Extensional Lower Cretaceous volcanism in the Coastal Range (29°20′–30°S), Chile: geochemistry and petrogenesis

Journal of South American Earth Sciences 16 (2003) 459–476 www.elsevier.com/locate/jsames

Extensional Lower Cretaceous volcanism in the Coastal Range (298200 –308S), Chile: geochemistry and petrogenesis D. Morata*, L. Aguirre Departmento de Geologı´a, Fac. Ciencias Fı´sicas y Matema´ticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile Received 1 May 2003; accepted 1 June 2003

Abstract Lower Cretaceous volcanism in the Coastal Range (298200 – 308S) of Chile is mainly represented by highly porphyritic (20 – 30% phenocrysts) lavas with unzoned Ca-rich plagioclase (An57 – 54Ab40 – 42Or3 – 4), clinopyroxene (Wo40En43Fs17), magnetite, and minor idiomorphic, altered olivines. Geochemically, these lavas are characterized by a relative homogeneity with high Al2O3 and low MgO contents, and classified as high-K to shoshonitic basaltic andesites to andesites generated in an intra-arc extensional setting due to oblique subduction. Their isotopic geochemistry is characterized by highly homogeneous low initial Sr ratios ((87Sr/86Sr)0 , 0.7036) and positive 1Nd values (1Nd ¼ þ2:9 to þ4.7 (143Nd/144Nd)0 , 0.5127) that are very different from those proposed as representative of ‘Andean-type’ magmatism. A non-Andean modern setting dominated by subduction associated with intra-arc extension is proposed. On a (87Sr/86Sr)0 versus 1Nd diagram, these lavas fit a model mixing curve for which the end members are Pacific MORB and Jurassic plutonic rocks from the Coastal Range. Coeval granitoids from the Coastal Range and lavas from the High Andes plot on the same field. Isotopically depleted mafic magmas could be metasomatized by the subducted sediments, which would increase their LILE content, and then partially contaminated by Jurassic plutonic rocks. The genesis of this magmatism may be related to a global low-spreading rate of 5 cm yr21 in the southeast Pacific during 125– 110 Ma. q 2003 Elsevier Ltd. All rights reserved. Keywords: Coastal Range; Geochemistry; Lower Cretaceous; Petrogenesis; Volcanism

Resumen El volcanismo del Creta´cico Inferior en la Cordillera de la Costa de Chile (298200 –308S) esta´ representado principalmente por lavas altamente porfı´dicas (20 – 30% fenocristales), con plagioclasa ca´lcica no zonada (An57 – 54Ab40 – 42Or3 – 4), clinopiroxeno (Wo40En43Fs17), magnetita, y menores proporciones de cristales idomo´rficos de olivino completamente alterados. Geoquı´micamente, estas lavas se clasifican como andesitas basa´lticas y andesitas altas en potasio a shoshonı´ticas, y esta´n caracterizadas por contenidos de Al2O3 relativamente altos y homoge´neos, y bajos valores de MgO. La ge´nesis de estas lavas se interpreta en relacio´n con un re´gimen extensional intra-arco, generado como consecuencia de una subduccio´n oblı´cua. Isoto´picamente son lavas con bajos valores iniciales en las relaciones de Sr ((87Sr/86Sr)0 < 0.7036) y valores positivos del 1Nd (1Nd ¼ þ2:9 a þ4.7 (143Nd/144Nd)0 < 0.5127). Se propone un contexto geotecto´nico diferente al del magmatismo andino reciente, controlado no so´lo por la subduccio´n sino tambie´n por una extensio´n intra-arco. En un diagrama (87Sr/86Sr)0 vs 1Nd ; las lavas creta´cicas se proyectan a lo largo de una curva de mezcla entre el MORB del Pacı´fico y las rocas pluto´nicas del Jura´sico de la Cordillera de la Costa. Adema´s, rocas granı´ticas del Creta´cico de la Cordillera de la Costa, ası´ como lavas creta´cicas de la Cordillera de los Andes, se proyectan en el mismo campo que el de las lavas creta´cicas. Este modelo petrogene´tico involucra a magmas ma´ficos, isoto´picamente empobrecidos, que serı´an metasomatizados por sedimentos subducidos, aumentando sus contenidos en LILE, y posteriormente contaminados parcialmente por las rocas pluto´nicas del Jura´sico. Este magmatismo podrı´a estar relacionado con una baja tasa de expansio´n ocea´nica de 5 cm yr21 en el SE del Pacı´fico durante el intervalo 125–110 Ma. q 2003 Elsevier Ltd. All rights reserved. Palabras clave: Volcanismo; Creta´cico Inferior; Cordillera de la costa; Geoquı´mica; Petrogenesis

* Corresponding author. Tel.: þ 56-2-6784539; fax: þ 56-2-6963050. E-mail address: [email protected] (D. Morata). 0895-9811/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2003.06.001

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D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

1. Introduction In classical petrology textbooks (e.g. Thorpe et al., 1982; Wilson, 1989), magmatism dominated by orogenic andesites generated in an active thick continental margin as a consequence of ocean – continent convergence and having a crustal component is called ‘Andean-type’ magmatism. Specific petrological and geochemical signatures, different from those of magmas generated in other tectonic settings, characterize this type of magmatism (Davidson et al., 1991). The subduction of oceanic lithosphere below the continental lithosphere has been continuous along the Chilean margin from the Mesozoic to the present, along with the development of magmatic arcs. Nevertheless, the petrology and geochemistry of some active continental margin magmas are different from those that characterize the modern Andes. Many of these differences probably are related to the different tectonic regimes that dominated during subduction across time. An ensialic marginal basin or intra-arc subsiding basin formed by extension and progressive crustal attenuation has been proposed as the setting for the Lower Cretaceous in central Chile (Levi and Aguirre, 1981; Levi and Nystro¨m, ˚ berg et al., 1984; Charrier and Mun˜oz, 1994; 1982; A Vergara et al., 1995). This geodynamic setting, which differs from that of modern Andean magmatism, accounts for the unique geochemical characteristics of Lower Cretaceous volcanism in the Coastal Range (Vergara et al., 1995). High-K, calc-alkaline to shoshonitic orogenic andesites and basaltic andesites, locally known as ‘ocoites’ (highly plagioclase porphyritic lavas), are the most conspicuous rocks generated during this time. Aguirre et al. (1989) refer to the 3 –13 km thick sequence of volcanic rocks that extends , 1200 km along the Coastal Range in central and north central Chile as the Ocoite Group. On the basis of trace element geochemistry, Herna´ndez et al. (1999) show subduction-related geochemical affinity for Early Cretaceous lavas north of La Serena. In this paper, we characterize the petrology and geochemistry of Lower Cretaceous lavas (ocoites) exposed in the Coastal Range of Chile in the La Serena region (308S) and propose a petrogenetic model based on geochemical data. We also present a comparison with the petrology and geochemistry (major, trace elements, and isotopic ratios) of other ocoites of the Ocoite Group in the Coastal Range. Previously published geochemical data correspond to those from lavas of the Veta Negra Formation (Vergara et al., 1995) located at the latitude of Santiago (338S) and those from the Pabello´n Formation (Cisternas et al., 1999) at the latitude of Copiapo´ (278300 S).

2. Geological setting The Lower Cretaceous stratified sequences crop out in a double, almost continuous, north-trending belt , 1200 km

long and 100– 150 km wide with a synclinorium structure extending from Taltal (258300 S) to Talca (358300 S) (SERNAGEOMIN, 1982). Thick, predominantly volcanic sequences occur along the western border of the belt (Coastal Range), whereas sedimentary and subordinate volcanic rocks appear to the east (Andean Range) (Aguirre, 1985; Vergara et al., 1995; Vergara and Nystro¨m, 1996). An intra-arc extensional basin, subsiding at high rates (100 – 300 m/m yr), has been proposed as the geodynamic setting for the Lower Cretaceous belt in the Coastal Range (Vergara et al., 1995), whereas a backarc setting has been postulated for the Andean Range (Vergara and Nystro¨m, 1996). Different names have been given to the Lower Cretaceous units of the Coastal Range at different latitudes (Aguirre, 1985). Thick volcanic sequences predominate at the latitude of Santiago (338S), whereas sedimentary sequences (carbonates and detritic rocks) are more abundant north and south of Santiago. In all cases, the volcanic rocks are systematically characterized by the presence of plagioclase-rich porphyritic basaltic andesites and andesites known as ocoites. At La Serena (298300 – 308000 S; Fig. 1), the Lower Cretaceous volcanic rocks of the Coastal Range are in the Arqueros Formation (Hauterivian – Barremian; Aguirre and Egert, 1965), a , 1500 m thick sequence of porphyritic andesites and fossiliferous marine limestones with manganese-rich beds at the top, and the Quebrada Marquesa Formation (Upper Barremian – Albian; Aguirre and Egert, 1965), a , 1900 m thick sequence of mainly continental clastic rocks and andesitic lavas, tuffs, and breccias with minor marine limestone intercalations that conformably overlies the Arqueros Formation. Above the Quebrada Marquesa, a 1000 – 1500 m thick sequence of continental clastic rocks, lavas, and breccias—the Vin˜ita Formation of late Cretaceous age—is unconformably located (Aguirre and Egert, 1965, 1970). All these formations are slightly folded, block faulted, and locally intruded by calc-alkaline and oversaturated granitoids (e.g. the Santa Gracia granitoids, with K –Ar ages in biotite ranging from 89 ^ 0.6 to 98.0; Aguirre et al., 1974). Five members of the Arqueros Formation have been identified (Aguirre and Egert, 1965): three volcanic members (Ka1, Ka3, Ka5) separated by two sedimentary ones (Ka2, Ka4) containing shallow-water marine fauna. In the Quebrada Marquesa Formation, volcanic rocks are mainly present in the lower units (Aguirre and Egert, 1965). Both formations were sampled in the Llano de Arqueros (area A, Fig. 1), a 1200 –1400 m plateau north of the Rio Elqui Valley. Lower Cretaceous volcanic rocks that correlate with the Arqueros and Quebrada Marquesa Formations (Moscoso, 1976) were also sampled in a region 100 km north of La Serena, along the Rı´o Los Choros Valley and close to Tres Cruces village (area B, Fig. 1). In this area, the volcanic rocks interfinger with clastic and carbonate marine Neocomian sedimentary rocks.

