Time interval between volcanism and burial metamorphism and rate of basin subsidence in a Cretaceous Andean extensional setting

ELSEVIER

Tectonophysics 313 (1999) 433–447 www.elsevier.com/locate/tecto

Time interval between volcanism and burial metamorphism and rate of basin subsidence in a Cretaceous Andean extensional setting L. Aguirre a,Ł , G. Fe´raud b , D. Morata a , M. Vergara a , D. Robinson c a

Dpto. Geologı´a, Facultad de Ciencias Fı´sicas y Matema´ticas, Universidad de Chile, Casilla 13518, Correo 21, Santiago de Chile, Chile b Ge ´ osciences Azur, UMR 6526. CNRS – Universite´ de Nice–Sophia Antipolis, 06108 Nice Cedex 02, France c Department of Geology, Wills Memorial Building, University of Bristol, Queen’s Road, Bristol BS8 1RJ, UK Received 3 February 1999; accepted 16 July 1999

Abstract 40 Ar=39 Ar ages were obtained from basaltic flows belonging to a 9-km-thick sequence generated in an extensional ensialic setting of an arc=back-arc basin type during the Early Cretaceous and presently exposed along the Coastal Range of central Chile. The basalts have been affected by very low- to low-grade burial metamorphism, mostly under prehnite– pumpellyite facies. Age values obtained from primary (volcanic) and secondary (metamorphic) minerals permit to quantify the time interval between volcanism and burial metamorphism. A plateau age of 119 š 1:2 Ma from primary plagioclase represents the best estimation of the age of the volcanism, whereas adularia, in low-variance assemblages contained in amygdules, gave a plateau age of 93:1 š 0:3 Ma which is interpreted as the age of the metamorphism. Considering the P–T conditions estimated for this metamorphic event, the c. 25 Ma time interval between volcanic emplacement and prehnite–pumpellyite facies metamorphism, the rate of basin subsidence in this extensional geodynamic setting would be comprised in the interval 150–180 m=Ma.  1999 Elsevier Science B.V. All rights reserved.

Keywords: Andes; metamorphism; geochronology; Cretaceous; extension; subsidence

1. Introduction Quantifying the time interval between deposition of volcanic materials in a regional subsiding basin and the generation of low-grade metamorphic assemblages in those same materials due to burial is ˚ berg et al., 1984; a scarcely explored subject (see A ˚ berg, 1985), mainly because of the difficulty to A obtain reliable ages of these two events. In this paper we report results obtained from the study of a Cretaceous volcanic sequence of central Chile metaŁ Corresponding

author.

morphosed under very low- to low-grade conditions. This work intends to contribute to the understanding of the arc=back-arc geodynamic processes, in particular to the evolution in time of ensialic marginal basins generated under spreading-subsidence conditions along active continental margins. A volcanic belt of Early Cretaceous age, 3–13 km thick and c. 1000 km long, is exposed along the Coastal Range of central Chile (Fig. 1). At the latitude of Santiago (33ºS) the belt is represented by a 9-kmthick pile (Fig. 2), the Ocoite Group (Aguirre et al., 1989), whose lower third (Lo Prado Formation) consists of marine and continental volcanic sediments,

0040-1951/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 1 9 5 1 ( 9 9 ) 0 0 2 1 7 - 6

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Fig. 1. (A) Location map and main physiographic features of central Chile (Arg. D Argentina; C. Valley D Central Valley Graben. (B) Simplified geologic map of the Coast Range between 32º300 and 34ºS. LK D Lower Cretaceous; UJ, MJ, and LJ D Upper, Middle, and Lower Jurassic, respectively (taken from Vergara et al., 1995).

limestones, and a bimodal sequence of dacitic ignimbrites to interbedded basalts. The central part of the pile is made up of continental porphyritic basalts and basaltic andesites (Veta Negra Formation, Ocoa Member) whereas the uppermost level consists of continental flow-breccias of basaltic andesite to andesite composition (Veta Negra Formation, Noviciado Member). Most of the basic lavas of the Ocoite Group

belong to the high-K calc-alkaline and shoshonite series (Levi et al., 1988; Vergara et al., 1995). The rocks of the Ocoite Group have been affected by very low- to low-grade, non-deformative metamorphism, which preserved the primary structures and textures. Its grade increases with stratigraphic depth ranging from zeolite facies at the top to lower greenschist facies at the very bottom of

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by different authors as representing an extensional ˚ berg et ensialic setting either of a marginal basin (A al., 1984; Aguirre et al., 1989; Aguirre, 1993) or of an island arc (Vergara et al., 1995) type.

2. Primary and metamorphic mineralogy; metamorphic facies

Fig. 2. Stratigraphic column of the Lower Cretaceous in the Coastal Range of central Chile at about 33ºS (modified from ˚ berg et al., 1984) Metamorphic facies are Levi et al., 1982 and A represented with increasing shades of gray (light gray D zeolite facies; medium gray D prehnite–pumpellyite facies; dark gray D greenschist facies). A D sampled level, this paper; 1 D sampled level, Lo Aguirre copper deposit (Munizaga et al., 1988); 2 D sampled level, El Soldado copper deposit (Boric and Munizaga, 1994). Lithological symbols as follows: a D continental flow-breccias of basaltic andesite to andesite composition; b D continental porphyritic basalts and basaltic andesites; c D dacitic ignimbrites; d D marine and continental volcanic sedimentary rocks; e D limestones.

the pile (Levi, 1969; Aguirre et al., 1989; Levi et al., 1989) (Fig. 2). Regional facies boundaries are parallel or subparallel to bedding and not to contact with contemporaneous or younger granitoids in the area demonstrating that the metamorphism is unrelated to the intrusions. These characteristics conform to the classical definition of burial metamorphism (Coombs, 1960) justifying that the Ocoite Group be referred as a typical example of this metamorphic type (Bucher and Frey, 1994). The various characteristics of the Ocoite Group and of its geologic framework have been interpreted

