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Andean geology

versión On-line ISSN 0718-7106

AndGeo v.36 n.2 Santiago jul. 2009

http://dx.doi.org/10.4067/S0718-71062009000200001 

Andean Geology 36 (2): 153-171. July, 2009
formerly Revista Geológica de Chile
www. scielo. cl/andgeol. htm

 

Fission track thermochronology of Neogene plutons in the Principal Andean Cordillera of central Chile (33-35°S): Implications for tectonic evolution and porphyry Cu-Mo mineralization

Termocronología mediante trazas de fision de plutones neógenos en la Cordillera Principal Andina de Chile central (33-35°S): Implicancias para la evolución tectónica y mineralización de pórfidos de Cu-Mo.

 

Víctor Maksaev1, Francisco Munizaga1, Marcos Zentilli2, Reynaldo Charrier1

1Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile. vmaksaev@ing.uchile.cl; fmunizag@cec.uchile.cl; rcharrie@cec.uchile.cl
2
Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J1. marcos.zentilli@dal.ca


ABSTRACT. Apatite fission track data for Miocene plutons of the western slope of the Principal Andean Cordillera in central Chile (33-35°S) define a distinct episode of enhanced crustal cooling through the temperature range of the apatite partial annealing zone (~125-60°C) from about 6 to 3 Ma. This cooling episode is compatible with accelerated exhumation of the plutons at the time of Pliocene compressive tectonism, and mass wasting on the western slope of the Principal Andean Cordillera in central Chile. The timing coincides with the southward migration of the subducting Juan Fernández Ridge and the development of progressive subduction flattening northward of 33°S. It also corresponds to the time of active magmatic-hydrothermal processes and rapid unroofing of the world class Río Blanco-Los Bronces and El Teniente porphyry Cu-Mo deposits. Zircon fission track ages coincide with previous 40Ar/39Ar dates of the intrusions, and with some of the apatite fission track ages, being coherent with igneous-linked, rapid cooling following magmatic intrusion. The thermochronologic data are consistent with a maximum of about 8 km for Neogene exhumation of the plutons.

Keywords: Thermochronology, Andes, Fission track, Apatite, Porphyry copper, Exhumation.


RESUMEN.

Los datos de trazas de fision en apatita de plutones miocenos del flanco oeste de la Cordillera Principal de Chile central (33-35°S) definen un episodio distintivo de enfriamiento acelerado a través del rango de temperatura de la zona de acortamiento parcial de trazas en apatita (~125-60°C) entre los 6 a 3 Ma. Este episodio de enfriamiento es compatible con exhumación rápida de los plutones al tiempo del tectonismo compresivo plioceno y remociones en masa en el flanco oeste de la Cordillera Principal en Chile central. El período de tiempo coincide con la migración hacia el sur de la subducción de la Dorsal de Juan Fernández y con el desarrollo de un aplanamiento progresivo de la subducción hacia el norte de los 33°S. También corresponde al tiempo de actividad magmático-hidrotermal y rápido desenterramiento de los pórfidos de Cu-Mo de clase mundial de Río Blanco-Los Bronces y El Teniente. Las edades de circones por trazas de fision coinciden con datos geocronológicos 40Ar/39Ar previos a las intrusiones y con algunas de las edades de trazas de fision en apatitas, siendo coherentes con el enfriamiento rápido relacionado con procesos ígneos después de la intrusion magmática. Los datos son consistentes con un maximo de aproximadamente 8 km para la exhumación neógena de los plutones.

Palabras claves; Termocronología, Andes, Trazas de fision, Apatita, Pórfido cuprífero, Exhumación.


