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Andean geology

versión On-line ISSN 0718-7106

AndGeo vol.42 no.3 Santiago set. 2015

http://dx.doi.org/10.5027/andgeoV42n3-a01 

 

Detrital-zircon U-Pb geochronology of the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists and spatially-related granitoids of the Sierra Castillo Batholith

Geocronología U-Pb de circones detríticos del Complejo Metamórfico Quebrada del Carrizo y Esquistos El Jardín y granitoides espacialmente relacionados del Batolito Sierra Castillo

 

Víctor Maksaev1, Javier Arancibia1, Francisco Munizaga1, Colombo Tassinari2

1 Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, casilla 13518, Correo 21, Santiago, Chile. vmaksaev@ing.uchile.cl; onegaiarthas@hotmail.com; fmunizag@ing.uchile.cl
2 Instituto de Geociencias, Universidade de Sao Paulo, Rua do Lago, 562-Cidade Universitária CEP 05508-080, Sao Paulo, Brasil. ccgtassi@usp.br


ABSTRACT. U-Pb detrital-zircon geochronology of two discrete outcrops of mica schists of the western border of the Domeyko Cordillera in the Region of Atacama, northern Chile, indicates that the maximum age of sedimentation of their protolith corresponds to the Late Carboniferous to Early Permian. The Early Permian granitoids of the Sierra Castillo Batholith that intruded the metamorphic rocks show ductile deformation and were emplaced within hot crust not much later than the greenschist-facies metamorphism peak that affected their host rocks. Therefore, the metamorphism of the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists is constrained temporarily between the maximum age of sedimentation of detrital zircons (314±11 to 291±5 Ma) and the crystallization of Early Permian intrusions (292.2±6.6 to 278.3±5.8 Ma), thus pointing to Early Permian metamorphic peak. Concentration of U-Pb ages between 400 and 600 Ma indicate eastern detrital input sources, such as the Pampean and Brasiliano orogenies and the Ordovician-Silurian Famatinian magmatic arc of northwestern Argentina. Other concentration of detrital-zircon U-Pb ages between 900 to 1,200 Ma reflect contributions of magmatic rocks of age of the Proterozoic Sunsas orogeny (Grenville). Whereas, only few grains of zircon with U-Pb ages older than 1,200 Ma occur and these may correspond to a minor contribution zircon from South American cratonic areas. Zircon grains of Devonian age are scarce in populations of zircons analyzed, consistent with a passive margin and a lull of magmatic activity during this period in the paleo-Pacific border of Gondwana. The U-Pb detrital zircon data from the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists coincide with detrital-zircon U-Pb data previously published for other metamorphic complexes of central-northern Chile, which are part of a Late Paleozoic subduction complex or accretionary wedge developed in the western edge of Gondwana. Consequently, the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists are relics of the same Paleozoic accretionary wedge, which constituted the substratum for the emplacement of the Permian plutons of the Sierra Castillo Batholith.

Keywords: Geochronology, U-Pb dating, Detrital zircon, Gondwana, Paleozoic, Carboniferous, Permian, Metamorphic rocks, Atacama.


RESUMEN. Dataciones U-Pb de circones detríticos de dos afloramientos de esquistos micáceos del borde oeste de la cordillera de Domeyko en la Región de Atacama, norte de Chile, indican que la edad máxima de sedimentación de su protolito corresponde al Carbonífero Tardío-Pérmico Temprano. Los granitoides del Pérmico Temprano del Batolito Sierra Castillo situados en los esquistos muestran deformación dúctil y probablemente se emplazaron en corteza caliente, no mucho más tarde que el máximo de metamorfismo de facies de esquistos verdes que afectó a sus rocas de caja. Por lo tanto, el metamorfismo de las rocas del Complejo Metamórfico Quebrada del Carrizo y los Esquistos El Jardín está ubicado temporalmente entre el Carbonífero Tardío-Pérmico Temprano que es la edad máxima de sedimentación de circones detríticos (314±11 a 291±5 Ma) y la edad de cristalización de intrusivos del Pérmico Temprano (292,2±6,6 a 278,3±5,8 Ma), consecuentemente indicando un máximo metamórfico del Pérmico Temprano. Las concentraciones de edades U-Pb entre 400 y 600 Ma indican aporte de detritos de fuentes orientales como las orogenias Brasiliana y Pampeana y del arco magmático Famatiniano del Ordovícico-Silúrico del noroeste de Argentina. Otras concentraciones de edades U-Pb de 900 a 1.200 Ma reflejan aportes de rocas magmáticas de la edad de la orogenia Sunsas (Grenville) del Proterozoico, mientras que hay solo escasos granos de circón con edades U-Pb mayores de 1.200 Ma y estos pueden corresponder a un aporte menor de zonas cratónicas sudamericanas. Los granos de circones de edad devónica son escasos en las poblaciones de circones analizadas, en consonancia con un margen pasivo y una pausa de la actividad magmática durante este período en el borde paleo-Pacífico de Gondwana. Los datos U-Pb de circones detríticos del Complejo Metamórfico Quebrada del Carrizo y los Esquistos El Jardín son coincidentes con datos U-Pb de circones detríticos publicados anteriormente para otros complejos metamórficos del centro-norte de Chile, los que son parte de un complejo de subducción Paleozoico Tardío o cuña de acreción desarrollado en la borde occidental de Gondwana. Por lo tanto, el Complejo Metamórfico Quebrada del Carrizo y los Esquistos El Jardín son relictos de la misma cuña de acreción del Paleozoico Tardío, la que constituyó el sustrato donde se emplazaron los plutones del Pérmico Temprano del Batolito Sierra Castillo.

