1. Introduction
The Eastern Andean Metamorphic Complex and the Chonos Metamorphic Complex, our study area in the Southern Patagonian Andes (Fig. 1), are composed of abundant metapelites, which have not been dated until now. The age determination of different metamorphic and/or tectonic eents in sedimentary rocks is mainly based on dating alumino-silicate clay minerals such as illite/muscovite or illite/smectite mixtures. Geochronological studies on diagenetic to very low-grade metamorphic pelitic rocks have been generally undertaken by applying the conventional K–Ar technique, but the heterogeneous character of grain-size, composition and origin of the clay minerals leads to serious limitations on the geological meaning of the obtained age.
Figure 1. Schematic geological map of Aysén region. The asterisks indicate the location of samples for dating by 40Ar–39Ar encapsulation technique. Unfilled circles indicate existing geochronological data in intrusive bodies.
The conventional 40Ar–39Ar dating technique usually reveals higher precision and more detailed information on the rock itself, but its application has been limited by reactor-induced 39Ar loss due to recoil in very fine-grained clay material of sedimentary rocks (e.g. Turner & Cadogan, Reference Turner and Cadogan1974; Onstott, Miller & Ewing, Reference Onstott, Miller and Ewing1994). Nevertheless, other detailed studies (Hunziker et al. Reference Hunziker, Hurley, Clauer, Dallmeyer, Friedrichsen, Flehmig, Hochstrasser, Roggwiler and Schwander1986; Kligfield et al. Reference Kligfield, Hunziker, Dallmeyer and Schamel1986; Reuter & Dallmeyer, Reference Reuter and Dallmeyer1987, Reference Reuter, Dallmeyer, Daly, Cliff and Yardley1989; Dallmeyer et al. Reference Dallmeyer, Reuter, Clauer, Liewig and Gayer1989) have shown that recoil-related 39Ar loss is not of major importance in every case.
The introduction of the encapsulation technique for 40Ar–39Ar geochronology allows the collection of the total of 39Ar loss from the sample during irradiation and permits recalculation of the obtained age (retention age, omitting the portion of 39Ar loss) and the encapsulated total gas age or so-called integrated age (including 39Ar loss) (Dong et al. Reference Dong, Hall, Peacor and Halliday1995). In this way, the recoil effect is monitored and the encapsulated total gas age yields an age comparable to a conventional K–Ar age. Hence, an estimate of the true crystallization age of the sample can be made (York, Evensen & Smith, Reference York, Evensen and Smith1992). On the other hand, the retention age is equivalent to a traditional unencapsulated metapelite 40Ar–39Ar age. In contrast, Dong et al. (Reference Dong, Hall, Peacor and Halliday1995) suggested that the obtained encapsulated total gas age might be younger than the time of mineral growth during low-grade metamorphic conditions. They postulated that K (40Ar*) occupies similar incoherent boundary sites to 39Ar in illite packages and therefore will be released as easily under room temperature as 39Ar during irradiation. This implies that the retention age (Dong et al. Reference Dong, Hall, Peacor and Halliday1995) is closer to the true (re-)crystallization age, in disagreement with the assumption made earlier by York, Evensen & Smith (Reference York, Evensen and Smith1992). More recently, Lin, Onstott & Dong (Reference Lin, Onstott and Dong2000) employed microanalytical techniques on fine-grained minerals and suggested that 39Ar recoil loss occurs from within coherent lattice domains and as a consequence, the assumption by Dong et al. (Reference Dong, Hall, Peacor and Halliday1995) should overestimate the retention age. They favour the encapsulated total gas or integrated age as the true crystallization age of the mineral.
Discussion on whether or not 39Ar or 40Ar* has been released during irradiation or under room temperature requires more detailed microanalytical studies, which has not been the aim of the study presented here. The emphasis was rather on comparing new data with the existing geochronological information of the study area and its implications.
This study presents the first 40Ar–39Ar encapsulation data on metapelites from the Eastern Andean and Chonos metamorphics complexes. The Kübler Index (KI) of <2 μm illite fractions from metapelitic rocks of each study area allows comparison of the KI values with the obtained 40Ar–39Ar encapsulation ages. This allows the latest thermal event affecting each metamorphic complex in the southern Andes to be interpreted.
