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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/S0718-71062009000200005 

Andean Geology 36 (2): 264-287. July, 2009
formerly Revista Geológica de Chile
www. scielo. cl/andgeol. htm

 

LA-ICP-MS zircon U-Pb geochronology to constrain the age of post-Neocomian continental deposits of the Cerrillos Formation, Atacama Region , northern Chile: tectonic and metallogenic implications

Geocronología U-Pb en circón por LA-ICP-MS para circunscribir la edad de depósitos continentales post-neocomianos de la Formacion Cerrillos, Región de Atacama, norte de Chile: implicancias tectónicas y metalogénicas

 

Víctor Maksaev1, Francisco Munizaga1, Víctor Valencia2, Fernando Barra2

1 Departamento de Geología, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile. vmaksaev@ing.uchile.cl; fmunizaga@cec.uchile.cl
2 Department of Geosciences, University of Arizona, 1040E. 4th Street, Bldg. #77, Tucson.AZ 85721, USA. victorv@email.arizona.edu; fbarra@email.arizona.edu


ABSTRACT. New U-Pb zircon dating of volcamc intercalations in the lower conglomeratic part of the Cerrillos Formation shows that its deposition extended in time at least from 110.7±1.7 to 99.7±1.6 Ma. The significantly younger U-Pb zircon age of 69.5±1.0 Ma obtained for the upper volcamc part of the Cerrillos Formation suggests recurrence of volcanism in the Late Cretaceous instead of continual volcanic activity. A minimum late Maastrichtian age for the Cerrillos Formation and its initial deformation was determined by the U-Pb zircon age range from 66.9±1.0 to 65.2±1.0 Ma for the lower part of the unconformably overlying Hornitos Formation. The new U-Pb data for the Cerrillos and Hornitos formations poses questions about the Campanian-Maastrichtian age range currently ascribed to the latter. The lower part of the Cerrillos Formation represents a major change in the sedimentary regime from previous marine carbonate sedimentation in a back-arc basin until the late Aptian to subsequent coarse alluvial sedimentation and volcanism since the early Albian. The lower part of the Cerrillos Formation is interpreted as the development of coalescent alluvial fans thinning inland, accompanied by volcanism. These developed as the result of transpressive deformation and uplift of the area of the current Coastal Cordillera by late Aptian, leading to subsequent active erosion and sedimentation inland, along with the eastward shift of the magmatic foci in the Región . Amineralizing period of Cu-Au porphyries overlaps in time with the deposition of the Cerrillos Formation in northern Chile; marking also a significant change in the metallogeny of the Andes of northern Chile.

Keywords: Cretaceous, Andes, U-Pb Geochronology, Cerrillos Formation, Porphyry copper.


RESUMEN. Nuevas dataciones U-Pb obtenidas para intercalaciones volcánicas en la parte inferior conglomerádica de la Formacion Cerrillos muestran que su depositacion se extendió en el tiempo al menos desde 110,7±1,7 hasta 99,7±1,6 Ma. La edad U-Pb significativamente más joven de 69,5±1,0 Ma obtenida para la parte superior volcánica de la Formacion Cerrillos, sugiere la recurrencia del volcanismo en el Cretácico Tardío, más que la continuidad de la actividad volcánica. Una edad mínima Maastrichtiano tardío para la Formacion Cerrillos y su primera etapa de deformacion se determinó por el rango de edades U-Pb de 66,9±1,0 a 65,2±1,0 Ma para la parte inferior de la suprayacente, discordante Formacion Hornitos. LosdatosU-Pb para laFormacion Hornitosplanteaninterrogantesrespecto al rango Campaniano-Maastrichtiano actualmente asignado a esta unidad. La parte inferior de la Formacion Cerrillos representa un cambio mayor en el régimen sedimentario desde la sedimentacion previa calcarea marina en una cuenca de trasarco hasta el Aptiano tardío a una subsecuente sedimentacion aluvial gruesa y volcanismo a partir del Albiano temprano. La parte inferior de la Formacion Cerrillos se interpreta como conos aluviales coalescentes que se adelgazan hacia el interior continental acompañados de volcanismo. Su desarrollo estuvo ligado a la deformacion transpresiva y alzamiento del area de la actual Cordillera de la Costa en el Aptiano tardío, lo que condujo a su erosion y sedimentacion hacia el interior continental, junto con la migracion hacia el este del eje magmático en la Región . Un período de mineralizacion de pórfidos de Cu-Au se traslapa en el tiempo con la depositacion de la Formacion Cerrillos y marca también un cambio importante en la metalogénesis de Los Andes del norte de Chile.