Palabras clave: Geocronología, Datación U-Pb, Circones detríticos, Gondwana, Paleozoico, Carbonífero, Pérmico, Rocas metamórficas, Atacama.


 

1. Introduction

We have investigated the U-Pb detrital-zircon geochronology for two discrete outcrops of metamorphic rocks of sedimentary protolith that are present in the western border of the Domeyko Cordillera in the Atacama Region of northern Chile. These are the Quebrada del Carrizo Metamorphic Complex (Cornejo et al., 2009) and El Jardín Schists (Tomlinson et al., 1999), respectively (Fig. 1), which are relics of the host rocks of the composite Permian Sierra Castillo Batholith, exposed east of El Salvador, northern Chile (Tomlinson et al., 1999; Cornejo et al., 2009). We aim to determine the maximum age of sedimentation of detrital zircons, the provenance of the detrital deposits that constitute the protolith of the micaceous schists, and their regional geological significance. In addition, we also provide U-Pb dates for igneous zircons from granitoids emplaced within the metamorphic rocks to constrain the minimum age of metamorphism. The studied metamorphic rocks are exposed in the Jardín and del Carrizo creeks, in relatively small areas (<0.25 km2) which occur on the border of the eastern block of the Sierra Castillo thrust fault (cf. Perelló and Müller, 1984; Tomlinson et al., 1993, 1999; Cornejo et al., 1993, 2009) and are bounded to the west by this sub-vertical, east-dipping, regional fault that extends along the western flank of the Cordillera de Domeyko in the northern portion of the Atacama Region (Perelló and Müller, 1984; Tomlinson et al., 1999) (Fig. 1).

 

FIG. 1. Schematic geological setting of the Quebrada
del Carrizo Metamorphic Complex and the El Jardín
Schists relative to the Late Paleozoic, fault-bounded
Sierra Castillo Batholith. Modified after Niemeyer,
1999, Tomlinson et al., 1999, and Cornejo et al., 2009.

 

The first description of the metamorphic rocks exposed in the Jardín creek (or lower Asientos creek) was published by García (1967), referred to as "Mica esquistos de Quebrada Asientos". Later the same unit was described as a "Metamorphic Complex" by Pérez (1982) and Perelló and Müller (1984), and then renamed as "El Jardín Schists" by Tomlinson et al. (1999). While, the metamorphic rocks exposed in the del Carrizo creek about 54 km farther north, were described by Cornejo et al. (1993) as "Quebrada del Carrizo Metamorphic Complex", a further structural study was completed by Niemeyer (1999), and a more recent description published by Cornejo et al. (2009).

2. Quebrada del Carrizo Metamorphic Complex and El Jardín Schists

The metamorphic rocks at the del Carrizo creek are exposed on the both flanks of the east-west-trending canyon-valley (Figs. 2 and 3), under a flat cover of Miocene welded tuffs (Llano Las Vicuñas ignimbrites; Cornejo et al., 2009), and overlying Upper Miocene to Pliocene alluvial gravels (Atacama Gravels; Cornejo et al., 2009).

 

FIG. 2. Geological map of the Quebrada del Carrizo metamorphic complex
(base topography after Google Earth image; lithostratigrapic units after
Cornejo et al., 2009).

 

FIG. 3. Quebrada del Carrizo metamorphic complex: (left) general view (to the SE) of the
outcrops in the del Carrizo creek underlying Miocene ignimbrite and gravels, (right) local
appearance of folded mica schists (view to the W).

 

The Quebrada del Carrizo Metamorphic Complex include gray-colored mica schists and actinolite-rich greenschists; the mica schists are muscovite-bearing with variable amounts of granoblastic bands of quartz and albite that alternate with lepidoblastic muscovite-rich bands and less abundant chlorite. The greenschists show a subparallel foliation of actinolite with albite and scapolite porphyroblasts in a nematoblastic matrix of actinolite with biotite and epidote; interstitial titanite and opaque minerals; a mafic volcanic protolith is inferred for these rocks based on their mineralogy. The greenschists occur as massive dark greenish gray outcrops, but display a distinct greenish light blue color in enclaves of these rocks within granite or near contacts with the intrusion, due to a pervasive chloritization of the amphiboles.

The schists of Quebrada del Carrizo in part occur as enclaves within sheared, medium-grained granite and are bounded by the east-dipping Sierra del Castillo thrust fault to the west and by another west-dipping thrust fault to the east (Fig. 2). These faults have been interpreted by Niemeyer (1999) as forming a sinistral, transpressive, tectonic wedge developed during the Eocene-Oligocene. The schists of the Quebrada del Carrizo are deformed into isoclinal folds with superimposed asymmetric folds and crenulation with fold axis near parallel with the main foliation that strikes NE and dips from 35° to 70° to the east.

The El Jardín Schists correspond to metamorphic rocks that are exposed in the northern flank of the Jardín creek (or lower Asientos creek) within a structural wedge formed by the southward convergence of the east-dipping Sierra Castillo thrust fault and the west-dipping Barrancas thrust fault (cf. Perelló and Müller, 1984; Tomlinson et al., 1999) and under a 250 m thick, flat cover of Miocene alluvial gravels with minor volcanic ash intercalations (Atacama Gravels; Tomlinson et al., 1999) (Fig. 4).

