Bulletin of the Geological Survey of Japan，voL35（11），p．547－563，1984 552．3：550．4（83）

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Moyra GARDEwEG＊，Shunso IsHIHARA＊＊，Yukihiro MATsuHIsA＊＊＊， Ken SHIBATA＊＊＊＊an（1Shigem TERAsHIMA＊＊＊＊

GARDEwEG，Moyra，IsRIHARA，Shunso，MATsuHlsA，Yukihiro，SHIBATA，Ken and TERA－ smMA，Shigem（1984） Geochemical studies of Upper Cenozoic igneous rocks from the Altiplano of Antofagasta，Chile．，翫〃．Gεol．S麗7v．1卯αn，voL35（11）， p．547－563。

Aめ謝滅3Magnetic susceptibility，tin，rubidium and strontium contents，and oxygen and strontium isotopic ratios were measured for UpPer Cenozoic igneous rocks from the Guatiquina area，Altiplano of Antofagasta，northem Chile。The rocks are mainly andes－ ites，dacites an（1ignimbrites of calc－alkaline series，and their equivalents of subvolcanic rocks．The analytical results were examined in view of contribution of crustal material to the magmas at a distructive continental plate margin． Magnetic susceptibility of the studied rocks are mostly between600and1，600× 10｝5SI，corresponding to the lower half of the range for the magnetite－series rocks．An E1：Laco andesite，which contains many phenocrysts of polygonal and some cubic magnetite，has the highest value of2，066×10－5SI．Some of the rocks，including both e丘usive and subvolcanic rocks，from the area north of latitude230S may possibly belong to the ilmenite－series． The Sn contents of the rocks are generally as low as1。1to3。7ppm．Sn－rich igneous rocks equivalent to“tin－granite”do not widely exist in the area．Therefore， southwestem prolongation of the Bolivian tin belt to this area is not plausible． Theδ180values an（1initial87Sr／86Sr ratios of the rocks range from十7．8to十11．2 permilandO．7069toO．7131，respectively。Apositivecorrelationisobservedbetween the180／160an（187Sr／86Sr ratios，while the87Sr／86Sr ratios increase with decreasing in Sr concentration．These results suggest involvement of180－and87Sr－rich crustal material in the andesitic magmas．General tendency of increasingδ180and87Sr／86Sr ratio toward north is observe（1in the studie（1area with the highest values at around230S．This accords with that the ilmenite－series a伍nity of the rocks is observed in the area north of23QS．Contribution of crustal materi＆1to the andesitic magmas would be large in the area around23。S．This may be attributed to either the cmstal thickness under the area or tectonic environment in which the magmas have formed，or both．

northem zone in southern Colombia and 恥廠o伽面o聰 Ecuador（50N－2。S），a central zone in south－ em Peru，southwestem Bolivia，northem Chile Late Cenozoic volcanism of the An（1ean and northwestem Argentina（160－280S），a orogenic belt is restricted to three zones： a southem zone in southern Chile and Argen－ tina （33。一520S）． The （班ferences in bulk ＊Servicio Nacional de Geologla y Minerla，Casilla 10465，Santiago，Chile and trace element compositions among these ＊＊R．esearch Planning O伍ce，＊＊＊Mineral Deposits zones suggest that the magma generation Department， ＊＊＊＊Geochemistry ＆ Technical processes are much more complicated in the Services ：Department， Geological Survey of Andean continental plate margin than those Japan，Higashi1－1－3，Yatabe，Tsukuba，Ibaraki， 305Japan along intra－oceanic island arc systems．

一547一

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The Central Andes where the studied area The present paper reports new geochemi－ is locate（i is characterized by two main groups cal data of UpPer Cenozoic igneous rocks of of volcanic roc：ks：composite stratovolcanoes the Guatiquina area，in the High Andes of of mainly andesitic－dacitic composition and Antofagasta，Ch虹e，including and．esites，ig－ large volume of ignimbrite sheets of dacitic nimbrites，glassy dacite－rhyolites and subvo1－ and rhyolitic composition．As in most parts canic rocks，Some of them were taken from of the Andes，the volcanics of this zone are isolated bodies but others，1ike the ign血brites mainly of calc－alkaline association，but in the and porphyries of Cerros Chamaca，Arenoso， north of latitu（ie 260S parallel to the Ch五e－ Chivato Muerto and Corral de Coquena，ap－ Peru trench，there is a continuous gradation pear to be genetically related．Magnetic sus－ toward east from calc－alkaline rocks to a ceptib量ity and tin content of all the ava1【able shoshonitic volcanic association（MuMzAGA samples were determined，because of the prox一 and MARIN’ovl6，19791DERuELLEラ1981）． ㎞ity of the stu（iied area to the“Bolivian t血 Together with this gradation，the volcanics belt”and their（iistinct relationship to t血ore vary systematically in their chemical and iso－ deposits of granitic amnity （IsHIHARA an（玉 topic compositions with SiO2，K20，Rb，87Sr／ TERAsHIMA，1977）．Age and petrochemic＆l 86Sr increasing from west to east（HARMoN’ characteristics of the subvolcanic rocks of the 8オα1．，1981）．The Central Andes volcanics studied area made us think of a possibility of are generally enriche（i in K20，Rb，87Sr／86Sr fin（iing a southwestem prolongation of the and180，as compare（i with the Upper Ceno－ tin belt． zoic volcanic rocks of both northern and southem Andes． Ge⑪畏09董輪且Se舗期g o置Ce蹴甑鍾A翻es Volcanism of the Central Andes has been for a long time＆matter of controversy with The widely extended Upper Cenozoic vo1－ regard to the petrogenesis of these rocks， canic rocks of the Central Andes hi（ie the especially in relation to the parental magma previous geological history of the area。The under the destructive plate margin along volcanic activity has been developed along which they are developed。During the last the Westem Cord皿erαand the Altiplano。 昼ve years，the number of trace element and The main centers of eruption are along the isotopic studies of these rocks has markedly bor（iers between Chile and Bolivia and Chile 血creased。 Nevertheless， the reported 87Sr／ and Argentina （Fig。1）．Toward the east， 86Sr ratios an（1 δ180 values differ not only eruptions took place all the way across to between those of the northem and southem the foothHls of the Eastern Cor（i皿era in Peru， Andes（FRANcIs8厩Z．，19771EAwKEswoRTH Bolivia and Argentina． ααZ．，19791MuEHLENBAcHs and STERN， The Upper Cenozoic volcanic materials， 1980），but also with血nearby areas in the together with some sedimentary clastic conti－ Central Andes（HAwKEswoRTH6如Z．，1982）， nental series， have been deposited over and even within the same volcanic complex strongly folded，faulted and eroded Mesozoic （JAMEsασZ．，19763FRANcIsααZ．，1980）． and．Lower Cenozoic rocks（：LAHsEN，1982）． Most of these studies have been done with Few outcrops of Precambrian rocks have been samples of isolated points，frequently study－ foun（l un（ier younger rocks in the volcanic ing one or two volcanic complexes．They chεしhl of southern Peru，Bolivia and northern may represent a very small portion of the Chile（PAccIααZ．，1980），while in northem large volcanic chain of the Central Andes． Argentina large outcrops of Precambrian and Theh2 apParently contra（iictory results and LOWer PaleOZOiC Sedimentary and igneOUS conclusions lea（i us to think necessity ofεし（i（ii－ rocks underlie an（i surround the young vo1－ tional and more detailed works in the volcanic canic rocks。The occurrence of Precambrian cha。in． rocks west of the volcanic belt in the Penlvian