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

461

Fig. 1. Geological map showing the two studied areas (A and B) of the Lower Cretaceous volcanic rocks from the Coastal Range in the La Serena region (308S). Modified and simplified from SERNAGEOMIN (1982).

Paleontological and biostratigraphical data allow correlation of the sedimentary – volcanic sequences of both areas studied (Fig. 1.). A Late Hauterivian – Late Barremian age is assigned to the formations (Aguirre and Egert, 1965; Mourgues, 2000a,b). The first whole-rock K – Ar ages for the volcanic rocks of the Quebrada Marquesa Formation, which directly overlie the Arqueros Formation, show considerable scatter with dates of 61.3– 91.9 Ma, probably as a consequence of

the secondary alteration pervasively present in these rocks (Palmer et al., 1980). A minimum age of 73 ^ 4 Ma (K – Ar in plagioclase) was obtained by Empara´n and Pineda (1999) in intrusive andesites that cut the Quebrada Marquesa Formation in area A, and K – Ar ages of 108 ^ 3 (in amphibole) and 109 ^ 3 Ma (in biotite) were recently obtained from granodiorites intruding lavas from the Arqueros Formation in the same area (Empara´n and Pineda, 2000). Preliminary 40Ar/39Ar ages obtained by

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D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

the step-heating procedure were recently reported by Morata et al. (2001) for selected primary Ca-plagioclase populations. Concordant weighted ages of 117.0 ^ 0.6 (from the Ka1 member of the Arqueros Formation) and 114.3 ^ 0.7 Ma (from the Ka3 member of the Arqueros Formation) were obtained. These ages correlate with those obtained in lavas from the Veta Negra Formation (Lower Cretaceous at the latitude of Santiago, 338S), in which a plateau 40Ar/39Ar age of 119.4 ^ 1.2 Ma from primary plagioclase phenocrysts represents the best estimation for the age of the volcanism (Aguirre et al., 1999).

plagioclase, augite, and magnetite make up the intergranular to cryptocrystalline groundmass. Sericite, prehnite, mafic phyllosilicates, epidote, and minor pumpellyite (together with calcite and silica varieties) fill veinlets and vesicles or partially replace primary phenocrysts, mainly plagioclase. Secondary alteration is more intense in the upper, vesicular part of each lava flow, whereas it is weaker in the inner, massive part, where the primary mineralogy has been almost completely preserved.

4. Whole-rock geochemistry 3. Petrology

4.1. Analytical methods

The Lower Cretaceous volcanic rocks form thick piles of mafic lava flows, some of which are several meters thick. They are porphyritic to highly porphyritic (20 – 30% phenocrysts) massive flows with a vesicular top and interbedded with sedimentary sequences. At some lava tops, structures similar to those described as peperites by Hanson and Hargrove (1999) are identifiable, which implies an interaction between hot magma and wet, unconsolidated or poorly consolidated sediments (White et al., 2000). Some of the stratigraphically lowermost lava flows show pillowlike volcanic structures (Aguirre and Egert, 1965), which indicates a subaqueous emplacement. Feeder dykes and subvolcanic intrusive andesites, with petrographical and mineralogical characteristics similar to those of the lavas, intrude the Lower Cretaceous units. The basic lavas are characterized by large (2 – 3 cm), mostly unzoned plagioclase (An60 – 54Ab37 – 42Or3 – 4; Fig. 2a, Table 1) phenocrysts (, 80% of total phenocrysts) and compositionally homogeneous, up to 1 cm clinopyroxenes (Wo37 – 41En45 – 42Fs18 – 17; Fig. 2b, Table 1). Minor idiomorphic olivine—pseudomorphosed by iron oxide and mafic phyllosilicates—idiomorphic magnetite, and Timagnetite are also found as phenocrysts. Microcrystalline

Twelve samples of mafic lava flows—mostly unaltered and free of calcite and vesicles—one feeder mafic dyke, and one intrusive andesite from area A (Fig. 1) were selected for whole-rock geochemistry (Table 2). From area B (Fig. 1), six lava flows and two intrusive andesites (sills) were also analyzed (Table 2). Geochemical analyses (major and trace elements) were carried out by inductively coupled plasma atomic emission spectrometry (ICP-AES, Perkin– Elmer P430) at the Departamento de Geologı´a, Universidad de Chile (J. Martı´nez, analyst) and by X-ray fluorescence at the Centre de Recherches Pe´trographiques et Ge´ochimiques, Nancy, France (samples ELQ- and MAR-). A subset of eight samples (Table 2) was also analyzed for trace elements by inductively coupled plasma mass spectrometry (ICP-MS, Perkin– Elmer Sciex Elan 5000) at the Centro de Instrumentacio´n Cientı´fica, Universidad de Granada, Espan˜a. Finally, sample MAR-76 (J. Nystro¨m, unpublished information) was analyzed by ICP-MS at the Centre de Recherches Pe´trographiques et Ge´ochimiques, Nancy, France. Sr and Nd isotope data (Table 3) were obtained for the eight ICP-MS analyzed samples at the Centro de Instrumentacio´n Cientı´fica, Universidad de Granada, Espan˜a,

Fig. 2. Chemical variations of primary mineralogy in the Lower Cretaceous volcanic rocks from La Serena. (a) Projection of plagioclase chemistry on an An– Ab–Or diagram. (b) Projection of the pyroxene chemistry on a Wo–En–Fs diagram (1, diopside; 2, hedenbergite; 3, augite; 4, pigeonite; 5, clinoenstatite; 6, clinoferrosilite; Morimoto et al., 1988). Open circles represent lavas from the Arqueros Formation in area A; open squares are lavas from area B.

Table 1 Selected microprobe (CAMECA SX-50, Universidad de Granada, 20 nA, 20 Kv, 5 m as analytical conditions) analyses of plagioclase and clinopyroxene phenocrysts Plagioclases

Pyroxenes

Sector A

Sector B

ARQ994 A 1 r

ARQ994 A 5 c

ARQ997 A 28 c

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O

53.63 0.08 27.92 0.00 0.89 0.02 0.11 11.21 4.55 0.73

52.83 0.05 28.02 0.00 0.83 0.03 0.14 11.47 4.36 0.70

52.40 0.08 28.97 0.00 0.75 0.01 0.14 11.91 4.22 0.40

Total

99.15

98.43

98.89

Si Ti Al Cr Fe2þ Mn2þ Mg Ca Na K Ba

Structural formulae to 2.458 2.441 0.003 0.002 1.508 1.526 0.000 0.000 0.034 0.032 0.001 0.001 0.008 0.009 0.551 0.568 0.404 0.391 0.043 0.041 0.000 0.000

P cat An Ab Or

TC0013 B 145 r

TC0013 B 151 c

TC995A B 31 c

TC995A B 32 r

Sample Zone Analysis Type

ARQ0018 A 79 c

54.27 0.07 27.93 0.01 0.98 0.03 0.12 11.32 4.85 0.68

52.95 0.06 28.59 0.00 0.86 0.03 0.12 12.15 4.29 0.61

52.72 0.06 28.84 0.01 0.89 0.01 0.13 12.24 4.15 0.55

53.10 0.04 28.33 0.01 0.94 0.02 0.11 11.78 4.46 0.46

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O NiO

50.96 0.62 2.51 0.00 10.92 0.37 15.50 18.30 0.32 0.00 0.00

100.26

99.66

99.61

99.26

Total

99.51

eight oxygens 2.408 2.462 0.003 0.003 1.569 1.493 0.000 0.001 0.029 0.037 0.001 0.001 0.009 0.008 0.586 0.550 0.376 0.427 0.024 0.039 0.000 0.000

