LA-ICP-MS U-Pb zircon ages of the khondalites from the Wulashan and Jining high-grade terrain in northern margin of the North China Craton: Constraints on sedimentary age of the khondalite

Abstract

The Wulashan and Jining Complexes in northern margin of the North China Craton are high grade terrains that are composed predominantly of TTG gneisses, granulites, amphibolites and khondalites. Traditionally, all these rocks were considered as metamorphic supracrustals and their ages were arbitrarily assigned according to their metamorphic grade, i. e. the higher grade rocks were older than those lower grade rocks. Recently, the complex has been divided into metamorphosed plutons and supracrustals, but their ages remain controversial, with most workers believing that the plutons formed in the middle Archean, whereas the khondalites formed in the late Archean. To resolve this important issue, we applied the LA-ICP-MS U-Pb zircon dating technique, combined with CL images of zircons, to determine the ages of detrital and metamorphic zircons in the khondalites from the Wulashan and Jining Complexes. The results show that the metamorphic zircons of the khondalites from the Wulashan and Jining Complexes formed at ∼1.8 Ga, consistent with the metamorphic ages of ∼1.85 Ga obtained using the TIMS method. Detrital zircons in the khondalites from the Hadamengou, Taoerwan and Yaoziwan areas of the Wulashan Complex yielded LA-ICP-MS U-Pb zircon ages ranging from 2.0 to 2.3 Ga, and the detrital zircons in the khondalites from the Huangtuyao area of the Jining Complex yielded the same age range. These ages demonstrate that the protoliths of khondalites from the Wulashan and Jining Complexes formed in the Paleoproterozoic, not in the Archean as previously considered. In addition, a kf-granite sample collected from the Damiao area of the Wulashan Complex yielded a LA-ICP-MS U-Pb zircon age of ∼2.5 Ga, similar to those ages already obtained for the granulites and TTG gneisses in the Wulashan and Jining Complexes. Although these Archean granulites, TTG gneisses and granites are spatially associated with the khondalites, the former are much older in age than the latter, which suggests that the protoliths of the khondalites were not in-situ deposition, but were allochthonous sediments whose interdigitation with the Archean granulites and TTG gneisses formed by the tectonic processes of the Lüliang Movement.