| The Jiamusi Block of NE China is located between the North China and Siberia cratons. It collided with the Songliao Block to the west in the early Mesozoic as one of the circum-pacific accreted blocks, and formed the unified Eurasian eastern boundary as a result of subduction of the paleo-Pacific plate. Huge amounts of igneous rocks were produced in the whole of eastern Eurasia, including the Jiamusi Block, caused by back-arc intraplate extension triggered by paleo-pacific subduction. Study of the petrogenesis of these igneous rocks is important for understanding the nature of the late Mesozoic magmatism and geodynamic evolution of the Jiamusi block and eastern China.For this research, typical late Mesozoic geological sections were selected from the Jiamusi Block for field geology, petrographic study, zircon SHRIMP U-Pb geochronology, geochemistry, Sr-Nd isotope, zircon Lu-Hf isotope and zircon O-isotope study, in order to accurately determine the age of the late Mesozoic igneous rocks and their petrogenesis. This study also summarizes previous research of subduction processes in the region and compares the geochronological and geochemical results with the igneous rocks in the Great Xing’an Range, the Songliao Basin, and other areas of East Asia. Finally the temporal relationship of the igneous rocks in NE China is discussed and illustrated by utilizing several models.The main achievements and conclusions of this thesis are as follows:(1) The late Mesozoic magmatism of the Jiamusi Block mainly occurred in the mid-Cretaceous between104±1and100±2Ma. Zircon SHIRMP U-Pb dating shows that the Muling Yilin Formation rhyolite and Wulaga granite porphyry have the same age of104±1Ma; the Hegang Songmuhe Formation basalt erupted between103±2and100±2Ma; and the Huanan composite dyke and its country rock and the Jiamusi bimodal dykes were all emplaced at100±2Ma.(2) The mid-Cretaceous igneous rocks of the Jiamusi Block belong to the high-K calc-alkaline series, with a bimodal signature. They are all rich in LILE and HREE, depleted in HFSE, and formed in an active continental margin or intraplate tectonic setting.(3) The Yilin Formation rhyolite is a high-Mg adakite, with geochemical signature of high-Sr and La/Yb, low-Yb and Y, and high Mg#(-0.57). Positive εNd(t)(-+0.75), low zircon δ18O (5.6-6.7) and positive εHf(t)(5.8-12.7) suggest that the source of the adakite is mantle peridotite metasomatized by slab derived melt/fluid. Negative Eu and Ba anomalies indicate residual plagioclase in the source and low melting pressure. Some samples have low Sr contents and abnormal87Sr/86Sr values, possibly caused by magma-ground water interaction. The Wulaga pluton has two types of granite porphyry. One is biotite granite porphyry which invaded into the early Cretaceous Ningyuancun Formation sandstone and tuff. The geochemical features of high Sr (>300ppm), low Y (-8ppm), high Mg#(-0.57), positive εHf(t)(6.3-12.7) and εNd(t)(-+0.5) show that the biotite granite porphyry is also a high-Mg adakite, similar to the Yilin Formation rhyolite. However, the crustal zircon δ18O (～8.0) and lower Y content (7.5-8.3ppm) suggest that it was derived from the partial melting of a subducted slab but experienced minor mantle contamination, while the melting pressure was relatively high, with garnet, hornblende and rutile as residual minerals. The other type of granite porphyry has only minor biotite. It invaded into khondalitic rocks of the pan-African Mashan Complex. Higher aluminum saturation index but consistent log (Na2O/MgO) values compared with the biotite granite porphyry suggests that it was derived from partial melting of upper continental crust triggered by adakitic magma upwelling and emplacement.(4) The Hegang Songmuhe Formation basalt has relatively high SiO2contents (52.1-53.2%), with4.25-4.36%Na2O and1.32-1.35%K2O, putting it into the alkaline basaltic andesite field in the TAS diagram. The high Al2O3(18.0-19.0%) and low MgO (3.0-4.0%) contents and Mg#of0.42-0.45are features indicative of magma that experienced high degrees of crystal fractionation. No Eu anomalies (Eu/Eu*=0.98-1.00), high εNd(t) values (2.9-3.0), and flat LILE patterns suggest that crustal contamination was minor. High sNd(t) and relatively low (87Sr/86Sr)i suggest that the mantle source was depleted. The (87Sr/86Sr)i ratios (0.70565-0.70571) are higher than expected for normal asthenosphere-derived basalt, indicating that the mantle source was affected by subducted oceanic crust basalt which was altered by sea water. In tectonic discrimination diagrams, the Songmuhe Formation basalt plots in the active continental margin and intraplate field. It represents the beginning of the mafic magmatism in the Jiamusi Block, and extention of the lithosphere in the mid-Cretaceous.