Geochemistry and tectonics of the Chilean 80uthern basaltic Quaternary volcanism (37-46°8)

Leopoldo López-Escobar

José Cembrano Departamento de Geologla, Universidad de , Casilla 13518, Santiago 21 , Chile Hugo Moreno

ABSTRACT

Between latitudes 37° and 46°S 01 the Andes, the Nazca-South America Plate convergen ce Is currently slightly oblique, Postglacial volcanism has been continuous and intense, being expressed as numerous composite stratovolcanoes (SV) and hundreds 01 minoreruptive centers (MEC). The overall trend 01 the volcanic are is NNE (- N100E), and its main structural leature is Ihe 1,000 km long, also NNE-trending, Uquiñe-Olqui lault zone (lOFZ). Some MEC are spatially associated wilh the main NNE-trending lineaments 01 the lOFZ; others, such as Ihe Carrán-los Venados volcanic group (40.3°S), lorm N50-700E clusters, oblique to the overall trend 01 the volcanic are. Between 37° a,d 41.5°S, Central Southem Volcanic Zone (CSVZ), the SV lorm either N50-600W or N50-700E alignments. Between 41.5° and 46°S, South Southem Volcanic Zone (SSVZ), Ihe distribution 01 SV is similar. The alignment 01 parasitic vents, MEC and SV suggests that the direction O'H .. in Ihe CSVZ and SSVZ is roughly N50-700E, which may rellect a transpressional tectonic regime, 01 m resulting lrom a combination 01 dextral strike-slip and shortening across the are. Most young extensional NE-trending volcanic chains (late Pleistocene-Holocene), including SV ± MEC contain mainly basal tic rocks. This is consistent with a short residen ce time 01 magmas in the crust. Older volcanic complexes (Eariy Pleistocene) and volcanic edil ices controlled by NW-trending contractional Iractures and laults present not only basaltic rocks, but also medium andesites, and even rhyolites. This is probably the result 01 a longer.intracrustal magma residence, yielding a more mature and differentiated composition. Basaltic rocks from eilher N50-600W and N50-700E transverse fractures tend to increase their incompatible elemenls abundances, BaIla, and 87Sr¡eeSr ratios and decrease their lalYb and ' 43 Nd/''''Nd ralios in going from wesl lo east, suggesling that these geochemical features are mainly controlled by subcrustal features and processes; fractures and faults would only facilitate the ascent of magmas. Similargeochemical behaviours are observed between MEC basaltic rocks located al similar latitude, but different longitude.

Keywords: Arc basa/tic magmas, Southern Andes, Tectonism, Geochemistry.

RESUMEN

Geoquímica y tectónica del volcanismo cuaternario basáltico de los Andes del Sur de Chile (37-46°5)_ Entre las latitudes 37° y 46°S de los Andes, la convergencia entre las placas de Nazca y Sudamericana es levemente oblicua. El volcanismo posglacial, que ha sido continuo e intenso, ha originado numerosos estratovolcanes compuestos (EV) y cientos de centros eruptivos menores (CEM). El arco volcánico tiene una orientación general NNE (-N100E), y su principal característica estructural es la zona de Falla Liquiñe-Olqui (ZFlO), de 1.000 km de longitud y orientación NNE. Algunos CEM están asociados espacialmente con los lineamientos NNE de la ZFlO; otros, como el grupo volcánico Carrán-los Venados (40,3°5), están orientados oblicuamente (N50-700E) con respe·::to a la orientación general del arco volcánico. Entre los 37° y 41 ,50S, Zona Volcánica Sur Central (ZVSC), los EV presentan alineamientos N50-600Wo N50-700E; algo similar acontece entre los 41 ,5° Y 46°S, Zona Volcánica Sur Sur (ZVSS). El alineamiento

de los conos parásitos, de los CEM y de los EV sugiere que la dirección de O'H max en las regiones central y surde los Andes

Revistll GeoldglclI d

del Sur es aproximadamente N50-700E, lo cual reflejaría un régimen tectónico transpresional, que resultaría de la combinación de una rotación dextral, tipo 'strike-slip', y un acortamiento del arco. La mayoría de las cadenas volcánicas jóvenes (Pleistoceno tardío-Holoceno), de orientación noreste, son extensionales y contienen principalmente rocas basálticas, lo cuales consistente con un tiempo de residencia cortical corto. Complejos volcánicos antiguos (Pleistoceno temprano) y edificbs controlados por fracturas y fallas compresionales, de orientación noroeste, presentan basaltos y también rocas volcánicas más ácidas. Esto resultaría de un tiempo de residencia más prolongado en la corteza, lo que permitiría la genelación de magmas más maduros y diferenciados. Las rocas basálticas, tanto de alineamiento de orientación N50-600W como N50-700E, tienden a aumentar su abundancia en elementos incompatibles y sus razones BaiLa y 87Sr/86Sr y a disminuir sus razones LalYb y ' 43Nd/'44Nd en dirección oeste-este, sugiriendo que estas características dependen fundamentalmente de procesos subcorticales; las fracturas y fallas sólo facilitarían el ascenso de los magmas.

Palabras claves: Magmas basáfflccs de arco, Andes del Sur, Teclonismo, Geoqulmica.

INTRODUCTION

least four distinetive petrographie provinees have The Southern Voleanie Zone (SVZ) of the Andes been reeognized in this Quaternary voleanie are: is located along the western margin of the South north (NSVZ=33-34.5°S), transitional (TSVZ=34.5- Ameriea Plate, between latitudes 33°S and 46°S. At 37"S). central (CSVZ=37-41.5°S) and south (SSVZ=41 .5°-46°S) (López-Eseobar and Moreno, 1994 and referenees therein; Fig. 1). The NSVZ provinee eonsists mainly of andesites and daeites with OH' bearing ferromagnesian minerals, sueh as amphiboles and biotite. In this provinee, the voleanie eenters exhibit a northsouth alignment. The TSVZ provinee ranges in eomposition from to rhyolites, with a predominanee of andesites and daeites. Pyroxenes±amphiboles±biotite is a eommon mineral assoeiation in most silieie produets. In this provinee, northwest alignments of the voleanie eenters predominate over northsouth and northeast alignments (Katsui and Katz, 1967; Katz, 1971;

