Alkali-rich dykes of the Late Permian in Ulungur (乌伦古) area are representative products of granitic magmatism in the evolution of the Paleozoic orogenic belt in the East Junggar (准噶尔), North Xinjiang (新疆). We selected two representative samples for geochemical analysis (major and trace elements), and twenty-two zircon grains for zircon dating. Isoplot (ver3.0) was used to calculate isotopic age and make concordia diagrams. This study shows that the trachy porphyry dykes, featuring low concentration of Al2O3, CaO, MgO and high alkali contents, are metaluminous alkaline rock and belong to A-type granitoids. The dykes have low concentration of the REE and incompatible elements, and the REE patterns show clear negative Eu anomalies (δEu=0.74–0.58), enriched LREE (LREE/HREE>4, (La/Yb)N=5.97–4.63) and undifferentiated HREEs. Similar normalized REE and incompatible element patterns are also showed in the dykes from Yemaquan (野马泉) and granites from Ulungur, suggesting that they are possibly originated from the same source and formed in the same tectonic environment, but the trachy porphyry dykes are more evolved. The age of the trachy porphyry dykes is 255.3±2.4 Ma, which is probably the crystallization time of the trachy porphyry. The dykes formed in late-orogenic phase of post-collision process or within-plate environment, which suggested that the trachy porphyry dykes possibly crystallized in the transition period during which the tectonic setting changed from post-collision to within-plate environment. So we consider that the age when the post-collision ended and the crustal cratonization begun in the East Junggar is 255 Ma, Late Permian.
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