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Numerous granitic intrusions crop out in the eastern segment of the North Qaidam block (NQ), NW China. To evaluate their ages, petrogenesis and genetic relationships to other granitoids in the NQ, we present geochemical and geochronologic data for six intrusive bodies and review regional data. Zircon U-Pb (SHRIMP) dating yielded ages of 413 ± 3 Ma for the Hadesengou granite; 254 ± 3 Ma for the Xugeigou granite; 251 ± 1 Ma for the Qiluoshan granite; 249 ± 1 and 248 ± 2 Ma for the Chahannuo hornblende diorite and granite, respectively; 240 ± 2 Ma for the Chahanhe granite; and 250 ± 1 and 244 ± 3 Ma for the Shailekegoulei granodiorite and granite, respectively. Consequently, the Wulan plutons can be divided into two petrologic groups: Early Devonian (D1) quartz monzonite and syenogranite, and Late Permian to Early Triassic (P3-T1) hornblende diorite, granodiorite, and granite. The D1 granitic intrusions have geochemical affinities with A-type granites (A2-type) characterized by low Ca, Sr, Ba and Nb, and high Fe, Ga, Y and Rb, consistent with derivation by partial melting of metapelitic source rocks containing a small amount of metagraywacke. The P3-T1 I-type granitic intrusions are geochemically typical of active continental margin rocks, consistent with derivation by partial melting of metabasalt and clay-poor metagraywacke. Combined with previous studies, we recognize five periods of granitic magmatism in the NQ: (1) 465–473 Ma; (2) 423–446 Ma; (3) 391–413 Ma; (4) 372–383 Ma; and (5) 240–271 Ma. Based on the temporal-spatial distribution of granitic intrusions in the NQ and the regional tectonic evolution, we interpret the first and second periods of granitic magmatism as related to normal plate subduction, and the third period to slab break-off and exhumation of the subducted plate. The fourth stage of granitic magmatism is attributed to large-scale lithospheric mantle delamination, involving the differential movement of orogenic blocks. The fifth period of granitic plutonism probably reflects northward subduction of the East Kunlun Paleotethys oceanic crust and southward subduction of Zongwulong oceanic crust beneath the Oulongbuluke continental block.

The petrology, geochemistry, geochronology, and Sr–Nd–Hf isotopes of the backarc granitoids from the central part of the Qilian block are studied in the present work. Both S- and I-type granitoids are present. In petrographic classification, they are granite, alkali feldspar granite, felsic granite, diorite, quartz diorite, granodiorite, and albite syenite. The SHRIMP ages are 402–447 Ma for the S-type and 419–451 Ma for the I-type granitoids. They are mostly high-K calc-alkaline granitoids. The S-type granitoids are weakly to strongly peraluminous and are characterized by negative Eu anomalies (Eu/Eu* = 0.18–0.79). The I-type granitoids are metaluminous to weakly peraluminous and are characterized mostly by small negative to small positive Eu anomalies (Eu/Eu* = 0.71–1.16). The initial (87Sr/86Sr) values are 0.708848–0.713651 for the S-type and 0.704230–0.718108 for the I-type granitoids. The eNd(450 Ma) values are − 8.9–−4.1 and − 9.7–+ 1.9 for the S-type and I-type granitoids, respectively. The TDM values are 1.5–2.4 Ga for the S-type and 1.0–2.3 Ga for the I-type granitoids. For the Qilian block, the backarc granitoid magmatism took place approximately 60 million years after the onset of the southward subduction of the north Qilian oceanic lithosphere and lasted approximately 50 million years. Partial melting of the source rocks consisting of the Neoproterozoic metasedimentary rocks of the Huangyuan Group and the intruding lower Paleozoic basaltic rocks could produce the S-type granitoid magmas. Partial melting of basaltic rocks mixed with lower continental crustal materials could produce the I-type granitoid magmas. Major crustal growth occurred in the late Archean and Meso-Paleoproterozoic time for the Qilian block. The magma generation was primarily remelting of the crustal rocks with only little addition of the mantle materials after 1.0 Ga for the Qilian block.

Adakitic intrusive rocks of ∼ 430–450 Ma were discovered in the North Qilian orogenic belt, the western section of the Central Orogenic System (COS) in China. These adakitic rocks were lower crust melts rather than slab melts as indicated by their crustal Ce/Pb, Nb/U, Ti/Eu, and Nd/Sm ratios and radiogenically enriched (87Sr/86Sr)i of 0.7053–0.7066 and eNd(t) of − 0.9 to − 1.7. While they are all characterized by low Yb ( 65) and (La/Yb)N (> 13.7) ratios, these adakitic rocks are classified into the low-MgO–Ni–Cr and high-MgO–Ni–Cr groups. The low-MgO samples were derived from partial melting of thickened lower crust, whereas the high-MgO samples were melts from delaminated lower crust, which subsequently interacted with mantle peridotite upon ascent. Adakitic rocks from the adjacent North Qinling orogenic belt also originated from thickened lower crust at ∼ 430 Ma. In addition, the North Qilian and North Qinling orogenic belts both consist of lithological assemblages varying from subduction-accretionary complexes at south to central arc assemblages, which include adakitic rocks, then to backarc phases at north. Such a sequence reflects northward subduction of the Qilian and Qinling oceans. In these two orogenic belts, the occurrence of adakitic rocks of common origin and ages together with the similarities in tectonic configurations and lithological assemblages are considered to be the evidence for the continuity between eastern Qilian and western Qinling, forming a > 1000 km Early Paleozoic orogenic belt. In such a tectonic configuration, the Qilian and Qinling oceans that subducted from south possibly represent parts of the large “Proto-Tethyan Ocean”. This inference is supported by the coexistence of Early Paleozoic coral and trilobite specimens from Asia, America and Australia in the North Qilian orogenic belt. Post-400 Ma volcanic rocks occur in the North Qinling orogenic belt but are absent in the North Qilian orogenic belt, indicating that these two orogenic belts underwent distinct evolution history after the closure of the Proto-Tethyan Ocean (∼ 420 Ma).