The oldest known rocks in south-western China: SHRIMP U-Pb magmatic crystallisation age and detrital provenance analysis of the Paleoproterozoic Dahongshan Group

Abstract

The Paleoproterozoic Dahongshan Group outcrops along the present day southwest margin of the Yangtze Craton. It is the oldest rock found in the region and forms the basement for the late Mesoproterozoic sedimentary successions. The Dahongshan Group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist-lower amphibolite facies. Some of the original sedimentary features, such as unimodal cross-bedding in quartzite and rounded conglomerate clasts, are still preserved, suggesting deposition in a fluvial environment. Abundant carbonate rocks, also found within the sedimentary sequence, indicate periods of shallow marine or lagoonal conditions. A weighted mean SHRIMP 206Pb/207Pb age of 1675 8 Ma for a tuffaceous schist unit confirms the Paleoproterozoic formation age of the group. SHRIMP and LAM ICP-MS U-Pb analysis of detrital zircon grains from metasedimentary units indicate a late Archean to Paleoproterozoic (ca. 2780-1860 Ma) source region.The age spectrum of the detrital zircons is similar to that of the late Archean and early Paleoproterozoic basement rocks previouslyidentified in the Yangtze Craton, but includes a population of Paleoproterozoic grains (ca. 2400-2100 Ma) which have no known source region in the exposed Yangtze Craton.