2. Geological setting
Between 46° and 49°S latitude, the Aysén region of southern Chile is characterized mainly by two metamorphic complexes, which are covered by Mesozoic and Cenozoic volcanic and sedimentary units and intruded by the Meso-Cenozoic North Patagonian Batholith (Fig. 1).
The Eastern Andean Metamorphic Complex is composed of schists, marbles, metasandstones and etapelites. This complex is divided into two units (Lagally, Reference Lagally1975), Lago General Carrera and Lago Cochrane, according to lithological differences and structural deformation styles. The oldest stratified unit unconformably overlying the Eastern Andean Metamorphic Complex rocks is the Late Jurassic Ibañez Formation (rhyolitic to dacitic pyroclastic rocks and lava flows).
The Chonos Metamorphic Complex is composed of metasedimentary and metabasaltic rocks. Hervé et al. (Reference Hervé, Mpodozis, Davidson and Godoy1981) distinguished two zones or belts: an eastern belt in which primary structures are preserved and a western belt with no preserved primary structures. The North Patagonian Batholith (Meso-Cenozoic) is characterized by a granitic to monzonitic composition and intrudes the eastern margin of the Chonos Metamorphic Complex and the western margin of the Eastern Andean Metamorphic Complex.
Both complexes were affected by greenschist facies metamorphism (Willner, Hervé & Massonne, Reference Willner, Hervé and Massonne2000; Ramírez & Sassi, Reference Ramírez and Sassi2001) of yet unknown age and show evidence of multiple deformational and metamorphic events revealed by complex folding and deformation of at least two foliation planes.
The Chonos Metamorphic Complex is considered to represent a subduction zone assemblage. In contrast, the Eastern Andean Metamorphic Complex is considered to represent a passive margin succession (Augustsson et al. Reference Augustsson, Münkers, Bahlburg and Fanning2006; Faúndez, Hervé & Lacassie, Reference Faúndez, Hervé and Lacassie2002).
According to ages compiled from Bell & Suarez (Reference Bell and Suárez2000) and Thomson & Hervé (Reference Thomson and Hervé2002), the depositional ages from the Eastern Andean Metamorphic Complex vary between Early Devonian and Triassic, whereas the age of metamorphism is not clearly constrained. Thomson & Hervé (Reference Thomson and Hervé2002) determined a post-metamorphic cooling age of 250 Ma (fission track – FT, zircon) and a U–Pb SHRIMP age on detrital zircon of 354±10 Ma interpreted as a possible maximum depositional age.
In the Chonos Metamorphic Complex, Fang et al. (Reference Fang, Boucot, Covacevich and Hervé1998) determined a Late Triassic (c. 220 Ma) depositional age through the presence of monotis fossils. A U–Pb SHRIMP age on a detrital zircon grain of 207±6 Ma was interpreted as indicative of a maximum stratigraphic age of 213 Ma (Thomson & Hervé, Reference Thomson and Hervé2002). Thomson, Hervé & Fanning (Reference Thomson, Hervé and Fanning2000) determined an age of 200 Ma (FT, zircon) which they and Thomson & Hervé (Reference Thomson and Hervé2002) interpret as indicating post-metamorphic cooling below 260±40°C.
3. Methodology
Seven metapelitic samples were analysed. Two samples from the western belt of the Chonos Metamorphic Complex, one from Italia Island (ITA8-2) and one from Leucayec Island (LEU5-3), and five (50302, 2404, 2105, 1903 and 3-2-3) from the Eastern Andean Metamorphic Complex (Fig. 1).
Sample preparations for Kübler Index determination and encapsulated 40Ar–39Ar geochronology were undertaken at the Geology Department, University of Chile. Samples of less than 2 μm grain-size were separated from the metapelites employing the methodology for Kübler Index determination described in Ramírez et al. (Reference Ramírez-Sánchez, Hervé, Kelm and Sassi2005). KI standards SW-1, SW-2, SW-4 and SW-6 were used, as recommended and described by Warr & Rice (Reference Warr and Rice1994).