The basic flows have porphyritic textures with large (up to 2 cm) and abundant phenocrysts of plagioclase (An56 Ab40 Or4 to An70 Ab28 Or2 ), subordinate augite (Wo38 En45 Fs17 ), titanomagnetite and small amounts of totally altered olivine crystals. Albitization and sericitization of the primary plagioclase and chloritization of the ferromagnesian minerals are widespread. The metamorphic minerals are found in different habitats (metadomains) among which: (1) partially or totally replaced primary minerals; (2) altered glassy material from the groundmass; (3) amygdule filling; (4) veinlets and microfractures. The proportion of secondary minerals increases downwards in the pile and the mineral assemblages characterize a gradual change of metamorphic facies from a pumpellyitebearing zeolite facies at the top to greenschist facies at the very bottom (Fig. 2). This implies a thermal gradient of around 20–30ºC km 1 (Aguirre et al., 1989). This gradient has been contrasted with temperature data obtained from chlorite compositions applying the method of Cathelineau and Nieva (1985) and Cathelineau (1988) which indicates values from c. 150ºC (top) to c. 260ºC (bottom) for the 5 to 6 km thick Ocoa Member (Morata et al., 1996). Characteristic amygdule metamorphic assemblages in the zeolite and prehnite–pumpellyite facies in lava flows are: (a) laumontite C chlorite C pumpellyite C prehnite C epidote C K-feldspar and (b) pumpellyite C epidote C chlorite C prehnite C K-feldspar.

3. 40 Ar=39 Ar dating of the magmatic and metamorphic events 3.1. Description of the analyzed material Materials employed for dating were feldspar crystals (Ca-plagioclase and K-feldspar), and whole rock

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samples, from the lower third of the Veta Negra Formation=Ocoa Member (Fig. 2). Primary Ca-plagioclase is present as large phenocrysts of transparent aspect although in most samples these crystals have been totally or partially albitized leaving only some patchy relicts with primary calcic composition. Secondary (D metamorphic) feldspars appear (1) replacing primary phenocrysts and microliths of plagioclase and, (2) filling open spaces e.g. amygdules and veinlets. In (1) they correspond to pure albite resulting from albitization processes and to K-feldspar (adularia) as patches inside totally albitized plagioclase phenocrysts; in this last case sericite flakes accompany adularia. In (2), adularia (Or97 Ab3 ) is abundantly present filling amygdules in low-variance assemblages together with pumpellyite, chlorite and low albite (Morata et al., 1997). Crystal and rock sampling was carried out at the slopes of the Bustamante Hill (Fig. 1) along a section of c. 400 m comprising several basalt flows (SiO2 52%, Al2 O3 16–17%, K2 O 2–2.5% as a mean in ‘fresh’ samples) with an average thickness of 25 m. These flows (labeled as 96BU6, 96BU10, 96BU14 and 96BU15) are petrographically and chemically similar to one another. In each of them, the strongest metamorphic effect is manifested at the highly vesicular flow tops, whereas the bottom and central parts are less altered. However, a wholly unaltered specimen was not available in the section sampled. A basaltic dike (96BU9) cutting across the sequence was also collected for analysis. 3.2. Previous radiometric age determinations on rocks of the Ocoite Group Rb=Sr analyses of six ‘unaltered’ samples of basalts taken from the same stratigraphic level of the flows reported here, did not display an isochron, but scattered data which fit around a so-called ‘refer˚ berg et al., 1984). This figure ence line’ at 117 Ma (A was deduced from the mean value of the paleontological age interval, Hauterivian to Albian, assigned to a sequence of flows and marine limestones which crops out 400 km north of the Bustamante Hill area, and correlatable with the middle part of the Ocoite ˚ berg et al. (1984) Group (Aguirre and Egert, 1965). A also studied samples of strongly altered basalt and of

epidote in amygdules in these same flows establishing that the whole rock system closed at 102 š 3 Ma which was interpreted as representing the age of a metamorphic event occurring about 10–20 Ma after the flows were extruded. Munizaga et al. (1988) carried out whole rock Sr isotopic analyses on volcanic rocks hydrothermally and pervasively altered belonging to the stratabound copper deposits of El Soldado and Lo Aguirre (Fig. 1) located stratigrafically below the flows studied by us (Fig. 2). Rb=Sr isochrons of 109 š 4 Ma and of 113 š 3 Ma were obtained for these samples respectively. K=Ar whole rock analyses performed on variously altered rocks of the same deposits gave ages of 113 š 4 Ma and 110 š 4 Ma for the ‘freshest’ rocks and from 96 š 5 to 100 š 3 Ma for the most altered samples. This difference in age was interpreted by Munizaga et al. (1988) as indicative of the minimum time interval during which the hydrothermal activity operated. Boric and Munizaga (1994) reported 40 Ar=39 Ar data from other altered rocks of El Soldado copper deposit. Disturbed age spectra with apparent ages mainly varying from 115.6 to 134.0 Ma were obtained from more or less albitized plagioclase from an altered=mineralized trachytic dike and from a non-mineralized andesitic dike, respectively. Integrated ages of 105 š 2:8 Ma (Lo Prado Fm.) and 101:4 š 1:7 Ma (Veta Negra Fm.), calculated from 2 step experiments, were obtained on metamorphic albites filling amygdules of andesites whereas integrated 40 Ar=39 Ar (from 2 step experiments), K=Ar whole rock, and Rb=Sr whole rock ages cover the interval 113–99 Ma. Boric and Munizaga (1994) concluded that the main mineralization at El Soldado took place at c. 131.8 Ma linked with the waning phase of the Neocomian magmatism and was followed by a new mineralization stage related to metamorphic processes occurred during the Albian, around 113–99 Ma. Paleontological information exists concerning the stratigraphic levels dated. It comes from an ammonite fauna contained in limestones from the uppermost levels of the Lo Prado Formation at the locality of Loma Guayaca´n, 27 km NNW from the Bustamante Hill area. This fauna has been assigned to the Hauterivian (132–127 Ma according to Gradstein et al., 1995) by Pe´rez (1989). This locality can

