~(40)Ar/~(39)Ar isotopic ages of Qitianling granite and their geologic implications

摘要

This paper has reported the first application of ~(40)Ar/~(39)Ar dating to orthoclase from Qitianling granite. The resultant plateau ages yielded by three orthoclase specimens 2KL-17, 99LQ-2 and 2KL-31 (Note: The last one was taken from the part of granite which had been attributed to Calling super-unit of the Indosinian Period by the former researchers) collected from the said granite are (139.57+-2.79) Ma, (140.55+-2.81) Ma and (144.91+-2.90) Ma respectively. The above-mentioned ages represent the closed ~(40)Ar/~(39)Ar age of the orthoclase. The consistency in age dating results, the similarity in geochemical characteristics and rock textures, and the NW-SE orientation of orthoclase phenocrysts almost throughout the granite, provide evidence for the intimate relationship between the Furong super-unit and the Cailing super-unit that form the main part of the granite, suggesting that they are products of comagmatic conjugate differentiation during the Late Jurassic. This paper also makes a comparison between the Qitianling granite and the Qianlishan granite.