島根大学地球資源環境学研究報告 24，53～58ページ（2005年12月） Geoscience Rept．Shimane Univ．，24，p．53～58（2005） 國Majoranαtraceelementabundancesin＜180and180－2000μmfractionsofstreamsediments fromtheHiiRiver，ShimanePref◎cture，Japan Edwin Ortiz＊and Barry Roser＊ Abstr3ct Stream sediments collected from4710calities in the Hii River of Shimane Prefecture were hand sieved to separate the＜ 180μmand180－2000μmfractions．M司orandtracecompositionsofbothfractionsineachsampleweredeteminedbyX－ ray fluorescence analysis．Elemental abundances contrast significantly between the two size fractions．Average abundances of SiO2，K20，Ba，and Rb are greater in the l80－2000μm fractions，whereas the remaining elements are generally more abundant in the＜180μm fractions．Loss on ignition values are also greater in the＜180μm fractions． Average compositions of both fractions show similar nomlalized distributions to average upper continental cnlst．The greatest depart皿e from upPer continental cnlst composition is observed in the＜1801αm ffaction，with enrichment for several ferromagnesian elements（V，TiO2，Fe203t，Sc）and for Zr，Y，and ThJn both fractions，Nb，CaO，Na20，Sr，Rb， MgO andNi＆re slightlydepletedrelativetoUCC． Key words：Stream sediments，Hii River，geochemistry，major and trace elements and specific discussion will be published at a later date．The IntrO（IUCtiOn dataset presented here is part of a regional geochemical Although elemental abundances in the composition of database for the northem San’in region．This project began stream sediments reflect the geochemical characteristics of with the study of the Kando and the Hino Rivers（Ortiz and lithotypes present in their catchment，they are also Roser2003；Ortiz and Roser2004a，b）and greatly increases innuenced by a variety of processes that operate between （by over ten times）the density of geochemical data the source and the sediments produced．Such processes available for stream sediments in the area． include weathering，size sorting，heavy mineral placering effects，and the effects of addition of detritus from Lithology ofthe Hii River Basin geochemically disparate source rocks．The factors involved in these processes have been the subject of numerous In the regional context the distribution of lithological studies（e．g．Basu et al．1990；Johnson，1993；Ohta et al． units in the Hii River drainage basin is relatively simple． 2004；Avila et a1．2005）because they provide information Cretaceous to Paleogene felsic to intennediate granitoids crucial for the interpretation of phenomena of geological and volcanic rocks extend throughout most ofthe catchment and environmental interest． （Fig．1）．Granitoids occupy the m勾ority ofthe central basin， This article reports X－ray fluorescence（XRF）analyses of whereas less extensive volcanic rocks are concentrated ＜180μmand180－2000μmfractionsof47streamsediment towards the southem edge of the watershed．Lithotypes samples collected from the Hii River．The Hii drainage among the plutonic rocks include granodiorites，granites， system is located in eastem Shimane Prefecture，and drains granitepolphyries，granophyresandquartzdioritesto an area of about930km20ver a main channel length of gabbros，whereas the volcanic rocks are mainly dacite to approximately82km，stretching from the headwaters in rhyolite lavas and pyroclastics （Editorial Board of Mount Sentsu to the westem end of Lake Shir“i（Fig．