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be stratigraphically correlated with that represented by 2 (El Soldado) in Fig. 2. 3.3. New results Single grains of plagioclase and adularia, clusters of 29 and 37 grains of plagioclase, a bulk sample of plagioclase and whole rocks were analyzed by the 40 Ar=39 Ar step heating procedure. Grain sizes for single grain analyses are on the order of 500 ð 1500 µm for plagioclase and 400 ð 500 µm for adularia. The samples were irradiated in the nuclear reactor at McMaster University in Hamilton, Canada, in position 5c. The total neutron flux density during irradiation is 8:8 ð 1018 n cm 2 , with a maximum flux gradient estimated at š0.2% in the volume where the samples were included. We used the Hb3gr hornblende as a flux monitor with an age of 1072 Ma (Turner et al., 1971). The analytical procedure is described in detail by Ruffet et al. (1991). The gas extraction was carried out by a Coherent Innova 70-4 continuous laser and the mass spectrometer is a VG 3600 working with a Daly detector system. The typical blank values of the extraction and purification laser system are in the range 34–90, 0.3–1.7, 0:3–1ð 10 14 ccSTP for the mass 40, 39, 36, respectively, measured every third step, whereas argon isotopes measured on the plagioclase single grains were on the order of 34–3000, 260–80,000, and 2–37 times the blank level, respectively. The criteria for defining plateau ages were the following: (1) it should contain at least 70% of released 39 Ar; (2) there should be at least three successive steps in the plateau; and (3) the integrated age of the plateau should agree with each apparent age of the plateau within a 2 sigma (2 σ) error confidence interval. All errors are quoted at the 1σ level and do not include the errors on the age of the monitor. The error on the 40 Ar* =39 ArK ratio of the monitor is included in the plateau age error bar calculation. 3.3.1. Age of the basalt flows In order to obtain the age of the basalt emplacement, we analyzed one bulk sample (heated in a high-frequency furnace), two restricted populations (29 and 37 grains, heated with the laser) and single grains of primary Ca-plagioclase phenocrysts. The bulk sample 96BU6 displayed a U-shaped age spec-

437

trum characterized by apparent ages ranging from 112.2 to 132.8 Ma, if we exclude the very high ages at low temperature (Fig. 3a). Whereas one single grain gave a similar age pattern, the two restricted populations and two single grains of plagioclase from the same rock, carefully selected following the transparency criterium, displayed concordant age spectra, characterized by high and variable ages at very low temperature, low apparent ages at intermediate temperature and very concordant ages at high temperature, including one plateau age at 119:4š1:2 Ma (Fig. 3b). The low-temperature ages excluded, the age spectrum of the sample giving the plateau age displays apparent ages ranging from 111.8 to 119.9 Ma, this domain of variation being sensibly lower than that given by the bulk sample. Two single grains (partially milky white) from two other lava flows (96BU14 and 15) displayed lower intermediate temperature ages but the last steps gave apparent ages converging towards the 119.4-Ma plateau age (Fig. 3d). The 37 ArCa =39 ArK ratio obtained from the gas fraction corresponding to (1) the plateau age of 96BU6 plagioclase, the high-temperature apparent age obtained on the second restricted population and the two single grains of 96BU6 plagioclase, (2) the plagioclase single grain from the flow 96BU15 and (3) the plagioclase single grain from the flow 96BU14 are 13.3, 8.12 and 13.7 (Fig. 3b), respectively. They closely approach the corresponding ratios calculated from microprobe analyses (using the relationship 37 ArCa =39 ArK D 0.453 ð CaO=K2 O) on fresh plagioclase phenocrysts of the same rock samples, which are 12.2, 8.8 and 13.3, respectively. The similarity of the apparent ages and the 37 ArCa =39 ArK ratios at high temperature for the plagioclases from the three investigated lava flows (Fig. 3b,d; the one single grain and the bulk sample from 96BU6 in Fig. 3a are excepted) supports the validity of the plateau age at 119:4 š 1:2 Ma which probably represents the best estimate for the formation age of the lava flow. The single grain of plagioclase from the sample 96BU6 (Fig. 3a), partially milky white, displayed a disturbed age spectrum characterized by higher ages at high temperature, similar to those obtained on the bulk sample (132–136 Ma). This will be discussed later (Table 1).

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Fig. 3. (a) 40 Ar–39 Ar age and 37 ArCa =39 ArK ratio spectra obtained on bulk sample and one plagioclase single grain (number 5 in Table 1) from one lava flow (96BU6) from the Bustamante section. The error bars are given at the one-sigma level. (b) 40 Ar–39 Ar age and 37 Ar =39 Ar ratio spectra obtained on two single grains and two clusters of 20 to 35 grains of plagioclase from one lava flow (96BU6) Ca K from the Bustamante section. (P) D plateau age. The numbers included in boxes are indicated in Table 1. mp D 37 ArCa =39 ArK ratio deduced from the CaO=K2 O ratio obtained by microprobe analysis. Same legend as (a). (c) 40 Ar–39 Ar age and one 37 ArCa =39 ArK ratio spectrum obtained on two single grains of adularia included in amygdules from two lava flows (96BU10 and 96BU12) from the Bustamante section. Same legend as (b). (d) 40 Ar–39 Ar age and 37 ArCa =39 ArK ratio spectra obtained on two single grains of plagioclase from two lava flows (96BU14 and 96BU15) from the Bustamante section. The age and 37 ArCa =39 ArK ratio spectra displayed by the plagioclase BU6 and the adularia 96BU10 are given for comparison. mp14 and mp15 D 37 ArCa =39 ArK ratios deduced from the CaO=K2 O ratios obtained by microprobe analysis of plagioclases from 96BU14 and 96BU15. Same legend as (b). (e) 40 Ar–39 Ar age spectra obtained on two whole rock samples from one lava flow (96BU6) and one dike (96BU9) crossing the lava flows from the Bustamante section. The age and 37 ArCa =39 ArK ratio spectra displayed by the plagioclase BU6 and the adularia 96BU10 are given for comparison. Same legend as (b).

