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The Qilian Caledonian orogenic belt on the north margin of the Qinghai-Tibet Plateau were formed by convergence and collision of the Alxa terrain, Qilian terrain and Qaidam terrain. The 350-km-long, WNW–ESE-trending North Qaidam ultrahigh-pressure (UHP) metamorphic belt, lying between the Qilian and Qaidam terrains, was formed between 495 and 440 Ma by deep subduction of the South Qilian Sea and the Qaidam continental crust beneath the Qilian terrain. The UHP belt was exhumed by a process of ‘oblique extrusion’ during transformation from ‘normal’ to ‘oblique’ intracontinental subduction between the Qilian and Qaidam terrains. Exhumation began at 470–460 Ma and was completed by 406–400 Ma. Exhumation structures are well-preserved in the UHP rocks and record extensive retrograde metamorphism.

A suite of Early Paleozoic island arc volcanic rocks, consisting mainly of basaltic lavas and some intermediate-silicic rocks, crops out in the North Qaidam basin, northeastern Tibet Plateau. These rocks have been extensively metamorphosed under greenschist facies conditions, and hence differ from the older Proterozoic intermediate-mafic volcanic rocks, which have generally experienced amphibolite facies metamorphism. The Early Paleozoic volcanic rocks are divided into three groups based on their geochemical characteristics: (1) VTG-I, consists of island arc tholeiitic basalts, basaltic andesites and andesites that crop out in Jilusu and Shuangkoushan; (2) VTG-II, which are high-Al, alkaline and sub-alkaline basalts in Jilusu; and (3) VTG-III, an assemblage of highly depleted, MORB-type basalts, andesites and dacites that occurs mostly in Shuangkoushan. Groups VTG-I and -II are considered to be the products of mature island arcs produced in two stages. In the first stage, following initiation of ocean–continent subduction, the island arc tholeiites were generated by partial melting of the subsiding oceanic crust and mantle wedge beneath the island arc. As subduction progressed and accelerated, calc-alkaline and high-Al basalts were formed and the crust beneath the island arc was thickened. The highly depleted MORB-type lavas (VTG-III) have boninitic affinities and are believed to have been generated from depleted mantle material modified by subduction zone fluids, possibly in the fore-arc region. The island arc tholeiites are dated at 514.2±8.5 Ma whereas UHP eclogite in the North Qaidam Belt has been dated at 494 Ma. These ages suggest that ocean–continent subduction took place in the North Qaidam Basin in the Early Paleozoic and was followed shortly by continent–continent collision and subduction, which produced the UHP rocks.

Using both TIMS and SHRIMP methods, U–Pb isotopic dating was carried out on zircons from eclogites and their host gneisses in the Xitieshan area of the north Qaidam mountains, western China. Zircons from both eclogites and retrograded eclogites with metamorphic characteristics gave ages of 485–500 Ma, representing the age of eclogite facies metamorphism. Euhedral zircons from the eclogites, which show a magmatic character (Th/U>1), yielded ages of 750–800 Ma, which are believed to represent the magmatic crystallization age of the protlith. TIMS dating of zircons from granitic gneiss enclosing retrograded eclogite gave an upper intercept age of 952±13 Ma and a lower intercept age of 478±44 Ma. The former probably represents the magmatic crystallization age of the granitic protolith, and the latter is in agreement with the age of metamorphic zircons from the eclogite within the margin of error. This implies that the eclogites and their enclosing gneisses in Xitieshan area as a whole underwent HP–UHP metamorphism during an early Paleozoic orogeny to form one part of the north Qaidam HP–UHP metamorphic belt. These results also suggest an ‘in situ’ rather than ‘tectonic'relationship between the eclogites and their host gneisses.