Single zircon grains record two Paleoproterozoic collisional events in the North China Craton

Abstract

The Western Block of the North China Craton consists of the Yinshan Block in the north and the Ordos Block in the south which were amalgamated along the east-west trending Khondalite Belt at ∼1.95 Ga. The Western Block then collided with the Eastern Block to form the coherent basement of the North China Craton along the north-south trending Trans-North China Orogen at ∼1.85 Ga. The Huaian Complex, a high-grade terrrane located at the conjunction of the Khondalite Belt and Trans-North China Orogen, records metamorphic events associated with both collisions. The complex consists of lithologies from both the Khondalite Belt and Trans-North China Orogen, of which the former consist of graphite–garnet–sillimanite gneiss, garnet quartzite, felsic paragneiss, calc-silicate rock and marble, together called the Khondalite series. Zircons in the graphite–garnet–sillimanite gneiss can be divided into three types: (1) spherical grains without internal structures, (2) grains with a core-and-rim structure; and (3) grains with a dark core surrounded by double rims. Except for the dark core in type 3, all other types of zircon domains are structureless and highly luminescent, with very low Th/U ratios, typical of a metamorphic origin. Analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 ± 26 and 1947 ± 22 Ma, similar to previously determined metamorphic ages from the Khondalite Belt and thus interpreted as the time of collision between the Yinshan and Ordos Blocks. Analyses on type 1 zircons, rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 ± 15 and 1857 ± 16 Ma, interpreted as the time of collision between the Eastern and Western Blocks. Thus, zircons in the graphite–garnet–sillimanite gneiss of the Huaian Complex record both of the Paleoproterozoic collisional events in the North China Craton.