Zircon U-Pb Chronology and Geochemistry of the Late Ordovician-Early Silurian K-Bentonite from the Western Segment of the South Dabashan, Northern Margin of the Yangtze Block and Its Regionally Tectonic Implications

The Ordovician-Silurian bentonite is an ideal target for probing complex geological events such as biological extinction, glaciations, and volcanic events at that time due to its global distribution and multiple intercalary strata. In this paper, Late Ordovician-Early Silurian K-bentonites from the western segment of the South Dabashan are sampled for zircon U-Pb dating and geochemical analyses. A mean 441.4 +/- 4.2 Ma zircon U-Pb age is obtained from the K-bentonite and is regarded as volcanic eruptive time. Geochemical data show characteristics of intermediate volcanic rocks and typical K-bentonites elsewhere. Chondrite-normalized REE+Y distribution patterns display LREE (light REE) enrichment and HREE (heavy REE) depletion with distinct negative Eu anomalies. Primitive mantle-normalized spider diagram shows similar concave-up patterns with relatively enriched LILE (Ba, U, and Pb), HFSE (Nb, Ta, Zr, and Hf) and distinctly depleted elements such as P, Sr and Ti, indicating these K-bentonites derive from high-K and alkaline primary magma and represent the product of within-plate volcanic eruption. High-frequent and periodic volcanic eruptions from the Yangtze block, its peripheral areas, and the southern margin of the North China block began with Early Ordovician and lasted to Late Silurian. In the Dabashan region, ca. 440-448 Ma K-bentonites may primarily originate from the within-plate volcanic setting, but volcanic ashes of ca. 450-460 Ma K-bentonites are perhaps island arc products connected with north-dipping continuous subduction of the Paleo-Qinling Ocean during Early Paleozoic.