Zircon LA-ICP-MS U-Pb Dating of the Porphyritic Monzonitic Granite from Southwest Hongyanjing in Beishan Area, Gansu Province and Its Significance

In this paper, the chronologic and geochemical researches were carried out on the porphyritic monzonitic granite in Southwest Hongyanjing.The zircon LA-ICP-MS U-Pb dating results show that the zircon U-Pb age of the intrusions is (402.2±4.4)Ma, which indicates that this intrusion was formed in the Early Devonian.Petrochemical and Geochemical data indicate that the porphyritic monzonitic granite are characterized by high Si (68.12%~75.76%)and high alkali (7.69%~9.91%), with A/CNK value of 1.4~1.5.Such geochemical characteristics, such as the rittmann index in sigma(σ)=1.8~3.8 (average 2.38), indicate that this intrusions belongs to the calc-alkaline strong peraluminous series.It displays higher total REE content(ΣREE=175.4~296.76).Trace elements HFSE and Y are well concentrated, but Ba, Sr, Eu, P, and Ti are poorly concentrated.The comprehensive analysis also suggests that the granite is strongly peraluminous S-type granite of the calc-alkaline suite,which was formed in the post-orogenic tectonic environment.