Late Paleoproterozoic to early Mesoproterozoic Dongchuan Group in Yunnan, SW China: Implications for tectonic evolution of the Yangtze Block

Abstract

The Dongchuan Group including the Yinmin, Luoxue, Etouchang, and Luzhijiang Formations from the base upward has extensive outcrops in the western Yangtze Block. It was previously thought to be the lower part of the late Mesoproterozoic to early Neoproterozoic Kunyang Group, but there was a lack of precise age determination. The whole sequence is more than 4 km in thickness and consists of conglomerate, sandstone, slate, dolostone, carbonaceous slate, and minor tuffaceous volcanic rocks. The basal unit, the Yinmin Formation, is a continental red-bed deposit thought to have been formed in an intra-continental rift basin. Zircons from the sandstone and volcanic rocks of the Yinmin Formation are analyzed for U–Pb and Lu–Hf isotopes in this study. Detrital zircons from the sandstone have U–Pb age populations at 1.8–1.9 Ga, 2.25–2.35 Ga, ∼2.5 Ga, and 2.7–2.9 Ga with the oldest age of ∼3.7 Ga. Most zircons have high Th/U ratios (>0.1) and were probably derived from igneous rocks, whereas a few 2.05–1.95 Ga grains with low Th/U ratios (<0.1) may have a metamorphic origin. The age groups are roughly consistent with those of known tectonothermal events in the northern Yangtze Block. The youngest detrital zircons of the Yinmin Formation have 207Pb/ 206Pb ages of ∼1780 Ma and dolerite dykes intruding the formation have a zircon 207Pb/ 206Pb age of 1690 ± 32 Ma (2 σ). A zircon 207Pb/ 206Pb age of 1742 ± 13 Ma (2 σ) was obtained from a tuff sample of the Yinmin Formation and is considered to represent the deposition age. In combination with other age data of the Dongchuan Group, the newly obtained U–Pb age data thus constrain the deposition of the Dongchuan Group to the period between ∼1.7 Ga and 1.5 Ga, significantly older than previously thought. Presence of late Paleoproterozoic continental rift in the western Yangtze Block is contemporaneous with the break-up of the Columbia supercontinent, and provide further evidence to support the idea that the Yangtze Block was likely part of the supercontinent. Detrital zircons of the Yinmin Formation mostly have Hf continental modal ages ( T DM C) between 3.5 Ga and 2.6 Ga with the oldest T DM C of 3.9 Ga, indicating that the western Yangtze Block experienced significant continental crustal growth during the Paleo- to Neo-archean and contains Eoarchean crustal rocks. In combination with data from others, our results suggest that the Yangtze Block have a widespread Archean basement. The Archean zircons have both positive and negative ɛ Hf( t) values (−10.1 to +6.5) suggesting both juvenile crustal growth and reworking of older materials. In contrast, the Paleoproterozoic zircons have highly variable but mostly negative ɛ Hf( t) values implying episodic crustal reworking of Archean crustal rocks. Unlike the Paleoproterozoic detrital zircons, igneous zircons from the tuff and dolerite dyke samples mostly have positive ɛ Hf( t) values (−0.8 to +10.1), indicating contributions of juvenile materials from a depleted mantle source due to continental rifting.