First ~(40)Ar/~(39)Ar Age Determinations on the Malkhan Granite-Pegmatite System: Geodynamic Implications

摘要

The Malkhan granite-pegmatite system located in Central Transbaikalia, in the southwestern portion of the Malkhan-Yablonovy structure-formational zone of the Caledonian folding comprises two granite massifs (Bolsherechensk and Oreshny) and a miarolitic pegmatite field of the same name, which adjoins the Chikoi deep-seated fault and Lower Cretaceous Chikoi rift depression in the north. The first ~(40)Ar/~(39)Ar data were obtained on porphyritic biotite granites of the Oreshny massif and on K-feldspar, mus-covite, and lepidolite from the Oktyabrskaya pegmatite vein. According to these data, the age of the granite-pegmatite system is 123.8-127.6 Ma, which is consistent with the age of Lower Cretaceous rocks from the Chikoi depression. The intimate spatial relationship and isochronism between the Chikoi depression and the Malkhan granite-pegmatite system are strongly suggestive of a rift regime that affected its evolution, thus highlighting the need to regard the evolution of this system as being intimately related to depression development. Such a model can easily be realized within the framework of the concept of a metamorphic core complex, which was used to explain the nature of Transbaikal-type rift depressions and conjugate granite-gneiss swells.