 

FIG. 4. Geological map of the El Jardin Schists (base topography after
Google Earth image; lithostratigrapic units after Tomlinson et al., 1999).

 

The metamorphic rocks are gray-colored, mica schists, with a main foliation striking N20-30° E and dipping from 25° to 50° to the west, which have superimposed asymmetrical folds with semiperpendicular axis strike direction and local crenulation cleavage (kink bands) with wavelength less than 3 cm (Fig. 5). There are 2 to 5 cm thick segregation quartz veins and heterogeneity in the grain size, with protruding quartz-rich, coarser-grained layers that are more resistant to weathering, which alternate with mica-rich fine-grained levels that are friable and more weathered (Fig. 5).

 

FIG. 5. El Jardm Schists: (left) general appearance of the outcrops, (right) crenulation
cleavage in the mica schists.

 

The schists have granoblastic texture when quartz is the dominant phase over micas, whereas lepidoblastic textures characterize the more micaceous metamorphic rocks. Poikiloblastic textures are less frequent with albite porphyroblasts with inclusions of fine grains of quartz, albite and white micas, plus some carbonaceous material. Bands formed of crystalline aggregate of quartz alternate with muscovite-rich or chlorite bands, and when present, albite porphyroblasts occur within the micaceous bands that are deformed on the edges of the larger albite grains. Chlorite and opaque minerals occur in some samples, which appear to have completely replaced biotite; also there are late calcite veinlets.

The mineral assemblage of actinolite±albite± chlorite±epidote±quartz of the greenschists and quartz±albite±muscovite±chlorite of the mica schists indicate that the protoliths of the two exposures (El Jardín and Quebrada del Carrizo) were subjected to greenschists facies metamorphism. The characteristics of the micaceous schists are consistent with a sedimentary protolith, which is corroborated by rounded to subrounded zircon grains separated from these rocks, some of which have graphitic coatings (Fig. 6).

 

FIG. 6. Cathodoluminiscence images of detrital zircon grains from the dated samples
of schists; note the internal zoning typical of igneous zircon, but the rounded shape
of the grains due to wear during sediment transport. Black circles in the lower images are
laser beam pits. Upper images from Centro de Pesquisas Geocronológicas, Brazil and
lower images from the laboratory of the Universidad de Chile.

 

Local light pink, granitic injections occur within fold hinges of the El Jardín Schists and a dark green, foliated tonalite is intruded crosscutting the schist foliation, these granitoids are part of the southern end of the Sierra Castillo Batholith, exposed mostly to the north, along the Domeyko Cordillera east of El Salvador (Tomlinson et al., 1999) (Figs. 1 and 4).

The granite intruded within a fold hinge in the Jardín Schists (CHY-74; Fig. 7) is medium-grained, leucocratic rock with hypidiomorphic granular texture and composed of quartz, plagioclase and orthoclase, with less than 10% of chlorite, muscovite, and opaque minerals that appear to have completely replaced biotite. The quartz grains show undulatory extinction and subgrains, some plagioclase are bent and show wedge twins, but also microfractures, indicating ductile and fragile deformations of the granite. The tonalite (CHY-19a) is a medium-grained, mesocratic rock with hypidiomorphic granular texture and composed mainly by plagioclase and hornblende and minor biotite and quartz; it has a foliation defined by preferential orientation of tabular minerals (Fig. 7). The quartz grains show undulatory extinction and plagioclase grains have wedge twins and microfractures, indicating also ductile and fragile deformation of the tonalite.

 

FIG. 7. Intrusions into the El Jardín Schists: (left) contact between schists and granite
emplaced within a fold hinge, (right) aspect of the foliated tonalite.

 

The sheared granite (CHY-83) that crosscuts the metamorphic rocks at the del Carrizo creek is composed of rounded and partly corroded grains of quartz, orthoclase, and plagioclase with deformation twins, and minor chloritized biotite, within a felsitic microcrystalline groundmass composed of quartz, plagioclase and muscovite. Wedge twins, recrystallization and microfractures of the minerals evidence ductile and fragile deformation of this granite.

3. Previous geochronological data

A whole rock K-Ar age of 196±5 Ma was reported for mica schist by Cornejo et al. (1993) and a muscovite K-Ar age of 278±6 Ma by Tomlinson et al. (1999) for an amphibole, scapolite and muscovite schist of the Jardín Schists. In addition, Tomlinson et al. (1999) reported a sericite K-Ar age of 264±6 Ma for muscovite-bearing granite body that crosscut these metamorphic rocks. Besides, a whole-rock K-Ar age of about 151 Ma was originally reported for the Quebrada del Carrizo Metamorphic Complex by Cornejo and Mpodozis (1996), but later Cornejo et al. (2009) published a muscovite K-Ar age of 272±6 Ma for a mica schist, a biotite K-Ar age of 269±6 Ma for a biotite and amphibole schist and an amphibole 40Ar/39Ar age of 277±6 Ma for an arfvedsonite and scapolite schist. While, there are a number of Permian K-Ar dates that range from 281±6 to 254±8 Ma for the granitic rocks of the Sierra Castillo Batholith (Tomlinson et al., 1999; Cornejo et al., 2009) and Rb-Sr isochrones of 278±4, 270±10 and 269±4 Ma (Halpern, 1978; Brook et al., 1986).

