The Early Paleozoic Magmatic Events In The Tectionc Conjunction Of The Qinlin-Qilian Orogen And Its Geological Significance

Recent progresses on the early Paleozoic ophiolites,subdution-related igneous rocks and(ultra)high pressure metamorphic rocks gave a better understanding on the geological evolution of the North Qilian orogenic belt and adjacent North Qinling orogenic belt.The junction of the Qinling-Qilian orogenic belts between Baoji and Tianshui,is a key area for the study of continental dynamics.The Qinling-Qilian orogenic belt is divided into the eastern part of the North Qilian and the northern part of the West Qinling by the Xinyang-Yuanlong fault.The northern part of the West Qinling has an early Paleozoic ophiolite complex,which is absent in the eastern part of North Qilian where,however,a complete trench-arc-basin system is developed.Accordingly,it is obvious that there are a great of differences in material composition among the two.Therefore,the investigation for them is of great geological significance,which in turn will provide insights into the study in the early Paleozoic magmatism in the Qinling-Qilian junction,the exploration of the early Paleozoic structural framework of the Qinling-Qilian junction,and the comparation on tectonic evolution history of the eastern North Qilian and the northern West Qinling.In this thesis,detailed petrographic and geochemical studies on the slightly metamorphosed Hongtubao metavolcanic rocks near the eastern part of the North Qilian orogenic belt,are carried out to constrain the petrogenesis and tectonic background of Hongtubao metavolcanic rocks.Combined with previous studies on Early Paleozoic magmatism at the eastern part of the North Qilian orogenic belt,this contribution has a further discussion on the early Paleozoic tectonic evolution of the North Qilian orogenic belt.The main rock associations of Muqitan suite in Tianshui area of West Qinling mountains include amphibolite and amphibolite-facies gneiss.The major and trace element and zircon U-Pb isotopes are analyzed for the amphibolite in Muqitan suite to determine the formation age,petrogenesis,characters of magmatic source area and tectonic setting.Combining previous studies on the early Paleozoic magmatism in the northern margin of the West Qinling with results in this paper,some knowledge has been obtained as follows:(1)The Hongtubao metavolcanic rocks is a set of slightly metamorphosed rock series of which the metamorphic grade can reach greenschist facies,and its original rock is mainly basic volcanic rock(basalt),with a small amount of shallow diabase intrusion.Such metabasite belongs to the tholeiite series.The content of rare earth elements is relatively high with a weak right-leaning type distribution curve of rare earth elements,light rare earths are slightly more enriched than heavy rare earths,and the large ion lithophile elements,Rb,Ba,K,and Sr,are changed obviously.Additionally,this metabasite is rich in Th and LREE elements and depleted in Nb,Ta,Zr,Hf and Ti elements.The ratios of Ti/Zr and Th/Ce are different from continental crust,indicating that it is not contaminated by crustal materials.The samples show a positive correlation between Ni,Cr and Mg O,as well as a positive correlation between Al₂O₃and Mg O.These relationships indicate that the fractionation of olivine and clinopyroxene plays an important role in the Hongtubao metavolcanic rocks.The unconspicuous relations between Ca O/Al₂O₃and Mg O and weak Eu anomaly[?Eu=0.92-1.01]indicate that plagioclase does not undergo crystal differentiation.The back-arc basin basalt is different from the simple decompression melting of the mid-ocean ridge basalt(MORB),and it is also different from the island arc basalt(IAB),which has a large amount of subducting fluid.However,the back-arc basin basalt is formed by different degrees of melting,and it often has both mid-ocean ridge basalt(MORB)and island arc basalt(IAB)geochemical characteristics.The characteristics of the HREE of the Hongtubao metavolcanic rocks are similar to the Enriched mid-ocean ridge basalt(E-MORB).And the light rare earths(LREE)and trace elements Th and U are significantly enriched,and they have strong negative anomalies of Nb and Ta,comparable with island arc basalt(IAB).The Th/Nb ratio of the metabasite is positively correlated with the Th/Yb ratio,which is similar to the characteristics of the back-arc basalt.Meanwhile,the weak Eu anomalies(?Eu=0.92?1.01)indicates slight crystallsation of plagioclase.Therefore,it is speculated that the Hongtubao metavolcanic rocks studied in this thesis was formed in the environment of the back-arc basin and related to the back-arc extension.(2)In this thesis,the amphibolite of Muqitan suite is mainly exposed in Xiajiaping-Dongjiaping area.The ratios of trace elements La/Nb and Nb/U indicate that the source area of the Muqitan amphibolite is contaminated by the continental crust.The Muqitan amphibolite has a lower Th/Yb ratio(0.19?1.93)and high Sr/Nd ratio(6.68?171.05),indicating that the magma source area has experienced the metasomatism of subducted slab fluid.Zircon U–Pb dating by LA–ICP–MS yielded 488.9±4.4 Ma magmatic age,which is interpreted to represent the formation age of amphibolite.The rock geochemical characteristics of amphibolite show that the large ions lithophile elements such as Rb,Ba,Th,U,and K are enriched but the high field strength elements such as Nb,Ta,Zr,Hf,and Ti are depleted,which is similar to the geochemical characteristics of the depleted island arc-type volcanic rocks,demonstrating that this amphibolite was formed in the tectonic setting of the active continental margin island arc or the island arc in the ocean.Accordingly,the Muqitan amphibolite is the product of crystallization differentiation of island arc type magma which is attributed to the oceanic crust subduction along the Guanzizhen-Liziyuan subduction collision zone in Tianshui area on the northern margin of West Qinling in the Early Paleozoic.