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Table 1 40 Ar–39 Ar analytical results obtained on primary and metamorphic minerals and whole rocks from the Bustamante section of lava flows (central Chile) Step

Atmospheric contamination (%)

39 Ar

37 Ar

39 Ca = ArK

40 Ar* =39 Ar

(%)

K

Age (Ma)

96BU6 plagioclase bulk sample 1 83.56 2 89.29 3 84.98 4 72.84 5 31.84 6 26.27 7 23.47 8 26.93 9 29.22 10 33.53 11 31.03 12 40.65 13 52.67 fuse 47.63

0.01 0.28 1.11 6.20 11.69 8.38 13.89 12.58 9.70 9.96 5.79 7.31 4.07 9.02

0.001 6.744 9.782 12.111 12.489 12.011 11.836 12.566 12.702 12.585 12.753 12.709 12.771 12.916

88.993 17.296 8.092 4.272 3.605 3.644 3.689 3.948 4.057 4.012 4.130 4.284 4.292 4.274

1713.1 š 81.0 484.4 š 23.3 242.8 š 8.2 132.3 š 2.9 112.2 š 1.1 113.4 š 1.1 114.8 š 1.1 122.6 š 1.1 125.8 š 1.2 124.5 š 1.2 128.0 š 1.2 132.6 š 1.3 132.8 š 1.7 132.3 š 1.4 Integrated age D 125.9 š 0.4 Ma

96BU6 plagioclase single grain (5) 1 82.10 2 34.79 3 72.03 4 73.45 fuse 38.84

7.81 10.09 18.27 15.42 48.41

9.983 11.350 10.286 10.129 12.049

12.512 3.511 4.103 4.359 4.452

360.0 š 18.4 108.5 š 8.9 126.1 š 4.4 133.7 š 6.1 136.5 š 2.0 Integrated age D 149.9 š 2.4 Ma

96BU6 plagioclase (29 grains) (3) 1 84.59 2 95.04 3 81.03 4 48.49 5 51.51 6 16.19 7 5.84 8 5.34 9 8.80 10 10.24 fuse 14.22

0.02 0.17 0.97 2.52 5.82 6.54 7.50 5.84 7.71 5.64 57.29

8.479 9.180 11.737 12.402 12.786 12.915 13.116 13.139 13.091 13.090 13.003

75.052 9.024 4.580 3.763 3.670 3.591 3.665 3.783 3.794 3.784 3.859

1530.9 š 598.4 268.8 š 122.8 141.4 š 26.0 117.0 š 7.3 114.2 š 3.4 111.8 š 2.9 114.0 š 2.9 117.6 š 3.5 117.9 š 3.4 117.6 š 2.8 119.9 š 1.5 Integrated age D 118.9 š 1.1 Ma

96BU6 plagioclase (37 grains) (4) 1 71.63 2 92.64 3 82.53 4 66.87 5 44.32 6 36.88 7 23.57 8 17.37 9 8.59 10 35.91 11 19.07 12 10.55

0.08 0.60 1.19 1.59 2.36 3.07 3.20 3.51 2.38 3.51 3.03 2.28

5.324 8.455 11.040 11.892 12.279 12.125 12.272 12.233 12.451 12.440 12.568 12.571

64.893 8.903 4.392 3.907 3.503 3.560 3.649 3.563 3.533 3.722 3.595 3.565

1378.0 š 48.6 263.5 š 26.7 134.8 š 9.2 120.4 š 3.0 108.3 š 3.0 110.0 š 2.7 112.7 š 1.8 110.1 š 1.6 109.2 š 2.0 114.9 š 1.9 111.1 š 2.1 110.2 š 1.8

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Table 1 (continued) Ar* =39 ArK

Age (Ma)

12.401 12.518 12.883

3.719 3.950 3.913

114.8 š 2.2 121.7 š 1.6 120.6 š 1.1 Integrated age D 120.7 š 0.7 Ma

6.57 14.36 10.67 11.35 57.04

11.158 10.679 11.667 11.573 12.663

8.052 3.473 3.776 3.775 3.928

239.8 š 15.4 107.3 š 3.4 116.4 š 6.3 116.4 š 6.0 120.9 š 1.6 Integrated age D 126.1 š 1.8 Ma

96BU6 plagioclase single grain (1) 1 80.41 2 27.62 3 36.44 4 37.74 5 30.72 fuse 14.10

5.81 6.04 13.19 14.76 1.98 58.21

13.600 11.511 13.883 12.595 14.349 12.993

11.790 3.584 3.627 3.889 3.196 3.873

341.1 š 23.0 110.7 š 7.8 112.0 š 5.4 119.8 š 5.8 99.0 š 28.2 119.3 š 1.3 Integrated age D 131.2 š 2.1 Ma

96BU10 adularia single grain 1 57.70 2 53.44 3 41.35 4 34.41 5 18.22 6 24.5 7 30.05 8 31.48 9 33.23 10 35.15 fuse 37.08

8.76 3.08 3.10 4.12 3.06 4.46 8.43 7.33 10.60 10.98 36.08

0.006 0.000 0.000 0.005 0.007 0.005 0.002 0.002 0.001 0.001 0.001

2.913 2.973 3.022 2.954 2.968 2.976 2.973 2.950 2.976 2.987 3.007

90.93 š 1.05 92.76 š 1.50 94.24 š 1.41 92.18 š 0.94 92.61 š 0.56 92.85 š 0.51 92.76 š 0.51 92.08 š 0.62 92.86 š 0.60 93.19 š 0.67 93.81 š 0.45 Integrated age D 93.0 š 0.2 Ma

96BU12 adulaire single grain 1 98.64 2 90.86 3 57.51 4 29.63 5 21.00 6 10.6 7 13.552 8 17.83 9 23.19 10 24.47 11 30.56 12 38.17 13 60.70 14 73.27 fuse 35.50

0.10 1.69 5.35 11.26 5.67 4.51 3.50 2.23 2.17 1.91 1.97 2.56 20.53 14.27 22.28

0.013 0.003 0.002 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.003 0.001

1.672 3.408 2.887 2.960 2.934 2.939 2.947 2.923 2.902 2.972 2.917 2.978 3.023 3.001 3.031

52.80 š 37.03 106.05 š 6.26 90.24 š 1.05 92.45 š 0.50 91.67 š 0.46 91.84 š 0.57 92.09 š 0.77 91.33 š 1.14 90.70 š 0.97 92.83 š 1.33 91.14 š 1.89 93.02 š 1.53 94.40 š 1.02 93.71 š 1.60 94.63 š 0.52 Integrated age D 93.4 š 0.4 Ma

Atmospheric contamination (%)

39 Ar

35.67 27.05 22.90

4.76 6.93 61.50

96BU6 plagioclase single grain (2) 1 83.23 2 32.98 3 45.99 4 57.87 fuse 17.65

Step 13 14 fuse

37 Ar

39 Ca = ArK

40

(%)

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Table 1 (continued) Step

Atmospheric contamination (%)

39 Ar

37 Ar

39 Ca = ArK

40

Ar* =39 ArK

Age (Ma)

(%)

