PLlOCENE TO PRESENT MIGRATION OF THE VOLCANIC FRONT, ANDEAN SOUTHERN VOLCANIC ZONE

University 01 Colorado, Department 01 Geologica Sciences, CHARLES R. STERN Boulder, Colorado 80309, USA.

ABSTRAeT

Between the Pliocene and the Present the volcanic belt from 33-34°S, at the northern end of the Southern Volcanic Zone (SVZ) of the Andes, narrowed into a single chain 01 volcanos as the volcanic lront migrated 35 km to the east and the boundary separating the volcanically quiescent zone to the north from the SVZ moved 25 km southward. These changes can be related to progressive flattening 01 the angle of subduction of oceanic lithosphere below the northern end of the SVZ. This resulted in 1. an increase in the rate of tectonic erosion 01 the continental margin causing eastward migration of the axis of the Chile Trench; and 2. a decrease in the volume and cooling of the asthenospheric mantle wedge above the subducted slab, and consequently. an increase in the dep!h of magma generation in !he subarc mantle below this region. In contrast, the volcanic belt lrom 38-39°S, near the middle 01 the SVZ. broadened as the volcanic front migrated 35-80 km to the west over the same time periodo At the northern end 01 the SVZ, increased crustal thickness caused by compressive delormation accentuated the decrease in the volume and the cooling of the subarc mantle associated with Ilattening 01 the angle of subduction. Increased importance of tectonic erosion of the continental crust towards the northern end of the SVZ, combined with !he smaller volume and cooler subarc manlle wedge, resulted in 1. an increase in the importance of continental crust in the subducted slab-derived components added to the subarc mantle; 2. an increase in the relative importance of the slab-derived components in the smaller subarc manlle; and 3. conditions of greater depths and smaller degrees of par­ tial melting, and consequently, increased significance of residual garnet in !he subarc mantle. These conditions are 87Srf86Sr ratios in magmas erupted at !he northern end compared to farther south in the SVZ, and they may have been important factors in producing both the Pliocene porphyry copper deposits between 33-34°S as well as other Cenozoic porphyry copper mega-deposits farther north in !he Chilean Andes.

Key words: Andean vo/canism, Pliocene, Quaternary, Vo/canic front, Subduction geometry, Tectonic erosion, Subarc mant/e, Porphyry copper.

RESUMEN

Desde el Plioceno al Presente el cinturón volcánico comprendido entre 33° y 34°S, en el extremo norte de la Zona Volcánica Sur (ZVS) de los Andes, se enangostó hasta formar una sola cadena volcánica a medida que el frente volcánico migraba 35 km hacia el este y el límite que separaba la zona volcánica inactiva al norte de la ZVS se movía 25 km hacia el sur. Estos cambios podrían estar relacionados con la disminución del ángulo de subducción de la litósfera oceánica bajo el extremo norte de la ZVS. Esta disminución en el ángulo de subducción resultó en : 1. un aumento en la velocidad de la erosión tectónica del margen continental causando la migración hacia el este del eje de la Fosa Chilena; y 2. una disminución en el volumen y el enfriamiento de la cuña del manto astenoslérico sobre la placa subductada y, consecuentemente, un aumento de la profundidad de la zona de generación de magmas en el manto, bajo el arco en esta región. En contraste, el cinturón volcánico desde 38-39°S, cerca de la mitad de la ZVS, se ensanchó a medida que el frente volcánico migraba 35-80 km hacia el oeste durante el mismo período. En el extremo norte de la ZVS, el aumento del espesor de la corteza causado por deformación compresiva acentuó la disminución en el volumen y enfriamiento del manto bajo el arco asociado a la disminución del ángulo de subducción.

Revista Geológica de Chile, Vol. 16, No. 2, p. 145-162. 8 Figs. 1989. 146 PLlOCENE-PRESENT MIGRATION, VOlCANIC FRONT, SOUTHERN ANDES

JUAN FERNANDEZ RIDGE Í\ Í\ /\1\/\

NAZCA PLATE SOUTH AMERICAN SOUTHERN

PLATE VOLCANIC

ZON E

--- , ~ ,.1 .'- .. 4.

ANTARCTIC AUSTRAL VOLCANIC PLATE ZONE

FIG. 1. Schematic map 01 Ihe distribution 01 lale Qualemary volcanlc centers 01 the Andean orogenic are (triangles) lorming Ihe Southem and Austral Volcanic Zones. The figure also shows platas and plate boundaries, depth in kilometers to Ihe center 01 the Benioff zone 01 seismic aetivity (Bevis and Isacks, 1984), and the outcrop 01 Qualernary back-are basalts (blaek) and Paleozoie and L,)wer Mesozoic pre-Andean basement (diagonally lined). Upliftad area, including Ihe Coastal and Main Andean cordilleras and the Sierras Pampeanas Aanges, are shaded. The areas wilhin the boxes, between 33-34°S and 38-39°S, are shown in more detail in figures 2 and 3. C.A. Stern 147

El aumento de la importancia de la erosión tectónica de la corteza continental hacia el extremo norte de la ZVS, junto a un volumen menor y al enfriamiento de la cuña del manto bajo el arco, resultó en: 1. un aumento en la importa'lcia de la corteza continental en la componente derivada de la placa subductada añadida al manto bajo el arco; 2. un aumento en la importancia relativa de los componentes derivados de la placa subductada en el manto disminuido bajo el arco; y 3. condiciones de mayor profundidad y grados menores de fusión parcial y, consecuentemente, un aumento del granate residual en el manto bajo del arco. Estas condiciones son consistentes con un modelo de contaminación de la fuente en el manto propuesto para explicar el contenido alto de Sr y las razones altas de LalYb y 87Sr/B6Sr en magmas que hicieron erupción en el extremo norte en comparación con los de más al sur en la ZVS. Ellos podrían haber sido factores importantes en la génesis de los depósitos de pórfido cuprífero pliocenos entre 33° y 34°S y también de otros mega-depósitos cenozoicos de pórfido cuprífero más al norte en los Andes de Chile.

Palabra claves: Volcanismo andino, Plioceno, Cuaternario, Frente volcánico, Geometrfa de subducción, Erosión tectónica, Manto, Pórfido cuprlfero.

