Barren Miocene granitoids in the Central Andean metallogenic belt , Chile : Geochemistry and Nd-Hf and UPb isotope systematics

Four Middle-to-Late Miocene barren plutonic complexes that occur between the giant porphyry copper deposits of the central Chilean Andes were selected for U-Pb LA-ICPMS geochronology and Hf-isotope systematics on single zircon grains. Major and trace elements and Sr-Nd-Hf isotope whole rock geochemical studies were undertaken to compare with slightly younger or coeval barren and fertile intrusive rocks between 32° and 34°S. The studied granitoids yield resolvable crystallization ages of 11.3±0.1 Ma (Cerro Mesón Alto massif), 10.3±0.2 Ma (La Gloria pluton), 14.9±0.2 Ma/14.9±0.1 Ma (Yerba Loca stock) and 11.2±0.1 Ma/14.7±0.1 Ma (San Francisco Batholith). Major and trace elements discard an adakitic signature as suggested for coeval porphyric intrusions at 32°S, slightly younger mineralized porphyries at Río Blanco-Los Bronces deposit and other Cenozoic adakites. Volcanic host rocks are less fractionated than the intrusive rock units. The same observation can be made for the unmineralized northern plutons compared to the southern ones. Initial Sr-Nd isotope data show insignificant variation (0.703761-0.704118 and 0.5127580.512882), plotting in the mantle array. Trace element enrichment can be explained by addition of subducted-slab fluids and/or terrigenous sediments to the mantle wedge prior to and/or slight crustal input during magma ascent. Zircon grains separated from these barren intrusives share a similar initial εHf-data variation for the younger age group (10-12 Ma; 7.04-9.54) and show a more scattered range for the older one (14-15 Ma; 8.50-15.34); both sets plot between the DM and CHUR evolution lines. There is evidence that magma evolution was slightly distinct through time from older to younger barren magmatism, compared to a few fertile porphyritic rocks from Río Blanco-Los Bronces porphyry copper deposit. It is suggested that chronological inconsistencies within these complexes might be related to differential shortening across the NE-SW-trending Yeso Valley transfer fault, assumed as coeval, which also explains the local lack of easterly magmatic arc front migration.


Introduction
Miocene-Pliocene magmatism in the Andean Cordillera of central Chile between latitudes 32ºS and 34ºS has been of major interest since it hosts world-class porphyry Cu(-Mo) deposits.Studies of magma source based on the Sr and Nd isotopic systematics of mineralized plutonic complexes have revealed the involvement of magmas having component signatures characteristic of mantle, subduction, lower and/or upper crust.Several authors (e.g., Stern, 1991;Stern and Puig, 1991;Skewes et al., 1991;Skewes and Stern, 1994;Kay and Mpodozis, 2001) have discussed the migration and petrogenetic evolution of the magmatism, suggesting that it was directly linked to the changing geometry of the subduction zone.The aim of these discussions was to develop a model for mineralization occurring in flat-slab areas of the Andean Cordillera.In contrast, the associated barren magmatism has generally not been considered, having been investigated separately by Kurtz et al. (1997), Kay et al. (1999) and Kay et al. (2005).This paper adds geochronological, geochemical and isotopic data to the discussion of petrogenetic processes involved in the Miocene-Pliocene barren magmatism of the central Andean Cordillera.The new data set pretends to help and better understand the evolution of barren to fertile magmas related to subduction dynamics.
The geochronological and geochemical data presented here focus on four selected barren intrusive units of Miocene age, from south to north: Cerro Mesón Alto massif (CMA, 33°40'S), La Gloria pluton (LGP, 33°27'-35'S), Yerba Loca stock (YL, 33°15'-18'S) and San Francisco Batholith (SFB, 33°10'S).They were emplaced between two supergiant Mio-Pliocene porphyry Cu (-Mo) deposits in the central Andes (Fig. 1).The first U-Pb and Hf-isotope LA-ICPMS data on single zircon grains and major and trace elements as well as Sr-Nd-Hf isotope data on whole rocks from these barren plutonic complexes presented here, allow to make progress on understanding the nature of the involved magma source(s) and their evolution in time.This new data set is compared with published analyses and interpretations for coeval, barren intrusions and slightly younger fertile porphyric rocks from the central Andean Cordillera.
Sample collection was undertaken in a limited region between 33° and 34°S, but the discussion addresses the entire Miocene-Pliocene metallogenic belt of the Main Andean Cordillera (Principal Cordillera; 32°-34°S) in central Chile.

Plutonism at 33°-34°S: current interpretation
Miocene-Pliocene central Andean plutonic rocks are located at the southern limit and immediately south of the flat-slab segment (ca.29°-34°S), which separates the central volcanic zone from the southern volcanic zone.This ca.32°-34°S-70°00'-30'W belt hosts three major Miocene-Pliocene porphyry Cu(-Mo) deposits, from north to south, Los Pelambres, Río Blanco-Los Bronces and El Teniente (Fig. 1).Barren granitoids or smaller apophyses align with the three deposits.Mineral deposits along the Andean Cordillera may be linked to the convergence of the oceanic Nazca and South American continental plates (e.g., Sillitoe, 1974;Davidson and Mpodozis, 1991) and a causal relation between subduction and the nature (barren or fertile) of the associated magmatism has been proposed by e.g., Richards (2005).