1. INTRODUCTION

The subduction-related Miocene-Pliocene volcanism and plutonism along the Principal Andean Cordillera of central Chile (33-35°S) developed synchronously with crustal thickening and tectonic uplift related to compressive tectonism (Miocene Quechua, and Pliocene Diaguita phases, described originally by Steinmann (1929) in Perú and by Salfity et al. (1984) in northwestern Argentina; see also the review of González-Bono riño et al., 2001). This Andean segment occurs immediately southeast of the place where the Juan Fernández Ridge is being subducted (Fig. 1), whichhas migrated from northto south during the Late Cenozoic (Yáñez et al., 2001. 2002). The progressive decrease in the angle of subduction of Nazca oceanic lithosphere northward from 3 3 °S has been attributed to the Juan Fernández Ridge subduction (Yáñez et al., 2001,2002). Crustal deformation resulting in increased crustal thickness and possibly dehydration and/or melting of thickened lower continental crust have also been credited to the effects of the ridge subduction (Kay et al., 1999, 2005; Kay and Mpodozis, 2002), and higher exhumation rates during the Neogene (Skewes and Holmgren, 1993; Kurtz et al., 1997). In addition, the giant porphyry Cu-Mo deposits of Río Blanco-Los Bronces and El Teniente (Fig. 1) were formed in this Andean segment during the late Miocene to early Pliocene (Deckart et al., 2005,2006; Maksaev et al., 2004; Cannell et al., 2005) and were rapidly unroofed (Skewes and Holmgren, 1993; Serrano et al., 1996). Their origin has been attributed to crustal thickening, uplift and erosion that accelerated crystallization and devolatilization of crustal magma chambers above which the giant Cu-Mo deposits were formed (Skewes and Stern, 1994; Stern and Skewes, 2005). Thus, understanding the Neogene exhumation history of the Principal Andean Cordillera of central Chile has implications for both the tectonic and metallogenic evolution of this segment of the Andean orogen.


A number of plutons of the Principal Andean Cordillera of central Chile were previously dated by the 40Ar/39Ar method by Kurtz et al. (1997). These authors modelled the exhumation of two Miocene plutons (La Obra and Nacimiento Cortaderal) based on differences of biotite and K-feldspar plateau 40Ar/39Ar ages that were ascribed to unverified ex-humation-linked cooling. We have analyzed zircon and apatite crystals from the same Miocene intrusivebodiesby fissiontrackthermochronology (Fig. 2; Table 1), establishing their low temperature cooling histories with the goal to improve the understanding of the exhumation history of the western slope of the Principal Andean Cordillera in central Chile, as a contribution to constrain the tectonic and metallogenic evolution of this Andean segment. Analyses were done on some of the remaining samples from the 'Proyecto Geodinámico El Teniente' of CODELCO-Chile, which were originally dated by Kurtz et al. (1997), plus additional samples collected specifically for this study. The extended geographical spread of the samples in the región (Fig. 2; Table 1) allows assessment of variations of the cooling histories of the plutons. The fission track low-temperature thermochronology is evaluated relative to the published geochronological data of the Miocene plutons to constrain their time-temperature history and the implications of the new data for tectonics and metallogeny are discussed.



2. GEOLOGICAL FRAMEWORK

The segment of the Andes between 33° and 35°S is situated immediately south of the flat-slab segment of the subducted oceanic Nazca Plate (28-33°S; Cahill & Isacks, 1992; Yáñez et al., 2002). Thus the subducting Nazca slab sharply flexes southwards from subhorizontal to a dip angle 25-30° (Pardo et al., 2002). The rough mountainous western flank of the Principal Andean Cordillera in central Chile is about 60 km wide, varying from about 1,000 mupto 6,969 mabove sealevel (Mount Aconcagua; Fig. 1). Most of the highest peaks are Andean are volcanoes of the currently active Southern Volcanic Zone, which extends from 33°S southward along the continental watershed (e.g., 6,570 m at the summit of the Tupungato volcano). A Miocene-Pliocene fold-thrust belt and a foreland basin system extends farther east into Argentinean territory (Fig. 1), and a 50 km wide alluvial plain (the Central Depression; Fig. 2) borders the Principal Andean Cordillera to the west; the plain slopes westward from about 700 down to 400 m, where it adjoins the Coastal Cordillera (Fig. 2).

