Upper Cretaceous intrusives in the Coastal Cordillera near Valdivia: forearc magmatism related to the passage of a triple junction?

Upper Cretaceous intrusives of limited extent crop out in the Coastal Cordillera near of Valdivia (39o48’ S), 100 km west of the main topographic divide of the Andean Cordillera. Given that plutonic rocks of the same age crop out at the same latitudes in the high Andes the coastal intrusives emplaced in a forearc position in the upper plate of a subduction setting. They correspond to hypabyssal intrusives displaying mainly porphyritic texture and lithological variations with microtonalites (minor), porphyritic microgranodiorites (main) and microgranites. They intrude the Upper Paleozoic-Triassic accretionary complex of the Bahia Mansa Metamorphic Complex. These intrusives, that comprise the Chaihuín Pluton and minor stocks of porphyritic felsic rocks, have calc-alkaline affinities with metaluminous and peraluminous character. They are geochemically similar to the contemporaneous main arc-related plutonic rocks of the Gualletué Plutonic Group. The microgranitoids and dacitic rocks from Los Boldos, the low and Loncoche are peculiar because they show an apparently adakitic affinity in Sr/Y and LaN/YbN discriminant diagrams; nevertheless Sr contents of these rocks (<<400 ppm) preclude a true adakitic character. The petrogenesis of the Chaihuín Pluton, is consistent with an evolution from tonalite to granite by successive fractional crystallization of plagioclase, amphibole, biotite, Fe-Ti oxides, apatite and zircon. The initial 87Sr/86Sr ratio (0.70411-0.70745), εNd (+4.24 to -3.09) and present-day Pb isotopic ratios (206Pb/204Pb: 18.616 to 18.708; 207Pb/204Pb: 15.620 to 15.635; 208Pb/204Pb: 38.573 to 38.662) of these rocks indicate that depleted mantle derived-magmas were contaminated by assimilation of crustal material at the base of the paleo accretionary prism or by subduction erosion. The heat required to explain mantle melting beneath the forearc crust could be supplied by the subduction of a young and hot oceanic slab and/or an active spreading ridge, as attested in similar settings in the world. According to plate reconstruction models the studied forearc intrusives would be generated during the migration of a triple junction that passed near Valdivia between 100 and 70 Ma.


Introduction
In the Coastal Range of South Central Chile, between 39° S and 40° S, outcrops of isolated intrusive bodies of limited extension (< to 24 km long) and of Upper Cretaceous age (mainly 92-83 Ma; Table 1) have been identified. At that time and at same latitudes, the corresponding subduction-related magmatic arc was approximately 100 km to the east, in the present Main Andean Cordillera position (Fig. 1, Table1). The existence of such magmatic rocks in the Coastal Range represent magmatic suites emplaced in the forearc crust which is typically a nonmagmatic zone with low heat flow (Gill, 1981). However, occurrences of magmatic activity in forearc positions have been reported recently elsewhere in the world. Examples of this magmatism comprises the semicontinuous igneous belts that extends from Alaska to Oregon (e.g., Madsen et al., 2006;Ayuso et al., 2009) and the Taitao granite intrusions related to the subduction of the active Chile ridge beneath the Taitao Peninsula in southern Chile (e.g., Forsythe et al., 1986;Guivel et al., 2003;Anma et al., 2009).
In the study area, the BMMC is locally intruded by small shallow-level granitoids and dacitic porphyries of Upper Cretaceous age (Table 1). In general, these FIG. 1. Location of Upper Cretaceous forearc intrusives (black) together with contemporary arc plutons emplaced in the Principal Cordillera (gray), between latitudes 38°30' S and 40° S, indicating their age (more details in Table 1). The study area (box) is magnified in figure 2. Gualletué Plutonic Group (Suárez and Emparan, 1997) is indicated in the Principal Cordillera from which samples were taken for comparison. In addition, Caburga Granite and Reigolil Granitoids (Moreno and Lara, 2008), and Paimun Granitoid (Lara and Moreno, 2004) are also indicated. Based on the Chilean Geological Map 1:1.000.000 (Digital version, Sernageomin, 2004).