In the study area, barren and mineralizationhosting granitoids of the San Francisco Batholith and the Teniente Plutonic Complex (cf.Kurtz et al., 1997;Kay et al., 2005), as well as smaller apophyses FIG. 1. Geological map with sample locations.Modified after Rivano et al. (1990).intrude either the Miocene Farellones Formation or the Eocene to earliest Miocene Abanico Formation.
The generally assigned age of this plutonic magmatism is Middle Miocene (Table 1), but various unmineralized stocks lack precise geochronological information.
Of the studied plutonic units, the Cerro Mesón Alto massif (CMA), La Gloria pluton (LGP) and San Francisco Batholith (SFB) have been previously dated (Table 1), but not the Yerba Loca stock (YL).A single K-Ar age on biotite of 9.8 Ma (R.E.Drake, unpublished) has been presented for the La Gloria pluton by Cornejo and Mahood (1997) and interpreted as a cooling age, possibly close to its crystallization age.The Cerro Mesón Alto granitoid, which represents the intrusive lower unit of the andesitic mountain peak, was dated by Kurtz et al. (1997), who named it after its outcrop south of 'Lago Yeso' rather than after its outcrop as a part of the Cerro Mesón Alto, and assigned it to the Teniente Plutonic Complex on the basis of a hornblende 40 Ar/ 39 Ar age of 12.4±2.5Ma.Published U-Pb ages on the San Francisco Batholith show a range of 8 to 12 Ma (in Deckart et al., 2005); the younger zircon ages of 8-9 Ma were for samples from the Río Blanco-Los Bronces mine area and were interpreted as late, but still pre-mineralization (mineralization from ca. 6.3-4.3Ma; Deckart et al., 2005), barren magmatic pulses of the San Francisco Batholith complex.Isotopic data indicate a decrease of 87 Sr/ 86 Sr and an increase of εNd isotopic signatures for Pleistoceneto-Holocene volcanic centers from the northern end of the Southern Volcanic Zone (NSVZ) (33°S) to the south (SSVZ, >36°S), coincident with crustal thinning from north to south (e.g., Kay et al., 2005).Regarding El Teniente and Río Blanco-Los Bronces Cu(-Mo) deposit, isotopic signature are increasing in 87 Sr/ 86 Sr but decreasing in εNd ratios with decreasing ages of magmatic rocks (Kay et al., 2005;Stern and Skewes, 1995).However, for the flat-slab segment north of the NSVZ, isotope studies demonstrated an increase of radiogenic 87 Sr/ 86 Sr signatures and a decrease of εNd values in west-to-east and north-tosouth directions, which coincide with the postulated younging of mineralization-hosted intrusive rocks in the same directions (e.g., Stern et al., 1984;Stern and Puig, 1991;Stern and Skewes, 1995;Skewes and Stern, 1996).This agrees with a southward younging of the mineralized centers in the central Cordillera between Los Pelambres and the coeval Río Blanco-Los Bronces and El Teniente Cu(-Mo) deposits (Deckart et al., 2005).
The geochemical patterns of the barren plutons have been compared with the contemporaneous, mainly volcanic, Farellones Formation and were interpreted to be possibly cogenetic (cf.López-Escobar et al., 1979;Kay et al., 1995).In the more recent study by Kay et al. (2005) it is proposed that both magmatic trends might be linked to tectonic erosion, yet not to the inception of subduction of the Juan Fernández Ridge near 33°S at 10 Ma, as reported by Yáñez et al. (2001).

Zircon U-Pb ages and Hf-isotopes
Three unmineralized granitoids and one intrusive, which belongs to the San Francisco Batholith complex, hosting the Río Blanco-Los Bronces porphyry copper deposit, were dated by the U-Pb LA-ICPMS method on single zircon grains.A summary of intrusion-related published ages of the area is presented in table 1. Analytical procedures are described in Appendix 1. U-Pb age data are documented in tables 2a-d.Hf-isotopic data for single zircon grains are summarized in tables 3a-d.
The Cerro Mesón Alto (CMA) massif was sampled south of the 'Lago Yeso' dam, about 90 km southeast of Santiago (Fig. 1).This massif intrudes the Eocene to earliest Miocene volcaniclastic Abanico Formation with sharp contacts.Twenty-five spot analyses on different zircon grains yield a mean crystallization age of 11.29±0.10Ma with a MSWD of 0.91 and a probability of 0.59 Figs. 2a,2b;Table 2a).This age is slightly younger but within error of the one (12.4±2.5 Ma; 40 Ar/ 39 Ar) from Kurtz et al. (1997).These authors ascribed the low precision of the latter to the low potassium content and/or a slight excess of 40 Ar, which is common in amphiboles and can easily lead to ages older than the 'true' crystallization age.Consequently, it is suggested that the U-Pb zircon age of 11.29±0.10Ma obtained here is the best estimation of crystallization age of the Cerro Mesón Alto massif.
