Detrital zircons of the Meitan Formation during Ordovician in Northeastern Guizhou and its significance for provenance-tectonic and implications for metallogenic chronology

Located in northeastern Guizhou of middle Yangtze area, Yanhe County has many low-temperature hydrothermal deposits. It is a good place for studying the provenance-tectonic response of the basin due to its back-bulge position during Ordovician. However, in most area of middle Yangtze, stratigraphic sequence is composed mainly of carbonate rocks intercalated with mudstone, which restricts the development of quantitative analytical methods such as the detrital zircon geochronology. Recently, the authors noticed that there are mudstone and carbonate intercalated with sandstone around Yanhe in the Metitan Formation (O_1-2m) of lower-middle Ordovician. A detrital zircon sample from the sandstone shows that only 8 zircons (12%) are older than 1.0 Ga and they have no age peaks, and the weighted average of the youngest group of zircons ((457.8 ±8.1) Ma) is close to the upper limit of the Meitan Formation. This means that the detritus of the Meitan Formation was mainly derived from a newly formed source area and had a relatively high deposition rate. The age of zircons can be divided into five groups according to their age distribution and peaks: ~461 Ma (Ⅰ), ~580 Ma and ~606 Ma (Ⅱ), ~722 Ma (Ⅲ), ~865 Ma (Ⅳ₁) and ~936 Ma (Ⅳ₂). On the basis of isotope ratio and REE, the authors consider that the ages can be divided into three parts:Ⅰ, Ⅱ and Ⅳ₂, Ⅲ and Ⅳ₁, which are mainly derived from three different source areas. Combined with the previous study, the authors hold that the three source areas are Qianzhong uplift, Kangdian ancient land, and Wuling-Xuefeng uplift, with Qianzhong and Kangdian being newly formed source area. What's more, there are many (73%) hydrothermal zircons in age group Ⅰ (450 ~511 Ma), and this age matches well with low-temperature hydrothermal mineralization, suggesting that it was caused by the activation of the fluid, probably due to the regional tectonic regime converted from extension to compression. This implies that the Caledonian movement had begun to affect the Yanhe area, but was mainly manifested as hydrothermal activity, with no large scale of magmatism.