Late Mesozoic magmatism in the East Qinling Orogen,China and its tectonic implications

The Qinling Orogen in Central China records the history of a complex geological evolution and tectonic transition from compression to extension during the Late Mesozoic, with concomitant voluminous granitoids formation. In this study, we present results from petrological, geochemical, zircon U-Pb-Lu-Hf isotopic studies on the Lengshui felsic dykes from Luanchuan region in the East Qinling Orogen. We also compile published geochronological, geochemical, and Hf isotopic data from Luanchuan region and present zircon Hf isotopic contour maps. The newly obtained age data yield two group of ages at ～145 Ma and 140 Ma for two granite porphyries from the Lengshui felsic dykes, with the ～145 Ma interpreted as response to the peak of magmatism in the region, and the ～140 Ma as the timing of formation of the felsic dykes. The corresponding Hf isotopic data of the granite porphyries display negative ε_Hf(t) values of –16.67 to –4.61, and Hf crustal model ages (T_DM^C) of 2255–1490 Ma, indicating magma sourced from the melting of Paleo- to Mesoproterozoic crustal materials. The compiled age data display two major magmatic pulses at 160–130 Ma and 111–108 Ma with magmatic quiescence in between, and the zircon Hf isotopic data display ε_Hf(t) values ranging from –41.9 to 2.1 and T_DM^C values of 3387–1033 Ma, suggesting mixed crustal and mantle-derived components in the magma source, and correspond to multiple tectonic events during the Late Mesozoic. The Luanchuan granitoids are identified as I-type granites and most of these are highly fractionated granites, involving magma mixing and mingling and crystal fractionation. The tectonic setting in the region transformed from the Late Jurassic syn-collision setting to Early Cretaceous within-plate setting, with E–W extension in the Early Cretaceous. This extension is correlated with the N–S trending post-collisional extension between the North China Craton and Yangtze Craton as well as the E–W trending back-arc extension triggered by the westward Paleo-Pacific Plate subduction, eventually leading to lithospheric thinning, asthenospheric upwelling, mafic magma underplating, and crustal melting in the East Qinling Orogen.