petrography and geochemistry of quaternary rocks from the southern volcanic zone of the andes, chile

7/30/2019 Petrography and Geochemistry of Quaternary Rocks From the Southern Volcanic Zone of the Andes, Chile

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Petrography and geochemistry of Quaternary rocks from theSouthern Volcanic Zone of the Andesbetween 41 30' and 4600 'S, Chile

Leopoldo Lpez-EscobarRolf KilianPamela D_ KemptonMichio Tagiri

Departamenlo de Geologla, Universidad de Chile, Casilla 13518, Corre:> 21, Santiago, ChileMineraloglsch-Pelrographisches Inslnul, Universlliil Tbinge" Wilhemslrasse 56,

0-7400 Tbingen 1, GermanyNERC isolope Geosciences Laboralory, Keyworth, Noningham, NG12 5GG, Unned Klngdom

Departmenl 01 Earth Sciences, Ibaraki Universny, Mno 310, Japan

ABSTRAeT

Rocks from thirteen stratovolcanoes, belonging to the Quaternaryfron tofthe Southern Volcanic Zone :SVZ) of the Andes(41 "30'S-46"00'S) are mainly low- to medium-K basalts and basal ic andesites. Andesites and dacites are less abundant,and rhyolites are found only at Chaitn volcano. Ple istocene volcanic rocks range in composition from basalt to dacite, butHolocene volcanic rocks are predominantly basalts and basaltic andesites. In general, stratovolcano-basalts from the41 "30'-46"00'S regio n of the SVZ are geochemically similar to stratovolcano-basal ts from the 37"00'-41 "30'S region, butexhibit a wider ranga of Pb-isotope ratio s which is close to that ofvolcan ic rocks from the 33-37S region. In detail, two typesof basalts, depleted (type-1) and enriched (type-2) in incompatible elements, are distinguished in this -egion of the SVZ.Compared to type-1 basalts, type-2 basalts have a lower degree of olivine zoning and higher FeO(l)/MgO, LalYb, and 87Sr/8"Sr ratios. In type-2 basalts, olivine rarely coexists with augite. Plagioclase phenocrysts in both type of basalts exhibit strongnormal zoning, from Anu3 (cores) to Ans8 (rims).ln spite of being located in an area of presumably thin continental crust, theChaitn rhyolites are geochemically simila rto rhyolites from fu rther north (33-37"S) where the continental crust is thick, butare notably enriched in radiogenic Pb, particularly in 206Pb. Compared to the nearby Michinmahuida basalts, the Chaitnrhyolites have higher Sr-, Pb-, and 0- isotope ratios, but lower Nd- isotope ratios, are notably depleted in Ca, Sr, Eu, Ti, Zr,Hf, y and middle and heavy rare-earth elements, and have lower KlRb ratios. On the basis of their chemical and isotopicdifferences, type-1 basal ic magmas are not parental magmas of type-2 basalts. These differences seem to reflect differentdegrees of partial melting of the asthenosphere followed by contamination of these melts with lowerto intermediate crustalmaterial. Atuppercrustal levels , basalticmagmas evolveto produce intermediate magmas, eitherbyfracti:mal crystallizationcombined with different degrees of crustal contamination o r by mixing with rhyolitic liquids generated b'l partial melting ofcrustal material.Key words: Petro/ogy, Geochemistry, Quaternary volcanism, Southern Andes, Southern Chile.

Revista Geolgica de Chile, Vol. 20, No. 1, p.33-55, 16 Figs . 3 rabies, July 1993.


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34 PETROGRAPHV ANO GEOCHEMISTRV OF QUATERNARV ROCKS FROM THE SOUTHERN VOLCANIC ZONE