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Upper Cretaceous intrusives in the Coastal Cordillera near Valdivia... plutonic to subvolcanic rocks are strongly altered and weathered, exhibiting spheroidal weathering with less altered cores of up to 1 m in diameter. In addition, there are numerous mafic and felsic dikes intruding the host rocks near the intrusives. The localities were these intrusions crop out are (Fig. 2): • Chaihuín Pluton (Arenas et al., 2005): corresponds to one NE-SW elongated intrusion which has the greatest extension (24 km long; Fig. 3). It is mostly composed of porphyritic microgranodiorites that are intruded by the Pan de Azúcar Granite in its central part (Mella et al., 2012). Contacts of the pluton with BMMC are by intrusion (Sernageomin, 1998) but in its northern edge ( Fig. 3), it is a faulted contact (the Corral Fault; Mella et al., 2012). • Los Boldos Granitoids (Munizaga et al., 1988): corresponds to two small intrusions (<2 km, each in lateral extend) that are locally strongly weathered and have whitish aspect. • Loncoche dacitic porphyries: corresponds to two bodies (1 and 9 km long) of porphyritic felsic rocks, with whitish aspect. Their shape is like a laccolith with dip to the south (Quiroz et al., 2007). • Oncol dacitic porphyry: corresponds to a strongly weathered intrusive body with spheroidal relicts. • Laurel dacitic porphyry: corresponds to one intrusion (2 km long) (Arenas et al., 2005;Mella et al., 2012) with spheroidal relicts. The above mentioned intrusives are partially covered by the Pupunahue Strata (Oligocene-Miocene; Illies, 1970) of continental origin, Santo Domingo Formation (Lower to Middle Miocene; Martínez and Pino, 1979;Mella et al., 2012) of marine nature, and by glacial, fluvial and coastal Quaternary deposits (Arenas et al., 2005;Quiroz et al., 2007).
These coastal intrusives will be compared with the Gualletué Plutonic Group, which is a belt of plutonic rocks in the Andean Cordillera, south of Lonquimay (Fig. 1). This plutonic Group is composed of different intrusions from Upper Jurassic to Upper Cretaceous, in at least 3 subunits of 148-140, 126 and 108-73 Ma (Suárez and Emparán, 1997; Table 1).
Quartz-bearing andesitic porphyry has only been observed in the Oncol Porphyry; seriated phenocrysts (up to 2 mm) are dispersed in a fine-grained hypidiomorphic granular groundmass (0.1 mm).

Geochemistry
A total of twenty two samples representative of the five studied intrusive complexes were analyzed for their major and trace elements compositions (Tables 2, 3, 4). In addition, a total of eigth samples were also analyzed for their Sr, Nd and Pb isotopic compositions (Table 5). Three samples from the Gualletué Plutonic Group were included as representatives of the Upper Cretaceous magmatic arc, contemporary with the forearc magmatism in the Coastal Cordillera.

Analytical methods
Geochemical analyses were conducted at the Geochemistry Laboratory of the Chilean Geology and Mining National Service (Sernageomin), by means of X-ray fluorescence spectrometry for major elements and by ICP-AES and ICP-MS for trace and rare earths elements. Isotopic analyses were conducted at the "Laboratoire de Géochimie isotopique" (ULB, Bruxelles, Belgium), with the TIMS method (Thermo-Ionization Mass Spectrometer) for Sr and Nd isotopes, and MC-ICP-MS (Inductively Coupled Plasma Mass Spectrometer) for Pb isotopes (see Ashwal et al., 2002 for detailed analytical methods).

Major elements
The Upper Cretaceous felsic plutons and stocks of the Coastal Range have typical calc-alkaline signature in the AFM diagram (Fig. 7A); they have metaluminous to peraluminous affinity (Fig. 7B), with a slightly more peraluminous composition than intrusives of the Gualletué Plutonic Group. As the Chaihuín Pluton displays a wide compositional spectrum (from diorite to granite) and has been the most largely analyzed, the Harker diagrams ( Fig. 8) will be focused on this pluton and its pattern will be compared to those of the others intrusions.
Rocks from the Chaihuín Pluton show linearly decreasing TiO 2 , Fe 2 O 3 , CaO, MgO, MnO and P 2 O 5 contents with increasing SiO 2 values (0.82<R 2 <0.94). The Al 2 O 3 contents also decrease with increasing SiO 2 but the trend is more scattered, especially in the less differentiated rocks. This dispersion is probably related to the variable proportions of plagioclase phenocrysts. The K 2 O contents increase linearly with SiO 2 content, whereas Na 2 O is scattered.
The other stocks present major elements variations similar to those of the Chaihuín Pluton, with the following exceptions: Oncol Porphyry (sample GdO-1) is significantly enriched in Al 2 O 3 and K 2 O for the same SiO 2 content but has lower values of MgO, MnO, Na 2 O and P 2 O 5 . A sample from Los Boldos Granitoids (sample DiLb-1) has a higher Na 2 O content and a lower K 2 O concentration. The 3 samples from Gualletué Plutonic Group are quite similar to the Chaihuín Pluton variation trend.