Palabras claves: Cretácico, Andes, Geocronología U-Pb, Formacion Cerrillos, Pórfidos cupríferos.


1. INTRODUCTION

The accurate dating of continental volcano-sedimentary lithostratigraphic units inthe Andes has traditionally been complicated by the lack of suitable mineral phases for reliable isotopic determinations and/orabsenceofpaleontological material. Thus, the estimation of accurate ages for this type of units persisted as a significant problem of Chilean geology. In this way, the actual age range of the Cerrillos Formation remained uncertain, and as a matter of controversy, since its definition by Segerstrom and Parker (1959). The Cerrillos Formation is a succession of red conglomerate beds and volcanic rocks overlying Valanginian to late Aptian marine sedimentary strata of the Chañarcillo Group in the Atacama Region of northern Chile (Segerstrom and Ruiz, 1962; Marschik and Fonboté, 2001; Arévalo, 2005a, b; Mourgues, 2004, 2007). The continental coarse clastic alluvial sedimentation and coeval subaerial volcanism represented by the Cerrillos Formation marks an abrupt change from the previous marine carbonate sedimentationwithinanextensionalback-arc basin until late Aptian (e.g., Segerstrom and Parker, 1959; Zentilli, 1974; Jurgan, 1977; Pérez et al, 1990; Arévalo, 2005a, b; Mourgues, 2004, 2007). In recent publications the Cerrillos Formation was ascribed to the Albian-Turonian, based on its stratigraphic position over the marine fossiliferous Pabellón Formation of late Aptian minimum age, and unconformable underthe volcanic Hornitos Formation attributed to the Campanian-Maastrichtian (Arévalo, 2005a, b). To better constrain the age of this unit and stress its significance in the geological evolution of the Andes of northern Chile, we dated volcanic rocks of the Cerrillos Formation and the overlying Hornitos Formation by the LA-ICP-MS U-Pb method in zircon. The U-Pb system in zircon is a robust method for accurately assessing the crys-tallization ages of igneous rocks, due to the highest known closure temperature for Pb diffusion, which exceeds 900°C for zircons of typical sizes (Cherniak and Watson, 2000 and references therein), and the overall chemical resistance of zircon to alteration or metamorphism {e.g., Davis and Krogh, 2000). In addition, the laser ablation approach allows analyzing 35-50 Lim diameter spots on individual zircon crystals and restricts the isotopic analyses to the portions of interest.

2. GEOLOGICAL BACKGROUND

The Cerrillos Formation was definedby Segerstrom and Parker (1959) in the area of the current Carrizalillo creek (formerly Cerrillos creek), 27 km southwest of Copiapó city in the Atacama Region (Fig. 1). The lower part of this formation is about 2,000 m thick; it is composed predominantly of red orthoconglomerate, paraconglomerate, and sands-tone beds dipping east, with intercalations of tuffs, andesitic lava flows, breccias, agglomerates and volcanic avalanche deposits, and local levels of fresh-water limestone and siltstone. The conglomerate clasts are poorly sorted, roundedto subrounded, withboulders up to 1 m in diameter, which are composed primarily of andesitic porphyritic lavas, with a volcanic wacke matrix. The red sandstone beds are volcanic wackes containing andesitic material; ripple marks and desiccation cracks are common in the finer clastic intercalations (Segerstrom and Parker, 1959; Segerstrom, 1959). About 1,000 m above the base of the formation, in addition to andesitic clasts, the conglomerates contain limestone clasts with reworked marine fossils of Hauterivian-Barremian age (Segerstrom, 1968); these indicate denudation of Neocomian limestone strata during the sedimentation of the Cerrillos Formation. The red conglomeratic succession represents high-ener-gy alluvial deposits, which are interpreted as coa-lescent alluvial fans. The provenance of the coarseclastic material sof the Cerrillos Formation is clearly from aprominent ground composed primarily of andesitic volcanic rocks, but where eventually Neocomian carbonate rocks were also exposed to erosion , either by progressive denudation and/or tectonism. The composition of the source terrain corresponds well to the rocks of the Jurassic to Early Cretaceous volcanic are and its easterly interfingering with Neocomian carbonate strata (La Negra, Punta del Cobre and Bandurrias formations; e.g., Moscoso et al, 1982; Arévalo, 2005a, b), which are exposed immediately west from the Early Cretaceous back-arc basin represented by the Chañarcillo Group. In contrast, the Región to the east is formed of Upper Paleozoic to Triassic granitoids and mainly felsic volcanic rocks, and Jurassic sedimentary strata, none comparable to the composition of the clasts of the lower part of the Cerrillos Formation.


