Nd isotopic composition of Late Devonian seawater in western Thailand: Geotectonic implications for the origin of the Sibumasu terrane

Conodonts collected from sections near the small towns of Thong Pha Phum and Mae Sariang in the westernmost part of Thailand are used to reconstruct the neodymium (Nd) isotopic composition of seawater during the Late Devonian. The study provides the first Devonian seawater signatures recognized within the Australian shelf of northeastern Gondwana and the adjacent Paleotethys Ocean. At Thong Pha Phum site, the seawater was characterized by very low εNd values (from − 13.1 to − 18.2) and very high Sm/Nd ratios (between 0.36 and 0.66). In contrast, the seawater at the Mae Sariang site was characterized by much more radiogenic signatures (εNd values from − 8.7 to − 11.1) and uniform, low Sm/Nd ratios (between 0.20 and 0.23). Extremely low εNd values recognized at Thong Pha Phum attest to a passive margin continental setting and a paleogeographic position very close to a continental area where Paleoproterozoic and Neoarchean rocks were eroded. Thus, the isotopic data provide strong evidence that during Late Devonian time the Sibumasu terrane was situated in the proximity to the Archean cratons of Western Australia, presumably adjacent to the Carnarvon intracratonic basin. Moreover, Sibumasu may not have been situated in an outboard position on the shelf, as previously suggested, but could have been directly attached to the Australian continental crust. By contrast, low and uniform Sm/Nd ratios of seawater at Mae Sariang resemble those of the Variscan and the present-day oceanic seawaters. Therefore, a pelagic setting within the Paleotethys Ocean is postulated for the Mae Sariang succession. This conclusion is also constrained by minor temporal changes in εNd values and suggests that the Paleozoic of Mae Sariang is not part of the Sibumasu terrane but belongs to the Inthanon Zone.