Trace elements
Trace element contents are reported versus SiO 2 content in figure 9. Chaihuín Pluton rocks display in general smooth and continuous trends. Incompatible elements show contrasting behaviors: Rb content is continuously increasing; Pb also increase but with some scatter while Zr content remains roughly constant from 58 to 68 wt% SiO 2 , then decreases significantly. 34 Upper Cretaceous intrusives in the Coastal Cordillera near Valdivia...
The transition elements (Ni, Cr, V) which are generally compatible, decrease strongly and rapidly with increasing SiO 2 ; the mafic enclave Ch-A2 has the highest content. Although Co is generally compatible in magmatic differentiation, in Chaihuín Pluton its contents augment with increasing SiO 2 content which is puzzling. Sr, which is compatible when plagioclase is fractionating, also decreases linearly. Ba trend is more scattered. Y and La increase but at Chaihuín Pluton two samples have Y contents over 40 ppm.
The samples from the Laurel and Loncoche porphyries grossly follow the Chaihuín trends, although those of Los Boldos Granitoids have lower contents of La. The only sample analyzed for Oncol Porphyry

Rare earths elements (REE)
The chondrite-normalized REE patterns of the intrusives are shown in figure 10. All the rocks of the Chaihuín Pluton display sub-parallel patterns (Fig. 10A), with LREE enrichments (La/Yb: 9-14), rather flat HREE and negative Eu anomaly (Eu/Eu*=0.05-0.30), except for the andesitic dike (sample Ch-B5). This anomaly is significantly more pronounced in the most differentiated rocks. REE content slightly increases with increasing SiO 2 . The mafic enclave sample (Ch-A2) exhibits more enriched pattern, especially in MREE. Oncol and Laurel porphyries show similar patterns (Fig. 10B) which fall in the range of the Chaihuín samples. Granitoids from Los Boldos and dacitic porphyries from Loncoche (Fig. 10C) are characterized by a notorious HREE depletion and thus by higher La/ Yb ratios (up to 25) and by the absence of significant Eu anomalies. Sample GdLb-2 (Los Boldos Granitoid) shows comparatively HREE enrichment and a slight negative Eu anomaly. The granitoids from the Gualletué Plutonic Group present moderately inclined and sub-parallel patterns (La/Yb=7 to 10), with flat slope for HREE, similar to the Chaihuín Plutons.

Isotopic geochemistry
The Sr, Nd and Pb isotopic compositions (Table 5) have been measured for 10 samples: 8 from the forearc intrusions and 2 from the Gualletué Plutonic Group.