The published K-Ar and 40Ar/39Ar data for the Jardín and Quebrada del Carrizo metamorphic rocks clearly indicate their Early Permian minimum age, but the actual age of the protolith remained indefinite, just as any actual correlation of these metamorphic rocks with other units and their significance in the regional geological context.

4. Analytical methods

Zircon grains were obtained from rock samples following normal mineral separation procedures at the Geology Department, University of Chile, which included crushing, washing, and heavy liquid and magnetic separation. Zircon grains were mounted in epoxy and later polished. The zircon mounts were cleaned with 3% HNO3 and later washed in ultraclean water and cathodoluminescence (CL) images were prepared for all samples prior to U-Pb analyses.

U-Pb analyses were performed at the Isotope Geochemistry laboratory, in the Andean Geothermal Center of Excellence, Department of Geology of the Universidad de Chile. Analyses were performed using a Photon Machines Analyte G2 192 nm ArF excimer laser ablation system with a HelEx 2 ablation cell, coupled to a Thermo Scientific Neptune Plus multicollector ICP-MS. The laser system is normally operated at 7-8 Hz with an ablation diameter of 25-30 µm. In order to reduce elemental fractionation and maximize the sensitivity, the ablated material is carried in helium gas (0.5 l/min) prior to entering the ICP torch (Günther et al., 1999). The plasma conditions for U-Pb geochronology are typically 1200 W. Faraday cup detectors and ion counters are used simultaneously in static mode to measure 202Hg, 204Hg+204Pb, 206Pb, 207Pb, 208Pb, 235U, 232Th and 238U. Data are collected with an integration time of 1.084 s per cycle. The ablation time is about 50 s which is equivalent to 50 cycles. The first 10 cycles are not used in the data reduction (pre-ablation) to achieve better signal stability and to eliminate surface contamination.

The measure sequence among standard -sample is 3 standards followed by 4 unknowns. A 20s blank is taken between every ablation. Normalization was carried out using the Plesovice zircon standard (337.13±0.37 Ma; Slama et al., 2008) as primary standard, and SL2 or 91500 as secondary standard. The 238U intensity and the 206Pb/238U, 207Pb/235U, 206Pb/208Pb ratios are recorded every session and are compared with the values reported in the literature. For the data reduction we use the Iolite software (Paton et al., 2011), which allows to correct for downhole fractionation and choose the type of spline used to correct for instrument bias. Age calculation and plots were made using ISOPLOT software (Ludwig, 2003).

U-Pb analyses for sample CHY-39 were also performed by the Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) with a high-resolution, multiple collector mass spectrometer with a plasma source (MC-ICP-MS; Finnigan, Neptune), connected to an Excimer ArF laser (λ=193 nm) in the Geochronological Research Center (Centro de Pesquisas Geocronologicas) of the Sao Paulo University in Brazil. The ablated material was carried by Ar (~7 l/min), and He (~0.6 l/min) in analyses of 60 cycles of 1 second. The measure sequence among standard, blank and unknown was: 2 blanks, 3 standards, 13 unknowns, 2 blanks and 2 standards All analyses were conducted in static mode with a laser beam diameter of ~29 µm, operated with an output energy of 6 mJ and a pulse rate of 6 Hz. Normalization was carried out using Temora standard zircon (416.75±0.24 Ma, 95% confidence limits; Black et al., 2003) and age calculation were performed using ISOPLOT V3 (Ludwig, 2003). Corrections for common Pb were applied to samples with 206Pb/204Pb lower than 1000, using Stacey and Kramers (1975) model at the age of crystallization and for polarization displacement. U-Pb data were plotted using ISOPLOT V3 (Ludwig, 2003). Errors for isotopic ratios are presented at ±1o level in Appendix 1a and 1b.

The U-Pb age for the tonalite sample CHY-19a was obtained using a sensitive high resolution ion microprobe (SHRIMP II) in the Geochronological Research Center (Centro de Pesquisas Geocronologicas) of the Sao Paulo University in Brazil, following the procedures described by Maksaev et al. (2014).

Interpreted ages are based on 206Pb/238U for <1.0 Ga grains and on 206Pb/207Pb for >1.0 Ga grains. U-Pb analyses that were >30% discordant or >5% reverse discordant were excluded from interpretations and the respective concordia plots are shown in figure 8. Sample location and analytical U-Pb data is included in Appendix 1a for detrital-zircon and 1b for igneous zircon grains. The numerical ages are assigned to the International Stratigraphic Chart of the International Commission on Stratigraphy (Cohen et al., 2013).

 

FIG. 8. Concordia diagrams of the analyzed samples of mica schist (discordant analyses
excluded). Ages of oldest and youngest zircon grains are given in each plot.