96BU14 plagioclase single grain 1 98.0 2 94.962 3 96.89 4 91.62 5 73.14 6 34.82 7 39.82 8 58.80 9 74.02 10 62.15 11 64.92 12 72.57 13 45.80 fuse 72.30

1.16 3.77 5.38 8.01 9.05 7.34 6.41 5.74 9.25 3.89 4.19 9.63 0.63 25.54

1.649 3.872 8.248 6.606 6.205 8.051 8.949 8.444 7.324 7.295 7.753 9.861 14.253 13.734

3.921 3.874 3.485 3.349 3.258 3.116 3.239 3.258 3.126 3.113 3.282 3.482 3.285 3.660

119.93 š 37.15 118.53 š 18.99 106.96 š 18.16 102.91 š 6.80 100.18 š 2.71 95.9 š 2.18 99.620 š 2.29 100.21 š 2.85 96.235 š 3.04 95.87 š 4.48 100.92 š 3.14 106.89 š 2.87 101.01 š 21.85 112.19 š 2.22 Integrated age D 105.6 š 1.6 Ma

96BU15 plagioclase single grain 1 99.69 2 96.28 3 91.09 4 91.45 5 91.47 6 76.34 7 59.56 8 52.23 9 46.02 10 45.7 11 41.285 12 29.61 13 27.45 14 28.56 15 30.03 16 30.58 17 41.27 18 46.07 19 41.09 fuse 15.58

0.21 0.32 0.38 0.90 1.88 2.23 1.77 2.16 2.49 3.71 4.32 3.73 3.66 3.28 2.79 2.46 5.46 13.58 9.59 35.10

0.571 0.584 0.662 0.678 0.840 1.333 2.322 3.627 4.447 4.318 4.088 4.517 4.935 5.065 5.153 5.305 4.742 4.277 5.420 8.125

0.427 3.811 3.856 3.618 2.714 3.372 3.527 3.451 3.473 3.444 3.400 3.396 3.493 3.474 3.421 3.491 3.444 3.464 3.553 3.875

13.44 š 43.44 116.66 š 21.63 118.00 š 12.83 110.94 š 10.45 83.84 š 7.92 103.60 š 3.48 108.21 š 2.78 105.96 š 2.71 106.62 š 2.69 105.75 š 1.42 104.43 š 1.16 104.31 š 0.99 107.21 š 1.16 106.65 š 1.35 105.06 š 1.63 107.16 š 1.75 105.76 š 1.05 106.34 š 0.93 108.99 š 0.96 118.55 š 0.69 Integrated age D 111.3 š 0.4 Ma

96BU9 whole rock 1 99.16 2 93.33 3 79.13 4 69.85 5 67.76 6 55.26 7 53.10 8 47.74 9 41.88 10 39.68 11 40.59 12 40.49 13 44.09 fuse 54.81

1.02 2.93 4.73 6.44 5.60 7.25 4.60 8.70 10.51 12.42 11.45 6.03 5.70 12.63

1.877 1.771 1.672 1.994 2.173 1.939 1.715 1.227 1.088 1.204 1.478 1.808 3.115 6.052

2.478 2.928 3.066 2.980 2.982 2.954 2.845 2.903 2.887 2.900 2.914 2.925 2.940 3.038

77.38 š 54.40 91.12 š 8.89 95.29 š 2.99 92.68 š 1.96 92.75 š 1.78 91.89 š 1.35 88.58 š 1.83 90.35 š 1.14 89.85 š 0.94 90.24 š 0.83 90.69 š 0.83 91.01 š 1.51 91.48 š 1.31 94.44 š 1.35 Integrated age D 91.4 š 0.7 Ma

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Table 1 (continued) Step

Atmospheric contamination (%)

96BU6 whole rock 1a 98.79 2 91.62 3 85.98 4 65.32 5 48.49 6 36.21 7 36.72 8 34.16 9 30.610 10 32.18 11 31.31 12 30.70 fuse 40.82

39 Ar

37 Ar

39 Ca = ArK

40

Ar* =39 ArK

(%) 0.42 1.66 4.94 5.77 5.47 6.97 6.77 7.69 8.85 7.43 6.41 6.14 31.48

1.614 1.125 0.607 0.571 0.617 0.648 0.657 0.635 0.512 0.664 0.950 0.790 2.558

4.516 2.279 2.806 3.115 3.069 3.038 2.956 2.906 2.906 2.890 2.861 2.911 2.988

Age (Ma) 138.58 š 68.24 71.27 š 5.83 87.33 š 3.76 96.69 š 1.56 95.33 š 0.81 94.37 š 0.66 91.89 š 0.93 90.38 š 0.70 90.37 š 0.69 89.88 š 0.67 89.00 š 0.72 90.51 š 0.89 92.85 š 0.61 Integrated age D 91.8 š 0.4 Ma

40 Ar*

D radiogenic 40 Ar. Ca and K D produced by Ca and K neutron interference, respectively. The numbers within brackets following some of the sample numbers are indicated in Fig. 3b. Correction factors for interfering isotopes were ( 39 Ar–37 Ar)Ca D 7:06 ð 10 4 , (36 Ar–37 Ar)Ca D 2:79 ð 10 4 , (40 Ar–39 Ar)K D 3:6 ð 10 2 .

3.3.2. Age of the metamorphism Crystals of adularia from amygdules in two different flows (96BU10 and 96BU12) displayed flat and concordant age spectra and a plateau age (96BU10) at 93:1 š 0:3 Ma (Fig. 3c). A slight increase in the apparent ages with the temperature of degasing (not known) is observed. The 37 ArCa =39 ArK ratio (proportional to the Ca=K ratio) spectrum given by the sample 96BU10 shows that, at low temperature, the Ar released corresponds to impure adularia (Ca=K ratio ranging from 0.004 up to 0.011), whereas at high temperature a more stable composition, closer to pure K-feldspar, is reached (low Ca=K c. 0.002). It is then possible that the true ages correspond to those obtained on the high-temperature steps with average values of 94:2 š 0:6 Ma (96BU12) and 93:5 š 0:3 Ma (96BU10), this last concordant with the plateau age. The two partially altered, milky white, plagioclase single crystals, from flows 96BU14 and 96BU15 (Fig. 3d), displayed low apparent ages at medium temperatures with values c. 99 and 107 Ma, respectively, intermediate between those of fresh transparent plagioclase 96BU6 and adularia. A clear relationship between apparent ages and the 37 ArCa =39 ArK ratio shows that the low–intermediate temperature apparent ages are the result of a mixture of pure

plagioclase and younger sericite and=or K-feldspar as observed in the plagioclase under the microscope. Two whole rock analyses were performed on the lava flow 96BU6 and the basaltic dike 96BU9 cutting across the flow sequence (Fig. 3e). The age spectra are disturbed but, in both cases, the values obtained are close to that of the adularia, even for the sample 96BU6 for which the plagioclase displayed a higher plateau age. Alteration in these rocks is intense and it is likely that the age spectra mostly reflect the metamorphic component rich in K-feldspar and sericite, the fresh plagioclase component being negligible.