INTRODUCTION

Barazangi and Isacks (1976) showed that in and the Pliocene, of the locus of Andean volcan­ regions of the 80uth American continental margin ism at latitude 30°8, to progressive flattening of below which the angle of subduction is very low the angle of subduction below this region during (Iess than 20 degrees), active volcanism is ab­ this time periodo sent. Volcanism in the Andes is currently occurr­ This paper discusses the migration of the vol­ ing only in those regions below which the subduc­ canic front from the Pliocene to the Present within tion angle is somewhat greater (20-30 degrees). two segments of the 80uthern Volcanic Zone of These differences suggest a close relation be­ the Andes (SVZ, 33-46°8, Fig. 1): between 33-34°8 tween the generation of Andean volcanism and the at the northern end of the SVZ (Fig. 2) and between geometry of subduction of oceanic lithosphere, as 38-39°8 near the middle of the SVZ (Fig. 3). In do es the near uniformity of the depth to the Benioff these two regions the volcanic front has migrated zone of seismic activity below the volcanic front as m uch as 30-80 km over the last 1 to 2 Ma, but in within the volcanically active segments of the different directions. Both the differences in the Andes. sense of migration of the volcanic front, as well as Accordingly, changes in the location of the An­ differences in the chemistry of the magmas erup­ dean volcanic front with time have been interpre­ ted along the current front within these two re­ ted to indicate changes in the geometry of subduc­ gions, can be related to changes occurring in the tion. For example, Kay et al (1987, 1988) attribute geometry of subduction below these different por­ the eastward migration, between the Miocene tions of the SVZ of the Andes.

THE VOLCANIC FRONT BETWEEN 33-34°5

GENERAL TECTONIC SETTING Jordan et al., 1983). These changes across this segment boundary have been related to differen­ The northern end of the SVZ, at 33°8, cor­ ces in the angle of dip of the subduction of the responds to a major Andean tectonic segment Nazca plate, which beco mes significantly shallow­ boundary spatially associated with the impinge­ er to the north, possibly due to the subduction of ment of the Juan Fernández Ridge upon the Chile the Juan Fernández Ridge (Barazangi and Isacks, Trench (Fig. 1). North of this latitude Quaternary 1976; Pilger, 1984; Bevis and Isacks, 1984). volcanism is absent, the Chilean Central valley Bevis and Isacks (1984) concluded that the disappears, and the 8ierras Pampeanas structural change in the angle of subduction of the Nazca province of compressive crystalline basement Plate in the vicin~y of 33°8 does' not reflect an uplifts appears in the Argentine foreland (Fig. 1; abrupt tear in the subducted oceanic plate, but 148 PlIOCENE-PRESENT MIGRATION. VOLCANIC FRONT. SOUTHERN ANDES

N

k:' ~ :'1 Holocene

r:::?l Mid to Late Pleisto­ t ~ cene-Holocene(?)

0.45 Ma rhyolitic P'777:l ~ pyroclastic flow from Maipo caldera

~ Pliocene-Early to ~ Mid(?) Plaistocene

( Caldera

~ K-Ar age in Ma

¿4.7-3.91 Ttc /

o 10 20 km ... !we ,

FIG. 2. Map 01 tha Pliocene/aarly lo mid(?) Pleistocena. mid to lata Pleistocene/Holocene(?). and Holocene volcanic units between 33-34°S. The ligura shows the location 01 the Holocene volcanos: TT. Tupungatito; SJ. San José de Maipo; MA. Maipo. Mid to late Pleistocene/Holocene(?) centers: T. Tupungato; CA. Cerro Alto; M. Marmolejo, as well as the outcrop 01 the late Pleistocene rhyolitic ashflow lrom the Maipo caldera (diagonally lined). Plio­ cene/early to mid(?) Pleistocene centers: LB/RB. Los Bronces-Río Blanco; Tte. El Teniente; C. Cerro Castillo; L Listado; and PB. Picos de Barroso, and outcrop 01 CC. Formation Colón-Coya. Ages shown in boxes are K-Ar and fission-track determinations, in Ma, Irom Charrier and Munizaga (1979), Stern et al. (1984a); Warnaars et al. (1985), and Cuadra (1986). C.R. Stern 149

rather a smooth transition achieved by flexure of a ªnd thus post-dates, a 15 by 20 km caldera coherent slab. The contours of the depth to the depression formed 450,000 years ago (Stern er deeper parts of the Benioff zone east of below the al., 1984a). Mid Pleistocene pre-caldera lavas are volcanic front (Fig. 1) indicate that the northward exposed in the wall of the Maipo caldera (Harring­ flattening of the angle of subduction may, in fact, ton and Stern, 1987), and these lavas overlie and begin as far south as 36°S. Crustal deformation, post-date older volcanic rocks associated with the thickening, and uplift north of 33°S have been Nevado de Argüelles, Listado, and Picos de Barro­ related to the development of the low angle of sub­ so volcanic centers which may be early Pleisto­ duction below this region (Jordan et al., 1983) and cene or Pliocene in age (Schneider et al., 1988). the gradual northward increase in elevation of the Main Cordillera north of 36°S (Hildreth and Moor­ THE PLlOCENE ANO EARLY PLEISTOCENE bath, 1988) is presumably caused by crustal thick­ VOLCANIC FRONT ening related to the gradual flattening of the angle of subduction towards the northern end of the SVZ. The Pliocene and early Pleistocene volcanic front between 33-34°S was located to the west of THE CURRENT VOLCANIC FRONT the current volcanic front (Fig. 2). Andesite and da­ cite lavas, rhyolite pyroclastic flows, and subvol­ Between 33-34°S the current volcanic front, and canic intrusions of dacite, latite, and quartz por­ in fact the entire volcanic belt, consists of the four phyries, tourmaline breccias, and andesite and predominantly andesitic volcanic complexes Tu­ lamprophyre dikes, which have been dated by K-Ar pungatolTupungatito, Cerro Alto, Marmolejo/Espí• techniques as being Pliocene and early Pleisto­ ritu Santo/San José de Maipo, and Maipo/Casimiro cene in age, occur in the vicinity of the Río Blanco (Fig. 2; Thiele and Katsui, 1969; López-Escobar er and Los Bronces mines (4.9 - 3.9 Ma; Ouirt er al., al., 1977, 1985; Stern et al., 1984a, b; Hickey et 1971; Drake et al., 1976; López-Escobar and Ver­ al., 1984,1986; Futa and Stern, 1988; Stern, 1988, gara, 1982; Vergara and Latorre, 1984; Warnaars and in press; Hildreth and Moorbath, 1988). These et al., 1985; Blondel et al., 1988), the El Teniente volcanos are located along the crest of the Andes, mine (4.7 - 2.9 Ma; Ouirt et al., 1971; Clark er al., 288 ± 8 km east of the axis of the Chile Trench and 1983; Cuadra, 1986), and in the valley of the Río approximately 100 ± 10 km aboye the top of the Cachapoal (2.3 - 1.8 Ma; Charrier and Munizaga, Benioff zone of seismic activity (Bevis and 1979). The Pliocene and early Pleistocene vol­ Isacks, 1984; Hildreth and Moorbath, 1988). South canic rocks near the latter two localities have been of 34°S the volcanic belts widens (Fig. 1) to include grouped together as the Formation Colón-Coya both the volcanic front and an eastern belt of (Cuadra, 1986). This formation includes lava flows, Andean stratovolcanos, as well as fields of laharic deposits, and layers of volcanic ash. Cua­ scattered and fissure aligned alkali basalt eones dra (1986) reported a mid Pleistocene age (1.3 Ma) and flows to the east of the Main Cordillera (Muñoz for a sample of ash intercalated within the forma­ and Stern, 1988, 1989). tion. Although the source of this ash is uncertain, The age of the oldest volcanic activity in the and it may have been derived from a volcanic regions of the TupungatolTupungatito, Cerro Alto, center along the curren! volcanic fron! as were the and Marmolejo/Espíritu Santo/San José de Maipo 0.45 Ma rhyolitic pyroclastics which rest on the volcanic centers is not well determined. Thiele and Formation Colón-Coya near Coya (Stern et al., Katsui (1969) suggested that the Tupungato and 1984a) this date indicates that lava flows within Marmolejo volcanos may have begun to be active the Formation Colón Coya may range in age to as in the Pliocene. The Maipo volcano occurs within, young as mid Pleistocene. 150 PLlOCENE-PRESENT MIGRATION, VOLCANIC FRONT, SOUTHERN ANDES