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Fig．2 Bolivian tin belt（shaded area）and polymetallic Ag－Pb－Sn＿Sb－Cu mineralizations （triangles）occurring to the south．After GRANTα召！．（1976）．

coastラ and east of it， under the Bolivian changes of the subduction zone determine（1 Altiplano suggests that the volcanic chain of by offsets and changes in dip of the seismic the Central Andes is underlain by Precam－ zone（SwIFT and CARR，1974）．SILHToE brian and／or Paleozoic “continenta1” crust （1974） propose（i that the longitu（iinal sub－ （CoBBING an（I PITcHER，1972 血 SILHToE， divisions of Chile and their boun（iaries are 19721CoIRAααZ．，19821ALLMENDINGERα comparable to the transverse geological fea－ αZ．，1983）．The thickness of this crust under tures in Japan and are also coincident with the volcanic centers is of approximately discontinuities on the underlying deep seismic 70km in southem Peru and northem Chile， zone and reflect transverse boun（1ary zones where the Cenozoic volcanics rest on the cen－ between separate segments of oceanic litho－ tra1“kee1”of the Andes（HARMoN8オαZ．， sphere which are subducte（l as in（1ividual 1981，Fig．1）。 units，pe士hapsevenatd近erentrates。 Although the Upper Cenozoic volcanic The volcanic activity of the Central Andes prod．ucts of Central Andes are considered＆s shows a continuous development since Mio－ one geological unit（Fig．1），N－S variations cene unt皿today（LAHsEN，1982）．It has been in the onset，evolutionラintensity and hthology characterized by the eruption of extensive of the activity are locally observed along the sheets of ignimbrites and the development of volcanic belt（LAHsEN，1982）．These vari－ large composite stratovolcanoes・of apProxi－ ations and other major tectonic changes along mately the same age．There are also sma11 the Cord皿era de los Andes correlate with intrusions of acidic subvolcanic bodies that

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are insigni且cant in volume but are interesting big groups can be recognized： 1） andesites， because of their relation with some important 2）glassy rhyolites an（i dacites．The andesites mineral deposits（8。g．，Bolivian tin porphy－ have variable amounts of plagioclase and ries）．At Poquis near the Ch丑e－Argentina pyroxene phenocrysts，opaques an（i apatite as border，polymetallic mineralization（Sb－Sn－ minor mineral phases．丁紅e older andesites Ag－Mn－Pb－Cu）is known associated with a （GMC－16）also have quartz，homblende Miocene rhyolitic porphyry（TRAvIsANY and an（l biotite phenocrysts． In the Jachi vo1－ DIAz，1978）．S㎞丑ar mineralization is also cano the andesites have big rounded quartz known to occur in the Argentine side（Fig。2）． xenocrysts and pyroxene clots。The andesites from the EI Laco volcano have opaques as Geo亙ogy盟観S躍p亙eDesc顧P伽n phenocrysts and amygdules of massive cha1－ ce（iony（？）． The andesites of the Hua丑itas The Guatiquina area is located in the Cen－ volcano occasionally contain big olivine tral Andes in the Altiplano （or Puna）of phenocrysts with reaction rim and plagioclase－ Antofagasta，Chile （Fig。3）。It is formed pyroxene－opaques clots。The groundmass is mainly of volcanic rocks of Upper Tertiary， usually glassy．Interbedded with these andes－ under which only small outcrops of older ites some玉ithic welded tuffs are recognized rocks are exposed in erosion windows． （GARDEwEG，in prep。）．The glassy（lacites Ma血e clastic sediments of Ordovician age and rhyolites have Pliocene ages（GARDEwEG crop out near the Chilean－Argentinian border an（1RAMIREz，in press）and they form domes in the Poquis area（MARINovl6，1979）， like Cerro Chamaca and Loma Negra，1ava south of the Curutu volcano（MARINovI6α domes like Cerro Bola（2．7±0．2Mal K－Ar αZ。，1976）and northwest of the Guaitiquina on biot．） or r血g structures like Corral de ：Pass（GAR．cIAααZ．，1962，Fig。3）．Younger Coquena（4．4±05Mal K－Ar on biotite） marine sed㎞ents of Cretaceous age and and Cerros de Jarellon where black and， continental sediments of Tertiary age have brownish red obsidian also appears（Fig．3）． also been recognized（GARDEwEG and RAMI－ The subvolcanic rock：s are bodies of irregu－ REZ，in press）。 lar shape，usually covered by ignimbrites or The Upper Cenozoic volcanics can be lavas。 They are porphyries of da，citic and divided into three main rock types：ignim－ rhyolitic composition whose ages range from brites as very extensive sheets，1avas associ－ Upper Miocene to Lower Pliocene（Table1） ated with stratovolcanoes and small eruptive an（1they become older from west to east．In centers，and subvolcanic porphyries。 They Poquis，nea．r the Bohvian border，crops out the are mainly andesites an（i dacites that belong oldest quartz porphyry（12。88±0．46Mal to calc－alkaline association although the K－Ar on biotite）with big quartz，sanidine eastemmost samples（Fig．3）present chemi－ and biotite phenocrysts in a groundmass of cal characteristic similar to those of shosho－ quartz and feldspars with small crystals of nitic association（MuMzAGA and MARINovI6， zircon，apatite and opaques．It has metallic 19791GARDEwEG and RAMIREz，in press）． mineralization associated and is locally highly The ignimbrites have Pliocene ages（4．1± altered（MARINovl6，1979）．West of Poquis， O．4Mal4．5±0．1Mal4．8±0．6Mal K＿Ar in the Cordones Quilapana and Ceja Alta on biotites）and dacitic composition（GARDE－ （8．0±2。O Mal：K－Ar on plagioclase）the wEG and RAMIREz，in press）。In general they subvolcanic outcrops are altered dacitic por－ ha』ve big Phenocrysts of quartz，PIagioclase， phyries with big phenocrysts of plagioclase biotite and usually some homblende，pyrox－ and biotite oxidize（i or replaced by chlorite enes，，sphene，zircon，opaque minerals and and sericite aggregates．The groun（imass is pumice fragments of dif［erent sizes and shapes． very fine an（i usually argillized．East of the Among the stratovolcano－related lavas，two Salar de Aguas Calientes（Fig。3）crop out the