RESUMEN

Petrografay geoqufmica de rocas cuaternarias de aZona Volcnica Sur de os Andes entre los 41 30' Y 4600'S,Chile. Los centros volcnicos del frente cuatemario de la Zona Volcnica Sur (ZVS) de los Andes, ubicados entre laslatitudes 41 3O'S 4600'S, son predominantemente baslticos y andestico-baslticos, de contenidos bajos a medios deK. Andesitas y daeltas son menos abundantes que las rocas baslticas y se encuentran riolitas slo en el volcn Chaitn.Las rocas volcnicas pleistocenas varan en composicin de basalto a dacita, pero las holocenas son fundamentalmentebasaltos y andesitas baslticas.En general, los basaltos de los estratovolcanes de la regin comprendida entre 41 30'-46"00'S de la ZVSson geoqufmicamente semejantes a los basaltos de los estratovolcanesde a regin 37 0 00'-41 30'S, perosus razones isot:icas de Pb presentan un intervalo ms amplio de variacin, siendo ste cercano al presentado por lasrocas volcnicas de la regin 33-37' S. En detalle, dos tipos de basaltos pueden distinguirse en esta regin de la ZVS delos Andes: basaltos empobrecidos (tipo-1) y enriquecidos (tipo-2) en elementos incompatibles. En comparacin con losbasaltos del tipo- '" los del tipo-2 presentan un grado ms bajo de zonacin del olivino y sus razones FeO(T'lMgO, La/Yb,Y 87Sr}"Sr son ms elevadas. En los basaltos del tipo-2, el olivino raramente coexiste con augita. Los fenocristales deplagioclasa de ambos tipos de basaltos exhiben una fuerte zonacin normal, que vara de Anu3 (centro) a Ans8 (bordes).A pesar de estar ubicadas en un rea de corteza continental presumiblemente delgada, las riolitas del Chaitn songeoqumicamente semejantes a las riolitas de los 33-37"S (donde la corteza continental es gruesa), pero son ms ricas enPb radiognico, en especial en 2OPb. En comparacin con basaltos vecinos del Michinmahuida, estas rioHlas tienen razonesisotpicas de Sr, Pb y s elevadas y de Nd ms bajas, estn notablemente empobrecidas en Ca, Sr, Eu , Ti , Zr, Hf, yY en tierras raras medianas y pesadas, y presentan razones KlRb ms bajas . Sobre la base de sus diferencias decomposicin, los basaltos del tipo-1 no son magmas parentales del tipo-2. Sus diferencias parecen ref lejard iferen tes gradosde fusin parcial de la astensfera seguido de diferentes grados de contaminacin a nivel de la corteza inferior o media.En niveles corticales superiores, los magmas baslticos evolucionan a intermed ios ya sea por cristalizacin fraccionada,acompaada de contaminacin cortical o por mezcla con magmas riolticos generados por fusin de material cortical .Palabras claves: P6Irologla, Geoqulmica, Volcanlsmo cuatemario, Andes del Sur, Sur de Chile.

INTRODUCTlON

The Quaternary volcanic front of the SouthernVolcanic Zone (SVZ) of the Andes extends betweenlatitudes 33 and 46S, and is a product of thesubduction of the oceanic Nazca plate beneath thecontinental Sou:h American plate. Its northern end isassociated wi:h the intersection of the JuanFernndez Ridge with the Chile-Per Trench and itssouthern end is the triple junction Nazca-AntarcticSouth American plates (Fig.1).

While numerous geochemical and petrologicalstudies have been carried out in the Quaternaryvolcanic front of the SVZ of the Andes betweenlatitudes 330(' and 4130'S (see Hildreth andMoorbath, 198E , and references therein; Tormey etal., 1991, and references therein; Ferguson et al.,1992, and references therein), similar studies arecomparatively scarce between 41 30' and 4600'S(see Stern et al., 1976, and references therein,Onuma and Lpez-Escobar, 1987, and referencestherein; Futa and Stern,1988, and references therein) ,

mainly because of serious logistic problems (Iack ofroads, bad climatic conditions, dense vegetation,massive glaciers).

In the 4130'-4600'S region of the SVZ, theunderthrusting of the Nazca plate beneath the SouthAmerican plate has produced thirteen major volcaniccenters (Fig.1; Table 1). Most of them are basahic incomposition and Pleistocene to Recent in age (Kilianand Lpez-Escobar, 1989,1991). Rhyolites occur onlyat Chaitn volcano. Many of these centers are related,not only to the subduction of the oceanic Nazca Platebeneath the continental South American plate, butalso to the Liquie-Ofqui fault zone, that extends forca. 1,000 km, between latitudes 38 and 47S.

The aim of this study is to present and to discusspetrographical and geochemical data (major and trace elements, and isotopic compositions) obtainedfrom samples recovered at the thirteen major volcanoes located in the 4130'-4600'S region of theQuaternary volcanic front of the SVZ of the Andes .


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L. Lpez-Escobar, R. Klllan, P.D. Kempton and M. Tagiri

Nazca Plate

---- ---. . . - . . . - ~ \

" . - lRise '

......

35

TSVZ

SSVZ

FIG.1. The Southern Volcanic Zone (SVZ) ot the Andes and its three main regions: northern (NSVZ 33"OO'-34"30'S),transition (TSVZ; 34"30' -37"00'S) and southern (SSVZ; 37 -46S).The location ofthe SVZauatemaryvolcanic tront,Chile trench, Chile Rise and the ages ot the oceanic Nazca pi ate (Herron et al.,19B1) are also 3hown.


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TABLE 1. MINERAL COMPOSlTION OF ROCKS FROM THE SSVZ OF THE ANDES BETWEEN 4130' ANO 46OO'S.