TABLE 4. REE CONTENTS OF THE STUDIED SAMPLES, GROUPED IN CHAIHUÍN (CHA), ONCOL (ONC), LAUREL (LAU), LOS BOLDOS (BOL), LONCOCHE (LON) AND GUALLETUÉ (GUA) INTRUSIVES. VALUES ARE EXPRESSED IN PPM.
Unit Sample La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ∑ La/Yb Eu/Eu* Initial Sr and Nd isotopic compositions, recalculated for an average age of 90 Ma, have been plotted in the classical Nd-Sr isotopic diagram (Fig.11), including the data of metamorphic basement rocks (Lucassen et al. (2004). Samples from the Chaihuín Pluton show a rather restricted ranges of initial isotopic compositions ( 87 Sr/ 86 Sr) t ratios vary from 0.70415 to 0.70551 and εNd (t) values from +2.07 to +3.04. The mafic enclave Ch-A2 has a distinct signature, 0.70745 and -1.13 respectively. The isotopic compositions of the Los Boldos granitoid and Loncoche dacitic porphyry ( 87 Sr/ 86 Sr) t =0.70411 and 0.70466; εNd (t)=+4.24 and +1.09 respectively) fall in the Chaihuín domain. By contrast, the andesitic porphyry from Oncol and the Laurel dacitic porphyry are grossly similar (0.70711 and 0.70612; -3.09 and -0.94 respectively) and more evolved than the Chaihuín rocks. The two samples from the Gualletué Plutonic Group have isotopic compositions (0.70424; +4.13 and +3.62) that are quite similar to those in the Chaihuín Pluton.
As U and Th concentrations have not been measured, it is not possible to recalculate the initial Pb isotopic compositions but for rather young rocks (~90 Ma), the measured Pb compositions are presumably still close to the initial compositions.
On the other hand, these intrusives are quite similar in petrography, SiO 2 range and Sr and Nd isotopes to others forearc granitoids around the world (Table 7), although the Kodiak batholith (Farris, 2010) is strongly peraluminous due to the occurrence of kyanite and andalusite xenocrysts. In the following sections the petrogenesis of these rocks will be addressed with the aim to discuss whether these rocks formed during the subduction of an active oceanic spreading ridge. M=MgO), that differentiates between tholeitic and calc-alkaline rocks (Irvine and Baragar, 1971). B. ASI diagram (Aluminum Saturation Index) determined for A/C+N+K (Al 2 O 3 /CaO+Na 2 O+K 2 O) molar versus SiO 2 , which divides peraluminous and metaluminous fields (Shand, 1927); samples enclosed by dotted line belong to Gualletué Group (arc plutons).

Source and contamination of the forearc magmas
The MORB-normalized multi-element diagram for the studied rocks (Fig. 12) shows typical subduction related patterns (calc-alkaline signature, LILE enrichment, HFSE depletion and Nb negative anomaly), which suggests the participation of two components in the genesis of the forearc and arc magmas: the asthenospheric mantle wedge and LILE-rich fluids derived from the subducted plate and/or eroded forearc, which metasomatized the mantle wedge and induced its partial melting (e.g., Iwamori, 1998).
Beneath the forearc, mantle-derived magmas would rise up to the base of the crust, formed by a fossil accretionary prism consisting mainly of siliciclastic meta-sediments and minor metabasite intercalations (e.g., Aguirre et al., 1972;Hervé, 1988;Willner, 2005). Due to density differences, the mantle-derived magmas would stagnate at the base of the fossil (paleo) accretionary prism, forming an assimilation zone in a situation similar to that proposed for the magma evolution in continental magmatic arcs (Hildreth and Moorbath, 1988;Annen et al., 2006). Another possibility is mantle contamination by subduction erosion (Stern, 2011). Such processes would create differentiated and heterogeneous magmas (mainly of granodioritic composition) at regional scale, with fluctuating signals between crustal reservoirs and depleted asthenosphere (εNd=+4.24 to -3.09). Involvement of a MORB-type mantle derived basaltic melts and metasediments has been proposed for the genesis of forearc granitoids, for  example in the Sanak-Baranof Belt, which intruded into the accretionary prism (Farris, 2010;Ayuso et al., 2009;Sisson et al., 2003).
In the Nd-Sr isotopic diagram (Fig. 11), the coastal granitoids display a narrow isotopic field extending from a MORB-type pole (εNd from +4.24 to +1.1) for Los Boldos porphyry, Loncoche dacitic porphyry and Chaihuín pluton to a slightly enriched pole (εNd from -0.94 to -3.1) for Laurel dacitic porphyry, Oncol porphyries and the mafic enclave from Chaihuín. The isotopic data show (Fig. 11) a mixing line between the MORB domain and meta-psammopelites of the accretionary paleo-prism, showing different degrees of assimilation. FIG. 12. A. MORB-normalized multi-element diagram (Sun and McDonough, 1989), for Upper Cretaceous forearc and arc (Gualletué) intrusives. B. Multi-element diagram normalized to Ga-M6 sample (SiO 2 =67%) from Gualletué Plutonic Group, for the Upper Cretaceous forearc intrusives. The silica content for each sample is indicated in parenthesis next to each sample.
Contamination of the mantle wedge-derived magmas with continental crust would explain the further LILE enrichment and the more peraluminous nature of the forearc intrusives in comparison to those from Gualletué Plutonic Group. The rather wide ranges of Pb isotopic composition of the forearc rocks ( 206 Pb/ 204 208 Pb/ 204 Pb: 38.50-39.57), roughly define a scattered linear array in the 208 Pb/ 204 Pb versus 206 Pb/ 204 Pb diagram; this array extends from the MORB field to high values corresponding to the "Marine Sediments/Upper Crust" (Fig. 13). This confirms the crustal contribution in the genesis of the forearc magmas. Crustal assimilation processes would also occur in shallow magmatic reservoirs, as attested by xenoliths of metamorphic rocks found in the intrusives.
The Loncoche and Los Boldos intrusive rocks (SiO 2 between 67-71 wt%) have low HREE contents and consequently high La/Yb ratios (up to 25), reflecting an apparently adakitic signature as shown in the Sr/Y -Y and (La N /Yb N ) discrimination diagrams (Drummond and Defant, 1990;Fig. 14). Nevertheless their low Sr contents (<<400 ppm) preclude they are truly adakites.
The microdioritic mafic enclave (Ch-A2) is one of the least evolved rocks of the Chaihuín series (62 wt% SiO 2 , with rather high Ni and Cr contents comparative to the others rocks) but it has a significantly higher initial 87 Sr/ 86 Sr (0.7075), a negative εNd value (-1.1) and high MREE content. These features suggest that mafic enclaves possibly represent the initial pulses of the forearc magmatic event, which could correspond to mantle magmas markedly contaminated by molten material from the fossil accretionary prism.
The andesitic dike (Ch-B5) that cuts the Chaihuín intrusives could have evolved from residual magmas trapped in an independent magma chamber.