Oligocene-Miocene volcanic successions, tota-ling about 2,600 to 4,900 m in thickness, are exposed on most of the western flank of the Principal Andean Cordillera in central Chile. Traditionally the 1,300 to 1,900 m thick, folded lower section (Oligocene to early Miocene in age) has been mapped as the Abanico Formation or the equivalent Coy a-Machalí Formation south of 34°S (Klohn, 1960; Aguirre, 1960; Thiele, 1980; Charrier et al., 2002, 2005), whereas the upper unconformable subhorizontal volcanic rocks (1,300 to 3,000 m thick) are considerad to be part of the Farellones Formation (Charrier et al., 2002). In fact, a progressive unconformity (time-transgressive) separates these units and the K-Ar ages of the two volcanic units overlap between 22 and 16 Ma (Nyström et al., 2003; Charrier et al., 2002, 2005, 2007). The volcanism of the Principal Andean Cordillera of central Chile appears to have initially developed during the Eocene-Oligocene under an extensional tectonic regime (Charrier et al., 2002). However, during the Miocene, volcanic activity took place simultaneously with compressive pulses of the Quechua tectonic phase and related crustal shortening. Subsequently, volcanic activity practically waned, and only very localized igneous activity was recorded in the región during the Pliocene Diaguita compressive tectonic phase [e.g., González-Bonorino et al., 2001). The igneous activity completely ceased by the late Pliocene along the western flank of the Principal Andean Cordillera, except for the currently active Southern Volcanic Zone of the Andes, which developed since the latest Pliocene farther east along the continental watershed. Basin inversion and significant tectonic uplift of the Principal Andean Cordillera took place as a result of the Neogene compressive tectonism (Godoy et al., 1999; Charrier et al., 2002, 2005), but also the sy nchronous development of a foreland basin system farther east in Argentina (Giambiagi et al., 2001).

A number of Miocene granodioritic to dioritic plutons are scattered throughout the Cordillera at these latitudes (e.g., Kurtz et al., 1997); some of them are composite intrusions and locally display sill geometry within the Oligocene-Miocene volcanic succession. In addition, this segment of the Andes encompasses two of the largest porphyry Cu-Mo deposits in the world, the Río Blanco-Los Bronces and El Teniente. These deposits occur within hydrothermal alteration zones related to late Miocene-early Pliocene multiphase porphyritic stocks (Skewes et al., 2002; Maksaev et al., 2004; Deckart et al., 2005, 2006) (Figs. 1 and 2). The waning stage of Pliocene are-related igneous activity and the formation of giant Cu-Mo deposits preceded a 50 km eastward are migration, to the currently active Southern Volcanic Zone of the Andes (Kay et al., 2005).

Remnants of Pliocene rock debris (avalanche deposits) occur scattered along the westernmost section of the Principal Andean Cordillera between 33° and 34°S in central Chile (e.g., SERNAGEOMIN, 2002) and within sections of the Central Depression (Marangunice et al., 1979). The Pliocene rock debris records massive landslides of upper Miocene volcanic materials from the Principal Andean Cordillera (e.g., Encinas et al., 2006).

3. FISSION TRACK THERMOCHRONOLOGY

Zircon and apatite fission track analysis are methods that provide quantitative information on the thermal histories of rocks (e.g., Gallagher et al., 1998). The temperatures at which fossil fission tracks within the apatite and zircon mineral groups partially anneal (i.e., partial age resetting) are not sharply definedbutprogressive (Green et al., 1986). The temperature range where partial track annealing oceurs is known to be a function of the phase composition (Green et al., 1986; Carlson et al., 1999), cooling rate, and possibly the symmetry group of the mineral (Spikings et al., 2005 and references therein).

Unannealed track lengths in apatite range between ca. 14.5 and 15.5 µm relative to standard Durango apatite (Gleadow et al., 1986), and henee apatite samples that have mean track lengths in this range, combined with narrow track length distribu-tions, have experienced rapid monotonic cooling from temperatures of ≥ 125-100°C to temperatures of ca. ≤60°C at the time indicated by the respective apatitefissiontrackage (Lasletteía/., 1987). Broad track length distributions with shorter mean lengths reveal instead that the sample experienced a more complex thermal history, spending a significant amount of time within the partial annealing zone (Gleadowe et al., 1986; Spikings et al.,2005). Similar principies apply to zircon fission track data, but the lack of well-characterized annealing kinetics of tracks within zircon precludes the determination of a thermal history from the track length distributions. In most geological settings zircon has an effective closure temperature of about 240±50°C (Hurford, 1986; Brandon et al., 1998; Bernet et al., 2002). However, experimental data show that there is a wide temperature range, from 160 to 380°C, for the temperature bounds for the zircon partial annealing zone, which are strongly dependent on cooling rates and alpha radiation damage of zircon crystals (Bernet and Garver, 2005; Tagami, 2005; Reiners and Brandon, 2006). Thus, as a reasonable approximation for this work, a temperature of 280±30°C has been plotted against the zircon fission track age in study to produce a single temperature-time point on a thermal history path (e.g., Tagami and Shimada, 1996; Thomson et al., 2001; Reiners and Brandon, 2006; Adriasola and Stökhert, 2008).

