1. Introduction

Thermo-chronological data are usually utilized to track the cooling/exhumation of basement rocks, and sampling surveys must have the availability to measure temperature changes in the older rocks (Murray et al., 2018). In the eastern portion of the Altiplano plateau (14-22° S), the basement is mainly Paleozoic in age and largely exposed, and abundant fission-track and (U-Th)/He data therein indicate major exhumation events in the Eocene-Oligocene   (40-25 Ma) and Miocene (20-2 Ma) (Gillis et al., 2006; Ege et al., 2007; McQuarrie et al., 2008; Barnes et al., 2012; Perez et al., 2016; Anderson et al., 2018). In the western Altiplano, volcanic and sedimentary Cenozoic rocks are dominant (Fig. 1) and the geologic evolution indicates major deformation and uplift during the Eocene Incaic orogeny and Miocene Quechua orogeny (Charrier et al., 2013; García et al., 2017). In the Western Cordillera, older rocks are locally exposed and their thermochronological record is poor, whereas in the forearc (from the Coastal Cordillera to Chilean Precordillera), the basement is Jurassic-Paleocene in age and their thermochronological data are largely older than 24 Ma (Fig. 1) and indicate very slow exhumation since the Cretaceous (Schlunegger et al., 2006; Schildgen et al., 2007; Avdievitch et al., 2018).

 

Fig. 1. Simplified geological map of the northernmost Chile, westernmost Bolivia and southernmost Peru showing distribution of rock units, main structures, morpho-structural domains, available thermochronological ages (in Ma), and location of the figure 2 (Belén area) and figure 3 (structural cross-section). Geology based on maps of the Geological Surveys of Chile, Bolivia and Peru. Proterozoic-Paleozoic rocks are metamorphic, Mesozoic-Eocene rocks are igneous and sedimentary, and Oligocene-Holocene units are continental volcanic and sedimentary rocks and unconsolidated deposits. Note the absence of Proterozoic to Eocene bedrock outcrops in the Western Cordillera. Thermochronological ages are previous to this study. (U-Th)/He ages are from Avdievitch et al. (2018). Apatite fission-track ages are from Schlunegger et al. (2006) in Chile and Barnes et al. (2012) in Bolivia. Some zircon and apatite fission-track ages and AHe ages are reported in the Belén and Uyarani areas, but their sample locations and other details are unknown (see text).

Two outcrops of Proterozoic-Paleozoic metamorphic rocks occur in the Western Cordillera at 18-19° S, the Uyarani and Belén complexes (Fig. 1). In westernmost Bolivia (Uyarani), apatite fission-track (AFT) and apatite (U-Th)/He (AHe) data in Proterozoic rocks suggest mostly Eocene cooling (Horton et al., 2006; Barnes et al., 2012). In the Proterozoic-Paleozoic Belén Metamorphic Complex (BMC) of northernmost Chile (Figs. 1 and 2; Salas et al., 1966; Wörner et al., 2000a; García et al., 2004; Charrier et al., 2013), the thermochronological record consists of one zircon fission-track age of 75±5 Ma (Damm et al., 1994) and AFT and AHe ages of 11-13 Ma (Horton et al., 2006). Analytical data, errors and sample locations are not reported, and thermal modeling not developed for the BMC rocks. Zircon (U-Th)/He (ZHe) data are not available today. The AFT and AHe ages were interpreted as provoked by rapid cooling (Horton et al., 2006) or thermal reset (Wotzlaw et al., 2011).

Fig. 2. Geological map of the Belén area in northernmost Chile at ca. 18.5° S, showing location and results of the samples analyzed; map based on García et al. (2004) and Arcos et al. (2016). The geochronological and thermo-chronological ages are in Ma. For the new (U-Th)/He ages: A, apatite and Z, zircon. For the AHe ages, the mean weighted values are indicated (see text), whereas for the ZHe ages, the value ranges. The U-Pb, 40Ar/39Ar and K-Ar ages are from Muñoz and Charrier (1996), Wörner et al. (2000b), García et al. (2004) and Arcos et al. (2016), and the apatite fission-track age from Schlunegger et al. (2006). For the BMC rocks, available isotopic ages are not showed for simplification (see text).