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youngest subvolcanic rocks（4．8±：0。2Ma三 K－Ar on biotite）．They are three subrounded Ma騨電ic S胆sce凶嚇琶y bodies （Chamaca，Arenoso and Chivato Muerto）of porphyrytic texture and dacitic Magnetic susceptibility of the ava五able composition with big phenocrysts of quartz， samples were measure（1by a Geoinstruments Plagioclase， red biotites an（i sphene 量n the ky TH－1meter and shown in SI unit（Table groundmass of variable textures，but frequ－ 1）。Opaque minerals were observed on thin ently spherulitic。The bo（iies are petrographi－ sections and three samples were studied on cally very sim虹ar to the ignimbrites of the polished sections。 area and probεしbly genetically relate（1． Magnetic susceptib且ity of the studied rocks Among these rocksラ35samples mainly of are mostly between600an（口，600×10一5SI． lavas and porphyries were chosen for geo－ This range is lower than that of the thole廿te chemical studies．Their locations are shown series of the Hachijoj㎞a volcanic rock：s and in：Fig。3and then main char＆cteristics and higher than that of the ilmenite－series and the analytical results are liste（l in Table L intermediate－series of the Setouchi volcanic

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Sample No． Rock type Locality Alteration Sio2

GMC－1 Glassy dacite Bola hill Fresh 63．31 －3 Dacitic porphyry Cor（ion Qui1εしpana Limonitization，chlorite，altered K－feld，calcite，silicification 一5 Welded tu昼（ignimbrite） Pampa Redonda Fresh 61．61 一Rhy －6 ・1iticbandedand Jarellon hills Limonite stained 73．09 spherulitic glass

一6－1 Rhyolitic obsidian 〃 Loca，11y devitrified 73．12 －7 Quartz porphyry （rhyo1．） Poquis Oxidized biotite，groun（1mass slightly 71．49 altered 一10 Dacitic porphyry Cordon Ceja Alta（N） Limonite，clay minerals and sericite 67．78 in groundmaSS

一11 〃 〃 Slightly altered，similar to GMC－1 68．45

＿12 〃 〃 Fresh plagiocla，se，chlorite in groun（1－ 68．29 maSS 一16 Acidic andesite 〃 （S） Fresh，except for oxidized biotite 63．54 －19 Welded tu∬（ignim．） Arenoso hi11（N） Relatively fresh，hematite 65．74

－20 Quartz porphyry（dacitic） 〃 Relatively fresh，hematite 67．83

－21 〃 〃 （E） Fresh 66．94 －22 Welded tu任with pumice 〃 （SE） Fresh 66．88 fragments（ignim．） 一23 Quartz porphyry（dacitic） Chivato Muerto h皿 Altered in certain degree，very low 63．07 －24 Glassy dacitic dome Chamaca h皿（NE） Relatively fresh，hematite 69．15 －26 Welded tu仔（ignim．） 〃 （SE） Fresh（but not completely） 68．28 －28 Spherulitic rhyolite Corral de Coquena Fresh 74．66 －121 Quartz porphyry Poquis ～GMC－7 69．43

PG－119 Glassydome Loma Negra（N） Oxidized biotites and pyroxenes 69．89 －126 Orthopyroxene an（i clino－ Jachi Small amount of calcite an（11imonite 59．49 pyrOXeneandeSite stained on groundmass

一127 〃 Jachi Fresh，only small amount of chlorite 59．87 －200 Pyroxene andesite La．co volcano Fresh，except for some amygdules 60．89 of undetermined composition 一211 Pyroxene and olivine Huailitas volcano Fresh 58．07 andesite

一212 Glassy pyroxene an（iesite 〃 Small amount of limonite in fractures 62．57 －214 Acidic andesite Morado hill Oxidized hornblende 62．15 －217 Lithic welded tuff Huailitas volcano Fresh，but with some rock fragments 68．00

－221 Orthopyroxene and clino－ 〃 Very small amount of chlorite in 59．85 pyroxene andesite amygdules

一222 Pyroxene andesite 〃 Fresh 62．72 －225 Lithic welded tu登（～PG－217） 〃 Fresh，but with some rock fragments 68．37

－227 Pyroxene an（1esite 〃 （S） Sma111imonite veins 60．17 －229 Pyroxene an（lesite 〃 Abundant opaques 63．29 －230 Pyroxene an（iesite 〃 （tOP） Fresh 61．78 －231 Fe glassy andesite 〃 （top） Limonite stained 57．55

－234 Py』roxene an（iesite 〃 （top） Locally limonitized groundmass and 59．75 pyroxene with opacitic rim

＊Age determined in other sample of the same body，＊＊Average，＊＊＊Assumed age．

一554一 σεo・hε而oαZ5鰯∫θs・ノUρ画Cεnozo∫018nε・配s．R・oんs（σα74εwθ8αα1．）

Cenozoic igneous rocks of Guatiquina area，Antofagasta，Chile．

Rb＊＊『 Sr＊＊ Magneticsuscept．＋』 K－Ar age（Ma） 87RLb／86Sr＊＊ （87Sr／86Sr）P÷（87sr／86sr）o δ180SMow＋ Sn＋ （ppm） （ppm） （％o） （ppm） （×10－5） SI＊＊

2。7±0．2（biot。） 125 269 1．35 一 2．0 1261 － 1．9 1115

4．1±0．4（biot。） 140 362 1．12 － 1．1 1409 222 154 4．17 － 3．4 225

215 130 4．79 － 3．7 245 12．88±0．46＊ 193＋ 255＋ 2．19 0．71348 0．71307 十10．9 1．7 589

156 324 1．39 十7．8 1．5 1186

160 344 1．35 － 1．3 1417 8．0±2。0（plag。） 146 363 1．16 十 7．5plag． L3 1298 十8．5w．r． 117 562 0．60 一 1．6 1032 108 573 0．55 － 1，9 24 （5．0）＊＊＊ 168＋ 273＋ 1．78 0．70988 0．70975 十8．6 1．5 135 163 266 1．77 － 2．0 37 4．5±0．4（biot．） 148＋ 294＋ 1．46 0，70951 0．70942 十8．6 2．0 950