4. ANALYTICAL METHODS

Apatite and zircon were separatedby conventional methods at the facilities of the Universidad de Chile. The zircon and most of apatite fission track analyses were done by Dr. RB. O'Sullivan in the 'Apatite to Zircon, Inc.' Laboratory, Idaho, USA, and some of the apatite fission track analyses were done by A.M. Grist at the Fission Track Laboratory of Dalhousie University, Halifax, Nova Scotia, Canada. Etching of natural fission tracks in apatite was attained by immersion in 5.5N HN03 for 20.0 seconds (±0.5 seconds), whereas etching of zircon grains was done by immersion in an eutectic melt of NaOH+KOH at 210°C (±10°C). Total etching times for zircons varied between 25 and 69 hours, but for most samples was 44 hours and 15 minutes. The external detector method was used for fission track dating (Gleadow, 1981), ages were calculated using the zeta calibration method (Hurford and Green, 1983), and errors were calculated according to conventional methods (Green, 1981). Analyses done at the 'Apatite and Zircon, Inc.' included the systematic measurement of the maximum fission track etch pit diameters oriented within 5 o of the c axis of the apatite crystal (Dpar) in order to considerfission-track annealing variability among different apatite species in thermal history modelling (Carlson et al., 1999). Irradiation of apatite samples with 252Cf was used to increase the amount of etched confined track for length measurement. The precision of each track length is estimated to be ±0.20 µm.

The AFTsolve multi-kinetic inverse modelling program of apatite fission track data (Ketcham et al., 2000) was used to derive time-temperature histories from the fission track data of apatite samples. This program implements various laboratory calibrations of the behaviour of fission tracks in apatite in response to heating and cooling histories, and calculates the range of thermal histories that are potentially consistent with the measured age and the measured frequency distribution of confined track lengths. Full details concerning these calibrations and the various uses of AFTsolve are publicly available (Carlson et al., 1999; Donelick et al., 1999; Ketcham et al., 1999, 2000). 20,000 random time-temperature paths are created by a Monte Cario scheme, and for each path the resul-ting fission track age and track length distribution are calculated, and the goodness-of-fit between calculated and measured data is evaluated by a Kolmogorov-Smirnov test. The program maps out the time-temperature regions that envelop all thermal histories with 'good' and 'acceptable' fit, corresponding to goodness-of-fit values from 0.5 to 1 and from 0.05 to 0.5, respectively.

5. RESULTS AND DISCUSSION

Apatite fission track ages from 3.2±0.7 to 21.1±3.4 Ma were obtained for eighteen samples from Miocene plutons that are spread on the western slope of the Principal Andean Cordillera of central Chile (33-35°S) and span elevations between 760 and 3,790 m (Table 2; Fig. 2). In addition, 14 zircon fission track ages, which range between 5.3±0.3 and 13.8±0.7 Ma (Table 3; Fig. 2), were obtained for the same plutons.


Most of the zircon fission track ages are similar to their respective biotite 40Ar/39Ar or K-Ar ages, except for the Extravio pluton with a zircon fission track ~1 myrs. younger than a biotite 40Ar/39Ar age (Table 4), but in this case the ages are for separate samples 4.5 km apart, respectively. The 13.8±0.7 Ma zircon fission track for the Rosario de Rengo pluton appears to be younger than the respective hornblende 40Ar/39Ar of 16.2±1.3 Ma, but these dates overlap at ±2o error limits. On the other hand. the samples from the Río Blanco granodiorite yield-ed zircon fission track ages between 8.7±0.7 and 5.3±0.3 Ma, which are significantly younger than the 11.96±0.40 Ma zircon U-Pb age of their host rock (Table 4). These samples were obtained from drill cores within the Río Blanco porphyry copper deposit (TT-188, TT-189) and its surroundings (TT-190), where the thermal history is complicated by a succession of younger intrusions that are associated with porphyry copper mineralization and hydrother-mal alteration (e.g., Deckart et al., 2005). In fact, the zircon fission track ages for the Río Blanco granodiorite coincide with zircon U-Pb ages of other younger intrusive bodies emplaced within this intrusion at the Río Blanco porphyry copper deposit area (the Cascada Granodiorite, Quartz Monzonite, and Don Luis Porphyry; Table 4). Consequently thermal aureoles of other intrusions might have reset the zircon fission track system of the Río Blanco granodiorite at the homonymous porphyry copper deposit. As a reference, Tagami and Shimada (1996) have shown that zircon fission track ages in sand-stones surrounding a rapidly cooled granitic intrusion become totally reset within a distance of ~3 km from a steep-dipping contact, although this distance can vary depending on several factors, including the size and shape of the intrusives, the permeability of the wallrock, the depth of emplacement, and the presence of fluids (e.g., Adriasola et al., 2006).


