Geochronology, geochemistry and geological significance of the Wulan composite pluton in the southern Xing'an block

There are some existing debates about the Late Paleozoic tectonic framework of the northern margin of the Xing'an-Mongolian orogenic belt, especially the final closing time of oceanic basin. Wulan composite Pluton in the southern Xing'an block provides a good medium to address this problem. In this paper, the Zircon LA-ICP-MS U-Pb ages and geochemical characteristics of Wulan composite Pluton were studied to determine its formation epoch and petrogenesis, and to reveal the regional tectonic setting. Wulan composite Pluton is composed of fine grained monzogranite and porphyritic monzogranite, the former constitutes the main body of the rock mass, with the diagenetic age of (312±2) Ma, which was formed in early Late Carboniferous. Geochemical analyses indicates that both of the fine grained monzogranite and porphyritic monzogranite have the characteristics of medium total alkali content, rich in K and peraluminous, belonging to highly fractionated S-type granite of high-K calc-alkaline series. The total REE contents of main rock mass of fine grained monzogranite are relatively low, with obvious negative Eu anomalies, showing high differential evolution character. Both the fine grained monzogranite and porphyritic monzogranite are rich in large ion lithophylic elements (Rb, Th and K), and obviously depleted in Ba, Sr and Ti, with slightly depleted in Ta and Nb. These two types of rocks have characterastics of similar trace and rare earth elements distribution, indicating that they come from the same source region. The formation of the rock mass is related to the evolution of the ancient Asian ocean and was formed in the late orogenic extensional environment after the oceanic basin was closed.