Farther south at the latitude of La Serena (30°S), a 1,000-m-thick succession of continental coarse and fine sedimentary rocks andpyroclasticbreccias is exposed. This succession contains fossil dinosaur bones, the remains of crocodiles and turtles, and abundant silicified tree stems (Quebrada La Totora Beds); this unit has been correlated with the Cerrillos Formation and assigned to the late Albian to Cenomanianby Pineda and Emparan (2006).

The upper part of the Cerrillos Formation is about 2,000 m in thickness and is dominated by andesitic lavas, breccias, agglomerates, and tuffs; the lavas are mostly porphyritic pyroxene-bearing andesites. The rocks are often chloritized, have abundant epidote, and appear greenish-grey to reddish-brown in outcrop. The volcanic rocks of the upper part of the Cerrillos Formation represent the development of a new subduction-related magmatic are approximately located along the site of the for-merEarly Cretaceous back-arc marine sedimentary basin of the Chañarcillo Group, thus marking an eastern shift of the magmatic foci.

The aggregate thickness of the Cerrillos Formation has been estimated to exceed 4,000 m (Segerstrom, 1959; Arévalo, 2005a, b). However, the Cerrillos Formation has strong thickness and lateral facies variations, owing to its coarse clastic continental and volcanic nature.

Segerstrom and Parker (1959) considered that the Cerrillos Formation unconformably overlies the Lower Cretaceous marine carbonate rocks of the Chañarcillo Group and also the underlying Bandurrias Formation farther north in the Paipote creek. In fact, the basal stratigraphic relationship of the Cerrillos Formation is obscured by thrust faults related to the Paipote thrust and fold system, which trucate strata (Arévalo, 1999, 2005a, b; Arévalo et al, 2006). At its type-locality there is no angular unconformity at the base of the Cerrillos Formation, but an erosional surface was inferred from the occu-rrence of limestone clasts with Neocomian marine fossils withinthe conglomerates (Arévalo, 2005a, b; Arévalo et al, 2006). On the other hand, the Cerrillos Formation is unconformably covered by the Hornitos Formation, which consists of a succession 2,200 m thick of breccias, tuffs, lavas and conglomerates with coarse sandstone matrix and lenticular intercalations of continental red sandstone, calca-reous mudstone and limestone. Dacitic domes and minor rhyolitic ignimbritic intercalations are also present. At its top, a continuous member of basal-tic and trachybasaltic lavas is developed (Arévalo, 2005a, b). Although separated by an unconformity, the similar continental sedimentary and volcanic nature, and marked lateral facies variations of the Cerrillos and Hornitos formations complicate the rightful identification and correlations of rocks of these units in the field.

The end of marine carbonate sedimentation in the back-arc basin and basin inversion by late Aptian correlates in time with geological evidences forthe onset of left-lateral transpressionat regional scale along the western Coastal Cordillera of northern Chile, where the magmatic are was located at that time (Aguirre, 1985; Taylor et al, 1998; Randall and Taylor, 1996; Marschick and Fontbote, 2001; Marschick and Sóllner, 2006). In the Coastal Cordillera area the north-south-trending Atacama Fault System, a major intra-arc, sinistral, strike-slip fault system, was active during the Early Cretaceous; itextendsformorethan 1,000 kmfrom latitude 20° to 30°S, as attestedby numerous faults and associated mylonite belts and foliated intru-sions (Arabasz, 1971; Scheuber and Adriessen, 1990; Boric et al, 1990; Scheuber and González, 1999; Dallmeyer et al, 1996; Grocott and Taylor, 2002). However, a debate persists about the actual regional stress conditions on the continental border during the latest Early Cretaceous, because sinistral, transtensional faults provided conditions for emplacement of intrusions between -120 and 93 Ma, as inferred from emplacement models of intrusive bodies and 40Ar/39Ar and K-Ar dating of foliated intrusions and mylonites along the Coastal Cordillera (Grocott and Taylor, 2002; Arévalo 2005a, b; Arévalo et al, 2005,2006). Accordingly, these authors favor instead a regional sinistral trans-tensionduringthistime-span, and considerthe onset of sinistral transpression to be post 77 Ma (e.g., Grocott and Taylor, 2002). Although tensional and compressive zones may develop simultaneously along shear zones, the deposition of the Cerrillos Formation was interpreted as filling a narrow, subsident basin related to active extensional tectonics (Arévalo 2005a, b; Arévalo et al, 2005). This event was correlated with a poorly constrained in time, mid-Cretaceous extension oblique to the orogen, interpreted by Mpodozis and Allmendinger (1992, 1993) in the Sierra de Fraga of the Atacama Region In addition, the lack of a 'strong' angular unconformity at the base of the Cerrillos Formation, and the fact that most contractional structures post-date this unit east of Copiapó, were taken by Arévalo et al. (2006) as evidences for discarding the major inversion of the back-arc basin at the time of the deposition of the Cerrillos Formation as proposed by Marschick and Fontbote (2001) and Marschick and Sóllner (2006).