2.420 0.002 1.540 0.000 0.033 0.001 0.008 0.595 0.380 0.036 0.000

2.411 0.002 1.554 0.000 0.034 0.000 0.009 0.600 0.368 0.032 0.000

2.433 0.001 1.530 0.000 0.036 0.001 0.008 0.578 0.396 0.027 0.000

5.015

5.010

5.012

Si AlVI Ti AlVI Fe3þ Cr Fe2þ Mn2þ Mg Ca Na K Ni P cat

54.154 58.884 41.990 37.600 3.855 3.517

59.955 36.813 3.232

57.723 39.565 2.713

(mg) En Fs Wo

5.009

5.010

5.005

55.198 40.502 4.300

56.795 39.106 4.099

59.464 38.129 2.407

5.021

Sector B ARQ0018 A 86 c

ARQ994 A 6 c

ARQ996 A 17 c

ARQ996 A 18 r

TC0013 B 158 r

51.09 0.64 2.41 0.01 10.55 0.40 15.48 19.11 0.36 0.01 0.02

51.10 0.59 2.36 0.01 9.92 0.38 15.49 18.95 0.37 0.00 0.00

51.36 0.73 1.82 0.01 11.41 0.34 15.01 18.33 0.34 0.01 0.02

50.39 1.00 2.93 0.00 10.79 0.40 14.71 18.85 0.33 0.01 0.00

50.98 0.61 2.35 0.00 10.53 0.40 15.76 18.52 0.34 0.00 0.04

100.07

99.17

99.39

99.40

99.54

Structural formulae to six oxygens 1.902 1.895 1.909 0.098 0.105 0.091 0.017 0.018 0.017 0.012 0.000 0.013 0.073 0.096 0.071 0.000 0.000 0.000 0.270 0.234 0.241 0.012 0.012 0.012 0.863 0.856 0.863 0.732 0.759 0.759 0.023 0.026 0.027 0.000 0.000 0.000 0.000 0.001 0.000

TC0013 B 159 c

TC995A B 40 r

TC995A B 41 c

TC995A B 51 c

50.99 0.67 2.67 0.00 9.99 0.34 15.62 19.51 0.29 0.00 0.03

51.39 0.58 2.31 0.01 10.91 0.39 15.72 18.12 0.31 0.00 0.03

51.35 0.56 1.95 0.00 11.30 0.44 15.49 18.26 0.32 0.00 0.01

51.01 0.55 1.98 0.01 11.08 0.38 15.62 18.33 0.28 0.00 0.03

100.11

99.77

99.68

99.25

1.927 0.073 0.021 0.008 0.048 0.000 0.311 0.011 0.839 0.737 0.025 0.001 0.001

1.889 0.111 0.028 0.018 0.061 0.000 0.278 0.013 0.822 0.757 0.024 0.000 0.000

1.899 0.101 0.017 0.002 0.089 0.000 0.241 0.013 0.875 0.739 0.025 0.000 0.001

1.887 0.113 0.019 0.004 0.092 0.000 0.219 0.011 0.862 0.774 0.021 0.000 0.001

1.912 0.088 0.016 0.013 0.065 0.000 0.277 0.012 0.872 0.722 0.023 0.000 0.001

1.917 0.083 0.016 0.002 0.072 0.000 0.282 0.014 0.862 0.730 0.023 0.000 0.000

1.910 0.087 0.015 0.000 0.082 0.000 0.267 0.012 0.872 0.735 0.020 0.000 0.001

4.002

4.002

4.002

4.002

0.797 0.759 44.016 44.765 16.462 18.159 39.522 37.075

0.753 43.959 18.808 37.233

0.765 44.286 18.355 37.358

4.002

4.003

4.002

4.001

4.002

4.002

0.762 44.249 18.205 37.545

0.785 43.726 17.477 38.796

0.782 44.354 16.638 39.008

0.730 43.122 19.022 37.856

0.747 42.563 18.253 39.184

0.784 44.717 17.525 37.758

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

Sample Zone Analysis Type

Sector A

c, core; r, rim. 463

464

Table 2 Geochemical analyses of the Coastal Range Lower Cretaceous volcanic rocks of the La Serena region (308S) Sector A: La Serena-Arqueros (298500 S) Sample Type Subunit

Ba Rb Th Nb Sr Zr Y Cr V Ni Co Sc Pb Cu Zn Ta Hf U

Ka1 49.63 1.11 16.92 4.37 6.00 0.53 4.37 5.91 3.73 3.63 0.49 3.14 99.83 0.56

1600 8 8 560 135 31 13 319 9 26 36 19 835 4.2

ARQ99-7 Dyke Ka1 49.10 1.31 17.31 3.41 7.12 0.21 3.86 7.96 4.62 1.51 0.26 3.00 99.67 0.49

MAR-76 Ocoite Ka1 54.05 1.12 17.31 5.97 1.91 0.23 2.13 4.79 4.57 4.37 0.77 2.07 99.29 0.67

ICP-MS

ICP-MS

888 50.7 3.74 3.0 576 134 30.5 56 346 23 29 35.8 5.66 277 188 0.22 3.67 0.94

1038 155.0 12.3 9.9 470 216 35.5 25 209 6 15 8.43 36 190 0.81 6.92 3.56

MAR-99 Lava Ka1 50.06 1.06 16.51 6.08 3.38 0.55 4.39 5.02 3.21 4.36 0.64 4.04 99.3 0.70

999 130 6 454 184 38 , 10 254 14 29

11

ARQ99-4

ARQ00-13

ARQ99-3

Ka3

Ka3

Ka5

53.90 0.76 19.11 4.32 2.44 0.10 1.99 6.35 4.29 4.04 0.49 1.79 99.58 0.59

54.50 0.79 19.00 4.11 2.52 0.11 2.14 6.14 4.11 3.84 0.47 2.00 99.73 0.60

ICP-MS

ICP-MS

987 144.7 16.20 8.9 673 175 25.2 29 183 9 16 19.4 14.64 34 86 0.67 4.59 4.15

1046 139.0 16.40 8.6 644 173 24.0 4 178 4 16 18.0 14.68 27 72 0.65 4.55 4.07

53.50 1.03 16.93 7.79 0.68 0.15 0.90 4.19 5.09 5.89 0.56 2.85 99.56 0.70

MAR-92 Ocoite Ka5 51.43 1.00 17.59 3.92 4.44 0.39 3.83 8.02 2.75 2.57 0.52 3.23 99.69 0.61

ARQ00-9 Kqm1a 51.53 0.99 16.95 5.77 3.40 0.16 3.88 6.45 3.84 3.93 0.42 2.60 99.92 0.67

ELQ-1 Ocoite Kqm1a 50.65 1.18 16.82 4.38 5.81 0.23 4.06 6.72 3.60 3.04 0.68 2.42 99.59 0.55

24 59 33 7.37 1.80 6.11 5.80 1.19 3.27 3.19 0.51

15.0 35.9 4.98 22.3 5.81 1.92 5.35 0.80 4.83 1.06 2.82 0.45 2.72 0.43

34.4 75.0 9.59 41.1 8.89 2.07 7.39 1.03 6.29 1.28 3.34 0.52 3.54 0.45

32.9 68.7 8.66 34.2 7.02 1.85 5.94 0.79 4.24 0.84 2.30 0.36 2.18 0.33

34.3 73.8 9.56 36.6 7.04 1.66 5.39 0.82 4.51 0.88 2.37 0.35 2.29 0.35

47.97 0.82 20.78 4.18 4.09 0.34 4.37 6.68 3.54 2.33 0.39 2.84 98.33 0.66

MAR-77 Ocoite Kqm1a

MAR-80 Lava Kqm1a

51.68 0.91 17.33 6.05 2.45 0.16 4.17 6.17 3.95 3.84 0.62 2.55 99.88 0.75

52.33 0.96 19.55 6.74 1.19 0.32 2.89 2.04 2.95 7.05 0.62 3.72 100.36 0.81

ICP-MS 768 12 12 220 111 34 7 183 12 13 22 213 95 5.8

910 69 9 594 165 27 16 251 29 45

2327

1066 172.3 15.18 9 751 171 25.9 20 245 14 25 24.3 17.19 29 103 0.76 4.92 4.10

1397 163

545 80

563 213

4226 409

2 637 150 37 11 254 15 28

,1 786 49 14 ,10 249 15 28

12 806 162 28 26 206 21 23

3 771 64 13 ,10 194 ,10 ,10

140

10

89

12

ICP-AES La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

ELQ-2 Lava Kqm1a

31 78 41 8.29 1.92 7.58 6.76 1.36 3.82 3.73 0.58

27.7 59.7 7.74 32.2 6.47 1.49 5.42 0.84 4.68 0.98 2.73 0.41 2.75 0.40

ARQ00-19

TC99-5a

TC99-5b

TC99-6b

TC00-20

TC00-13

TC00-16

TC99-2

TC00-9

KTai

Ka1

Ka1

Ka1

Ka1 (?)

Ka3 (?)

Ka3 (?)

Ktai?

Ktai?