The Western Cordillera at 18-19° S, in northernmost Chile, is affected by Neogene west-vergent faulting and folding, which have been dated from 18 to 6 Ma, according to cross-cutting relationships and absolute age of syn-kinematic sequences (García et al., 2004; Charrier et al., 2013). This age range is consistent with the today available AFT and AHe ages for the Proterozoic-Paleozoic BMC rocks; however, the Chilean Precordillera is also affected by Neogene deformation and the AFT and AHe data in the basement rocks are older than 24 Ma (Cretaceous-Paleogene), similar to that the Coastal Cordillera. As the Western Cordillera deformation involves the BMC rocks, combined zircon/apatite He ages on these rocks would permit to track their youngest cooling history, and infer an absolute age for the last activity of the fault that uplift the BMC rocks. In order to contribute on the understanding of these topics, we present and discuss new 10 ZHe ages and 11 AHe ages from two samples of the BMC, and we develop inverse T-t models obtained with these ages and geological constraints.

2. Geological setting

2.1. Stratigraphy

The BMC consists of two lithologic units, one of amphibolites, schists and gneisses, and one of granitic orthogneisses (Fig. 2; e.g., García et al., 2004). Numerous and diverse isotopic ages have been interpreted as related to protolith formation or metamorphism and intrusion events. Concerning the metamorphism age, two Rb-Sr isochrons on schists indicated ca. 1,000 Ma and ca. 544 Ma values, respectively (i.e., Proterozoic to early Paleozoic; Pacci et al., 1980; Basei et al., 1996). Several U-Pb zircon dates, from 473±3 to 366±3 Ma, have been considered as magmatic-crystallization ages (of the protolith units) and metamorphism ages (Wörner et al., 2000a; Loewy et al., 2004; Arcos et al., 2016). In fact, duplicates of the same samples analyzed by (U-Th)/He in this work (PM-116 and PM-118) have been dated by U-Pb zircon (Arcos et al., 2016): the major zircon populations from two samples gave 472±2 and 465±2 Ma, respectively, and were interpreted as magmatic-protolith ages, whereas alternative-minor zircon populations gave 444±4 and 442±4 Ma, respectively, and were considered as related to high-temperature metamorphism events (Arcos et al., 2016).

The BMC is locally overlain unconformably by Carboniferous-Permian sedimentary rocks and Jurassic-early Cretaceous marine sedimentary rocks (Fig. 2; Wörner et al., 2000a; García et al., 2004). These three units are largely covered in angular unconformity by ca. 2,500 m thickness of continental volcanic and sedimentary rocks of the Lupica Formation, which consists roughly of a lower unit of andesitic-dacitic lavas and breccias, a middle unit of rhyolitic tuffs, and an upper unit of sandstones and lacustrine rocks (García et al., 2004). The formation has a late Oligocene to early Miocene age (25-18 Ma), according to numerous U-Pb, ⁴⁰Ar/³⁹Ar, and K-Ar determinations (Wörner et al., 2000b; García et al., 2004; Arcos et al., 2016). In Chapiquiña, this unit overlays an andesite lava dome, which has been dated by U-Pb zircon at 31.1±0.5 Ma (early Oligocene; Arcos et al., 2016). The depositional contact relationship between the BMC and Lupica Formation indicate that the first was exposed to erosion prior to 25 Ma (i.e., during the Oligocene). In addition, clasts of the BMC in conglomerates of the Oligocene (35-23 Ma) Azapa Formation (García et al., 2004), and provenance studies based on heavy mineral analysis (Pinto et al., 2007) and detrital zircon U-Pb ages on sandstones of the Azapa Formation (Wotzlaw et al., 2011) also evidence an Oligocene exposition of the BMC rocks.

The Lupica Formation rocks are intruded by middle Miocene monzodioritic-dioritic stocks, with available U-Pb and K-Ar ages from 17.5±0.2 to 12.5±0.6 Ma (Muñoz and Charrier, 1996; García et al., 2004; Arcos et al., 2016). For the Miocene intrusion north of Chapiquiña, one AFT age of 25±5 Ma has been reported by Schlunegger et al. (2006) (Figs. 1 and 2). This intrusion was dated at 15.3±0.2 Ma by U-Pb zircon (Arcos et al., 2016) almost in the same location where the AFT age was obtained; also, the intrusion was dated at 12.5±0.6 Ma by K-Ar biotite, nearby (Muñoz and Charrier, 1996). Thus, the fission-track age is inconsistent because is older than the crystallization age.

