Petrography and geochemistry of Quaternary rocks from the Southern Volcanic Zone of the between 41 °30' and 46°00'S,

Leopoldo López-Escobar Departamenlo de Geologla, Universidad de Chile, Casilla 13518, Corre:> 21, Santiago, Chile

Rolf Kilian Mineraloglsch-Pelrographisches Inslnul, Universlliil Tübinge" Wilhemslrasse 56, 0-7400 Tübingen 1, Germany

Pamela D_ Kempton NERC isolope Geosciences Laboralory, Keyworth, Noningham, NG12 5GG, Unned Klngdom Michio Tagiri Departmenl 01 Earth Sciences, Ibaraki Universny, Mno 310, Japan

ABSTRAeT

Rocks from thirteen stratovolcanoes, belonging to the Quaternaryfrontofthe Southern Volcanic Zone :SVZ) of the Andes (41 "30'S-46"00'S) are mainly low- to medium-K and basal tic . Andesites and are less abundant, and rhyolites are found only at volcano. Pleistocene volcanic rocks range in composition from to , but Holocene volcanic rocks are predominantly basalts and basaltic andesites. In general, -basalts from the 41 "30'-46"00'S regio n of the SVZ are geochemically similar to stratovolcano-basalts from the 37"00'-41 "30'S region, but exhibit a wider ranga of Pb-isotope ratio s which is close to that ofvolcanic rocks from the 33-37°S region. In detail, two types of 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 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 strong normal zoning, from Anu3 (cores) to Ans8 (rims).ln spite of being located in an area of presumably thin continental crust, the Chaitén rhyolites are geochemically similarto rhyolites from fu rther north (33-37"S) where the continental crust is thick, but are notably enriched in radiogenic Pb, particularly in 206Pb. Compared to the nearby basalts, the Chaitén rhyolites 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 isotopic differences, type-1 basal tic magmas are not parental magmas of type-2 basalts. These differences seem to reflect different degrees of partial melting of the asthenosphere followed by contamination of these melts with lowerto intermediate crustal material. Atuppercrustallevels, basalticmagmas evolveto produce intermediate magmas, eitherbyfracti:mal crystallization combined with different degrees of crustal contamination or by mixing with rhyolitic liquids generated b'l partial melting of crustal material.

Key words: Petro/ogy, Geochemistry, Quaternary volcanism, Southern Andes, Southern Chile.

Revista Geológica de Chile, Vol. 20, No. 1, p.33-55, 16 Figs .. 3 rabies, July 1993. 34 PETROGRAPHV ANO GEOCHEMISTRV OF QUATERNARV ROCKS FROM THE SOUTHERN VOLCANIC ZONE •••

RESUMEN

Petrografía y geoqufmica de rocas cuaternarias de la Zona Volcánica Sur de los Andes entre los 41 °30' Y 46°00'S, Chile. Los centros volcánicos del frente cuatemario de la Zona Volcánica Sur (ZVS) de los Andes, ubicados entre las latitudes 41 °3O'S ~ 46°00'S, son predominantemente basálticos y andesítico-basálticos, de contenidos bajos a medios de K. Andesitas y daeltas son menos abundantes que las rocas basálticas y se encuentran riolitas sólo en el volcán Chaitén. Las rocas volcánicas pleistocenas varían en composición de basalto a dacita, pero las holocenas son fundamentalmente basaltos y andesitas basálticas. En general, los basaltos de los estratovolcanes de la región comprendida entre 41 °30'- 46"00'S de la ZVSson geoqufmicamente semejantes a los basaltos de los estratovolcanes de la región 37 0 00'-41 °30'S, pero sus razones isotó:¡icas de Pb presentan un intervalo más amplio de variación, siendo éste cercano al presentado por las rocas volcánicas de la región 33-37' S. En detalle, dos tipos de basaltos pueden distinguirse en esta región de la ZVS de los Andes: basaltos empobrecidos (tipo-1) y enriquecidos (tipo-2) en elementos incompatibles. En comparación con los basaltos del tipo-'" los del tipo-2 presentan un grado más bajo de zonación del olivino y sus razones FeO(T'lMgO, La/Yb, Y 87Sr}"Sr son más elevadas. En los basaltos del tipo-2, el olivino raramente coexiste con augita. Los fenocristales de plagioclasa de ambos tipos de basaltos exhiben una fuerte zonación normal, que varía de An u3 (centro) a Ans8 (bordes). A pesar de estar ubicadas en un área de corteza continental presumiblemente delgada, las riolitas del Chaitén son geoquímicamente semejantes a las riolitas de los 33-37"S (donde la corteza continental es gruesa), pero son más ricas en Pb radiogénico, en especial en 2°OPb. En comparación con basaltos vecinos del Michinmahuida, estas rioHlas tienen razones isotópicas de Sr, Pb y °más elevadas y de Nd más bajas, están notablemente empobrecidas en Ca, Sr, Eu , Ti , Zr, Hf, y Y en tierras raras medianas y pesadas, y presentan razones KlRb más bajas. Sobre la base de sus diferencias de composición, los basaltos del tipo-1 no son magmas parentales del tipo-2. Sus diferencias parecen reflejardiferentes grados de fusión parcial de la astenósfera seguido de diferentes grados de contaminación a nivel de la corteza inferior o media. En niveles corticales superiores, los magmas basálticos evolucionan a intermedios ya sea por cristalización fraccionada, acompañada de contaminación cortical o por mezcla con magmas riolíticos generados por fusión de material cortical.

Palabras claves: P6Irologla, Geoqulmica, Volcanlsmo cuatemario, Andes del Sur, Sur de Chile.

INTRODUCTlON

The Quaternary volcanic front of the Southern mainly because of serious logistic problems (Iack of Volcanic Zone (SVZ) of the Andes extends between roads, bad climatic conditions, dense vegetation, latitudes 33° and 46°S, and is a product of the massive ). of the oceanic beneath the In the 41°30'-46°00'S region of the SVZ, the continental Sou:h American plate. Its northern end is underthrusting of the Nazca plate beneath the South associated wi:h the intersection of the Juan American plate has produced thirteen major volcanic Fernández Ridge with the Chile-Perú Trench and its centers (Fig.1; Table 1). Most of them are basahic in southern end is the triple junction Nazca-Antarctic­ composition and Pleistocene to Recent in age (Kilian South American plates (Fig.1). and López-Escobar, 1989,1991). Rhyolites occur only While numerous geochemical and petrological at Chaitén volcano. Many of these centers are related, studies have been carried out in the Quaternary not only to the subduction of the oceanic Nazca Plate volcanic front of the SVZ of the Andes between beneath the continental , but latitudes 33°0(' and 41°30'S (see Hildreth and also to the Liquiñe-Ofqui zone, that extends for Moorbath, 198E, and references therein; Tormey et ca. 1,000 km, between latitudes 38° and 47°S. al., 1991, and references therein; Ferguson et al., The aim of this study is to present and to discuss 1992, and references therein), similar studies are petrographical and geochemical data (major and tra­ comparatively scarce between 41 °30' and 46°00'S ce elements, and isotopic compositions) obtained (see Stern et al., 1976, and references therein, from samples recovered at the thirteen major volca­ Onuma and López-Escobar, 1987, and references noes located in the 41°30'-46°00'S region of the therein; Futa and Stern, 1988, and references therein), Quaternary volcanic front of the SVZ of the Andes. L. López-Escobar, R. Klllan, P.D. Kempton and M. Tagiri 35

TSVZ

......

Nazca Plate

SSVZ

-- ".-l Rise '

. --­ ...-...- '~\ ---

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 -46°S). The location ofthe SVZ auatemaryvolcanic tront, Chile trench, Chile Rise and the ages ot the oceanic Nazca pi ate (Herron et al.,19B1) are also 3hown. TABLE 1. MINERAL COMPOSlTION OF ROCKS FROM THE SSVZ OF THE ANDES BETWEEN 41°30' ANO 46°OO'S.

