Sub-micron trace elemental distributions and U-Pb dating of zircon from the oldest rock in the Anshan area, North China Craton

Abstract

The formation ages of ancient rocks are controversial because of the co-existence of several generations of zircon within a single sample. This study presents BSE imaging, CL imaging, Raman spectroscopy, NanoSIMS trace element imaging and U-Pb dating measurements of zircon from the trondhjemitic enclave and its host leucosome of the Archean Anshan Complex, which were suggested to be the oldest rock (3.8 Ga) in the North China craton. Unlike previous studies, zircon grains were in situ analyzed in polished thin sections instead of mineral separations from the rocks. Three types of zircon have been identified. All zircon in the leucosome share similar morphology and textural relationships observed within thin sections, marked as type I. Zircon in the trondhjemitic enclave can be divided into two groups, marked as type II and type III zircon, respectively. Type II zircons share similar occurrence, texture and U-Pb ages of Type I. They are euhedral prismatic or rhombus in shape with oscillatory zonings. Type I and II zircon yield weighted mean 207Pb/206Pb ages of 3134 ± 34 Ma (n = 14, MSWD = 6.2) and 3132 ± 30 Ma (n = 7, MSWD = 2.1), respectively, which can be referred to as formation ages of the leucosome and the trondhjemitic enclave. Type III zircons generally occur with rounded shapes and core-rim textures. The cores yield ages of 3.7–3.8 Ga, while the rims give discordant ages of 3.3–3.2 Ga, indicating that these zircons are inherited with altered narrow rims during the later magmatism. Therefore, 3.7–3.8 Ga cannot represent the formation age of the trondhjemitic enclave. This study demonstrates that in situ measurements of zircon in polished thin sections can preserve the petrographic settings of zircon and provide strict constraints for the zircon formations and hence the host rocks. More ancient rocks with 3.8 Ga zircon from the Archean Anshan Complex should be studied by this method to further clarify whether there is 3.8 Ga rock in the North China craton.