The La Gloria pluton (LGP) is located in the Andean foothills, 50 km east of Santiago.It also intrudes the volcaniclastic Abanico Formation, dated west of the pluton at 21.1 to 30.9 Ma (Ar/Ar on plagioclase, Fock, 2005) (FA-4; Fig. 1; Table 2b).The weighted mean age calculated from nineteen ablations on different zircon grains is 10.34±0.15Ma with a MSWD of 0.46 and a probability of 0.97 (Figs. 2d,2e).Combined with the above-mentioned 9.8 Ma K-Ar biotite cooling age this indicates a rapid cooling history for the La Gloria pluton.
The north-south orientated San Francisco Batholith was sampled at its southern border ca.40 km northnortheast of Santiago.Published K-Ar and Ar/Ar ages of the San Francisco Batholith range between 18.5 Ma and 11.3 Ma (e.g., Warnaars et al., 1985;Deckart et al., 2005).A recently published zircon U-Pb IDTIMS age of 11.96±0.40Ma (Deckart et al., 2005) was interpreted as the crystallization age of an unmineralized rock unit close to the Río Blanco mine area, a few kilometres northeast of the localities sampled for this study.Two new U-Pb LA-ICPMS ages on single zircon grains from two distinct sites were obtained here.The first sample was taken from a locality adjacent to the mine site Los Bronces and gave a crystallization age of 11.16±0.07Ma (FA-7, Table 2c, Fig. 3a).The second was collected ca. 8 km to the southeast and yielded a crystallisation age of 14.74±0.13Ma Table 2c,Figs. 3b).
The Yerba Loca (YL) stock is located ca. 30 km northeast of Santiago and close to the southeastern exposed limit of the San Francisco Batholith.In this as yet undated intrusive two U-Pb LA-ICPMS ages have been obtained.A total of eleven spots were used to calculate a crystallization age of 14.88±0.16Ma with an MSWD of 0.45 for sample FA-9 and an age of 14.96±0.15Ma (n=11) with a MSWD of 0.58 for sample .Sample FA-11 was collected ca.20 m below and 100 m laterally from sample FA-9.
Thus the Cerro Mesón Alto massif has a slightly older age than the La Gloria pluton despite its location further east.On the other hand, only the younger age obtained from the San Francisco Batholith is coeval with the Cerro Mesón Alto massif U-Pb age.The Yerba Loca stock and most of the San Francisco Batholith are older than the Cerro Mesón Alto and La Gloria plutons.This pattern only partly confirms the postulated eastward younging of the magmatic arc (e.g., Stern and Puig, 1991;Skewes and Stern, 1996).However, because westward-vergent backfolding is restricted to the southern slopes of the Yeso Valley, Godoy (2005) proposed a sinistral transfer fault along that valley, which could have been active during emplacement of the Cerro Mesón Alto massif (Fig. 1).The study area therefore shows a complex structural history with Late Miocene thrust faults trending mainly north-south, northeast-southwest and northwest-southeast, which might indicate differently displaced block structures in the metallogenic belt between 32-34°S.
Hf isotopes for zircon minerals from all four plutonic complexes were also analyzed.Initial isotope ratios were calculated using the U-Pb zircon crystallization ages obtained during this study.The Cerro Mesón Alto massif and La Gloria pluton zircon samples Calculated Hf model ages (Hf-T DM ), which are interpreted in the same way as Nd model ages, range from 300 to 390 Ma for sample FA-3 from the Cerro Mesón Alto massif and between 320 and 380 Ma for sample FA-4 from the La Gloria pluton.The younger    Since there is no significant variation in the 176 Hf/ 177 Hf ratio data set for the samples from Cerro Mesón Alto, La Gloria pluton and the youngest San Francisco Batholith, magma mixing or inheritance do not seem to have been important.The older samples (FA-8, -9, -11) however, do show a slightly greater isotopic variation, but no analyzed zircon grain shows evidence of older nuclei (Figs.2-3) or reveals inherited U-Pb ages (Tables 2a-d).

whole rock geochemical characteristics and
Sr-Nd-Hf systematics
Low MgO (<2.5 wt%), Ni and Cr concentrations from the Cerro Mesón Alto massif and La Gloria pluton are indicative of their fractionated character.The San Francisco Batholith and Yerba Loca igneous rocks show higher MgO (>2.5 wt%), Ni and Cr contents compared to the others.There is an overall increase in transition metal element (Cu, Cr, Co, Ni, V, Zn, Sc) concentrations in the more mafic lithologies and at decreasing distance to the mineral district Río Blanco-Los Bronces.FA-8 from A slightly negative-to-no-Eu anomaly (Fig. 4) and CaO/Al 2 O 3 ratios correlated with Sc, suggest clinopyroxene fractionation in all samples.Rare earth element (REE) plots show a 60-to 100-fold enrichment in light REE, around 20-fold enrichment for middle REE and a 5-to 7-fold enrichment for heavy REE relative to chondritic values (Fig. 4a).The slight enrichment of the heavy REE relative to chondrite suggests that amphibole and possibly garnet might be minor residual phases in the magma source.(La/Yb) n ratios range from 11.95 to 15.34 in the Cerro Mesón Alto massif, 8.96 to 9.33 in the La Gloria pluton, 6.73 to 7.76 in the Yerba Loca stock, and 8.07 to 9.16 in the San Francisco Batholith.Cerro Meson Alto and La Gloria have a higher potassic and SiO 2 content indicating a higher degree of fractionation.Similar major and rare earth element observations have been outlined in Kay et al. (2005) on whole rock geochemical analyses of contemporaneous rock units.