Lavatypes Phenocrysts Groundmass

PI (An) Bt Am Opx Cpx 01 (Fo) Or Q PI (An) Opx Cpx 01 Ap Or GI -o m -i ~ Gl :IJ ·Basaltic +++ (81·65) ++ + + (79-72) + ++ (57-49) + + +- > ." :lO Hornopirén·Basalt ++ (87·73) + ++ + (81-70) ++ (70-61) + + + -< >z O Hualaihué-Basalt ++ (84-69 + ++ + (82-74) +- ++ (59-48 + + + + +- (j) m -Andesite +. +- O ++ (72-48) + ++ +- + (76-75) + ++ (67-49) + + + () :lO m Michinmahuida-Basalt +++ (92-58) + + + (72-69) +- + (84-61) +- + + + +- ~ ~ :IJ Chaitén-Rhl'olite + (43-36) + +- + + + (37-36) + + +++ -< O Corcovado-Basalt "TI ++ (92-56) + + + (80-71) +- ++ (57-52) + + o -Basalt ++ (90-69) +- + + (82-66) +- + (67-46) +- + + m~ :IJ -Andesite (89-72) +- +- (53-40) +- + + ++ + ++ ~ :IJ -Andesite +++ (92-47) +- +. + +- (76) +- + ++ (63-47) +- + + + +- -< :IJ O () Cay-Basalt + (93-61) ++ ++ (85-69) + (84-53) +- + + + " '""TI Maca-Basalt + (89-48) ++ ++ (87-73) + (72-63) +- + + + :IJ O ~ Hudson-Basalt ++ (86-70) +- ++ + (75) + (58-49) +- ++ + + -i :lO m en O e Am=; An=Anorthite; Ap-Apatite; Bt=Biot~e; Cpx=Clinopiroxene; Fo=Forster~e; GI=Glass; OI=Olivine; Or=Ore; Opx=Orthopyroxene; PI=Plagioclase; Q=Ouartz -i :lO m :IJ Ouantity: +++ main constituent; ++ frequent; + accessory I +- occasionalty; - not detected z ~ h >z O

S'z ~ L. López-Escol1ar, R. Kllian, P.D. Kempton and M. Tagiri 37

Ca 50 50

Mg 50 Fa Fa Olivine-series

50

Andesite (Ol-Opx-Hbl)

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

Type-1 basalts (Fa 85 -65)

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

FIG. 2. Ca-Fe(1)-Mg compositions of the orthopyroxene, clinopyroxene, olivine and amphibole in different rock types of the 88VZ between 41 "30' and 46"00'8, ordered from north to south. 38 PETROGRAPHY ANO GEOCHEMISTRY OF QUATERNARY ROCKS FROM THE SOUTHERN VOLCANIC ZONE ...

GEOLOGIC OUTLlNE

One of the main structural features of the Andean units are intensely eroded and partially region under study is the Liquiñe-Ofqui fault zone covered by material erupted by Holocene volcanic (Hervé et al, 1978; Hervé et al, 1979; Hervé, 1984; cones. These cones are either developed within the Thiele et al., 1986), which is a dextral transcurrent Pleistocene or as satellite cones. dislocation (Hervé, 1984; Pankhurst et al., in press; Yate, Michinmahuida, Corcovado, Yanteles, Cembrano, 1992), represented by a N-S trending belt Melimoyu, Mentolat, , Maca, and Hudson are of cataclastic and mylonitic rocks. South of 38° S, this large composite stratovolcanoes, whose geology is fault zone controls a large number of SVZ centers now under a detailed study. Hualaihué is a small (Hervé et al., 1978; Thiele et al., 1986; Fig. 1). cinder cone, emplaced in the center of a volcanic Many centers belonging to the 41°30'-46°00'S caldera structure. Hornopirén is also a relatively region of the Quaternary volcanic front of the SVZ of small cinder and flow center, with almost a the Andes have Paleozoic to Mesozoic basement, perfect conical shape, located 15 km north-east of but Yate, Michinmahuida and Mentolat overlie the Hualaihuévolcano. Chaitén volcano is a relatively Miocene to Pliocene volcanic formations (Kilian and small center, that apparently suf1ered a caldera López-Escobar, 1989; 1991). Yate, Hualaihué, Michin­ collapse; it has a resurgent dome and is located 20 mahuida and Hudson show evidence of having km south-west of the Michinmahuida volcano. developed Pleistocene calderas. Relicts of the pre-

PETROGRAPHY

The petrography of the studied samples was tolat commonly contain plagioclase+clinopyro­ determined by optical microscope and the chemical xene+orthopyroxene±olivine phenocrysts. Andesites compositions of the constituent minerals and glass contain subhedral plagioclase+clinopyroxene+ were obtained by electron microprobe (ARL-Applied orthopyroxene phenocrysts. Acid andesites and Research Laboratories) atthe Mineralogicallnstitute dacites from Yate and Mentolat have plagioclase+ of the Tübingen University. A total of 6 electron­ orthopyroxene±amphibole phenocrysts. The Huequi microprobe analyses were made of amphibole, 3 of andesites and dacites and some Mentolat andesites biotite, 289 of plagioclase, 48 of olivine, 15 of opaques, have a plagioclase+olivine+orthopyroxene+ 62 of , and 12 of glass present in the amphibole assemblage that reflects disequilibrium, groundmass, as well as glass occurring as inclusions confirming the observation and interpretation of Fuen­ in phenocrysts. Cross sections of the plagioclase, zalida (1979). In addition, crystal clots of clinopyrox­ olivine and pyroxene phenocrysts were made with ene+olivine are common in most basalts, and crystal different intervals down to 4 mil. The results are clots of plagioclase+clinopyroxene+ orthopyroxene+ summarized in table 1 and figure 2. opaques are common in most andesites. Thefirst petrographic investigations ofthe 41 °30'- Plagioclase is the most abundant phase, both as 46°00'S volcanic rocks (López-Escobar et al., 1985 b) phenocryst and in the groundmass. It is an early indicated that while a clinopyroxene+orthopyroxene phenocryst in the Pleistocene andesites of Yate, +opaques assemblage was common in the northern Hornopirén, Michinmahuida, Yanteles and Mentolat, part of this region, an olivine+clinopyroxene assem­ but was formed after olivine and clinopyroxene in the blage was common in the southern parto The current basalts of Mentolat, Cay, Maca and Hudson. investigation modifies this general view. Plagioclase phenocrysts in the basalts exhibit strong Basaltic roc

FIG. 3. Strong no mal zoned plagioclase phenocryst of a Michinmahuida basalt (type-2 basalt). cores and An'2 to Anso in their rims. Normal zoning 01 and andesites from Yate, Hualaihué, Hornopirén, plagioclase is even more marked (An to ) in the 83 An4S Huequi, Mentolat, Cay an~ Maca vdcanoes (Fig. 4). groundmass. Similar zoning is observed in the Coexisting ortho- and clinopyroxenes have similar plagioclase 01 Corcovado and Melimoyu andesites. FelMg ratios in from Yate, HOf'nopirén, Corco­ Oscillatory and reverse zoning are observed in the vado, Melimoyu, Mentolat and Maca (Fig. 5). The plagioclase crystals of Huequi andesites and dacites, lowest Fe/Mg ratios occur in orthopyroxenes from and in some Mentolat andesites. Alkali feldspars Mentolat lavas. Zonations were not clearly observed formed in the groundmass of some basalts, and in (Fig.2). Yate and Hudson andesites, as a very late-stage Olivine is presenf in most basalts, basaltic product of crystallization. andesites and andesites from this region of the SVZ. Ortho- and clinopyroxene coexist in most 41 °30'- On the basis of its degree of zonation and association 46°00'5 volcanic rocks. Figure 2 shows their com­ with augite, the basalts have been subdivided into positions in the Ca-Mg-Fe2+pyroxene quadrilateral. two types(1 and 2). In type-1 basalts (Hualaihué, The orthopyroxene is gene rally hypersthene, being Corcovado, Cay and Maca); large euhedral olivine Fe-hypersthene in the Chaitén rhyolites. The phenocrysts (1-4 mm), showing a comparatively clinopyroxene is normally augite, being diopside in a strong normal zoning (Fo,s-Foes), coexist with large basaltic sample from Hornopirén and salite in a phenocrysts of augite (2-4 mm; Figs. 2, 4). By contrast, sample from Mentolat. Orthopyroxene commonly in type-2 basalts (Yate, Michinmahuida, Melimoyu occurs as a phenocryst phase (1-3 mm), but is also and Hudson), olivine phenocrysts have a weaker normal zonation (Fo -Fo ) and rarely coexist with present in the groundmass of Yate, Hornopirén, 75 ss Huequi, and Mentolat andesites. In most other rocks augite (Fig. 5). In both types, hypersthene coexists in studied, orthopyroxene appears as smallxenocrysts. disequilibrium with olivine and augite (dashed lines in Euhedral clinopyroxene phenocrysts occur in basalts figure 2), but in some type-2 basalts (Yate and 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-1 basalt).

