U-Pb Geochronology and Hf-O Isotopes of zircons from the Pennsylvanian Coastal Batholith, South-Central Chile

Katja Deckart1, Francisco Hervé1, 2, C. Mark Fanning3, Valeria Ramírez1,
Mauricio Calderón1 Estanislao Godoy1

1 Departamento de Geología, Universidad de Chile. Plaza Ercilla 803, Santiago, Chile.
kdeckart@cec.uchile.cl; mccaldera@gmail.com; egodoyster@gmail.com; valramirezm.82@outlook.com

2 Escuela de Ciencias de la Tierra, Universidad Andrés Bello, Sazié 2315. Santiago, Chile.
fherve@unab.cl

3 Research School of Earth Sciences, The Australian National University, Canberra 0200, Australia.
mark.fanning@anu.edu.au

 

 

The Chilean Coastal Cordillera (33°-40°S) com­­prises a distinctive lithotectonic association representative of subduction zone processes. Critically important indicators of subduction are the late Paleozoic accretionary complex and the calc-alkaline Coastal Batholith. Although their origin within a subduction setting as exemplified for the Neogene Chilean active margin (e.g., Barazangi and Isacks, 1976; Hager and O’Connell, 1978; Jordan &et al., 1983; Cahill and Isacks, 1992) seems straightforward (Hervé et al., 1988; Hervé et al., 2007a; Parada et al., 2007), the exact timing, nature and geodynamic significance of the Coastal Batholith in south-central Chile warrants further detailed investigations.
The Coastal Batholith is a large and elongate composite intrusive body within slightly older metamorphic rocks of the accretionary prism (Hervé et al., 2013). The sedimentary protolith of these country rocks was deposited and then shortly after buried to a depth of about 8 to 10 km (at about 3 kbar; Willner et al., 2005) where it was intruded by quartzdioritic to granitic magmas. Trench-ward shift of arc magmatism from an easterly location apparently occurred as Mississippian (early Carboniferous) detrital zircons are present in the sedimentary rocks of the accretionary prism (Hervé et al., 2013). This shift may be explained by global tectonic plate reorganization, or tectonic processes such as the steepening of the subducted oceanic slab due to subduction of older and colder oceanic lithosphere. Other possible geodynamic scenarios (e.g., subduction rollback), however, are explored.
In this study we report new SHRIMP U-Pb zircon ages on distinct portions of the largely north-south elongated Paleozoic Coastal Batholith and the first Hf-O isotope data for this batholith. The aim is to present new aspects to the timing and petrogenesis of the batholith and magmatism along this portion of the proto-Pacific Gondwana margin. Our goal is to demonstrate that the Coastal Batholith was emplaced in a short period of time and shows different petrogenetic features compared to those other cordilleran batholiths which were formed along tens or hundred million years through multiple pulses of magmatism (cf. Hervé et al., 2007b).

2. Geologic Background

The Coastal Batholith of south-central Chile crops out to the east of the accretionary metamorphic complex between 33°S and 38°20’S, jumping at 40°S further east into the Principal Cordilleran range (Fig. 1).

FIG. 1. Geological sketch map of central Chile between 33° and 40°S (modified from SERNAGEOMIN, 2003). Patterned zone represents the Coastal Batholith and rocks of the accretionary metamorphic prism. Additional age data are U-Pb zircon crystallization ages (*) of the plutonic protolith of gneisses from the main trace of the Liquiñe-Ofqui fault zone Hervé et al., 2013).