 

5. Results

Sample CHY-79; Quebrada del Carrizo Metamorphic Complex: lepidogranoblastic albite-quartz-muscovite schist (Fig. 9a) composed of 35% anhedral albite (0.5 to 1.5 mm long), 30% anhedral quartz (0.1 to 1.0 mm), 30% subhedral deformed muscovite (0.1 to 0.5 mm long), 10% opaque minerals, traces of epidote and tourmaline. 40 U-Pb ages were used from 61 individual grains analyzed (20 discarded). The largest group of zircon U-Pb ages is from 405 to 560 Ma (n=17; peak at 470 Ma; Fig. 10) that represent 42.5% , followed by the youngest population from 271 to 317 Ma (n=10; peak at 303 Ma) that represents 25%; other smaller groups are from 615 to 680 (n=4; peak at 650 Ma; 10%); from 790 to 841 (n=4; peak at 840; 10%), and from 1098 to 1,163 Ma (n=4; peak at 1,130 Ma; 10%; Fig. 10).

 

FIG. 9. Microphotographs (cross-polarized light) of samples of metamorphic
rocks: a. CHY-79: albite-quartz-muscovite schist; b. CHY-80: quartz-albite-chlorite
schist; c. CHY-19: albite-quartz-chlorite schist; and d. CHY-39: albite-quartz-muscovite schist.

 

Sample CHY-80; Quebrada del Carrizo Metamorphic Complex: lepidogranoblastic quartz-albite-chlorite schist (Fig. 9b) composed of 60% anhedral quartz (0.5 to 1.5 mm long), 18% anhedral albite (0.1 to 0.9 mm), 10% subhedral chlorite (0.1 to 0.5 mm long), 10% opaque minerals (0.1 to 0.3 mm), minor muscovite and calcite. 54 U-Pb ages were used from 69 individual grains analyzed (15 rejected).

The youngest group of zircon U-Pb ages from 269 to 321 is the largest one (n=18; peak at 292 Ma; Fig. 10) and represents 33.3%, followed by a population from 405 to 544 Ma (n=13; peak at 470 Ma) that represents 24%, and a group from 966 to 1,237 (n=11; peaks at 970 and 1,200 Ma) that represents 20.4%; a smaller group from 618 to 628 (n=3; peak at 620 Ma) represents 5.5% (Fig. 10).

 

FIG. 10. U-Pb age histogram distribution of analyzed zircons (grey) with black probability
curves from the mica schist samples.
238U/206Pb ages for all zircon <1 Ga were used
and 207Pb/206Pb ages were used for all zircon grains >1 Ga (discordant analyses excluded).

 

Sample CHY-19; El Jardín Schists: lepidograno-blastic albite-quartz-chlorite schist (Fig. 9c) composed of 37% of anhedral albite (0.05 to 1.0 mm long), 35% anhedral quartz grains (0.1 to 0.5 mm), 25% subhedral chlorite (0.1 to 0.5 mm long), 2% muscovite and 1% opaque minerals. 59 U-Pb ages were used from 71 individual grains analyzed (12 discarded). The largest group of zircon U-Pb ages is from 460 to 716 Ma (n=41; peak at 540 Ma with a secondary peak at 480 Ma; Fig. 10) that represents 69.4%, followed by group from 1,006 to 1,092 Ma (n=7; peak at 1,060 Ma) that represents 11.9%. The youngest group is from 319 to 336 Ma (peak at 331 Ma) that represent 8.5% (Fig. 10).

Sample CHY-39; El Jardín Schists: lepidograno-blastic albite-quartz-muscovite schist (Fig. 9d) composed of 35% anhedral albite (0.5 to 1.0 mm long), 28% anhedral quartz (0.1 to 1.5 mm), 20% subhedral muscovite (0.1 to 0.3 mm long), 15% subhedral chlorite, minor calcite and tourmaline. 56 U-Pb ages were used from 65 individual grains analyzed (9 discarded). The largest group of zircon U-Pb ages is from 401 to 673 Ma (n=24; peak at 530 Ma with a secondary peak at 480 Ma; Fig. 10) that represents 42.9%, followed by a population from 749 to 842 Ma (n=7; peaks at 800 Ma) that represents 12.5%; another group from 1,074 to 1,228 (n=6; peak at 1,080) that represents 10.7%; another population from 931 to 988 Ma (n=4; peak at 960) that represents 7.1%, and; some individual zircon grains yielded U-Pb ages from 1,364 to 3,435 Ma. The youngest population is from 304 to 328 Ma (n=4; peak at 313) that represents 7.1% (Fig. 10).

Most of the analyzed zircon grains from the schists show regular internal zonation pattern characteristic of a magmatic origin (Corfu et al., 2003) (Fig. 6) and 50% of the zircon grains have Th/U ratio values >0.5, which are consistent with composition of igneous zircons according to Hoskin and Schaltegger (2003). The complete Th/U ratio ranges from 0.04 to 2.15 (Fig. 11), so that near half of zircon grains cannot be unambiguously classified by their Th/U ratio (Fig. 11), but only two zircon grains have Th/U ratio of 0.04 that is consistent with metamorphic-melt zircon composition (Hoskin and Schaltegger, 2003), and these are of zircon grains that yielded Proterozoic U-Pb ages of 1,005 and 2,409 Ma, respectively.

 

FIG. 11. Th/U ratio versus U-Pb zircon age plot of the analyzed zircon grains;
the 0.004 and 0.5 black lines mark according to Hoskin and Schaltegger (2003)
the upper and lower limits for zircon of metamorphic or magmatic origin, respectively.