4．8±0．2（biot。） 161 301 1．55 十10．2 1．7 134 （5。0）＊＊＊ 206＋ 220＋ 2．71 0．70973 0．70954 十11．2 1．9 164 4．8±0，6（biot．） 178 218 2．36 － 2．3 1044 4．4±0．3（biot．） 188 105 5．18 － 2．3 447 12．88±0．46＊ 201 232 2．51 － 1．6 732

134 240 1．62 1．6 164 4．9±0．4（w．r．） 89 380 0．68 1．2

（4。9）＊＊＊ 83．1＋ 410＋ 0．587 0．70695 0．70691 十8．3 1．8 1539 （2．0）＊＊＊ 97．1÷ 468＋ 0，601 0．70772 0．70770 十8。2 1．3 2066

（3．5）＊＊＊ 90．8＋ 383＋ 0．687 0．70962 0．70959 十9．1 1．7 1376

168 300 1．62 十7．8 9．8 681 107 390 0．79 2．3 328 168 290 1．68 2．4 1534 4．5±0．7（w．r。） 112＋ 395÷ 0。82 0．70998 0．70993 十10．4 1．9 1562

（3．5）＊＊＊ 162÷ 315＋ 1．49 0，71067 0．71060 十8．7 2．7 840 174 290 1．74 2。7 1069 （3，5）＊＊＊ 127÷ 308＋ 1．19 0．71071 0．71065 十10．7 2．4 1553 157 290 1．57 3．0 1018 154 280 1．59 2．6 1081 129 380 0．98 2．0 1564 114 360 0．92 2．3 624

＋Determined in the GSJ，the rest of the analyses were done in the Servicio Nacional de Geologia y Mineria，Chile．

一555一 BμZZα∫n oノ‘hεσ召olo8∫cαZ5μ7vεly o∫16ψαn，1VoZ．35，ハ～o．11 rocks in Japan （IsHIHARA，1979a）． That is， ite having little magnetite（PG－214）or on the Chilean rocks occupy the lower half of rocks whose matrix is glassy（PG－212，234）， the magnetite－series且eld in magnetic sus－ indicating some of the gτoundmass magnetite ceptibility vs。SiO2plot（Fig。4）。 formed du血g the devit面cation。 Subvolcanic rocks which are the oldest in Lithic welded tuf王s interbedded with the the stu（iied areas are divi（ied into three groups andesites have high magnetic susceptibility in their magnetic susceptib丑ity。The oldest， （1069，1534×10－5S亙）．These rock：s contain 13Ma group in the Poquis area has an inter－ abundant phenocrysts of magnetite and the mediate value of589－732×10－5SI．The biotite color is greenish brownl thus showing nextgroup of8Mainthe Cordones Qu丑apana tmely magnetite－series character． and Ceja Alta areas has the highest values Among lava dome felsic rocks，fresh one as1115－1417×10－5SI．The youngest5Ma （GMC－1，1261×10－5SI）has high magnetic group at Arenoso and Chivato Muerto has susceptibility and contains phenocrystic mag－ the lowest values as37－135×10－5SI． netite．But low ones（GMC－6，6－1ラ225－ The highest group（GMC－3，10，11and 245×10－5SI）contain only few magnetite 12）contains many polygoml magnetites as phenocrysts and the groundmass is glassy。 phenocryst and also且ne crystals in the ground－ Welded tu仔s are highly magnetic as950－ mass．But color of the biotite is Z≒Y＝ 1409×10－5SI。 The rocks contain many reddish brown．The intermediate one con－ phenocrystic magnetite and Z≒Y color of the tains lesser amount of phenocrystic magnetite biotite is greenish brown to brown and some which is primary and also secondary mag－ （GMC－5ラ19）have reddish brown color． netite which formed after decomposition of Most of the studied rocks have magnetite biotite by alteration（GMC－7）or only且ne as phenocryst，so that they may be called crystals of magnetite in the groundmass magnetite－series rock：s．However，their mag－ （GMC－121）。The lowest group（GMC－20， netite contents are less than those of typical 21and23）also contains phenocrystic mag－ magnetite－series rocks and Z≒Y color of the netite but has corroded outline．This mag－ biotite is often reddish brown．Thus，they netite is converted to hematite by alteration have an intermediate character between typi－ to lower the magnetic susceptibility。The cal magnetite－series and ilmenite－series rocks． biotite of Z≒Y color is abnormally reddish Although further deta丑ed studies need to be brown which may be partly due to limonite done，some of the studied rocks may possibly sta血ing。 belong to ilmenite series。They are quartz The subvolcanic rock：s contain genera皿y porphyries at Poquis（GMC一一7，121），andes－ magnetite and the variety of the magnetic ites at Morado H皿and glassy rhyolite dome susceptibility is resulted from the origiml low at JareUon H丑1（GMC－6，6－1）l a1110cated content of magnetite （Poquis rocks）and at north of latitude23000／S． hematitization over magnetite by hydro－ thermal alterations （Arenoso and Chivato 丁量醜Co鵬e説s Muerto）． Among stratovolcano－related rocks，the El Trace amount of tin in igneous rocks and Laco andesite has the highest value （2066× its relation to Sn mineralization are very well 10－5SI）．This contains many phenocrysts of established in granitic terrains （IsRIHARA an（1 polygonal and some cubic magnetite．Most TER．AsHIMA，1977）．Even in volcanic areas of the others are also high in the range of where the Kurok：o type（leposits occur，Sn－ 840＿1，600×10－5SI． These rockls contaj㎞ rich volcanic rocks are observe（I relate（1 to magnetite as phenocryst and且ne crystals in cassiterite－bearing massive sulfide deposits in the groundmass．A few of the low values the Canadian shield，wh且st tin is very poor （624－681．×10－5 SI） are obta血ed on andes一 in volcanic rocks around the Sn－free Kuroko

一556一 G60chεアn∫oαZ5伽6漉s o∫U卯召7C8nozo∫018nεoμs Rocんs（σα74θwε8「αα1・）

10 息

！ ＼ ／ ＼ ＼ ／ ＼ ／ ＼ 8 ／ ／ CGnqdlqnConGdion Sn－relq†edSn relo†ed ／∠二＿Precqmbriqn＿volconic＿rocks＿／ ／ －－ 6⊆ の ／ ／ 1 JGpon 〆 5Qu騨eSヤeて“ E ／ ／∠＿＿＿一＿＿／ α ／ α ／一一一一r〆 曳てeいd O｛ 4 ／鯉、、ne ／ 8 ㌦．詮 △ ブ 盛 可「「 十 濡蘭 皇一彪 塩 Kuroko 2腔 十・ JGpqnese 十holeii†e †rend

50． 60 70 80 ％ SiO2 Fig．5 Tin vs．SiO2content of the studied volcanic rocks．The areas for the Japanese and Canadian volcanic rocks are taken from IsHIHARA an（1TERAs田MA（1982，1983）．Symbols are the same as those of Fig．4．