--""\'­\ Nakamura, 1973; Moreno, 1974, 1976). The CSVZ \ provinee eonsists of a series of basalts to rhyolites, with a predominanee of basalts and basaltie ande­ sites. Two pyroxenes±Fe-olivine is a eommon min­ eralogy in the daeites and rhyolites. Northeast and northwest, but not northsouth, alignments of the composite stratovoleanoes (SV) are observed in this provinee; northsouth alignments are observed among minor eruptive eenters (MEC=seoria eones ± lava flows and maars). Finally, the SSVZ provinee is les s well known, but two types of basalts, 'normal' ande­ FIG.l . Geolectonic setting 01 the Southern Volcanic Zone (SVZ) sites+daeites, 'mixed' andesites+daeites and searee 01 Ihe Andes and ~s lour main proYinces (modilied Irom rhyolites have been deseribed (López-Eseobar et L6pez-Escobar e/ al., 1993 and relerences Iherein). The al., 1991, 1993). The intermediatetosilicie members trench is shown by a Ihick, barbed line; the Chile Ridge intersecls tha continental margin al lhe southern end 01 exhibit OH ' bearing ferromagnesian minerals. In this lhe SSVZ. Naz<:a-South ArnelÍCaPlate convergence is slighlty province, the alignment of the SV is mainly northeast. dexlral-obllqtle and occurs al 9 cmlyr (Ciluentes, 1989). The CSVZ and SSVZ (37- 46°S; Fig. 1), as the rest L. López·Escobar. J. Cembrano and H. Moreno 221

of the Southern Andes volcanic range, are the result andesites, daeites and scarcely rhyolites). of the subduction of the oceanic Nazca Plate beneath NNE-trending regionallineaments correspond to the continental South America Plate. The northern the main basement structural feature of the CSVZ boundary of this Andean segment is the prolongation and SSVZ provinces, the 1,000 km long Liquiñe­ into the continent of the intersection of the Mocha Ofqui Fault Zone (LOFZ; Hervé, F. el al., 1979; Fig. Fracture Zone with the Chile-Perú Trench; its 2), previously designated as the Liquiñe-Reloneaví southern end is the intersection of the Chile Ridge Fault Zone (Hervé, F. el al., 1974). The LOFZ is with the continen!. In this region of the Andes, the represented in the field by northsOl.. th trending ductile Nazca-South America Plate convergence is currently shear zones and brittle faults (Hervé, M. 1976; oblique (_25°) with respect to the orthogonal to the Cembrano, 1992; Pankhurst el al.. 1992; Cembrano trench (Jarrard, 1986; Dewey and Lamb, 1992). and Hervé, F., 1993). Other regional-scale linea­ Ouaternary volcanism is intense and very active in ments are obliq ue to the overall trend of the magmatic both provinces (Moreno, 1974, 1976). Between 37° arc and show a predominant N50-600W and N50- and 41.5°S (CSVZ), the width of the volcanic arc, 70 0 E orientation (Moreno, 1974, 1976). Some of the whose axis is located about 270 km from the trench, northeast trending lineaments are :hought to join the is aproximately 80 km, with a maximum of 120 km two right-stepping NNE trending main lineaments of and a minimum of 70 km; between 41 .5°S and 46°S the LOFZ, giving rise to a strike-slip duplex (Cembrano (SSVZ), the width of the volcanic are reduces to and Hervé, 1993). about 40 km. The volcanic activity is expressed as Geologieal, geochemical and petrological studies numerous SV and hundreds of MEC. During post­ carried out in the CSVZ and SSVZ of the Andes have glacial times (Iast 15,000 years), volcanic activity been mostly centered in the SV, although some MEC has been eontinuous with eruptions in both kinds of olivine basalts eould represent some of the most volcanoes. Postglacial rocks of SV and MEC from primitive magmas erupted in this Andean region. the CSVZ and SSVZ are predominantly basalts and The aim of the present study is to discuss some basaltic-andesites (Moreno, 1974, 1976; López• geochemical characteristics of basalts erupted by Escobar, 1984; Hickey el al., 1986; Hickey-Vargas SV and MEC, spatially associated with the LOFZ, in el al., 1989; Tormey el al., 1991 a; López-Eseobar el the 37- 46°S region of the SVZ, to ·:ritically evaluate al., 1991,1993), although the SV may al so exhibit these features as a function of their tectonic setting. intermediate to silicic products (andesites, silicic-

SPATIAL DISTRIBUTION OF VOLCANIC CENTERS

The spatial arrangement of individual SV and (39.25°S), Huililco (39.soS), Antlcura (4 centers; clusters of MEC within the SVZ arc, between 3]0 and 40.6°S), Cayutué (17 centers; 41.3°S), La Viguería 46°S, seems to define regular patterns at a regional (41.4°S), Pocoihuén (41.soS), Palena (43.00S), Río scale (Fig. 2). Even parasitic volcanoes (vents) Frío (43.soS) and Puyuhuapi (9 centers; 44.3°S). within individual SV show a systematic distribution, However, at a larger scale, other MEC, such as those which is consistent with the regional pattern shown of the Carrán-Los Venados vol·:anic group (70 by SV and MEC (Katsui and Katz, 1967; Katz, 1971; centers; 40.3°S; Figs. 2, 3; Morooo, 1977, 1980), Nakamura, 1973; Moreno, 1974, 1976; Moreno and form clusters whose long axes are oriented in a N50- Parada, 1976; Nakamura, 1977; Cembrano and 70 0E direction (Fig. 3). Moreno, 1994; Figs. 2, 3). The distribution of the SV in the CSVZ (37- In the CSVZ and SSVZ provinces, the distribution 41.5°S) is mainly restricted to alignments whose of so me MEC is spatially associated with the main trends are oblique with respect to the magmatic arc NNE-trending lineaments of the LOFZ, which are axis. In fact, the SVform either N50-600W alignments parallel to the overall trend of the volcanic arc. Such (T olhuaca-, -Ouetrupillán-Lan ín, is the case 01 Loica (38.5°S), Caburga (5 centers; Puyehue-Cordón Caulle; Moreno, 1974, 1976, 1977) 39.2°S), Huelemolle (3 centers; 39.2°S), Pichares or, more commonly, N50-70oE ali{;nments (- 222 GEOCHEMISTRY ANO TECTONICS OF THE CHILEAN SOUTHERN ANDES ...