Trace element patterns are typical of continental margin subduction-related arc magmas.The samples are enriched (2-38 times) in large ion lithophile elements (LILE; e.g., Rb, Ba, Th, U, Ta, K, La, Ce) relative to island arc basalts (IAB; Ewart et al., 1998;Fig. 4b).High field strength elements (HFSE), Ti, Sr, and the heavy REEs show similar and/or slight enrichment or depletion relative to IAB.The strong negative anomaly for Zr and likewise for Hf (not shown) and Pb (Cerro Mesón Alto massif, La Gloria pluton) may be due to incomplete analytical acid digestion of zircon minerals.Differences between IAB and the active continental margin basalts are seen in element concentrations such as K, Sr, Rb, Ba, Zr, Th and U, which are more enriched in the latter.
The same rock concentrations normalized to average N-MORB show an even stronger enrichment in more or less the same elements (Rb, Ba, Sr, Th, U, K, Nd, La, Ce and Pb) compared to the IAB normalization and confirm a LILE transfer to the magma source area.Enrichment of these elements is usually explained through partial melts in the mantle wedge or fluids liberated from the subducted slab.
Harker diagrams of oxides versus SiO 2 content (Fig. 5a-h) show a negative correlation of Al 2 O 3 , MgO, CaO, FeO*, TiO 2 and P 2 O 5 with increasing SiO 2 concentrations and an overall positive correlation of Na 2 O and K 2 O versus SiO 2 for the barren magmatism.Major element patterns are similar for the fertile rock sample group from the Río Blanco-Los Bronces deposit except for Na 2 O indicating a negative correlation with an increasing SiO 2 content.Barren andesites from the Coya Machalí and Farellones formations are characterized by the lowest SiO 2 content (50-61 wt%), followed by the barren igneous groups (58-71 wt%) without any tendency within the rock groups.The Río Blanco-Los Bronces porphyric rock group shows the highest SiO 2 content ranging from 66 to 71 wt%.
Trace elements (Ni, Sc, Cr) versus SiO 2 content (Fig. 6a-e) show a negative correlation, whereas Ba indicates a positive pattern and Sr yields little FIG. 5. Major element (wt%) Harker diagrams with new obtained data and selected data from Kay et al. (2005) and Stern and Skewes (1995)  correlation for the andesitic and barren plutonic rock groups.Porphyric Río Blanco-Los Bronces rocks (LOI: 0.85-2.0wt%) show a positive correlation for Ba, a negative for Sr and rather no correlation for Sc with increasing SiO 2 content.(La/Eu) n ratios versus La n concentrations (normalization after Sun and McDonough, 1989) rules out a break between volcanic and plutonic rocks, with ratios under 3 for the former and greater than 3 for the latter at a similar La n concentration (Fig. 7a).Río Blanco-Los Bronces porphyric rocks show the highest (La/Yb) n ratios (14-28), implying either a strongly fractionated character or possible residual garnet in the source of this group (Fig. 4).These major and trace elements show that the volcanics are less differentiated than the coeval to younger barren and fertile porphyric rocks, which requires differences in magma source regions and/or differences in the entire tectonomagmatic framework at the time of their magma formation.The trace elements ratios Ba/Th versus Ta/Yb (Pearce et al., 2005;Fig.8a) clearly separate the herein selected volcanic from the plutonic barren groups.The higher Ba/Th ratio of the volcanic group (low Ta/Yb) may be linked to a low-temperature fluid component, whereas all barren plutonic groups have a higher Ta/Yb with lower Ba/Th ratios indicating an enriched mantle influence (N-MORB: Ta/Yb=0.04;Ba/Th=52.5;Sun and McDonough, 1989) and/or recycled terrigeneous sediments rather than a shallow subduction (low-T, Ba-bearing, no significant Th addition) component (Pearce et al., 2005).In contrast, Kay et al. (2005) demonstrated that the Teniente Volcanic Complex group has similar characteristics with plutons of the immediate vicinity of El Teniente area.In the trace element ratio diagram (La/Yb versus Ba/La; Fig. 8b) the high Ba/La and low La/Yb ratios suggest rather dominating subduction fluids (i.e., La Obra and La Gloria plutons) during genesis (e.g., Tollstrup and Gill, 2005;Elliott, 2003), whereas a low Ba/La with increasing La/Yb ratios (Cerro Mesón Alto massif, San Francisco Batholith, and Yerba Loca stock, and 12-8 Ma Teniente Plutonic Complex [TPC-1]) might indicate a slightly increasing terrigenous sediment involvement during genesis.Río Blanco-Los Bronces porphyric intrusives show a tendency to both, slightly elevated La/Yb and Ba/La ratios pointing to a more complex magma genesis.(Sr/P) n ratios ranging from 3.63 to 4.46 for the Cerro Mesón Alto massif and La Gloria pluton and from 2.11 to 3.83 for the San Francisco Batholith and Yerba Loca stock, suggest enrichment through a fluid component rather than sediment influence (Borg et al., 1997).Pelagic and ODP Leg 141 sediments (Kilian and Behrmann, 2003) yield (Sr/P) n ratios of 1.65 and 1.43, respectively.On the other hand, the higher Pb/La ratio (about 1-2) in the Yerba Loca stock could suggest a possible sediment influence during genesis or slight crustal contamination by an elevated Pb signature in the older magmatic group.Trace element ratios of La/Ta>25 (Hickey et al., 1986) indicate a typical arc-like signature for all available data from the here presented rock units.Furthermore, ratios such as Th/Yb and Ta/ Yb usually indicate the involvement of an enriched mantle source with a slight subduction component and/or slight crustal contamination during the genesis of these magmas, but do not discriminate unambiguously between them (Pearce, 1983).
