Phlogopite ~(40)Ar/ ~(39)Ar geochronology of mantle xenoliths from the North China Craton: Constraints on the eruption ages of Cenozoic basalts

摘要

Mantle xenoliths provide direct information about lithospheric evolution and asthenosphere-lithosphere interaction, and therefore precise dating of the host basalts which carried the xenoliths is important. Here we report ~(40)Ar/ ~(39)Ar geochronology of phlogopite separates from five spinel lherzolite xenoliths collected from the North China Craton (Hannuoba of Hebei Province, Sanyitang of Inner Mongolia Autonomous Region and Hebi of Henan Province), as well as the groundmass of the host basalts. Argon extraction was performed by conventional step heating technique and ultra-violet laser ablation microprobe (UVLAMP) technique. ~(40)Ar/ ~(39)Ar incremental heating results on groundmass yielded geologically meaningless ages. However, conventional step heating on phlogopites produced Miocene cooling ages, identical to the eruption ages obtained from the K-Ar dating methods of the Hannuoba and Sanyitang basalts. Adopting procedures to exclude potential influence of excess radiogenic Ar from a deep fluid source on a phlogopite separate from lherzolite yielded results with a good agreement of ages suggesting that the argon isotopes are distributed homogenously in this mineral, with no influence of excess argon. Phlogopites from Hebi yield ages between 6.43 and 6.44Ma which are slightly older than those obtained from K-Ar method on whole-rocks. The discrepancy in the K-Ar ages obtained from the altered whole-rock samples suggests partial loss of ~(40)Ar. As a consequence, phlogopite Ar-Ar ages are considered more accurate than that of the whole-rocks. These results suggest that 40Ar/ 39Ar chronology of phlogopite provides reliable and precise ~(40)Ar/ ~(39)Ar ages of host basalts.