FIG. 5. Nonnal zonad plagioclase phanocrysts, hypidiomorphic augita and hypersthana in a basalt Irom Michinmahuida volcano (~pe-2 basalt).

L. López-Escobar, R. Kilían. P.D. Kempton and M. Tagiri 43

Hudson), hypersthene coexists in equilibrium with sometimes hematite. Early titanomagnetite pheno­ pigeonite. The latter mineral is common in those crysts have higher TVFe ratios (ca. 0.1) than titano­ type-2 basafts having holocrystalline groundmass. magnetite crystals present in the groundmass (ca. The Yate, Hornopirén and Michinmahuida andesites 0.05). usually contain small olivine xenocrysts with Fo Chaitén rhyolites are highlyvitrophyric, containing content in the range F076 -F066 . The highest Fe/Mg only 5% volume of plagioclase microphenocrysts ratio is observed in an andesite samplefrom Melimoyu, together with ferrohypersthene and biotite. The An

whose olivine composition is Foso and that of the content (An'2 -An a7) as well as the degree 01 zoning of hypersthene is M9.s (Fig. 2). the plagioclase are low. Apatite and zircon are present Opaques are gene rally titanomagnetite and as accessory minerals.

CHEMISTRY

MAJOR ANO TRACE ELEMENTS stratovolcanoes can be classified as high-alumina basalts. Andesites and dacites are found in the The geochemical data 01 32 volcanic rocks of the Pleistocene units ofYate, Hualaihué, Michinmahuida,

41 °30' -46°00'S of the SSVZ are presented in table 2. and Hudson. Rhyolites (70-75% Si0 ) are present 2 This data constitute the basis of the following only at Chaitén volcano. discussion. In addition, other 35 whole roe k analyses The type-1 basalts (Hualaihué, Corcovado, Cay (major and trace elements) of volcanic rock were and Maca) tend to be depleted in some large ion obtained and are shown in figures 6-13. lithophile elements (LILE = Nap, KP, Rb, Cs, Ba, Based on the distribution 01 data on a Kp-Si0 2 RE E, Y, Th and U) and high field strength elements classification diagram (Fig. 6) and on an AFM diagram , (HFSE= Ti02 Zr, Hf andTa) in comparison withtype- (Fig. 7), the analyzed samples are low- to medium-K 2 basalts (Michinmahuida and Hudson), but tend to calcalkaline volcanic rocks, with tholeiitic affinities. be enriched in AIPa (Fig. 8), MgO, Cao, Cr and Co Afthough they range in composition from basalt to (Table 3). In addition, type-1 basalts tend to have rhyolite (Table 2), there is a predominance of basalts higher K/Rb (Fig. 9) and Baila (Fig. 10) ratios than and basaltic andesites, particularly in the Holocene type-2 basalts, but lower FeO(T)/MgO (Fig. 11) and eones. The basalts showvariable AIP3 concentrations la/Yb (Table 3; Fig. 12) ratios. However, both types in the range 16 to 21 wt % (Fig. 8). According to their of basalts have similar Zr/Hf, Th/U and AIPlCaO high AIPa and intermediate alkali contents, basalts ratios (Table 3). 01 Corcovado, Maca, Cay, Mentolat and Melimoyu

, I I 3 - ¡ ~ I I : Rhyolite I I I 2 - I I + Yate o Cha~én Ó MichlOmahuida • Mentolat : ----­ • Cayo Maca ----¡--- * Hudson --- I - I FIG.6. K,O versusSiO, diagramlor rocks Irom differenl I I volcanlc cen1ers 01 lhe SSVZ be1ween 41°30' I and 46<00'S. WHhin each cenler, 1here exlsls a I I good K o-SiO,correlallon, bul lhe relallonshlp , al\óesile I belwee~ bolh oxides varies from one cenler lo Basalt : \..0"'-1<. I ,I I lhe olher. o 50 60 70 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 41°30' ANO 46°00'S COMPAREO WITH THE COMPOSITIONAL RANGE OF THE SSVZ STRATOVOlCANO-BASAlTS BETWEEN 37° ANO 4toS.

TYPE-l BASALTS RAHGE' TYPE-2 BASAL TS RAHGE' 37°OO-41 °3O'S BASALTS RANGE'

Si02 48.96 51 .68 49.56 51.90 48.39 52.08 Ti02 0.84 1.13 1.55 2.33 0.43 1.22 AI20a 17.95 20.48 16.62 1827 14.64 22.37 cn Fe20 a 8.45 9.98 10.68 12.18 7.69 10.96 MnO 0.13 0.15 0.16 0.19 0.12 0.19 MgO 4.92 7.14 3.78 4.97 3.11 14.32 cae 9.35 10.12 7.18 8.87 8.55 11.96 Na20 2.70 3.31 3.42 4.53 2.19 4.19 KP 0.57 0.85 1.03 1.58 0.29 0.88 pps 0.18 0.21 0.37 0.80 0.08 0.31

K 4731 7055 8549 13114 2407 7304 Rb 10.7 16.0 28.6 31.4 6.0 18.7 Cs 0.40 0.63 0.44 1.43 0.30 1.60 Sr 414 632 395 643 247 541 Ba 155 265 315 380 96 232 Ga 14.6 19.8 Pb 2.0 10.6 Se 26.0 32.5 28.0 32.6 26.4 35.7 V 173 226 215 248 163 248 Cr 84 191 8 52 46 640 Co 29.7 34.7 25.4 27.7 23.0 64.5 Ni 9 302 Zn 81 118 98 112 68 96 Y 19 23 30 41 12 22 Zr 85 190 188 243 41 92 Nb 0.9 3.8 Hf 1.90 2.28 3.98 4.67 1.12 2.80 Ta 0.16 0.29 0.56 0.74 0.06 0.59 Th 1.20 2.22 2.75 3.68 0.01 2.50 U 0.31 0.70 0.67 0.86 La 7.8 10.9 21 .1 30.0 3.3 9.8 Ce 18.0 24.6 45.6 64.9 8.6 24.6 Nd 12 21 30 38 6.3 13.9 Sm 3.13 3.82 6.57 8.63 1.92 3.40 Eu 1.02 1.2 1.71 2.43 0.71 1.17 Tb 0.49 0.55 0.84 1.23 0.27 0.68 Yb 1.65 1.94 2.86 3.68 1.21 2.35 Lu 0.28 0.34 0.43 0.60 0.17 0.36

87/86S r 0.70356 0.70421 0.70413 0.70451 0.70381 0.70433 14:J114-4Nd 0.512710 0.512826 0.512771 0.512818 0.512808 0.512891 2

KlAb 371 444 273 332 249 543 RIYCs 18 39 21 65 9 24 Baila 17 24 13 15 21 33 LalYb 4.4 5.6 5.8 8.3 2.2 5.4 ZrIHf 44.7 83.3 40.3 61.1 27.9 43.7 ThIU 3.17 3.87 4.10 4.33 AI2°'¡CaO 1.9 2.1 1.9 2.0 1.5 2.1 FeQlTVMgO 1.2 1.5 2.2 2.9 0.7 2.9

, Hualalhué, Corrovado, Cay, Maca 2 Michlnmahulda, Hudson • , Llalma, VHlarrlca. Puyehue. . Osomo L. López·Escobar, R. Kllian, P.D. Kemplon ami M. Taglrf 4S

F 1986). 80th the 3]oOO'-41°30'S and the 41°30'- 46°00'S stratovolcano-basalts have KlRb ratios (T able 3) comparable to those 01 oceanic island basal1s (018) but with Rb/Cs ratios that are almost three times lower, as they are strongly emiched in Cs. While the MgO contents 01 type-1 basalts are among the highest reported for basalts from the SVZ of the Andes, the MgO contents of type-2 basalts are among the lowest (Tables 2,3). However, differences in MgO contents, almost as large as those existing between both types 01 basalts fromthe 41 °30'-46°00'S region 01 the SVZ, have been reported in basalts 1rom the volcano (39°S; Hicke}'-Vargas et al., 1989). Likewise, type-1 basalts have some 01 the lowest HFSE contents among basalts 1rom the SVZ 01 the Andes (Table 3), and type-2 basalts are among F the richest in these elements (see also: López-Esco• bar et al. , 1977; Hickey et al. , 1986; Hildreth and Moorbath, 1988).