52.12 0.90 16.55 7.91 1.48 0.15 3.25 8.52 2.85 2.70 0.33 2.96 99.72 0.80

55.89 0.95 16.30 3.43 4.56 0.16 3.32 7.80 2.60 2.16 0.27 2.16 99.60 0.56

56.60 1.02 17.51 3.78 3.12 0.11 3.26 5.48 4.17 2.67 0.25 1.99 99.96 0.65

51.20 1.00 17.05 5.35 4.60 0.17 4.30 8.68 3.09 2.35 0.29 1.84 99.92 0.62

ICP-MS

ICP-MS

629 83.9 13.77 10.1 463 250 28.5 19 171 9 18 20.8 6.95 47 71 0.79 7.14 3.66

674 78.5 11.93 6.6 503 132 24.7 59 282 18 29 33.7 13.84 297 100 0.49 3.73 3.18

27.1 63.0 8.32 34.2 7.35 1.63 5.84 0.91 5.17 1.08 3.01 0.46 2.95 0.40

23.0 48.9 6.22 25.1 5.62 1.55 5.02 0.69 3.90 0.85 2.39 0.38 2.19 0.34

51.10 1.02 17.19 5.90 4.40 0.21 4.68 8.65 2.69 2.47 0.33 1.61 100.25 0.65

52.80 0.93 16.94 5.00 4.20 0.19 4.04 7.54 3.21 2.60 0.31 1.92 99.68 0.63

54.01 0.80 19.41 4.22 2.56 0.17 2.61 6.15 3.08 4.27 0.39 2.27 99.94 0.65

ICP-MS 690

770

1700

11 9 480 119 22 28 269 17 24 31

9 9 526 110 22 20 255 16 25 31

12 14 650 149 20 17 176 8 15 18

144 116

123

34 224

4.0

4.1

2.7

900 76.9 11.51 5.7 638 122 21.7 15 262 7 25 28.61 17.07 35 79.67 0.46 3.62 2.90

54.56 0.77 17.58 2.58 4.48 0.12 3.61 6.87 2.92 3.11 0.18 3.24 100.02 0.59

52.80 0.80 19.31 4.91 3.68 0.18 3.59 7.28 3.93 1.20 0.29 1.72 99.69 0.63

ICP-MS 605 7 37 504 121 22 19 267 12 21 28 1248 160 1.9

462 83.8 9.98 6.5 396 185 23.3 55 187 20 22 22.8 6.58 90 62 0.52 5.14 2.76

340 3 4 620 90 21 13 158 9 18 17 62 87 2.7

ICP-AES

24 59

25 56

35 8.37 1.78 6.68

29 6.29 1.54 5.17

5.79

4.55

3.49

2.47

3.17 0.59

2.52 0.41

25 54

25 54

27 65

28 5.60 1.44 5.09

29 5.55 1.42 5.00

37 6.08 1.61 4.38

4.32 0.90 2.43

4.43 0.93 2.45

4.25 0.89 2.38

2.42 0.36

2.44 0.39

2.34 0.36

25.3 53.0 6.92 29.5 6.01 1.45 4.84 0.72 4.20 0.84 2.38 0.36 2.30 0.32

17 42 25 4.83 1.20 4.24 4.51 0.90 2.36 2.33 0.35

18.0 42.0 5.41 21.6 4.93 1.22 4.48 0.62 3.74 0.78 2.24 0.34 2.15 0.33

13 33 23 4.78 1.42 3.97 4.28 0.86 2.30 2.26 0.33

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Sum (mg)

ARQ99-8

Sector B: Tres Cruces (298200 S)

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

465

Table 3 Sr–Nd isotopic data for the lavas of the Lower Cretaceous from the Coastal Range in the La Serena region (308S) Sample

Type

Age (Ma)

Formation

Rb

Sr

Sm

Nd

87

Sr/86Sr

143

Nd/144Nd

(87Sr/86Sr)0

(143Nd/144Nd)0

1tNd CHUR

tDM (Ga)

Sector A MARQ-76 ARQ99-7 ARQ99-4 ARQ00-13 ARQ00-9 ARQ00-19

b–a b b–a b–a b–a b–a

117 117 115 115 110 100

Ka1 Feeder dyke Ka3 Ka3 Kqm1 Ktai

155.0 50.7 144.7 139.0 172.3 83.9

470 576 673 644 751 463

8.89 5.81 7.02 7.04 6.47 7.35

41.1 22.3 34.2 36.4 32.2 34.2

0.705150 0.704145 0.704457 0.704453 0.705012 0.704104

0.512831 0.512847 0.512744 0.512745 0.512805 0.512865

0.70356 0.70372 0.70344 0.70343 0.70397 0.70336

0.512731 0.512726 0.512651 0.512658 0.512718 0.512780

þ 4.7 þ 4.7 þ 3.1 þ 3.3 þ 4.3 þ 5.3

0.47 0.63 0.58 0.53 0.47 0.41

Sector B TC99-5a TC00-13 TC99-2

b–a b–a a

117 115 100

Ka1? Ka3? Ktai?

78.6 76.9 83.8

503 638 396

5.62 6.01 4.93

25.1 29.5 21.6

0.704370 0.704183 0.704231

0.512750 0.512732 0.512873

0.70362 0.70361 0.70336

0.512646 0.512639 0.512783

þ 3.1 þ 2.9 þ 5.3

0.64 0.59 0.44

Initial ratios were calculated on the basis of an assumed age of 117 and 115 Ma for the lavas of the Arqueros Formation (Ka1 and Ka3), 110 Ma for the Quebrada Marquesa Formation lava (Kqm1), and 100 Ma for the intrusive andesites (Ktai). b, basalt; b– a, basaltic-andesite; a, andesite. Concentrations of Rb, Sr, Sm, and Nd are in ppm. Model ages (tDM ; in Ga) are calculated with reference to the depleted mantle reservoir.

using a Finnigan MAT 262 thermal ionization mass spectrometer (TIMS) with variable multicollector and RPQ. The normalization value for 87 Sr/ 86Sr was 88 Sr/86Sr ¼ 8.375209, and the reproducibility under successive determinations of the NBS-987 dissolved standard was better than 0.0007% ð2sÞ: For the Nd determinations, the 143 Nd/ 144Nd was normalization value for 146 144 Nd/ Nd ¼ 0.7219 with precision better than 0.0016% ð2sÞ calculated under successive measures of the WSE power standard. The reproducibility of successive measurements of the La Jolla dissolution standard was better than 0.0014% ð2sÞ: For sample MAR-76, the Sr and Nd isotopic data were obtained on a Finnegan MAT 261 multicollector mass spectrometer at the Swedish Museum of Natural History, Stockholm (J. Nystro¨m, unpublished information) (for analytical considerations, see Parada et al., 1999). Initial isotopic ratios were calculated on the basis of ages of 117, 115, and 110 Ma for the basic lavas and 100 Ma for the intrusive andesite (Table 3).

K2O contents and plot in the high-K series. The high-K content of the Lower Cretaceous lavas has been noted as one of their major characteristics and used to classify them as high-K calc-alkaline to shoshonitic (Levi et al., 1988; Vergara et al., 1995). The variations of major elements with respect to the SiO2 content are plotted in Fig. 4. Because no differences can be established between the two studied areas, they are treated together. Despite the incipient alteration (loss on ignition, LOI, ranges 1.5– 3.5 wt%) always present in these rocks, a relatively good negative correlation between SiO2 content and MgO, TiO2, and FeO and a poor one with CaO can be observed. Alkali elements could be considered relatively unaffected by the relatively high volatile content, and similar K and Na contents can be observed in rocks with different

4.2. Major element geochemistry According to the SiO2 content (50 – 57 wt% on anhydrous bases), the rocks analyzed are mainly basaltic andesites with minor basalts and andesites. Total alkalis show scattered values ranging from 4.98 to 11.35% (on anhydrous bases) without any correlation with SiO2 content. With such alkali values, the rocks classify as intermediate between subalkaline and alkaline, with Na2O/K2O normally # 1 or . 1 in feeder dyke and intrusive andesites (Table 2). In the SiO2 versus K2O diagram (Fig. 3), most of the studied rocks classify as shoshonitic to high-K calc-alkaline basaltic andesites, with a relatively good positive correlation between the two oxides. No major differences have been found between the lavas from the two studied areas, though the feeder dyke (sample ARQ99-7) and intrusive andesites (samples ARQ00-19 and TC00-9) have lower

Fig. 3. SiO2 versus K2O (anhydrous base) recommended classification diagram for subduction zone magmas (Tatsumi and Eggins, 1995) for the Lower Cretaceous volcanic rocks of the La Serena region. Open circles represent lavas from the Arqueros Formation in area A; open squares are lavas from area B; open triangles are lavas from the Quebrada Marquesa Formation in area A; the black circle represents the feeder dyke (ARQ997); and the black square is the intrusive andesite (ARQ00-19).

466

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

Fig. 4. Major element variations with respect to SiO2 content (anhydrous base). Symbols are as in Fig. 3.

LOI values (Table 2). Two geochemical characteristics of these rocks are worth noticing: the relatively high and constant Al2O3 content around 17% and the relatively low (, 5%) MgO values. The poor correlation observed in the CaO values can be attributed to the incipient albitization of some plagioclase crystals, whereas no major dispersion is observed in the Na2O, which mainly ranges 3 –4%. No correlation appears between the LOI and SiO2 values. Most rocks have Fe2O3/FeO ratios . 1, and a negative correlation with the LOI content is observed (Fig. 5). The high constant Al2O3 and low MgO contents seem general characteristics of the Mesozoic volcanic rocks in the Andean margin (Vergara et al., 1995) and will be discussed subsequently. In terms of

Fig. 5. Variations of the Fe2O3/FeO ratio with respect to the loss on ignition (LOI) values.