In our view, the change from marine carbonate sedimentation in the Early Cretaceous back-arc extensional basin to the subsequent subaerial deposition of coarse reddish conglomerates and volcanic rocks of the Cerrillos Formation during the late Aptian certainly represents an abrupt change in the sedimentary regime with time, which cannot be attributable to a variation of the sea-level according to the patternsof sealevel change (Haqetal, 1988), neither to continual subsidence. It represents the sudden onset of vigorous erosion of andesitic volcanic rocks from the former Jurassic-Early Cretaceous magmatic are, which provided clastic materials for the lower part of the Cerrillos Formation derived from the west, interpretation that is coincident with previous views for the provenance of clastic materials of this unit (e.g., Arévalo, 2005a, b). Therefore, the deposition of the Cerrillos Formation is not consistent with persistence of crustal attenuation related to extensional or transtensional tectonics in the Región as interpreted by Arévalo (2005a, b). On the contrary, the deposition of the Cerrillos Formation is regarded here as consequence of orogenic tectonism on the active continental margin, leading to initiation of vigorous erosion of the area of the former subduction-related magmatic are, accompanied by continental alluvial sedimentationinland, back-arc basin inversion , and eastward migration of the magmatic front. In fact, this change was related to a major reorganization of the Andean orogen, which involved the closure of the back-arc basins all along the South American active continental margin, from Colombia to Southernmost Chile (Dalziel, 1986; Bourgois et al, 1987; Mpodozis and Ramos, 1990).

3. GEOCHRONOLOGY

3.1  Sampling strategy and zircon concentration

To constrainboththe initiationandthe minimum age of the Cerrillos Formation, samples of volcanic rocks from this unit and from the lower part of the overlying unconformable Hornitos Formation were collected. Sampling was concentrated between latitudes 27° and 29°S (Fig. 1), mostly within the areas of the Copiapó and Los Loros 1:100.000 scale geological maps (Arévalo, 2005a, b); the boundaries of the lithostratigraphic units sampled are those defined in these regional geological maps. Zircon separation was done by conventional methods in the Department of Geology of the Universidad de Chile; only half of the collected samples yielded zircon for U-Pb dating.

3.2  Analytical procedure for U-Pb dating

The analytical work was done at the University of Arizona. Zircon concentrates of the <350 micron fraction were separated magnetically. Inclusion-free zircons from the non-magnetic fraction were then handpicked under a binocular microscope. At least fifty zircons from each sample were mounted in epoxy and polished until sectioned in half for laser ablation analyses. Previous to laser ablation ICP-MS analysis, zircons were studied under Scanning Electron Microscopy (SEM) by B SE and cathodoluminesence imaging in order to determine the complexity of the zircons. Zircon crystals were analyzed in polished section with a Micromass Iso-probe multicollector ICP-MS equipped with nine Faraday collectors, an axial Daly detector, andfour ion-counting channels (Dickinson and Gehrels, 2003). The Isoprobe is equipped with an ArF Ex-cimer laser, which has an emission wavelength of 193 nm. Analyses were conducted on 35-50 micron spots with output energy of ~50 mJ (at 23kV) and a repetition rate of 8 Hz. Each analysis consisted of one 20-second integration on peaks with no laser firing and twenty 1-second integrations on peaks with the laser firing. Hg contribution to the 204Pb mass position is accordingly removed by subtracting the on-peak background value. The depth of each ablation pit was -15-20 microns. Total measurement time was ~90 s per analysis. The collectors were configured for simultaneous measurement of 204Pb in an ion counting channel while 206Pb, 207Pb, 208Pb, 232Th, and 238U were measured with Faraday detectors. All analyses were conducted in static mode. Inter-element fractionation was monitored by analy zing fragments of a large concordant zircon crystal from Sri Lanka with a known (ID-TIMS) age of 564±4 Ma (2o) (Gehrels et al, 2008). This reference zircon was analyzed once for every three unknown samples. The reported ages for zircon grains are based on 206Pb/238U ratios, because they are more re hable foryoung rocks than the 207Pb/235U and 206Pb/207Pb ratios that present significantly higher errors. This is due primarily to the low intensity (commonly <1 mV) of the 207Pb signal from these young, low-U grains. The 206Pb/238U ratios are corrected for common Pb by using the measured 206Pb/204Pb, common Pb composition from Stacey and Kramers (1975), and respective uncertainties of 1.0 and 0.3 for 206Pb/204Pb and 207Pb204Pb. For each sample, the 206Pb/238U ages are plotted with 2 sigma error bars that reflect only the error from determining 206Pb/238U and 206Pb/204Pb. The weighted mean of each sample was calculated using the Isoplot program (Ludwig, 2003). For the final age of each sample, additional uncertainty from the calibration correction, decay constant, common lead composition and variation in measured 206Pb/238U and 206Pb/207Pb of the standard are considered. These systematic errors (1.8-2.2%) were added quadratically to the measurement error (-0.98-2.2%). The reported ages are based primarily on 206Pb/238U ratios for <1000 Ma grains and 206Pb/207Pb for >1000 Ma grains. All reported final ages and weighted mean ages have uncertainties at the two-sigma level. The results are summarized on table 1, the respective bar graphs of 206Pb/238U zircon ages for individual LA-ICP-MS analyses are shown in the figure 2, and analytical data are included in the Appendix 1.



