First 40Ar/39Ar age determinations on the Malkhan granite-pegmatite system: Geodynamic implications

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 ⁴⁰Ar/³⁹Ar data were obtained on porphyritic biotite granites of the Oreshny massif and on K-feldspar, muscovite, and lepidolite from the Oktyabrskaya pegmatite vein. According to these data, the age of the granitepegmatite 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.