The 40Ar/39Ar analyses were carried out at the Radiogenic Isotope Geochemistry Laboratory at the University of Michigan, Ann Arbor, following the preparation and analytical procedure described in Dong et al. (Reference Dong, Hall, Peacor and Halliday1995). The standard used was the MMhb-1 hornblende of Samson & Alexander (Reference Samson and Alexander1987). Isotopic ratios were analysed using a VG-1200s Mass Spectrometer with Daly detector. Duplicates were determined for samples 1903, 2105, 2404, ITA8-2 and LEU5-3. Absolute and stratigraphic ages are referenced according to the Geological Time Scale (Gradstein et al. Reference Gradstein, Ogg, Smith, Agterberg, Bleeker, Cooper, Davydov, Gibbard, Hinnov, House, Lourens, Luterbacher, McArthur, Melchin, Robb, Shergold, Villeneuve, Wardlaw, Ali, Brinkhuis, Hilgen, Hooker, Howarth, Knoll, Laskar, Monechi, Plumb, Powell, Raffi, Röhl, Sadler, Sanfilippo, Schmitz, Shackleton, Shields, Strauss, Van Dam, van Kolfschoten, Veizer and Wilson2005).
4. Results
4.a. Kübler Index (KI)
The analysed material of each sample comprises a fine 2 μm grain fraction of chlorite and mica (illite/muscovite), with some quartz and albite (Fig. 2).
Figure 2. (a) Microphotography of SEM from sample 50302. Mica and chlorite are orientated to the foliation plane; quartz band is recrystallized. (b) Microphotography of SEM from sample 2404, item A. (c) Photography of optical microscope from sample LEU5-3, showing two foliations S1 and S2.
The metamorphic grade of the Eastern Andean Metamorphic Complex determined by the KI method indicates epizone grade for samples 50203, 2404, 2105 and 1903 ((0.22°Δ2θ); (0.23°Δ2θ); (0.15°Δ2θ); (0.22°Δ2θ) Kübler Index, respectively). The southernmost sample (3-2-3) yields a KI of 0.45°Δ2θ, representing diagenetic grade (Table 1).
Table 1. 40Ar–39Ar encapsulation ages obtained from the Eastern Andean Metamorphic Complex (samples listed from north to south)
KI – Kübler Index.
From the Chonos Metamorphic Complex, sample LEU5-3 indicates an epizonal grade (0.18°Δ2θ) and ITA8-2 shows an anchizone grade (0.26°Δ2θ) (Table 2).
Table 2. 40Ar–39Ar encapsulation ages obtained from the Chonos Metamorphic Complex (samples listed from north to south)
KI – Kübler Index.
According to the thermal data inferred by the Kübler Index, the epizonal samples were subjected to temperatures over 300°C, whereas the diagenetic samples were subjected to temperatures under 200°C. Anchizone samples represent temperatures between 200 and 300°C (Warr & Rice, Reference Warr and Rice1994).
4.b. 40Ar–39Ar encapsulation data
4.b.1. Eastern Andean Metamorphic Complex
The amount of 39Ar loss corresponds to the 39Ar which was captured in a quartz vial during irradiation and then analysed. For the five metapelite samples of the Eastern Andean Metamorphic Complex with KI index values ranging between 0.15 and 0.45 (°Δ2θ), the amount of 39Ar loss varies from 21 to 25%, and shows no correlation with increasing metamorphic grade (Fig. 3).
Figure 3. Correlation diagram of Kübler Index (KI) and % 39Ar released from analysed samples of Eastern Andean Metamorphic Complex (EAMC) and Chonos Metamorphic Complex (CMC). Also shown are data from Dong et al. (Reference Dong, Hall, Peacor and Halliday1995) and Hall et al. (Reference Hall, Higueras, Kesler, Lunar, Dong and Halliday1997).
The retention ages from the Eastern Andean Metamorphic Complex are listed from north to south in Table 1. Samples located to the north of Lake Cochrane show Middle Jurassic ages, whereas samples located to the south of Lake Cochrane show Early Jurassic, Late Triassic and Permian ages. The diagenetic grade sample (0.45°Δ2θ) yields an age of 281.7±1.6 Ma and represents the oldest age obtained.
The total gas ages from the same samples show similar variation but are generally younger than the retention ages, varying from Early Cretaceous to Late Triassic (Table 1).