71 N

fr~~('i; 1 Holocene

t 1":0:::1 Mid lo Lole P II ~ j', tn~~n.,_1 Holocene ( ?)

1, 'a" '1 Pl iocene- Early lo Mid (?) Pleislocene

r Caldera

@]] K-Ar age in Ma

o 10 20 30km I

FIG. 3. Map, modilied alter Muñoz and Stem (1989), 01 the Pliocenelearly lo mid(?) Pleistocene, mid lo late Pleisto­ cene/Holocene(?), and Holocene volcanic units be!ween 37°30'-39°5. l he figure shows the localion 01 lhe Holo­ cene volcanos: AN. Antueo; CP. Copahue; ca. Callaqui ; La. Lonquimay; LL. Laima; LC. Uallicupe; and eones near LB. Laguna Blanca. Mid to late Pleistocene/Holocene(?) centers: SV. Sierra Velluda; lH. Tolhuaca; SN. Sierra Nevada; PM. Palao Mahuida; QM. Queli Mahuida. Pliocene/earfy to mid(?) Pleistocene centers : BO. Bonete; TM. Trocoman; Cl. Cerro Trolon; M. Las Monjas; R. Rahue; B. Butahuao; PS. Pino SololTralihue; PH. Pino Hachado. Ages shown in boxes are K-Ar determinations, in Ma, from Drake (1976), Drake et al. (1976) , Muñoz and Stem (1988, 1989), and Muñoz et al. (1989). C.R.Stem 151

THE VOLCANIC FRONT BETWEEN 38-39°S

GENERAL TECTONIC SETTING the volcanic front at 37°30'S, also rests on the edge of an older volcanic edifice, Sierra Velluda, a The region between 38-39°S occurs at the south­ lava 1rom the base of which has been dated as late ern boundary of the segment 34-39°S of the vol­ Pleistocene (0.3 Ma; Drake, 1976a, b). These rela­ canic belt of the southern Andes which is charac­ tions suggest that, near the middle of the SVZ, terized by intra-arc extension (Fig. 1; Muñoz and volcanic activity in the vicinity of the current vol­ Stern, 1988 and 1989; Muñoz et al., 1989). North canic front was initiated in the mid to late Pleisto­ of 39°S, orogenic are volcanos occur both in the cene and the volcanic front has not significantly Main Cordillera and to the east on preeordilleran changed its position since that time (Drake, 1976 uplifts separated from the Main Cordillera and from b; Muñoz and Stern, 1988, 1989). The Tolhuaca each other by extensional valleys within which volcano and the base 01 the Callaqui, Lonquimay, intra-arc alkali basaltic volcanism is occurring and lIaima volcanos are assumed to be mid to late (Muñoz and Stern, 1988, 1989). South of 39°S, the Pleistocene in age as well (Fig. 3). orogenic are is restricted to the Main Cordillera Drake (1976b) described outcrops, which occur while back-arc basaltic volcanism occurs well to to the west of the current volcanic front at !atitude the east. This change near 39°S has been attribut­ 35°30'S, of Pliocene and early Pleistocene lavas ed to the change in age, from older to younger res­ belonging to what Vergara and Munizaga (1974) pectively, of the oceanic lithosphere being sub­ termed the 'andesitic plateau series' and which ducted below the regions north and south of the more recently have been included in the Formation intersection of the Valdivia Fracture Zone with the Cola de Zorro (Vergara and Muñoz, 1982). The Chile Trench (Muñoz and Stern, 1988). eruptive centers for these and other Pliocene and early Pleistocene lavas in central-south Chile have THE VOLCANIC FRONT been shown to occur to the east of the current volcanic front, as described in more detail below The current volcanic front between 38-39°S eon­ (Drake, 1976b; Pesce, 1987; Muñoz and Stern, sists of the three predominantly basaltic through 1988, 1989). andesitic volcanic centers Callaqui, lonquimay, and lIaima (Fig. 3; lópez-Escobar et al., 1977; THE PLlOCENE ANO EARL Y PLEISTOCENE Hickey et al., 1984, 1986; Muñoz and Stern, 1988, VOLCANIC FRONT 1989). These volcanos are located within but to the west of the crest of the Andes, 285 ± 5 km east The Pliocene and early Pleistocene volcanic of the axis of the Chile Trench and approximately front between 38-39°S was Iocated well to the east 90 ± 10 km aboye the top of the Benioff zone of of the current volcanic front along the Copahue­ seismic activity, which is slightly shallower than at Pino Hachado precordilleran uplift (Fig. 3; Muñoz the northern end of the SVZ (Bevis and Isacks, and Stern, 1988, 1989; Muñoz et al., 1989). Basalt­ 1984). The Holocene volcanic belt between 38- ic through andesitic lava flows and rhyolitic pyro­ 39°S also includes stratovolcanos east of the vol­ clastic deposits dated by K-Ar techniques as be­ canic front, such as the Copahue volcano, and ing Pliocene and early-mid Pleistocene in age have fields of small fissure aligned and scattered eones been described from the base of the inner wall of of alkali basalts to the east of the Main Cordillera the Copahue caldera (4.3 Ma; Muñoz and Stern, (Muñoz and Stern, 1988). 1988), from the pyroclastic outflow related to the lonquimay volcano is located just to the south­ 10rmation 01 this caldera (1.1 Ma; Muñoz and east 01 an older inactive(?) center, the Tolhuaca Stern, 1988), and from the Rahue volcanic center volcano, and the lIaima volcano is located just to (4.1-1.0 Ma; Muñoz and Stern, 1988) the Tralihue/ the southwest of an older inactive volcano, Sierra Pino Solo volcano (2.4-2.1 Ma; Muñoz and Stern, Nevada. A sample from Sierra Nevada has been 1988; Muñoz et al., 1989), the Pino Hachado vol­ dated as being less than 1 Ma, implying a mid canic eomplex (2.45-1.4 Ma; Vergara and Muni­ Pleistocene age for this center (Muñoz and Stern, zaga, 1974; Muñoz and Stern, 1988; Muñoz et al., 1988). The Holocene Antuco volcano, located on 1989), and the Queli Mahuida volcano (1.0-0.9 Ma; 152 PLlOCENE·PRESENT MIGRATION, VOLCANIC FRONT. SOUTHERN ANDES