 

The youngest U-Pb ages of zircon in populations of detrital zircon are commonly used to constrain maximum depositional ages of stratigraphic units (e.g., Dickinson and Gehrels, 2009 and references therein), but there is lack of consensus about the methods to do so (Barbeau et al., 2009). Thus, we use four alternate measures of the youngest age which vary from least to most statistically robust as follows: (a) youngest single grain age, (b) youngest graphic peak controlled by more than one grain age, (c) weighted mean age of the youngest three or more grains that overlap at ±2c (d) weighted mean age of all grains that correspond to the youngest peak (Table 1).

 

TABLE 1. MAXIMUM DEPOSITIONAL AGE INTERPRETATIONS FROM YOUNGEST
DETRITAL ZIRCON ANALYZED FROM EACH SAMPLE.

 

The maximum depositional age estimates of table 1 consistently correspond to Late Carboniferous for the El Jardín Schists and Early Permian for the Quebrada del Carrizo Metamorphic Complex; 314±11 Ma and 291±5 Ma, respectively, if the youngest most statistically robust values of table 1 are considered.

The U-Pb zircon ages obtained for intrusive rocks emplaced within the schists are of 278.3±5.8 Ma for granite (CHY-83) of the del Carrizo creek and of 285.7±6.8 for foliated tonalite (CHY-19a) of the Jardín creek (Fig. 12). A granite injection (CHY-74) within a fold hinge in the El Jardín Schists contains a group of inherited Neoproterozoic to Cambrian zircon grains (Fig. 12) that is analogous to the prominent maxima of U-Pb ages of the metamorphic rocks, thus probably incorporated from them. The youngest peak (5 spot analyses) defines a mean U-Pb age of 292.2±6.6 Ma for the granite (Fig. 12), which coincides within error limits with the age obtained for the foliated tonalite in the same area.

 

FIG. 12. Concordia diagrams of the analyzed samples of granitoids that were emplaced
within the metamorphic rocks. The U-Pb age histogram distribution of analyzed zircons
(grey) with black probability curves of the granite sample CHY-74 is also included to
illustrate a population of inherited zircon grains.

 

6. Discussion

The results of U-Pb dating of detrital zircons from four samples of mica schists are comparable to each other, as they show significant concentrations of U-Pb ages in the range of 400-600 Ma (Early Devonian to Late Proterozoic), which constitute the largest population in 3 of the samples, and define graphical relative probability peaks at 470 Ma for the zircon grains from the Quebrada del Carrizo and peaks at 460 and 480 Ma for those from El Jardín Schists (Fig. 10); the later also include higher peaks at 530-540 Ma (Fig. 10) with an ill-defined bimodal population. The four analyzed samples include youngest population of Late Paleozoic zircons, which are of ~269-321 Ma for Quebrada del Carrizo and of ~304-336 Ma for El Jardín Schists. Besides, the four samples include significant concentrations of U-Pb ages from 1,000 to 1,200 Ma, plus less prominent peaks centered at about 800 and 620-650. The overall coincidence of zircon grain age populations indicates a common provenance of the sedimentary material of the protolith of the mica schists of Quebrada del Carrizo Metamorphic Complex and El Jardín Schists.

The provenance of sedimentary material of other metamorphic complexes of central-northern Chile has been extensively discussed in previous studies (e.g., Willner et al., 2008; Bahlburg et al., 2009; Álvarez et al., 2011). These authors have shown that most of detritus have been derived from eastern sources, as U-Pb ages reflect input of zircon grains from all major orogenic cycles connected to the stepwise southwestward growth of the Amazonia craton in the southwestern Amazonia Orogenic System and the subsequent Terra Australis orogen between 2 Ga and 0.25 Ga, including additional input from the Arequipa Massif. Overall, the largest zircon contributions were derived from the Rondonia-San Ignacio, Sunsas, Famatinian and Late Paleozoic orogenic belts, with lesser input, in decreasing order of magnitude, from Brazilian-Pampean, Transamazonian and older Paleoproterozoic and Archean sources (Bahlburg et al., 2009, and references therein). In our samples the youngest U-Pb age populations indicate detrital input from Late Paleozoic magmatism of the paleo-Pacific border of Gondwana (e.g., Maksaev et al., 2014), and the concentration of U-Pb ages within the 400 to 600 Ma range imply contributions of the Famatinian and Pampean orogenies or the Braziliano cycle orogenesis. Although there is a larger proportion of Pampean age zircon grains in the Jardín Schists compared to those of the Quebrada del Carrizo Metamorphic Complex (Table 2). In addition, age concentrations between 900 and 1,200 Ma indicate a contribution of the Sunsas orogeny of the southwestern Amazonia margin of Grenville age (Bahlburg et al., 2009, and references therein). Whereas there are scarce Devonian grains (Table 2), consistent with the development of a passive margin in this region and a lull of magmatic activity during this period (e.g., Bahlburg et al., 2009), and also only few zircon grains older than 1,200 Ma (Table 2).

 

TABLE 2. PERCENTAGES OF U-Pb AGE DATES REPRESENTING KNOWN AND
DISTINCT GEOCHRONOLOGICAL EVENTS IN PROBABLE SOURCE REGIONS ON THE
AMAZONIA CRATON, THE AREQUIPA MASSIF AND THE CENTRAL ANDES.*

* Temporal ranges of events according to Bahlburg et al. (2009 and references therein).