4. Discussion and conclusions Most of the previous geochronological data obtained on correlatable or spatially related rock samples of the Ocoite Group are included between the adularia plateau age of 93:1 š 0:3 Ma and the plagioclase plateau age of 119:3 š 1:2 Ma obtained in the present study. Moreover, it is generally observed that the age values decrease with the degree of rock alteration. The ages that we obtained on adularia are, nevertheless, lower than those measured by previous authors on metamorphic materials: 102 š 3 Ma ˚ berg et al. (1984); 99–113 Ma by by Rb=Sr, by A

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Boric and Munizaga (1994), deduced from Rb=Sr and 40 Ar–39 Ar integrated ages. In this last study (El Soldado mine) the higher age values could be explained by the fact that the dated rocks are deeper than the 96BU basalts in the Ocoite Group stratigraphic pile (see Fig. 2). Consequently, they could have reached prehnite–pumpellyite facies conditions at an earlier time during subsidence. Nevertheless, because of the high scatter of the data obtained by Boric and Munizaga (1994) on albitic plagioclase (a mineral commonly considered inappropriate for K=Ar dating), the discrepancy with our data is better explained by our possibility to select and analyze tiny pieces of pure adularia. It remains to discuss the values obtained on the bulk sample and one single grain from 96BU6 (Fig. 3a), characterized by apparent ages up to 136:5 š 2:0 Ma at high temperature. Similar age values were previously obtained by Boric and Munizaga (1994) but in their case they correspond to apparent ages on disturbed age spectra obtained on albite and altered plagioclase. Our single plagioclase grain was partially milky white, and since the 37 ArCa =39 ArK spectrum does not show a strong contribution of potassic phases, e.g. sericite (Fig. 3a), it is likely that the milky aspect of the grain is due to albitization. This is also probable in the case of the bulk sample, which shows a highly disturbed age spectrum (previously described in detail) without strong variation of the Ca=K ratio. Therefore, these high age values (132–136 Ma) must be considered as suspect, whereas the existence of one plateau age (at 119:4 š 1:2 Ma) and the convergence of data around the plateau age (on the more transparent grains) provide strong arguments to consider this age as representative of the emplacement of the basalts. Therefore, the 40 Ar–39 Ar results obtained in our study permit to identify: (1) an extrusive magmatic event which took place c. 119 Ma ago originating the basalt flows of the middle part of the Ocoite Group; (2) a metamorphic episode at 93–94 Ma giving rise to mineral assemblages of the prehnite–pumpellyite facies with conspicuous presence of adularia. The interval of c. 25 Ma between these two events would measure the time elapsed between deposition of the volcanic material in the extensional ensialic setting characterizing the Pacific margin of this region during the Early Cretaceous and the setting of the

physico-chemical conditions allowing the appearance of prehnite–pumpellyite assemblages through processes of subsidence and burial. The apparent ages obtained on plagioclase and whole rock are much younger in the cases where the metamorphic effects are more intense. Thus, in the most pervasively altered rock samples (96BU6 and 96BU9 whole rock analyses), the age spectrum tends to be identical with the adularia spectrum. The adularia analyzed belongs to a low-variance amygdule assemblage and is here considered as representing the closest approximation to the thermodynamic equilibrium reached during this prehnite– pumpellyite facies metamorphism. The plateau age of 91:5 š 0:4 Ma obtained for the basaltic dike (96BU9) cutting across the lava flows is very close to the age of the adularia, suggesting that the dike was also affected by the metamorphic event, a conclusion supported by the alteration of most of the ferromagnesian phenocrysts as observed in thin section. The rocks of the Ocoite Group in the region studied have been intruded by granitoid plutons (Fig. 1) which according to Drake et al. (1982) were emplaced under conditions of crustal spreading. The K=Ar ages of these plutons are comprised in the interval (120–90 Ma) (Drake et al., 1982; Gana et al., 1996). They produced local contact metamorphism on the volcanic rocks of the Ocoite Group generating small hornfelsic aureoles but seem to be totally unrelated to the regional low-grade metamorphic pattern described in this paper which is quite persistent along the Lower Cretaceous volcanic belt through outcrop distances of hundreds of kilometers.

5. Geodynamic considerations The chronological results obtained shed light on the geological evolution of the active continental margin of South America where a similar geodynamic framework prevailed during the Early and Middle Cretaceous. They also permit to evaluate the rate of subsidence in basins with similar characteristics to the one in the Coastal Range of central Chile. Widespread rifting occurred along the western border of South America during the Cretaceous,