A slightly distinct evolution regarding the younger and slightly older plutonic rock groups may however be suggested considering the trace element ratios reported in this paper.Fluid dominance related to subduction processes are recognized in Cerro Mesón Alto and La Gloria pluton whereas sediment influence is manifested in magma evolution in the Yerba Loca stock and possibly the San Francisco Batholith.

Isotopic characteristics
Whole rock Sr, Nd and Hf isotope ratios were measured in two samples from the Cerro Mesón Alto massif, three from La Gloria pluton, two from the Yerba Loca stock and two from the San Francisco Batholith (Tables 3-5).Initial ratios were calculated back to the individual intrusive ages ranging between 15-10 Ma.Since the number of samples involved is  (2005) and Río Blanco-Los Bronces rocks (Stern and Skewes, 1995), average sediment compositions from Kilian and Behrmann (2003), N-MORB and E-MORB (Niu and O'Hara, 2003).Symbols as in figure 6.
rather small, isotope modelling was not attempted.Instead, isotopic ratios are plotted in conventional diagrams or combined with trace element ratios (Figs.9-11).Initial Sr-and Nd-isotope ratios (0.703761-0.704118and 0.512758-0.512882)show a restricted range for all four intrusions of the study area.The similarity in Hf-isotope ratios for whole rock and zircons, probably especially in the case of the younger intrusive rocks, indicates that zircon is most possibly controlling the Hf-isotope values.
Isotope diagrams show that initial Nd-Sr isotopic ratios of the new data plot in the mantle array between Depleted Mantle (DM) and Bulk Silicate Earth (BSE) (Fig. 9).The southern intrusives (Cerro Mesón Alto and La Gloria pluton) are slightly more radiogenic in their 87 Sr/ 86 Sr i ratio than the northern group (San Francisco Batholith and Yerba Loca stock) at equivalent 143 Nd/ 144 Nd i values.As a result of this characteristic the southern group plots inside many fields: the Chilean Coastal Batholith of central Chile (31°-34°S) (Parada et al., 1999), where its depleted portion overlaps with the SSVZ volcanic field (Kilian and Behrmann, 2003), Teniente Volcanic Complex (Kay et al., 2005) and in parts with the Farellones and Abanico formations (Nyström et al., 2003).Río Blanco-Los Bronces porphyry (Stern and Skewes, 1995)  87 Sr/ 86 Sr i ratios are similar with the SSVZ volcanic field but 143 Nd/ 144 Nd i values are less depleted compared to the rest of the rock groups of the depleted quadrant.
Ba/Nb versus Sr and Nd initial isotopic compositions confirm the relative stability of the Sm-Nd ( 143 Nd/ 144 Nd) i : 0.512758-0.512882)system in a subduction environment and suggests a fluid dominated process (increased Ba/Nb) for Cerro Mesón Alto, La Gloria pluton and Río Blanco-Los Bronces porphyric intrusives compared to the San Francisco Batholith and Yerba Loca stock (Fig. 10a-b).
significantly higher Th/La ratios.Whereas higher Th/ La ratios might account for recycling subarc mantle lithosphere or continental crust into the mantle source, lower Th/La ratios of the new data and characteristic for Río Blanco-Los Bronces porphyries indicate recycling of oceanic crust through the subduction zone into a mantle source (Kerrich et al., 1999).