18

16

14

50 5~ 60 6~ 70 1~ 5;02

a Yate b a Yantetes *• Hualaihué Ó Melimoyu o Hornopirén .. Mentolat O Michinmahuida • Maca • Chaftén • Cay .. Corcovado O Hudson Ó Huequi

FIG. 7. AFM diagram lor rocks Irom dillerenl volcanic cenlers 01 the SSVZ between 41 "30' and 46"OO'S showlng boundary 14 between tholel~lc and calcalkaline series lavas (aller Irvlne and Baragar, 1971). Accordlng to thls dlagram, many cenlers 01 the SSVZ reglon between 41 °30' and 46°00'S 50 ~~ 60 10 1~ have tholelltlc allinitles. 5;Oz

Despite the differences between the two types of FIG. 8. AI,o. versusSIO.dlagramlorSSVZrocksbetween41°JO' basalts of the studied region the overall compositional and 46°00'S. Corcovado, Menlolat, \1eUmoyu, Cay and MacabasaRsandbasanlcandesles (twe-l) have relaUvely range is geochemically similartothat 01 stratovolcano­ hlghAI.O. contents. However, MlchlnnahuldaandHudson basalts from the 37°00'.41 °30'S region (Hickey et al., basaltlc rocks (type-2) have relatlvely low AI,o. contents. 46 PETROGRAPHY AND GEOCHEMISTRY OF QUATERNARY ROCKS FROM THE SOUTHERN VOLCANIC ZONE ...

• Yanleles D. Melimoyu ~ Mentolat • Maca IJ Cay () Hudson + Yate Q Hualaihué O Hornopirén O Michinmahuida ID Michinmahuida dacite 9 Chaijén .A. Corcovado 'V Huequi * Michinmahuida Miocene

FIG. 9. KlRb versus SiO. diagram lor rocks lrom different volcanlc centers 01 the SSVZ between 41 °30' and 46°OO'S. The KI Ab ratio Is highly variable arnong basans and basaltlc andesltes, but not In the rnos! sillclc rocks. Thls behavlour 50 60 70 is eommon In SVZ Ouaternary volcanic rocks (see, lor exarrple, Hlldreth and Moorbath, 1988 and Davldson eta/., 1988). The generaltrend In passlng lrom basalts to daenes mimlcs that 01 an AFC trend.

Like the 37°00'-41°30'S stratovolcano-basalts, from the SVZ. Likewise, the Hudson andesites are the 41°30'-46°00'S stratovolcano-basalts have BaI also among the richest in those elements among SVZ La ratios (Tab~ 3) that are intermediate between andesites. Similar relationships between basic and those of intraoceanic island arcs (IAB: 30-50; Davidson more silicic rocks are observed in other centers, such et al., 1988, and referencestherein), andthose ofOIB as Michinmahuida, Yate and Mentolat, although the (8-13; Hildreth and Moorbath, 1988, and references degree of enrichment varies from one center to the therein) and MORB (Fig. 10). In general, both the other. 41 °30'-46°00'S stratovolcano-basalts and the 37°00'- As expected, the Chaitén rhyolites are notably 41 °30'S stratovolcano-basalts have similar rare-earth enriched in KP, Rb and Cs, and have relatively low elements (REE; abundances (Table 3; Fig. 13; López• KlRb and Rb/Cs ratios (Table 2; Figs. 9,12). Actually, Escobar and Fray, 1976; López-Escobar et al.,1977; the KlRb ratios ofthe Chaitén rhyolites are lowerthan Hickey et al., 1986; Futa and Stern, 1988). However, those expected in rhyolites from Andean SVZ regions while the type-1 and 3]000'-41 °30' basalts exhibit with thin continental crust (3]000'-41 °30'S; see figure relatively low LalYb ratios (3-6; Table 3) and normal 11 of Hildreth and Moorbath, 1988), being similar to Eu abundances, type-2 basalts have higher LalYb those of silicic rocks from the 33-37°S region, where ratios (5.8-8.3; - able 3) and display aslightly negative the continental crust is relatively thick. The Chaitén Eu anomaly (Fig.13). rhyolites are also similar to the 33-3]oS rhyolites in The chemical characteristics of the petro­ the abundances of Sc, V, Cr and Co. graphically identified 'normal' andesites parallelthose ofthe basalts wth which they are associated (Figs. 6, 40~~~~~------~ 8,9,11). For example, the Hudson basalts are among .IAS::. the richest in incompatible elements among basalts Ba/la

• Yanteles D. Melimoyu ~ Mentolat • Maca IJ C¡;y () Hudson + Yale Q Hualaihué O Hcrnopirén O Michinmahuida e C~aijén .A. Corcovado \l HL~qui

FIG. 10. BaA..a versus LalSm diagram lor SSVZ roeks between 41 °30' and46°oo'S corrpared with NSVZ (33°00'-34 °30'S) roeks, SSVZ roeks between 37° and 41°S, IAB (island are 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' and46°00'S lorm diff¡;rent elusters. PM = average 01 prlmitive manlle 2 4 6 8 aHer Hol-nan (1988). la/Sm L. López-Escobar, R. Kifian, P.D. Kempton and M. Tagiri 47

6 r------~ + Yale Q Hualaihué feO , n Hornopirén \J Huequi MgO- o Yate Pleistocene a Cha~én • Miocene·Pliocene 1 ~ Pleistocene I Mlchlnmahutda 4 ( ~ Holocene 3 . ~: . JJJ:-\l I .~+ + HUA

50 55 60 65 70 Si0275 6 ... Corcovado o Yanteles FeO 6 Melimoyu ~ Mentolat MgO _ • Maca () Cay () Hudson

4

3

2

50 55 60 65 70 Si0275

FIG . 11. FeOlMgO versus Si02 diagram lor SSVZ rocks between 41 "30' and 46"00'S. Michinmahuida (MIC), Hualaihué • Yanteles , Melimoyu (HUA), Hudson (HUD), Hornopirén (HOR) and Maca & Mentolat • Maca (MAC) show a strong increase 01 Fea whhin the 50-55 wt o Cay (t Hudson Hualaihué % Si0 range, which could rellect the ellect 01 olivine * Yale ~ 2 [1 Hornopirén O Michinmahuida Iractionation. a Cha~én A Corcovado Huequi

The REE patterns of the Chaitén rhyolites (Fig. ChOltén _ 9rhyollte 13) are notably different from those of the basaltic and andesitic rocks fromthe 41 °30'-46°00'S region of the SVZ. They are depleted in MREE and HREE and exhibit larger negative Eu anomalies (Eu/Eu*=0:59). Their REE pattern resembles those of 33-3rs rhyolites ratherthan rhyolites fromthe 37°00'-41 °30'S region (see López-Escobar and Munizaga, 1983; Frey et al., 1984; Gerlach et al., 1988; Hickey-Vargas et al., 1989). Compared to the nearby Michinmahuida basalts (type-2), the Chaitén rhyolites are enriched in Ta ORB (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; López-Escobar et al., 1991). Analogous to the REE data, HFSE in the Chaitén rhyolites are also more similar to rhyolites from 33- FIG. 12. LaNb versus Rb/Cs diagram lor SSVZ rocks between 41 "30' and 46"00'S compared with SSVZ rocks between 3rS than to rhyolites from the 3]oOO'-41°30'S 37" and 41"S and MORB. Symbols are t'19 same as in region. ligure10. 48 PETROGRAPHV ANO GEOCHEMISTRV OF QUATERNARV ROCKS FROM THE SOUTHERN VOlCANIC ZONE •• •