4.b.2. Chonos Metamorphic Complex
The amount of 39Ar loss in samples ITA8-2 (epizone) and LEU5-3 (anchizone) is 13 and 22%, respectively (Table 2).
The retention age obtained from the Leucayec Island metapelite indicates an Early Jurassic age and that from Italia Island a Middle Jurassic age. The total gas ages from the same samples are both Late Jurassic–Early Cretaceous (142–146 Ma). Retention ages from the same samples are thus 20–40 Ma older than the total gas ages.
5. Discussion
5.a. KI versus 39Ar loss
The relation between KI and the amount of 39Ar loss shows a very limited correlation. This observation clearly contradicts the findings of Dong et al. (Reference Dong, Hall, Peacor and Halliday1995) and Hall et al. (Reference Hall, Kesler, Simon and Fortuna2000), who found that a decrease in the amount of 39Ar loss is correlated with increasing metamorphic grade (decrease in KI).
Our studied samples were most likely affected by several metamorphic overprints, such as burial, regional and/or contact metamorphism. Therefore, KI values obtained from the smallest minerals will indicate the metamorphic grade from the latest metamorphic or, in a more general sense, latest thermal event affecting each study area. Various metamorphic overprints registered in one sample might contradict the relatively straightforward relation of 39Ar loss to changes in the associated metamorphic grade as presented by, for example, Dong et al. (Reference Dong, Hall, Peacor and Halliday1995). Furthermore, since nearly all analysed samples (except 2105 and 3-2-3 from the Eastern Andean Metamorphic Complex) were collected close to plutons or small intrusions, they might not reflect burial or regional metamorphism. Instead, they are suggested to represent contact metamorphic conditions which might not produce KI in the same way and therefore do not fit into the graphical interpretation presented by Dong et al. (Reference Dong, Hall, Peacor and Halliday1995) (Fig. 3).
5.b. Geochronology of the area
5.b.1. Age of Eastern Aandean Metamorphic Complex
The retention ages from the Eastern Andean Metamorphic Complex vary from north to south (Fig. 4). The Cochrane Lake approximately divides the Eastern Andean Metamorphic Complex into two areas (Lagally, Reference Lagally1975). Towards the north (Puerto Tranquilo), the retention ages are 164.8±0.4 Ma (50302) and 166.0±0.3 Ma (2404; Lago Plomo) (Middle Jurassic) and total gas ages are 131.1±0.3 Ma (50302) and 128.6±0.2 Ma (2404), all younger than in the southern area. The oldest ages are obtained for the southern area (3-2-3; Bravo River), with a retention age of 281.7±0.8 and a total gas age of 223.2±0.7 Ma (Permian–Triassic).
Figure 4. Schematic variation of 40Ar–39Ar encapsulated ages, north to south from Eastern Andean Metamorphic Complex. The small graph shows the retention age (Ma) versus fraction of 39Ar released (Hall et al. Reference Hall, Kesler, Simon and Fortuna2000), including total gas age and Kübler Index (KI) value of the sample. The height of the small square indicates the approximate error of the age of the intrusive. 1 – K–Ar age on biotite minerals collected near Puerto Tranquilo. 2, 3 – K–Ar age on biotite next to Lake Plomo (Suarez & De la Cruz, Reference Suárez and De La Cruz2001). 4 – Ar–Ar age on biotite at Cerro Esmeralda (Parada, Palacios & Lahsen, Reference Parada, Palacios and Lahsen1997). Minimum depositional ages from Augustsson et al. (Reference Augustsson, Münkers, Bahlburg and Fanning2006) and maximum depositional ages from Thomson, Hervé & Fanning (Reference Thomson, Hervé and Fanning2000); units according to Augustsson et al. (Reference Augustsson, Münkers, Bahlburg and Fanning2006). Ages of volcanic units from Pankhurst et al. (Reference Pankhurst, Riley, Fanning and Kelley2000).
