Chronology, Geochemical Characteristics of the Intermediate Acid Intrusives in Dangjinshankou, Eastern Altun and Their Tectonic Significance

The Dangjinshankou intrusive body in eastern Altun is mainly composed of granite, granodiorite and diorite. LA-ICP-MS zircon U-Pb isotopic dating shows that the weighted average age of ²⁰⁶Pb/²³⁸U of Dangjinshankou granite is (518±13) Ma, indicating that the emplacement age is early Cambrian. Geochemistry shows that the rock mass has a high content of sio₂, and the average content of total alkali Na₂O + K₂O is 6.46%. The rock is rich in both sodium and potassium, but mainly rich in sodium, with low content of Fe₂O₃, P₂O5 and MgO. The total amount of rare earth elements ∑REE is 47.28×10^-6-368.74×10^-6, with an average value of 225.37×10^-6. As a whole, the heavy and light rare earth fractionation is obvious, δEu is 0.45-1.08, with an average value of 0.83. Negative europium is slightly anomaly， high field strength elements are enriched, and large ion lithophile elements are depleted. In HFSE elements Th, Ce, Zr, Hf are enriched while Nb, Ta are slightly depleted. In LILE elements Ba, Sr, Eu appear depleted. Rb shows strong enrichment. The granite belongs to quasi aluminous peraluminous rocks of calc-alkaline high-K calc-alkaline series. The analysis of petrogenesis and tectonic environment indicates that the intrusions in this area may form under the environment of subduction volcanic arc and later syn-collision, with the partially melted basic rocks of amphibolite facies and the participation of mantle materials. The regional geological data shows that Dangjinshankou intrusive body has a lot in common with the granitic magma activities and tectonic setting in the ophiolite belt and the Qilian orogenic belt.