ISOTOPIC COMPOSITIONS those of the more basic volcanic rocks (Stern et al., 1984). This situation contrasts with that observed in Sr-isotope analyses were made on 32 samples of other centers of the SVZ, Villarrica for example, the 41 °30'-46°00'S region of the SVZ by Notsu et al. where rocks ranging in composition from basalt to (1985, 1987; Table 2). According to these authors, rhyolite have similar 87S rf8eSr ratios (Déruelle et al., their results indicate that the basalts to dacites have 1983; Hickey-Vargas et al., 1989). 87S r/86Sr ratios of 0.70394-0.70482, with the majority The '43Nd/'44Nd ratios of basalts to dacites range ranging from 0.70404 to 0.70434; exceptions to this from 0.51271-0.51290. The few data available, are the andesit3s from Huequi, Yanteles and Corco­ suggest that type-l basalts have similar '43Nd/'44Nd vado wh ich have 67Srf8aSr ratios of 0.70455 to 0.70482. ratios to type-2 (Tables 2, 3). The lowest value is The 87Sr¡aeSr ratio of the Chaitén rhyolites (0.70570) observed for the Chaitén rhyolite (0.51259; Table 2). is the highest among the analyzed rocks by Notsu et This ratio is similar to those of rhyolites from the al. (1985; 1987). Fifteen of these samples were re­ northernmost region ofthe SVZ (33°00'-34°30'S; Fig. checked isotopically at the Open University (Table 2: 14), where the continental crust is comparatively NBS=071 024), the 87S r/86S r relations being in gene­ thick (Futa and Stern, 1988; Hildreth and Moorbath, rallower. The Sr isotope ratios of Hualaihué amount 1988). to 0.70356 (-O 00039), those of Chaitén rhyolite to In a plot of 207Pb/204 Pb versus 208Pbp04Pb (Fig.15), 0.70560 (-0.00010). According to the new de­ as well as inthe 208Pbf204Pb versus 207Pbf204Pb diagram terminations, type-l basalts have slightly lower 87Sr/ (Fig. 16), the 41 °30' -46°00'S region basaltic to dacitic 86Sr ratios than type-2 basalts (Tables 2, 3, and Fig. rocks (Table 2) fall within the fields of the 33-37°S 14), and the rbyolites of Chaitén have significantly Quaternary volcanic rocks determined by Hildreth higher Sr isotope ratios than the basalts and andesites and Moorbath (1988). This field also includes the of its neighbour Michinmahuida. Asimilar relationship volcanic rocks from 3]oOO'-41°30'S analyzed by is observed at the are a (34°S), where the 67Sr/ Harmon et al. (1984) and Hickey et al. (1986). Thus, 86Sr ratios of the rhyolites are significantly higherthan all the SVZ (33-46°S) rocks, regardless of latitude or

100 r:-1r------:J IOO~------~

50

20 !! ..,.¡: e o Huequl andeslfa ~ 10 10 "- JC U O 11: 5 5

"* ChOlfen .. Rn olite ~ ~~!.:f ~hUwjO {t~&~~~~\fJ!~;:' o lio lochue 2 [J COrco .... odo • "'onteles Type4 tIOlOlts • Co)' • r.IJoco

Lo:. Nd Sm Eu Tb Vb lu 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 Challén rhyolltes. L. López-Escobar. R. Kllian, P.D. Kempton and M. Taglri

0 .5130

0.5128 FIG. 14. 14"Nd/14'Nd versus·7Srl""Sr lor SSVZ rocks between 41"30' and 46°OO'S co~ared wtth SVZ between 33° and 37°S, SSVZ between 37" and 41°S, MORB, IAB and OIB. Data sources: 0 .5126 Hlckey el al. (1986) and relerences thereln; Futa and Stem (1988) and relerences thereln; Hlldreth and Moorbath (1988) and relerences thereln. Vlllarrlca stratovolcano and 0.5124 L--__---'- ____--' _____ '-- ____"-----' mlnor eruptlve centers (MEe) 01 the Vlllarrlca area rocks afler Hlckey­ 0.703 0.704 0.705 0706 Vargas et al. (1989). 8 7 S.r/86 Sr

crustal thickness, have similar Pb isotope com­ (1987). The 0'80 values of the basaltic andesites and positions. The only exception is the Chaitén rhyolite, dacites fall in the range +6.4 to +8.4%0, which is which is notably enriched in radiogenic Pb, particularly significantly higherthan that of mantle derived rocks. in 206Pb. The O-isotope composition of the Chaitén rhyolite is The 0'80 values of the 41 °30'8-46°00'8 8VZ +9.4%0, which is typical of upper crustal rocks. The basalts vary from +5.8 to + 7. 7%0(Table 2). However, O-isotope ratio of this rhyolite is s milar to those onlythe Hudson basalts have values within the range exhibited by rhyolites from the northernmost region of of mantle derived rocks (5.8-6.2%0) reported by the 8VZ (8tern et al., 1984; Hildreth and Moorbath, Kyser (1986), Ito et al. (1987) and Woodhead et al. 1988).

DISCUSSION

Although the data presented in this paper are not crysts exhibiting normal zoning; d- 'mixed' ande­ sufficient to evaluate in detail any particular sites are characterized by disequilibrium mineral petrogenetic model, they, at least, improve our assemblages (e.g. plagioclase+oliv;ne+orthopyro­ petrographic and chemical knowledge of 8VZ xene+amphibole) that are compatible with mixing of Quaternary volcanism between latitudes 41 °30' and basaltic and acidic magmas; e- rhyolites, represented 46°00'8, and place some constraints on the origin by the Chaitén rhyolites, which, to the best of our and evolution of volcanic rocks from this region. knowledge, are the only rhyolites reported in this Petrographically, the rocks ofthe 41 °30'8-46°00'8 region of the 8VZ. 8VZ Quaternary volcanic front can be grouped into Chemically, type-1 basalts have comparatively five categories: a- type-1 basalts are characterized low LILE and HF8E, high MgO and CaO, and low by the coexistence of highly zoned olivine (F08S -Foss) FeOm/MgO, La!Yb and 878r/8S8r ratios. Type-2 basalts, and augite; b- type-2 basalts are characterized by on the other hand, have comparatively high LILE and weakly zoned olivine (F07S-F070 ), which commonly HF8E, low MgO and CaO, high FeO(T)/MgO, La!Yb does not coexist with augite; c- 'normal' andesites and 878rfBs8r ratios. The normal andesites follow the and dacites are characterized by plagioclase pheno- chemical trends observed in their coexisting basalts. 50 PETROGRAPHY ANO GEOCHEMISTRY OF Q UATERNARY ROCKS FROM THE SOUTHERN VOLCANIC ZONE ...

15.7.------, 207 P b 204Pb CHA-'* 15.6

FIG. 15. "''PbI''''Pb versus ""'PbP"'Pb lor SSVZ rocles belween 41 "30' and 46"00'S compared wilh SVZ belween 33" and 37' S. SSVZ belween 37" and 41 ' S. SVZ rhyoliles oelween 35" and 36°S. Nazca plale basalls and Nazca plate IB.5 IB .6 IB.7 16 .6 206 P b/ 204 Pb sedlmenls. Dala sources: Hildrelh and Moar­ bath (1988 and relerences Illerain) and Hickey­ Vargas e/ al. (1989) and relerences Iherein.

20B pb .-----.~~~~~~ __,, __ ~~ ~~~7<

204 P b

368

38.6

FIG. 16. ""'Pbl""'Pb versus "''PbI'''''Pb lor 5SVZ 38.4 rocks belween 4 1"30' and 46"00'S comparad wllh SVZ belween33" and 37"S. SSVZ between 37" and 41 "5. MEe 38.2 VUlarrlca area, Nazca plate basalts. Nazca plate sedlmenls. detrilal oceanic sedímenls, MOAB and OIB. Dala sources: Hildrelh and Moorbalh (1 988. and relerences 15.50 15.55 1560 15.65 207Pb/ 204 Pb Ihereln) and Hlckey-Vargas e/ al. (1989, and relerences Illerein).