3.3   Results

A U-Pb zircon age of 110.7±1.7 Ma was ob-tained for a light pink, trachyte, vitric, tuff breccia (KP-613; Table 1) intercalated within conglomerate beds dipping 10-15°E, about 15 meters above the base of the Cerrillos Formation, near the locality of Molle Bajo (Fig. 1). This is the oldest isotopic age obtained for the Cerrillos Formation and probably representative of the initiationof clastic sedimenta-tionandvolcanic activity. Inaddition, a U-Pb zircon age of 102.2±2.0 Ma (KP-606) was obtained for a large amygdaloidal (amygdules filledby celadonite and chalcedony) porhyrytic andesite block (1.5 m in diameter) from a coarse volcanic breccia horizon, intercalated into conglomerate beds dipping 30°E. stratigraphically about 900 m above the base of the Cerrillos Formation atPaipote creek. Inthe same locality, another intercalation of reddish, dacitic, tuff breccia (KP-605), about 36 m higher in the stratigraphic succession relative to the previous sample, yielded a U-Pb zircon age of 99.7±1.6 Ma.

AU-Pb zirconage of 69.5±1.0 Ma was obtained for a porphy ritic andesite block from an agglomerate (KP-607) from a volcanic succession gently dipping east (15°E), immediately south of the Paipote creek, mapped as the upperpart of the Cerrillos Formation by Segerstrom (1960) and Arévalo (2005a); this sample was collected about 2,900 m above the base of the succession exposed along the Paipote creek.

A U-Pb zircon age of 65. 2± 1.0 Ma was obtained for a thick (~30 m) massive horizon of grey andesite breccia (KP-608) that has been mapped by Arévalo (2005a) as the basal stratum of the Hornitos Formation in the San Miguel creek, near the confluence with the Paipote creek (Fig. 1). A reddish, vitric, welded tuff from the lower part of the Hornitos Formation (KP-610) yielded a U-Pb zircon age of 66.9±1.0 Ma and a similar U-Pb zircon age of 66.0±1.3 Ma was obtained for another, reddish brown, vitric, dacitic welded tuff (KP-614) at the type-locality of the Hornitos Formation. The stratigraphic position of the two last samples is not exactly determinedbecause the Hornitos Formation is folded and faulted and its base is not exposed at the sampling locations.

A U-Pb zircon age of 65.2±1.2 Ma was obtained for an andesitic green breccia (abundant epidote and chlorite in the groundmass) with lithic fragments of porphyritic andesite and fragmented plagioclase crys-tals (KP-611). This sample was collected immediately above the Chañarcillo Group, from the basal part of an andesitic volcanic succession with conglomerate intercalations that overlies with angular unconform-ity the marine carbonate strata of the Neocomian Chañarcillo Group at Algarrobal creek (Fig. 1). This succession was mapped as the Cerrillos Formation by Abad (1980) and Moscoso et al. (1982), but its basal portion yielded a U-Pb zircon age that is identical to those that we obtained for the Hornitos Formation.

