Abstract
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.
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