GENESIS OF THE 41°30'-46°00'S STRATOVOLCANO-BASALTS

In terms of major and trace element abundances, basalts mimic those of basalts from some minor type-1 basalts are among the most primitive basalts eruptive centers (MEC) situated along the lOFZ in in the SVZ, and type-2 basalts among the most the 37°00'-41 °30'S region (e.g. MEC of the Villarrica evolved. With the exception of the Hudson basalts, and areas; Hickey-Vargas et al., 1989; L. the 41 °30'-46°00'S stratovolcano-basalts and those lópez-Escobar, MA Parada, R.L. Hickey-Vargas, from the 37°00'-41°30'S region of the SVZ have FA Frey, H. Moreno'. similar KlRb, Rb/Cs, Baila, laIYb ratios and isotopic The compositional similarities between the 41 °30'- compositions. Comparea to the latter basalts, the 46°00'S andthe 3]000'-41 °30'S stratovolcano-basalts Hudson basalts have higher Rb/Cs and lalYb, but suggest that the main source of the former is also the lower Baila and 207Pb/2G4 Pb ratios (Tables 2, 3). asthenospheric mantle wedge aboye the subduction Actually, the laIYb and Baila ratios of the Hudson zone as proposed by lópez-Escobar and Frey (1976);

I 1993.Contrasting origin 01 adjacent andesitic and basaltic volcanism in the Southern Andes: case 01 Cal buco volcano and minar eruptive centers distributed alon9 the Liquiñe·Ofqui Fau~ Zone, 41-42"S. Universidad de Chile. Departamento de Geologla (Unpublished). 40 p. L. López-Escobar, R. Kilian, P.D. Kempton and M. Tagiri 51

López-Escobar et al. (1977) and Hickey et al. (1986) subducted sediments (Futa and Stern, 1988), both 10r the 37°00'-41°30'S rocks. The BaiLa ratios of sources could be in the asthenosphere. However, i1 these rocks, which are slightly higher than those 01 both the asthenospheric mantle under the 41 °30'- oceanic basalts, suggest that melting was triggered 46°00'S region and the primary magmas are by 1'uids derived from the subducted oceanic slab homogeneous, another mechanism is required to (Hickey et al., 1986; Tormey et al., 1991) explain the chemical differences between types-1 A contribution from oceanic sediments to the and -2 basalts. Possible mechanisms inelude: a­ composition of the slab derived fluids is suggested by dif1erent degrees 01 partial melting 01 the astheno­ the relatively high Cs/Rb and 207Pbf2G4Pb ratios of the sphere 1ollowed by interaction 01 these melts with Michinmahuida, Corcovado, Cay and Maca basalts, heterogeneous lithosphere and b- crustal contami­ which are similar to the values exhibited by the nation. In the lattercase, the corrpositional differences 37°00'-41 °30'S stratovolcano-basalts. The Cs/Rb and cannot be generated in the upper crust, because 207Pbf204Pb ratios of the latter basalts correlate with di11erentiation 01 basaltic magmas in this region pro­ l°Be (Morris et al., 1985; Hickey et al., 1986), which is duces more silica rich magmas (Tormey et al.,1991), another indicator 01 subducted sediment involvement. and since most 01 the ma1ic type-2 basalts are enrich­ U-Th disequilibrium studies (Tormey, 1989; Sig­ ed in incompatible elements, the differences must be marsson et al., 1990; Tormey et al., 1991) also produced at lower or intermediate c'ustal levels. confirm the participation 01 subducted sediment in According to Hildreth and Moorbath (1988), the the genesis of · the 3]oOO'-41°30'S stratovolcano­ Zr, H1 and Ta abundances in crustallilncontaminated basalts. However, the relatively low Cs/Rb and 207Pb/ SVZ basalts would be respectively 125 ppm, 3 ppm 204Pb ratios of the Hudson basalts in conjunction with and 0,2 ppm. In this context, type-1 'Nould appear to the similarity between their O-isotope signature and be practically uncontaminated by crustal material that of mantle-derived rocks suggest that sediments (Tables 2, 3). There10re, their 87Sr,"·Sr and 143Nd/ were not significantly involved in the genesis of their 144Nd ratios, respectively higher and lower than those primary magmas. Considering that Michinmahuida, 01 middle ocean ridge basalts (MOR3), suggest that Corcovado, Cay and Maca are large stratovolcanoes, the mantle under the 41 °30'-46°00'S region is, whose magmas probably underwent one or more isotopically, an OIB-type mantle, as it was postulated, stages 01 homogenization during their ascent to the 10r the mantle under the 37°00'-41 °30'S region 01 the sUrface, the relatively high and variable 0180 values SVZ, by Hickey et al (1986). 01 their basalts could reflect variable degrees 01 The comparatively low Ca and Sr abundances 01 upper crustal contamination. type-2 basalts, as well as their negative Eu-anomaly The differences in LILE, HFSE, FeO

GENESIS OF THE 41°30'-46°00'S ANDESITES

As discussed by Tormey et al. (1991), the basalt­ degrees of upper crustal contamination are required andesite-dacite differentiation in the SVZ would take to explain their modified and variable Sr, O and Pb place in the upper crust. As seen in figure 6 (KP isotope compositions.

Si0 ), most of the 41°30'-46°00'S region Huequi andesites and dacites, and some versus 2 'normal' andesites and dacites are derived from their andesites from Mentolat, can be distinguished from respective parental basaltic magmas (either of type- 'normal' andesites and dacites because they show 1 or -2) by crystal fractionation of those phases petrographical evidence of originating by mixing of present as phenocrysts (mainly olivine, type-1 basaltic and rhyolitic magmas. The Sr-isotope and plagioclase). This process results in enrichment composition of the Huequi samples supports this in elements tha: are incompatible with those phases hypothesis, since their 87Sr¡aeSr ratios are intermediate (LILE and HFSE), and depletion in those that are between those of Hualaihué basalts (type-1) and compatible (Se, Co, Cr, Eu). However, different those of Chaitén rhyolite.

GENESIS OF THE CHAITEN RHYOLITES

The Chaitén rhyolite differs significantlyfromtype- are different. The isotopic differences suggest that and -2 basalts in incompatible trace element the rhyolite has a crustal origino Its low AIP3' CaO, Sr, abundances and ratios, and in isotopic ratios (Table and Eu-depletion on one hand, and its low abundances 2, Figs. 9-16). The incompatible trace element pattern of Cr, Co, MREE, y and HREE, on the other, suggest and Sr and Nd isotope ratios of the Chaitén rhyolite that plagioclase and amphibole were residual phases resemble those of rhyolites from the 33-3]oS region in its source. On this basis, the Chaitén rhyolite ofthe SVZ (Stem et al., 1984; Hildreth and Moorbath, primitive magma would have been generated by 1988), which are situated in an are a of relatively thick melting of an amphibolitic source. The low KlRb ratio continental crust, rather than those from the 3]000'- seems to confirmthis hypothesis.ln fact, as suggested 41°30'S region (Deruelle et al., 1983; Gerlach et al., by Hanson (1978) and Davidson et al. (1988), low KI 1988; Hickey-Vargas et al., 1989), which are situated Rb ratios are characteristic of liquids generated by in an area of relativelythin continental crust. However, low degrees of partial melting of an amphibolitic in Pb-isotope compositions, the Chaitén rhyolite is source within the crust. The high 0180 value of the unique in relation to rhyolites from the 33°00'-41 °30'S Chaitén rhyolite sample suggests, in addition, that region of this volcanic arc (Figs. 15, 16). this source probably originated by metamorphism of The differences, in incompatible trace element rocks that were once near the surface of the earth. patterns and Sr. Nd, Pb and O isotope compositions, The presence of accreted metabasalts west of the between the Chaitén rhyolite and the 41 °30'-46°00'S volcanic arc (Charrier et al., 1991, and references region basalts strongly indicate thattheir main sources therein) supports this hypothesis.