710 ) o .. .. ") Hornoplren ( Anlueo (.) Los Angel ... VOle~ ~ ~ Voleano ) ~ r---'v""" I ~~cono ~':1 'l 8 '0,.~ V~leano ~ :\3 , ( Chri!énl : o cleano:-·• oo.: I\ ·Miehinmahuida Voleono (.) t i \ 'tI "1 '\ \ -~ TEt.lUCO. \l:)\eano. r ,..."._ ..-J. \\ ...rlJ>

(.) o \,: ?90 , r" l' )", I/~ ~ villarrico\mlcan~'./ : ° ¡'rJ Mehmayu J} \; 44° (.) .....~ c:s~ '-..JI Volcono ' z Quelrupillón \mle~no •• : o~ ¿J ro A).1 I & I Lanín Voleano ~;~.~ D'\i' I /1j (' J> A' e¿)~~"ol> l ~uyul\uopi ~ __ Choshueneo vOlean$ ~ • Voleanlc Groü~ '-, 40° '~r~: ? Carrón _ :¡\., G> C-f'.-'-'-/ Lo, Nevados .' :> '" Voleanie Group ~ ! ;z \.r·, '\~ ~!50 P'Jyehue VOlcon~ j. ""1 N \~ =14. \ ;z- ~ .. "1,.. \ 1- COIHAIQUE 5" " PunliagJdo Voleano . )' Llonquihw Lok, • : / ~ o 30 60km albueo VOlean~~ I I I ~" ~; ó \ • Hudson Voleono ! ;

Holocene stratovolcanoes ~ Alignments and/or fractures • " ó Pleistocene stratovolcanoes /''/ Liquiñe-Ofqui Fault Zone (LOFZ) Holocene minor eruptive centers i' Pleistocene minor eruptive centers '''' Pleistocene Caldera

FIG. 2. Spatial dislribulion 01 Slralovotcanoes (SV) and minor eruplive cenlers (MEC) 01 Ihe CSVZ and SSVZ 01 Ihe Andes. Slralovolcanoes lorm northeasl and northwesl-lrending allgnmenls. MEC lorm NSO-70oE Irending, en echelon, elongaled cluslers or lie along regional scale NNE Irending lineamenls 01 Ihe Liquiñe-Olqul Faull Zone. Modllled Irom Kalz (1971), Moreno (1976), Cembrano and Moreno (1994) .

Sierra Velluda, Llaima-Sierra Nevada, Osomo-Pun­ According to Cembrano and Hervé, F. (1993), the tiagudo-Cordón Cenizos). In the SSVZ (41.5-46°S), last four SV are distributed along N400E lineaments some SV lie along N50-700E lineaments (Hualaihué­ that join the two N1 OOE branches of the LOFZ (Fig_ , , Maca-), some lie within 2), but LANDSAT images show a well-defined the LOFZ (Horn:>pirén), and finally, Corcovado, northsouth alignment of , Corcovado, , Melimcyu and SV are located 30 Yanteles and volcanoes. km westward of the main lineament of the LOFZ. a b r­ -,-', .... , ro- 3"'30' , , , , 72°10' , \ I t \ \. ... ')t \ "'-,--'1 \ [¡1 \ r, ,0 .. \ \ '----'--~ \ \ \ ( ~ \ \ .-' '). ' .... , ~ " '\ PQo-qulNocionoI~"1G /~ \',- ~~-.... ' ... " (-- \ \C~~ ...... 1...-~ "7 (~ J i --- LOZ.t.. c5v.. í iil ;:_-"-J'--"..... S:- ovJ-Ó-~J~o·.o.r ).!o 5 lb , ) - ~ .... __,,' C" 6. '--t) ,¡e" ,,@t\ ;t :)_ (j17 .. ,', '\\, ~ ti! \ t'~\ " o r" 1 5 '-, ( I 1 I I 72doo' -- ...... 1 1 \ , J J ( ___--....:v~.I,..-- CARRAN VOLeA.'C SUBGROUP--'¡ -,' /------) , o 25 km 72"00' \ ...... ,,/ .. ,'j, .... -¡¡·-¡;;-.;;.t ...... f , !

e -- ") d o tOkm --~ .J I \' J'\', '\"J'I...I ~ , ,\ ,,,-.... C' , FIG.3. Spatial distribution 01 pa­ ~\ I :J ••~ > rasitic eones (llank erup­ .,.JI '--...,..,1 • • :. J 1 tion sites) and MEC within { ~. c' the CSVZ and SSVZ 01 ~PUNTIAG.UDO u I the Andes. The long axis r..-J 41° • o .~,:t,Ot3 01 clusters 01 parasitic \ ec.'C,_'\ , ", Cr \ I eones 01 the stratovol­ I '-- ..... ( " , .J \ / canoes trends roughly ; N50-70oE; B- Llaima vol­ cano (Naranjo and More­ no, 1991); b- Carrán-Los Venados volcanic group (Moreno,1977); c- Osor­ LlDnquihu. no- Puntiagudo-Cordón Lo" ~ N Cenizos volcanic chain (Moreno. 1976) and d­ "" Maca-Cay volcanic re­ gion. N '"c.> GEOCHEMISTRY ANO TECTONICS OF THE CHILEAN SOUTHERN ANDES ...

VOLCANISM ANO TECTONICS: IS THERE A CAUSAL RELATIONSHIP?

