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Integrated zircon U-Pb dating and whole rock geochemical analyses have been carried out for two typical S-and I-type granitoids in the northQinling. Zircon dating by SIMS of the Piaochi S-type granitoids yields an emplacement age of 495±6 Ma. The granitoids show whole-rock ε Nd (t)=−8.2-−8.8, zircon ε Hf (t)=−6-−39. The Huichizi I-type granitoids have emplacement ages of 421±27 Ma and 434±7 Ma established by LA-ICP-MS and SIMS methods, respectively. Their whole-rock ε Nd (t)=−0.9-0.9 and zircon ε Hf (t)＝−11-8.4. Combined with statistical analyses of 28 zircon ages of granitoid plutons collected from the literature, Paleozoic magmatism in the north Qinling can be divided into three stages. The first-stage magmatism (~505-470 Ma) mainly occurred in the east part of the north Qinling and has features of an I-type arc, associated with which are S-type granitoids such as Piaochi pluton. The early granitoids (~505-490 Ma) have close spatio-temporal relations to ultra-high-pressure (UHP) rocks, and thus are interpreted as an oceanic subduction system along a continental margin. The second-stage magmatism (~450-422 Ma) occured through the whole north Qinling, and is characterized by I-type granitoids represented by the Huichizi pluton. The magma is interpreted as partial melting of lower crust mixed by mantle-derived magma in a collisional setting with the uplift of terranes. The third-stage magmatism (~415-400 Ma) is dominated by I-type granitoids and only took place in the middle part of the north Qinling, and is regarded as a late-stage collision. The spatial and temporal variations of the Qinling Paleozoic magmatism reveal protracted subduction/collision. The subduction was initiated from the east part of the north Qinling, earlier than that in the Qilian-northern Qaidam, Kunlun, and northern Dabie regions. This demonstrates variations in time of subduction, accretion and collision of separate blocks or terranes in the orogenic systems in central China.

Zircon U-Pb dating of early Paleozoic granitoids in North Qinling yields three age peaks of ~500, ~452 and ~420 Ma. They can be temporally correlated with high-pressure to ultrahigh-pressure metamorphism at ca. 500 Ma, retrograde granulite-facies metamorphisms at ca. 450 Ma and amphibolite-facies metamorphism at ca. 420 Ma, respectively. The first episode of granitic magmatism is considered to have resulted from continental collision, whereas the second and third episodes of magmatism are attributed to crustal uplifting. Combined with the regional geological setting and new results from high-pressure and ultrahigh-pressure metamorphic rocks, the ca. 500 Ma magmatism is interpreted as the result of partial melting of sedimentary rocks in accretionary wedge between the south Qinling microcontinent and the north Qinling belt including the southern margin of the North China Craton. The ca. 450 Ma intensive magmatism is ascribed to dehydration melting of deeply subducted continental crust at thickened conditions in response to slab breakoff, and the final magmatism in ca. 420 Ma is interpreted as the product of partial melting during the tectonic transition from contraction to extension.

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