1）． GeologicalMap ofShimanePrefecture‘‘EBGMSP”1997）． The Hii River supplies a m司or proportion of the sediments A second group of lithological units including Miocene entering the Lake Shinji－Lake Nakaumi coastal lagoon volcanic and sedimentary rocks，principally of the Hata， system．The historical，cult皿al，socia1，environmental and Kawai，Kuri，Omori，and Fujim Formations，crops out economic importance of this area has been reviewed by along the westem boundary and in the northeastem Tokuoka et al．（1998）． extremity ofthe drainagebasin（Fig．1）．The main lithotypes The pu耳）ose of this report is to present the data obtained arerhyolitetodaciticlavas，andesites，andpyroclasticrocks， by XRF analysis，along with a brief description of the along with locally derived sandstones，conglomerates and general variations of elemental abundances in two size shales（EBGMSP1997）． fractions of sediments from the Hii River．More extensive The characteristics and interrelationships among most of theunitscroPPingoutinthisdrainagebasinhavebeen ＊Dept．ofGeoscience，Shimane University，Matsue690－8504，Japan described in a number of studies（e．g．Iizumi et a1．，1984， 53 M司orandtraceelementabundances in＜180and180－2000μmfractions ofstream sediments 54 from the Hii River，Shimane Prefect皿e，Japan Oonφ｝ 嶋｝。φφ｝ ’＼、、8 STUDY ￥ ￥￥ 6 SEAOFJAPAN ￥ 、 ￥ 、、 、、 ￥、 AREA 、 MlHO BAY ■ 35030， o 35030， 、 ￥ 婚 愚㍉、、 、、 、￥ 羅 、 LAKE ￥ ￥￥ 鎚……齢 ～ NAKAUMI 、、 ’ ミミミゆミ畿N 、 ￥、 ￥￥ ￥、 ￥￥ LAKE g ：．￥＼（ ￥￥ SHlNJ1 ぎヤ ヤヤ ヤヤ ヤヤ ヤペ ヌヤ ペペ ヤヤ ヤ ヒ ペ ヤ ヤ ヤ ヤヤ ヤヤ ヤ ．‘￥ 、 、 ．’、￥ ￥￥ ￥、 、 ￥￥＼賦￥． ヤ ヤ ペヤ ニ ii魏…、聯 Q＼hミ・ミ”モミ c莇：’製 灘、輝 ￥ミ・ 舞iiil・……… ￥ ￥ ： 覇灘溜鐡㎜ キ 、 、￥ ：．． ￥ ヤヤ ペヤ 、 ￥￥ ヤヤ ヤ ニいぎ V〉… ￥ ￥ 漉ン離’儀一撚層￥’ 羅 ＝血：：＝＝．…層一≒一…一 8 ，・曙 掘3謎 ’4 R：：：騨一・一一 35020， ]襯、 ・認45 4 ・認 8 謙 43 37 4 一 ．・ 4 峨膏．一．職灘攣 15 39、・ v 〜」汐鴇諦燐榊葉：．．…一・ 囎、 、、、 v 〉 ・隔 勢 、 ・： 、 VV 」 “蓋．2 ．． ，そ 》濯 」 塗 、撚．“」亀継 φ 、 “ ． V の ミキ 23『．＝一一 一可2二 鰻 騒難 @ 2 22 @ 、、 R騰張護附鶴難 ￥ 、 『40 9 @ 5… @ 18」 雛嚢議 @蕪鱗・■し：．冊．・．■ ， 、〉）y 禦王塾……モ上皇繋累。1 ・簸 6 3 6 、 V 洋 、、＝〉 35010， 10・愚 35010， 4 嶋，ooり“，｝ 11 12 繋一 藻9 灘聯・Q§・・ ，〜〉》 》 V 》V7 磨r麟 〉 一6》 鱗 VV yVv 〉〉〉 V Vv v》 V Ooの。り甲 〉 》v｝ 〉 ｝v・ v 》） 秘 vV〉 鳩 v しEGEND 》Stream ●10Samplingsite 澱 Omori，Fujina，Matsue Fm ・……Drainagedivide 彫 Kawai－Kuri Fm 匿蜀A”uvialdep・sits 匪動 Hata Fm 012 5 10Km …ヨWakurayamaAndesite 翻 Granitoids ㎜ Alkali basalts，dolerite，rhyolite（R）鰯 Felsic Volcanic Rocks Fig．1．Map showing thegeneralizeddistribution oflithotypes in thecatchmentofthe HiiRiverand location ofsample sites．Geologybasedon the1：200，000geological map ofShimane Prefecture（Editorial Board ofGeological map ofShimane Prefecture，1997）． Kagami et a1．，1992；Yamauchi and Yoshitani，1992；Kano 2004．Weather conditions were clear and dry，and stream et a1．，1994；Roser et al．，2001）． flows norma1．Sub－samples were collected with a plastic water scoop from active channels over a length of20－50m， Sampling and SamplePreparation andthen storedinplasticzip－topbagsas single representative samples foreach site． ．F’ε14Sα即ling Selection of sampling sites was made with the aim of Sampling was carried out on five days during autumn achieving a unifom distribution that would cover a11 Edwin Ortiz and Barry Roser 55 1ithological units present in the draimge system．