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

467

major element variations, the Lower Cretaceous lavas, feeder dyke, and intrusive andesites conform to a unique trend without major chemical differences between outcrops. 4.3. Trace element geochemistry As in the case of the major elements, no important differences have been found among the lavas, feeder dyke, and intrusive andesites. Furthermore, no significant differences between the lavas from the two studied areas or from different members of the Arqueros Formation are observed. The large ion lithophile elements (LILE) generally show a higher dispersion than the high field strength elements (HFSE) due to their higher mobility with alteration. However, despite this dispersion, relatively good correlations are observed. The Ba (up to 1000 ppm) and Sr (up to 800 ppm) contents are remarkably high, and the dispersion in Rb is strong. Sr shows a negative correlation, in agreement with the behavior of the CaO, which indicates their substitution in the Ca-bearing minerals. Positive correlations of SiO2 with Zr, Nb, Th, Hf, and La are found. The lower Th contents correspond to the feeder dyke (ARQ99-7), whereas the higher Th values are from lavas of the Ka3 member of the Arqueros Formation. The compatible elements Cr, Ni, V, and Sc show a negative correlation with SiO2, with a major dispersion in the case of Cr and Ni and a relatively high correlation factor for V and Sc. Yb shows a slight negative correlation with SiO2 and, consequently, a slight increase in the (La/Lu)N ratios with differentiation. In agreement with low MgO values, the Cretaceous lavas have low Cr and Ni contents. In spite of their high Ca-plagioclase content, no positive Eu anomaly or decrease in the (Eu/Eup)N ratio with SiO2 is observed. Trace elements normalized to the normal mid-ocean ridge basalts (N-MORB) show the typical pattern of subduction zone magmatism (Fig. 6), with enrichment in the incompatible elements with respect to the N-MORB. Patterns are characterized by an increase from Sr to Rb, Rb < Th, and Th . Ba and a general decrease in the values from Th to Yb. Ta –Nb constitutes a marked trough and, except for the feeder dyke and two samples from the Quebrada Marquesa Formation, has values greater than 1. In all samples, TiO2, Y, and Yb present values close to or slightly lower than the N-MORB. The compatible elements Sc and Cr have values lower than 1. The feeder dyke (ARQ99-7) has Ba . Th < Rb and, together with the two samples from the Quebrada Marquesa Formation, a more pronounced Ta – Nb trough. The intrusive andesites (ARQ00-19 and TC00-9) show no difference with respect to the other lavas plotted. In general terms, the trace element patterns of the Lower Cretaceous volcanic rocks from the La Serena region are similar to those shown by Vergara et al. (1995) for the basaltic andesites of the Veta Negra Formation. Differences are related to the higher Ba, Th, Ta –Nb, and Ce contents in La Serena. The lavas from the Veta Negra Formation at the latitude of Santiago have

Fig. 6. N-MORB (Pearce, 1982) normalized multi-element pattern diagrams for Lower Cretaceous volcanic rocks of the Coastal Range of the La Serena region. For samples for which no ICP-MS data are available, Ta was calculated according to the ratio Na/Ta ¼ 16 (Wood et al., 1979).

Rb . Ba . Th, and the Ta– Nb trough is slightly below the N-MORB values (BUS-35; Fig. 7). Only the feeder dyke (ARQ99-7) has a pattern similar to those of the Veta Negra Formation lavas; the pattern of the intrusive andesite (ARQ00-19) is identical to those of the lavas from La Serena. The rare-earth element (REE) contents normalized to chondrite (Fig. 8) show homogeneous patterns, with enrichment in light REE (La to Sm) with respect to the heavy REE. The LREE have 40 –100 times the chondritic values, whereas the HREE have 8 –20 times. In general, the higher REE values correspond to lavas richer in SiO2 and

468

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

Fig. 7. N-MORB (Pearce, 1982) normalized multi-element pattern diagrams for selected basaltic-andesitic lavas of both studied areas compared with the basaltic-andesite sample BUS-35 from the Veta Negra Formation (Vergara et al., 1995). All samples were analyzed by ICP-MS. Numbers in parentheses are the SiO2 contents of each sample.

lower in MgO and CaO. The (La/Lu)N ratio ranges from 5.6 to 11.2 in the lavas, with lower values for the feeder dyke (4.2). The intrusive andesite has a (La/Lu)N value of 8.1, in the same range as those of the lavas. In all the patterns, LREE enrichment is observed, though for the Gd to Lu, an almost flat pattern is present (Fig. 8). Again, as shown in Fig. 7, only the feeder dyke (ARQ99-7) has a close similarity to the basaltic andesites from the Veta Negra Formation (BUS-35, Vergara et al., 1995) with an almost flat REE pattern (Fig. 8) and a (La/Lu)N ratio of 3.92. A remarkable feature observed in the REE patterns of these plagioclase-rich porphyritic lavas is the systematic absence of a positive Eu anomaly. All have a slightly negative Eu anomaly, with (Eu/Eup) ranging from 0.73 to 0.90. This absence of a positive Eu anomaly is also observed in the Lower Cretaceous lavas from the Pabello´n (Fig. 5 in Cisternas et al., 1999) and Veta Negra (Table 2 in Vergara et al., 1995) Formations. 4.4. Sr and Nd isotope geochemistry Sr and Nd isotope ratios appear in Table 3. In lavas, the (87Sr/86Sr)0 ratio ranges 0.70344 –0.70397 in area A and 0.70361 – 0.70362 in area B, whereas the (143Nd/144Nd)0 ratio ranges 0.512651– 0.512731 (1Nd ¼ þ3:1 to þ 4.7) in area A and 0.512639 –0.512646 (1Nd ¼ þ2:9 to þ 3.1) in area B. The feeder dyke (ARQ99-7) has a higher initial 87 Sr/86Sr ratio (0.70372), with 1Nd ¼ þ4:7: The intrusive andesites show a similar lower (87Sr/86Sr)0 ratio (0.70336 in samples ARQ00-19 and TC99-2) and higher (143Nd/144Nd)0 ratio (0.512780, 1Nd ¼ þ5:3 in sample ARQ00-19; 0.512783, 1Nd ¼ þ5:3 in sample TC99-2). The Sr isotopic ratios for these Lower Cretaceous rocks are almost coincident with previously published values for rocks of the Veta Negra Formation (0.70379 ^ 0.00020, Vergara et al., 1995). A general decrease in the (87Sr/86Sr)0 ratio with SiO2 content is apparent, but no correlation between SiO2 and the (143Nd/144Nd)0 ratio is observed.

Fig. 8. Chondrite-normalized patterns (Nakamura, 1974) for the REE content of the Lower Cretaceous volcanic rocks in the Coastal Range of the La Serena region. Basaltic andesite from the Veta Negra Formation (BUS35 in Vergara et al., 1995) is plotted for comparison.

5. Tectonic setting and conditions of emplacement of the Lower Cretaceous lavas According to new 40Ar/39Ar ages obtained for the Early Cretaceous lavas in the Coastal Range of the La Serena region (Morata et al., 2001), the older lavas from the Ka1 and Ka3 members were emplaced 117 – 115 Ma ago, and lavas from the Quebrada Marquesa Formation consequently were emplaced after this period. Lavas from the Arqueros Formation were intruded by granitoids to which a minimum age of 108 –109 Ma can be assigned on the basis of recent K –Ar ages (Empara´n and Pineda, 2000). Plutonic rocks of the Illapel Complex farther south (318150 S 718150 W – 318530 S 708570 W), which intrudes Lower Cretaceous volcanic rocks, range in age from 85.9 ^ 2.2 to

D. Morata, L. Aguirre / Journal of South American Earth Sciences 16 (2003) 459–476

113 ^ 3 Ma (K– Ar data, Rivano et al., 1985). These values constrain the emplacement of the Lower Cretaceous lavas in the La Serena region to 117– 110 Ma. These ages are slightly younger than those obtained for the emplacement of lavas of the Veta Negra Formation, 400 km to the south of the studied area. 40Ar/39Ar plateau ages from primary plagioclase phenocrysts of 119.4 ^ 1.2 and 118.7 ^ 0.6 Ma have been obtained for the Bustamante region (338250 S) (Aguirre et al., 1999) and the Chacana area (338S) (Fuentes et al., 2001), respectively. Model ages, calculated with reference to the depleted mantle reservoir (tDM ; Table 4), range from 0.41 to 0.64 Ga. These ages put these Lower Cretaceous volcanic rocks in the same magmatic event, with evolution in time from south to north. An extensional geodynamic regime characterized by spreading/subsidence and strong crustal attenuation, without the generation of oceanic crust, has been proposed as the predominant tectonic setting along the Pacific margin of South America during the Early Cretaceous from the southernmost Andes in the Magallanes area (Dalziel, 1981) to Peru, with the development of ensialic, aborted, marginal basins (Atherton et al., 1983). For the Pacific ˚ berg et al. (1984) postulate the existence Chilean margin, A of an aborted marginal basin generated as the outcome of that tectonic regime, whereas Charrier and Mun˜oz (1994) and Vergara et al. (1995) propose an intra-arc basin. Volcanic rocks dominate in the central part of the basin (Veta Negra Formation), whereas sedimentary rocks predominate to the north (the Arqueros Formation) and south ˚ berg et al., 1984) of the basin. In this scenario, the (A emplacement of Cretaceous granitoids is consistent with crustal spreading. The projection of the geochemical features of the La Serena Lower Cretaceous volcanic rocks into various geotectonic discriminant diagrams confirms the calcalkaline orogenic signature of these lavas (Fig. 9). It is worth noting the similarities among the Lower Cretaceous lavas from the Veta Negra Formation, the studied area, and the Pabello´n Formation. As was shown for the trace elements, REE, and initial isotopic ratios, the feeder dyke (ARQ99-7) overlaps the Veta Negra lavas. Some trace element ratios are useful for discriminating the tectonic regime in which the lavas were erupted. According to Pearce and Norry (1979), lavas with Zr/Y ratios . 3 are characteristic of continental volcanic arcs, whereas those with Zr/Y ratios , 3 are typical of oceanic volcanic arcs. Vergara et al. (1995) show a trend in time from continental to oceanic character during the Mesozoic volcanic sequences at Santiago’s latitude. According to these authors, the Veta Negra lavas would have erupted in a highly attenuated, immature, continental volcanic arc. Because the Lower Cretaceous lavas from La Serena and Copiapo´ have Zr/Y . 3 (Table 4), a slightly different tectonic setting with respect to those that dominate 338S must be invoked. Gorton and Schandl (2000) propose a geochemical index to discriminate between continental and

