Origin of Late Palaeozoic bauxites in the North China Craton: constraints from zircon U–Pb geochronology and in situ Hf isotopes

Abstract

Bauxite deposits that lie between Middle Ordovician limestones and Late Carboniferous clastic sediments (known as the ‘G’ layer bauxites) are widespread throughout the northern part of the North China Craton. The origin of these bauxites has remained uncertain and has been attributed either to alteration of the underlying limestone or to tephra deposited on the limestone erosional surface. We use U–Pb ages and in situ Hf isotopic analyses of detrital zircons from the ‘G’ layer bauxite deposits (both by laser ablation inductively coupled plasma mass spectrometry) to constrain the major source materials of the bauxites. The zircon ages are dominated by Palaeozoic ages, especially Carboniferous to Early Permian (338–291 Ma), coeval with the Palaeozoic subduction-related volcanism in the Inner Mongolia Palaeo-Uplift on the northern margin of the North China Craton. Zircon ϵ Hf ( t ) values range from 2.2 to −24.5 and are dominated by negative values, similar to those of Palaeozoic magmatic rocks in the Inner Mongolia Palaeo-Uplift, but distinct from those of Palaeozoic magmatic rocks in the Central Asian Orogenic Belt with positive ϵ Hf ( t ) values. Thus, we argue that the bauxites were derived mainly from volcanic dust produced during the Palaeozoic, particularly in the Carboniferous to Early Permian (338–291 Ma), by volcanism in the Inner Mongolia Palaeo-Uplift along the northern margin of the North China Craton, rather than in the Central Asian Orogenic Belt to the north. The underlying Middle Ordovician limestones, however, had provided only minor, if any, source materials for the bauxites.