The zircon fission track ages are interpreted to represent the time of post-emplacement cooling to temperatures of around 280±30°C (e.g., Tagami and Shimada, 1996; Bernet and Garver, 2005; Reiners and Brandon, 2006), and the biotite 40Ar/39Ar closure temperature can be estimated as 320±30°C using Dodson (1973) and parameters presented by Harrison et al. (1985) and McDougall and Harrison (1999). Therefore, the overall coincidence of the zircon fission track ages with biotite 40Ar/39Ar ages (Table 4) is consistent with in situ post-magmatic rapid cooling of the host intrusions. The dated plutons should have remained at a cool (shallow) level in the crust since their emplacement, where temperatures have not been high enough to significantly anneal the fission tracks in the zircon grains (within the zircon fission track retention zone); except for subsequent heat input from other intrusions that may have lead to annealing of zircon fission tracks at Río Blanco. Therefore Neogene exhumation has not been sufficient to expose Miocene intrusive rocks from depths corresponding to the zircon partial annealing zone. The lower temperature bound of the zircon partial annealing zone may be as high as 250°C for non damaged zircons and rapid cooling rates (Tagami, 2005; Reiners and Brandon, 2006), implying that the Neogene exhumation has been limited to a column of less than ~8 km, assuming a paleogeothermal gradient of 30°C/km. This establishes a maximum for the Neogene exhumation in this part of the Andes that is consistent with the preservation of a succession of more than 3,000 m of Oligocene-Miocene volcanic rocks along the western slope of the Principal Cordillera of central Chile.

The above geothermal gradient is speculative, but not unreasonable as a first approximation, since Giese (1994), onthe basis of thermal measurements and calculations for the present-day crust in the Central Andes, estimates a temperature of 141°C at a depth of 5 km (average gradient of 28.2°C/km), and regional surface heat flow measurements yield currentvalues of-80-100 mW/m2 (Hamza and Muñoz, 1996; Muñoz, 1999). Thermal conductivities for granitoids vary between 2.5 and 3.5 W/(m°K) (Seipold, 1998). Thus, present-day geothermal gradients of 32±8°C/km can be estimated for the upper crust (Adriasola et al., 2006); if a higher geothermal gradient was prevalent during the Neogene are mag-matism it would signify even less exhumation.

The apatite fission track ages obtained for the La Obra, San Gabriel and Cruz de Piedra plutons are similar to zircon fission track ages and/or previous 40Ar/39Ar or K-Ar ages of the respective intrusions (Table 4). However, most of the apatite fission track ages are 2 to 7 myrs. younger than their respective 40Ar/39 Ar, K-Ar or zircon fission track ages (plutons: Río Blanco, Jeria, Extravío, Alfalfalito, Río Pangal, Estero Crucero, Rosario de Rengo, and Nacimiento Río Cortaderal; Table 4). Their apatite fission track ages fall within the restricted range from 5.6 to 3.2 Ma (Table 4) that represents the period when the host plutons cooled through the temperature range of the apatite partial annealing zone (~125-60°C; Gleadow et al., 1986; Green et al., 1986; Carlson et al., 1999).