4. DISCUSSION

The U-Pb zircon age of 110.7Ü.7 Ma for a light pink tuff breccia intercalation near the base of the Cerrillos Formation indicates that the subaerial volcanic activity and alluvial sedimentation that formed this unit started in the early Albian. This is consistent with its stratigraphic position overlying the Pabellon Formation of late Aptian mínimum age (Arévalo, 2005b). Inaddition, this is coherent with the U-Pb zircon age of 90.4±0.5 Ma reported by Arévalo (2005b) for a dacite intrusion that crosscuts the lower part of the Cerrillos Formation near the locality of Molle Bajo. The alluvial sedimentation and volcanism of the lower part of the Cerrillos Formation extended in time at least to the latest Albian according to the U-Pb zircon ages of 102. 2±2.0 and 99.7±1.6 Ma obtained for volcanic rocks intercalated within conglomerates of the mid-part of the lower part of the Cerrillos Formation in the outcrops at the Paipote creek.

The U-Pb zircon age of 69.5±1.0 Ma obtained for the upper volcanic part of the Cerrillos Formation immediately south of the Paipote creek is at least some 30 m.yrs. youngerthanthe ages obtained in the lower part of this formation. Although the east-dipping stratigraphic succession appears to be continuous along the Paipote creek, the large time-span suggests a recurrence rather than continuity of the volcanism extending up to the latest Campanian-earliestMaastrichtian Onthe other hand, this zircon U-Pb 69.5±1.0 Ma for volcanic rocks mapped by other authors as the upper part of the Cerrillos For-mationis atvariance withthe Campanian-Maastrich-tian age ascribed to the unconformable, overlying Hornitos Formationby Arévalo (2005a, b), and with the occurrence of Late Cretaceous fossil dinosaur remains described by Chong (1985) and Arévalo (2005b) within strata of the Hornitos Formation. Thus, either there is an erroneous delimitation of the Cerrillos Formation in the available geological maps or the age range assigned to the Hornitos Formation would need a revision .

We obtained a U-Pb zircon age of 65.2±1.0 Ma (KP-608) for a thick massive stratum of andesitic breccia mapped by Arévalo (2005b) at the base of the Hornitos Formation in the San Miguel creek, near the confluence with the Paipote creek. The age is consistent with the higher stratigraphic position of this breccia horizon relative to the previous volcanic rocks thatyielded 69.5±1.0 Ma. However, this radiometric age for the basal stratum of the Hornitos Formation indicates that the volcanic activity of this unit began at about the Cretaceous-Tertiary boundary, at variance with the overall Campanian-Maastrichtian age ascribed to it by Arévalo (2005a, b), but also with strata containing Late Cretaceous fossil dinosaur remains described by Chong (1985) and Arévalo (2005b) as part of the Hornitos Formation. In addition, south of the Copiapó river valley we obtained a U-Pb zircon age of 66.9±1.0 Ma (KP-610) for a reddish welded tuff horizonintercalationbetweenconglomerateandred sandstone beds from the lower part of the Hornitos Formation. The exact stratigraphic position of this sample is hard to pinpoint due to the folding and faulting at this location, but the age is identical to other U-Pb zircon age of 66.1±0.5 Ma reported by Arévalo (2005b) for a similar tuff level some 8 km north. We also obtained a U-Pb age of 66.0±1.3 Ma (KP-614) for another vitric, dacitic welded tuff from the type-locality of Hornitos in the Copiapó River valley, which is identical (within analytical error) to the above mentioned U-Pb ages, but also to a U-Pb zircon age of 65.6±0.2 Ma obtained by Arévalo (2005b) for a dacite dome some 17 km to the NNE from Hornitos (Fig. 1).

Our new U-Pb dates from 67 to 65 Ma, coupled with a dozen of K-Ar dates between 65 and 50 Ma compiled by Arévalo (2005a, b) strongly suggest that the Hornitos Formation represents volcanism and continental sedimentation that began in the latest Maastrichtian and developed mainly during the Paleogene, conflicting with the Campanian-Maastrichtian age ascribed to this unit by Arévalo (2005a, b). In addition, the strata containing Late Cretaceous fossil dinosaur remains ascribed to the Hornitos Formationby Chong (1985), and Arévalo (2005b) is at variance with the new geochronolo-gical data obtained for the Cerrillos and Hornitos formations. Actually, the conflicting U-Pb ages may partly be reconciled if rocks mapped as the upper volcanic part of the Cerrillos Formationby Segers-trom (1960) and Arévalo (2005a, b) would really belong to the Hornitos Formation. However, this alternative would involve changes to the geological cartography that are beyond the scope of this paper It is apparent that the new U-Pb data raise questions about the chronology of Hornitos Formation, the stratigraphic position of fossil dinosaur remains described by Chong (1985) and Arévalo (2005b), and to the delimitation of the continental stratified units of the Cerrillos and Hornitos formations.

