. The Chondrite-normalized rare earth element (REE) pattern for the Early Permian volcanic rocks shows that The volcanic rocks are characterized by enrichment in light rare earth elements (LREEs), relatively depletion in heavy rare earth elements (HREEs), and no Eu anomalies, similar to the Andean arc volcanic rocks. Besides, On the primitive mantle (PM)-normalized spider diagram, the Early Permian volcanic rocks are characterized by enrichment in large ion lithophile elements (LILEs) and relatively depletion in high field strength elements (HFSEs) such as Nb, Ta, Zr, Hf and Ti, similar to the volcanic rocks from island arc or active continental margin settings, too.2.2 The Middle Permian volcanic rocksThe Middle Permian volcanic rocks in the study area are mainly composed of rhyolite and dacite with high SiO2(77.23%~77.52%), rich in K(Na2O/K2O< 0.80), and low MgO, TiO2, TFe2O3 and CaO. They belong to the subalkaline series, have peraluminous geochemical affinity (ACNK=1.2~1.4) and is plotted within (high-K) calc-alkaline series in SiO2 vs. K2O diagram.The Middle Permian volcanic rocks have lower REE abundance (ΣREE=74, 79 and 162μg/g) and intensively negative Eu anomalies as well as less LREE/HREE fractionation ((La/Yb)N=2.0, 2.6 and 10.3). On the primitive mantle (PM)-normalized spider diagram, the Middle Permian volcanic rocks are strongly depleted in Ba, Sr, Eu, Ti and P, and highly enriched in U, Th, K, Rb, implying its crust-derived origin.3 Petrogenesis and tectonic implications of late Paleozoic volcanic rocks in eastern and southeastern margins of the Jiamusi Massif3.1 The Early Permian volcanic rocksThe existences of phenocrysts such as pyroxene, plagioclase and quartz in the late Paleozoic 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, implying that mineral fractional crystallization is the dominant mechanism to control magmatic evolution. Besides, the 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 melt-related metasomatism. Therefore, it is suggested that the Early Paleozoic volcanic rocks could be derived from the partial melting of the lithosphere mantle wedge modified by the subducted oceanic slab.The basalt-basaltic andesite-minor dacite and rhyolite association for the Early Permian volcanic rocks in the study area belongs to subalkaline series, shows a calc-alkaline evolutionary trend, is plotted within (medium-K) calc-alkaline series in SiO2 vs. K2O diagram, and displays the geochemical characteristics of arc volcanic rocks. Taken together with rock association, major and trace element data, it is proposed that they could form under the tectonic setting of the active continental margin.3.2 The Middle Permian volcanic rocksHigh SiO2, rich K2O, low MgO, peraluminous geochemical features (ACNK=1.2~1.4) of the Middle Permian volcanic rocks imply that the primary magma could be derived from the partial melting of the middle-upper crustal material, which is also supported by their strong enrichment in Th, U and Pb and depletion in Eu, Sr, P and Ti, consistent with the character that the Middle Permian volcanic rocks fall into the crustal magma source on Rb-Ba-Sr relationship diagram.The Middle Permian volcanic rocks consists mainly of rhyolite and minor dacite, belongs to the subalkaline series and (high-K) calc-alkaline series. On Rb-(Y+Nb) variation diagram, the Middle Permian volcanic rocks are located chiefly in the volcanic arc (VA)—syn-collisional tectonic settings. Taken together, it is suggested that the Middle Permian volcanic rocks could be related to continent-continent collision.3.3 Tectonic implications of late Paleozoic volcanic rocksLA-ICP-MS zircon U-Pb dating and geochemical data for the Late Paleozoic volcanic rocks from eastern and southeastern margins of the Jiamusi Massif indicate that an active continental margin setting could exist in the eastern margin of the Jiamusi Massif in the Early Permian. But, the Middle Permian volcanic rocks with high SiO2, low MgO and enrichment in K2O suggest that it could form under the continent-continent collision setting between the Jiamusi and the Khanka Massifs, and that the Paleo-Asiatic oceanic plate between the two Massifs could be disappeared in the Middle Permian. Therefore, it can be considered that the tectonic evolution between the Jiamusi-Khanka Massifs and the North China Craton began in the study area after the Middle Permian.