The mean length and distribution of confined tracks in apatite provide important additional infor-mation on the cooling history below ~125°C (e.g., Gleadow et al., 1986; Green et al., 1989). In general, the measured apatite fissiontrack length distributions have long mean track lengths (≥ 14.0 µm) and standard deviations under 1.8 µm (apatite samples from Río Blanco, Extravío, Alfalfalito, Cruz de Piedra, Río Pangal, Estero Crucero, Santa Rosa de Rengo, and Nacientes Río Cortaderal plutons; Fig. 3), indicative of rapid cooling through the temperature range of the apatite partial annealing zone (~125-60°C; Gleadow et al., 1986). However, six apatite samples (from San Francisco, La Obra, San Gabriel plutons) have shorter mean track lengths (between 13 and 14 um) and negative-skewness towards shorter tracks (Fig. 3), which indícate that their host rocks spent some time in the temperature zone where fission tracks in apatite were partially annealed (Gleadow et al., 1986). The relationship between the apatite fission track age, and the mean track length and standard deviation for all samples, which yielded adequate length data (Table 2) are plotted in figure 3. The overall 'boomerang' pattern of the plotted fission track data in figure 3 is indicative of partial and total resetting, or exhumation of a fossil partial annealing zone. The long track lengths (<14 µm) and small standard deviations (<1.5 um) of a group of apatite samples from these plutons and their age data define a distinctive rapid cooling episode between ~6 and 3 Ma; whereas another group of apatite samples displays higherdegrees of track shortening (<14 µm) and larger standard deviations (<1.5 µm), thus their apatite fission track data suggest a more complex cooling and/or thermal history.


Figure 4a shows sample apatite fission track ages with track lengths above 14 µm plotted versas altitude. Despite scattering, probably due to the extended geographical spread of the samples in this región, there is a general tendency for older apatite fission track ages to correlate with high altitude above sea level, as would be expected for exhumation-related cooling. The same tendency is apparent in the subset of three drill core samples of the Río Blanco granodiorite (Fig. 4b). As a reference, dashedlines correspondingto hypothet-ical exhumation rates of 0.45 mm/y are shown on figures 4a and 4b, but the actual exhumation rates could have been somewhat higher or lower than this given the errors on the ages. Alternatively, the apatite fission track ages, which are 2 to 7 rayrs. younger than their respective 40Ar/39Ar, K-Ar or zircon fission track ages in the same intrusions, may represent the time lag required for complete relaxation of the perturbations of the isotherms in the geothermal field imposed by intrusion of magmatic bodies, instead of exhumation-related cooling. However, the above age-elevation relationship favors the latter.



The cooling histories of intrusive bodies depend on their volume, shape, and level of emplacement. According to numerical simulations, ascent and emplacement of granitic intrusions in the upper crust typically occurs in less than 1 m.y rs. (Kukowski and Neugebauer 1990; Cathles et al., 1997; Adriasola et al., 2006). Such rapid cooling is inferred for the Cruz de Piedra intrusion, which is an unaltered quartz monzonite stock that crops out cióse to the continental watershed at 3,790 m of altitude (nearthe Chilean-Argentineanborder; Fig. 2). We have obtained an apatite fission track age of 6.1±0.6 Ma (mean track length: 14.95±0.10 µm), and an identical zircon fission track age of 6.2±0.4 Ma for this stock (ETP-14; Table 4). Previously, Kurtz et al. (1997) reported a biotite 40Ar/39Ar plateau age of 5.5±0.1 Ma for the same sample (ETP-14), which is equivalent to these fission track ages (within ±2σerrorlimits). Consideringthe differing temperature ranges of the apatite and zircon fission track annealing zones, and the closure temperature of biotite for the K-Ar system, the similar ages imply very rapid in situ post-magmatic cooling, as shown by the temperature-time path below 350°C, which is consistent with high cooling rates (> 200°C/m.yrs; Fig. 5).


 

At shallow levels in the upper crust (~5 km), heat transfer from a magma chamber is likely to occur through convection of hydrothermal fluids. Depending on the permeability of the wallrock, the anomalous heat flow affects the shape and distribution of the geothermal gradients in their surroundings, lifting the isotherms in the area above the chamber towards the surface of the Earth (e.g., Adriasola et al., 2006). This situation is recognized at the Río Blanco porphyry copper deposit, which was formed by hydrothermal fluids in association with an epizonal, multistage intrusive complex from 6.3±0.1 Ma to 4.3±0.1 Ma according to U-Pb, 40Ar/39Ar, K-Ar and Re-Os ages (Deckart et al., 2005). Our apatite fission track ages for underground samples of the hydrothermally altered/mineralized Río Blanco granodiorite are 3.7±0.4 and 3.2±0.4 Ma (Tables 2 and 4). In addition, apatite (U-Th)/He ages of 3.5±0.1 and 2.4±0.1 Ma were reported by Mclnnes et al. (2005) for the quartz monzonite and Don Luis porphyries of the Río Blanco orebody These data attest to exceedingly rapid cooling of the hydrothermal system after cessation of the igneous pulses; Le., within~l m.yrs. for cooling through the temperature range of the apatite partial annealing zone (~125°-60°C) andwithin~2 m.yrs. through the temperature range of the apatite He partial retention zone (~85°-40°C; Wolf et al., 1998).