Initial εHf-isotope data plotted against εNd i ratios (Fig. 11a) separate one sample (FA-1) of the Cerro Mesón Alto massif from the rest of all plutons.The more radiogenic εNd i value of the Cerro Mesón Alto massif (FA-1) is not accompanied by a higher εHf i value.This result can be considered to represent an outlier since major and trace elements do not indicate any unusual behaviour compared to the other Cerro Meson Alto sample.Plotting 176 Hf/ 177 Hf i versus 87 Sr/ 86 Sr i isotopic data a split between the Cerro Mesón Alto and La Gloria pluton group (about 10-11 Ma) and the San Francisco Batholith and Yerba Loca stock rocks (about 14.8 Ma) becomes evident, with the first group being slightly more radiogenic in Sr isotopes (Fig. 11b).Variations in initial isotopic composition are still less in Nd and smallest in the case of Hf.This observation underlines the fact that Hf isotope systematics represents the most robust system in subduction weakly or not affected by dehydration.It possibly represents original or residual mantle source values.Figure 11c shows whole rock and zircon initial Hf-isotope ratios versus age indicating that all samples plot above the CHUR and below the DM evolution lines.This observation indicates a residual depleted mantle involved at an early stage of magma evolution.'Mantle'-T DM ages calculated for the Hf whole rock data from the Cerro Mesón Alto and La Gloria pluton granitoids, however, lie in the future (Table 3a-b).Since 176 Lu/ 177 Hf ratios are high and samples are young, as confirmed by geochemical data, they most probably did not derive directly from a depleted mantle source.An estimated 'crustal' T DM (two-stage model) with a 176 Lu/ 177 Hf ratio=0.015(average continental crust; bulk silicate earth, Goodge and Vervoort, 2006) should give a more realistic age of separation of the host magma source from depleted mantle (DM) or crustal forming events.The widest whole rock crustal-T DM age range is given by the Yerba Loca stock (510-590 Ma), whereas the San Francisco Batholith lies between 520 and 530 Ma.Both, the Cerro Mesón Alto massif and the La Gloria pluton values range between 550 and 570 Ma. Figure 11c shows that whole rock and zircon single grain initial 176 Hf/ 177 Hf isotopic data are principally overlapping except for the ca.14.8 Ma zircon samples from the Yerba Loca stock and San Francisco Batholith (zircon: 110-540 Ma; WR: 510-590 Ma) with initial Hf-ratios slightly more radiogenic than the entire whole rock and zircon mineral data set.(1993) and Kilian and Behrmann (1997), Teniente Volcanic Complex (TVC) (Kay et al., 2005), Río Blanco-Los Bronces porphyries (Stern and Skewes, 1995).Farellones (FF) and Abanico formations (AF) (Nyström et al., 2003) and, Kilian and Behrmann (2003), respectively.Symbols as in figure 6.

Middle-to-Late Miocene source rock
In a subduction environment, element enrichment is often explained through either varying amounts of essentially lower crustal components, metasomatic fluid circulation from the subducting ocean plate, dehydration of subducted sediments, or partial melting of sediments.Adakitic geochemical rock signatures (high Sr/Y (>40), La/Yb(n) subscript (>20) ratios; low high field strength element concentrations; depleted (<0.704) 87 Sr/ 86 Sr ratios (Defant and Drummond, 1990)) have been described as relatively common in the Neogene of the central Andes, but can be excluded for the granitoids herein presented.
The ages of all samples versus Sr/Y ratios (Fig. 12) indicate a slight-to-no tendency to increasing Sr/Y ratios with decreasing ages in the temporal interval from 20-5.5 Ma.It is notable that there is a higher Sr/Y ratio (88 and 113) detected for the two dacite porphyry samples (Stern and Skewes, 1995) from the Río Blanco-Los Bronces deposit as observed by several authors (e.g., Thieblemont et al., 1997) who suggested that adakitic magmas (Sr/Y >40) are involved in giant porphyry copper deposits.Furthermore, La Gloria pluton and San Gabriel granitoids are barren, yet plot on the limit or inside the adakite field.
An arc-like signature generally characterizes the Middle to Late Miocene rocks.Ba/Th, Ta/Yb and Th/Yb ratios indicate for all plutonic rocks an enriched mantle influence, which might be explained through a slight subduction component in the source area and /or crustal contamination during magma ascent.Higher Ba/La ratios show a tendency to subduction fluids as enrichment factor (La Gloria pluton and the older La Obra pluton), whereas higher La/Yb might suggest terrigeneous sediment influence (Cerro Mesón Alto).The remaining samples (e.g., Río Blanco-Los Bronces porphyries) plot intermediate to these trends and might indicate both processes for the enrichment of especially large ion lithophile elements (LILE).
The available isotopic compositions are compatible with two distinct mantle sources for two distinct rock and age groups: the 10-11 Ma group and the older, 14.8 Ma group (Fig. 9).All samples plot in the mantle array between N-MORB and Bulk Silicate Earth compositions.Whereas the first group shows  5) whereas the second group (YL, SFB) indicates with its slightly positive εOs (i) values (average 187 Os/ 188 Os (i) =0.17; εOs (i) =+28 and +37%; Table 5) that the magma source was enriched in Re, presumably being the result of ocean floor sediments recycled into the source area or by the way of crustal contamination during magma ascent.These isotope characteristics agree with the trace element ratios (e.g., Ta/Hf, Ta/ Yb, Th/Yb) characteristic of the intrusive bodies, as presented above.Increased Ba/Nb ratios might suggest preferential trace element enrichment by fluid processes or pelagic sediments or, when combined with increasing initial 87 Sr/ 86 Sr ratios, to crustal input.1995).The adakite field is taken after Defant and Drummond (1990).Symbols as in figure 6.
the same crustal Hf-T DM ages for whole rock analyses but a range from 540 Ma to 110 Ma (Cambrian to Early Cretaceous) for zircon single grains.This rather identical Neoproterozoic to Early Cambrian crustal Hf-T DM age for zircon and whole rock from the younger plutons suggests that zircon grains originated from the same magma as the zircon-bearing intrusives and, therefore, the modelled crustal residence age may be used as an estimate for a crustal-type magma separation from a residual depleted mantle source.Hf isotopes from the older zircon samples suggest contribution from a more juvenile mantle residual.These results do not agree with the 2 Ga Hf-model ages and their interpretation presented in Montecinos et al. (2008) on coeval (15-10 Ma) intrusions at 33ºS.They described their obtained model ages as consistent with Sm-Nd model and upper intercept U-Pb ages from zircons of distinct Proterozoic and Phanerozoic lithologies (1.9-2.1 Ga) in the central Andes between 21ºS and 32ºS and 36º and 41ºS (in Franz et al., 2006).Consequently, this inconsistency outlines a distinctive petrogenetic evolution for the herein studied 33ºS to 34ºS plutons compared to the one proposed in Montecinos et al. (2008) using the much older lithologies reported by Franz et al. (2006).
