Abstract: In this paper, the geochronology, geochemistry and Hf isotopic composition of the Harasu porphyritic syenogranite in Zalantun, Inner Mongolia were studied.LA-ICP-MS zircon dating indicates that the Harasu porphyritic syenogranite was emplaced during the Late Triassic(213.17±0.93 Ma).The mineral assemblage is mainly composed of quartz, alkaline feldspar and plagioclase.The Harasu porphyritic syenogranite is characterized by high SiO₂(72.56%~74.36%), K₂O(4.74%~5.49%), low MgO(0.12%~0.34%), CaO(0.54%~0.95%), TiO₂(0.19%~0.29%) and P₂O₅(0.042%~0.053%), A/CNK=1~1.05, < 1.1.It is strongly depleted in Ba, Sr, Eu, P and Ti, showing the geochemical characteristics of aluminous A-type granite.The Harassu A-type granites have high ε_Hf(t) values(+9.08~+15.3), which may due to the partial melting of new meso-basic crustal materials.The Harasu A-type granite is recognized as the product of a post-orogenic tectonic.The crust in Zalantun area was thickened and transformed into a post orogenic extension mechanism, which may be affected by the remote effect of the closure of the Paleo-Asian Ocean, and then the Late Triassic Harasu porphyritic syenogranite in Zalantun area was formed.