き deposits in Japan（IsHIHARA an（i TERAsHIMA， not represent the original values because of 羅

1983）． the hydrothermal alteration observed on these 馨 In the studied areas， tin contents are porphyries。In the Akenobe subvolcanic tin

generally as low as L1－3。7ppm（Table1）． Iodes area in southwestem Japan，two con－ き This range is similar but slightly higher than trasting values are seen on fresh rocks （avg． 講 that of volcanic rocks relate（i to the Kuroko 6。O ppm）and altered rocks（avg。1。7ppm， mineralization in Japan（Fig．5）．Among the TERAsHIMA and IsHIHARA，1982），and．the studie（玉 rocks， the Hua且itas an（iesites are tin is considered to have been leache（l out somewhat enriched in tin as andesite，giving through the hydrothermal alteration due to rise to an average of32ppm（nニ9）ラbe－ meteOriC Water CirCUlatiOn dUring a deUteriC cause of particularly high value of9。8ppm stage。 on a glassy andesite（PG－212）。If this rock The general paucity of tin ill the Chilean is excluded，the average is2。3ppm（n＝8）． rocks in（iicates that Sn－rich igneous bo（1ies The Jachi andesites are especially、10w in Sn equivalent to“tin granite”do not widely exist （average15ppm）．Among other volcanic in the studied area．It folows that Sn－rich rocksラrhyolite dome of the Jarellon Hill gives continental crus，t which wou1（l have been a high values as3。4and3。7ppm，which could source for the Bolivian Sn－carrying igneous well be an銭menite series． rocks（ioes not continue to the south，into the All the subvolcanic porphyries are depleted northern Chile． in tin as1。3－2．O ppm．The values are much lower than those of porphyries in the Bolivian ⑪xyg艦韻西s鵬聡重置囎亘so重o睡¢ tin belt of2。7to2LO ppm Sn （average C丑亘ar2駐c苞e買茸s琶量cs 85PPm，nニ5ラIsHIHARA and TERAsHIMA， unpublished data）．The cassiterite－minera1－ Oxygen isotopic ratios （180／160） were ized porphyries at Poquis are not particularly determined for14samples including6sub－ emiched in tin。Eowever，these analyses may volcanic rocks and related ignimbrite，and8

， 一557一 β那ZZα∫n oノ‘h8σεolo8∫cαZ3配7vθy o／1αραn，1VoZ、35，1〉o。11

12

懸 H 十 3 螺＋芸 器 IO α’me耐e－ H ≧ serles2 騰 ○ L騰H △＋←甚 Σ J翻騰 （ノ） 8 0 璽 一線黒砿鷹㌧羅鞠 ％ 6

ソ。ICσηlcr。CκS。f」σρση se「le5 4

50 60 70 80 ％SiO2

十 Subvolcqnic rOcks 懸Lqvqs △Welded十uffs 醗GlqssyrOcks Fig．6． δ180 vs．SiO2 content of the studie（1 rocks． Letters with the data points for lavas indicate the stratovolcanoes from which the lavas came out： J，JachivolcanolL，Laco volcanol and H，Huailitas volcano．Data for altered rocks are indicated by asterisk． The compositional ranges of Quatemary volcanic rocks，and ilmenite－series and magnetite－series granitoids of Japan were taken from MATsuHIsA（1979）and MATsuHlsAααZ．（1982）．

volcanic rocks．The analytical method is de－ parison．It is obvious that the rocks from scribed in MATsuHlsA（1979）．The data are the Guatiquina areaラexcept for a glassy rock presented in terms ofδ一notation relative to （PG－212）and an altered subvolcanic rock SMOW．A plagioclase sample separated from （GMC－10），are distributed in the compo－ GMC－12dacite porphyry was analyzed to－ sition range of the ilmenite series granitoids， gether with several whole－rock samples of havingδ180values of十8to十11per mi1． altered subvolcanic rocks（GMC－10，12and These values are comparable to those of an－ 23），a㎞ing at detecting hydrothermal activity desites from northem Chile and northwestem after crystallization・ Argentina reported by HARMoN6短Z・（1981）， 87Sr／86Sr ratios were determined for10 but higher than those for Ecua（lor （HARMoN’ fresh whole－rock samples for which oxygen ααZ．，1981） and central and southern Ch皿e isotopic data were available．The analytical （LoNGsTAFFE8厩1．，19831MuE肌ENBAcHs method is the same as described in SRIBATA and STERN，1980）． The high δ180 values εごαZ．（this volume）．The present ratios were suggest involvement of180－rich crustal ma－ corrected to the initial values by apPlying the terial in the andesitic magmatism． observed or assumed K－Ar ages of the rocks． Theδ180values of the altere（1subvolcanic ：Fresh whole－rock samples would provide iso－ rocks GMC－10，12and23range from十7。8 轟 topic compositions of their magmas and sub－ to十10。2per mi1．Among those，GMC－10 畿 sequently constraint on their source materia1． and12are from a subvolcanic complex，Cor－ 建 The δ180 values are plotted against wt don de Ceja Alta．There is no distinct cor－ ｛ percent SiO2 0f the rocks in Fig。6． The relation between isotopic composition and composition ranges of Quatemary volcanic apParent degree of alteration。 Hlowever， rocks and the magnetite－and ilmenite－series plagioclase of GMC－12has aδ180value of granitoids from Japan（MATsuHlsA，19791 十75per mi1，being l per mil lower than the MATsuHIsAααZ．，1982）are shown for com一 whole－rock value． Since plagioclase is the

一558一 G60chα痂oαZ S砺4∫8s o／U卯67C8nozo∫c18nθo配s Rooんs（Gα74εw88ααZ．）

mineral which reacts most read且y with ther－ relation with Sr concentration（：Fig。7）．Since mal waters in respect of oxygen isotopes，its magmatic processes do not produce any 180－depletion suggests hydrothermal activi－ signi且cant isotopic fractionation of strontium， ty with low－180 waters （consequently of these results suggest contamination or ass㎞i－ meteoric origin）after crystallization．It is lation of Sr－isotopically d置erent material to supposed that potential mineralization might the magmas．This relationship is interpreted exist relating to this subvolcanic complex。 by either mixing of a mantle－derived com－ The initia1 87Sr／86Sr ratios range from ponent of low87Sr／86Sr and．high Sr with O．7069to O。7131，and show a negative cor一 a crustal component of higher 87Sr／86Sr an（110wer Sr （FRANcls αα1．，1980），or combined processes of wa11－rock assimilation O，715 an（i fractional crysta11並ation （BR．IQuEu an（l LANcELoT，19791DEPAoLo，1981）． ○．713 十 HARMoNε緬Z．（1981）discussed combined ＼ ass㎞ilation－fractional crystallization models ＼H ○．7H for the An（1ean lavas on theδ180－87Sr／86Sr o 嘩H H diagram，in which end－member compositions

qり ⑧ ＋△＼讃 り were taken asδ180＝5．7，87Sr／86Sr＝0．703 ① ○．709 ＼H ＞ for hypothetical mantle andδ180＝19，87Sr／ ρ ＼ 灘L し 86Sr＝0。735for isotopically evolve（l cnlst． In 囮J＼ O。707 their models the ratio of Sr concentration in the end－members（mantle：cmst）was assumed to be5：L The present results also show a O．705 positive relationship betweenδ180and87Sr／ 86Sr （Fig。8）。 Therefore，roughly speaking， O．703 I OO 200 300 400 500 they accord with the mixing model of low一 ppm’Sr δ180，一87Sr／86Sr mantle－derived material with