L. Aguirre et al. / Tectonophysics 313 (1999) 433–447

from Colombia in the north to the North Scotia ˚ berg Ridge in the South Atlantic (Dalziel, 1981; A et al., 1984; Aguirre and Offler, 1985). These rifting conditions have been attributed to asthenospheric mantle upwelling leading to extension, crustal attenuation, subsidence and bimodal arc-volcanism (Aguirre et al., 1989). This Cretaceous magmatic event could be correlated with abnormal heat flux from the mantle associated with the final dispersal of Gondwana, notably with the 160–85 Ma Pangean thermal stage (Veevers, 1989). The volcanic products were deposited in intra-arc or back-arc basins subsiding at high rates (Atherton and Aguirre, 1992; Vergara et al., 1995). In the Coast Range of central Chile, the combined mechanism of extension and subsidence resulted in a low-relief topography close to sea level and deposition on a flat basin floor (Vergara et al., 1995). According to Vergara et al. (1995), the eruption rate in central Chile during the formation of the Veta Negra lava pile was about 500 km3 my 1 . In all the Cretaceous basins along the western margin of South America, the infilling materials, predominantly volcanics, were affected by low-grade metamorphism which varied from ocean-floor to burial type depending mainly on the extent of crustal attenuation, the volume of mantle upwelling and the rate of subsidence. The intracontinental events described above have been correlated with spreading rates in the SE Pa˚ berg et al., 1984). Thus, the age of 119 Ma cific (A obtained for the effusion of the Bustamante basalts is comprised in the interval 125–110 Ma which, according to Larson and Pitman (1972), corresponds to an episode of spreading of 5 cm yr 1 in the SE Pacific. The age of 94 Ma, representing the metamorphism induced by burial (subsidence), falls in the interval 110–85 Ma, characterized by fast spreading in the ocean (18 cm yr 1 ) according to the same authors. Although this correlation must be extended to other basins, our data seem to confirm a relationship between the subsidence event in an intra-arc basin and the spreading rate of the subducting plate, showing the potentiality of such geochronological studies. The time interval between lava emplacement and prehnite–pumpellyite facies metamorphism reported here allows to evaluate the rate of basin subsidence in the Coast Range of central Chile during the Early

445

Cretaceous. Approximate P–T values for the transition from the zeolite to the prehnite–pumpellyite facies, typified by the reaction laumontite C pumpellyite C quartz D prehnite C epidote C chlorite C H2 O, were obtained by Aguirre et al., 1989 (Fig. 3) using: (1) a schematic pseudobinary T – X Fe diagram (after Cho et al., 1986) based on the compositions of pumpellyite and epidote in adularia-bearing lowvariance assemblages contained in amygdule of lavas from the Bustamante area; and (2) the petrogenetic grid by Liou et al. (1985). The figures obtained were: T D 175 š 5ºC and P D 1:06 š 0:05 kbar. In terms of depth (with a baric gradient between 250 and 280 bar=km) the pressure value amounts to c. 4.0 km. Thus, the investigated lava flows, characterized by mineral assemblages of the prehnite–pumpellyite facies, would have taken 25 Ma to reach a depth of 4.0 km marking, in this particular terrain, the onset of that facies. These figures allow to calculate a rate of basin subsidence comprised in the interval of 145–180 m=Ma. However, the metamorphic mineral assemblages in the Bustamante lava dated suggest a metamorphic grade higher than the one corresponding to the zeolite=prehnite–pumpellyite facies boundary. Consequently, the amount of basin subsidence calculated above should be taken as a minimum value. Vergara et al. (1995) have given figures in the interval of 100–300 m=Ma for the subsidence in the intra-arc basin of the Coast Range of central Chile during the Jurassic and Early Cretaceous and, more precisely, of >250 m=Ma for the Early Cretaceous. In accordance with our present results, the thermal gradient acting during the metamorphic event which affected the rocks of the Ocoite Group would have been higher than the value of 20–30ºC km 1 estimated by Aguirre et al. (1989). The materials deposited in the Early Cretaceous basin were rapidly uplifted during the mid-Cretaceous (the late part of the Early Cretaceous) with a rate of erosion exceeding 300 m=Ma between the final deposition of the Veta Negra Formation and a thick conglomerate unit of Late Cretaceous age unconformably covering it (Vergara et al., 1995). The Early Cretaceous basin of the Coast Range of central Chile could be compared with the tectonic model proposed by Busby-Spera (1988) for the early Mesozoic arc of the southwest Cordilleran United States. This structure, which evolved over

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40 Ma, occupied an extensional or transtensional graben depression similar to the modern extensional arc of Central America. According to Busby-Spera (1988), the relatively short time, as put in evidence by geochronological work, in which thick volcano-sedimentary sequences were deposited along the Cordilleran arc graben depression required rates of intra-arc subsidence of at least 200 m=Ma. This figure is in close agreement with those obtained for the central Chilean basin. Finally, the long standing process of subsidence and burial metamorphism of volcanic materials in the intra-arc basin of the Coast Range of central Chile might have important metallogenic implications. It has been suggested (Sato, 1984) that dehydration processes due to advanced burial metamorphism might be responsible for the formation of the typical Chilean manto type copper deposits which are particularly concentrated in the Cretaceous rocks from the region studied.

Acknowledgements This research was supported by the FONDECYT Project 1961108 and the Universidad de Chile DTI Grant-2834-9445. Contribution UMR Ge´osciences Azur nb. 000. D.M. acknowledges the Universidad de Ca´diz (Spain) for supporting a research stage during 1997 in the Departamento de Geologı´a, Universidad de Chile. We thank V.A. Ramos and P.H. Reynolds for reviews.

References ˚ berg, G., 1985. Open Rb–Sr systems due to burial metaA morphism and some implications for dating. Geol. Fo¨ren., Stockholm Fo¨rh. 107, 127–132. ˚ berg, G., Aguirre, L., Levi, B., Nystro¨m, J.O., 1984. SpreadA ing-subsidence and generation of ensialic marginal basins: An example from early Cretaceous of central Chile. In: Kokelaar, B.P., Howells, M.F. (Eds.), Volcanic and Associated Sedimentary and Tectonic Processes in Modern and Ancient Marginal Basins. Geol. Soc. London Spec. Publ. 16, 185–193. Aguirre, L., 1993. Compositional variations of Cretaceous pumpellyites along the western margin of South America and their relation to an extensional geodynamic setting. J. Metamorph. Geol. 11, 437–448. Aguirre, L., Egert, E., 1965. Cuadra´ngulo Quebrada Marquesa,