Lavatypes Phenocrysts

PI (An) Bt Am Opx Cpx 01 (Fo) Or Q PI (An) Opx

YateBasaltic Andesite +++ (8165) ++ + + (79-72) + ++ (57-49) +HornopirnBasalt ++ (8773) + ++ + (81-70) ++ (70-61)Hualaihu-Basalt ++ (84-69 + ++ + (82-74) +- ++ (59-48 +Huequi-Andesite ++ (72-48) + ++ +- + (76-75) +. + ++ (67-49) +Michinmahuida-Basalt +++ (92-58) + + + (72-69) +- + (84-61) +-Chaitn-Rhl'olite + (43-36) + +- + + + (37-36) +Corcovado-Basalt ++ (92-56) + + + (80-71) +- ++ (57-52)Yanteles-Basalt ++ (90-69) +- + + (82-66) +- + (67-46) +-Melimoyu-Andesite ++ (89-72) +- + +- ++ (53-40) +-Mentolat-Andesite +++ (92-47) +- +. + +- (76) +- + ++ (63-47) +-Cay-Basalt + (93-61) ++ ++ (85-69) + (84-53) +-Maca-Basalt + (89-48) ++ ++ (87-73) + (72-63) +-Hudson-Basalt ++ (86-70) +- ++ + (75) + (58-49) +-

Am=Amphibole; An=Anorthite; Ap-Apatite; B t = B i o t ~ e ; Cpx=Clinopiroxene; F o = F o r s t e r ~ e ; GI=Glass; OI=Olivine; Or=Ore; Opx=Orthopyroxene; PI=Plagioclase; Q=OuartzOuantity: +++ main constituent; ++ frequent; + accessory +- occasionalty; - not detected

Groundmass

Cpx 01 Ap Or

++ + ++ + +

+ ++ + +

++ ++ ++ ++ + ++ + ++ + +++ + +

GI

+-

+-+-+-

+++

+-

-om-iGl:IJ>.":O-zO(j)mO():Om:IJ-zOS'z


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L. Lpez-Escol1ar, R. Kllian, P.D. Kempton and M. Tagiri

MgFa

50

50

Ca

50

Olivine-series

50

Fa

Andesite(Ol-Opx-Hbl)

Mg L - - + - - - r l ~ " " ' = 9 " " : : : - - ' T : ' = " ' : ' = - - . - - - r - - . - - . r - - ~ Fe 2+50~ ~

Type-1 basal ts(Fa 85 -65)

Mg L - ~ ~ L , - - " - ~ . - - - r - - ' - - - r - - . - - - ~ ~50

Fa

37

FIG. 2. Ca-Fe(1)-Mg compos itions of the orthopyroxene, clinopyroxene, olivine and amphibole in different rock types of the88VZ between 41 "30' and 46"00'8, ordered from north to south.


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38 PETROGRAPHY ANO GEOCHEMISTRY OF QUATERNARY ROCKS FROM THE SOUTHERN VOLCANIC ZONE ..

GEOLOGIC OUTLlNE

One of the main structural features of the Andeanregion under study is the Liquie-Ofqui fault zone(Herv et al, 1978; Herv et al, 1979; Herv, 1984;Thiele et al., 1986), which is a dextral transcurrentdislocation (Herv, 1984; Pankhurst et al., in press;Cembrano, 1992), represented by a N-S trending beltof cataclastic and mylonitic rocks. South of 38S, thisfault zone controls a large number of SVZ centers(Herv et al., 1978; Thiele et al., 1986; Fig. 1).

Many centers belonging to the 4130'-4600'Sregion of the Quaternary volcanic front of the SVZ ofthe Andes have Paleozoic to Mesozoic basement,but Yate, Michinmahuida and Mentolat overlieMiocene to Pliocene volcanic formations (Kilian andLpez-Escobar, 1989; 1991). Yate, Hualaihu, Michinmahuida and Hudson show evidence of havingdeveloped Pleistocene calderas. Relicts of the pre-

caldera units are intensely eroded and partiallycovered by material erupted by Holocene volcaniccones. These cones are either developed within thePleistocene calderas or as satellite cones.

Yate, Michinmahuida, Corcovado, Yanteles,Melimoyu, Mentolat, Cay, Maca, and Hudson arelarge composite stratovolcanoes, whose geology isnow under a detailed study. Hualaihu is a smallcinder cone, emplaced in the center of a volcaniccaldera structure. Hornopirn is also a relativelysmall cinder and lava flow center, with almost aperfect conical shape, located 15 km north-east ofthe Hualaihuvolcano. Chaitn volcano is a relativelysmall center, that apparently suf1ered a calderacollapse; it has a resurgent dome and is located 20km south-west of the Michinmahuida volcano.

PETROGRAPHY

The petrography of the studied samples wasdetermined by optical microscope and the chemicalcompositions of the constituent minerals and glasswere obtained by electron microprobe (ARL-AppliedResearch Laboratories) atthe Mineralogicallnstituteof the Tbingen University. A total of 6 electronmicroprobe analyses were made of amphibole, 3 ofbiotite, 289 of plagioclase, 48 of olivine, 15 of opaques,62 of pyroxene, and 12 of glass present in thegroundmass, as well as glass occurring as inclusionsin phenocrysts. Cross sections of the plagioclase,olivine and pyroxene phenocrysts were made withdifferent intervals down to 4 mil. The results aresummarized in table 1 and figure 2.