Fig．7 （87Sr／86Sr）o vs．ppm Sr of the studied high一δ180，一87Sr／86Sr crustal materia1．How－ rocks．Symbols are the same as in Fig．6． ever，the slightly convex downward．curvature

12

＠ I I ／灘H ン 灘 H ／ l O ／ 分 ／ ぷ ／ ／ シ ／鹸H ／ Σ 9 の ／ ／ の ／ ／ ／ ○ ／レー！パ十 H 囲 里 」鰹 り 8 斎 L

7

6 O．705 O，707 0，709 O．711 O．7［3 （87Sr／86Sr）。

Fig．8 δ180 vs．（87Sr／86Sr）o of the stu（1ied rocks． Symbols are the same as in：Fig．6．

一559一 βμZZα’n o1∫hθσεolo8’6αZ S配7vεy o／1αραnンγoZ．35，ハ10。11

rather gra（iua1， reεしch血g the highest values

12 （q） of O．710to O．713 at around230S． The high 許H ⑱ increase in 87Sr／86Sr at230S is strengthene（i 3ぷ H，1 by a quartz porphyry from Poquis（GMC－7）． l O ≧ This tren（1accor（is with the overall variation o ＼ Σ of the87Sr／86Sr ratios of the Quatemary calc－ の 購H＼氏騰～灘 ○ 里 8 ⑲ 」 し alkaline rocks証ong the Andean belt：the 幼 highest values in the border area of Peru， Bolivia，Chile and Argentina，decreasing to－ 6 wards north and south along the Andes （KLERKxααZ．，19771McNuTTααZ．，19751 （b） 十 O．713 JAMEs ε∫ αZ．， 19765FRANcls εオ αZ．， 1977； 1

＼ HAwKEswoRTHααZ．，19791THoR．PEααZ．， O O．711 H騨H 19791FRANclsααZ．，19801HAwKEswoR．TH

（∫） 騰＼ ε短1．，1982）．The variation of isotopic com－ 8 H醸H 嘘 ＼ ＼ positions of the volcanic rocks may reHect （∫）」 O．709 お ＼ thickness of the crust along the Andes，which ＼ 鰯L acts as the contaminant to the magmas。 ＼＼ O、ア07 羅 HAwKEswoRTHαα1．（1982）reported high J 87Sr／86Sr ratios of O．7069 to O．7095 for the volcanic rocks from the Cerro Purico area， O．705 22。5σ 23。Oσ 23。3σ 24。Oσ which is Iocated west of Cerro Hua五itas Lq†1†ude， degrees sou†h analyze（i in the present study （Fig。1）， It is Fig。9 Plots ofδ180（a）and（87Sr／86Sr）o（b）vs． noticed that these volcanoes are aligned in 1atitude south of sample locations。Data for east－west direction，crossing the general trend altere（i rocks are not included．Symbols are the of the Andean volcanism．A transverse fault same as in Fig．6． system crossing the sub（1uction zone may exist along this line，an（1provide conditions favor－ of the postulated mixing line suggests that the able for melting of180－and87Sr－rich crustal Sr－concentration of the crustal end－member materia1． would be comparable to or slightly higher than that of magma．This means that the s灘m町我翻Co聰d翻o皿 conta㎞inating material would be igneous rocks of the o1（ier crust． The negative cor－ The Upper Cenozoic igneous rocks in the relation between Sr concentration＆nd87Sr／ Guatiquina areaラAltiplano of Antofagasta， 86Sr ratio would be interpreted by combined nortkern Ch丑e，are divided into tぬree main processes of ass㎞ilation and fractional crys－ rock types：（1）ignimbrites as very exten－ tallization in this case（DEPAoLo，1981）． sive sheets，（2）1avas associated with strato－ General tendency of increase inδ180and volcanoes and（3）small eruptive centers and 87Sr／86Sr toward north is observed in the subvolcanic porphyries。They are mainly an－ studie（i area except for glassy rocks （Fig． desities and． dacites of calc－alkaline series 9a，b）。Glassy rocks may not maintain their with a minor amomt of shoshonitic associ－ pr㎞ary isotopic compositions because of the廿 ation in the eastemmost part of the area． 1ess resistance to deuteric processes． The The ignimbrites are dated at4．1to4．8Ma δ180values remain virtuaHy constεしnt at about by the：K－Ar method．The lavas of the 十8per mil in the area of24。to23030／S， stratovolcanoesarealsodatedats㎞近arages， then sharply increase to 十11per m五at although the number of age－determinations is around 230S． The increase in 87Sr／86Sr is very limited．The ages of the subvolcanic