Muñoz and Stern, 1988). sce, 1987), and even further north Volcán Campa­ Other volcanic centers which formed the Plio­ nario (36°S) and Cerro San Francisqu~o (Drake, cene and early Pleistocene volcanic front along 1976a, b). Pliocene and early to mid Pleistocene the Copahue-Pino Hachado uplift between 38-39°S lavas outcropping at Paso Pichachen (37°30'S; inelude the Butahuao, Huarenchenque, and Palao 3.6 Ma; Muñoz et al, 1989) and in the region north Mahuida volcanic complexes (Pesce, 1987; Mu­ of Laguna del Maule (36°S; 4.2-0.6 Ma; Drake, ñoz and Stern, 1988), while to the north this chain 1976b) were derived from these centers which all includes the Cerro Trolon, Trocoman, Bonete, Gua­ occur east of the current volcanic front. naco, and Los Niches (37°S) volcanic centers (Pe-

DISCUSSION

The chronological information summarized a­ tant - of the volcanic fronts at 33-34°S and 38- boye indicates that between 33-34°S, at the north­ 39° S are discussed below, along with their pos­ ernmost end of the SVZ, and between 38-39°S, sible role in producing the petrochemically distinct near the middle of the SVZ, the Andean volcanic magmas erupted in these two regions of the SVZ of front has changed its position significantly, but in theAndes. different directions, from the Pliocene to the Pre­ sent. MIGRATION OF THE VOLCANIC FRONT Between 38-39°S the front has migrated west­ AT 33-34°S ward, a distance which ranges from 30-80 km over the last 1-2 Ma (Fig. 3), as was recognized pre­ The eastward migration, between the Pliocene viously by Vergara and Munizaga (1974), Drake and the Present, of the volcanic frant at 33-34°S, (1976a, b), and Muñoz and Stern (1988, 1989). In could be related to 1. a decrease in the angle of contrast, between 33-34°S the volcanic front has subduction of the oceanic plate; 2. eastward mi­ migrated eastward, approximately 35 km, over this gration of the Chile Trench due to tectonic erosion same time period, and the boundary between the of the continental margin; and/or 3. caoling of the volcanically inactive zone to the north and the SVZ mantle wedge overlying the subducted plate, re­ to the south has migrated southward at least 25 sulting in a deepening of the zone of magma gener­ km (Fig. 2). ation below the volcanic front (Fig. 4). As discuss­ Changes in the position of the Andean volcanic ed below it is likely that all three of these proces­ front with time are likely to reflect changes in 1. ses are occurring at the northern end of the SVZ of the angle of subduction of oceanic lithosphere be­ the Andes. Iow the western margin of the continent; 2. the Flattening of the angle of subduction of oceanic pos~ion of the trench relative to the continental lithosphere below the Andean segment north of margin; and/or 3. the thermal regime in the mantle 33°S began in the mid Tertiary and continued into wedge aboye the subducted oceanic crust which the Pliocene, w~h a major change occurring be­ determines the depth of magma generation below tween 11-8 Ma as indicated by eastward migration the volcanic front. These changes may be caused and ultimate cessation of volcanism in this region by 1. the subduction of bouyant features such as (Kay et al., 1988). Flattening of the angle of sub­ ase ismic ocean ridges; 2. changes in the rate duction north of 33°S must have affected the geo­ and/or orientation of plate convergence; 3. accre­ metry of subduction to the south, if, as suggested tion to or tectonic erosion of the continental mar­ by Bevis and Isacks (1984), the subducted ocean­ gin; and 4. changes in the vigor of convection in ic lithosphere beJow these adjacent regions forms the asthenospheric manlle wedge aboye the sub­ a coherent slab. Continued flexture and flattening ducted oceanic plate (Cross and Pilger, 1982). The of the angle of subduction below the northern end probable causes for the migration - from the Plio­ of the SVZ is consistent with the southward cene to the Present during which time the con­ migration of the boundary between the volcanically vergence rate and direction between the Nazca quiescent zone to the north and the SVZ to the and South American plates apparenlly was cons- south, as well as the eastward migration of the C.R. Srern 153

EASTWAR D ARe MIGRATION POSSI BlE CAUSES A Decreoslng subduction ongle I Pliocene / Eorly Pleistocene

Montle 100 km ------B Tectonic erosion

Mid Pleistocene/ Present Chile

o~Trr~ .. ,~ .... ,... I __ ...... e Cool ing of subarc mantle ' : ..,' ~ .. ':i'':, ,. .' . ~r~~,t ,~~i