Fractional crystallization
The asthenospheric wedge-derived magmas first assimilate crustal material from the fossil accretionary prism, generating magmas of intermediate composition. Mantle heterogeneity and subducted sediments could also play a role in the observed compositional variations. Then, they ascend and differentiate mainly by fractional crystallization. Based on the empirically calibrated geobarometer of the Al-content in hornblende Seifert et al. (2005) estimated the crystallization depth for this forearc intrusives at less than 3 km. This crystallization process resulted in the progressive and gradual mineralogical changes documented in the Chaihuín Pluton.
Amphibole predominates over biotite in the tonalites; the reverse is observed when SiO 2 increases, as reflected by the abundant biotite and rare amphibole in late-stage granites. Similarly, the anorthite-content of plagioclase decreases TiO 2 , V), apatite (decrease of P 2 O 5 ) and zircon (decrease of Zr in most differentiated rocks). The spatial distribution of tonalites and microgranites in Chaihuín Pluton suggests that the differentiation occurred from the borders to the center of the intrusion (Fig. 2). Field evidence and fractional crystallization models suggest that the microgranites would be the product of the differentiation of microgranodiorites (Mella et al., 2012). A progressive enrichment of REEs is observed for Chaihuín rocks (Fig. 10), as well as a deepening of the Eu negative anomaly (related to the fractionation of plagioclase) as the silica content of whole rock increases. A fractional crystallization model has been quantitatively evaluated (using the Rayleigh equation) for the Chaihuín series of rocks based on their REE contents and their modal proportions (Fig. 15). Calculations were performed in two stages: 1) tonalite (Co) → microgranodiorite (R1), and 2) microgranodiorite (C'o) → microgranite (R'1). For the first stage, the best fit of the calculated curve was obtained for F=0.7 (= fraction of melt remaining after fractional crystallization) with a small discrepancy for the HREE (Yb and Lu). For the second stage, the calculated curve has good fit for F=0.75, with a discrepancy for Eu.