469

island arcs in volcanic rocks of felsic to intermediate compositions. Lavas with Th/Ta . 6– 20 are characteristic of active continental margins, whereas a Th/Ta ratio . 20– 90 would be typical of oceanic arcs. The Th/Ta ratio (# 20) of the Lower Cretaceous lavas considered here (Table 4) is in the range of the active continental margins, close to the boundary with the oceanic arcs. The same tectonic setting can be proposed using the Ta/Yb versus Th/Yb diagram (Fig. 10). According to the original boundaries proposed by Pearce (1982, 1983), lavas from the Veta Negra Formation belong to oceanic arcs (Ta/Yb , 0.1); those from the La Serena and Copiapo´ regions erupted in an active continental margin (Ta/Yb . 0.1). Applying the tectonic field divisions proposed by Gorton and Schandl (2000), the boundary between oceanic arcs and active continental margins is an approximately 458 line (Fig. 10), and the studied Lower Cretaceous lavas plot on the active continental margin field, close to the oceanic arc boundary. According to Vergara et al. (1995), extension is indicated by high rates of subsidence (100 – 300 m/m yr), as inferred from thick piles of predominantly volcanic rocks deposited in elongated basins that remain close to sea level. Subsidence was probably initiated by rifting processes controlled by a regional block-fault pattern imposed by uplift during the Late Jurassic. In this context, the eruption of voluminous, mantle-derived, fissural flood basalts along ˚ berg et al., north-trending faults would have been favored (A 1984). These north-trending fault systems could have been related to major tectonic structures subparallel to the Pacific margin. In this sense, it is worth noting the presence, along the Chilean Pacific margin, of two long-lasting, major N – S fault systems known as the Liquin˜e-Ofqui and Atacama, both of which are clearly related to plutonism and volcanism (Herve´ and Thiele, 1987). In particular, the Atacama fault system, 208300 –298300 in northern Chile, was active during the Late Jurassic – Early Cretaceous, coincident with the greatest plutonic activity in the arc (Gonza´lez, 1999; Scheuber and Gonza´lez, 1999). Intra-arc normal crustal extension with emplacement of large volumes of calcalkaline plutons took place during 160 – 150 Ma as a consequence of oblique NW –SE subduction (Scheuber and Gonza´lez, 1999). Because a strongly oblique SE subduction operated in this part of the Chilean Pacific margin during the Jurassic– Early Cretaceous (Jaillard et al., 1990), the fissural volcanic activity of the Coastal Range could have been controlled by similar major N – S fault systems. In short, intra-arc extension, combined with control by major N – S fault systems, would account for the emission of the Lower Cretaceous lava flows and the intrusion of their associated plutons. ˚ berg et al. (1984), basin formation with the As noted by A eruption of mantle-derived basalts was an outstanding feature of the western margin of South America during the Early Cretaceous. In some of these basins, oceanic crust was generated, though most were of the aborted type. A progressive decrease in age of these Early Cretaceous

470

Table 4 Geochemical features of the Lower Cretaceous volcanic rocks from the Coastal Range in the La Serena region (298300 –308S) Sector A: La Serena-Arqueros (298500 S) Formation

Arqueros

Feeder dyke

Sector B: Tres Cruces (298200 S) Quebrada Marquesa

Ktai

Arqueros?

Ktai?

TC99-2

TC00-9

278300

31– 348

Veta Negra

Pabello´n

Andean Cordillera

SiO2 Al2O3 MgO K2O (mg) Na2O/K2O Fe2O3/FeO

52.44 ^ 2.03 17.62 ^ 1.03 2.82 ^ 1.37 4.10 ^ 1.00 0.63 ^ 0.05 0.98 ^ 0.13 3.06 ^ 3.79

49.10 17.31 3.86 1.51 0.49 3.06 0.48

50.83 ^ 1.71 18.29 ^ 1.78 3.87 ^ 0.58 4.04 ^ 1.81 0.69 ^ 0.10 1.03 ^ 0.40 2.32 ^ 1.98

56.60 17.51 3.26 2.67 0.65 1.56 1.21

53.06 ^ 1.67 17.54 ^ 1.19 3.68 ^ 0.65 2.61 ^ 0.87 0.64 ^ 0.07 1.35 ^ 0.80 1.67 ^ 1.52

56.38 18.16 3.73 3.21 0.59 0.94 0.58

53.87 17.10 3.36 2.79 0.80 3.28 1.33

51.10 18.52 3.58 2.33 0.58 1.39 1.12

54.29 ^ 0.95 16.91 ^ 0.70 4.15 ^ 1.07 3.09 ^ 0.38 1.00 ^ 0.00 1.22 ^ 0.17

54.47 ^ 1.73 18.64 ^ 1.49 3.34 ^ 0.48 1.98 ^ 0.73 0.58 ^ 0.06 2.09 ^ 0.88 0.98 ^ 0.42

(La/Lu)n Eu/Eup La/Nb Nb/Y Th/Yb Ta/Yb Ba/La Zr/Nb Zr/Y Ba/Nb Ba/Zr Ba/Th Rb/Nb Th/Nb Th/La Ti/Zr La/Ta La/Yb Sr/Y Th/Ta

8.88 ^ 2.89 0.79 ^ 0.05 3.3 ^ 0.5 0.3 ^ 0.1 4.76 ^ 2.35 0.23 ^ 0.06 36.7 ^ 17.0 19.3 ^ 6.4 6^2 123.5 ^ 45.2 7^2 94.99 ^ 59.41 8.8 ^ 9.2 1.37 ^ 0.41 0.41 ^ 0.07 39 ^ 11 47.93 ^ 6.91 11.09 ^ 3.62 18 ^ 8 19.97 ^ 6.01

4.18 1.04 5.1 0.1 1.38 0.08 59.3 45.3 4 300.0 7 237.24 17.1 1.26 0.25 61 67.88 5.51 19 16.96

6.42 ^ 1.34 0.77 ^ 0.05 5.6 ^ 5.7 0.3 ^ 0.2 4.48 0.19 ^ 0.13 40.8 ^ 17.6 31.6 ^ 29.0 5^2 566.0 ^ 633.8 19 ^ 26 91.67 58.9 ^ 62.4 1.20 0.46 64 ^ 34 89.53 ^ 90.50 9.08 ^ 1.25 38 ^ 18 19.20

8.11 0.78 2.6 0.4 4.80 0.25 24.2 25.6 11 63.0 2 52.50 0.0 1.20 0.46 24 41.60 10.40 20 19.20

7.99 ^ 1.28 0.83 ^ 0.05 3.1 ^ 0.9 0.3 ^ 0.2 4.47 ^ 0.94 0.21 ^ 0.12 37.0 ^ 13.1 15.5 ^ 4.8 6^1 108.7 ^ 32.1 7^2 85.18 ^ 30.23 5.1 ^ 7.0 1.38 ^ 0.51 0.44 ^ 0.05 46 ^ 7 44.30 ^ 8.73 10.15 ^ 1.48 25 ^ 5 19.71 ^ 4.96

6.50 0.78 2.8 0.3 4.64 0.24 25.7 28.4 8 71.1 3 46.34 12.9 1.53 0.55 26 34.59 8.39 17 19.15

4.67 0.97 3.3 0.2 1.33 0.11 26.2 22.5 4 85.0 4 113.33 0.0 0.75 0.23 54 52.00 5.75 30 12.00

3.86 0.87 6.1 0.1 1.28 0.07 42.8 41.1 3 261.8 6 173.43 44.4 1.57 0.26 71 72.78 4.92 28 18.86

6.80 ^ 0.99 0.80 ^ 0.05 3.6 ^ 0.6 0.3 ^ 0.1 3.13 ^ 0.56 0.19 ^ 0.05 24.3 ^ 3.9 30.0 ^ 4.6 8^1 87.0 ^ 18.7 3^0 74.89 ^ 12.08 10.1 ^ 4.0 1.16 ^ 0.20 0.33 ^ 0.05 28 ^ 8 51.79 ^ 11.10 9.60 ^ 1.29 22 ^ 4 16.73 ^ 3.09