The U-Pb geochronological data for the basal stratum and the lowermost part of the Hornitos Formation determine a latest Maastrichtian mi-nimum age for the deposition and a first stage of deformation of the Cerrillos Formation, assuming correct delimitation of this unit in the current geological maps.

The U-Pb zircon age of 65.2±1.2 Ma obtained for a green andesite breccia from the basal part of a volcano-sedimentary succession at the Algarrobal Creek coincide with the U-Pb zircon ages for the basal part of the Hornitos Formation (Fig. 1). This succession was mapped as the Cerrillos Formation by Abad (1980) and Moscoso et al. (1982), but the isotopic age disagrees with this stratigraphic assignation. It is apparent that the continental se-dimentary andvolcanic nature of the Cerrillos and Hornitos formations and their marked lateral facies variations complicate mapping and correlations. particularly for extensive outcrops of rocks of the Cretaceous-Tertiary boundary, which unconformably overlie different older strata in the Atacama Region .

Our new geochronological U-Pb data substantia-te that the sedimentation and coeval volcanism of the Cerrillos Formation began in the early Albian. The volcanism may have extended in time until ~70 Ma. but the large time-span suggests recurrence instead of continual volcanic activity or misinterpretation in the identification of this unit in current geological maps. In addition, it is apparent that the deposition and initial deformation of the Cerrillos Formation were completed prior to ~66 Ma, according the U-Pb dates that we obtained for the basal rocks of the unconformably overlying Hornitos Formation (Fig. 1).

The lower part of the Cerrillos Formation represents continental, alluvial sedimentation during the Albian over the carbonate rocks of the Chañarcillo Group previously deposited within a marine back-arc basin. This significant change in the sedimentary regime cannot be attributable to a variation of the sea-level according to the patterns of sea level change (Haq et al, 1988). In our view, the coarse alluvial sedimentation is not consistent with persistence of extensional tectonics in the Región as interpreted by Arévalo (2005a, b), because it signals surface uplift from marine to continental deposition, and consequent active erosion of the source Región of the clastic materials of the Cerrillos Formation. Therefore, the lower part of this formation is regarded as the actual sedimentary record of orogenic tectonism on the active continental margin. The composition of the alluvial materials is consistent with a western provenance, as previously inferred by Arévalo (2005a, b), which corresponds to the area of the former magmatic are currently forming the Coastal Cordillera. During the Albian active sinistral strike-slip faulting along the Coastal Cordillera coincides in time with the onset of compressive deformation that led to a major inversion of the Early Cretaceous intra- and back-arc basins in the late Aptian to Albian, in agreement with previous interpretations (e.g., Aguirre, 1985; Marschik and Fontboté, 2001; Marschick and Sóllner, 2006). The simultaneous development of sinistral transtensional faults at that time (e.g., Grocott and Taylor, 2002; Arévalo 2005a, b; Arévalo et al, 2005, 2006) is considered to be a normal part of regional shearing deformation. It is apparent that the tectonism during the Aptian-Albian was concentrated within the magmatic are on the active continental border, but not inland where the Cerrillos Formation was being deposited at that time, which explains the lack of an angular unconformity at the base of the Cerrillos Formation at its type locality. The oceurrence of subsequent compressive structures in this unit is the result of the pattern of eastward migration of deformation with time, which follows the inland migration of the magmatic front in northern Chile (i. e., Rutland, 1971; Mortimer and Saric, 1975; Maksaev, 1990).

The lower clastic part of the Cerrillos Formation was interpreted by Arévalo (2005a, b) as tapering eastward in a series of steps limited by hypothetical normal faults related to active extensional tectonics. In our interpretation these faults are unnecessary, as the clastic rocks of the Cerrillos Formation repre-sent coalescent alluvial fans thinning inland, which developed during the Albian, following the tectonic uplift of the former western magmatic are and the concurrent emergence of the back-arc basin above the sea level. An eastward shift of the magmatic foci is also apparent from the volcanic intercalations in the lower conglomeratic part of the Cerrillos Formation and particularly by the upper volcanic part of this unit, but the ~30 m.yrs. difference between them suggest that the actual main eastward migration of the are significantly postdated the alluvial sedimentation.

According to Segerstrom (1959; p. 12) the stratigraphic position of the Cerrillos Formation is equivalent to that of the Las Chucas Formation in central Chile. This was confirmed by the Early Cretaceous age inferred by Rivano et al. (1986) for Las Chucas Formation from K-Ar dating of intru-sive rocks that crosseut the unit. In addition, Wall et al. (1999) and Selles and Gana (2001) reponed U-Pb zircon ages in felsic volcanic rocks of the lower section of the Las Chucas Formation in the 116. 1±0.3 to 106. 5±0.4 Ma range, whole-rock K-Ar agesof 101±3 and 100±3 Ma and a plagio clase K-Ar age of 95±3 Ma for lavas of the upper stratigraphic levels of the Las Chucas. These radiometric ages for Las Chucas Formation are in the same range of the U-Pb zircon ages that we obtained for the lower part of Cerrillos Formation.

