THE MALKHAN GRANITE-PEGMATITE SYSTEM

Abstract

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 deepseated fault and Lower Cretaceous Chikoi rift depression in the north. The first 40Ar/39Ar data were obtained on porphyritic biotite granites of the Oreshny massif and on Kfeldspar, muscovite, 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 high lighting 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 Transbaikaltype rift depressions and conjugate granite–gneiss swells.