Abstract:

Step-wise heating gas-released Ar-Ar method was applied for dating of biotite from the granulite and amphibole from the amphibolite enclave in the adjacent granitic pluton, Huangtuling area, North Dabieshan. The amphibole indicates a plateau age of (124.9±4.6) Ma, consistent with the timing of the Cretaceous(120—130 Ma)metamorphic and magmatic events regionally. However, the biotite gave a plateau age of (176.9±0.8) Ma, much older than that of the amphibole which has a higher closure temperature for Ar than biotite does, suggesting that excess argon exists in the biotite of the granulite. Late generation biotites growing in lower temperature condition and fluid-rich environment are the main carriers of excess argon. Therefore, when applying the Ar-Ar dating method to reconstruct the cooling rate of geologic rocks containing multi-genetic cogeneric potassium-bearing minerals, the only appropriate choice should be using of laser probe to in situ determining the age of the generation mineral of metamorphic peak.

Key words: excess argon, biotite, thermo-chronology, granulite, North Dabie Unit