CONCLUSIONS

Five types of petrographically and chemically With the exception of the Hudson basalts, the distinct Quatemary volcanic rocks are distinguished stratovolcano-basalts from this region are in the 41 °30'-46~00'S region of the SVZ of the Andes: geochemically similar to stratovolcano-basalts from depleted basalt;¡ (type-1), enriched basalts (type-2), farther north (3]oOO'-41°30'S region of the SVZ). In normal andesites and dacites, mixed andesites and some geochemical features (relatively high laIYb dacites, and rhyolites. and low Baila ratios), the Hudson basalts are similar L. López-Escobar, R. Kilian, P.D. Kempton and M. Tagiri 53

to basalts from some minor eruptive centers (MEC) accompanied by some kind of crustal assimilation in situated along the LOFZ in the 37°00'-41 °30'S region. orderto raise their 87S r/86S r ratios, keeping their 143Nd/ Both types of basalts are consistent with 144Nd ratios approximately constant relative to type-1 generation by melting of OIB-type asthenospheric basalts. material. Melting would be triggered by fluids derived At upper crustal levels, some basaltic magmas fromthe subducting oceanic lithosphere (as indicated evolved by fractional crystallization, involving phases by the relatively high BaiLa ratios in these basalts). present as phenocrysts, accompanied by assimilation Subducted sediments are commonly involved in this of upper crustal material, to create intermediate rocks; process (as suggested by the high Cs/Rb and 207Pb/ others, however, mixed with rhyolitic magmas to 20-4pb ratios of most basalts). generate the so-called mixed andesites and dacites. Primary asthenospheric magmas could be The Chaitén rhyolites have chemical and isotopic modified, to various extents, within the mantle characteristics that suggest that their primitive lithosphere or within the lower or intermediate crust. magmas were generated by partial melting of crustal This contamination process could have created some material of amphibolitic composition. This partial of the compositional differences observed between melting was probably induced by the intrusion of the type-1 and type-2 basalts. The modified magmas relative hot «1200°C) type-1 basaltic magma into would represent the parental magmas of erupted the lower crust. A mixing of basaltic and rhyolitic basalts. The depletion in MgO, CaO and Sr, observed magmas could have produced the Huequi andesites. in type-2 basalts in relation to type-1, and their This process would explain the occurrence of basalts, negative Eu-anomalies, could be also due to rhyolites and mixed andesites in the smalllocal zone fractionation of plagioclase and olivine from their of the SVZ between 42° and 43°S. parental magmas. However, this process had to be

ACKNOWLEDGEMENTS

LLE acknowledges the financial support of the gratefully thanks the Deutsche Forschungs­ following grants: FONDECYT 1051-89 and 1221-91, gemeinschaft for their support and fu,ding. Wethank DTI- Universidad de Chile E-2834_ The Fundación Drs. K. Notsu, of the University of Tokyo (Laboratory Andes-Chile is also acknowledgedforthe Sabbatical for Earthquake Chemistry), Wes Hildreth of the U.S. Period Program grant, andthe hospitality ofthe NERC Geological Survey, Jon Davidson of the University of Isotope Geoscience Laboratory, Keyworth, California at Los Angeles, and Russell Harmon of the Nottingham, United Kingdom is greatly appreciated. NERC Isotope Geosciences Laboratory, Keyworth We also acknowledge the grants N° 58041009 and (Nottingham), fortheir kind collaboration in obtaining 59043009 of the Ministry of Education, Science and isotopic and trace element data. We also thank the Culture of Japan, obtained by Dr. N. Onuma, former critical suggestions of Drs. Moyra Gardeweg, Robert Professor of the Ibaraki University, deceased on Drake, Francisco Hervé and Robert Pankhurst that January 15, 1985, that made possible the sampling improved the manuscript. This work is a contribution by helicopter, of thirteen volcanoes belonging to the to the IGCP Project N°345 'Lithospheric Evolution of 41°30'-46°00'S region of the SVZ of the Andes. RK the Andean Continental Margin'.

REFERENCES

Catanzaro, E.J.; Murphy, T.J.; Shields, W.R. 1968. Absolute Vol. 49, No. 1, p. 348. isolopie abundance ralios 01 three leadisotope standards Cembrano, J. 1992. The Liquiñe-Olqui laollt zone (LOFZ) in (Abstraet). American GeophysicaJ Union. Transactions, !he province 01 Palena: field and mierostruetural evidence 54 PETROGRAPHY ANO GEOCHEMISTRY OF QUATERNARY ROCKS FROM THE SOUTHERN VOl CANIC ZONE ...

01 a ductile-brittle dextral shear zone. Universidad de Hervé, F.; Araya, E.; Fuenzalida, J.L.; Solano, A. 1978. Chile, Departamento de Geología, Comunicaciones, Nuevos antecedentes acerea de la geologra de la costa No. 43, p. 3-27. norte de Chiloé Continental. Chile. In Congreso Charrier, R.; Godoy, E.; Rebolledo, S.; Hervé, F. 1991. Is Geológico Argentino, No. 7, Actas, Vol. 1, p. 629~38 . earty PaleozoiC accretion 01 the Choapa metamorphic Hervé, F.; Araya, E.; Fuenzalida, J.L.; Solano, A. 1979. complexin certral Chile compatible with Ihe pracordillera Edades radiométricas y tectónica neógena en el sector terrane? UniV9rsidad de Chile, Departamento de Geo­ costero de Chiloé continental, X Región. In Congreso logIa, Comun.'caciones, No. 42, p. 44-48. Geológico Chileno, No. 2, Actas, Vol. 1, p. F1-F18. Davidson, J.P.; Fe rgu son , K.M.; Colucci, M.T.; Dungan, Hickey, R.L.; Fray, F.A.; Gerlach, D.C.; López-Escobar. L. M.A. 1988. The origin and evolution 01 magmas lrom tIle 1986. Multiple sourees lor basaltic are rocks 110m tIle San Pedro-Pellado volcanic complex, S. Chile: mul­ Southem Volcanic Zone 01 the Andes (34°-41 OS): trace ticomponent sources and open system evolution. elament and isotopic evidence 01 contributions 110m Contributions to Mineralogy and Petrology, Vol. 100, p. subducted oceanic crust, mantle and continental crust. 429-445. Journal of Geophysical Research, Vol. 91, p. 5963- Déruelle, B.; Harnon, R.S.; Moorbalh, 5.1983. Combined 5983. Sr-O isotope relationships and petrogenesis 01 Andean Hickey-Vargas, R.L.; Moreno, H.; López-Escobar, L. ; Frey, Volcanics 01 South America. Nature, Vol. 302, p. 814- F.A. 1989. Geochemical variations in Andean basal tic 816. and silicic lavas from the Villarrica-Lanin volcanic chain Ferguson, K.M.; Dungan, M.A.; Davidson, J.P.; Colucci, (39.5"5): an evaluation 01 source heterogeneity, M.T. 1992. The Tatara-San Pedrovolcano, 36"S, Chile: Iractional crystallization and crustal assimilation. A chemically variable, dominantly malic magmatic Contributions to Mineralogy and Petrology, Vol. 103, p. system. Journal o( Petrology, Vol. 33, p. 1-43. 361-386. Frey, F.A.; Gertadl, D.C.; Hickey, R.L. ; López-Escobar, L. ; Hildretll, W.E.; Moorbatll, S. 1988. Crustal contribution to Munizaga, F. 1984. Petrogenesis 01 the Laguna del arc magmatism in the Andes 01 Central Chile. Maule volcanlc complex, Chile (36"S). Contributions to Contributions to Mineralogy and Petrology, Vol. 98, p. Mineralogy and Petrology, Vol. 88, p. 133-149. 455-489. Fuenzalida, J.L. 1979. Estudio geológico preliminar de Holman, A.W. 1988. Chemical dillerentiation 01 the Earth: Península Huaqui, X Región. Memoria de Titulo (Inédi­ tIle relationship between mantle, continental crust, and to). Universidad de Chile, Departamento de Geologla, oceanic crust. Earth and Planetary Sciences Lefters, 158 p. Vol. 90, p. 297-314. Futa, K.; Stem, C.R. 1988. Sr and Nd isotopic and trace Irvine. T.N; Baragar, W.R.A. 1971 . A guide to the chemical element compositions 01 Quatemary volcanic centers classilication 01 tIle common volcanic rocks. Canadian 01 the Southern Andes. Earth and Planetary Sciences Journal of Earth Sciences, Vol. 8, p. 523-548. Lefters, Vol. 88, p. 253-262. Ito, E.; White, W.M.; Gopel, C. 1987. The 0, Sr, Nd, and Pb Gerlach, D.C.; Fray, F.A. ; Moreno, H. ; López-Escobar, L. isotope geochemistry 01 MORB. Chemical Geology, 1988. Recentvolcanism in the Puyehue-Cordón Caulle Vol. 62, p. 157-176. Region, Soutllern Andes, Chile (40.5"S): Petrogenesis Killian, R.; López-Escobar, L. 1989. Volcanismocuaternario 01 evolved la\'as. Journal o( Petrology, Vol. 29, p. 333- en los Andes patagónicos (41 "S-55"S): aspectos geo­ 382. lógicos, petrográficos y geoquímicos. Medio Ambiente, Hanson, G. N. 1978. The application oftrace elements to tIle Vol. 10, p. 92-106. petrogenesis 01 igneous rocks 01 granitic composition. Killian, R.; López-Escobar, L. 1991. Petrology olthe Southem Earth and Planetary Sciences Lefters, Vol. 38, p. 26-43. Southandean Volcanic Zone (41-46"5) and its Harmon, R.S.; Barreiro, B.A.; Moorbath, S.; Hoels, J.; implication, with emphasis on Michinmahuida-Chaitén Francis, P.W.; Thorpe, R.S.; Déruelle, B.; McHugh, J.; complex (43"5). Zentralblatt für Geologie und Viglino, J.A. 1984. Regional 0-, Sr-, and Pb-isotope Pa/aontologie, Vol. 1, p. 1693-1708. relationships in late Cenozoic calc-alkaline lavas 01 the Kyser, T.K. 1986. Stable isotope variations in Ihe mantte. In Andean Corállera. The Geological Society of London, Stables isotopes in high temperature geological Journa/, Vol. 141, p. 803-822. processes (Valley, J.W.; Taylor, H.P.; O'Neil, J.R.; Herron, E.M.; Cande, S.C.; Hall, B.R. 1981. An active editors). ReviewofMineralogy, Vol. 16, p. 141-164. spreading cEWIter collides with a subduction zone: A López-Escobar, L. 1984. Petrology and chemistry olvolcanic geophysical survey 01 the Chile Margin triple junction. rocks 01 Ihe Southern Andes. In Andean magmatism: Geological Society o( America, Memoir, No. 154, p. chemical and isotopic constraints (Harmon, R.S.; 683-701. Barreiro, B.A.; editors). Shiva Publications Ud., p. 47- Hervé, M. 1984. La zona de lalla Liquiñe-Olqui en Liquiñe. 71 . Cheshire, United Kingdom. Universidad de Chile, Departamento de Geología, Co­ López-Escobar, L.; Frey, F. 1976. Rocas volcánicas cua­ municaciones, No. 34, p. 101-105. ternarias de Chile central-sur (33"-41 "S): Modelos L. López-Escobar, R. Klllan, P.D. Kempton and M. Tagirl ss