Geotectonic control on the genesis of forearc intrusives
Forearc areas usually have relatively low heat flow and no-or only minor-magmatic activity (Gill, 1981), due to the depression of the isotherms nearest the trench, as a result of the subduction of the partly cooled oceanic slab beneath the continental plate. However, this scenario can be modified by the interaction of the continental margin with an active oceanic ridge that could be responsible for the magmatic activity in the forearc (e.g., Marshak and Karig, 1977;DeLong et al., 1979;Sisson et al., 2003). In fact, the presence of igneous rocks, either FIG. 14. Discrimination diagrams between adakite versus normal arc rocks (Drummond and Defant, 1990). Data provided by Lucassen et al. (2004) Sun, 1995); modified from Martin et al. (1999). volcanic (basalts to dacites) or plutonic (tonalites to granites) in such position has been used as a key indicator of plate redistribution in converging margins (Groome et al., 2003). Between Cretaceous and Early Eocene, Farallon and Aluk (or Phoenix) plates subducted beneath South America (Fig. 16A). The triple junction (Farallon-Aluk-South American) would have migrated to the south from the northern edge of South America (Folguera and Ramos, 2002), to be positioned in the trench at northern Chile (~18º S) in the Cretaceous (~140-120 Ma, Müller et al., 2008;Somoza and Ghidella, 2012). Subsequently, the triple junction would have moved south along the Chilean edge until it finally arrived to the Patagonian Andes latitudes in the Paleogene (~42 Ma, Folguera and Ramos, 2002;~50 Ma, Müller et al., 2008;Breitsprecher and Thorkelson, 2009;Scalabrino et al., 2009;~45 Ma, Somoza and Ghidella, 2012;60 Ma, Seton et al., 2012). However, the exact chronology of the migration of the triple junction from northern Chile to Patagonia is still undetermined. On the other hand, a new geotectonic reconstruction (Seton et al., 2012;Müller et al., 2016) indicates the fracture of the Aluk plate about 120 Ma ago into four plates: Hikurangi, Manihiki, Chazca and Catequil; the latter two being the only ones that subducted beneath South America (Fig. 16A). Although the model of Müller et al. (2016) shows a more complex pattern described by the Catequil-Chazca ridge along the South America border, it also indicates the presence of an active spreading center at the considered latitudes during the range between 100-70 Ma. This is in agreement with the age of the studied intrusions of the Costal Range near Valdivia and it is therefore possible to attribute the origin of the magmatic activity to a thermal anomaly in the forearc region produced by subduction of an FIG. 15. Fractional crystallization modeling based on mantle-normalized REE contents (Sun and McDonough, 1989) and modes showed in the adjacent table, for Chaihuín Pluton rocks: Co: parent rock, R1: daughter rock, L1: calculated daughter rock.
The modeling was performed in two stages: tonalite → granodiorite (at the top) and granodiorite → granite (at the bottom). Partition coefficients were taken from Rollinson (1993). oceanic ridge, as proposed by Glodny et al. (2006). Nevertheless, in the plate reconstruction of Müller et al. (2016, Fig . 16A) it is observed that the Farallon plate did not migrate to Patagonia. Instead, the triple junction Chazca-Catequil-South America migrates from the south to the north (compare 100 and 70 Ma reconstructions from figure 16A). The subduction of an oceanic ridge involves the generation of a slab window (Dickinson and Snyder, 1979;Thorkelson, 1996), which could explain the decreasing activity of the arc magmatism or its replacement by a MORB-like or rift-type volcanism. The latter anomalous magmatism has not been observed in the arc rocks that are contemporaneous with the studied intrusions: indeed these arc rocks typically have a calc-alkaline signature. The lack of attenuation of the magmatic activity in the arc and the calc-alkaline signature of the forearc rocks are not in favour of the development of a slab window. This could be explained by the presence of a ridge with strongly oblique subduction to the trench which does not allow the development of a large slab window (Fig. 16B).
The subduction of a young and hot oceanic crust expectable in a ridge subduction environment would generate slab dehydration at areas near the trench, which would allow the mantle melting beneath the forearc, as suggested by Iwamori (1998). Therefore, slab dehydration would have been developed during the Upper Cretaceous, simultaneously under both forearc and arc regions (Fig. 16B).