7.49 ^ 2.31 0.84 ^ 0.16 4.4 ^ 2.1 0.2 ^ 0.1 3.16 ^ 1.19 0.19 ^ 0.08 19.5 ^ 10.7 30.0 ^ 10.7 7^2 80.3 ^ 42.0 3^1 62.22 ^ 34.14 6.8 ^ 2.7 1.47 ^ 0.87 0.32 ^ 0.04 44 ^ 21 56.51 ^ 26.42 9.75 ^ 2.78 24 ^ 12 18.90 ^ 11.09

0.70372 0.512726 þ4.7

0.70397 0.512718 þ 4.3

(87Sr/86Sr)0 (143Nd/144Nd)0 1Nd

0.70348 ^ 0.00007 0.512680 ^ 4.4 £ 1025 þ 3.7 ^ 0.9

0.70336 0.512780 þ 5.3

0.70362 ^ 0.00000 0.512643 ^ 0.000052 þ 3.0 ^ 0.1

0.70336 0.512783 þ5.3

0.70374 0.512701 þ 4.2

0.70354 ^ 0.00034

Data from the Veta Negra Formation (Vergara et al., 1995) at the latitude of Santiago (338S) and the Pabello´n Formation (Cisternas et al., 1999) in the Copiapo´ area (278300 ) are also shown. Data from Lower Cretaceous backarc volcanic rocks from the Andean Cordillera (31– 348S, Vergara and Nystro¨m, 1996) are shown for comparison.

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ARQ00-19

Ktai?

338S

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471

Fig. 9. Tectonic discrimination diagram (Cabanis and Lecolle, 1989) for the Lower Cretaceous volcanic rocks in the Coastal Range of the La Serena region: (1) orogenic domains; (2) intracontinental and postorogenic domains; (3) anorogenic domains. CA, calc-alkaline basalts; VAT, volcanic arc tholeiites; CB, continental tholeiites; BAB, basalts generated in fore- and backarc basins; E-MORB, enriched mid-ocean ridge basalts; N-MORB, normal mid-ocean ridge basalts. Symbols are as in Fig. 3. Open diamonds represent lavas from the Veta Negra Formation (338S, Vergara et al., 1995); crosses are lavas from the Pabello´n Formation (278300 S, Cisternas et al., 1999).

basins from south to north is postulated here. The geochemical signature found for the Lower Cretaceous lavas in the Coastal Range of the La Serena region enables us to include them as part of a major Lower Cretaceous magmatic province developed along the Pacific Chilean margin. Local variations in the geodynamic setting, probably related to oceanic crustal segmentation, can account for the geochemical differences found along it.

6. Constraints on the genesis of magmatism The geochemistry of the volcanic rocks generated in subduction-related zones reflects the contribution of several components, such as mantle wedge, oceanic crust, sediments, fluids, and hydrous melts, to the magmas that originated in this type of setting (Pearce and Peate, 1995; Pearce et al., 1995). Some petrogenetic trace element ratios have been calculated and are shown in Table 4. Geochemical similarities between the Lower Cretaceous lavas from both studied areas (Fig. 1A and B) and those from the Pabello´n Formation farther north (278300 S) are evident. However, differences have been found between these lavas and those from the Veta Negra Formation (338S), with which only the feeder dyke (ARQ99-7) presents similarities.

The Ta/Yb versus Th/Yb diagram (Fig. 10) highlights these similarities and differences. An arc signature is evident in this diagram, in which lavas from the La Serena region have higher Th contents than those from the Veta Negra Formation. In comparison with the coetaneous andesites and basaltic andesites from the Andean Cordillera at 318– 348S (Table 4), for which a backarc geotectonic setting has been proposed by Vergara and Nystro¨m (1996), minor geochemical differences can be established. The initial Sr and Nd isotopic ratios (Tables 3 and 4, Fig. 11) of the studied samples plot on the mantle array, above bulk earth, in the more primitive sector of the field proposed by Nystro¨m et al. (1993) for the Veta Negra lavas. Compared with recent basic lavas from the southern volcanic zone of the Andes (34 – 418S), most Lower Cretaceous lavas are more primitive (Fig. 11). Data from granitoids of the Illapel Complex (31 –328S), with ages ranging from 85.9 ^ 2.2 to 113 ^ 3 Ma (Rivano et al., 1985), are also plotted in Fig. 11. In general terms, the Lower Cretaceous rocks (granitoids and lavas from La Serena), together with Lower Cretaceous lavas from the High Andes, plot closely in the diagram (Fig. 11) and thereby confirm a Lower Cretaceous field distinguishable from that of the Jurassic volcanic rocks of the Coastal Range in central Chile. As shown by REE and trace elements,

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age ¼ 93.5– 96.5 Ma; Parada et al., 2000, 2001), emplaced in the Veta Negra volcanic sequence. To the north of the studied area (, 278S), McNutt et al. (1975) obtain unusually low (87Sr/86Sr)0 ratios (0.7022– 0.7030) for several Cretaceous plutonic rocks with K –Ar ages of 107 –125 Ma, which are interpreted as originating from a subcrustal source. These geochemical similarities suggest that comparable petrogenetic processes operate in the genesis of the Lower Cretaceous magmatic rocks, which would confirm a major igneous province. A potential relationship with a mantle plume or superplume, as Vergara et al. (1995) and Parada et al. (1999) propose, may explain some of the geochemical features of Lower Cretaceous magmatism in north central and central Chile. 6.1. Magma source

Fig. 10. Ta/Yb versus Th/Yb diagram (Pearce, 1982, 1983) for the Lower Cretaceous volcanic rocks from the Coastal Range in the La Serena (308S) region. Data from the Pabello´n Formation (Cisternas et al., 1999) in the Copiapo´ area (278300 ), Veta Negra Formation (Vergara et al., 1995) at the latitude of Santiago (338S), and Lower Cretaceous volcanic lavas from the Andean Cordillera (Vergara and Nystro¨m, 1996) are also plotted. C, crustal contamination vector; W, within-plate enrichment vector; S, subduction zone enrichment vector; f, fractional crystallization vector; SHO, shoshonitic series; CA, calc-alkaline series; TH, tholeiitic series. Midocean ridge and within-plate volcanic rocks plot along the diagonal shaded band. The vertical boundary between oceanic arcs and active continental margins (Ta/Yb ¼ 0.1) from Pearce (1983). Diagonal dotted lines divide the fields of oceanic arcs (OA), active continental margins (ACM), and within-plate volcanic zones (WPVZ) (Gorton and Schandl, 2000). UC, average composition of the upper continental crust (Wedepohl, 1995). Primitive mantle (PM), N-MORB, E-MORB, and OIB values from Sun and McDonough (1989). Open circles represent lavas from the Arqueros Formation in area A; open squares are lavas from area B; open triangles are lavas from the Quebrada Marquesa Formation in area A; open diamonds represent lavas from the Veta Negra Formation; crosses are lavas from the Pabello´n Formation; the black circle is the feeder dike (ARQ99-7); the black square represents the intrusive andesite (ARQ00-19); and black triangles are lavas from the Andean Cordillera.

the feeder dyke (ARQ99-7) shows the closet isotopic similarities with the lavas of the Veta Negra Formation. Sample TC99-2 shows the most primitive isotopic characteristics ((87Sr/86Sr)0 ¼ 0.70336, 1Nd ¼ þ5:3) and compares with those of the Cretaceous Caleu Pluton (( 87Sr/ 86Sr)0 , 0.703, 1Nd ¼ þ5 to þ 6.5, 39Ar – 40Ar

One remarkable feature observed in all Lower Cretaceous volcanic rocks from the Coastal Range is the predominance of basaltic andesites and andesites with high-K and low MgO contents and the absence of primary Mg-rich basaltic liquids. High-K primary basaltic liquids found in some subduction zones (Table 3.1 in Tatsumi and Eggins, 1995) are characterized by Al2O3 content . 17% and MgO . 9%. These primary magmas are mainly generated in the mantle wedge above the subducted oceanic crust. For the genesis of the Lower Cretaceous lavas, olivine fractionation from mantle-derived melts, before lava emplacement, could explain their low (mg), MgO, Cr, and Ni, but we discuss other genetic alternatives. The REE patterns of the studied lavas (Fig. 8) are characterized by Yb contents < 10 times the chondritic values, which exclude the possibility of garnet as a residual phase in the source. The similarities in some incompatible trace element ratios (Table 4) point to a homogeneous source. In this sense, the roughly constant Na8.0 values (Na2O abundances recalculated, using a fractional crystallization model, to an MgO content of 8 wt%) in most samples (2.85 ^ 0.35 in area A and 2.23 ^ 0.16 in area B) could be interpreted, according to Plank and Langmuir (1992), as indicative of a relatively high and constant degree of partial melting from a homogeneous source. In comparison, lavas from the Veta Negra Formation in the Cerro Bustamante area (338250 S) have Na8.0 values of 2.40 ^ 0.25. In addition, the rather flat pattern of the REE indicates a relatively high degree of partial melting. High K, Ba, Sr, and Th contents (Figs. 6 and 7) are a primary feature of these volcanic rocks, as can be shown using the trace element composition of the plagioclase phenocrysts (Morata et al., 2000). The abundance of these high incompatible trace elements could be related to the ability of the K, Ba, and Sr to fractionate into a fluid phase associated with the subducted sediments or with the dehydration of altered oceanic crust. Another geochemical characteristic of the Lower Cretaceous volcanic rocks is the absence of a positive Eu anomaly,