Nakamura (1977) proposed that the systematic Uyeda, 1980). More recent work on the causal alignments of MEC andflankvolcanoes (vents) within relationship between tectonism and volcanism major SV in subduction zone margins may be used (Bellier and Sebrier, 1994; Ventura, 1994) have em­ as a tool to determine the direction of the maximum phasized the close spatial and temporal relationship

)' existing between volcanoes and pull-apart basins, horizontal stress (aH max at both local and regional scales. Both flank crater distribution within indivi­ releasing bends, tension fractures and Riedel shears, dual SV and the elongated distribution of MEC are within and overall strike-slip deformation zone. In believed to represent a natural analog of hydrofrac­ addition, focal mechanism studies of crustal earth­ ture experiments resulting from magma overpres­ quakes, occuring in many volcanic arcs, show that suring. In fact, the volcanic centers are supposed to strike-slip deformation predominates over. com­ be the surface expression of vertical feeder dykes pression or extension (Mc Caffrey, 1992). The latter located underneath (Nakamura, 1977). According stress regimes are recorded in forearc and backarc to Lister and Kerr (1991) deviatoric stress plays a regions, respectively. The hypothesis of magma key role in the initiation and orientation of dykes. For ascending as dykes through a composite system of low viscosity magmas, dykes are expected to be tensional and shear fractures, in an overall strike­ oriented perpendicular to the minimum principal slip deformation regime (cr1 and cr3 horizontal), is stress axis at all dephts within the lithosphere (Emer­ consistent with the dynamic models of fracture man and Marrett, , 990). Hence, spatially associated propagation proposed by Hill (1977) and Shaw (1980) volcanic alignments would reflect the direction of the for the crust and lithospheric mantle. maximum horizontal stress, which may be either cr1 Following the model proposed by Nakamura

• or cr2. Minor complications to this general model (1977), the direction of crH ma in the CSVZ and SSVZ arise from: 1- gra\iity forces acting on the volcano in is roughly N50-70oE, as indicated by the alignment addition to the regional tectonic stress. According to of most parasitic vents, MEC and SV. The fact that Nakamura (1977), gravity forces add up to the not all volcanic centers form northeast alignments regional tectonic stress, and thus, may modify the can be explained as pre-existing crustal fractures of trajectory of the principal stress axes, producing different orientations may also serve as channelways less well-defined, usually curved, volcanic center for magma ascent, regardless of the dominant alignments, and 2- pre-existing anisotropies in the tectonic stress field. Among these pre-existing dis­ basement rocks which may also serve as channel­ continuities in the basement, the most important ways for ascending magma (non-andersonian dikes). ones trend NNE and WNW. The NN E features corre­ In this case, volcanic alignments will not indicate by spond to crustal scale anisotropies related to Mio­ themselves the orientation of the stressfield. cene-Pliocene ductile shear zones and brittle faults Nakamura's model, which can be extended to of the LOFZ. The WNW trending lineaments can be alignments of SV, has been succesfully tested in the regional in scale and are thought to be pre-Andean Aleutians (Nakamura et al., 1978). In this place, the in origin (Munizaga et al., 1988). Although both NNE ' expected direction of the crH max obtained by the and WNW trending basement discontinuities may inversion of fault-slip data, is consistent with that play an important role in magma ascent and em­ suggested by the systematic orientation of volcanic placement, the predominant ENE alignments of structures. MEC and CSV document the more likely pathways for The fact thatllolcanic arcs occur along active low viscosity magma to ascent through the crust. convergent margins ('compressive' in a loose sense) This N50-70oE direction is oblique to the main trend led Nakamura (1977) to state that the overall tectonics of the volcanic arc (-N 1OOE), suggesting that the arc of volcanic arcs should be strike-slip (cr1 and cr3 domain is not under an overall trench orthogonal horizontal; cr2 vertical), instead of compressional shortening, as might be expected in a 'compres­ (Fig. 3; cr1 and cr2 horizontal; cr3 vertical). This would sional', two-dimensional Chilean-type' subduction allow magma to aseent through vertical dykes having model (í.e., Uyeda and Kanamori, 1979). a direction parallsl to that of crH mu (Nakamura and As stated before, since feeder dikes of volcanic L. López-Escobar, J. Cembrano and H. Moreno 225

centers are thoughtto be vertical, 03 has to be horizon­ Therefore, field and seismic e'lidences suggest tal, and then aH,.,.. corresponds etther to 01 or 02. In that the tectonic regime within the volcanic arc has the first case, the overall tectonics of the volcanic arc been dextral strike-slip for the last few million years is strike-slip.ln contrast, if aH,.,.. corresponds to 02, the (Pankhurst et al., 1992; Cembrano. 1992). An overall volcanic arc tectonic is extensional. -N1 OOE-trending dextral strike-slip :leformation zone, Independent evidence of the geometry of the corresponding to the volcanic arc domain, should principal stress axes for the Quaternary is limited. have a -N55°E trending instantaneous shortening However, structural studies in the Southern Chilean axis. This trend coincides closely with the direction Andes show that the long-term (Ma) ductile-brittle of aH,.,.. obtained from the volcanic alignments. The kinematics, recorded in high-strain rocks and brittle fact that the direction of aHmu is at a larger angle faults underlying the Quaternary volcanoes along than expected, with respect to the :leformation zone the Liquiñe-Ofqui FaultZone (LOFZ), has been dextral boundary may reflect a transpressional tectonic strike-slip, for the late Miocene-Pliocene times regime, resulting from a combinaticn of dextral strike­ (Hervé, M., 1984; Cembrano, 1992; Cembrano and slip and shortening across the arco This tectonic Hervé, F., 1993; Nelson et al., 1994). Inversion of regime is consistent with plate kinematic constraints fault slip data from 3.3 Ma 4°Ar-39Ar age plutonic as the Nazca-South America Plate convergence rocks, forming the basement of the volcanic arc at has been slightly dextral oblique for the last 20 Ma, 42°S, is also consistent with a dextral strike-slip producing deformation partitioning into torearc deformation (Lavenu and Cembrano, 1994). shortening and intra-arc transpressive dextral strike­ The short-term kinematic picture is only poorly slip deformation (Dewey and Larrb, 1992) (Fig. 4). constrained by a single focal mechanism solution of In the above scenario, and following the fracture a crustal earthquake, which occurred at -46°S within propagation model of Shaw (1 98C), rapid ascent of the volcanic arc. The earthquake focal mechanism magma should be expected along Inherited or newly indicated dextral strike-slip deformation along a created northeast-trending tensional fractures and NNE-trending fault parallel tothe surface expression faults within the volcanic arc (Fig. 5). This agrees of the LOFZ (Chinn and Isacks, 1983; Cifuentes, with geochemical interpretations based on U-Th 1989). disequilibrium considerations (T ormey et al., 1991 b).