 

Previously published detrital-zircon U-Pb data for other metamorphic rocks of northern Chile, such as: El Tránsito Metamorphic Complex, Huasco Metamorphic Complex, Choapa Metamorphic Complex, Las Tórtolas Formation and El Toco Formation (Willner et al., 2008; Bahlburg et al, 2009; Álvarez et al., 2011) (Fig. 13), are in agreement with our new detrital-zircon U-Pb data as they reveal eastern input of the Pampean and Famatinian or Braziliano orogenies, but also of Sunsas orogeny of Grenvillian age (Willner et al., 2008; Bahlburg et al., 2009; Álvarez et al., 2011). In contrast, Late Paleozoic zircon grains are not always present, even in samples from the same metamorphic complex; for example Bahlburg et al. (2009) showed that 2 samples from Las Tórtolas Formation either have or have not Late Paleozoic zircon grains, respectively. A similar situation is documented by the contrasting U-Pb data for samples from the El Tránsito Metamorphic Complex, east of Vallenar, obtained by Bahlburg et al. (2009) that show that a prominent maximum between 359 and 250 Ma (57%) with the youngest grain at 254±7 Ma and the data obtained by Alvarez et al. (2011) for the same metamorphic complex that lack of Late Paleozoic zircon grains.

 

FIG. 13. Distribution of the Late Paleozoic
metamorphic and sedimentary complexes of
northern Chile (N of 32°S; modified after
Hervé et al., 2007) and location of the
 Quebrada del Carrizo Metamorphic Complex
and El Jardín Schists.

 

The prominent maximum between 359 and 250 Ma of detrital-zircon U-Pb data for a mica schist of the El Tránsito Metamorphic Complex obtained by Bahlburg et al. (2009) has been questioned by Alvarez et al. (2011) and Salazar et al. (2013) due to the contrast with the U-Pb data of Alvarez et al. (2011) and the occurrence of Carboniferous rhyolitic tuffs of the Cerro Bayo Formation (U-Pb from 324.7±4.0 to 300.8±4.6 Ma; Salazar et al., 2013; Maksaev et al., 2014), which were thought to be deposited unconformably over the El Tránsito Metamorphic Complex. However, these Carboniferous rhyolitic rocks are actually in contact by a thrust fault with the metamorphic rocks (Salazar et al., 2013), and this condition of very different crustal levels juxtaposed is not unique; for example, the high-pressure Limón Verde Metamorphic Complex in the Antofagasta Region of northern Chile, which have a maximum depositional age of 300 Ma and peak metamorphism dated at 280 Ma (Soto, 2013; Morandé, 2014), is in structural juxtaposition against penecontemporaneous supracrustal rhyolitic volcanic and sedimentary sequences of the Late Carboniferous to Early Permian Collahuasi Formation (Tomlinson and Blanco, 2008; Tomlinson et al., 2012).

The Late Carboniferous to Early Permian maximum depositional age for the El Jardín Schists and Quebrada del Carrizo Metamorphic Complex are well in agreement with the maximum depositional age determined by Bahlburg et al. (2009) for the El Tránsito Metamorphic Complex, but also for Las Tórtolas Formation, Huasco Metamorphic Complex and Huentelauquén Formation. In addition, our U-Pb data are in agreement with the maximum depositional age determined by Alvarez et al. (2011) for the Choapa and Huasco metamorphic complexes and also with the maximum depositional age estimated by Willner et al. (2008) for the Choapa Metamorphic Complex (308 Ma), the Huentelauquén Formation (303 Ma) and coastal accretionary systems (344-308 Ma) of central Chile. Consequently, the incorporation of Late Paleozoic material is common-place in the protolith of the above mentioned metamorphic and metasedimentary rocks of north-central Chile (Fig. 13) and its absence in some samples reflect only variations in the input of these materials, rather than equivocal U-Pb data.

The U-Pb data for intrusions that were emplaced within the deformed metamorphic rocks of the Quebrada del Carrizo and El Jardín range from 292.2±6.6 to 278.3±5.8 Ma (Early Permian). The zircon U-Pb age range obtained for the intrusive rocks is a bit older, but overlaps within error with the 278±6 to 269±6 Ma time-span indicated by a subset of the oldest previously published K-Ar and 40Ar/39Ar dates for the metamorphic rocks (Tomlinson et al., 1999; Cornejo et al., 2009), which obviously recorded only the minimum age of the latest thermal effect of the intrusions of the Sierra Castillo Batholith. It is worth mentioning that Perelló and Müller (1984) interpreted that the intrusions and the metamorphic rocks were part of the same intrusive complex that was affected by dynamic metamorphism related to the Cenozoic activity of the regional Sierra Castillo and Barrancas Faults. However, this interpretation is not supported by our new U-Pb data and is at variance with our and other author’s geological observations (e.g., Pérez, 1982; Tomlinson et al., 1999; Cornejo et al., 2009) that recognized the crosscutting relationship of the Permian intrusions in the metamorphic rocks.

The dated intrusive rocks have been affected by ductile deformation, but show no obvious evidence of metamorphism. Thus, the greenschists facies metamorphism is constrained between 291±5 and 278.3±5.8 Ma for the Quebrada del Carrizo Metamorphic Complex and between 314±11 and 292.2±6.6 Ma for El Jardín Schists, considering the respective maximum depositional age and the crystallization age of the granites; this point to Early Permian age for the metamorphic peak of the schists. The ductile deformation of the Early Permian granitoids suggests an emplacement within hot crust (relatively at a deep level), which probably took place not long after the greenschist facies, metamorphic peak of their country rocks, as further sustained by the closeness between the maximum depositional age for the schists and the crystallization ages of the intrusive rocks.

