Spatial–Temporal Distribution, Petrogenetic Evolution and Tectono–Magmatic Evolution in Qilian Mountains and Northern Margin of Qaidam

Types of intrusions with different formation ages are widely exposed in Qilian Mountains and its adjacent areas, which record multi–stage tectono–magmatic events in Qilian Mountains, Northern Margin of Qaidam and Zongwulong tectonic belt. According to the spatial distribution, chronological framework and petrogenetic types, the intrusive magmatism in Qilian–Northern Margin of Qaidam region can be divided into three stages: Precambrian (2470～561 Ma), Early Paleozoic to Devonian (517～360 Ma), and Middle–Late Permian to Triassic (271～210 Ma). ① The Precambrian granites (2470 Ma～561 Ma) recorded the process of convergence and dispersion of ancient continents and the opening of the North Qilian Ocean. ② Since the opening of the Sinian ocean basin, a large number of intrusions have been formed between 517 and 359 Ma in Qilian Mountains and the Northern Margin of Qaidam. Since the Middle Devonian, the Qilian Mountains, the Northern Margin of Qaidam and the Zongwulong region have began to shown relatively independent tectonic–magmatic evolution process. Since 517 Ma, granitoids with geochemical characteristics of slab subduction, adakitic rocks and alkaline granites have been formed in Qilian Mountains, together with the volcanic rocks formed in the same period can constitute a volcanic–intrusive rock association of the arc–basin system. The strongly peraluminous S–type collision–related granites widely formed in the Ordovician (452～444 Ma) and Early Silurian (431 Ma) in Central and Southern Qilian, and the molasse sedimentary formation occurred in the Late Ordovician, indicate that the Qilian collision orogenic event occurred during the Late Ordovician and Early Devonian (452～431 Ma). In addition, the Northern Margin of Qaidam may have entered the collision–post–collision orogenic stage at ca. 463 Ma, slightly earlier than that of Qilian Mountains. In 440～425 Ma stage, a large number of high Mg neutral intrusive rocks, adakitic rocks with positive εNd(t) and εHf(t) values and the A-type granites formed during 430 Ma and 420 Ma, represent the post-collision orogenic extension of Central and Southern Qilian. To the Devonian, Qilian Mountains and the Northern Margin of the Qaidam Basin showed some differences for tectonic and magmatic evolution and have relatively independent evolution processes. The intrusive magmatism began to weaken sharply in Qilian Mountains. The occurrence of a small amount of A–type granite formed at 409～402 Ma in North Qilian, diabase dikes at 385.9Ma in Central and Southern Qilian and molasses in Laojunshan Formation is a geological record of Qilian Mountains from post–collision to intra–continental extension in the Middle Devonian. In Northern Margin of Qaidam, high Sr and low Y–type granites were developed during 402～367 Ma, and a small amount of A–type granite (413～391 Ma) was formed by slab broken–off almost at the same time. In addition, the continental conglomerate and rift–related volcanic rocks in Late Devonian Maoniushan Formation, and the ultramafic–mafic rocks intermittently exposed at southern Zongwulong–Longwu Gorge, Xinghai–Saishitang–Kuhai during the Early–Middle Devonian and the Late Permian may be related to the intracontinental extension and the evolution of Zongwulong Ocean. ③ From the Middle and Late Permian to Triassic (271～210 Ma), the granitic magma in Qilian Mountains is still weak. However, the Zongwulong belt formed a distinct and unique magmatic activity period in the Mesozoic. The Late Permian–Early Triassic granite was formed in the southward subduction of the Zongwulong Ocean. The geochemical futures of granites formed at ca. 240～220 Ma is similat to that of the collision or post–collision granite in Westeren Qinling. By 215 Ma, the discovery of A–type granite in Zongwulong belt marked them and its adjacent areas had all entered the intra–continental stage, and the main orogenic stage of the whole Qinling–Qilian region had ended. After the Triassic (＜201 Ma), the magmatic activity in Qilian mountains and the northern margin of Qaidam was weakly.