The accretionary complex is subdivided into two metamorphic series with distinct P/T conditions (Godoy, 1970; Aguirre et al., 1972). The high P-low T Western and the low P-high T Eastern Series (Hervé, 1977; Willner et al., 2005) located to the north of 40ºS have Carboniferous depositional ages (Hervé et al., 2013). The Eastern Series shows a metamorphic overprint at around 300 Ma (Willner et al., 2005) which is related to the intrusive event of the Coastal Batholith on its eastern side. To the south of 40ºS the Western Series shows a Permian depositional age and Permian to Triassic age range for metamorphism (Duhart et al., 2001; Hervé et al., 2013).
The Coastal Batholith is composed of calc-alkaline granitoid suites developed during late Paleozoic time, mainly between late Carboniferous and Early Permian (e.g., Cordani et al., 1976; Hervé, 1976; Hervé et al., 1988; Parada et al., 1991; Martin et al., 1999a; Ramírez, 2010). Also present in the Coastal Cordillera are gabbros and fayalite-bearing granites of Triassic age and classified as an anorogenic A-type granitoid suite (Vásquez and Franz, 2008) and some late to middle Jurassic two-pyroxene diorites, gabbros and hornblende-biotite tonalites (Godoy and Loske, 1988; Parada et al. et al., 1991; Hernández, 2006).
Furthermore, the subduction related plutonic event represented by the Subcordilleran Batholith in Patagonia, Argentina, indicates ages of the Triassic-Jurassic boundary (Rapela et al., 2003).

3. Previous age and isotopic data of the late Paleozoic Coastal Batholith

The Coastal Batholith intrusives 30 km north of the Quintay study area at Reñaca, yielded a Rb-Sr isochron age of 299±31 Ma (Hervé et al., 1988). At Valparaíso, 10 km to the south of Reñaca, a Rb-Sr isochron age of 296±5 Ma (Shibata et al., 1984) was obtained. South of Quintay, at Algarrobo, an age of 292±2 Ma and at Santo Domingo an age of 308±15 Ma was published by Hervé et al. (1988). Both represent ages of the northern part or the outcropping Coastal Batholith. For the same dated samples ⁸⁷Sr/⁸⁶Sr whole rock initial ratios range between 0.70582 and 0.70605. Earlier, Cordani et al. (1976) presented a late Paleozoic age of 319±17 Ma (K-Ar recalculated) and a Juarssic age of 169±12 Ma (K-Ar recalculated) at Quintay. The Paleozoic sample yields an initial ⁸⁷Sr/⁸⁶Sr isotope ratio of 0.7132, the Jurassic sample is less radiogenic with an initial ratio of 0.7038. Two more Paleozoic K-Ar ages are presented in Hervé et al. (1988) for Algarrobo intrusives on biotite and hornblende yielding 287±7 Ma and 296±7 Ma, respectively. At Quintay and Reñaca, two further Jurassic ages on biotite are published (Hervé et al., 1988), at 159±4 Ma and 156±1 Ma, respectively. An additional whole rock Rb-Sr age was likewise obtained yielding 167±14 Ma with an initial ⁸⁷Sr/⁸⁶Sr whole rock isotope ratio of 0.70412. Furthermore, Gana et al. (1996) and Wall et al. (1996) published for the coastal area between Valparaíso and Cartagena several Middle to Late Jurassic K-Ar mineral ages on plutonic rock units. The Jurassic ages were attributed to thermal rejuvenation in rocks associated to the intrusion of Jurassic bodies.
A U-Pb zircon crystallization age of 299±10 Ma for the Pomaire pluton further east (Gana and Tosdal, 1996), inland of Quintay, confirms the strong presence of Carboniferous (Pennsylvanian) rocks in this area (Cochoa Unit after Rivano et al. (1993). Earlier, Godoy and Loske (1988) published two U-Pb dates on zircon fractions in two distinct localities at Quintay; dates are 290 Ma (tonalite) and 309 Ma (gneissic granite). Additionally, Gana and Tosdal (1996) obtained a U-Pb zircon age of 214±1 Ma (Late Triassic) from the Cartagena gneissic diorite unit at Punta Suspiro, directly north of San Antonio harbor, about 25 km south of Algarrobo.
Hervé et al. (1988) published a late Carboniferous, Pennsylvanian age for a coastal Santo Domingo granite sample, some 35 km south of Algarrobo. The Rb-Sr age is 308±15 Ma, with a whole rock initial ⁸⁷Sr/⁸⁶Sr isotope ratio of 0.70582.
Further to the south, ca. 35 km northeast from the city of Concepción at the Dichato locality, a K-Ar biotite age of 306±6 Ma was published (Hervé et al., 1988). The same age (Rb-Sr isochron of 306±6 Ma) was obtained by Lucassen et al. (2004) in a diorite from the Cantera Giacomo, north of Concepción. Additionally at Concepción a biotite K-Ar age of 215±4 Ma (Triassic; Hervé et al., 1988) was indicated 85 km southwest of the city of Los Angeles and 70 km from the coastline at the Traiguén locality, a biotite K-Ar age of 298±4 Ma was published (Hervé et al., 1988).
Furthermore, in the same area, the Nahuelbuta Mountains National Park, Hervé et al. (1988) obtained a biotite K-Ar age of 284±5 Ma. Glodny et al. (2008) present two-point Rb-Sr ages of 286.3±4.2 Ma (biotite-feldspar age) and 306.8±4.5 Ma (feldspar-muscovite age) on two intrusive igneous rocks of the same Nahuelbuta National Park region.
Initial ⁸⁷Sr/⁸⁶Sr ratios (ranging between 0.7057 and 0.7098) and Nd values (between -2 and -4) were obtained for granitoids and enclaves in the Santo Domingo Complex (Parada et al., 1999). Similar enriched isotopic compositions were determined for granitoids of ca. 294 Ma (Hervé et al., 1988) from Nahuelbuta with initial ⁸⁷Sr/⁸⁶Sr ratios varying in the range of 0.705 to 0.715 and Nd values in the range of -2.5 and -7.5 (Lucassen et al., 2004). These results indicate a mixing of at least two isotopically different lithospheric sources, with crust similar in composition to the exposed metasedimentary host rocks (Lucassen et al., 2004).
In the high Andes of Central Chile, Pineda and Calderón (2008) established through U-Pb zircon data a Carboniferous age of 328.1±2.8 Ma for a biotite-muscovite granite (Los Molles granite) and a Permian age of 294.2±2.3 Ma for a hornblende-biotite tonalite (El Pangue Tonalite), which belongs to the Elqui Limari Batholith (30-31ºS).
Willner et al. et al. (2008) presented U-Pb zircon ages of 326±12 Ma and 294±4 Ma, Carboniferous and early Permian times, for two leucogranite pebble ages from the scarcely developed coastal accretionary prism between 31°-32°S. These authors indicate that initial Hf signatures on the same zircons range between +3.6 to -3.1, respectively. The crustal residence times are of 1.3-0.97 Ga, Meso- to Neoproterozoic, with slightly older model ages for the somewhat younger leucogranite samples. Furthermore, they suggest that Carboniferous samples are either the result of mixing of juvenile, mantle-derived magma with older crust or of recycling of crustal material distinct from the older magmatic products of the coastal accretionary system.
It is noteworthy that Martin et al. (1999b) dated late Paleozoic to Early Jurassic intrusive, volcanic and sedimentary rocks in the El Indio region close to the Argentinean border at 29-30°S. They interpreted their occurrence to be related to extensional processes that followed the cessation of Carboniferous to early Permian subduction along the western edge of Gondwana.