In the southern section, Thomson, Hervé & Fanning (Reference Thomson, Hervé and Fanning2000) interpret the U–Pb age of c. 350 Ma on detrital zircon collected close to O'Higgins Lake, southern Bravo River, to represent the maximum depositional age of the Cochrane unit from which sample 3-2-3 was collected. The minimum depositional age of this area is 270 Ma (FT, detrital zircon) (Thomson, Hervé & Fanning, Reference Thomson, Hervé and Fanning2000). The latter age is close to but not concordant with the retention age of sample 3-2-3 and can be interpreted as a metamorphic age of the lithological unit. In contrast, the total gas age of 3-2-3 is younger than the minimum depositional age and cannot be related to any known geological or thermal event in the area.
The northernmost sample (2105) from the area south of Lake Cochrane yields a retention age of 222.0±0.3 Ma and a total gas age of 176.2±0.3 Ma. Whereas the total gas age cannot be related to any known geological event, the retention age is only slightly younger than the minimum depositional age given by zircon FT data of 238 Ma (Augustsson et al. Reference Augustsson, Münkers, Bahlburg and Fanning2006) and is concordant with the recognized metamorphism of Late Permian to Middle Triassic age.
At about 5 km distance from sample 1903, an intrusive tonalite yields U–Pb zircon and K–Ar (biotite) ages of 155±10 Ma and 156–157 Ma, respectively (Middle Jurassic–Late Jurassic: Parada, Palacios & Lahsen, Reference Parada, Palacios and Lahsen1997), which were interpreted as crystallization ages. Two younger whole rock K–Ar ages from the same tonalite of 140±4 and 142±5 Ma have been interpreted as ages of a later stage of mineralization (Parada, Palacios & Lahsen, Reference Parada, Palacios and Lahsen1997). The latter ages are concordant with the total gas age of 145.3±1.4 Ma of sample 1903, whereas the retention age of the metapelitic sample (191.6±0.9 Ma) is older than the intrusion ages and apparently has no geological significance.
Intrusive bodies located about 10 km to the north of metapelite samples 50302 and 2404 (Fig. 4) suggest a close relationship between this intrusive event and the obtained encapsulation 40Ar–39Ar ages, suggesting contact rather than burial or regional metamorphism. Jurassic plutons of the General Carrera and Plomo Lakes yield ages varying from 140 Ma to 155 Ma (Fig. 4) (Suárez & De la Cruz, Reference Suárez and De La Cruz2001). The total gas ages of samples 50302 and 2404 (128.6 Ma to 131.1 Ma), whereas the retention ages (164.8 Ma to 167.3 Ma) are older than the postulated intrusion ages and are thus apparently geologically meaningless.
5.b.2. Age of the Chonos Metamorphic Complex
A zircon FT age of 210±12 Ma (Thomson, Hervé & Fanning, Reference Thomson, Hervé and Fanning2000) was obtained in the eastern belt, which is interpreted to represent the minimum depositional age of the Chonos Metamorphic Complex. The metapelite sample LEU5-3 yields a retention age of 180.5±0.4 Ma, whereas the total gas age is 142.3±0.3 Ma. The latter age is concordant with geochronological Rb–Sr (whole rock) ages of 137.5±6.5 Ma, and 140.1±6.3 Ma obtained from plutons at Melchor and Luz Islands, respectively (Fig. 5; Pankhurst et al. Reference Pankhurst, Weaver, Hervé and Larrondo1999) and in consequence reflects the time of this intrusion event of the North Patagonian Batholith. The retention age from Chonos Metamorphic Complex sample LEU5-3 is slightly younger than the suggested minimum depositional age and apparently without geological meaning.
Figure 5. Schematic variation of 40Ar–39Ar encapsulated ages from Leucayec and Italia islands, Chonos Metamorphic Complex. The small graph shows the retention age (Ma) versus fraction of 39Ar released (Hall et al. Reference Hall, Kesler, Simon and Fortuna2000) including total gas age and Kübler Index (KI) value of the sample. Depositional ages according to Fang et al. (Reference Fang, Boucot, Covacevich and Hervé1998), and Hervé, Fanning & Pankhurst (Reference Hervé, Fanning and Pankhurst2003). Intrusive ages of Victoria (132.8±6.5), Melchor (137.6±6.0) and Luz (140.1±6.3) islands from Pankhurst et al. (Reference Pankhurst, Weaver, Hervé and Larrondo1999).