The Early Permian age of the greenschist facies peak metamorphism is somewhat younger, but may be assigned to the so called Toco orogeny; a meta-morphic and plutonic event related to subduction of the paleo-Pacific or Panthalassian Ocean and the formation of an associated subduction complex or accretionary wedge on the edge of Gondwana, which was originally ascribed to the Early to Late Carboniferous (Bahlburg and Breitkreuz, 1991; Hervé et al., 2007), but according to the U-Pb data should be extended to the Early Permian. It should be noted that the onset of late Paleozoic magmatism in northern Chile occurred at ca. 330 Ma according to published U-Pb data (Maksaev et al., 2014), thus the later stages of sedimentation, integration into the accretionary wedge, deformation and metamorphism of the studied rocks may overlap in time with the initial stages of the Late Paleozoic magmatism, represented by the Early Permian plutons of the Sierra Castillo Batholith, which were emplaced within the former accretionary wedge of the southwestern border of Gondwana.

7. Conclusions

Detrital-zircon U-Pb data from two discrete outcrops of mica schists in the Atacama Region of northern Chile indicate that the maximum depositional age of their protolith corresponds to the Late Carboniferous to Early Permian, indicating input of detritus from the Late Paleozoic magmatism of the western border of Gondwana. The Early Permian granitoids of the Sierra Castillo Batholith crosscut the schists and show ductile deformation, and probably were emplaced in hot crust, shortly after the green-schist facies metamorphic peak of their metamorphic country rocks. Therefore, the peak metamorphism for the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists is constrained between the Carboniferous to Early Permian maximum depositional U-Pb ages of detrital zircon grains (314±11 and 291±5 Ma) and the crystallization of Early Permian intrusions (292.2±6.6 to 278.3±5.8 Ma).

The concentrations of detrital-zircon U-Pb ages within 400 and 600 Ma indicate significant detritus input from eastern sources including the Brazilian and Pampean Orogenies and the Ordovician to Silurian Famatinian magmatic arc of northwestern Argentina. Other concentrations of detrital-zircon U-Pb ages from 900 to 1,200 Ma are reflecting input from Sunsas (Grenville) age magmatic rocks, while only scarce U-Pb ages older than 1200 Ma occur and these may correspond to a minor input of zircon grains from South American cratonic areas. Grains of Devonian age are scarce in the analyzed zircon populations, consistent with a passive margin and lull of magmatic activity during this period on the paleo-Pacific border of Gondwana.

The detrital-zircon U-Pb data for the Quebrada del Carrizo Metamorphic Complex and El Jardín Schists are in agreement with detrital-zircon U-Pb data previously published for other metamorphic complexes of central-northern Chile, which are part of a Late Paleozoic subduction complex or accretionary wedge developed at the western border of Gondwana. Therefore, the El Jardin Schist and Quebrada del Carrizo metamorphic complex are relics of the same Late Paleozic accretionary wedge, where Early Permian plutons of the Sierra Castillo Batholith were emplaced.

Acknowledgements

This study was supported by CONICYT, Chile, through Fondecyt 1110093 grant to V. Maksaev and F. Munizaga (Universidad de Chile). We thank the analytic work of F. Barra of the Isotope Geochemistry laboratory of the Andean Geothermal Center of Excellence, Department of Geology of the Universidad de Chile and K. Sato of the Geochronological Research Center (Centro de Pesquisas Geocronologicas) of the Universidade de Sao Paulo, Brazil. Part of the data of this paper constituted the Bachelor's Thesis of J. Arancibia. A review by Dr. J. Alvarez contributed to improve this paper.

 

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Manuscript received: December 12, 2014; revised/accepted: July 10, 2015; available online: July 13, 2015.

 

APPENDIX 1a. DETRITAL ZIRCON U-PB ANALYTICAL DATA OF MICA SCHISTS.

SAMPLE CHY-19; 26°23’22.05”S/69°29’09.62”W, h. 2134 m

 

Appendix 1a. continued.

 

Appendix 1a. continued.

SAMPLE CHY-79; 25°55’25.37”S/69°20’13.91”W, h. 2866 m

 

Appendix 1a. continued.

 

Appendix 1a. continued.

SAMPLE CHY-80; 25°55’25.37”S/69°20’13.91”W, h. 2866 m

 

Appendix 1a. continued.

 

Appendix 1a. continued.

SAMPLE CHY-39; 26°23’22.52”S/69°29’13.92”W, h. 2140 m; Analysis at CPGeo, Brazil.

 

Appendix 1a. continued.

CROSS out data = analytically inaccurate; Con. = % of concordance.

 

APPENDIX 1b. ZIRCON U-PB ANALYTICAL DATA OF GRANITOIDS.

SAMPLE CHY-83 Pink, sheared granite; 25°55’36.50"S/69°19’58.70"W,
h. 2868 m; 49 spots; LA-ICP-MS U-Pb age=278.3±5.8 Ma (95% conf.).

 

Appendix 1b. continued.

 

Appendix 2b. continued.

SAMPLE CHY-19a; Dark green, foliated tonalite; 26o23’22.05"S/69o29’09.62"W,
h. 2134 m; 14 spots; SHRIMP U-Pb age = 285.7±6.8 Ma (95% conf.).

 

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