4. Analytical Procedures

4.1. SHRIMP U-Pb

Zircon grains were separated from whole rock samples using standard crushing, washing, heavy liquid (Sp. Gr. 2.96 and 3.3), and paramagnetic procedures. Hand selected zircon grains were placed onto double-sided tape, mounted in epoxy together with chips of the Temora reference zircon, sectioned approximately in half, and polished. Reflected and transmitted light photomicrographs were prepared for all zircons, as were cathodoluminescence (CL) Scanning Electron Microscope (SEM) images. These CL images were used to decipher the internal structures of the sectioned grains and to ensure that the ~20µm SHRIMP spot was wholly within a single age component within the sectioned grains.
The U-Th-Pb analyses were made using SHRIMP II at the Research School of Earth Sciences (RSES), The Australian National University, Canberra, Australia following procedures given in Williams (1998, and references therein). Each analysis consisted of 6 scans through the mass range, with Temora reference zircon grains analyzed for every three unknown analyses. The data have been reduced using the SQUID Excel Macro of Ludwig (2001). The Pb-U ratios have been normalised relative to a value of 0.0668 for the Temora reference zircon, equivalent to an age of 417 Ma (see Black et al., 2003). Uncertainty in the reference zircon calibration for each analytical session are given in the Table footnotes for each sample. Uncertainties given for individual analyses (ratios and ages) are at the one sigma level (Appendix 1). Tera & Wasserburg concordia plots, probability versus density plots with stacked histogram and weighted mean ²⁰⁶Pb/²³⁸U age calculations were carried out using ISOPLOT/EX (Ludwig, 2003). Weighted mean ²⁰⁶Pb/²³⁸U age calculation uncertainties are reported as 95% confidence limits. The geological time scale used is that of IUGS-ICS January 2013 (www.stratigraphy.org).