, a , I I 1 I IU 1 u I 1- I u t I I N « "'" 1 '" 1/ -1 '" " I ~ a. ...o'" '" 1 1I1 /! o'" 1 ~,' ... 1 « ___ ~I I (J I N 1/ « I I ~:I I ~ 1 .-----'::::: I I ------1 ~I I ,I __ /1 1 I 1 I 1 u , 1 , I I I '" I 1 I " I I I 1 , I I

FIG. 4. SllTlllitied model tor the teetonie setting ot the Southern Andes Voeanie Zone (moditied trom Dewey and Lamb. 1992; Cembrano and Moreno, 1994). The voleanie are Is seen as a N100E trending broad zone ot dextral strike-slip detormalion resulting trom dHterenl degrees ot partltlonlng 01 oblique subduetlon into shortenlng In lhe toreare and Inlra-are shear. Non-partitioning (a) and cOlTlllele partnionlng (b) scenarios are shown. 226 GEOCHEMISTRY AND TECTONICS OF THE CHILEAN SOUTHERN ANDES ...

cr.~

'" d,

FIG . 5. Following Shaw' s Iracture propagalion model (1980), rapid ascenl 01 magmas is expectedlhrough InOOrlled or newly crea­ led Iractures which are close to para­ leUlsm with the Instantaneous shorten­ Ing dlredion (·r maxlmum principal stress axis. Too present locallon 01 volcanic cenlers would rellect the subvertical poslllon 01 underlylng leeder dykes.

GEOCHEMISTRY

Most SV and MEC of the CSVZ and SSVZ, Iying et al., 1992; López-Escobar et al., 1995). Unlike the along northsouth-trending lineaments or forming other CSVZ and SSVZ centers, Mocho-Choshuenco, N50-700E trending-en echelon alignments, such as a basaltic andesite to dacitic SV, located at 400S (Mc the -Puntiagudo-Cordón Cenizos volcanic Millan et al., 1989), is emplaced on a basement that, chain, are fundamentally basaltic to basaltic andesite in addition to Jurassic igneous rocks and Triassic in composition (Moreno et al., 1979; López-Escobar metasediments, also contains Paleozoic meta­ et al., 1992; Tagiri et al., 1993). However, N50-600W sedimentary rocks of the coastal western series trending volcanic chains, such as the Villarrica­ (Fig.1). SV MEC Quetrupil lán-Lanín and Puyehue-Cordón Caulle vol­ Most and basalts (Si02 S 52%) and CSVZ canic chains, include postglacial dacitic to rhyolitic basaltic andesites (Si02=52-56%), from the rocks in addition to basaltic products (Moreno, 1977; and SSVZ provinces of the Southern Andes, have Gerlach et al., 1988; Hickey-Vargas et al., 1989; MgO <11%, ranging from 3.0% to 10.5% (Fig. 6). López-Escobar et al., 1993; Moreno et al., 1994). These values are lower than those expected in Isolatedyoung an::lesitic SV, like and Huequi magmas in equilibrium with mantle peridotite volcanoes, líe on northwest-trending lineaments, (MgO=11-15%). Only one , from the SV tens of kilometers long (Fig. 2). It is also worthwhile Puyehue, has a MgO abundance equal to 14.32% pointing out that Lanín (Lara and Moreno, 1994) and (Gerlach et al., 1988; Fig. 6). However, this particu­ Tronador volcanoes are emplaced on an uplifted lar basalt is olívine rich and its high MgO content block, east of the LOFZ main trend. Calbuco volcano could be the result of an olivine accumulation. would be also emplaced on an uplifted block (Thiele The two main types of basaltic rocks, depleted in et al., 1986; Barrientos et al. , 1992; López-Escobar K (KP < 1%) and enriched in this element (Kp=1- L. López-Escobar, J. Cembrano and H. Moreno 227

orientation (Figs. 2, 3b) and cross the main trace of the lOFZ, those MEC located in the western part (Los • • Venados subgroup) are more depleted in incompa­ 2 • tible trace elements than those located in the eastern • part (Carrán subgroup; C. Rodríguez, personal com­ K,0 munication). Farther south, the Puyuhuapi MEC basalts (45°S), located in the eastern branch of the lOFZ (Iike Huililco and Pichares centers), are also enriched in incompatible elements, being richerthan those MEC basaltic rocks from Caburga-La Barda­ 2 4 6 8 10 12 14 16 Huelemolle (39°S), Los Venados (40.3°S) and Ca­ MgO yutué-La Viguería-Pocoihuén (41.5°S). Unfortuna­ FIG. 6. K.O versus MgO dlagram lor SV and MEC basaltic rocks tely, there are no data for MEC basaltic rocks as­ Irom Ihe CSVZ and SSVZ provlnces 01 Ihe SVZ 01 lhe sociated with the western branch :>f the lOFZ at the Andes. Symbols are as lollow: (1) K-poor basaltic rocks; latitude of the Puyuhuapi MEC group. (11) K-rlch basaKic rocks; (+) K-poor SV basaltic rocks, (o) K-poor MEC basaltic rocks, (A) K-rlch SV basaltic rocks, The behaviour ofthe incompatible trace elements (o) K-rlch MEC basaltlc rocks, and (.) Back-arc MEC suggests that their abundances in SV and MEC basaltlc rocks (Muñoz and Slem, 1989). The laller are basaltic rocks of the Andean CSVZ and SSVZ is shown lor comparison. Thls ligure and Ihe lollowing ones were bulll uslng lhe dalabase shown In table 1.