2.2. Structure

The Miocene and older rocks are involved in a 70 km long belt formed by NNW striking, west-vergent folds and reverse faults, and according to the age of deformed and non-deformed sequences, the most important shortening (ca. 7 km) occurred between 18 and 6 Ma (Figs. 2 and 3; García et al., 2004; Charrier et al., 2013). One of these structures is the Chapiquiña-Belén fault (ChBF), a 15 km long, N-S-trending, 50-60° east-dipping reverse fault that limits the BMC in the west (Fig. 2; García et al., 2004; Arcos et al., 2016). To the west, the footwall of this fault consists largely of gentle to moderately inclined beds of the Lupica Formation. To the east and covering the BMC, the beds of Lupica Formation dispose as a homoclinal panel with 10-30° dip to the east. The BMC is interpreted as located in the core of a major and complex fault-bend anticline of 15-20 km half-wavelength, with a detachment fault located at ca. 7-10 km depth (Fig. 3; García et al., 2004; Charrier et al., 2013). The anticline is faulted in the core, and has, to the west, a frontal transported syncline and two east-dipping thrusts, which are associated with two generations of syn-kinematic fluvial gravels dated at 18-15 and 12-8 Ma (García et al., 2004). To the west at this latitude, in the Precordillera, Miocene shortening is much less (ca. 100 m) and absorbed by a gentle, 50 km long fold, the Oxaya anticline (Muñoz and Charrier, 1996; García and Hérail, 2005; García et al., 2017).

Fig. 3. Structural cross-section in the northernmost Chile Andes (modified from Charrier et al., 2013), showing the main geological units and faults, as well as the available apatite (U-Th)/He ages in the basement of the Precordillera (Avdievitch et al., 2018; sample mean ages) and Western Cordillera (this study; single-grain ages). Trace location of the entire cross-section in figure 1.

3. Samples and methods

The (U-Th)/He thermochronological method is used to track the cooling history of rocks below He partial retention zone, which is of ~150-220 °C for zircon (ZHe) and ~55-90 °C for apatite (AHe) (e.g., Ehlers and Farley, 2003; Reiners et al., 2005). The two analyzed samples of the BMC were collected at a horizontal distance of 3 km between them, and located at more than 5 km from Miocene intrusions to prevent collection of samples thermally reseted by Miocene magmatism (Fig. 2). The samples are located at similar altitude and distance from the trace of the ChBF. The sample PM-116 (at 3,760 m a.s.l.) is a granodioritic orthogneiss of biotite and muscovite, with quartz, plagioclase and orthoclase. PM-118 (at 3,600 m a.s.l.) is an amphibolitic-biotitic gneiss (or amphibolite, or metadiorite), formed by plagioclase, amphibole, biotite and quartz. Heavy mineral standard separation was carried out with Wilfley Table, magnetic field application and dense liquids at the laboratory of the Universidad de Chile. Zircon and apatite concentrates were obtained with binocular lens observation.

The (U-Th)/He analyses were carried out at the University of Colorado Boulder. The grains were screened for quality, including crystal size, shape and presence of inclusions. Grains were placed into Nb tubes and heated at 800-1,100 °C for He extraction and measurement line. The degassed ⁴He is then spiked with ³He and analyzed on a Balzers Prisma Plus QME 220. Grains are attack with acid combination for dissolution, and the solutions are spiked with a ²³⁵U-²³⁰Th-¹⁴⁵Nd tracer. Sample solutions are analyzed for U, Th, and Sm content using ICP-MS. He ages are calculated using the methods described in Ketcham et al. (2011). The natural occurring ²³⁸U/²³⁵U ratio used in data reduction is 137.818 after Hiess et al. (2012). Every batch of samples includes standards run sporadically, as Fish Canyon Tuff zircons and Durango fluorapatites. Additional analytic procedures can be reviewed in http://www.colorado.edu/geologicalsciences/resources/research-facilities/u-thhe-thermochronology-lab.