Crust ' 100 ".', . km

~------290km------~

FIG. 4. 8chematic diagram of !he factors influencing the eastward migration of the volcanic front at 33-34°8. Possible causes: A, B, and C show, respectively, the effect on the location of !he volcanic front of A. decreasing angle of subduction; B. eastward migration of the trench axis relative to the coast caused by tectonic erosion along !he inner trench wall ; and C. cooling in the asthenosphere wedge over1ying the subducted slab as represented by !he change in position of the 1200°C isotherm. In diagrams A and B the original position 01 the top subducted slab is indicated by dashed lines and the final position by heavy solid lines. In C the original position of the 1200°C isotherm is represented by a dashed line and the final position by a Ihin solid line. The combined effect of !hese three factors is likely to have caused shrinking and deepening 01 the zone of subarc melting (horizontally lined area) and the eastward migration, between the Pliocene and the Present, of the volcanic front below the 8VZ at 33-34°8 as discussed in the text and iIIustrated on the right side 01 the diagram. volcanic front observed between the Pliocene and during the Mesozoic and Cenozoic development of the Present. the Andes, of the south American continental mar­ Tectonic erosion of the continental margin, gin of central and northern Chile, north of about 33° causing the trench axis to migrate eastward with S, is suggested by the 1. the lack of any Meso­ time, is likely to be a more significant factor at the zoic or Cenozoic accretionary complex and the al­ northern end compared to farther south in the 8VZ most complete absence of sediment within the because of both 1. the lower subduction angle Chile Trench north of this latitude (Scholl et al., below the northern end of the 8VZ, and 2. the lower 1970, 1977; Rutland, 1971; Plafker, 1972; Kulm et sediment supply to the Chile Trench caused by the al., 1977; Ziegler et al., 1981; Hussong and Wipper­ increasingly arid climate towards the northern end man, 1981; Lowrie and Hay, 1981; Thornburg and of the 8VZ. Ziegler et al. (1981), Shreve and Cloos Kulm, 1987); 2. the north-northwest strike into the (1986), and Cloos and Shreve (1988) have sug­ trench and essential disappearance, north of gested that lower subduction angle and decreased 33°S, of the pre-Andean upper Paleozoic subduc­ sediment supply to the trench will increase the pro­ tion-related accretionary terrain and Upper Paleo­ bability of tectonic erosion along the inner trench zoic to Lower Jurassic plutonic belts which make wall. Sediment subduction and tectonic erosion, up the western portion of the Coastal Cordillera in 154 PlIOCENE-PRESENT MIGRATION, VOLCANIC FRONT, SOUTHERN ANDES south-central Chile (Fig. 1; Hervé et al., 1988); and also suggested that beginning il1 the mid Tertiary, 3_ the progressive eastward migration of the mid a combination of crustal thickening and progres­ Jurassic to Cenozoic Andean plutonic belts 01 Cen­ sively decreasing angle of subduction below the tral Chile (Parada and Hervé, 1987). Andean segment north of 33°8 resulted in a de­ Allmendinger et al. (in press) recently conclud­ crease in the volume and cooling of the astheno­ ed that the total amount of observed crustal short­ sphere wedge aboye the subducted slab. The evo­ ening occurring over the last 15 Ma at 30°8, within lution of the volcanic belt at the northern end of the the Andean segment just north of the SVZ, is proba­ SVZ, between the Pliocene and the present, from a bly greater than can be accounted for with reason­ bread zone, involving magmatic activity both along able crustal thicknesses, thus implying the likely­ the volcanic front and to the east as well (Fig. 2), hood 01 tectonic erosion along the convergent into the single belt of volcanic centers which cur­ plate boundary. They consider 170 km of observed rently forms the volcanic belt at this latitude, is structural shortening at this latitude to be a con­ consistent with cooling of the subarc mantle over servative estimate, while they calculate, by crust­ this time periodo The restriction of the volcanic belt al area balance assuming a maximum reasonable at 33-34°8 to a single chain of volcanos, the lower value of 45 km for present crustal thickness, only magma production rates estimated for these cen­ 137 km of horizontal shortening. The difference in ters compared to farther south in the SVZ (8tern et these two estimates suggests the removal, by tec­ al., 1984a; 8tern, 1988, and in press), and the tonic erosion, of a minimum of 33 km of 38 km thick slightly greater depths to the 8enioff zone beneath crust over the last 15 Ma, implying a minimum the front below the northern end 01 the SVZ are all average rate of 84 km 3/Ma of tectonic erosion per consistent with a cooler subarc mantle below this km along the trench. This value is within the range region compared to 1arther south in the SVZ. reported by 80urgois (1989) of 75-535 km 3/Ma 01 crust removed over the last 6 Ma by tectonic MIGRATION OF THE VOLCANIC FRONT erosion per km along the Peruvian Trench between AT 38-39°S 5-12°8. Apparently subduction of the Nazca Ridge below this region of southern Perú locally acceler­ The westward migration, between the early ated rates 01 tectonic eresion (Van Huene et al., Pleistocene and the present, of the volcanic frent 1988). The abrupt thinning of the trench sediments at 38-39°8, could be related to 1. an increase in and disappearance of the pre-Andean basement the angle of subduction of the oceanic plate; 2. north of 33°8 suggests that the subduction of the westward migration of the trench axis due to Juan Femández Ridge below the Andean segment accretion along the inner trench wall; and/or 3. just north of the SVZ may have had a similar effect, heating of the mantle wedge overlying the which would be currently focused at the northern subducted slab resulting in a shallowing of the end of the SVZ. zone of magma generation below the volcanic frent 8ubduction 01 oceanic lithosphere will lower the (Fig. 5). AII three of these processes may have temperature in the mantle below the over-riding occurred to some extent, but the importance of the plate unless convection in the asthenosphere latter is particularly clear. Extensive back-arc wedge aboye the subducted slab causes the influx volcanism east of the Main Cordillera between 38- of hot mantle. The overall decrease in the volume 39°8 implies active convection and influx of hot of the asthenospheric wedge aboye the subducted asthenospheric material into the mantle wedge slab at the northern end of the SVZ, caused by the below this region (Muñoz and 8tern, 1988, 1989). progressive flattening of the angle 01 subduction, 80th the broad volcanic belt and the shallower probably acts to inhibit the extent of convection depths to the 8enioff zone of seismic activity and influx of hot asthenosphere into this region. below the volcanic front suggest a hotter subarc This effect may be accentuated by increasing mantle below this region compared to the northern crustal thickness towards the northern end of the end of the SVZ. Also, although no accretionary SVZ. The lack of back-arc volcanism and extension complexes have been described within the trench, east of the arc between 33-34°8 (Fig. 1) is consis­ the morphology of the trench at this latitude is tent with a lack of convection in the astheno­ masked by sedimentary infill (Lowrie and Hay, sphere wedge below this region. Kay et al. (1987) 1981; Thornburg and Kulm, 1987). This implies that C.R. Stern 155

WESTWARD ARe MIGRATION POSSIBLE CAUSES 38- 3905 A Increasing subduclian angla 11 1 Plioeene / Early Plelstaeene Crusl

