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

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 detritalzircon 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.


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 km 2 ) which occur on the border of the eastern block of the Sierra Castillo thrust fault (cf.Perelló and Müller, 1984;Tomlinson et al., 1993Tomlinson et al., , 1999;;Cornejo et al., 1993Cornejo et al., , 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).
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).

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).
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).
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 semiper pen dicular 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 micarich fine-grained levels that are friable and more weathered (Fig. 5).
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

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

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)   amphibole schist and an amphibole 40 Ar/ 39 Ar age of 277±6 Ma for an arfvedsonite and scapolite schist.
The published K-Ar and 40 Ar/ 39 Ar 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.

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% HNO 3 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) U. 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±0.37Ma; Slama et al., 2008) as primary standard, and SL2 or 91500 as secondary standard.The 238 U intensity and the 206 Pb/ 238 U, 207 Pb/ 235 U, 206 Pb/ 208 Pb 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 Geocronológicas) 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±0.24Ma, 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 206 Pb/ 204 Pb 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 ±1σ 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 Geocronológicas) of the Sao Paulo University in Brazil, following the procedures described by Maksaev et al. (2014).
Interpreted ages are based on 206 Pb/ 238 U for <1.0 Ga grains and on 206 Pb/ 207 Pb 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).
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.
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) The U-Pb zircon ages obtained for intrusive rocks emplaced within the schists are of 278.3±5.8Ma 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.

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  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).
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 Álvarez et al. (2011) for the same metamorphic complex that lack of Late Paleozoic zircon grains.
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 Álvarez 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 Álvarez 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±5.8Ma (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 40 Ar/ 39 Ar 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±5.8Ma 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 metamorphic 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) 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.

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 greenschist 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±5.8Ma).
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 Jardín 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.

FIG. 3 .
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).
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.
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.
weighted mean age of all grains that correspond to the youngest peak (Table1).The maximum depositional age estimates of table1consistently 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.
. 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 FIG.10.U-Pb age histogram distribution of analyzed zircons (grey) with black probability curves from the mica schist samples. 238/ 206 Pb ages for all zircon <1 Ga were used and 207 Pb/ 206 Pb ages were used for all zircon grains >1 Ga (discordant analyses excluded).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.grainsthat overlap at ±2σ (d)

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 toBahlburg et al. (2009 and references therein).
, 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 Chinches Formation 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.