In order to evaluate the dispersion of data, we calculated the weighted mean ages, associated errors and MSWDs, using the Isoplot ® routine for Excel ® (Ludwig, 2012). To evaluate the consistency of analyses in each sample, we studied the correlation between effective uranium and calculated crystal age. (U-Th)/He measures that successfully pass this consistency tests were used to obtain inverse thermal modeling. Here we use the QTQt v 5.6.0 software (Gallagher, 2012) to model the thermal history that include the (U-Th)/He analyses in apatite (4 grains). QTQt uses a Bayesian transdimensional Markov Chain Monte Carlo method. We ran 100,000 forward models in the exploratory phase (burn in), then an additional 10,000 (post burn in) to construct the posterior distribution for the model parameters considering the data of individual samples. This is an iterative process with the proposed model being drawn from a perturbation (i.e., add a T-t point) of the current model. This model is either accepted, thus becoming the current model, or rejected. See Gallagher (2012) for further details on the method.

4. Results

4.1. (U-Th)/He ages

4.1.1. Sample PM-116

The five picked zircon grains of the sample PM-116 are euhedral crystals, with widths >43 µm and lengths <221 µm. Crystals show some inclusions and two terminations with slightly rounded ends (Table 1). The (U-Th)/He measures in zircon produced individual ages of 20±1, 31±2, 32±1, 35±1 and 49±1 Ma. Three ages are in the range 31-35 Ma (early Oligocene). No clear trend is observed between single-grain ZHe ages and effective uranium (Fig. 4).

Fig. 4. Plots of the crystal age versus effective uranium, for zircons and apatites of the samples PM-116 and PM-118. Note the slight positive correlation for four measures of the apatites of sample PM-116. See text.

The five apatite crystals dated are euhedral, with widths >55 µm and lengths <236 µm. Broken ends and corners are present in grains, but have clear faces and are free of inclusions on the microscope (Table 2). Single-grain (U-Th)/He ages range from 10.4±0.6 to 15.1±1.3 Ma, belonging to the middle Miocene. The results give a weighted mean age of 12.1±2.2 Ma (at 95% confidence) with MSWD of 26 (Fig. 5). The single-grain age of 15.1 Ma (PM-116-a01) has low total He degassed (Re=99.1%), suggesting the presence of inclusions or other low-diffusivity zones within the crystal (Table 2). In addition, this date is out of the tendency between single-grain age and effective uranium (eU), displaying high AHe age and low eU value (Fig. 4), which suggests that implantation of alpha particles may have affected the apatite. If this single-grain AHe date is ignored, a slight positive correlation remains between these two variables (Fig. 4). In this case, with four ages, the weighted mean age results of 12.0±2.6 Ma, with a MSWD of 27.

Fig. 5. Plots of the apatite (U-Th)/He ages and weighted mean ages (WMA) for samples PM-116 and PM-118. The grain ages are ordered from least to greatest and the gray band indicates the range of the weighted mean age.

4.1.2. Sample PM-118

The five selected zircon crystals are euhedral, with widths >77 µm and lengths <310 µm. Two terminations and some inclusions are observed (Table 1). They yielded (U-Th)/He ages of 113±12, 139±5, 143±4, 220±9 and 226±10 Ma. The results belong to the Mesozoic and are statistically very scattered, with very low replicability. No correlation is observed between individual crystal ZHe ages and effective uranium (Fig. 4).

The six picked apatites are euhedral crystals, with widths >56 µm and lengths <157 µm. Some crystal show only one termination, but have clear faces and are free of inclusions on the microscope (Table 2). The (U-Th)/He measures in these grains generated individual ages from 12.2±0.6 to 18.7±0.7 Ma, i.e., from the early-middle Miocene. The crystals produce a weighted mean of 14.0±2.1 Ma (at 95% confidence) with MSWD of 60 (Fig. 5). No correlation exists between the single-grain age and eU (Fig. 4). Four ages at 12.2 to 14.6 Ma are observed with relatively similar eU (37.2-67.3 ppm). The individual crystal PM-118-a04 displays relatively high eU and higher AHe age (18.7 Ma; Fig. 4 and Table 2); it is the only that does not overlap with the weighted mean age within errors (Fig. 5). If this crystal date is ignored, the weighted mean age results of 13.6±1.7 Ma with MSWD of 28.

