关键词: 锆石U-Pb年龄, 晚石炭世, 含锰岩系, 玛尔坎苏锰矿带, 西昆仑

Abstract:

The recently discovered Malkansu Mn carbonate metallogenic zone in the West Kunlun orogenic belt, represents a significant breakthrough for economic prospecting in China. In this paper, based on the existing age of the strata in the Malkansu Mn carbonate belt, U-Pb dating is carried out on the zircons from volcanic-bearing breccia, sandy limestone and interbedded tuff in the manganese-bearing rock series in order to determine the depositional age of this strata. In combination with the background of regional structure and supergene system evolution in the same period, the geological significance of the Malkansu manganese ore belt is further discussed. Zircons from the volcanic-bearing breccia limestone in the first lithological member and sandy limestone of the second lithological member, and the youngest three zircons in interlayered tuff of the third lithological member in the manganese-bearing Kalaatehe Formation yield 206Pb/238U ages of (322.9±1.8) Ma and (322.0±5.3) Ma, respectively. In combination with the newly reported Re-Os ages of the manganese carbonate ores, the deposition of the manganese-bearing rock series can be roughly limited to the Late Carboniferous (about 320 Ma). The manganese-bearing rock series of the Kalaatehe Formation may represent a back-arc basin sedimentary sequence related to the northward subduction of the Paleo-Tethys Ocean in the Late Paleozoic. The manganese-bearing rock series deposited during the Late Paleozoic glacial period (340-285 Ma), and the manganese carbonate ores also have (REE+Y)PAAS distribution similar to modern oceanic ferromanganese deposits, indicating that the original manganese oxides precipitated slowly and were in equilibrium with seawater. This can be compared to the formation process of other sedimentary manganese carbonate deposits related to glaciers in the Neoproterozoic around the world, that is, the gradual melting of glaciers leads to the slow precipitation of dissolved Mn2+ in the form of manganese oxides. Based on this, it is speculated that the dynamic glacial activity developed in the Gondwana continent in the southern hemisphere in the Late Paleozoic had a certain impact on the geochemical cycle of Mn in the Paleo-Tethys Sea area.

Key words: zircon U-Pb age, Late Carboniferous, manganese-bearing sequence, Malkansu metallogenic zone, West Kunlun Mountains