Locahties were selected from1：50，000topographic base maps on Hii River 一一怦鼈黷P80－2000μm fraCtiOn 10 Fraction average！UCC 一一Z一一く180μm fraction which geology ffom the．1：200，000geological map of Shimane Prefecture（EBGMSP1997）had been overlain．At 1 a few localities samples could not be collected due to difficulty of access or paucity of sand－grade sediment，and sites were displaced slightly f士om those originally intended． 0．1 Location of the sampling sites is given in Figure1．Overall sampling density was one sample per l9．7km2． 0．01 Nb Pb U K Ca Zr Th Na Sr Rb Mg La Ce Si Al Ba Sc Y Fe Ti Ni Cr V Fig．2．Multi－element plot showing the average composition Sα即16Pr8四嬬ion （anhydrous normalized）of＜180and180－2000μm fractions from Bulk samples were dried at80－90℃the day after the Hii River（data from Table3）normalized against the Upper collection，and subsequently hand－sieved to remove pebbles Continental Crust（UCC）average ofTaylor＆McLennan（1985）． Elements are arranged from left to right in order of increasing and clasts coarserthan2mm．Average weight ofthe＜2mm normalized abundance in average Mesozoic－Cenozoic greywacke fractions was993g．These＜2mm fractions were split （Condie1993）relative to UCC，following the methodology of （mostly into eighths or quarters），and an appropriately sized Dinelli et aL（1999）．The m勾or elements are normalized as split was then passed though an83mesh stainless steel oxides． sievetoseparatethe＜180and180－2000μmffactions．On average，the＜180μm fraction comprised12wt％of the＜2 had LOI values＞5wt％，whereas in the180－2000μm mm bulk materia1．Only in four samples did the＜180μm fractions values were considerably lower，with most fraction exceed20wt％．The＜180μm fraction formed a samples＜2wt％，and only two exceeding5wt％． particularly large proportion （70wt％）of the sample Summary statistics for the fractions from analyses collected at site31． normalized to100％LOI－ffee are given in Table3．Contrast Both fractions were then crushed separately in tungsten in average elemental abundances between the fractions is carbide mills，applying the procedures described in Ortiz significant．Abundances of most of the elements analyzed and Roser（2004b）．Sub－samples（10－12g）of the crushed aregreaterinthe＜180μmaverage，exceptfor SiO2，K20， fractions were then placed in glass vials and dried for at Ba，andRb，whichhavehighervaluesinthe180－2000μm least24hours at110℃prior to dete㎜ination of loss on fraction．Although there are several anomalies，the average ignition（LOI）． composition over the entire suite is similar to that of average upPer continental crust（UCC），as shown in Figure 2．The180－2000μmfractionaverageisslightlydepletedin AmlyticalMethods most elements relative to UCC，and only shows slight LOI was estimated for each sample from the net weight enrichment in Pb，Ce and V．The pattem is relatively Hat loss after ignition in a muffle fumace at1000℃for at least overa11，and greatest depletion is seen in Nb，Ni，Cr，CaO twohours．Ignitedmaterials were retumed to glass vials and and MgO．In contrast，the＜180μm fraction average is a held at110℃for at least24ho皿s before the preparation of little less regular，exhibiting