Thefirst petrographic investigations ofthe 41 30'-4600'S volcanic rocks (Lpez-Escobar etal.,1985 b)indicated that while a clinopyroxene+orthopyroxene+opaques assemblage was common in the northernpart of this region, an olivine+clinopyroxene assemblage was common in the southern parto The currentinvestigation modifies this general view.

Basaltic roc


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L. Lpez-Esrobar. R. Kfffan. P.D. Kempton and M. Tagirf 39

FIG. 3. Strong nomal zoned plagioclase phenocryst of a Michinmahuida basalt (type-2 basalt).

cores and An'2 to Anso in their rims. Normal zoning 01plagioclase is even more marked (An 83 to An4S ) in thegroundmass. Similar zoning is observed in theplagioclase 01 Corcovado and Melimoyu andesites.Oscillatory and reverse zoning are observed in theplagioclase crystals of Huequi andesites and dacites,and in some Mentolat andesites. Alkali feldsparsformed in the groundmass of some basalts, and inYate and Hudson andesites, as a very late-stageproduct of crystallization.

Ortho- and clinopyroxene coexist in most 41 30'-4600'5 volcanic rocks. Figure 2 shows their compositions in the Ca-Mg-Fe2+pyroxene quadrilateral.The orthopyroxene is gene rally hypersthene, beingFe-hypersthene in the Chaitn rhyolites. Theclinopyroxene is normally augite, being diopside in abasaltic sample from Hornopirn and salite in asample from Mentolat. Orthopyroxene commonlyoccurs as a phenocryst phase (1-3 mm), but is alsopresent in the groundmass of Yate, Hornopirn,Huequi, and Mentolat andesites. In most other rocksstudied, orthopyroxene appears as smallxenocrysts.Euhedral clinopyroxene phenocrysts occur in basalts

and andesites from Yate, Hualaihu, Hornopirn,Huequi, Mentolat, Cay Maca vdcanoes (Fig. 4).Coexisting ortho- and clinopyroxenes have similarFelMg ratios in lavas from Yate, HOf'nopirn, Corcovado, Melimoyu, Mentolat and Maca (Fig. 5). Thelowest Fe/Mg ratios occur in orthopyroxenes fromMentolat lavas. Zonations were not clearly observed(Fig.2).

Olivine is presenf in most basalts, basalticandesites and andesites from this region of the SVZ.On the basis of its degree of zonation and associationwith augite, the basalts have been subdivided intotwo types(1 and 2). In type-1 basalts (Hualaihu,Corcovado, Cay and Maca); large euhedral olivinephenocrysts (1-4 mm), showing a comparativelystrong normal zoning (Fo,s-Foes), coexist with largephenocrysts of augite (2-4 mm; Figs. 2, 4). By contrast,in type-2 basalts (Yate, Michinmahuida, Melimoyuand Hudson), olivine phenocrysts have a weakernormal zonation (Fo75-Foss) and rarely coexist withaugite (Fig. 5). In both types, hypersthene coexists indisequilibrium with olivine and augite (dashed lines infigure 2), but in some type-2 basalts (Yate and


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40 PETROGRAPHY ANO GEOCHEMISTRY OF QUATERNARY ROCKS FROM THE SOUTHERN VOLCANIC ZONE .. .

FIG. 4. Large olilline and augita phenocrysts, in a matrix 01 plagioclase microlites, in a basalt lrom Maca vol cano (type-1basalt).

FIG. 5. Nonnal zonad plagioclase phanocrysts, hypidiomorphic augita and hypersthana in a basalt Irom Michinmahuidavolcano ( ~ p e - 2 basalt).


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L. Lpez-Escobar, R. Kilan. P.D. Kempton andM. Tagiri

Hudson), hypersthene coexists in equilibrium withpigeonite. The latter mineral is common in thosetype-2 basafts having holocrystalline groundmass.The Yate, Hornopirn and Michinmahuida andesitesusually contain small olivine xenocrysts with Focontent in the range F0 76 -F066 . The highest Fe/Mgratio is observed in an andesite samplefrom Melimoyu,whose olivine composition is Foso and that of thehypersthene is M9.s (Fig. 2).Opaques are gene rally titanomagnetite and

43

sometimes hematite. Early titanomagnetite phenocrysts have higher TVFe ratios (ca. 0.1) than titanomagnetite crystals present in the groundmass (ca.0.05).

Chaitn rhyolites are highlyvitrophyric, containingonly 5% volume of plagioclase microphenocryststogether with ferrohypersthene and biotite . The Ancontent (An'2-Ana7 ) as well as the degree 01 zoning ofthe plagioclase are low. Apatite and zircon are presentas accessory minerals.

