Detrital Zircon U-Pb Geochronology of the Early Paleozoic Strata in Eastern Guangxi and Its Constraint on the Caledonian Tectonic Nature of the Cathaysian Continental Block

A comprehensive U-Pb geochronological study of detrital zircons from the Cambrian and Devonian strata in Dayaoshan-Damingshan area,eastern Guangxi has been conducted.The strata contact through an unconformity surface,indicating an early Paleozoic depositing hiatus in the region.The Cambrian detrital zircons are characterized by age spectrums of 2.7-2.6,~1.8,1.4-1.1 and ~1.0Ga with minor Meso-to Neoarchean grains as well as hydrothermal event records of 250 and 105Ma.The Devonian detrital zircons,only few of which were formed in Paleoarcheozoic age,were dominated by ~1.7 and 1.0Ga grains and recorded a pronounced thermal event at ~100Ma.The geochronology suggests that the study area possessed a Cathaysian attribute in continental accretion and tectonic evolution during the Pre-Devonian,and thus infers that the continental boundary between the Yangtze and Cathaysian blocks was located northwestern to the Dayaoshan-Damingshan terrain.Contrasting characters in geochronology of the Cambrian and Devonian strata indicate a shift in sedimentary provenance after the depositing hiatus.However,zircons with timing intervening the Cambrian and Devonian are absent,which suggests that Caledonian magmatism was minimal or lacking in the region and its proximity.Combined with previous studies,it is inferred that Caledonian folding was likely resulted from an oblique collision between the Cathaysia and a continental block which had moved apart later.This collision also resulted in a development of passive basin in Dayaoshan-Damingshan area by extruding along the vertical direction of pressing.Isotopic resetting and hydrothermal reaction are thought to account for the Mesozoic ages recognized in a number of detrital zircons.This observation suggests that large scale convecting hydrothermal fluids were involved in the Indosinian and Yanshanian magmatic and tectonic activities.Thus the convecting hydrothermal liquids are regarded as one of the key factors resulting in the mineralization of the late Mesozoic Au-polymetallic ore deposits in this region and plausibly along the whole southeastern margin of the South China fold belt.