Abstract:

 The statistical study of eight postcollision volcanic rocks of different times in five areas (Santanghu Basin, LudongWucaiwan region, Northwest Zhaheba, Luliang uplift and Beilekuduke) in north eastern margin of East Junggar, Xinjiang shows that the volcanic rocks are mainly composed of basalts and basaltic andesites with a small amount of rhyolite, which belong to calcalkaline and high K calcalkaline series. The volcanic rocks have a wide range of MgO(111％1074％)and Mg＃(1275) values, relatively high alkali ((Na2O+K2O)=34％1016％) and high content of TiO2 (TiO2=02％33％). The rocks are enriched in LILE (like Rb and Ba) and LREE, intensely depleted in HFSE (like Nb and Ta), and weakly depleted in Zr, Hf and Ti, without significant Eu and Ce anomalies (δEu=060121, δCe=089115). The (87Sr/86Sr)ivalues of volcanic rocks are 070278070542, and the (143Nd/144Nd)i values are 05125170512648, with a high positive εNd(t)(+4+75); the La/Nb ratios (1053) is higher than that of primitive mantle(0981), and the Nb/U ratios (237, the vast majority of rocks <30) are lower than those of MORB and OIB. Between the primitive mantle and crust, the rocks show the depletion of the mantle source characteristics, but in some aspects the rocks also show the enrichment of source characteristics, which may be due to the involvement of the magma in the formation of crustal material. The formation times of volcanic rocks belong to the era of postcollisional deep magmatic activity in Junggar; the rocks not only show the characteristics of intraplate magmatic evolution, but also have traces of arc volcanic rocks geochemistry; maybe they are from extensional settings of postcollisional period, and relate to lithospheric mantle underplating; they match the postcollision granite in northeastern margin of East Junggar, and they all formed in the postcollision period. The latest available zircon SHRIMP UPb and wholerock RbSr isochron dating ages provide strong evidence for rebuilding tectonic framework, recovering magma evolution and  recognizing the closure time of Junggar ocean in East Junggar.

Key words: postcollisional period, volcanic rocks, geochemistry, petrogenesis, East Junggar, Xinjiang