Tracing Source Rocks Of The Late Permian Karstic Bauxite In The Western Guangxi By Geochemistry Of The Sedimentary Rocks From The Heshan Formation

Provenance of the karstic bauxite deposits worldwide is difficult to be constrained since they are products of extremely weathered rocks.The immobile elements,especially REE and Al2O3/TiO2 ratio are generally used for the constraints on source rocks of karstic bauxite deposits.However,the so-called immobile elements could be remobilized during extreme weathering process.Usage of the immobile elements in examination of provenance of the bauxite deposits can not gain a credible result.Karstic bauxite deposits worldwide are commonly interbedded with some clastic rocks that are less weathered than the bauxites,so that the compositions of the clastic rocks may have important bearings for the sources of bauxite deposits.This dissertation tries to trace provenance of karstic bauxite deposits in western Guangxi by geochemistry of interbedded clastic rocks from the Heshan Formation.The bauxite deposit in western Guangxi is one of important base of bauxites in China,which includes unique combination of karst type and accumulation type.The karstic bauxite deposits are studied intensively and lay a solid foundation for studying provenance of the karstic bauxite deposits.There are four different models for source rocks of the karstic bauxite deposits in western Guangxi(i.e.,Limestone of the Maokou Formation,old land,basalts and acidic rocks of the Emeishan large igneous province),but provenance of the karstic bauxite deposit remains hotly debated.The Late Permian karstic bauxite deposits in western Guangxi are interbedded with clastic rocks within a thick Paleozoic carbonate sequence.The bauxite ores are closely associated with clastic rocks of base part of the Heshan Formation and gradual variation of mudstone — claystone — bauxitic-claystone — clayey-bauxite — bauxite is observed in some sections,suggesting they were derived from the same source.Thus,less weathered clastic rocks that are commonly interbedded with the bauxite deposits are targeted in this study.This dissertation presents new geochemical,geochronological,and isotopic data for clastic rocks and detrital zircons associated with bauxite deposits in western Guangxi.These data are combined with a paleogeographic reconstruction to constrain the provenance of the deposits and discuss some implications for element mobility in extreme weathering and regional geological evolutions in Paleozoic.The U–Pb ages of detrital zircons from both three bauxitic samples and three clastic rocks exhibit a Gaussian distribution,peaking at ~ 260 Ma.The similarity between the whole-rock ?Nd(t)and zircon U–Pb–Hf–O values of the bauxites and clastic rocks,suggesting they were derived from the same source.The U–Pb ages of detrital zircons from both three bauxitic samples and three clastic rocks exhibit a Gaussian distribution,peaking at ~ 260 Ma.This peak age is within error of the sedimentary age(259.8 ± 0.4 Ma)of the bauxite ores in.The peak age,together with abundant fragments of volcanic quartz crystals in the clastic rocks,suggests the bauxite deposit was mainly derived from volcanic rocks.The clastic rocks yield Al2O3/TiO2 values of 25 ~ 132 and ?Eu = 0.23 ~ 0.58,showing arc-like REE trends,indicating the source rocks were felsic.All clastic rock are characterized by high TiO2/Zr,and versus Nb/Y diagram,clastic rocks fall within the rhyolites or dacite,therefore the results are consistent with the origin of acidic volcanic rock.The ?Nd(t)values of the bauxite ores and clastic rocks typically range from-7.6 to-3.5,and a narrow range of TDM2 model ages of 1318 Ma ~ 1652 Ma,with 130 out of 140 detrital zircons yielding ?Hf(t)values of-26.7 to-0.6 and ?18O values of +5.6‰ to +10.3‰,consistent with a dominantly crustal provenance.Diagrams of Th/Nb versus Hf/Th and Th/U versus Nb/Hf show that the detrital zircons of the bauxite and clastic rock in this study formed from arc-related(or orogenic)magmatism.This also is supported by the negative anomalies of Nb,Ta and Ti in clastic rock and REE patterns similar to the Permian Hainan island continental arc.This study reveals extensive weathering may have remobilized many elements.The so-called immobile elements in the extremely weathered rock may be remobilized.The bauxites of the present study are characterized by high Al2O3/TiO2 values,convex HREE trends,and positive Ce anomalies relative to the clastic rocks.These characteristics may be caused by the precipitation of accessory minerals during the advanced stages of karstic weatheringThe karstic bauxite deposits in western Guangxi are mainly sourced from felsic volcanic rock of the magmatic arc in the northern margin of the Paleo-Tethys rather than the Emeishan large igneous province.The magmatic rocks with arc-affinity yield ages of 280 Ma ~ 260 Ma are widespread in Hainan Island and North Vietnam,suggesting Permian magmatic arc may be developed in northern margin of Paleo-Tethys.In addition,Late Permian-Early Triassic pyroclastic rocks and tuffs widely distributed in southwest of South China are similar to geochemistry of the karstic bauxite in western Guangxi,further the Paleo-Tethys magmatic arc is developed at the southwestern margin of South China by the northward subduction of Paleo-Tethys plate beneath the South China Block.