一560一

繋 θθochε〃2i6αIS1配4iεso∫のPαCεnozoiclgnεoμsRo6んs（σα74ε肥9ε1α1．） rocks are comparatively variable，ranging from systematics and cmstal contamination mo（1－ the youngest of4。8Ma to the oldest of els for calc－alkaline igneous rocks． Eαπh 12．9Ma determ五ned by the K－Ar method． PJαn6∫．5『c∫．L（3π．，vo1．43，P，385－396． Magnetic susceptib皿ity of the studied rocks CoIRA，B．，DAvIDsoN，J．，MPoDozls，C。and：RAMos， V．（1982） Tectonic and magmatic evolu－ are mostly between600and1，600×10－5SI． tion of the Andes of Northem Argentina This range corresponds to the lower half of and Chile．Eαπh So’．R6v．，vo1。18，P。303－ the magnetite－series且eld in the magnetic sus－ 332． ceptib最ity vs。SiO2 Plot given by IsHIHARA DEPAoLo，D．J．（1981） Trace element and isotopic （1979aブb）， Among stratovolcano－related effects of combined wallrock assimilation rocks，an EI Laco andesite，which contains and fractional crystallization． Eαπh PZαnθム many phenocrysts of polygonal and some Sα．Lεπ．，vo1．53，P。189－202． cubic magnetite，has the．highest value o£ DERuELLE，B．（1978） Calc－alkaline and shoshoni・ 2066×10－5SI．Some of quartz porphyries， tic lavas from 且ve ノ㎞dean volcanoes （be－ andesites and glassy rhyolites from the area tween latitudes21。45／an（124。30’S） and north of latitude230S may possibly belong the distribution of the Plio－Qua．ternary vo1－ to the翌menite series． canism of the south．central an（1southem An（1es．10zぴ．Volo．σεo∫h87η3，R6s，，vo1．3， The Sn contents of the stu（lied rocks are p．281－298． generaly as low as1．1to3。7ppm。Sn－rich F踏Ncls，P．W．，MooRBATII，S．and THoRpE，R．S。 igneous rocks equivalent to “tin－granite” do （1977） Strontium isotope data for：Recent not widely exist in the studied area．There－ an（1esites in Ecuador an（1 north Chile． fore，southwestem prolongation of the Bo－ Eαπh PZαnα．Sc∫．L8π．，vo1．37，P．197－202． 1ivian tin belt to this area does not seem ，THoRpE，：R．S．MooRBATH，S．，KRETz－ plausible。 scHMAR，G．A．HAMMILL，M。（1980） Stron－ Theδ180values and initial87Sr／86Sr ratios tium isotope evidence for crustal contami－ of the rocks range from十7。8to十11．2per nation of calc－alkaline volcanic rocks from mil and O．7069to O．7131，respectively。These Cerro Galan，northwest Argentina．翫πh results suggest involvement of180－and87Sr－ P1αηα．5（ゴ．．乙8π。，voL48，P。257－267． GAR．cIA，F．，P首REzシE．y CEBALLos，E．（1962） El rich crustal material in the andesitic magmas。 Or（10vicico de Aguada（1e la Perdiz．San－ General tendency of increase inδ180 and tiago，Chile．R8v．ハ4∫nε7α1θs，vo1．77，P，52－ 87Sr／86Sr toward north量s observed in the 61． studied area with the highest values at around GARDEwEG，M．y RAM：IREz，C。F．（in press）Hoja 23。S．This accords with the fact that the Rio Zapaleri Regi6n de Antofagasta．Ser－ ilmenite－series a伍nity of the rocks is observed vicio Nacional de Geologla y Minerfa， in the same area．Contribution of crustal Carta Geo16gica（ie Chile． material to the andesitic magmas would be GRANT，J。N．，HALLs，C．，AvILA，W．and AvILA，G． large in the area aroun（123。S．This may be （1976） Igneous geology an（1the evolution attributed mainly to the crustal thickness of hydrothermal systems in some sub－vo1－ under the area，and partly be due to a pos－ canic tin deposits of Bolivia， 7010αnごo P70－ c6s38s ∫n O78 σεn6s∫s。 Special publication， sible transverse fault system crossing the sub－ no．7，Geo1．Soc．London，P．117－126． duction zone． HARMoN，R．S．，THoRPE，R．S．and F踏NcIs，P．W． （1981） Petrogenesis of Andean andesites Re£ere聡ces from combine（10－Sr isotope relationship． ALLMENDINGER，R．W．，RAMos，V．A．，JoRDAN，T． ハ1α∫祝78，vo1。290，P，396－399。 G．，PALMA，M．an（i IsAcKs， B．L。 （1983） HAwKEswoRTH，C．J．，NoRRY，M．J，，RoDDlcK，J．C．， Paleogeography and Andean structural geo－ BAKER，P．E．，FRANcls，P．W．and THoRpE， metry，Northwest Argentina。 Tεαon∫03， R． S． （1979） 143Nd／144Nd， 87Sr／86Sr， and vo1．2，p，1－16。 incompatible element variations in calc－ BR．IQuEu，L．an（l LANcELoT，J．R．（1979） Rb－Sr alkaline an（iesites an（1 Plateau lavas from