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Fig. 11. (87Sr/86Sr)0 1tNd CHUR diagram for the Lower Cretaceous volcanic rocks in the Coastal Range of the La Serena region. Values for the mantle array (shaded area) from DePaolo and Wasserburg (1979). In the open triangle, trondhjemite from the coeval Illapel plutonic complex in the Coastal Range of Chile plots (060496-1, Parada et al., 1999; ,109 Ma, Rivano et al., 1985). Data from Cerro Bustamante (Veta Negra Formation) and Jurassic volcanic rocks of the Coastal Range taken from Vergara et al. (1995). Field of the Lower Cretaceous volcanic rocks of the Veta Negra Formation at 328300 –348300 S from Nystro¨m et al. (1993). Field of the Jurassic volcanic rocks from the Coastal Cordillera in central Chile from Vergara et al. (1995). Field of the Pacific MORB from Peate et al. (1997). SVZ, field of the recent basic lavas from the southern volcanic zone of the Andes (34–418S) (Hickey et al., 1986). Two component mixing models have been calculated using N-MORB (Sr ¼ 9.6 ppm, 87Sr/86Sr ¼ 0.70255, Nd ¼ 0.67, 143Nd/144Nd ¼ 0.51311; Peate et al., 1997) and three end members. (A) Slab fluids (Sr ¼ 354 ppm, 87Sr/86Sr ¼ 0.7045, Nd ¼ 1.36, 143Nd/144Nd ¼ 0.51313; Peate et al., 1997); (B) average of the Carboniferous tonalites from the Coastal Range of Chile (Sr ¼ 480.5 ^ 261.5 ppm, 87Sr/86Sr ¼ 0.7063 ^ 0.0003, Nd ¼ 26.6 ^ 3.0, 143Nd/144Nd ¼ 0.51212 ^ 0.00003; Parada et al., 1999); (C) average of four Jurassic tonalites and granodiorites from the Coastal Range in Chile (Sr ¼ 317.5 ^ 55.4 ppm, 87Sr/86Sr ¼ 0.70398 ^ 0.00047, Nd ¼ 25.7 ^ 4.6, 143Nd/144Nd ¼ 0.51257 ^ 0.00003; Parada et al., 1999). In all curve models, ticks are 10%. Samples from the Lower Cretaceous backarc lavas in the High Cordillera at 31–348S (Vergara and Nystro¨m, 1996) are also plotted.

despite their highly plagioclase porphyritic nature. This feature can be related to high fO2 conditions during magma genesis (Morata et al., 2000) and is corroborated by the relatively high Fe2O3/FeO ratio observed in the lavas (Fig. 5). Parada et al. (1999) (Fig. 11) propose a tectonomagmatic evolutionary model for the plutonic rocks of the Coastal Range, in which delamination of the oceanic lithosphere, followed by collision of an oceanic ridge with the continental margin, controls the genesis of the plutonic rocks. Underplated subduction-derived basalts associated with an asthenospheric window may be the mechanisms involved in the genesis of the Cretaceous Illapel Complex. As shown in Fig. 11, isotopic similarities between this plutonic complex and Lower Cretaceous lavas would indicate a similar origin. According to the metabasalt dehydration melting experiments of Rapp and Watson (1995), a high degree of melting (< 40 –60%) in low-pressure conditions (8 kbar) of altered subducted oceanic basalts would generate mafic liquids that correspond to low MgO, high Al2O3 basaltic and basaltic andesite compositions, which coexist with granulitic residues without garnet as a residual phase. This mechanism, previously suggested by Parada et al. (1999) to explain the TTG affinity of the Cretaceous plutonic rocks of the Coastal Range, may account for the geochemical features observed in the Lower Cretaceous volcanic rocks of the Coastal Range of La Serena. Partial melting of subducted oceanic crust would also explain the relatively low

(87Sr/86Sr)0 and high 1Nd values of the rocks studied compared with lavas from other active subduction volcanoes. The presence of a plume (or plumes) in the Pacific realm during the Early and mid-Cretaceous could represent an efficient mechanism that accounts for the large amounts of thermal energy needed to produce massive melting anomalies from altered subducted oceanic crust and gave rise to the huge volume of volcanic and plutonic rocks spatially associated in the Coastal Range. 6.2. Crustal contamination? The typical subduction-related pattern, with a marked enrichment in LILE, a systematic depletion of the HFSE, and a characteristic Ta – Nb trough, is present in the NMORB normalized multi-element diagrams (Figs. 6 and 7). According to the processes that affect arc magma composition (Pearce and Peate, 1995), crustal contamination also should be considered. In light of the initial isotopic values of the Lower Cretaceous rocks of the Coastal Range, minor crustal contamination can be expected. With the aim of contrasting the end member components involved in the genesis of these lavas, three simple mixing models have been elaborated (Fig. 11). Slab-derived fluids would minimally affect the Nd isotopic composition of arc magmas, whereas Sr would be highly remobilized (Peate et al., 1997). Mixing between a depleted mantle source and these slab-derived fluids would plot on model curve A in

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Fig. 11. Crustal contamination is modeled using two different contaminants: the Paleozoic basement of the Coastal Range (B) and local Jurassic plutonic bodies (C). The addition of slab fluid would be reflected as an increase in the Ba/Th ratios, whereas sediment addition would increase the Th/Nb ratio (Leat et al., 2000). An influx of K, Rb, and Ba from the subducted lithospheric slab was proposed by Vergara et al. (1995) to explain the geochemistry of the Veta Negra lavas. The Th/ Nb and Ba/Th ratios (Table 4) support the influence of sediment in the genesis of the magmas. However, the initial Sr and Nd isotopic values of the Lower Cretaceous rocks plot within curve C. This simplicity could support the contamination of the Lower Cretaceous parental magmas by a crustal component. The sediment input could be restricted to the non-radiogenic elements (e.g. Ba). Nevertheless, the higher Th contents and lower 1Nd values in the lavas from La Serena with respect to those of the Veta Negra Formation at 338S could be explained if crustal contamination was more operative near La Serena, which could be attributed to a lower rate of crustal extension and less crustal attenuation at the intra-arc basin of La Serena compared with the basinal conditions at the latitude of Santiago. An opposite situation, with a thicker continental crust during the Early Cretaceous at the Santiago latitude, was proposed by Levi et al. (1988), but a low-spreading rate of 5 cm yr21 in the southeast Pacific during 125 –110 Ma (Larson and Pitman, 1972) favors the increase of the subduction angle, which would enable the delamination of the subducted lithosphere, the increase of the asthenospheric wedge, or low-pressure partial melting of an altered oceanic crust. These mafic magmas, isotopically depleted, would be metasomatized by the subducted sediments, which would increase their LILE content and be partially contaminated by the Jurassic plutonic rocks that conform the arc.

proposed for the Lower Cretaceous lavas from the La Serena region (308S). A combined process of plate subduction and spreading/subsidence during the Mesozoic – Paleogene has been hypothesized by Levi and Aguirre (1981), Levi and Nystro¨m (1982), Levi et al. (1988), and Vergara et al. (1995). This extension would have been effective from the Early Cretaceous to the Oligocene – Miocene, giving a Sr –Nd isotopic trend toward a MORB signature (Nystro¨m et al., 1993). Geochemical similarities among Lower Cretaceous lavas from the Coastal Range along a N – S belt of some 100 km, as well as similarities (at least in initial isotopic ratios) to the Coastal Lower Cretaceous plutonic rocks, enable us to include these centers in a major magmatic province. The N –S geochemical variations observed could be attributed to a lower rate of crustal extension and less crustal attenuation at the intra-arc basin of La Serena compared with basinal conditions at the southern latitude of Santiago during the Lower Cretaceous.

Acknowledgements This study was supported by the Departamento de Investigacio´n y Desarrollo (DID), Universidad de Chile, Project I001-99/2. The authors thank F. Bea and P. Montero (Univ. Granada) for the facilities for isotopic and ICP-MS analyses and B. Levi and J. Nystro¨m for their permission to use unpublished geochemical analyses of samples with the prefixes ELQ- and MAR-. This paper benefited from constructive comments from B. Levi, S.M. Kay, and L. Lo´pez-Escobar.

References 7. Conclusions The Lower Cretaceous volcanism in the Coastal Range at the latitude of La Serena (298200 –308S) is mainly represented by highly porphyritic (20 –30% phenocrysts), high-K to shoshonitic, low MgO, high Al2O3 basaltic andesites and andesites. The homogeneous isotopic geochemistry of these lavas, with low initial Sr ratios (, 0.7036) and positive 1Nd values (þ 2.9 to þ 4.7), is not typical of modern magmas associated with subduction. Slight differences in the geochemistry (trace element and isotopic signatures) between the Lower Cretaceous lavas from La Serena and coetaneous lavas from southern latitudes (e.g. Veta Negra Formation, 338S) have been found. Mafic magmas, isotopically depleted, would be metasomatized by the subducted sediments and have a different crustal contamination component. On the basis of geochemical data, a non-Andean setting dominated by subduction and controlled by intra-arc extension is

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