Provenance analysis of Cambrian sandstones in the western margin of Yangtze block:Implication for paleogeography reconstruction and tectonic setting
During the Cambrian period,a set of clastic and carbonate rock associations was mainly deposited in the western margin of the Yangtze block.These sedimentary deposits have commonly been interpreted as representing a vast and stable craton sourced from older rocks in the Oldland.However,the widely contemporaneous magmatism recorded within the sediments is obviously inconsistent with the craton basin identified earlier.Therefore,the tectonic background needs to be determined based on the material sources of the western margin of the Yangtze block.Field outcrops and other data are used to determine the source areas of the Cambrian sediments in the western margin of the Yangtze block through heavy mineral analysis,tourmaline electron probe microanalysis,and detrital zircon U-Pb dating of three sandstone samples from Huize and Huidong counties.We then discuss the tectonic background of the Cambrian period in combination with a comprehensive analysis of sedimentary successions.These successions show that the Canglangpu,Xiwangmiao,and Erdaoshui formations are mainly composed of sandstone and dolostone,which were deposited in shore and tidal flat environments.The sandstones are fine-to coarse-grained,with subangular to subrounded and poorly sorted fabric.Quartz is the dominant constituent,while lithic fragments consist mainly of cherts,and feldspar grains are very rare.The detrital heavy minerals are mainly zircon,hematite-limonite,tourmaline,ilmenite,rutile,and apatite,suggesting a magmatic rock source.The results of tourmaline electron probe microanalysis further confirmed that the Cambrian sediments are mainly derived from granitoids,their associated pegmatites and aplites,metapelites,and metapsammites.Detrital zircon dating provided valuable insights into the age of the parent rocks in the provenance area,indicating a distribution ranging from 983 Ma to 540 Ma.Comparisons of the geochronological data with detrital zircon ages from different blocks reveal that the sediments were derived from 663～540 Ma rocks of the East African orogeny,as well as the underlying old sediments(dated 983～708 Ma)in the Kangdian Oldland.Contrary to previous studies suggesting a simple craton setting for the Cambrian period,our analysis of detrital zircon spectra indicates an active continental margin setting during this time.The sedimentary successions,detrital zircon spectra,and detrital grain framework collectively suggest that the Cambrian sediments were formed in a collisional setting,especially a foreland basin.
detrital heavy mineralprovenance analysisCambrian sedimentscollisional settingforeland basinwestern margin of the Yangtze block
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