Secular variation in the Nd isotopic composition of Neoproterozoic sediments from the southern margin of the Yangtze Block: evidence for a Proterozoic continental collision in southeast China

Abstract

A systematic SmNd isotopic study is reported for Permian to Mesoproterozoic sediments from the southern margin of the Yangtze Block (smYB), southeast China. A coherent secular variation of T DM ages is identified in fine-grained clastic sediments from the smYB. Mesoproterozoic sediments with stratigraphic ages ( T strat. ∼ 1.6-1.0 Ga) have approximately constant T DM model ages of ∼ 1.8 Ga. In the mid-Neoproterozoic and early Sinian (900-770 Ma), there is a dramatic decrease in T DM ages from 1.8 Ga to 1.3 Ga. In the late Sinian, through to Permian sediments ( T strat. ∼ 0.66-0.27 Ga) the Nd model ages increase again to ∼ 1.8 Ga. This pronounced decrease in T DM ages in the mid-Neoproterozoic and early Sinian sediments requires a large increase in the proportion of juvenile mantle-derived materials from the provenance of the smYB sediments. It is argued that the “Nd isotopic shift” is closely related to the important Jingning Orogeny, a continent-island-arc-continent collision event that occurred during the Neoproterozoic in the Yangtze Block. In this orogenic event, the Nd isotopic evidence indicates that a large amount of newly mantle-derived materials has been incorporated in the sedimentary provenance after the collision between the Yangtze and Huanan Blocks. The Nd isotopic results presented here thus enable new constraints to be placed on the tectonic evolution in southeast China. The integrated geological, geochemical and chronological constraints on the Banxi rocks suggest that the Banxi is most likely part of the normal Proterozoic stratigraphic section in the smYB, rather than a Mesozoic melange on the northern margin of Huanan as proposed by Hsü et al. (1987, 1988, 1990). A modified model involving Proterozoic collision between the Yangtze and Huanan Blocks is shown to be consistent with these observations.