1.5%), that were distinguished in the SSVZ (López• 20,000 • • Escobar et al., 1993), are also present in the CSVZ • province of the SVZ of the Andes arc (Fig. 6). Both K • groups include SV as well as MEC basaltic rocks. In 10,000 rii~~~' 11 each one, K 0 variability is relatively narrow, even ¡tiI----, - 2 /~.' when MgO may vary from 3.5 to 10.5%. Only in the ~J;Y/ I K-rich group, MEC basaltic rocks are significantly richer in MgO than SV basaltic rocks. Compared to • K-rich basaltic rocks, K-poor ones are also depleted 30 /-.., . in other incompatible elements, such as Rb, La and /Ib \ 11 • Th (Fig. 7). La 20 ~ ~ 66\ At latitute 39°S, where the SV Villarrica-Que­ --...... /i/' trupillán-Lanín are aligned in a N50-600W direction ('ti-----, 10 'ej.' I and the lOFZ suffers a displacement of 18 km to the I ...... , t!y-/ I west (Fig. 2), the incompatible element abundances of the SV basaltic rocks increase from west to east, and MECbasaltic rocks relatedtothe eastern branch 8 • of the lOFZ (Huililco and Pichares) are richer in these elements than those associated with the 6 Th /~ western branch (Caburga, La Barda and Huele­ ,,'~/ / 11 4 molle). Even more, the Rucapillán basalts that are ~. o ~/ • ~ .. "é-"''''.,<'''S!.,''' the westernmost ones at this latitute tend to have 2 lower incompatible element contents than the i~i!~}1 1.....-...... previous rocks (Moreno and López-Escobar, 1994). o 20 40 50 80 100 Similar west-east trends are observed in SV basaltic Rb rocks belonging to N50-700E volcanic alignments, FIG. 7. K-Rb, La-Rb and Th-Rb (incompalitle elemenls) diagrams such as the Osorno-Puntiagudo-Cordón Cenizos lor SV and MEC basallic rocks Irom Ihe CSVZ and SSVZ volcanic chain (Moreno er al., 1979). At the Carrán­ provinces 01 Ihe SVZ 01 Ihe Andes. Back·arc MEe basattlc Los Venados MEC group, which has a N60-700E rocks are shown lor comparison. Symbols as in figure 6. 228 GEOCHEMISTRY ANO TECTONICS OF THE CHILEAN SOUTHERN ANDES •••

TABLE 1. NEW CHEMICAL ANALYSES OF BASAL TIC ROCKS FROM THE SOUTHERN ANDES VOLCANIC lONE.

COMPOSITE STRATOVOLCANOES (S V) MINOR ERUPTIVE CENTERS (MEC)

CASA BLANCA CASABLANCA CASABLANCA RUCAPILLAN RUCAPILLAN MIRADOR MIRADOR Sampl. 28028HIS 250483-02 120183.Q3 280281.Q3 PORPHYRITIC APHANITIC 120679.Q2 260479.Q3 Latitud. 40.8°S 40.8°S 40.8"5 40.8"5 39.0 0 S 39.0"5 40.3°S 40.3°S

510, 45.25 50.88 50.95 51 .95 49.10 51.37 52.73 53.56 TIO, 0.46 0.43 0.53 0.52 0.79 1.29 1.21 1.17 AI,O. 18.14 22.37 18.29 18.45 16.43 16.63 18.05 17.93 Fe,O. 9.54 8.50 10.13 9.83 9.67 11 .57 11 .64 11 .19 MnO 0.14 0.12 0.14 0.15 0.14 0.16 0 .18 0.19 MgO 4.21 4.51 7.16 6.34 9.44 5.73 4.57 4.29 CaO 7.70 10.43 9.11 9.76 10.35 8.15 8.42. 8.50 Na,O 3.98 2.93 3.09 3.49 2.93 3.66 3.93 3.88 K,0 0.93 0.50 0.36 0.53 0.44 0.75 0.47 0.51 I P,O, 0.22 0.15 0.23 0.15 0 .14 0 .29 0.19 0.18

Rb 21 .2 8.4 12.0 10.6 9.8 8.5 Cs 1.5 1.0 1.0 Sr 444 534 514 441 528 495 478 478 Ba 324 140 232 158 154 266 185 192 Ga 19.2 18.5 18.0 17.9 Pb 10.6 5.6 10.6 5.3 Se 25 30 36 35 27 27 V 198 180 196 220 231 305 241 224 Cr 44 100 320 125 5 3 Co 24 36 35 39 30 30 NI 30 17 79 53 116 55 7 4 Cu 94 79 Zn 81 70 81 80 86 117 95 96 y 20.8 13.4 16.2 16.1 15.0 26.0 Zr 107 57 88 62 53 114 72 78 I Nb 3.5 1.8 3.8 1.5 1.6 1.8 Hf 2.6 1.6 1.7 3.8 1.9 1.7 Ta 0.21 0.13 0.10 0.10 Th 3 .80 1.10 0.60 0.70 La 12.2 6.6 7.0 14.0 7.1 6.9 Ce 27.3 17.2 16.0 32.0 18.1 17.2 Nd 14.2 10.2 10.0 19.0 10.8 11.7 Sm 3.69 2.66 2.31 4.40 3.40 3.00 Eu 1.17 0.93 0.88 1.36 1.20 1.30 Gd 2.69 4.40 Tb 0.54 0.48 0.60 0.70 Dy 2.53 4.35 Ho 0.52 0.83 Er 1.51 2.40 Yb 2.14 1.64 1.50 2.41 2.20 2.20 Lu 0.33 0.24 0.23 0.35 0.30 0.40 llAl. Sr 0.70415 0.70410 0.70414 1U11.. Nd 0.512842 0.512858 0.512850 -Pb 3)'_ Pb 201lI204 Pb L. L6pez-Escobar, J. Cembrano and H. Moreno 229

tabla 1 continuad

MINOR ERUPTIVE CENTERS (MEC)

PICHIHUINCO CAYUTUE LAVIGUERIA LAVIGUERIA POCOIHUEN PUYUHUAPI PUYUHUAPI PUYUHUAPI Sample 220483-01 201282-01 040476-01 151284-04 160185-03 160390-01 160390-04 160390-05 Latltude 41.3°S 41.4°S 41.4OS 41.5OS 44.3°S 44.3°S 44.3°S 44.3OS

SiC. 51.52 51.79 SO.57 51.54 51.86 48.84 49.46 48.61 TiC. 0.78 0.87 0.66 0.83 0.91 1.50 1.55 1.55 AI.o3 20.58 17.34 17.64 17.58 16.90 16.60 16.30 16.30 Fe.03 8.25 8.52 10.81 8.93 9.19 9.73 10.13 10.53 MnO 0.14 0.17 0.16 0.15 0.16 0.15 0.15 0.15 MgO 4.88 6.54 7.33 7.47 7.67 8.06 7.30 8.12 CaO 10.45 9.90 10.05 9.86 9.83 10.46 10.36 10.10 Na.o 2.94 2.91 2.86 2.80 2.69 3.29 3.25 3.15 K.o 0.36 0.78 0.62 0.67 0.65 1.18 1.16 1.18 p.o. 0.10 0.14 0.20 0.19 0.18 0.38 0.35 0.39