provincia de Coquimbo. Carta Geolo´gica de Chile No. 15, Instituto de Investigaciones Geolo´gicas, Santiago, 92 pp. Aguirre, L., Offler, R., 1985. Burial metamorphism in the Western Peruvian Trough: its relation to Andean magmatism and tectonics. In: Pitcher, W.S., Atherton, M.P., Cobbing, E.J., Beckinsale, R.D. (Eds.), Magmatism at a Plate Edge: The Peruvian Andes. Blackie Halstead Press, London, pp. 59–71. Aguirre, L., Levi, B., Nystro¨m, J.O., 1989. The link between metamorphism, volcanism and geotectonic setting during the evolution of the Andes. In: Daly, J.S., Cliff, R.A., Yardley, B.W.D. (Eds.), Evolution of Metamorphic Belts. Geol. Soc. London Spec. Publ. 43, 223–232. Atherton, M.P., Aguirre, L., 1992. Thermal and geotectonic setting of Cretaceous volcanic rocks near Ica, Peru, in relation to Andean crustal thinning. J. S. Am. Earth Sci. 5, 47–69. Boric, R., Munizaga, F., 1994. Geocronologı´a Ar–Ar y Rb–Sr del depo´sito estratoligado de cobre El Soldado (Chile central). Comunicaciones 45, 135–148. Bucher, K., Frey, M., 1994. Petrogenesis of Metamorphic Rocks. Springer, Berlin, 318 pp. Busby-Spera, C.J., 1988. Speculative tectonic model for the early Mesozoic arc of the southwest Cordilleran United States. Geology 16, 1121–1125. Cathelineau, M., 1988. Cation site occupancy in chlorites and illites as a function of temperature. Clay Miner. 23, 471–485. Cathelineau, M., Nieva, D., 1985. A chlorite solid solution geothermometer. The Los Azufres geothermal system. Contrib. Mineral. Petrol. 91, 235–244. Cho, M., Liou, J.G., Maruyama, S., 1986. Transition from the zeolite to prehnite–pumpellyite facies in the Karmutsen metabasites, Vancouver Island, British Columbia. J. Petrol. 27, 467– 494. Coombs, D., 1960. Lower grade mineral facies in New Zealand. In: International Geological Congress Reports, 21 Session Norden, 13, 339–351. Dalziel, I.W.D., 1981. Back-arc extension in the Southern Andes: a review and critical reappraisal. Philos. Trans. R. Soc. London A 300, 319–335. Drake, R., Vergara, M., Munizaga, F., Vicente, J.C., 1982. Geochronology of Mesozoic–Cenozoic magmatism in central Chile, Lat. 31º–36ºS. Earth Sci. Rev. 18, 353–363. Gana, P., Wall, R., Gutie´rrez, A., 1996. Mapa geolo´gico del a´rea de Valparaı´so–Curacavı´. Escala 1 : 1,000,000. Servicio Nacional de Geologı´a y Minerı´a, Chile. Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., Van Veen, P., Thierry, J., Huang, Z., 1995. A Triassic, Jurassic and Cretaceous Time Scale. In: Geochronology Time Scales and Global Stratigraphic Correlation. Soc. Econ. Paleontol. Mineral. Spec. Publ. 54, 95–126. Larson, R.L., Pitman, W.C., 1972. World-wide correlation of Mesozoic magnetic anomalies, and its implications. Geol. Soc. Am. Bull. 83, 3645–3662. Levi, B., 1969. Burial metamorphism of a Cretaceous volcanic sequence west from Santiago, Chile. Contrib. Mineral. Petrol. 24, 30–49. Levi, B., Aguirre, L., Nystro¨m, J.O., 1982. Metamorphic gradients in burial metamorphosed vesicular lavas: comparison

L. Aguirre et al. / Tectonophysics 313 (1999) 433–447 of basalt and spilite in Cretaceous basic flows from Central Chile. Contrib. Mineral. Petrol. 80, 49–58. ˚ berg, G., 1988. GeochemiLevi, B., Nystro¨m, J.O., Thiele, R., A cal trends in Mesozoic–Tertiary volcanic rocks from the Andes in central Chile and tectonic implications. J. S. Am. Earth Sci. 1, 63–74. Levi, B., Aguirre, L., Nystro¨m, J.O., Padilla, H., Vergara, M., 1989. Low-grade regional metamorphism in the Mesozoic– Cenozoic volcanic sequences of the Central Andes. J. Metamorph. Geol. 7, 487–495. Liou, J.G., Maruyama, S., Cho, M., 1985. Phase equilibria and mineral paragenesis of metabasites in low grade metamorphism. Mineral. Mag. 49, 321–333. Morata, D., Vergara, M., Aguirre, L., Cembrano, J., Puga, E., 1996. Chemical characteristics of metamorphic minerals in Lower Cretaceous basic flows from the Coast Range, Central Chile. III Int. Symp. Andean Geodynamics, ISAG-96, St. Malo, Extended Abstr., pp. 605–608. Morata, D., Aguirre, L., Ruzziconi, Y., Fe´raud, G., Vergara, M., Puga, E., Dı´az de Federico, A., 1997. Feldspar chemistry and preliminary Ar=Ar data on the Lower Cretaceous basic lavas from the Coastal Range of Central Chile: petrogenetic implications. 8º Congreso Geolo´gico Chileno, Antofagasta, T-II, pp. 1385–1388.

447

Munizaga, F., Holmgren, C., Huete, C., Kawashita, K., 1988. Geocronologı´a de los yacimientos de cobre El Soldado y Lo Aguirre, Chile Central. V Congreso Geolo´gico Chileno, Santiago, T-III, pp. 177–193. Pe´rez, E., 1989. Informe paleontolo´gico preliminar sobre una fauna fo´sil proveniente de estratos superiores de la Formacio´n Lo Prado en Loma Guayaca´n, este de Colliguay. Internal Report No. 5185 c.2, Servicio Nacional de Geologı´a y Minerı´a (SERNAGEOMIN), Chile. Ruffet, G., Fe´raud, G., Amouric, M., 1991. Comparison of 40 Ar– 39 Ar conventional and laser dating of biotites from the North Tregor Batholith. Geochim. Cosmochim. Acta 55, 1675–1688. Sato, T., 1984. Manto type copper deposits in Chile — a Review. Bull. Geol. Surv. Jpn. 35, 565–582. Turner, G., Huneke, J.C., Podosek, F.A., Wasserburg, G.J., 1971. 40 Ar– 39 Ar ages and cosmic ray exposure ages of Apollo 14 samples. Earth Planet. Sci. Lett. 12, 19–35. Veevers, J.J., 1989. Middle=Late Triassic (230 š 5 Ma) singularity in the stratigraphic and magmatic history of the Pangean heat anomaly. Geology 17, 784–787. Vergara, M., Levi, B., Nystro¨m, J., Cancino, A., 1995. Jurassic and Early Cretaceous island arc volcanism, extension, and subsidence in the Coast Range of central Chile. Geol. Soc. Am. Bull. 107, 1427–1440.