Inheritance in zircon single grains in all intrusives bodies is not characteristic, as has been recognized in mineralized porphyric intrusions (6.3-5.0Ma) of the Río Blanco-Los Bronces Cu (-Mo) deposit (Deckart et al., 2005), where zircon core ages range mainly between the Carboniferous and Mesoproterozoic.Furthermore, zircon grains from andesites of the volcanic Farellones Formation (ca.17 Ma) cropping out in the Río Blanco mine sector yield one Neoproterozoic age for its upper discordia intercept (Deckart et al., 2005), whereas U-Pb SHRIMP dating of a Miocene ignimbrite in the El Teniente area indicated abundant Oligocene ages together with a few 114 Ma and 268 Ma ages (Godoy et al., in press).Zentilli et al. (2001) described as a common phenomenon xenocrystic zircons in older Cretaceous rocks of the Punta del Cobre Cu-Fe (Au) District and in the El Soldado Cu District of northern and central Chile, respectively.They suggested that during ascent felsic magmas associated with the mineralization picked up zircons from the Coastal Cordilleran granitic basement.However, zircon results in this study do not confirm this type of contamination process for our plutonic setting, which points to a different magma generation and emplacement process for the 15-10 Ma barren intrusions compared to the fertile Geochemical and isotope characteristics of subduction zone-related plutonic bodies in the central Andes between 33°and 34°S indicate an enriched subarc mantle source which was preferentially enriched in LIL and some HFS elements.It is suggested that element enrichment occurred either through dehydration fluids from subducted sediments which replenished the subarc mantle source (YL, SFB), or/and through partial melting or incorporation of a small amount of fragments of crustal material without homogenisation within the ascending magma (CMA, LPG).Finally, source enrichment processes do not appear to have been homogenous, and can be discarded as a process responsible for the separation of the younger age group (10-11 Ma), Cerro Mesón Alto massif, La Gloria pluton and the younger San Francisco Batholith from the older (14.8 Ma) San Francisco Batholith and Yerba Loca stock rocks.

Conclusions
Barren plutons occur between two supergiant porphyry Cu(-Mo) deposits in the Central Andes, Chile, between 33° and 34°S.Four of them (Cerro Mesón Alto massif, La Gloria pluton (33°27'-35'S), Yerba Loca stock and San Francisco Batholith) have U-Pb LA-ICPMS emplacement ages ranging from 10.3 to 15 Ma.In the study area magmatism does not reflect the general eastward arc front migration postulated by e.g., Kurtz et al. (1997) and Kay et al. (1999Kay et al. ( , 2005)).The Cerro Mesón Alto intrusive is slightly older than the La Gloria pluton but coeval with one age obtained from the more westerly located San Francisco Batholith.The magmatic arc younging hypothesis is applicable on a regional scale (e.g., from the coast to the Principal Cordillera W to E transect) but not on a local scale (70°30'-69°45').It, however, does not take into account either displacement by covered sinistral transfer faults, a complex tectonic control of plutonic emplacement or even younger intrusive events at about 1.1 Ma, as described for the Volcán valley by Godoy (1998) and Baeza (1999), mixed in the general Miocene-Pliocene magmatism of the study area.The latter observation points to a stationary (12-1 Ma) magmatic arc at these latitudes.
A quartz monzodioritic to quartz monzogabbroic composition and a metaluminous character characterize the plutonic complexes.
Rare earth element patterns suggest that amphibole and garnet were possibly minor components on the Late Miocene magma source in contrast with the Neogene adakites and the Río Blanco-Los Bronces porphyries.The (La/Yb) n ratios indicate an only slightly higher fractionation degree for the southern plutons compared to the northern ones.Río Blanco-Los Bronces porphyries indicate with their elevated ratio most probably residual garnet in the source.The adakite -like geochemical features outlined for Late Miocene porphyritic intrusions at Los Pelambres located ca. 100 km north of the study area (Reich et al., 2003) could not be supported by the coeval barren intrusions analyzed herein.When the Middle-to-Late Miocene plutonic rocks studied here are compared with their volcanic Miocene Farellones Formation host rocks, a direct genetic relation, as mentioned for the Early-to-Middle Miocene intrusion and the Eocene-Early Miocene Abanico Formation (cf.López-Escobar et al., 1979;Kurtz et al., 1997), could not be confirmed.Yet, an arclike signature is suggested for all rocks, including volcanics and mineralized rocks.Trace element ratios for all plutonic rocks indicate an enriched mantle influence with possibly a slight subduction component and/or crustal contamination during magma generation.
