DETRITAL ZIRCON CONSTRAINTS ON THE PROVENANCE AND TECTONIC SETTING OF THE LOWER DEVONIAN SANDSTONE IN SOUTHEAST YANGTZE BLOCK, CHINA

The South China was geologically formed by amalgamation of the Yangtze and the Cathaysia Block along the Jiangnan Orogen during the Neoproterozoic and subject to widespread tectonic and magmatic events which are collectively called the Caledonian orogeny during the Early Paleozoic. Opinions about the nature of this orogeny are primarily split into two groups: collisional or intracontinental orogenesis. The Guizhou Province was geologically located on the southeast margin of the Yangtze Block and to the west of the Jiangnan Orogen.

Ages of 215 detrital zircon samples were obtained from quartz sandstone in the Lower Devonian Danlin Formation exposed in Dushan County, south Guizhou Province. The detrital zircons are mostly of Neoproterozoic age, with 59.5% in the range of 997–557 Ma. According to the zircon age pattern, morphology, trace element and combined with paleogeographic reconstruction, it suggests that these detrital zircons were directly supplied from the igneous rocks in the Neoproterozoic Groups within the Jiangnan Orogen to the east of the study area. Mesoproterozoic zircons made up 20.9% of the samples which range from 1569–1055 Ma; the ages of most of these zircons coincided with those of zircons indicating in the Grenvillian orogeny (1300–1000 Ma) recorded in the Cathaysia Block. The morphological properties of the zircon grains and paleocurrent direction suggest that these zircons may have been delivered from the interior of the Cathaysia Block to the Yangtze Block through multiple cycles of denudation and transportation processes during the Middle Ordovician. Moreover, detrital zircon age data reveals that the study area, together with the surrounding area, had an intracontinental tectonic setting, both before and after the Caledonian orogeny.

A combination of all the aforementioned findings further demonstrate that during the Early Paleozoic, the Cathaysia Block was not separated from the Yangtze Block by an ocean and the study area received sediments primarily from the Cathaysia Block located to the southeast. By the Early Devonian, the sediments contained in the study area were supplied by the Jiangnan Orogen to the east. Although a source change for the study area was caused by Caledonian orogeny, our data implies that the Caledonian orogeny represented an intracontinental orogenesis.