Discovery and Significance of Cretaceous Lamprophyre Dike Group in Quanji Block of Northern Tibetan Plateau

Cretaceous formation records are rare in the northern part of the Tibetan plateau, except for terrestrial sedimentary basins, with only few magmatic activities reported, let alone mantle-derived magmatic activities. The lamprophyres exposed in the Shaliuquan area, the eastern part of the Quanji block, were firstly discovered by field work, which were subjected to a large scale mapping, major and trace element analyses, precise zircon SHRIMP and LA-ICP-MS U-Pb dating and Sr-Nd-Pb isotopic measurements, aiming to deepen the understanding of the Late Mesozoic geological process in the northern plateau. The results yield the average zircon U-Pb ages of 135.2±1.8 Ma and 132.9±1.3 Ma. The rocks are rich in potassium (K₂O=4.53%-5.25%), magnesium (MgO=7.23%-12.27%) and poor in titanium (0.85%-1.29%), showing potassium calc-alkaline. They are also enriched in large ion LILE elements (Rb, Ba, Th, U and Pb) and deficient in high field elements (Nb, Ta and Ti). (⁸⁷Sr/⁸⁶Sr)_i value is 0.718 0-0.718 6. ε_Nd(t) value is -14.2 to -14.4, ²⁰⁸Pb/²⁰⁴Pb is 38.414 to 39.334, ²⁰⁷Pb/²⁰⁴Pb is 15.632-15.681, and ²⁰⁶Pb/²⁰⁴Pb is 18.568-19.203. These data indicate the magma originated from the oceanic subduction-related EMⅡ mantle source and formed from local partial melting of the lithospheric mantle along the deep fracture. The high contents of Au (the mean of 6.8×10^-9) and F (the mean of 2 450×10^-6) of the lamprophyres are very favorable for gold mineralization in the eastern part of the Quanji block.