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

摘要

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 w145 Ma and 140 Ma for two granite porphyries from the Lengshui felsic dykes,with the w145 Ma interpreted as response to the peak of magmatism in the region,and the w140 Ma as the timing of formation of the felsic dykes.The corresponding Hf isotopic data of the granite porphyries display negative εHf(t) values of e16.67 to e4.61,and Hf crustal model ages (TDM^C) of 2255e1490 Ma,indicating magma sourced from the melting of Paleo- to Mesoproterozoic crustal materials.The compiled age data display two major magmatic pulses at 160e130 Ma and 111e108 Ma with magmatic quiescence in between,and the zircon Hf isotopic data display εHf(t) values ranging from e41.9 to 2.1 and TDM^C values of 3387e1033 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 EeW extension in the Early Cretaceous.This extension is correlated with the NeS trending post-collisional extension between the North China Craton and Yangtze Craton as well as the EeW 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.