Geochronology and geochemistry of Late Mesozoic igneous rocks inJifeng area of Northern Da Hinggan Mountains and their tectonic significance

The Da Hinggan Mountains in Northeast China are typically characterized by extensive and large-scale Mesozoic magmatism.The temporal-spatial distribution and petrogenetic type of the magmatic rocks offer a potential method for constraining tectonic background and geodynamic mechanism.This paper reports the LA-ICP-MS zircon U-Pb ages and geochemical data for the Late Mesozoic granitic rocks in Jifeng area of the Da Hinggan Mountains.U-Pb age data show that magmatism in Jifeng area can be subdivided into two stages:ca.145.2 Ma and ca.124.2~125.8 Ma.The magmatic rocks in two different stages show consistent geochemical characteristics of high SiO₂ (66.88%~75.03%), high Al₂O₃ (13.26%~16.44%), high Na₂O+K₂O (7.35%~9.30%) and high zircon saturation temperature as well as enrichment of LILE and depletion of HFSE.What's more, most of samples display characteristics of Ga/Al×10000>2.6 and Zr> 250×10^-6.These rocks were considered to be the highly-fractionated A-type granite, indicating a regional crustal extension background.In combination with a large number of precise geochronological data between 170~100 Ma published for the Da Hinggan Mountains and their vicinities in the last few years, the authors hold that the first stage A-type granites may be products of partial melting of the lower crust under the tectonic background of transformation from compression to extension controlled by Mongol-Okhotsk Tectonic Domain, while the second stage A-type granitic rocks might have resulted from delamination of thickened lower crust under the tectonic setting of strong stretching.