CHEMISTRY

MAJOR ANO TRACE ELEMENTS

The geochemical data 01 32 volcanic rocks of the41 30'-4600'S of the SSVZ are presented in table 2.This data constitute the basis of the followingdiscussion. In addition, other 35 whole roe k analyses(major and trace elements) of volcanic rock wereobtained and are shown in figures 6-13.

Based on the distribution 01 data on a Kp-Si02classification diagram (Fig. 6) and on an AFM diagram(Fig. 7), the analyzed samples are low- to medium-Kcalcalkaline volcanic rocks, with tholeiitic affinities.Afthough they range in composition from basalt torhyolite (Table 2), there is a predominance of basaltsand basaltic andesites, particularly in the Holoceneeones.The basalts showvariable AIP3 concentrationsin the range 16 to 21 wt % (Fig. 8). According to theirhigh AIPa and intermediate alkali contents , basalts01 Corcovado, Maca, Cay, Mentolat and Melimoyu

FIG.6.

+ Yate o C h a ~ n MichlOmahu ida Mentolat Cayo Maca * HudsonK,O versusSiO, diagramlor rocks Irom differenlvolcanlc cen1ers 01 lhe SSVZ be1ween 4130'and 46


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44 PETROGRAPHY ANO GEOCHEMISTRY OF QUATERNARY ROCKS FROM THE SOUTHERN VOLCANIC lONE ..

TABLE 3. COMPOSITIONAL RANGES OF TYPES 1 ANO 2 BASALTS FROM THE SSVZ OF THE ANDES BETWEEN 4130' ANO4600'S COMPAREO WITH THE COMPOSITIONAL RANGE OF THE SSVZ STRATOVOlCANO-BASAlTS BETWEEN 37ANO 4toS.

TYPE-l BASALTS RAHGE'

Si02 48.96Ti02 0.84AI20a 17.95Fe20 acn 8.45MnO 0.13MgO 4.92cae 9.35Na 20 2.70KP 0.57pps 0.18K 4731Rb 10.7Cs 0.40Sr 414Ba 155GaPbSe 26.0V 173Cr 84Co 29.7NiZn 81Y 19Zr 85NbHf 1.90Ta 0.16Th 1.20U 0.31La 7.8Ce 18.0Nd 12Sm 3.13Eu 1.02Tb 0.49Yb 1.65Lu 0.2887/86Sr 0.7035614:J114-4Nd 0.5127102


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L. LpezEscobar, R. Kllian, P.D. Kemplon amiM. Taglrf

F

F

a * Yate b a Yantetes Hualaihu Melimoyuo Hornopirn .. MentolatO Michinmahuida Maca Chaftn Cay.. Corcovado O Hudson Huequi

FIG. 7. AFM diagram lor rocks Irom dillerenl volcanic cenlers 01the SSVZ between 41 "30' and 46"OO'S showlng boundarybetween h o l e l ~ l c and calcalkaline series lavas (aller Irvlneand Baragar, 1971). Accordlng to thls dlagram, manycenlers 01 the SSVZ reglon between 41 30' and 4600'Shave tholelltlc allinitles.

Despite the differences between the two types ofbasalts of the studied region the overall compositionalrange is geochemically simi lartothat 01 stratovolcanobasalts from the 3700'.41 30'S region (Hickey etal.,

4S

1986). 80th the 3]oOO'-4130'S and the 4130'-4600'S stratovolcano-basalts have KlRb ratios (Table3) comparable to those 01 oceanic island basal1s(018) but with Rb/Cs ratios that are almost threetimes lower, as they are strongly emiched in Cs.

While the MgO contents 01 type-1 basalts areamong the highest reported for basalts from the SVZof the Andes, the MgO contents of type-2 basalts areamong the lowest (Tables2,3). However, differencesin MgO contents, almost as large as those existingbetween both types 01basalts fromthe 41 30'-4600'Sregion 01 the SVZ, have been reported in basalts 1romthe Villarrica volcano (39S; Hicke}'-Vargas et al.,1989). Likewise, type-1 basalts have some 01 thelowest HFSE contents among basalts 1rom the SVZ01 the Andes (Table 3), and type-2 basalts are amongthe richest in these elements (see also: Lpez-Escobar et al., 1977; Hickey et al., 1986; Hildreth andMoorbath, 1988).

18

16

14

50 60 70 5;02

14

50 60 10 5;Oz

FIG. 8. AI,o. versusSIO.dlagramlorSSVZrocksbetween41JO'and 4600'S. Corcovado, Menlolat, \1eUmoyu, Cay andMacabasaRsandbasanlcandesles (twe-l) have relaUvelyhlghAI.O. contents.However, MlchlnnahuldaandHudsonbasaltlc rocks (type-2) have relatlvely low AI,o.contents.