The clastic rocks of the Las Chucas Formation have been interpreted as syntectonic molasse de-posits that accompanied the tectonic inversion of the Early Cretaceous marine basin in central Chile during a change from a rifted to a compressional margin in central Chile (Parada et al., 2005). This conclusion is in agreement with our interpretation of the clastic part of the Cerrillos Formation in the Atacama Region .

In the easternmost part of the Coastal Cordillera between latitudes 26°30'to30°30'SabeltofCu-Au porphyry deposits were formed from 108 to 88 Ma (Maksaev et al., 2006a); this mineralizing period overlaps in time with the deposition of the Cerrillos Formation. Porphyry coppermineralizationwas vir-tually absent in earlier times of the evolution of the Andes of northern Chile, except for rare exceptions (i.e., Antucoya; Maksaev et al., 2006b). However, since the Albian porphyry-type deposits become the dominant type of mineral deposits. It appears that the orogenic tectonism that affected the continental margin during the late Aptian resulting in the inversion of the back-arc basin and an abrupt change in the sedimentary regime with time indicates also a significant change in Andean metallogeny.

5. CONCLUSIONS

The deposition of the conglomeratic lower part, 2,000 m thick, of the Cerrillos Formation extended in time at least from 110.7±1.7to Ma to 99.7±1.6 Ma according to our new U-Pb zircon age data for volcanic intercalations. The volcanic activity may have continued until the 69.5±1.0 Ma because of the U-Pb zircon age obtained for the upper volcanic part of the Cerrillos Formation. However, the ~30 rayrs. time-span relative to the above ages suggests a recurrence, rather than continuity of the volcanism. In addition, this U-Pb zircon age falls into the 80-65

Ma age range ascribedto the overlyingHornitos Formation by Arévalo (2005a, b), whichposes questions about the accuracy of the age range ascribed to the Hornitos Formation, or alternatively, to the assignation of the dated rocks to the Cerrillos Formation. In any case, a minimum late Maastrichtian age for the Cerrillos Formation is established by the U-Pb zircon age range from 66.9±1.0 to 65.2±1.0 Ma for the lower part of the overlying Hornitos Formation (including a sample from its type-locality).

The U-Pb geochronological data indicates that the volcanism represented by the rocks mapped as Hornitos Formation started at about 67-65 Ma (Cretaceous-Tertiary boundary) and the overall geochronological data available strongly suggest that it represents primarily a Paleocene-Eocene volcanism. Due to the inconsistency with the Campanian-Maastrichtian fossil dinosaur remains in strata ascribed to the Hornitos Formation, the actual age range of the Hornitos Formation is considered still an open issue.

The lower part of the Cerrillos Formation represents a high energy, clastic, sedimentary accumulation within coalescent alluvial fans thinning inland, derived from the sudden accelerated erosion of the volcanic area to the west during the Albian, accompanied by andesitic volcanism related to the eastward shift of the magmatic are. Therefore, the alluvial buildup becomes dominant in time in the area formerly oceupied by the Neocomian marine sedimentary back-arc basin, accompanied by volcanic activity.

The depositionof the Cerrillos Formationresult-ed from the tectonic uplift during the middle to late Aptian of the area formerly oceupied by the Early Cretaceous magmatic are (current Coastal Cordillera) and the concurrent marine regression within the back-arc basin. This deformative episode was related to a major change from a rifted (extensional or transtensional) to a left-lateral compressional tectonic setting of the Chilean active continental margin, which represents a major paleogeographic change in the evolution of the Chilean Andes. This change of the tectonic setting was followed by a period of Cu-Au porphyry mineralization along the easternmost part of the Coastal Cordillera of northern Chile, thus also marking a significant change inthe metallogeny of the Chilean Andes.

 

Acknowledgments

Conicyt, Chile, provided financial support for this study through Fondecyt Grant 1040492 to the first two authors. Petrographic observations of the dated rocks by M. Brockway are very much appreciated. Comments provided by the reviewers Drs. R. Charrier, M. Basei and C. Arévalo helped to improve and clarify the manuscript, yet the interpretations of the data remain the sole responsibility of the authors.

 

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Manuscript received: April 18, 2008; revised/accepted: March 9, 2009

 


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