Manlle 100 km

B Teclonic accrelion I1

Manlle Mid Pleisloeene / Presenl ...... Chile 100 ...... Trench 1\ km "

e Healing of suba re manlle Conveeling \ 1 aSlhenosphere

1---- 280 km -i

FIG. 5. Schematlce diagram illustrating the lactors inlluencing the westward migration 01 the volcanic Iront at 38-39°S. Possible causes: A, B, and e show, respectively, the eltect on the location 01 the volcanic Iront 01 A. increasing angle 01 subduction; B. westward migration 01 the trench axis relative to the coast caused by accretion 01 material to the inner trench wall; and C. heating in the asthenosphere wedge overlying the subducted slab due to convective input 01 hot material. The combined eltect 01 these three lactors is Iikely to have caused the expansion 01 the zone 01 subarc melting and westward migration, between the Pliocene and the Present, 01 the volcanic lront below the SVZ Irom 38-39°S as discussed in the text and iIIustrated on the right side 01 the diagram. Symbols as in ligure 4. the current rate of sediment supply to the trench is canic front, the four volcanic centers at the north­ greater than the rate of sediment subduction, ern end of the SVZ have distinctive petrochemical which may be in part responsible for causing the characteristics (López-Escobar et al., 1977, 1985; trench axis at this latitude to be significantly fur­ Stern et al. 1984b; Hickey et al., 1984, 1986; Futa ther from the coast compared to the region north of and Stern, 1988; Stern, 1988, and in press; Hil­ 34° S where tectonic erosion is believed to be more dreth and Moorbath, 1988), including 1. the rela­ significant. Finally, progressive steepening of the tive scarcity of mafic lavas resulting in a higher angle of subduction below this region may be a geo­ average Si02; 2. higher incompatible element con­ metric response to the flattening of the angle of centrations for samples of similar Si02; 3. higher subduction taking place to the north, or to the sub­ La!Yb (Fig. 6); and 4. higher 87Sr/ffiSr (Figs. 7, 8), duction of older oceanic lithosphere as the inter­ lower 143Ndf1 44Nd, more variable Pb isotopic compo­ section of the Valdivia Fracture Zone and the Chile sition, but similar 1)180 compared to the volcanic Trench migrates southward. centers farther south in the SVZ. These data have been interpreted by Stern et al. (1984b), Futa and IMPLlCATIONS FOR ANDEAN MAGMA Stern (1988), Stern (1988, and in press) and HiI­ GENESIS dreth and Moorbath (1988) to imply that, relative to farther south in the SVZ, 1. a greater proportion of Compared to farther south along the current vol- components derived from the continental crust are 156 PLIOCENE·PRESENT MIGRATION. VOLCANIC FRONT. SOUTHERN ANDES

87Srf86Sr values of the andesites from the northern BASALTIC ANOESITES ANO ANOESITES 33-34°S end compared to basalts from farther south in the SVZ indicate that the process of incorporation of crustal components in the more mafic magmas 3% mcntle portlol meltinCJ leavlnCJ 10% residuol (Jarnet erupted at the northern end of the SVZ did not ... occur in the mid to upper crust where combined ~., E assimilation and crystal-liquid fractionation are ex­ pacted to lower rather than increase the Sr content of more evolved magmas (Fig. 7). Hildreth and Moorbath (1988) suggested that crustal contamina­ ...... W tíon of all SVZ magmas occurs in the lower continen­ w a: 10 tal crust, and that the significance of such intra­ crustal cont21mination increases towards the north­ ern end of the SVZ because of the greater crustal thickness below this region. Atternatively, Stern et al. (1984b), Futa and Stern (1988), and Stern (1988, and in press) have argued that crustal components are incorporated in all SVZ magmas by the subduction of these materials into the subarc

