Weatheringofcharnockitesandsedimentproductioninthe catchmentareaoftheCauveryRiver,southernIndia A.Sharma,V.Rajamani * School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110 067, India Received2May2000;accepted30October2000 Abstract In the Cauvery catchment area, charnockites are making high hills compared to gneisses. In situ, static weathering of a charnockiticbouldershowslesserextentofchemicalweatheringrelativetothatdevelopedintoapro®le.Thisdifferenceisseen bothinclaymineralcomposition,theirREEpatternsandintheA±CN±KandA±CNK±FMtriangularplots.Basedonthe chemical changes relative to Ti, three groups of elements are recognized. Ba, Si, K, Na, Sr, Ca and Mg show increasing depletionastheweatheringintensityincreasesinlistedorder.Alremainedrelativelyimmobile,whereas,FefollowedbyMn, Cr,Ni,REEandPshowavariableextentofenrichmentwithincreasingchemicalweathering.Thesedimentoccurringonthe foothillsofthecharnockitehillshaveaveryuniformcompositionandtexturewithoutanydepositionalfeatures.Reworkingof thesesedimentsina®rstorderstreamhascausedonlyminorcompositionalchangesbythelocalsegregationofheavyminerals onthechannelbedandbytheremovalofsmallamountofclaymaterialsassuspendedload.TheREEpatternofthereworked channelbedsandisnearlyidenticaltothatofthechannelwallsediments,butwithlowerby25%)abundance.Althoughthe rocks are .2500Ma and, now located in tropical latitudes and probably had experienced several periods of warm and wet climate,thesoilsandthesedimentsderivedfromthemarechemicallyveryimmaturesuggestingtheirexposuretodenudational processesveryrecentlybytectonicuplift. q 2001ElsevierScienceB.V.Allrightsreserved. Keywords:Charnockites;Weathering;Cauverycatchment;Geochemistry;Neotectonics 1. Introduction Rock weathering and subsequent transport of weathered materials by ¯owing air and water are important geological processes that control the morphology of the earth's surface. Weathering acts through symbiotic physical, chemical and microbial processes and results in the conversion of rocks and mineralsintosoils,sedimentsanddissolvedsolutesof naturalwaters.Chemicalcompositionofclasticsedi- mentsisshowntobeafunctionofseveralgeological, climaticandbioticfactorsaffectingtheweatheringof thesourcerockstosoilsDuddy,1980;Suttneretal., 1981; McLennan, 1989; Cullers and Stone, 1991; Berner, 1992; White and Blum, 1995; Nesbitt and Young,1996).Speci®cally,thenatureandtheextent of source rock weathering, physical sorting during transport Sawyer, 1986; Nesbitt and Young, 1996; Singh and Rajamani, 2001) and environment of deposition exerts signi®cant control on sediment geochemistry Taylor and McLenan, 1985; Milliken and Mack, 1990; Fralick and Kronberg, 1997; McCann,1998).However,geochemicalfractionation attendant on weathering of rocks under different SedimentaryGeology1432001)169±184 0037-0738/01/$-seefrontmatter q 2001ElsevierScienceB.V.Allrightsreserved. PII:S0037-073801)00102-6 www.elsevier.com/locate/sedgeo * Correspondingauthor.Fax: 191-11-616-5886. E-mail addresses: rajamani44@hotmail.com V. Rajamani), vrm@jnuniv.ernet.inV.Rajamani).