4.2. SHRIMP Oxygen

Oxygen isotope analyses were made using the RSES SHRIMP II fitted with a Cs source and electron gun for charge compensation following methods described by Ickert et al. (2008). The SHRIMP U-Pb analytical spots, craters approximately 20 µm in diameter by 1-2 µm deep were polished from the mount surface. The oxygen isotope analyses were then made on exactly the same location used for the U-Pb analyses. Oxygen isotope ratios were determined in multiple collector mode using an axial continuous electron multiplier (CEM) triplet collector, and two floating heads with interchangeable CEM - Faraday Cups. The Temora 2, Temora 3 and FC1 reference zircons were analysed to monitor and correct for isotope fractionation. The measured ¹⁸O/¹⁶O ratios and calculated δ¹⁸O values have been normalised relative to an FC1 weighted mean δ¹⁸O value of +5.4‰ (Ickert et al. et al., 2008). Reproducibility in the Duluth Gabbro FC1 reference zircon δ¹⁸O value ranged from ±0.266‰ to 0.509‰ (2 σ uncertainty) for the analytical sessions. As a secondary reference, zircons from the Temora 2 and Temora 3 zircons analysed in the same analytical sessions gave δ¹⁸O values of +8.2‰ and +7.59‰ respectively (2σ uncertainty), in agreement with data reported by Ickert et al. (2008) and unpublished data for the Temora 3 reference zircon.

4.3. LA-MC-ICPMS Lu-Hf

Lu-Hf isotopic measurements were conducted by laser ablation multicollector inductively coupled plasma mass spectroscopy (LA-MC-ICPMS) using the RSES Neptune MC-ICPMS coupled with a 193 nm ArF Excimer laser; similar to procedures described in Munizaga et al. (2008). Laser ablation analyses were performed on the same locations within single zircon grains used for both the U-Pb and oxygen isotope analyses. For all analyses of unknowns or secondary standards, the laser spot size was either ~47 µm or ~37 µm in diameter. The mass spectrometer was first tuned to optimal sensitivity using a large grain of zircon from the Mud Tank carbonatite. Isotopic masses were measured simultaneously in static-collection mode. A gas blank was acquired at regular intervals throughout the analytical session (every ≈10 analyses).
The laser was fired with typically 5-8 Hz repetition rate and 50-60 mJ energy. Data was acquired for 100 seconds, but in many cases only a selected interval from the total acquisition was used in data reduction. Throughout the analytical session several widely used reference zircons (91500, FC-1, Temora-2 and Mud Tank) were analysed to monitor data quality.
Signal intensity was typically ca. 5-6 V for total Hf at the beginning of ablation, and decreased over the acquisition time to 2 V or less. Isobaric interferences of ¹⁷⁶Lu and ¹⁷⁶Yb on the ¹⁷⁶Hf signal were corrected by monitoring signal intensities of ¹⁷⁵Lu and ¹⁷³Yb, ¹⁷²Yb and ¹⁷¹Yb. The calculation of signal intensity for 176Hf also involved independent mass bias corrections for Lu and Yb.