Lo Ce Nd Sm Eu Ho mantle source region, and that the significance of such source region contamination increases to­ FIG. 6. ~are earlh element concentrations, normalized wards the northern end of the SVZ because of a relative to primitive upper manlle (LaJO.315; greater rate of sediment subduction and tectonic Ce/O.813; NdIO.597; Sm/O.192; Eu/O.072; erosion in this region. As discussed above, the Tb/O.049; Yb/O.208), lor basalts Irom the Llaima eastward migration, between the Pliocene and the volcano at 39°S in Ihe middle of Ihe SVZ (data from Hickey et al., 1986) and basaltic andesites Present, of the volcanic front at the northern end and andesites from the Tupungato, Marmolejo, of the SVZ is consistent with increased importance and Maipo/Casimiro volcanos located between 33- of tectonic erosion in this region compared to 34°S at the northern end of the SVZ (data from farther south in the SVZ. López-Escobar et al., 1977, 1985; Stern, 1988). Stern (in press) calculated that a typical SVZ Also shown are models, developed and described basalt would have to assimilate at least its own in detail by López-Escobar et al. (1977), involving weight in postulated high-Sr lower crustal litho­ 3% and 15% partial manlle melting with garnet logies to raise both its Sr content and Sr-isotopic persisting as a residual phase only in the first model. These models suggest that the higher La composition to the values observed in the andes­ and LaJYb of the volcanic centers at the northern ites erupted at the northern end of the SVZ (Fig. 8). end 01 the SVZ rellect lower degrees 01 mantle Such large amounts of intra-crustal assimilation partial melting combined with a more signilican! might be expected to significantly modify the 8180 role 01 garnet as a residual phase after melting. and trace element composition 01 the basalt, but the andesites erupted at the northern end of the incorporated in the magmas at the northern SVZ have 8180 and trace element ratios nearly end of the SVZ; and 2. garnet plays a more impor­ similar to typical SVZ basalts. In contrast, addition tant role in the evolution of the magmas at the to the subducted oceanic lithosphere of only 1% of northern end of the SVZ. López-Escobar et al. tectonically eroded Paleozoic basement could (1977), and Stern (1988, and in press) conclude raise the 87Srf86Sr of the subarc mantle to the that lower degrees of subarc mantle partial melting value of 0.705 observed in the andesites erupted is also a significant factor in increasing both the at the northern end of the SVZ (Fig. 8). For the LalYb (Fig. 6) and incompatible element concentra­ subduction of a 100 km thick oceanic lithosphere tions, particularly the Sr contents (Fig. 8), of the at 10 cm/yr, such as is occurring below the SVZ of magmas erupted at the northern end compared to the Andes, this corresponds to tectonic erosion farther south in the SVZ. and subduction of 100 km 3/Ma of Paleozoic base­ The relatively high Sr contents and high ment per km of trench. This rate is similar to that C.R.Stern 157 calculated to have occurred over the last 15 Ma in NORTHERN SVZ 33 - 34- s : combinej ossi ft'l ilotion 0.706 ~ the Andean segment just north of the SVZ, and is at and fr.lctiunol cr ys tollization ( A Fe ) RHYOLlTES the lower end of the range occurring along the Increoses I1Sr /"Sr os Sr decr.osu coast of southern Perú. Progressive decrease in the angle of subduc­ tion and the resultant decrease in volume and cool­ 0.705 ing of the subarG asthenosphere, which have been identified aboye as important factors in causing -::: /lower cruS: or laure. :!' the eastward migration of the volcanic front at the ::::::~=====J ~ reqion con"ominatían CI 0.704 [ I northern end of the SVZ, are also consistent with RH YOllTES BASALTS the conditions - involving not only more significant svz 38 ... 40·5 : fracl¡anol- crystollizotlon without source region contamination, but also lower per­ olSimila tían decrecn Sr 01 con,'Ord '75r/"S, cents of partial melting and increased participation 200 400 600 800 I()()() of garnet - that have been invoked by López et aJo Sr (ppm) (1977), Stern et al. (1984b), and Stern (1988, and in press) to explain the distinctive petrochemical FIG. 7. Sr content versus 87Sr//l6Sr for samples from the characteristics of the volcanos in this region. As volcanic centers at the northem end (data from discussed previously by Stern et al. (1984b), and López-Escobar et al., 1977, 1985; Stem er al., 1984a; Slern, 1988; Hildreth and Moorbalh, 1988) Stern (1988, and in press), the decreased volume compared to the southern part of the SVZ of the of the subarc mantle below the northern end of the Andes. The trend observed through the se­ SVZ, due to a combination of both thicker crust and quence basalt to rhyolite in the southern part of lower angle of subduction, increases the relative !he SVZ, with constant 87StlB6Sr as Sr decreases, proportion of slab-derived components in the inta­ is interpreted to result from fractional crystalliza­ grated slab + mantle contribution to magma gene­ tion either without crustal assimilation or with as­ sis in this region. Cooling in the subarc mantle similation of young granitic rocks with low B7Sr//l6Sr similar to the SVZ basalts (Hickley et al., would deepen the zone of magma generation and '1984, 1986; Futa and Stern, 1988). The trend ob­ lower the extent of partial melting, thus increasing served through the sequence basal tic andesite the importance of residual garnet. The importance to rhyolite at the northern end of the SVZ, with of residual garnet would increase in the deeper increasing 87$r/ll6Sr as Sr decreases, can be mo­ magma generation zone associated with a cooler deled by assimilation -of upper crustal lilhologies subarc mantle because 1. lower temperatures and wilh relalively low Sr and high 87$r/B6Sr - combined higher pressures act to increase the proportion of with fractional crystallization (AFe, see figure 6 of Hildreth and Moorbath, 1988). The higher Sr mantla garnet relative to spinel; 2. lower degrees and 87Sr//l6Sr of basaltic andesites and andesites of mantle partial melting increase the probability of from the northern end of the SVZ compared to garnet as a residual phase since garnet is ona of basalts from farlher south in !he SVZ can not be the first mantle phases to enter a melt; and 3. as explained by either of these processes , but must discussed by Stern (in press), tha experimental result instead from either (see Fig . 8) assimilation results of Johnston and Wyllie (1989) indicate that of high-Sr lilhologies in the lower crust, as sug­ garnet will crystallize and increase in modal abun­ gested by Hildreth and Moorbath (1988) , or by changes in both the degree of partial rrelling as dance as a slab-derived melt hydridizes at near iso­ well as contamination process in the subarc man­ thermal conditions with peridotite within a relatively ~e source region, as suggested by Stern et al. cool subarc mantle wedge. (1984b), Futa and Stern (1988), and Stem (1988, Kay et al (1987, 1988) also noted the in­ and in press) . creasing importance, between the Mid and Late Miocene, of garnet in the petrogenesis of Andean crust as well. Hildreth and Moorbath (1988) also magmas near 30 0 S as the angle of dip of the sub­ suggested that the increased importance of garnet ducted slab flattened and the crust thickened in the petrogenesis of the magmas at the northern below this region, thus decreasing the volume and end compared to farther south in the SVZ reflects cooling the subarc mantle. They suggested that the thicker crust below this region. Thickened garnet may have increased in importance not only crust and increased compressive stress, such as in the mantle, but at the base of the thickened occur at the northern end compared to farther 158 PLlOCENE-PRESENT MIGRATION, VOLCANIC FRONT, SOUTHERN ANDES

ChIIlon CoOSlol COrdlllero may inerease the extent to whieh mantle derived PolealOle 8os.menl magmas evolve as they rise through the erust, 0 .7'0 SQURCE REGIQN CONTAMINATIQN thus inereasing the proportion of evolved inter­ 5 ____~ \ mediate and silicie magmas erupted relative to 50 more mafie magmas (Stern et al., 1984b). How­ 0708 ever, the fundamental petrochemieal features of UPPER CRU$TAL all Andean magmas, as well as magmas from other / AFe ~,Poleazoie convergent plate boundaries both along continen­ 'high-Sr 75 ,,10.... Rb/S' tal margins and in intra-oceanic regions, is estab­ lower cruSlol obbros onó lished in the intergrated subarc slab + mantle sys­ 25 2 IO~~~~~~l!!...~ lOWER CRUSTAL SSVZM AS$IMILATION tem (Stern, in press). ssvz B0501l In contrast to the northern end of the SVZ, a mod­ el involving relatively high degrees of partial melt­ 2:lO 400 '00 800 1000 -1200 ing (Figs. 6, 8) of a subarc mantle contaminated by Sr (ppm) components derived dominantly from subducted FIG. 8. Sr content versus 87Sr/86Sr for basalts from the pelagic sediment and/or sea-water altered_ oceanic southern part of the SVZ (SSVZ) and andesites erust, has been proposed for the generation of the from the northern end 01 the SVZ (NSVZ), taken basalts erupted fron) along the volcanie front near from figure 7, and materials possibly involved in the middle of the SVZ (López-Escobar et al., 1977; the generation 01 these rocks, including 1. ocean­ Hickey et al., 1984, 1986; Futa and Stern, 1988). ic mantle (OM; Sr = 50 ppm and 87Sr/86Sr = The decreased probability of tectonic erosion with­ 0.7025); 2. oceanic sediment (Sr =1000 ppm and in a trench filled with sediment (Ziegler et al., 1981; B7Sr/ll6Sr = 0.709); 3. high-Sr lower crustal litholo­ gies (Sr - 1000 ppm and 87Sr/Il6Sr=0.706); and 4. Shreve and Cloos, 1986; Cloos and Shreve, 1988) Paleozoic basement 01 the Chilean Coastal Cordi­ and the hotter temperatures in the subarc mantle llera (Sr = 200 ppm and 87Sr/86Sr=0.73). The figure below the middle as compared to the northern end shows mixing curves between these materials, of the SVZ - as indicated by the presence of back­ with percent 01 mixing indicated by the small num­ are basa~ic volcanism and the shallower depth to bers next to tick marks. These mixing curves the Benioff zone below the are in this region - are show that a mantle source for the SSVZ basalts consistent with the conditions implied by these (SSVZM) can lorm by the contamination 01 ocean­ ic type mantle with 1.5% of oceanic sediment. models. NSVZ andesites can form by either 1. assimila­ tion, by an SSVZ basalt within the lower crust, 01 IMPLlCATIONS FOR ANDEAN an equal weight 01 high-Sr gabbro or anorthosite; METALLOGENESIS or 2. by melting 01 a mantle source region (NSVZM) which, compared to the mantle farther south in the SVZ, has been lurther modilied by Mineralized subvolcanic tourmaline breccia the addition 01 1% subducted Paleozoic Coastal pipes within the Río Blanco-Los Bronces and El Te­ Cordillera basement. As discussed in a separate niente porphyry copper deposits (33-34°S; Fig. 2) publication (Stern, in press) in which the details have been dated as Pliocene in age (5.2-3.9 Ma of each model and the values 01 Sr and 87Sr/ll6Sr and 4.7-4.5 Ma, respectively; Ouirt et al., 1971; used in their construction are described in more Drake et al., 1976; Clark et al., 1983; Warnaars et detail, the author concludes that the source re­ gion contamination model, involving as it does on­ al., 1985; Cuadra, 1986). These deposits are host­ Iy a small change in lhe composition 01 subduct­ ed in older Miocene Farellones Formation volcanic ed materials, is a more appropiate model lor ex­ rocks and their plutonic equivalents (18.5-7 Ma; plaining the generation 01 andesites at the Vergara et al., 1988), which were erupted and em­ northern end of the SVZ since these andesites placed in the region between 31°30'-34°35'S prior have 8180 and trace element ratios very similar to the establishment 01 the current tectonic and to the basalts erupted larther south in the SVZ. magmatic segmentation 01 the Andes. Fluid inclu­ sion data indicate a period of uplift following the south in the SVZ and developed across the conti­ cooling of the plutons of the San Francisco batho­ nental margin at 300 S in the Mid to Late Miocene, lith (20.1-8.6 Ma; Warnaars et al., 1985) and pro- G.R. Stern 159