Sample ITA8-2 experienced anchimetamorphic temperatures between 200 and 300°C, and gives a retention age of 168.8±0.4 Ma and a total gas age of 146.5±0.4 Ma. Whereas total gas ages from LEU5-3 and ITA8-2 are concordant and are very close to or partly overlap the intrusive ages quoted above, the retention ages are discordant and not related to any known geological or thermal event of the area and are therefore possibly meaningless.
6. Conclusions
The encapsulated 40Ar–39Ar ages obtained from the Eastern Andean Metamorphic Complex, except those from Lake Cochrane (2105) and the Bravo River (3-2-3), are younger than the expected metamorphic ages in both study areas. Both the Chonos Metamorphic Complex and the Eastern Andean Metamorphic Complex were polydeformed, and thus very possibly metamorphosed before the intrusion of the North Patagonian Batholith. The white micas in the analysed rocks are well oriented along the main foliation planes in the metapelites.
Previous investigations through FT dating of detrital zircons (Thomson, Hervé & Fanning, Reference Thomson, Hervé and Fanning2000) have determined that cooling below the 260±40°C closure temperatures of zircon takes place at 200 Ma in the Chonos Metamorphic Complex and 270 Ma in the Cochrane unit of the Eastern Andean Metamorphic Complex. In Thomson & Hervé (Reference Thomson and Hervé2002), it is noted that FT zircon ages are partly reset at a temperature above 240±20°C with a possible total reset at 310±20°C. The obtained results are in accordance with the Late Triassic depositional age of the Chonos Metamorphic Complex and with the Late Devonian–Early Carboniferous depositional age of the Eastern Andean Metamorphic Complex. The North Patagonian Batholith shows an Early Cretaceous age at its western margin (Pankhurst et al. Reference Pankhurst, Weaver, Hervé and Larrondo1999), where it is in contact with the Chonos Metamorphic Complex and is characterized by some Late Jurassic satellite bodies near its eastern margin, where it intruded the Eastern Andean Metamorphic Complex.
Closed system behaviour for Ar retention on fine-grained authigenic illite minerals is suggested at a temperature below 150°C (Hunziker et al. Reference Hunziker, Hurley, Clauer, Dallmeyer, Friedrichsen, Flehmig, Hochstrasser, Roggwiler and Schwander1986). In this case it can be expected that the thermal history of the Chonos Metamorphic Complex and Eastern Andean Metamorphic Complex metapelites close to and reset by intrusions (plutons and stocks) did not reach temperatures higher than 150°C, from the intrusive emplacement until present times.
The good correlation of the intrusive ages with the obtained encapsulated 40Ar–39Ar total gas ages in the metapelites suggests that the latter reflect the thermal resetting of the country rock ages produced during emplacement of nearby intrusions. However, this process did not generate obvious contact metamorphic recrystallization textures in the observed rocks, nor did it produce a resetting of the zircon FT ages. The latter is not necessarily contradictory since the analysed illite and zircon minerals are not from the same sample. Resetting did not affect the southernmost and lower grade sample in the Eastern Andean Metamorphic Complex. The encapsulated 40Ar–39Ar retention age might therefore indicate the ‘true’ age of diagenesis for this sample, that is, 281.7±1.6 Ma.
40Ar–39Ar encapsulation data on metapelites could not discriminate which of the retention or total gas ages generally represent geologically meaningful burial or regional metamorphic ages. Nevertheless, we can suggest with the data presented herein, that encapsulated total gas ages represent contact metamorphic ages and therefore are geologically meaningful in the metapelites of anchi- and epizonal grades that are in close relationship with intrusive bodies. In contrast, retention ages are related to a metamorphic event, in this case of diagenetic grade, only when the sample is at a great distance from any intrusion, and therefore might in that case represent ‘true’ stratigraphic ages.
Acknowledgements
This study was supported by a FONDECYT 2990024 Project grant to ER-S. The authors would like to thank Chris Hall for carrying out geochronological analytical work and for his comments. The authors are grateful to Carita Augustsson and Stuart Thomson for their comments and reviews of the manuscript. The authors would like to thank J. Le Roux for checking the English.
) spacing: a first contribution from Eastern Andean Metamorphic Complex (Andes, Chile)