4.2. Inverse thermal models

Because zircons on both samples display visibly inclusions and high dispersion of their ZHe ages, these data were not included in the thermal modeling. Four out of five AHe ages of the sample PM-116 show a slight positive correlation between single-grain age and effective uranium (Fig. 4), that suggests this sample has undergone a relatively slow cooling. The T-t model with these measures indicates a wide range of possible thermal paths. The more probable path shows slow cooling from 15 to 0 Ma, that left the rock below 20 ºC. A relatively more marked cooling period, from 80 to 50 °C , is observed at 11-7 Ma (Fig. 6A).

Fig. 6. A and B Inverse thermal models from the four selected ages of the sample PM-116 (see text), using the QTQt v 5.6.0 software (Gallagher, 2012). The withe line shows the most probable path whereas the black lines, the range of this path. In B, the red squares indicate geological constraint points: (1) 20 ±2 0 °C at 27.5 ± 2.5 Ma considering as the exposition period of the Belén Metamorphic Complex; and (2) 100±20 °C at 21.5±3.5 Ma, as the burial period provoked by the Lupica Formation deposition over the BMC and associate volcanic activity.

5. Discussion

The ZHe ages in one sample are from 113 to 226 Ma, whereas in the other sample are from 20 to 49 Ma. These extremely different ages are related to the inclusion abundance and possible zoning in crystals. Zoning and heterogeneities in zircons of the BMC rocks have been reported by Wörner et al. (2000a), Loewy et al. (2004) and Arcos et al. (2016). Therefore, the geological interpretation of the ZHe ages is not possible. The AHe ages reported here for the BMC rocks (from 10.4 to 18.7 Ma) are similar to AHe and AFT ages (from 11 to 13 Ma) obtained by Horton et al. (2006) for the same rocks. Both studies are the unique that present AHe ages in the Western Cordillera of the Altiplano (14-22° S). Unfortunately, the work of Horton et al. (2006) is an abstract and the analytical data, ages, errors and modeling are not available, and integration and discussion of data are not possible.

The sample PM-116 gives an AHe weighted mean age of 12.1±2.2 Ma (MSWD=26), and the PM-118, 14.0±2.1 Ma (MSWD=60). For the sample PM-116, if the higher single-grain age of 15.1 Ma (PM-116-a01) is ignored by low Re, the weighted mean age results of 12.0±2.6 Ma with a MSWD of 27. For the sample PM-118, if the higher single-grain age of 18.7 Ma is ignored, the weighted mean age results of 13.6±1.7 Ma with a MSWD of 28, a value considerably lower than 60. In any cases, the MSWDs of 26-28 are high and data fail the chi-square test, thus, the weighted mean ages should be viewed with caution.

The slight positive correlation between single-grain AHe age and eU for four selected mesures in sample PM-116 suggests that the rock cooled slowly. We explore models considering geological constraints in the thermal history of the BMC: (1) rock temperature near to the surface of 20±20 °C at 27.5±2.5 Ma, as the exposition period of the BMC, and (2) rock temperature of 100±20 °C at 21.5±3.5 Ma, as the burial period provoked by the Lupica Formation deposition (2,500 m thickness) over the BMC and associate volcanic activity (FIG. 6B). If the geological constraints are added, the paths of T-t model between 15 and 0 Ma become more restricted compared to the T-t model without geological constraints (FIG. 6A). In this case, slow heating at 27-19 Ma and slow cooling after 15 Ma are more defined. The T-t modeling performed without geological constraints shows a slow cooling from 15 to 0 Ma, with a relatively more marked cooling period at 11-7 Ma. This is compatible with exhumation induced by tectonism and would permit to date the last activity of the Chapiquiña-Belén reverse fault, which uplifted the BMC block. The age agrees with exhumation age of the BMC by erosion, contractional deformation and uplift affecting the Western Cordillera and dated by stratigraphy and cross-cutting relationships from 18 to 6 Ma (García et al., 2004; Charrier et al., 2013). Additionally, the exhumation period coincides in part with the deformation causing the Oxaya anticline in the Precordillera at this latitude, dated between ca. 11.7 and 10.7 Ma (García and Hérail, 2005; García et al., 2017).