5. Results

5.1. Petrography

The nine studied samples are distributed within ca. 800 km distance along the Coastal Batholith, south-central Chile. Most important locality names referred to in the text are shown in figure 1. For exact localities and a summarized sample description refer to the corresponding Table 1.
The gneissic granodiorite (sample FO10-18), from the Playa Chica of the Quintay neighborhood, is composed of mainly plagioclase phenocrysts (35%), subordinately potassic feldspar (15%) and quartz (20%) with deformation lamellae that forms ribbons and subgrains (Hervé, 1976). Mafic minerals (30%) usually are biotite, hornblende and a variable amount of pyroxene. Some of these rocks contain hornblende with pyroxene inclusions. Abundant accessory minerals like zircon and apatite, subordinate amounts of titanite and epidote are present. This rock belongs to the Cochoa Unit of late Paleozoic age defined by Rivano et al. (1993).
The coarse-grained biotite-hornblende tonalite (sample FO10-99) from the Algarrobo locality is composed of quartz (30%), potassic feldspar (< 3%) interstitial to plagioclase (34%), biotite (18%) in automorphic crystals and some with zircon and apatite inclusions, and 15% of amphibole minerals. Amphibole occurs in automorphic poikilitic crystals including biotite, plagioclase and opaque minerals. Alteration minerals are chlorite in biotite, and epidote and white mica replacing plagioclase. Accessory minerals are opaques, zircon, apatite and allanite.
At Alcones, to the east of the Pichilemu coastal town, a biotite granite with cataclastic texture affecting all mineral constituents (sample FO10-08) was collected. The rock is composed of about 30% heavily deformed quartz with undulose texture and many sub-grains. Further on, the rock sample consists of 35% potassic feldspar in large crystals with poikilitic and many myrmekitic structures, 10% subidiomorphic plagioclase with deformed twin planes. Furthermore, the granite contains about 15% biotite mainly in deformed laths along microcataclastic bands. Accessory minerals are titanite, allanite and zircon.
A coarse grained cataclastic biotite granite sample (sample FO10-20) from Llongocura, 15 km to the east of Constitución, consists of large rounded quartz (35%) and the same amount of microcline crystals cut by tectonic microbreccias. Quartz is characterized by its undulose extinction. Microcline minerals often show myrmekite in the periphery. Plagioclase (25%) is present in smaller crystals frequently crushed in the microbreccias. Some minerals show sericitic or argillic alteration. Biotite flakes (5%) which occur in the microbreccias are altered to chlorite and prehnite. They frequently include apatite and zircon crystals.
The biotite quartz diorite (sample FO09-59) from the Punta Cullin locality, 60 km to the north of the city of Concepción, is mainly composed of 35% quartz and 40% plagioclase minerals, and some isolated biotite (25%) crystals. Primary plagioclase is mostly transformed to a dark aggregate, composed of very fine-grained epidote, chlorite and white mica. Biotite shows alteration to chlorite and has apatite and zircon inclusions.
The biotite granite (sample FO09-54) from the Tres Pinos locality in the Parque Nacional Nahuelbuta, is coarse-grained with an isotropic texture. It is composed of 30% quartz occurring as large rounded crystals with undulose extinction, 25% plagioclase laths with some crystals replaced by epidote and white mica, 25% interstitial microcline and 15% biotite which occurs in laths with zircon and apatite inclusions. Hornblende is less abundant (5%) and corroded, partially replaced by biotite.
At Antihuala, located in the Parque Nacional Nahuelbuta and about 30 km from the city of Angol, a medium-grained isotropic biotite hornblende granodiorite (sample FO09-53), is composed of 30% quartz, 25% microcline and 20% plagioclase. Biotite (15%) occurs in aggregates of smaller crystals. Amphibole (10%) appears in crystal clusters, some associated with biotite. Some biotite shows inclusions of zircon and apatite. Some plagioclase crystals are totally saussuritized. Accessory minerals are epidote and titanite.
From the southernmost locality of Ilihue, southern shore of the Lago Ranco and 70 km east of the town of La Unión, a medium grained isotropic biotite granodiorite (sample FO09-38) was collected. This belongs to the Principal Cordillera rather than the Coastal Range. It is composed of 30% quartz, 25% plagioclase, which is often included in big interstitial potassic feldspar (15%) in a monzonitic texture. Furthermore, biotite (20%) is mostly chloritized and contains occasionally titanite, prehnite and epidote. Amphibole (10%) is present in small euhedral prisms, with some chloritized rims. Plagioclase is mostly sericitized and encloses occasionally epidote and chlorite.
From the northernmost locality at the southern tip of Playa Grande at Quintay, sample FO09-205, belonging to the Laguna Verde Unit (Gana et al., 1996) of Jurassic age, is a quartz-bearing gabbro with hornblende and pyroxene. The sample is composed of 42% plagioclase (42%), quartz (3%), hornblende (45%), pyroxene (8%) and titanite (1%). Accessory minerals are apatite, allanite and zircon. Mafic and felsic bodies of this rock unit are intrusive into orthogneiss and granodiorites of Paleozoic age. South of Quintay town small lenses of metasedimentary rocks crop out, belonging to the Paleozoic Valparaíso Metamorphic complex. They comprise paragneissic bodies with hornstones and metaconglomerates (Hervé, 1976; Godoy and Loske, 1988).

