Geochemical characteristics and LA-ICP-MS zircon U-Pb age of porphyritic granodiorite in Heihongshan area of Beishan region, Inner Mongolia, and their geological significance

The tectonic location of Late Carboniferous porphyritic granodiorite of Heihongshan area in Beishan region belongs to Beishan-Tianshan orogenic system and Ejina-Beishan arc basin system, but without reliable zircon U-Pb dating report. On the basis of the detailed internal structure analysis of zircons from the porphyritic granodiorite using the technology of cathodoluminescence, the authors conducted isotope age analysis using LA-ICP-MS zircon U-Pb dating method. The result shows that porphyritic granodiorite formed at 312.1±3.2Ma. Geochemical study shows that porphyritic granodiorite belongs to medium potash-calc alkaline, quasi-aluminous granite system, being I type granite. It has characteristics of light REE enrichment and characteristics of enrichment of Rb, Th, K, Hf and depletion of Ta, Nb, Ti, P. It formed in a pre-collision tectonic environment. Combined with regional geological background, It is considered that ocean basin, represented by Hongshishan-Baiheshan-Pengboshanophiolite, should not have been closed in Late Carboniferous period.