A similar evolutionis inferredforthe El Teniente porphyry copper deposit, which according to com-bined U-Pb, 40Ar/39Ar, K-Ar and Re-Os ages formed from 6.5±0.1 Ma to 4.3±0.1 Ma (Maksaev et al., 2004), and apost-ore hornblende-richandesitic dike intruded at 3.85±0.18 Ma marks the end of igneous activity within the orebody (Maksaev et al., 2004). An apatite fission track age of 4.2±2.8 Ma (Maksaev et al., 2004), and apatite (U-Th)/He ages from 3.4 to 2.7 Ma for the dacite porphyry of El Teniente (Mclnnes et al., 2005), provide evidence for faStöcooling of this giant porphyry system after the end of the igneous intrusions.

Temperature-time paths were constructed combining previous geochronological data, new zircon fission track ages, and inverse modelling of apatite fission track data using the AFTsolve multi-kinetic program of Ketcham et al. (2000) for the Río Blanco, Extravío, Alfalfalito, Río Pangal, Estero Crucero, Santa Rosa de Rengo, and Nacientes Río Cortaderal plutons (Figs. 6, 7 and 8). The fission track ages of these plutons range from 5.6±1.5 to 3.2±0.4 Ma, andaré 2 to 7 m.yrs. younger than their respective zircon fission track and 40Ar/39Ar ages (Table 4). Furthermore, their apatite grains ha ve long mean fission track lengths (>14.0 µm; Fig. 2) and narrow track length distri-butions (standard deviations <1.5 µm), consistent with rapid cooling through the temperature range of the apatite partial annealing zone (~ 125-60°C). In general, their temperature-time paths are consistent with rapid post-magmatic cooling, with delays of 2 to 7 m.yrs. for cooling through the temperature range of the apatite partial annealing zone (~ 125-60°C). Although extended igneous-linked cooling cannot be completely discarded, the relatively long time-lag for cooling below -125°C and the modelled cooling histories are strongly suggestive of a distinct cooling event at ~6 to 3 Ma, probably related to subsequent, enhanced exhumation of these intrusive bodies. The exhumation of the Miocene plutons along the western slope of the Principal Andean Cordillera (33-35°S) from~6to3 Ma may well be a consequence of enhanced erosion related to increased relief. due to crustal shortening and surface uplift related to the Pliocene Diaguita compressive tectonism of Salfity et al. (1984). It coincides with the time of the southward migration of the location of the colusion of the Juan Fernández Ridge against the South American margin at these latitudes accompanied by the progressive onset of flat subduction from north to south (Yáñez et al., 2002). It probably represents part of a period of accelerated denudation as proposed by Skewes and Holmgren (1993), and overlap in time with documented Pliocene massive landslides along the western slope of the Principal Andean Cordillera (Encinas et al., 2006; and references therein). In fact, rapid unroofing of the Río Blanco porphyry Cu-Mo orebody took place before being covered by rhyodacitic tuff deposits of the La Copa complex dated by K-Ar from 4.9±0.2 to 4.0±0.2 Ma (Vergara and Drake, 1978; Serrano et al., 1996; Deckart et al., 2005).






Supplementary data comes from the geomorphologic analysis of Farías et al. (2008), who inferred the onset of high and rapid surface uplift in the Andes of Central Chile (33-35°) between 10.5 and 4.6 Ma, and the thermochronological study of Spikings et al. (2008) that estimated the onset of exhumation at about 7.5 Ma for the segment of the Principal Andean Cordillera in Chile located immediately south between 35-38°S, based on 40Ar/39 Ar, apatite fission track and zircon and apatite (U-Th)/He data. Spikings et al. (2008) also showed that exhumation between 35-38°S occurred at progressively earlier times northward, to the región that is at present above the fíat slab segment, which hosts the Juan Fernández Ridge. This fits well with our apatite fission track data between about 6 and 3 Ma representing the youngest exhumation between 33-35°S.