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46 PETROGRAPHY AND GEOCHEMISTRY OF QUATERNARY ROCKS FROM THE SOUTHERN VOLCANIC ZONE ..

50 60 70

Like the 3700'-4130'S stratovolcano-basalts,the 4130'-4600'S stratovolcano-basalts have BaILa ratios ( T a b ~ 3) that are intermediate betweenthose of intraoceanic island arcs (IAB: 30-50; Davidsonet al., 1988, and referencestherein), andthose ofOIB(8-13; Hildreth and Moorbath, 1988, and referencestherein) and MORB (Fig. 10). In general, both the41 30'-4600'S stratovolcano-basalts and the 3700'-41 30'S stratovolcano-basalts have similar rare-earthelements (REE; abundances (Table 3; Fig. 13; LpezEscobar and Fray, 1976; Lpez-Escobar et al.,1977;Hickey et al., 1986; Futa and Stern, 1988). However,while the type-1 and 3]000'-41 30' basalts exhibitrelatively low LalYb ratios (3-6; Table 3) and normalEu abundances, type-2 basalts have higher LalYbratios (5.8-8.3; - able 3) and display aslightly negativeEu anomaly (Fig.13).

The chem ical characteristics of the petrographically identified 'normal' andesites paralle lthoseofthe basalts wth which they are associated (Figs. 6,8,9,11). For example, the Hudson basalts are amongthe richest in incompatible elements among basalts

Yanteles D. Melimoyu~ Mentolat MacaIJ C;y () Hudson+ Yale Q HualaihuO Hcrnopirn O Michinmahuidae C ~ a i j n .A. Corcovado\ l H L ~ q u i

FIG. 10. BaA..a versus LalSm diagram lor SSVZ roeks between41 30' and46oo'S corrpared with NSVZ (3300'-34 30'S)roeks, SSVZ roeks between 37 and 41S, IAB (islandare basalts), OIB (oeeanic island basalts), and MOAB(mid-oceiln ridge basalts). Types 1 (BI) and 2 (BII)basanslr.>m the SSVZ region between 41 30' and4600'Slorm diff;rent elusters. PM = average 01 prlmitive manlleaHer Hol-nan (1988).

Yanleles D. Melimoyu~ Mentolat MacaIJ Cay () Hudson+ Yate Q HualaihuO Hornopirn O MichinmahuidaID Michinmahuida dacite 9 Chaijn.A. Corcovado 'V Huequi* Michinmahuida Miocene

FIG. 9. KlRb versusSiO. diagram lor rocks lrom different volcanlccenters 01 the SSVZ between 41 30' and 46OO'S. The KIAb ratio Is highly variable arnong basans and basaltlcandesltes, but not In the rnos! sillclc rocks. Thls behavlouris eommon In SVZ Ouaternary volcanic rocks (see, lo rexarrple, Hlldreth and Moorbath, 1988 and Davldson eta/.,1988). The generaltrend In passlng lrombasalts to daenesmimlcs that 01 an AFC trend.

from the SVZ. Likewise, the Hudson andesites arealso among the richest in those elements among SVZandesites. Similar relationships between basic andmore silicic rocks are observed in other centers, suchas Michinmahuida, Yate and Mentolat, although thedegree of enrichment varies from one center to theother.

As expected, the Chaitn rhyolites are notablyenriched in KP, Rb and Cs, and have relatively lowKlRb and Rb/Cs ratios (Table 2; Figs. 9,12). Actually,the KlRb ratios ofthe Chaitn rhyolites are lowerthanthose expected in rhyolites from Andean SVZ regionswith thin continental crust (3]000'-41 30'S; see figure11 of Hildreth and Moorbath, 1988), being similar tothose of silicic rocks from the 33-37S region, wherethe continental crust is relatively thick. The Chaitnrhyolites are also similar to the 33-3]oS rhyolites inthe abundances of Sc, V, Cr and Co.

4 0 ~ ~ ~ ~ ~ - - - - - - - - - - - - - - - - - - - - - - - - ~

Ba/la . IAS::.

2 4 6 8 la/Sm


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L. Lpez-Escobar, R. Kifian, P.D. Kempton andM. Tagiri

6 r - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~feO ,MgO-4

3

6FeOMgO _4

32

50

50

(~ : . JJJ:-\l I~ ++HUA

55

55

60

60

65 70 S i 0275

65 70 S i0275

+ Yalen Hornopirno Yate Pleistocene MioceneP liocene

PleistoceneHolocene

... Corcovado6 Melimoyu Maca() Hudson

1Q Hualaihu\J Huequia C h a ~ nI Mlchlnmahutda

Yanteles& Mentolato Cay* Yale

o Yanteles~ Mentolat() Cay

47

, Melimoyu Maca(t Hud sonHualaihu

FIG. 11. FeOlMgO versus Si 02 diagram lor SSVZ rocks between41 "30' and 46"00'S. Michinmahuida (MIC), Hualaihu(HUA), Hudson (HUD), Hornopirn (HOR) and Maca(MAC) show a strong increase 01Fea whhin the 50-55 wt% Si 02 range, which could rellect the ellect 01 olivineIractionation. [1 Hornopirna C h a ~ n