Isotopic ratios for the plutons also indicate a slightly enriched mantle source with only small isotopic variations, which might be accounted for by a slightly heterogeneous source.Nd-Sr isotopic ratios show small variations, whereas Hf-Sr isotopic data separate the southern from the northern intrusions, with higher radiogenic Sr for the southern ones.Crustal Hf-T DM modelling indicates a magma source that separated from the depleted mantle residual around 550-500 Ma and evolved until 11-10 Ma ago when magma re-melted and was emplaced at crustal levels as indicated by both whole rock (entire data set) and single zircon Hf-isotope (CMA, LGP, SFB) data.The older group (14.8 Ma; SFB, YL) is characterized by similar Hf-isotopic whole rock data but shows a wide range of zircon Hf-data suggesting contributions from a more juvenile mantle residual.Magma ascent has been direct for partly coeval volcanics but shows slight LIL element enrichment for the entire middle to Late Miocene plutonic magmatism.
The data presented here supports a slightly distinctive magma evolution through time from older to barren younger magmatism, different from the one recognised in the later, fertile one.Further studies on barren intrusives as well as a larger geochemical data set on the fertile magma are necessary.Some features that need more investigation and may relate to differences in the magmatic evolution of barren to mineralized intrusives are: the barren plutons contain mafic enclaves and show no zircon inheritance, mineralization-associated or -hosting intrusion show generally no mafic enclaves and no zircon inheritance, whereas the mineralized intrusives lack mafic enclaves and contain inherited zircon grains.powder in 15 ml Teflon Savillex beakers mixed with HF/HNO 3 and HClO 4 acids over night.This procedure was repeated several times before finally mixed with 2%-HNO 3 and analysed on the ICP-MS.
For magmatic magnetite analysed 0.2-0.5 g of handpicked minerals were loaded in Carius tubes (Shirley and Walker, 1995).Re and Os spikes and 16 ml of Aqua Regia were added and kept under cold conditions to avoid evaporation and early chemical reaction, then sealed.Afterwards, the tube was placed into an oven at 240°C overnight.Os was separated from the solution in a two-stage distillation process (Nagler and Frei, 1997), then purified and loaded with Ba(OH) 2 on a Ptfilament for analysis by a negative thermal ion mass spectrometer (NTIMS-VG54 mass spectrometer; Creaser et al., 1997).Re was separated after Os by using the remaining acid solution, After drying the solution, it was re-dissolved in 0.1 HNO 3 .Re was extracted and purified by using AG1-X8 (100-200 mesh) resins.After-wards Re was loaded with Ba(SO) 4 on Pt-filament and analysed by NTIMS.Whole rock isotopic analyses were obtained by using the multi-collector Nu Plasma Instrument ICP-MS.The chemical separation for Sr-Nd isotopes from the whole rock samples was obtained by using cation and anion exchange resin columns.An exponential correction for mass fractionation was applied using 146 Nd/ 145 Nd=2.07204 to avoid an iterative correction for the interference of 144 Sm on 144 Nd.The value for 146 Nd/ 145 Nd is an average of results obtained on a 1 ppm JMC321 Nd solution and is normalised to 146 Nd/ 144 Nd=0.7219.The interference of 144 Sm on 144 Nd was corrected by measuring 147 Sm and using 144 Sm/ 147 Sm=0.2070.Corrections for mass fractionation of Sr and Rb isotope ratios are made using an exponential law using a normalizing value for 86 Sr/ 88 Sr=0.1194.The precision (2 sd) and accuracy of standard analyses generally average for Sr isotpes 0.000066 and for Nd 0.000040 a.

Pb zircon LA-ICPMS data for fA-9 and fA-11, Yerba Loca.
San Francisco Batholith zircon sample (FA-7) shows a similar T DM age range from 320 to 390 Ma.The older rock sample from the San Francisco Batholith (FA-8) ranges in T DM ages from 194 Ma to 340 Ma whereas the Yerba Loca stock zircons (FA-9, FA-11) indicate the widest T DM age range between 76 Ma and 310 Ma, pointing to an inhomogeneous source of more juvenile mantle residuals.If a direct separation of these magmas from depleted asthenospheric mantle is assumed, initial epsilon Hf-values should be all greater than +10.Since the values range generally around +10, the magmas most probably were not derived directly from a depleted MORB-like mantle source.

87 86 Sr/ Sr (i)
FIG. 11.Initial Hf-isotope plots, a. whole rock εNd versus εHf; b. whole rock 87 Sr/ 86 Sr versus 176 Hf/ 177 Hf and, c. age versus whole rock and zircon single grain 176 Hf/ 177 Hf data; CHUR parameters of Blichert-Toft and Albarede(1997).more enriched Sr-isotope characteristics, Hf isotopic signatures are relatively uniform in both data sets.A major difference is shown by 187 Os/ 186 Os (i) isotopic data from magmatic magnetite minerals of each of the four plutons.Samples from the first group (CMA, LGP) are clearly characterized by continental crust input (average 187 Os/ 188 Os (i) =0.33; εOs (i) =+151 and +168 %; Table slightly