The Chaidamushan granite rock mass，in the Dachaidan Town，Qinghai Province，is mainly composed of porphyritic monzonitic granite，granite porphyry and rapakivi granite. Representative rocks were selected for main and trace element study and LA- ICP- MS zircon U- Pb geochronology to explain the magma genesis，tectonic setting and their significance. The results show that in the Chaidamushan granite the K2O/Na2O ratio changed from 1. 11 to 4. 41，the Litman index ranged between 1. 31 to 2. 20，A/CNK between 1. 06 and 1. 66，indicating a high potassium calc- alkaline, peraluminous and highly fractionated I- type granite. In the primitive mantle- normalized trace element distribution pattern，they show different degrees of enrichment of large ion lithophile elements (LILE) Cs，Pb，K，etc. and light rare earth elements (LREE)，relative loss of high field strength elements (HFSE) Y, Yb，has obvious negative anomalies of Ba, Nb, Ta, Sr, P and Ti. The total amount of rare earth elements is 207. 94×10-6~418. 40×10-6，the ratio of LREE/HREE is 5. 67~10. 29，the ratio of (La/Yb)N is 5. 88~13. 84，δEu is 0. 09~0. 50，δCe is 0. 99~1. 36，Sr/Y is 0. 87~3. 47，Rb/Sr is 1. 22~15. 45，K/Rb is 0. 01~0. 02，and Nb/Ta is 0. 63~11. 88. Rare earth element chondrite- normalized distribution pattern is characterized by the relative enrichment of light rare earths，the right- dip type of heavy rare earth relative loss，and negative Eu anomaly. Four samples were selected for LA- ICP- MS zircon U- Pb dating; the results showed that the granite porphyry in D3755- 1 206Pb/238U age ranged from 400 Ma to 407 Ma，and the weighted average age was 404. 6±2. 9 Ma (MSWD=0. 094)；in PM11- 1 206Pb/ 238U age ranged from 413 Ma to 424 Ma，with a weighted mean age of 418. 4±3. 0 Ma (MSWD=0. 30)；in monzonitic granite D1028- 1 206Pb/238U age ranged from 426 Ma to 436 Ma，with a weighted mean age of 434. 3±2. 0 Ma (MSWD=0. 21)；in granodiorite D1506- 1 206Pb/238U age ranged from 434 Ma to 439 Ma，with a weighted mean age of 437. 2±2. 6 Ma (MSWD=0. 076)，suggesting that the crystallization age of the Chaidamushan granite is between 437. 2 Ma to 404. 6 Ma i. e. from the Early Silurian to the Early Devonian. The magmatism in the southern margin of the Qilianshan- North Chaidam lasted for a long time (372~473 Ma)，with multiple stages dated to 450~470 Ma，430~450 Ma，410~430 Ma，400~410 Ma，370~400 Ma，amongst which 430~450 Ma and 400~410 Ma represent the two main peak periods of granite intrusion closely related to the high- pressure or ultra- high pressure metamorphism. The early period (430~470 Ma) is a response to the northward subduction of the south- Qilian ocean plate beneath the Qilian block; as the Qaidam block continued to subduct，the Qilian block from the north to the south thrusted over the Qaidam block to form a series of continuous magmatic intrusion activities in continent- continent collision zones. The late period (370~430 Ma) is a response to the deep subduction plate after the Qaidam block and the Qilian plate collided, were dismantled, and re- entered a series of magmatic intrusion activities in the stretching and sliding between different blocks. Spatial distribution of the Saishitengshan and the Aolaoshan in the northern margin of Chaidam represents the early magmatic activity，mainly I- type granite. The Luliangshan- Dachaidan- Xitieshan- Dulan area represents the late magmatic activity and has the characteristics of I- and S- type granites. The Chaidamushan rock mass represents the multi- stage magmatic activity in the southern margin of the Qilian，and is simultaneous with the magmatic activity of the northern margin of the Qaidam Basin.