petrogenéticos sugeridos por las tierras raras. In Con­ Onuma, N.; López-Escobar, L. 1987. Possible contribution greso Geológico Chileno, No. 1, Actas, Vol. 2, p. F223- of the asthenosphere, below the subducted oceanic F225. Iithosphere, to the genesis of the are magmas: López-Escobar, L.; Munizaga, F. 1983. Caracterrsticas geochemical evidence from the Andean Southem geoqurmicas del centro volcánico Laguna del Maule, Volcanic Zone (33-46°S). Journal of Volcanologyand Andes del Sur, 36"OO'S. Revista Geológica de Chile, Geothermal Research, Vol. 33, p. 283-298. No. 19-20 p. 3-24. Pankhurst, R.J.; Hervé, F.; Rojas, L.; Cembrano, J. (In López-Escobar, L.; Frey, F.; Vergara, M. 1977. Andesites press). Magmatism and tectonics in cxmtinental Chiloé, and high alu mina basalts from the central-south Chile Chile (42°-42°3O'S). Tectonophysics. high Andes: geochemical evidences bearing on their Sigmarsson, O.; Condomines, M.; Morris, J.D.; Harmon, petrogenesis. Contributions to Mineralogy andPetrology, R.S. 1990. Uranium and toSe enrich-nents by fluids in Vol. 63, p. 199-228. Andean Are magmas. Nature, Vol. 346, p. 163-165. López-Escobar, L.; Tagiri, M.; Vergara, M. 1991. Stem, C.R.; Skewes, M.A.; Durán, A.M. 1976. Volcanismo Geochemical featu res of the Southern Andes orogénico en Chile Austral. In Congreso Geológico Quatemary voleanics between 41 0 50' and 43°00'S. Chileno, No. 1, Actas, Vol. 2, p. F195-F212. Geologieal Societyof Ameriea, Special Paper, No. 265, Stem, C.R.; Futa, K.; Muehlenbachs, K.; Dobbs, F.M.; p.45-56. Muñoz, J.; Godoy, E.; Charrier, R. 1984. Sr, Nd, Pband López-Escobar, L.; Moreno, H.; Tagiri, M.; Notsu, K.; Onuma, O isotope composition of Late Cenozoic volcanics, N. 1985a. Geochemistry and petrology of lavas from northemmost SVZ (33°-34"S). In Andean magmatism: San José volcano, Southern Andes (33°45'S). chemical and isotopic constraints (Harmon, R.S.; Geochemical Journa/, Vol. 19, p. 209-222. Ba rrei ro , B.A.; editors). Shiva Publications Ud., p. 96- López-Escobar, L.; Tagiri, M.; Notsu, K.; Hildreth, W.; 105. Cheshire, United Kingdom. Harmon, R.S.; Davidson, J. 1985b. Geochemical Thiele, R.; Hervé, F.; Parada, M.A.; Godoy, E. 1986. La characteristics of rocks from the Andean SVZ between megafalla Liquiñe-Ofqui en el Fiordo Reloncavr 41°30'S and 46"00'S. Universidad de Chile, Departa­ (41"30'S), Chile. Universidad de Chile, Departamento mento de Geologfa, Comunicaciones, No. 35, p.137- de Geologfa, Comunicaciones, No. 37, p. 31-47. 141. Tormey, D.R. 1989. Geology and gec<:hemistry of the Morris, J.S.; Tera, F.; Harmon, R.S.; López-Escobar, L.; active Azufre-Planchón-Peteroa volcanic center Klein, J.; Midleton, R. 1985. Be-10 in lavas from the (35°15'S, Southem Andes): implieations for cordilleran Andean Southem Volcanic Zone (35"-40 0 S): Evidence are magmatism. Ph.D. Thesis (Unpublished) for sediment subduction. Universidad de Chile, Depar­ Massachusetts Institute of Technolo~, 331 p. tamento de Geologfa, Comunicaciones, No. 35, p. 147- Torrney, D.A.; Hickey-Vargas, R.L.; Frey, F.A.; López• 155. Escobar, L. 1991. Recent lavas from tha Andean voleanic Notsu, K.; López-Escobar, L.; Onuma, N. 1985. Sehaviour front (33" to 42"S): interpretations of along strike of the strontium isotope ratios along the strike of the compositional variations. Geologica/ SocietyofAmeriea, Andean SVZ with emphasis in the 41 °30' -46°oo'S region. Specia/ Paper, No. 265, p. 57-77. Universidad de Chile, Departamento de Geología, Co­ Woodhead, J.D.; Harmon, R.S.; Fraser, D.G. 1987. O, S, municaciones, No. 35, p. 177-180. Sr, and Pb isotope variations in volcanic rocks from the Notsu, K.; López-Escobar, L.; Onuma, N. 1987. Along-arc Northem Mariana Islands: implications for crustal variation of Sr-isotope composition in volcanic rocks recycling in intra-oceanic ares. Eanh and Planetary from the Southem Andes (33"S-55"S). Geochemical Sciences Letters, Vol. 83, p. 39-52. Journa/, Vol. 21, p. 307-313.

Manuscript received: November 12, 1991; accepted: March 24,1993.