Where was the Ailaoshan Ocean and when did it open: A perspective based on detrital zircon U–Pb age and Hf isotope evidence

The Indochina-Simao and Yangtze blocks were separated by a branch of the Paleo-Tethys Ocean, commonly referred as the Ailaoshan Ocean in the Paleozoic. Remnants of this Ailaoshan Ocean have been variably suggested to locate along (from east to west) the Ailaoshan fault, Jiujia-Anding fault and Amojiang-Lixianjiang fault. In order to test these models, we have carried out comprehensive detrital zircon U-Pb dating and Hf isotope analyses on the Cambrian to Devonian sedimentary units in the Ailaoshan Belt and its adjacent western margin of the Yangtze Block. Our results indicate marked detrital zircon provenance variation on different sides of the Ailaoshan-Tengtiaohe fault: detrital zircons from east of the fault display a diagnostic age peak at 730-900 Ma, which is characterized by both positive and negative εHf(t) values with a Hf model age (T_DM^C) peak at ∼ 1.8 Ga, whereas detrital zircons from west of the fault display two major age populations of 400-500 and 900-1000 Ma, both characterized by mainly negative εHf(t) values with a Hf model age (T_DM^C) peak at ∼ 2.1 Ga. Our new data indicate that detritus from east and west of the Ailaoshan-Tengtiaohe fault may have been mainly derived from, respectively, the Yangtze Block and Indochina-Simao blocks, thus suggesting the fault may represent the actual suture between the two blocks. Our study also reveals that the Ailaoshan Ocean may have started its early continental rift in the Early Silurian.