5.2. Geochronology and Hf-O isotopes

The samples analyzed cover the north-south exposure of the Coastal Batholith between 33° and 40°S in south-central Chile (Fig. 1).
The gneissic granodiorite (FO10-18) yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 311.5±3.3 Ma (95% confidence limits); the MSWD=1.4 for nineteen of twenty-one areas analyzed on 18 zircon grains (Figs. 2a, b). The initial εHf_(i) values for FO10-18 are between -1.17 and -5.64. Two-stage Depleted Mantle model ages (T_DM2) range from 1.31 to 1.60 Ga indicating a significant crustal residence time with inferred separation from a Depleted Mantle source in the Mesoproterozoic. δ¹⁸O ratios range from 6.61‰ to 7.30‰ with an average at 6.9‰ (Table 2; Appendixes 1a, 2a).

FIG. 2. Weighted mean age calculation and Tera & Wasserburg concordia plot for the U/Pb SHRIMP II data of a+b) gneissic granodiorite (sample FO10-18, Quintay); c+d) tonalite (sample FO10-99, Algarrobo) and e+f) biotite granite (sample FO10-08. Alcones), respectively.

The coarse-grained tonalite with faint foliation (FO10-99) yielded an age of 308.9±2.4 Ma (2σ) with an MSWD of 1.2 for 19 of the 20 areas (Figs. 2c, d). The initial εHf_(i) values record a relatively restricted range between -0.06 and -1.72. TDM2 range from 1.24 to 1.35 Ga implying crustal separation from the depleted mantle (DM) was likewise in Mesoproterozoic time. The δ¹⁸O ratios are between 6.09 to 6.80‰, with an average of 6.4‰ for this sample (Table 2; Appendixes 1b, 2b).
The coarse grained biotite granite (FO10-08) has a calculated weighted mean age of 319.6±2.4 Ma (2) for 12 of the 22 analyzed zircon areas (Figs. 2e, f). The calculated MSWD is 1.1 represented by 12 of 22 analyzed zircon areas. A secondary age group lies around 329.9±3.7 Ma (MSWD=0.74; n=4). Initial Hf(i) values are negative ranging from -1.11 to -3.56 with a Mesoproterozoic TDM2. δ¹⁸O ratios vary mainly between 6.32 and 7.84‰ with an average value of 7.3‰, excluding from the average calculation two outliers of 8.79 and 9.05‰ (Table 2; Appendixes 1c, 2c).
The coarse grained biotite-granite with potassic feldspar phenocrysts of up to 3 cm in length (FO10-20) yielded an age of 300.8±2.4 Ma with an MSWD of 0.99 for 19 of 20 analyzed grains (Figs. 3a, b). Sample FO10-20 shows an initial Hf(i) values between +0.06 and -3.95 and TDM2 ranging from 1.23 to 1.46 Ga. The average δ¹⁸O ratio is 6.6‰ belonging to a range of 6.14 to 7.50‰ (Table 2, Appendixes 1d, 2d).

FIG. 3. Weighted mean age calculation and Tera & Wasserburg concordia plot of U-Pb SHRIMP II data of a+b) biotite granite (sample FO10-20. Llongocura); c+d) biotite quartz diorite (sample FO09-59, Cullín) and e+f) biotite granite (sample FO09-54, Tres Pinos), respectively.

