Geochronology and Genesis of K‐feldspar Porphyritic Granite from Eshan Pluton Yunnan, in Southwestern Margin of the Yangtze Block

The Origin of the Black Shale Series and Bentonites from the Wufeng Formation in the Southwestern Upper Yangtze: Implications for the Convergence of the Yangtze and Cathaysia Blocks in the Late Ordovician

“…In the 13 th -18 th layers, the maximum positive excursion of the K 2 O/Rb ratio is 0.031, while the minimum negative excursion is 0.024, with an average of 0.026, higher than that of all the samples (0.024) and the PAAS average (0.023), indicating that the composition of pyroclastic materials increased. According to the development of multi-layer bentonites in the Wufeng Formation, this might be related to frequent volcanic events during this period (Hu et al, 2009).…”

“…The maximum positive excursion of the U/Th ratio is 0.67, while the minimum negative excursion is 0.18, with an average of 0.31; the maximum positive excursion of the V/(V + Ni) ratio is 0.96, while the minimum negative excursion is 0.55, with an average of 0.84; the maximum positive excursion of the V/Cr ratio is 6.46, while the minimum negative excursion is 1.64, with an average of 2.94; the maximum positive excursion of the Ni/Co ratio is 5.83, while the minimum negative excursion is 1.22, with an average of 3.32. The periodic change in the paleo-oceanic redox indicates that the sea-level fluctuated frequently in this period, which may be related to the regional orogeny and volcanic activities (Hu et al, 2009;Yang et al, 2018). The U au , U/Th and V/Cr ratios in the 18 th -22 nd layers of the Wufeng Formation show a significant positive excursion: the content of the U au increased from 1.99 ppm to 15.90 ppm; the U/Th ratio increased from 0.23 to 1.20; the V/Cr ratio increased from 1.36 to 3.68.…”

“…It is generally believed that the main provenance region of the bentonites in the Late Ordovician black shale series was located in the suture zone of the subduction-collision between the Cathaysia and Yangtze blocks, but different scholars have also put forward alternative viewpoints. Hu et al (2009) compared the early Paleozoic island arc magmas in the Qinling area and believed that the Ordovician-Silurian K-bentonites distributed in the Yangtze area might be related to the subduction-collision of the Yangtze block with the North China block and the closure of the Qinling Ocean. However, Yan et al (2007) studied the volcanic rocks in the Xieyuguan and Caotangou groups in the North Qinling Mountains and determined that the subduction of the Qinling Ocean began in the Early Ordovician (472 ± 11 Ma).…”