Previous independent geological evidence of the late Miocene-Pliocene uplift in the Andes of Central Chile, lead Skewes and Holmgren (1993) to postúlate that the associated rapid exhumation of active plutonic systems duringthis regime triggered exsolution of magmatic fluids with copper, which produced hydrothermal brecciation, and stockwork mineralization of the supergiant Río Blanco-Los Bronces and El Teniente porphyry copper deposits during the late Miocene and Pliocene. Actually, according to our data these epizonal intrusions cooled and consequently crystallized too fast for being significantly affected by pressure release from long-lasting, steady exhumation. Then again. exhumation does not progress at constató rates, and our fission track data strongly suggest accelerated exhumation cooling between ~6 and 3 Ma, coinciding with mass wasting of the volcanic cover of porphyry deposits (e.g., Godoy et al., 1994; Encinas et al., 2006). Thus mineralizing processes. brecciation, and formation of diatreme vents at the Río Blanco-Los Bronces and El Teniente porphyry copper systems (e.g., Sillitoe, 1985), probably was related to the gravitational sliding of their volcanic roof. Actually, overpressure of hydrothermal fluids may have been fundamental to trigger the land-slides of the volcanic cover on top of the porphyry systems similarly to the model of Reid (2004), and sudden unroofing by gravitational sliding during the magmatic-hydrothermal evolutionof these deposits may have destabilized the deeper magma chambers and may have influenced magma degassing and the development of shallow, richly-mineralized, late hydrothermalbreccias and explosive diatreme vents within these world class Cu-Mo deposits.

6. CONCLUSIONS

Fission track thermochronology of Miocene plutons of the Principal Andean Cordillera of central Chile revealed that most intrusions yield younger apatite fission track ages relative to their 40Ar/39Ar ages, and zircon fission track ages, respectively. The long track lengths (>14 µm) and small standard deviations (<1.5 µm) of the apatite samples from most of these plutons and their age data define a distinctive episode of enhanced cooling between about 6 and 3 Ma. During this period the dated rocks of the western slope of the Principal Andean Cordillera cooled rapidly through the temperature range of the apatite partial annealing zone (-125-60°C), consistent with accelerated exhumationrates throughout the latest Miocene and early Pliocene as a consequence of surface uplift associated with the Diaguita compressive tectonism. The timing coincides with the southward migration of the locus of the collision of the Juan Fernández Ridge against the South American margin at these latitudes, accompa-nied by the progressive onset of fíat subduction from north to south. As a consequence, it also appears that there was a progressively older onset of exhumation southwards along the Principal Andean Cordillera as inferred from our recentiy published thermochronological data.

The 6 to 3 Ma period coincides with the rapid unroofing of some of the largest porphyry Cu-Mo deposits in the world (El Teniente, Río Blanco-Los Bronces) at the time of their intrusion-related mineralization processes and also with mass wasting processes on the largely volcanic western slope of the Principal Andean Cordillera in central Chile. Massive landslides during the Pliocene may have had an effect on magma degassing and the develop-mentof shallow, richly-mineralized, late hydrothermal breccias and explosive diatremes within these world class Cu-Mo deposits.

The thermochronologic zircon fission track data are consistent with in situ post-magmatic rapid cooling of the intrusions, and a maximum of about 8 km of Neogene exhumation of the dated plutons. The relatively limited exhumation contributed to the preservation of the shallow late Miocene-early Pliocene Cu-Mo deposits and Neogene volcanic succession along the western slope of the Principal Andean Cordillera in central Chile.

Acknowledgments

This study benefited from the Program for Incentive of International Cooperation 7000932 of CONICYT, Chile, and Fondecyt Grant 1000932. The analytical contribution of Dr. RB. O'Sullivan of Apatite to Zircon, Inc, USA, and Dr. A.M. (Sandy) Grist of Dalhousie University, Canadá, is sincerely appreciated. Special thanks to R Zúñiga and A. Arévalo for authorizing the analyses of the samples collected during a previous regional project in the región ('Proyecto Geodinámico El Teniente') of El Teniente Division, CODETCO, Chile. Comments provided by the reviewers S.N. Thomson, S.M. Kay and T. Barbero helped to improve and clarify the manuscript. This is a contribution to the Anillo ACT-18 project: 'Tectonomagmatic control of GiantOre Deposits in the subduction factory of the High Chilean Andes between 32°-36°S: A multidisciplinary Approach' (TECMA-GOD-MULTI).

 

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Manuscript received: September 09, 2008; revised/accepted: December 22, 2008.

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