Rb 9.0 11.7 8.8 8.9 11.5 22.2 21.4 21.4 Cs 0.6 0.7 0.5 0.3 0.5 0.6 Sr 415 557 594 584 416 744 738 747 Ba 120 204 190 188 218 62 69 49 Ga 18.7 18.2 17.6 16.7 Pb 8.3 8.8 Se 30 30 32 29 31 47 50 46 V 213 217 213 219 Cr 79 156 200 177 277 180 159 180 Ca 28 29 37 53 52 51 NI 30 50 53 78 91 Cu Zn 74 82 82 79 Y 13.6 18.1 17.2 20.1 Zr 40 95 76 82 Nb 4.0 3.7 1.9 2.0 Hf 1.2 2.1 2.0 1.8 2.0 2.9 3.1 2.9 Ta 0.06 0.21 0.13 0.86 0.99 1.20 Th 0.65 1.73 1.20 0.89 1.64 4.16 4.10 3.88 La 3.9 10.3 9.2 9.0 9.2 28.1 27.6 21.8 Ce 10.6 26.0 22.0 22.7 22.7 46.3 44.6 48.5 Nd 7.0 14.4 12.9 13.3 13.8 28.1 28.5 30.0 Sm 2.02 3.58 3.19 3.22 3.38 6.00 5.77 4.87 Eu 0.81 1.14 1.10 0.99 1.06 1.68 1.73 1.59 Gd Tb 0.33 0.41 0.53 0.44 0.50 0.72 0.62 0.61 Dy 3.49 Ho Er Yb I.SO 1.74 1.70 1.63 2.03 1.84 1.98 1.84 Lu 0.23 0.28 0.26 0.26 0.27 0.30 0.37 0.31

.,.... Sr 0.703686 0.703912 0.704021 0.704001 0.703995 '431'44 Nd 0.512828 0.512796 0.512762 0.512765 0.512770 -Pb 18.575 18.516 18.447 18.457 18.462 "'71""Pb 15.627 15.570 15.580 15.580 15.579 -Pb 38.513 38.337 38.459 38.443 38.420

The database for figures 6-10 are chemical analyses published in: Carrasco, 1995; Ger1ach el al., 1988; Hickey el al., 1986; Hickey-Vargas el al., 1989; Lallsen el al., 1994; L6pez-Escobar and Moreno, 1981; L6pez-Escobar el al., 1981, 1992, 1993, 1995; Moreno, 1977; Moreno and L6pez-Escobar,1994; Moreno el al., 1979; plus the new analyses In lable 1. The complete sel is available upon request with lhe maln aulhor. 230 GEOCHEMISTRY AND TECTONICS OF THE CHllEAN SOUTHERN ANDES ••.

independent 01 the orientation 01 the laults and magmas were in equilibrium with garnet either at fractures where they are emplaced. In other words, their mantle source or at lower crustal levels. the role of faults and fractures would be to allow the Andean CSVZ and SSVZ K-poor basaltic rocks ascent of basaltic magmas, whose geochemical are quite heterogeneous in Sr and Nd isotopic ratios imprints are probably determined by subduction (87Sr/8·Sr=0. 7036-0. 7044; '43Nd/'-'Nd=O.512700- related processes. 0.512900; Fig. 9). However, K-rich ones have 87Sr/ The Baila and laIYb ratios of Andean CSVZ and 86Sr (0.7039-0.7044) and '43Nd/'-'Nd (0.51275- SSVZ basaltic rocks al so seem to depend on sub­ 0.51283) ratios that are respectively concentrated in crustal features ard processes.ln fact, independently the uppermost and lowest parts ofthe range exhibited of the orientation of the volcanic align ments to which by K-poor basaltic rocks. In each group, MEe tend to they belong, K-poor basaltic rocks tend to have have lower Sr-isotope ratios than SV. When MEC higher Baila and lower laIYb ratios than K-rich­ basalts of similar latitude are compared, their 87Sr/ ones (Fig. 8). One of the highest Baila and lowest 8·Sr ratios tend to increase and their '43Nd/)-'Nd laIYb ratios are presented by basalts from the ratios tend to decrease with decreasing longitude. Pichihuinco maar (one of the westernmost MEC of This is observed, for example, at -39°S, where K­ this Andean region). On the other hand, one 01 the poor MEe basalts (Caburga) have significantly lower lowest Baila and highest laIYb ratios are exhibited 87Sr/86Sr and higher '43Nd/1-'Nd ratios than K-rich by the Puyuhuapi MEC basalts (Demant el al., 1994; MEe basalts (Huililco and Pichares). Puyuhuapi lahsen el al., 1994; lópez-Escobar and Moreno, MEe basalts (44.3°S), have similar 87S r/86Sr ratios 1994). In fact, the latter basalts have lalYb ratios as (about 0.70400; Table 1) than Huililco and Pichares high as those presented by andesites from the basalts (Hickey-Vargas el al., 1989), but their '43Nd/ northernmost part of the SVZ (33-34.5°S) (lópez• 144Nd ratios are significantly lower (average Escobar, 1984; lópez-Escobar el al., 1985), where 0.512766). the continental crust has a thickness of 55-60 km. In K-rich as well as in K-poor basalts, high 87S r/ The relatively low Baila ratios of the Puyuhuapi 86Sr ratios are generally accompanied by low 143Nd/ MEC basalts and, in general, of K-rich basaltic rocks '44Nd ratios. This trend is well defined in the former have been explained by subduction related pro­ cesses (lópez-Escobar el al., 1993). These pro­ cesses could also explain the enrichments of these 0.7044 magmas in incompatible elements andtheir relatively I~.\ /1>,\ high laIYb ratios. The extremely high lalYb ratios of 0.7042 I' + .,' \ " \ \ 11 : .,' ~ \ the Puyuhuapi basalts, which is mainly due to their 0.7040 : .t·~1 ?> \ low Yb contents, also suggest that their parental V; ['t+, OOótj .,'" I , '0_--" -... 0.7038 :+ I ... r+ I I .. \q;p! \ , 0.7036 \ , \tl 40 0 .7034 0.~'29 ,,-,\ ,+ +1 ,¿ 30 '+O I ., lb +! I ro 1 í1. I I 20 "O 'o1 , ... -ÓZl., z ,. I o o 4 ... I + I ' ... 0.5128 I l' I 10 '- \ \,' / '":: \ \ '~ ó/II \+ \ ,ól \ 1 -~ o 2 4 6 8 10 12 14 16 18 20 \ I \~} LalYb 0.5127 o 20 40 60 80 100 Rb FIG. 8. Baila versus LalYb dlagram lor SV and ME.C basaltic rocks Irom lhe CSVZ and SSVZ provinces 01 Ihe SVZ 01 FIG. 9. ·'Sr,...Sr and '4>Ndl'44Nd versusRb diagrams lor SV and Ihe Andes. Back-arc MEC basallic rocks are shown lor MECbasallic rocks Irom lhe CSVZ and SSVZ provinces 01 comparison. Symbols as in ligure 6. Ihe SVZ ollhe Andes. Symbols as in ligure 6. L. López-Escobar, J. Cembrano and H. Moreno 23 1