HuequiO Michinmahuida

The REE patterns of the Chaitn rhyolites (Fig.13) are notably different from those of the basalticand andesitic rocks fromthe 41 30'-4600'S region ofthe SVZ. They are depleted in MREE and HREE andexhibit larger negative Eu anomalies (Eu/Eu*=0:59).Their REE pattern resembles those of 33-3rsrhyolites ratherthan rhyolites fromthe 3700'-41 30'Sregion (see Lpez-Escobar and Munizaga, 1983;Frey et al., 1984; Gerlach et al., 1988; Hickey-Vargaset al., 1989).

Compared to the nearby Michinmahuida basalts(type-2), the Chaitn rhyolites are enriched in Ta(1 .7x), Th (3 .5x) and U (3.4x), depleted in Ti02 (4.2x),Zr (1.5x) and Hf (1.5x), but have similar Zr/Hf and Th/U ratios (Table 2; Lpez-Escobar et al., 1991).Analogous to the REE data, HFSE in the Chaitnrhyolites are also more similar to rhyolites from 33-3 rS than to rhyolites from the 3]oOO'-4130'Sregion.

ChOltn_ 9rhyollte

A Corcovado

FIG. 12 . LaNb versus Rb/Cs diagram lor SSVZ rocks between41 "30' and 46"00'S compared with SSVZ rocks between37" and 41"S and MORB. Symbols are t'19 same as inligure10.

ORB


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48 PETROGRAPHV ANO GEOCHEMISTRV OF QUATERNARV ROCKS FROM THE SOUTHERN VOlCANIC ZONE

ISOTOPIC COMPOSITIONSSr-isotope analyses were made on 32 samples of

the 41 30'-4600'S region of the SVZ by Notsu et al.(1985, 1987; Table 2). According to these authors,their results indicate that the basalts to dacites have87Sr/86Sr ratios of 0.70394-0.70482, with the majorityranging from 0.70404 to 0 .70434; exceptions to thisare the andesit3s from Huequi, Yanteles and Corcovado wh ch have67Srf8aSr ratios of 0.70455 to 0.70482.The 87SraeSr ratio of the Chaitn rhyolites (0.70570)is the highest among the analyzed rocks by Notsu etal. (1985; 1987). Fifteen of these samples were rechecked isotopically at the Open University (Table 2:NBS=071 024), the 87Sr/86S r relations being in generallower. The Sr isotope ratios of Hualaihu amountto 0.70356 (-O 00039), those of Chaitn rhyolite to0.70560 (-0.00010). According to the new determinations, type-l basalts have slightly lower 87Sr/86Sr ratios than type-2 basalts (Tables 2, 3, and Fig.14), and the rbyolites of Chaitn have significantlyhigher Sr sotope ratios than the basalts and andesitesof its neighbour Michinmahuida. Asimilar relationshipis observed at the Maipo are a (34S), where the 67Sr/86Sr ratios of the rhyolites are significantly higherthan

100 r:-1r-- ----- ------ - :J50

!!.:..,eo20

~ 10"-JCUO11:

5

"* ChOlfen .. Rn oli te~ ~ ! . : f ~ h U w j O f J ! ~

o lio lochue[J COrco ...odo "'onteles Type4 tIOlOlts Co)' r.IJocoLo:. Nd Sm Eu Tb Vb lu

those of the more basic volcanic rocks (Stern et al.,1984). This situation contrasts with that observed inother centers of the SVZ, Villarrica for example,where rocks ranging in composition from basalt torhyolite have similar 87Srf8eSr ratios (Druelle et al.,1983; Hickey-Vargas et al., 1989).The '43Nd/'44Nd ratios of basalts to dacites rangefrom 0.51271-0.51290. The few data available,suggest that type-l basalts have similar '43Nd/'44Ndratios to type-2 (Tables 2, 3). The lowest value isobserved for the Chaitn rhyolite (0.51259; Table 2).This ratio is similar to those of rhyolites from thenorthernmost region ofthe SVZ (3300'-3430'S; Fig.14), where the continental crust is comparativelythick (Futa and Stern, 1988; Hildreth and Moorbath,1988).