The middle-late Miocene thermal signal of this part of the Western Cordillera is relatively similar to that the eastern Altiplano (Eastern Cordillera and Subandean belt), according the AFT and AHe data reported there (Gillis et al., 2006; Ege et al., 2007; Barnes et al., 2012; McQuarrie et al., 2008; Perez et al., 2016; Anderson et al., 2018). However, this signal differs considerably from that the forearc (from the Coastal Cordillera to Precordillera) and the eastern Western Cordillera. At this latitude, in the first domain (forearc), the available AHe ages are from 24±8 to 43±4 Ma (Avdievitch et al., 2018), AFT ages are from 37±3 to 123±7 Ma (Schlunegger et al., 2006), and models indicates an extremely low exhumation rate, while in the second domain (eastern Western Cordillera, Uyarani basement), the available AHe and AFT ages are greater than 34 Ma (Horton et al., 2006; Barnes et al., 2012). The last ages are compatible with the early Oligocene exposition of the BMC and the Incaic orogeny that affected the arc and inner forearc (Steinman, 1929; Mégard, 1987; Noblet et al., 1996; Maksaev and Zentilli, 1999; Charrier et al., 2013; García et al., 2017). This suggests that during the Miocene the cooling-uplift of the western part of the Western Cordillera was coupled to the eastern Altiplano and decoupled from the forearc and eastern Western Cordillera. In contrast, before 18 Ma, in particular during the Incaic orogeny, the exhumation of the Western Cordillera seems to have coupled to the forearc. More (U-Th)/He analyses in the BMC and other geological units, and more thermo-chronometers, are needed to more robust conclusions on the exhumation and burial history of the Western Cordillera.

Note that 450 km south of the study area, in the Salar de Atacama basin (23-24° S), Oligocene-lower Miocene localized extension has been documented, probably associated with transtensional movements (Pananont et al., 2004; Jordan et al., 2007). This is apparently contradictory with the observed in our study area and surroundings. Higher precision on the age of the Oligocene (s.l.) events at 18-19° S and 23-24° S is necessary for correlation, as well as improved refinement on the spatial association between these different Andean segments.

6. Conclusions

The ZHe ages of the sample PM-118 belong to the Mesozoic, from 113 to 226 Ma, whereas the sample PM-116 yielded ZHe ages of the Paleogene, from 20 to 49 Ma, with three values at 31-35 Ma. The extremely different ages are related to inclusions and zoning in crystals, thus, their geological meaning is not clear.

The 11 AHe ages range from 10.4 to 18.7 Ma. The positive correlation between single-grain AHe age and eU in sample PM-116 suggests that the rock slowly cooled. The T-t modeling performed with the selected measures of sample PM-116 shows a slow cooling from 15 to 0 Ma, with a relatively more marked cooling period at 11-7 Ma. This is compatible with exhumation induced by tectonism and would permit to date the last activity of the Chapiquiña-Belén reverse fault that uplift the BMC rocks. The exhumation age agrees with period of contractional deformation and uplift dated independently from 18 to 6 Ma.

The middle-late Miocene thermal signal of the western part of the Western Cordillera is relatively similar to that the Eastern Cordillera and Subandean belt, and differs considerably from that the forearc (Coastal Cordillera to Precordillera) and eastern Western Cordillera. This suggests that during the Miocene the cooling-uplift of the western part of the Western Cordillera was decoupled from the forearc. In contrast, before 18 Ma, in particular during the Incaic orogeny, the exhumation of the Western Cordillera would have been coupled to the forearc.

Acknowledgments
This work resulted from a collaborative initiative between the AMTC-Universidad de Chile (ANID/PIA Project AFB180004) and Chilean Servicio Nacional de Geología y Minería (SNGM), in the context of the Geological Survey of the 1:100.000 scale Putre sheet. We thank especially to geologist M. Ladino from the SNGM for his interest in this study and collaboration. We thank to K. Gallagher for the access of QTQt v 5.6.0 (2017) program to infer thermal histories from low temperature thermochronology. Fructiferous discussions were held with colleagues A. Tomlinson and M. Farías. Edition of this paper was made by F. Martínez and W. Vivallo.

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