(5) The Huanan composite dyke is located in the centre of the Jiamusi Block. It consists of two3m wide andesite porphyry margins and one5m wide rhyolite porphyry interior. The country rock is granite porphyry with andesite enclaves. All enclaves have the same composition, and most of them are rounded; although some are sub-angular with fine-grained rim, caused by rapid cooling due to high temperature difference between the basic and acidic magma. Zircons from the andesite and rhyolite porphyry have abundant acicular apatite inclusions indicating fast cooling considering the crystalization temperature drop from apatite to zircon. There are also some fluid inclusions. Both granite porphyry and rhyolite porphyry have high SiO2and Al2O3, low MgO and Fe2O3, with enrichment of LILE and LREE, and depletion of HFSE, Eu, Ba, U, and Sr; their sources are most possibly upper continental crust. The andesite porphyry was contaminated to various degrees by both the rhyolite porphyry and granite porphyry:the centre of the andesite porphyry has higher CaO, MgO, TiO2, MnO, Al2O3, Fe2O3, Sr, V, Sc, Eu/Eu*, Nb/Ta, εNd(t) values but lower SiO2, Ba, Rb, La, Th, and U content than the border of the andesite porphyry. The Huanan composite dyke and its country rock represent interaction between basaltic magma and upper continental crust; it also represents evidence of extension of the Jiamusi Block in the mid-Cretaceous.(6) The Jiamusi bimodal dykes section is located in the west of the Jiamusi block. It consists of rhyolite and dolerite dykes. The rhyolite is characterized by enrichment in large-ion lithophile elements (LILE) and light rare earth elements (LREE), and depletion in high-field strength elements (HFSE). It shows a significant negative Eu anomaly, and has εNd(t) values ranging from0.49to1.66and two groups of initial87Sr/86Sr ratios at0.7045and0.7061. The rhyolite displays the compositional signature of Peraluminous Ferroan Granitoid, indicating it was derived by either differentiation of basalt and/or low pressure partial melting of crust. The dolerite is also characterized by enrichment in LILE and LREE, and depletion in HFSE. It has a weak negative Eu anomaly and has εNd(t)=-1.22to+3.26, and (87Sr/86Sr)i=0.7057-0.7074. The dolerite originated from partial melting and mixing of both asthenosphere and lithospheric mantle which was affected by residual oceanic slab or sediment, and experienced different amounts of lithospheric and crustal assimilation and contamination. The Jiamusi bimodal dolerite and ferroan (A-type) rhyolite dykes represent the lithospheric thinning, mantle upwelling and tectonic extention.(7) The igneous rocks of the Jiamusi block have a variety of types, including the high-Mg adakite, ferroan rhyolite, basalt and rocks related with mixing between basaltic magma and crustal acid magma or bimodal magmatism. These various types of magmatism show the processes of dehydration and partial melting of subducted slab; assimilation and contamination of mantle peridotite by slab fluid and melt; partial melting of both asthenosphere and lithosphere mantle triggered by fluid; basalt upwelling and emplaced into the crust forming partial melting of upper continental crust; mixing and mingling of basaltic magma and crustal magma; basalt upwelling and underplating at the bottom of thinned crust forming ferroan (A-type) rhyolite. Magma formed in these processes ascend to the surface or sub-surface, suggesting an extension and thinning of the lithosphere, which is most possibly related with oceanic plate subduction and roll back.(8) The late Mesozoic igneous rocks in NE China have eastward temporal migration, and can be divided into three separate areas, which are from west to east:(1) The Great Xing’an Range～160-120Ma,(2) The Songliao Block～120-110Ma,(3) The Jiamusi area～110-90Ma. This temporal migration can be interpreted by the subduction-accumulation-rollback model of the paleo pacific plate. This model indicate that the episodic evolution of intraplate structural and magmatism evolution can be interpreted by subduction accumulation-rollback of subducted slab, with or without the change of rate and direction of subduction. This model is possibly also helpful to interpret the late Mesozoic tectonic evolution of other areas of eastern China. |
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