ceeding the subsequent intrusion of the Los Bron­ and thickening, and just prior to the eastward mi­ ces tourmaline breccias (Holmgren et al., 1988; gration or cessation of igneous activity. They iden­ M.A. Skewes, personal commun.). These data, as tified relatively low degrees of mantle partial melt­ well as the lack of important Pliocene porphyry ing in the presence of garnet as important factors copper deposits north of 33°, suggest that the in the generation of the magmas with which these metal-rich tourmaline breccia pipes at Río Blanco­ deposits are associated and suggested that these Los Bronces and El Teniente do not represent a conditions are meet after episodes of crustal thick­ late stage in the process of solidification of the plu­ ening due to the consequent decrease in the vol­ tons associated with the Miocene Farellones For­ ume and cooling of the subarc mantle and subse­ mation, as suggested by Warnaars et al. (1985), quent deepening of the zone of magma generation. but rather that the magmas responsible for this As noted aboye, another important effect related mineralization belong to the Pliocene phase of vol­ to any decrease in the volume of the subarc man­ canism which developed after the current segmen­ tle, caused by either crustal thickening or a de­ tation of the Andes near latitude 33°S had been crease in the angle of slab subduction, is an in­ established. crease in the relative importance of slab-derived Apparently the generation of these Pliocene components in the integrated slab + mantle source metal-rich tourmaline breccia pipes is closely relat­ of Andean magmas. Thus, as the volume of the ed in time to the initiation of the flattening of the subarc mantle shrinks, the 'geostill' (Sillitoe, 1972) angle of subduction below the northern end of the processes a greater proportion of subducted mate­ svz. The late Miocene Los Pelambres porphyry rial relative to mantle peridotite. It is here suggest­ copper at 32°S also formed as subduction angle ed that this effect, in association with the de­ decreased below the Andean segment just north of crease in the extent of melting as the subarc man­ the SVZ (Skewes and Stern, 1988). As discussed tle source region cools and deepens after periods aboye, decreasing angle of subduction led to a of either crustal thickening or in responce to the decrease in the volume and cooling of the subarc flattening of subduction angle, was one of the key mantle, an increase in the rate of tectonic erosion, processes in generating the magmas that led to and eventually the eastward migration of the arc in the development of mega-porphyry systems, not these regions (Fig. 4). In this article it is suggested only between 32-34°S, but in northern Chile as that two effects of decreasing angle of subduction - well. the increased relative importance of slab-derived The extent to which flattening of subduction components in the integrated slab + mantle magma angle and tectonic erosion were al so important source region as the volume of the subarc mantle factors in the generation of the porphyry copper decreases and the increased proportion of conti­ deposits in northern Chile, as it has here been nental components added into the subarc mantle suggested for those deposits between 32-34°S, as the rate of tectonic erosion increases - are remains speculative, but a clear correlation exists significant processes, not only in the generation of between the regional distribution of Chilean por­ the distinctive petrochemical characteristics of phyry copper deposits and the zone of extensive the magmas erupted along the current volcanic tectonic erosion delineated by Ziegler et al. (1981). front between 33-34°S, but also in the genesis of Unfortunately, because tectonic erosion destroys the late Miocene and Pliocene porphyry copper rather than produces rock units, determining deposits of Los Pelambres, Río Blanco-Los Bron­ whether tectonic erosion was an episodic process ces, and El Teniente located between 32-34°S. like crustal deformation, uplift and thickening, and Maksaev and Zentilli (1988) showed that porphy­ whether episodes of increased rates of tectonic ry copper deposits farther north in Chile formed eros ion correlate with porphyry copper genesis, following episodes of crustal deformation, uplift, remains problematic.

ACKNOWLEDGMENTS

The author wishes to thank his many Chilean (University of Colorado), Jorge Muñoz (Compañía collaborators, especially Milka Alexandra Skewes Minera Río Tinto), Michael Dobbs (Universidad de 160 PLlOCENE ·PRESENT MIGRATION, VOlCANIC FRONT, SOUTHERN ANDES

Santiago), Reynaldo Charrier and Estanislao Go­ tor their insights and assistance, and the National doy (Universidad de Chile) and Constantino Mpo· Science Foundation tor financial support trom dozis (Servicio Nacional de Geología y Minería), grants EAR80-20791 and EAR83-13884.

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