Isotopic Ages of the Carbonatitic Volcanic Rocks in the Kunyang Rift Zone in Central Yunnan, China

Abstract

The Mesoproterozoic Kunyang rift, which is located on the western margin of the Yangtze platform and the southern section of the Kangdian axis, is a rare massive Precambrian iron‐copper polymetallic mineralization zone in China. The Mesoproterozoic Wulu (Wuding‐Lufeng) basin in the middle of the rift is an elliptic basin controlled by a ring fracture system. Moreover, volcanic activities in the basin display zonation of an outer ring, a middle ring and an inner ring with carbonatitic volcanic rocks and sub‐volcanic dykes discovered in the outer and middle rings. The Sm‐Nd isochron ages have been determined for the outer‐ring carbonatitic lavas (1685 Ma) and basaltic porphyrite of the radiating dyke swarm (1645 Ma) and the Rb‐Sr isochron ages for the out‐ring carbonatitic lavas (893 Ma) and the middle‐ring dykes (1048 Ma). In combination of the U‐Pb concordant ages of zircon (1743 Ma) in trachy‐andesite of the corresponding period and stratum (1569 Ma) of the Etouchang Formation, as well as the Rb‐Sr isochron age (1024 Ma) and K‐Ar age (1186 Ma) of the dykes in the middle ring, the age of carbonatites in the basin is preliminarily determined. It is ensured that all of these carbonatites were formed in the Mesoproterozoic period, whereby two stages could be identified as follows: in the first stage, carbonatitic volcanic groups, such as lavas, pyroclastic rocks and volcaniclastic sedimentary rocks, were formed in the outer ring; in the second stage, carbonatitic breccias and dykes appeared in the middle ring. The metamorphic age of the carbonatitic lavas in the outer ring was determined to be concurrent with the end of the first stage of the Neoproterozoic period, corresponding to the Jinning movement in central Yunnan.