The intensely fractured fine-grained biotite granodiorite with dark inclusions (FO09-59) yielded an age of 310.6±2.3 Ma (MSWD=1.3 for 21 of 22 areas analyzed: Figs. 3c, d). The initial εHf_(i) values range from -0.99 to -4.25 and yielded Mesoproterozoic TDM2 ages between 1.37 and 1.51 Ga. This granodiorite has the most elevated δ¹⁸O ratio ranging from 8.09 to 9.03‰; average of 8.6‰ (Table 2; Appendixes 1e, 2e).
Sample FO09-54, a biotite granite gives an U-Pb SHRIMP age of 309.2±2.2 Ma with an MSWD of 1.5 for nineteen of twenty analyzed grains (Figs. 3e, f). The initial εHf_(i) values are between +1.67 and -2.17 with slightly younger Mesoproterozoic TDM2 in the range 1.14 to 1.38 Ga. The δ¹⁸O ratios are mostly between 6.54 to 7.98‰, with one higher at 8.82‰; the overall mean value is 7.4‰ (Table 2; Appendixes 1f, 2f).
The biotite granite (FO09-53) yielded an U-Pb age of 310.2±2.5 Ma (2σ) with a MSWD of 1.3 calculated for 19 of 21 analyzed zircon areas (Figs. 4a, b). Sample FO09-53 indicates a narrow range of initial εHf_(i) values from +0.73 to -0.93. The calculated crustal residence time is the same as shown by the other sample sites indicating Mesoproterozoic TDM2 between 1.19 to 1.32 Ga. The δ¹⁸O ratio shows an average of 7.7‰ on all analyzed zircon crystals; the range is from 6.83 to 8.41‰ (Table 2; Appendixes 1g, 2g).

FIG. 4. Weighted mean age calculation and Tera & Wasserburg Concordia plot of U-Pb SHRIMP II data of a+b) biotite granite, (sample FO09-53, Antihuala); c+d) hornblende-biotite granitodiorite (sample FO09-38. Lilhue) and e+f) quartziferous gabbro (sample FO09-205. Quintay), respectively.

The biotite-amphibole granodiorite (FO09-38) yielded an age of 305.9±2.4 Ma with a MSWD of 0.98 from 22 over 24 areas analyzed (Figs. 4c, d). The initial εHf_(i) ranges from +0.14 to -2.40. The calculated T_DM2 record a similar Mesoproterozoic range (1.23 to 1.39 Ga) as seen in the other sample sites and, the overall mean δ¹⁸O ratio of 7.7‰ with a range from 6.98 to 8.71‰ (Table 2; Appendixes 1h, 2h).
The northernmost analyzed sample, a hornblende-pyroxene quartz-gabbro (FO09-205) contributed with an unexpected Middle Jurassic age 168.9±1.1 Ma for 18 selected spots (Fig. 4e, f; MSWD=1.04). The δ¹⁸O ratios range from 4.72 to 5.73‰ with an average of 5.3‰ equivalent to mantle value of 5.3‰. The initial εHf_(i) values range between +3.37 and +5.76. Crustal T_DM2 values are indicating Neoproterozoic residences times (Table 2; Appendixes 1i, 2i).
In summary, the eight Paleozoic granitoid samples corresponding to a north-to-south distance along the Coastal Batholith of ca. 800 km record a narrow range in crystallization ages between 300.8±2.4 Ma and 319.6±2.4 Ma, Pennsylvanian (late Carboniferous), with five of the granitoids within uncertainty at ~309 Ma (Fig. 5). The initial Hf-isotopic composition, εHf_(i) on the dated zoned igneous zircon areas range from +1.67 to -5.64 (Fig. 6a), but the majority of the analyses are between a more restricted range of +1 to -4 epsilon units. The T_DM2 all suggest Mesoproterozoic crustal residence ages, but with some subtle variations. The δ¹⁸O average ratios range from 6.4 to 8.6‰ for the Pennsylvanian samples (Fig. 6b) and in detail there is a more prominent group between 6.0 to 7.5‰ with a lesser group at 8.0 to 9.0‰. The observed minor data discrepancy within each isotope system does not validate any time-space tendency. Additionally, this data clearly shows that the magmas are derived from crustal sources, that is, those that have had a prominent sedimentary input in order to achieve the elevated δ¹⁸O values. However one cannot preclude the total absence of a mantle input. Together, the O and Hf data are in agreement with likely magmatic sources being older crustal components (Mesoproterozoic T_DM2), enriched in sedimentary components (elevated δ¹⁸O).