一561一 B配ZZεだn oノォhθσ60108’αzZ Sμ7v召y o／1αραnク701．35，〈70．11

South America．Eαπh PJαn8∫．50∫．Lεが．， 2g o south． Eαπh PZαnθ！．So∫．L6π．，vo1．27， vol．42，P。45－57。 P．305－313． RAwKEswoR．TH，C．J．，HAMMILL，M，，GLEDHILL，A MARINovl6，N．（1979） Geologia de los Cuadr義ngu－ R．，vAN CALsTEREN，P。 and RoGERs， G。 10s Zapaleri y Nevados（ie Poquis，II Regi6n， （1982） Isotope an（1trace element evi－ Antofagasta．ハ4召n¢o吻ραm oP卿αZ1伽Zo dence for late－stage intra－crustal melting in 4θ8ε6Zogo，Depto．de Geo1。，U。de Chile。 the High Andes。Eαπh PZαn8乙30∫。L6∫1。， ，DfAz，F．，RAMIREz，C。E y TRAvlsANY， vo1．58ラP．240－254． A．（1976） Prospecci6n y evaluaci6n de IsHIHARA，S．（1979a） Kappameter KT－3 and its elementos polimetゑ1icos en la Alta Cor－ application for some volcanic rocks in Ja． dillera de la II：Regi6n，Antofagasta，Chile． Pan． BκZl・ σ80Z。 S泥7v。 1αρan， vo1． 30， 1n46融011σ． p．513＿519． MATsuHlsA，Y．（1979） Oxygen isotopic compo・ （1979b） Lateral variation of magnetic sitions of volcanic rocks from the East Ja－ susceptibility of the Japanese granitoids。 pan islan（1arcs an（1their bearing on petro－ 10房7。σ80Z。Soo。1αραn，vo1。85，P．509－523． genesis． 10z〃．70Z6。σ60∫h67解。R6s．，vo1．5， and．TERAsHIMA，S．（1977） The tin con－ p．271－296． tent of the Japanese granitoids and its ，SAsAKI，A．，SHIBATA，K and IsHIH：ARA，S． geological significance on the Cretaceous （1982） Source （iiversity of the Japanese magmatism。10乙‘7．Gε01．500．∫αPαn，vo1． granitoids： ！㎞ O， S． an（1 Sr isotopic ap－ 83，p．657－664． proach． ∠〔わs∫7ααs 5オh ln陀7nαオ∫onαZ Con∫。 and （1981） Tin abundance of G60ch70n．Cosη¢06h70n．1so’oPθG召oZ．シNik－ CenozoicvolcanicrocksinJapan．肋sオ． ko，」’apan，P．239－240． 1ss祝8， 1981 1！17CE1 5yn3．，・！望7c 70Zoαn∫sηz， MuE肌ENBAcHs，K．and STERN，C．R．（1980） p．144－145。 δ180variation among calc－alkaline volcanic and （1983）Genesisoftin rocks from S．Chile：Implications for the miner＆1ization in kuroko mineralized belts： effects of ridge subduction on the source Japan an（i Cana（1a． ハ4in∫ng G60Z。 5ρ80． region of orogenic volcanism．EOS，vo1。61， 1ss祝6，no。11，P．103－110． p。401。 JAMEs，D．E．，BRooKs，C．and CuYuBAMBA，A．（1976） MuNIzAGA，F．and MARINovl6，N．（1979）Eviden－ Andean Cenozoic volcanism：Magma gene－ cias preliminares de un volcanismo shosho－ sis in the light of strontium isotopic com－ nitico Cenozoico superior en el area ｛1el position an（i trace element geochemistry。 volcan Zapaleri，A．Regi6n Chile。14ααs B配Zム σ召oZ。Soo．！歪規．，voL 87，p。592－600， Cong7．G80ム Ch惚70，Arica－Chile no。3， KLE舐x，∫．，DEuTscH，S．，PIcHLER，H：．and ZEIL，W． E237＿E255． （1977） Strontium isotoPic comPosition and PAccl，D．，MuMzAGA，F．，HERv重，F．，：KAwAsHITA，K． trace element data bearing on the origin of and CoRDAM，V．（1980） Acerca de la Cenozoic volcanic rocks of the central and e（1a（i Rb／Sr precambrica de rocas de la For－ southem Andes．1．701c．Gεo1h召7規．R6s．， maci6n Esquistos de Be16n，Departamento vo1．2ヲP．49－71． de Parinacota，Chile． Rεv．σ80」．Ch∫1ε，vol． LAHsEN，A．（1982） Upper Cenozoic volcanism and 11，p，43＿50． tectonism in the Andes of northem Chile． RAMIREz，C．F．y GARDEwEG，M．（1982）H：oja Eαπh 56ム R6v1ε”s，vo1。18，P．285－302． Toconao，Regi6n de Antofagasta。Servicio LoNGsTAFFE，F．J．，CLARK，A。H．，McNuTT，R．且． Nacional de Geologia y Minerla．Carta and ZENTILLI，M．（1983） Oxygen isotopic Geo16gica de Chile．no54，121p． compositions of Central An（1ean plutonic SmBATA，K．，IsHIHARA，S．and ULRIKsEN，C．E． an（i volcanic rocks，1atitudes26。一29。south． （1984） Rb－Sr ages and initia1 87Sr／86Sr Eαπh P1αnθ！。So∫．Lθπ．，vo1．64，P．9－18． ratios of late Paleozoic granitic rocks from McNuTT，R．H，，CRocKET，」．Hl．，CLARK，A．H．， northern Chile． B配〃． G60Z． S祝7v． ∫αραn， CAELLEs，J．C。，FARRAR，E．，HAYNEs，S．J． vo1．35，P．537－545． an（l ZENTILH，M．（1975） Initiεし187Sr／86Sr SILLIToEラR．H．（1972） Relation of metal pro－ ratios of plutonic and volcanic rocks of the vinces in westem America to subduction Central Andes between latitu（1es26。and of oceanic lithosphere． β躍1。 G60Z． 500．

一562一 （7600hθn7iαzl S1〃4iεs o∫ こンρ1）ε～ Cεnozoic lgnεo魂s Rooんs （（7α74θwε9 ε1 αム）

∠［ηz，，vo1。83，P．813－817． GθoZ．，vo1．32，P。73－76． （1974） Tectonic segmentation of the THoRpE，R．．S．，FRANcls，P．W．and MooRBATH，S． Andes：implications for magmatism and （1979） Rare earth and strontium isotope metallogeny。 ハ7α魏γε，vo1．250，P．542－545． evidence concerning the petrogenesis of SwIFT，S．A．and CARR，M．」．（1974） The seg－ north Chilean ignimbrites．E碗h P1αn召ム mented nature of the Chilean Seismic Zone． 50あLθπ、ラvo1．42ラP。359－367． Phy3．Eαπh P’αnα．1n‘．，vo1。9，P．183－191． TRAvIsANY，V．y D鼠z，F．（1978） Estud．io geo－ TERAsHIMA，S．and IsHmARA，S．（1982） Tin Abun－ 16gico econ6mico y geoqufmico del sector dance in the Cretaceous granitoids around Nevados de Poquis． 11σ，in8鼠 the Akenobe mine ＆rea，Japan． ハ4ごn加g

チリ，アントファガスタ・アルティプラノの上部新生代火成岩類の地球化学的研究

M．ガルデヴェク・石原舜三・松久幸敬・柴田 賢・寺島 滋

要 旨

北部チリ，アントファガスタ・アルティプラノのグァティキナ（Guatiquina）地域の上部新生代火成 岩類について，帯磁率，Sn，Rb，Sr含有量，及びO，Sr同位体比を測定した．岩石は，主としてカ ルク・アルカリ岩系の安山岩，デイサイト，イグニンブライト，及びそれらと等価の貫入岩類である． 測定結果に基づき，大陸プレート縁での火成作用における地殻物質の寄与を検討した． 帯磁率は大部分600－1，600×10『5SIの範囲で，磁鉄鉱系列の岩石の値の範囲の下半に相当する。多く の磁鉄鉱斑晶を含むEI Laco安山岩は最も高い2，066×10一5SIを示す．南緯23。以北の岩石は，恐ら くチタン鉄鉱系列に属する． Sn含有量は一一般に1。1－3。7PPmと低く，“tin－granite”に相当するSnに富んだ岩石の分布はみら れない。そこで，ボリビアのスズ・ベルトのこの地域への延長の可能性は少ない． 岩石のδ180値及び87Sr／86Sr初生比は，それぞれ，十7。8～十11・2パーミル及び0・7069－0・7131 の範囲である．180／160比と87Sr／86Sr比の間には正の相関がみられ，87Sr／86Sr比はSr含有量の減少 につれて増大する．これらの結果は，180と87Srに富む地殻物質が安山岩マグマに混入したことを示 唆する．調査地では北に向ってδ180値，87Sr／86Sr比が増大する一般的傾向がみられ，南緯23。付近で 最大となる．このことは，南緯23。以北で岩石がチタン鉄鉱系となることと調和的である．安山岩マグ マヘの地殻物質の寄与はこの地域で最大となるのであろう‘地殻物質の寄与の程度は，地殻の厚さや， マグマが形成される場の地質構造に依存していると考えられる．

（受付：1984年7月12日；受理＝1984年8月9日）

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