In a plot of 207Pb/204Pb versus 208Pbp04Pb (Fig.15),as well as inthe 208Pbf204Pb versus 207Pbf204Pb diagram(Fig. 16), the 41 30' -4600'S region basaltic to daciticrocks (Table 2) fall within the fields of the 33-37SQuaternary volcanic rocks determined by Hildrethand Moorbath (1988). This field also includes thevolcanic rocks from 3]oOO'-4130'S analyzed byHarmon et al. (1984) and Hickey et al. (1986). Thus,all the SVZ (33-46S) rocks, regardless of latitude or

I O O ~ - - - - - - - - - - - - - ~

10 Huequl andeslfa

5

2

lo C. Nd Sm Eu Tb Vb Lu

FIG. 13. Chondrile normalizad REE pal1eros 01 types 1 and 2 basahs, Huequi, Yale, and Menlolat andesltes . and Challn rhyolltes.


16/22

L. Lpez-Escobar. R. Kllian, P.D. Kempton and M. Taglri

FIG. 14. 14"Nd/14'Nd versus7Srl""Sr lo r SSVZrocks between 41"30' and 46OO'S

c o ~ a r e d wtth SVZ between 33and37S, SSVZ between 37" and 41S,MORB, IAB and OIB. Data sources:

0 .5130

0.5128

0 .5126Hlckey el al. (1986) and relerencesthereln; Futa and Stem (1988) andrelerences thereln; Hlldreth andMoorbath (1988) and relerencesthereln. Vlllarrlca stratovolcano andmlnor eruptlve centers (MEe) 01 theVlllarrlca area rocks afler HlckeyVargas et al. (1989).

0.5124 L --__ -- ' -____ - '_____ --____ -----'

crustal thickness, have similar Pb isotope compositions. The only exception is the Chaitn rhyolite,which is notably enriched in radiogenic Pb, particularlyin 206Pb.

The 0'80 values of the 41 30'8-4600'8 8VZbasalts vary from +5.8 to +7. 7%0(Table 2). However,onlythe Hudson basalts have values within the rangeof mantle derived rocks (5.8-6.2%0) reported byKyser (1986), Ito et al. (1987) and Woodhead et al.

0.703 0.704 0.705 07068 7S.r / 8 6 Sr

(1987). The 0'80 values of the basaltic andesites anddacites fall in the range +6.4 to +8.4%0, which issignificantly higherthan that of mantle derived rocks.The O-isotope composition of the Chaitn rhyolite is+9.4%0, which is typical of upper crustal rocks. TheO-isotope ratio of this rhyolite is s milar to thoseexhibited by rhyolites from the northernmost region ofthe 8VZ (8tern et al., 1984; Hildreth and Moorbath,1988).

DISCUSSION

Although the data presented in this paper are notsufficient to evaluate in detail any particularpetrogenetic model, they, at least, improve ourpetrographic and chemical knowledge of 8VZQuaternary volcanism between latitudes 41 30' and4600'8, and place some constraints on the originand evolution of volcanic rocks from this region.

Petrographically, the rocks ofthe 41 30'8-4600'88VZ Quaternary volcanic front can be grouped intofive categories: a- type-1 basalts are characterizedby the coexistence of highly zoned olivine (F08S-Foss)and augite; b- type-2 basalts are characterized byweakly zoned olivine (F07S-F070), which commonlydoes not coexist with augite; c- 'normal' andesitesand dacites are characterized by plagioclase pheno-

crysts exhibiting normal zoning; d- 'mixed' andesites are characterized by disequilibrium mineralassemblages (e.g. plagioclase+oliv;ne+orthopyroxene+amphibole) that are compatible with mixing ofbasaltic and acidic magmas; e- rhyolites, representedby the Chaitn rhyolites, which, to the best of ourknowledge, are the only rhyolites reported in thisregion of the 8VZ.

Chemically, type-1 basalts have comparativelylow LILE and HF8E, high MgO and CaO, and lowFeOm/MgO, La!Yb and 878r/8S8r ratios. Type-2 basalts,on the other hand, have comparatively high LILE andHF8E, low MgO and CaO, high FeO(T)/MgO, La!Yband 878rfBs8r ratios. The normal andesites follow thechemical trends observed in their coexisting basalts.


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50 PETROGRAPHY ANO GEOCHEMISTRY OF Q UA TERNARY ROCKS FROM THE SOUTHERN VOLCANIC ZONE ..

15.7 . - -- - ----- - -------- - - ,207 P b204P b

15.6 *HA-'

IB.5 IB .6 IB.7 16 .6

FIG. 15. "''PbI''''Pb versus ""'PbP"'Pb lor SSVZ roclesbelween 41 "30' and 46"00'S compared wilhSVZ belween 33 " and 37 ' S. SSVZ belween37 " and 41 ' S. SVZ rhyoliles oelween 35 " and36 S. Nazca plale basalls and Nazca platesedlmenls. Dala sources: Hildrelh and Moarbath (1988 and relerences Illera in) and HickeyVargas e/ al. (1989) and relerences Iherein.

206 P b/ 204 Pb

p b . - - - - - . ~ ~ ~ ~ ~ ~ __, , _ ~ ~ ~ ~ 7

petrography and geochemistry of quaternary rocks from the southern volcanic zone of the andes, chile

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