Fig. 5. Age probability plot for the Late Carboniferous Coastal Batholith granite to diorite samples between 30° and 40°S, Chile.

 

Fig. 6. a. Initial εHf versus δ18O diagram; b. Age (Ma) versus δ18O diagram.

 

The Middle Jurassic sample from the northernmost sample site provides a clear difference to the older intrusives: inferred crustal residence time is Neoproterozoic, initial εHf_(i) values are positive between +3.4 to +5.8 and δ¹⁸O average indicates a mantle value of 5.3‰.

6. Discussion and Conclusions

On the basis of our new data, the Pennsylvanian Coastal Batholith in south-central Chile is shown to have been emplaced in a very short period of time (20 Ma) and to be relatively homogeneous in crystallization age and both Hf and O isotopic compositions along 800 km length, regardless of rock types which vary from quartz diorite to granite. After a 150 Ma magmatic lull, Jurassic quartziferous gabbros of much more primitive isotopic composition were emplaced through the Paleozoic rocks of the Coastal Batholith in the northern study area. This evolution is very different to what is observed in the Mesozoic to Cenozoic batholiths in northern Chile (Dallmeyer et al., 1996) and in the South Patagonian batholith (Hervé et al., 2007b) which are typical Andean batholiths, the last with large age range -spanning from 155 to 5 Ma- and greater variation in Hf and O isotopic compositions (Fanning et al., 2010).
This suggests a relative homogeneous source with a significant continental crust component for these magmas. The two stage Depleted Mantle model age calculations all yield Mesoproterozoic ages which indicate that the source(s) for the magmas from which the zircon crystallized have had a significant residence time in the crust. If this was the case, then the magmatic arc would have been formed on continental crust (δ¹⁸O >5.3±0.6; Fig. 6), beyond the backstop of the accretionary complex to the west. This suggests that the Eastern Series, which the batholiths intrudes, was deposited over this continental crust or either has been tectonically transported from the west to its present position over the western edge of continental crust. Alternatively, part or all of the melting crustal material could correspond to sediments transported deep into mantle wedge by the subducted crust, as modeled by Gerya and Meilick (2011). The fast extinction of the arc, which appears to have lasted less than 20 Ma, was probably the consequence of changes in the subduction parameters, which displaced the melting conditions of the underlying crust from the site of the Coastal Batholith to the east. These changes could be related with the contraction of the Rheic Ocean that culminated with the collision of NW Gondwana and south-eastern Laurentia at ca. 320 Ma (McElhinny et al., 2003). It is interesting to consider that during the span in which the south-central Coastal Batolith was emplaced Gondwana remained in a relative stationary position (Oviedo and Vilas, 1984) in contrast with fast movements and rotations before and after that time (Geuna et al., 2010).
The Coastal Batholith was eroded to a similar level as the present one, before deposition of the Late Triassic (Norian) sedimentary rocks (Nielsen, 2005). Surprisingly, detrital zircon derived from the batholith are not found as constituents of the Permian accretionary prism which constitute the Western Series south of 38°S (Hervé et al., 2013), a probable indication that significant portions of the accretionary complex do not crop out. It is interesting to note that the evolution of the accretionary prism continued until the Triassic, when Late Triassic plutonic bodies beside some locally restricted post-subduction A-type granitoids intruded into the Western Series (Vásquez and Franz, 2008).

Acknowledgements
This work was financed through the Chilean Grant FONDECYT 1095099. We would like to thank J. Vargas and R.Valles at the Geology Department, Universidad de Chile, for the zircon separation. M. Solari and M. Vásquez accompanied some of the field trips. A. González, P. Castillo, E. Salazar, and F. Poblete helped taking difficult samples in the northern section of the Coastal Batholith. Thorough revisions by C. Casquet (Universidad de Madrid) and F. Lucassen (University of Bremen) greatly improved the original manuscript.

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