basaltic rocks. The isotopic trend of decreasing In general, CSVZ and SSVZ basalts are quite 143Nd/I44Nd as 87Sr/86Sr increases is commonly inter­ heterogeneous in their incompatible element ratios preted as evidence of contamination with crustal (Fig. 10). Although this heterogeneity could reflect material. At least at latitude 39°S, there is evidence source heterogeneities, it may be also due to crystal that K-rich basalts (lanín basalts) are emplaced on fractionation processes, involving ¡he principal min­ an uplifted block, so some influence of crustal mate­ erais present in those basalts (olivine, pyroxenes rial cannot be wholly denied. and plagioclase). In fact, although K, Rb, la and Ba are incompatible with respect to olivine, pyroxenes and plagioclase, their degree of incompatibility, ex­ pressed by their crystal/liquid partition coetficients, is quite different in some cases. For example, the 1,000 plagioclase/basaltic liquid partition coefficient for K is 0.20 and for Rb is 0.05, and the clinopyroxene/ 800 .o basaltic liquid partition coefficient for K is 0.002 and a: ~ 600 for la is 0.120. Petrographic and chemical evidences that crystal fractionation processes have affected 400 • CSVZ basalts are given by the Rucapillán MEC • .. basalts (Moreno and lópez-Escobar, 1994) . P', Samples from this MEC exhibit aphanitic lens of 1, basaltic composition (Si0 =51.37'%; Tabla 1) im­ 50 1," 2 1 1 mersed in aporphyriticbasaltic mass (Si0 =49.1 0%). 1 \ I ' 2 I \ In comparison with the porphyritic fraction, the apha­ ~ 40 I \ I \ I C, • nitic one is depleted in MgO, CaO, Sr, Cr and Ni, and 30 l.o C , • has lower Klla and Sr/la ratios. These geochemical ~~\~II I ~~. tó....E.!JI characteristics suggest that the aphanitic lenses 20 I ' p,_./ / • represent melts derived, by crystal fractionation, Irom a parental basaltic magma similar to the • porphyritic fraction. This is important because the 700 geochemistry of some SV basaltic rocks from this Andean region resembles that of the aphanitic fraction .. 600 01 the Rucapillán samples . ~ 500 • 400 FIG. 10. Sorne Incompalible Elemenl Ralios versus Rb diagrams lor SV and MEC basaltic rocks lrom lhe CSVZ and SSVZ provinces 01 lhe SVZ 01 lhe Andes. Back-arc MEC 40 60 80 100 o 20 basaltic rocks are shown lor comparison. Symbols as In Rb ligure 6.

SUMMARV AND CONClUSIONS

In summary, Andean CSVZ and SSVZ strato­ these geochemical characteristics seem to be volcanoes (SV) and minor eruptive centers (MEC) independent of the orientation of fractures and faults basaltic rocks, from either N50-600W and N50-700E where those centers are emplaced. suggesting that transverse alignments tend to increase their incom­ they are controlled mainly by subduction-related patible elements abundances, Baila and 87Srf86Sr processes. Even the most primitive basalts of this ratios, and decrease their lalYb and 143Nd/I44Nd Andean segment show evidences of undergoing ratios in going from west to east. In other words, fractional crystallization. Actually, they have MgO < 232 GEOCHEMISTRV ANO TECTONICS OF THE CHILEAN SOUTHERN ANDES ...

11%, and Ni and Cr abundances lower than those of mantle-derived magmas. On the other hand, expected in mantle-derived primary magmas. Tecto­ volcanic edifices controlled by northwest-trending nism seems to control the possibility that basaltic fractures and faults (e.g., Villarrica-Ouetrupillán­ magmas either reach the surface or evolve to more Lanín and Puyehue-Cordón Caulle volcanic chains) differentiated products at crustallevels.ln fact, most may be under a combination of shortening and young volcanic chains (Late Pleistocene-Holocene), strike-slip deformation, which would cause a longer including SVand/or MEC defining northeast-trending intracrustal magma residence, yielding a more alignments are characterized mainly by basaltic to mature and differentiated composition. This could basaltic andesites rocks (e.g., Osorno-Puntiagudo­ be a reason why the latter shows a wider rock suite, Cordón Cenizos volcanic chain). This is consistent with the occurrence of not only basalts and basaltic with a short resid:mce time of magmas in the crust, andesites, but also medium andesites, dacites and resulting in a limited contamination and fractionation even rhyolites erupted in historie times.

ACKNOWLEDGEMENTS

This study was possible thanks to the following R.S. Hickey-Vargas (Florida International University), grants: FONDECYT-CHILE No. 194-0431, 193-1096 W. Hildreth (U.S. Geological Survey), P. Kempton and 193-0992. It ¡S a contribution to the IGCP Project (NERC Isotope Geosciences Laboratory), R. Kilian No. 345, 'Andear Lithospheric Evolution' and to the (Mineralogisch Petrographisches Institut), K. Notsu Programa de Riesgo Volcánico de Chile (SERNA­ and M. Tagiri (Ibaraki University) for their kind col­ GEOMIN). The authors are indebted to Drs. J.P. laboration in obtaining valuable geochemical data, Davidson (University of California, L.A.), FA Frey and to two anonymous reviewers for their critical (Massachusetts Institute of Technology), R.S. comments. Harmon (NERC Isotope Geosciences Laboratory),

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Manuscript received: July, 3. 1995; accepted: November 23. 1995.