Geodynamic implications for the Andean margin
The knowledge of the configuration, ages and position of the ridge segments subduction beneath the south-western South American continent during the Upper Cretaceous is still in progress. Glodny et al. (2006) proposed that the origin of the forearc intrusives near Valdivia was the north-south passage of Farallon-Aluk oceanic ridge, which created a thermal anomaly. But if we consider the new plate reconstruction (Müller et al., 2016) this forearc magmatism should be related to the passage of the Chazca-Catequil-South America triple junction. In any case, the results of this work can be used to refine paleotectonics reconstruction, specifying the presence of active ocean ridge segments interacting with western South America at ca. 39º-40º S during the 100-70 Ma period.
Considering the migration of the Farallon-Aluk-South America triple junction, we would expect to find forearc intrusions with progressively decreasing ages from the north (Loncoche and Los Boldos intrusives) to the south (Chaihuín Pluton). To the south of the studied zone, on Chiloé Island (42º S) there are small and isolated sub-volcanic bodies of Eocene age (Metalqui granodioritic pluton: 39.6± 0.3 Ma, and Gamboa Dacite, of 37.2±1.2 Ma; Duhart and Adriasola, 2008) in the forearc area, which could account for the migration of the ridge to the south. However, zircon and apatite fission track studies in Paleozoic-Triassic metamorphic rocks of Chiloé suggest the existence of a thermal anomaly between ~110 and 70 Ma (Glodny et al., 2007;Thomson and Hervé, 2002), that is much earlier than the Paleogene igneous activity. Therefore, the magmatic bodies of Chiloé could be linked to a subsequent subduction episode, either from another segment of the ridge or from a different expansion system. In contrast, to the north of the studied area, forearc intrusions older than the Upper Cretaceous have not been reported. This could be attributed to a rapid advance of the ridge along northern latitudes (that never generated forearc magmatism), or, if they have ever existed, they are now hidden beneath the seafloor or they were eroded by subduction. The fact such a large area (from Valdivia to Chiloé) seemed to be thermally perturbed during the same period of time can be related to the geometry and configuration of the oceanic ridge: likely with parallel direction with respect to western border of South America (Fig. 16B), or it corresponds to the subduction of different segments of the ridge at different latitudes of the continental margin.

Conclusions
The Upper Cretaceous magmatic bodies of the Costal Range, in the Valdivia area, are in forearc position with respect to the contemporaneous magmatic arc located about 100 km to the East. Several intrusions were studied, the most representative being which consists mainly of granodiorites and granites the Chaihuín Pluton.
The forearc intrusive event in the Valdivia area is considered as anomalous in the converging margin dynamics of the Andes: it is indeed not linked to normal subduction processes; nevertheless, it displays typical calc-alkaline signature (AFM trend, LILE enrichment, negative Nb anomaly). Compared to the Gualletué Plutonic Group, which is representative of the contemporary arc magmatism, the forearc intrusions present some differences: their emplacement level, more peraluminous nature, LILE enrichment, relative depletion in HFSE, REE patterns and their more variable and more enriched Sr and Nd isotopic compositions.
The forearc intrusions were emplaced at shallow level (epizonal conditions). They dominantly consist of tonalites-microgranodiorites-microgranites with porphyritic textures; their composition is predominantly felsic (60<SiO 2 <77 %). The Sr and Nd isotopic compositions suggest the involvement of two components in the genesis of their parental magmas: a MORB-like depleted mantle and an enriched crustal component presumably the metasedimentary rocks of the Late Paleozoic-Triassic accretionary prism (Bahía Mansa Metamorphic Complex). Mantle melting was initially triggered by fluids derived from the slab dehydration; the generated magma would be mafic, hydrated and isotopically depleted. During its uprise, the magma would have stayed at the base of the accretionary prism and induced partial melting or assimilation of metasedimentay material, producing a more siliceous and isotopically more rediogenic, contaminated magma. Alternatively, contamination could result from subduction erosion. While further uprising, the contaminated magmas differentiated by fractional crystallization which gave rise to the compositional diversity of the studied plutons (from quartz diorite to granite).
The origin of the parental magma(s) of the investigated forearc intrusions is attributed to a thermal anomaly in the forearc related to the subduction of an active ridge beneath the South American continent; this ridge was thought to be the Farallon-Aluk-South American triple junction (Glodny et al. 2006), but new reconstruction (Seton et al., 2012;Müller et al., 2016) suggest instead the Chazca-Catequil-South American triple junction migration.
The studied forearc intrusives could tentatively be interpreted as reflecting the presence of an active oceanic ridge in the South American edge between 39° and 40°S, at around 100-70 Ma.
Severals intrusiones were estudied, the most representative being the Chaihuín Pluton with consists mainly of granodiorites and granites.