Zircon typology and geochemistry in the palaeogeographic reconstruction of the Late Ordovician of Sardinia (Italy)

The present study concerns the Upper Ordovician sedimentary succession which crops out in the Sulcis-Iglesiente area autochthon and in the: Sarrabus-Arburese allochthonous units, with the aim of contributing to a better understanding of the pre-Hercynian palaeogeography. This study is based on the typology of zircon and trace-element geochemistry of zircon and titaniferous minerals. The Upper Ordovician rocks cropping out in the Sarrabus-Arburese area (allochthon belt) mainly result from the erosion of pre-Caradocian volcanics (the so-called 'Porfidi grigi'), which were emplaced in a magmatic arc setting. The lateral and vertical homogeneity of the terrigenous supply during the Caradoc and early Ashgill, and the argillaceouscarbonate sediments ,with shallow shelf faunas during Ashgillian times, suggest that stable geodynamic conditions persisted throughout this time ,;pan. In the Sulcis-Iglesiente area (autochthon belt), the nature of the terrigenous influx and the source-rocks remained the same from the Late Cambrian to the early Ashgill. The constant nature of the detrital zircon grains rules out any possibility that the 'Porfidi grigi' might have contributed to the sedimentary supply; the source-rocks were probably located within an internal cratonic area, distinct from the magmatic arc of the Sarrabus area. In the early Hirnantian, new source-rocks contributed to the terrigenous supply. They were volcanics with a strong mantle component. This new material was later progressively diluted by terrigenous supply of cratonic origin. The significant differences in the composition and origin of the terrigenous material deposited in the autochthonous and the allochthonous units may reflect distinct source-areas with different geodynamic histories. It is therefore possible that the two depositional areas were geographically separated. Location of the Sulcis-Iglesiente area within the North Gondwanan shelf is ~ plausible scenario. But the occurrence of Baltic taxa in the Sarrabus area suggests that this area was originally located in an intermediate position between North Gondwana and Baltica.