Geochemistry And Geochronology Of Mafic Dykes In The Yelanggou Area, West Section Of East Kunlun And Its Tectonic Implication

The East Kunlun Orogenic Belt (EKOB), a compound orogenic belt with long and complex tectonic history, has been one of the hot topic of the domestic and foreign researchers for a long time. Previous studies have suggested that tectonic-magmatic event during the late Mesoproterozoic (?1000 Ma) in the EKOB was related with the formation of the Rodinia super continent, and rift events during the Neoproterozoic, comparable to the break up of the Rodinia was documented in the EKOB. However, there is still lack of research on the duration of this rift event in the EKOB. Aiming to provide a basis for the duration this late Neoproterozoic rift event in the EKOB and the early tectonic history of EKOB, this thesis targets the newly discovered mafic dykes in the Yelanggou area, in the west section of the EKOB, and researchs on the field geology, geochemistry, petrogenesis and the tectonic setting of the mafic dykes.The Yelanggou mafic dykes are located near the Central East Kunlun Fault in the west part of the EKOB, and mainly consist of sillite, diabase and gabbro-diabase. Zircon U-Pb geochronology indicates that the mafic dykes have a formation age of 602±2.9 Ma (MSWD=0.48).The Yelanggou mafic dykes have a SiO2 content ranging from 50.93% to 54.65%, and are characterised by high Al2O3 (14.08%?16.49%), low TiO2 (0.95%?1.13%) and variable MgO and Mg# values (4.43%?10.43% and 46.79?65.94, respectively). All the mafic dyke samples display similar rare earth elements, e.g., enrichment of light rare earath elements and lack of fractionation of heavy rare earth elements as well as Eu anomalies (with Eu/Eu*= 0.97?1.08). The samples show enrichment of large ion lithophile elements and depletion of high field strength element, indicating a affinity of subduction components. However, the mafic dykes have a higher Zr/Y and Ta/Hf ratios (4.16?4.74 and 0.11?0.12, respectively) than the magmatic rocks in subduction zone, and similar to those of continental rift magmatic rocks.The comprehensive analysis indicated that the Yelanggou mafic dykes were formed in a continent rift setting, deriving from a lithospheric mantle enriched by the earlier subducted oceanic/continental crust. And the mafic magma experienced fractional crystallization of olivine, spinel, clinopyroxene, and have barely contaminated by the crustal material during its formation and emplacement process. The occurrence of the mafic dykes represents a rifting event in the EKOB during the late stage of Neoproterozoic and indicates that the rifting continued to ?600 Ma. In addition, the time of this rifting event in EKOB is comparable with those of west section of North Qilian and the northern margin of Qaidam, indicative of the affinity of these blocks and the long duration of the fragmentation of the Rodinia.