Precambrian crustal growth of Yangtze Craton as revealed by detrital zircon studies

Abstract

Clastic sedimentary rocks are natural samples of the exposed continental crust over large areas. The age and evolution of the Yangtze craton is not well known, because much of the craton is covered by thick Nanhua (850 –635Ma), Sinian (635 –543Ma) and Phanerozoic sedimentary rocks. The Nanhua clastic sedimentary rocks including tillites provide ideal samples for studies of age and evolution of the craton. 1130 zircons in 8 sandstone and tillite samples of three Nanhua formations (Liantuo, Gucheng and Nantuo) from the Yangtze Gorges area were dated by LA-ICP-MS. 251 of the 711 concordant zircons were analyzed for Hf isotopic compositions by LA-MC-ICP-MS. The results reveal four major age groups of 720 to 910 Ma, 1.90 to 2.05 Ga, 2.40 to 2.55Ga and 2.60 to 2.70 Ga with few grains >3.2Ga. Although Hf isotopic compositions show both juvenile crustal growth and reworking of old crust for all the age groups, the Paleoproterozoic is a period of prominent crustal reworking with negative eHf(t) values. The Neoproterozoic is a period of significant juvenile crustal additions, accounting for 68 percent of zircons with positive eHf(t) values similar to those of the depleted mantle. Crustal additions at 3.2 to 3.8Ga are also significant, as indicated by the zircon Hf continental model ages. Our results highlight the importance of analysis of a sufficient number of zircons for provenance studies. In addition, our results illustrate that possible temporal and spatial provenance variations have to be taken into account for better characterizing formation and evolution of the related continental crust. Our obtained youngest age for each formation shows a strong negative correlation with stratigraphic height. Youngest ages provide good constraints on the maximum ages of the three Nanhua formations: ≤770Ma, ≤733Ma and ≤704Ma for the base, middle unit and top of the Liantuo Formation, respectively; ≤703Ma for the Gucheng Formation and possibly ≤600 to 606Ma for the Nantuo Formation. Although the North China and Yangtze cratons show some apparently similar zircon ages, the two cratons have distinct histories of formation and evolution. While the North China craton is dominated by Archean crustal growth, the Yangtze craton is characterized by prominent crustal additions in the Neoproterozoic, which is almost lacking in the North China craton.