Journal of South American Earth Sciences 15 (2002) 625–641 www.elsevier.com/locate/jsames

Middle dyke swarms in the North Patagonian Massif: the Lonco Trapial Formation in the Sierra de Mamil Choique, Rı´o Negro province, Argentina

Mo´nica G. Lo´pez de Luchia,*, Augusto E. Rapalinib

aInstituto de Geocronologı´a y Geologı´a Isoto´pica, Pabellon INGEIS, Ciudad Universitaria, 1428 Buenos Aires, Argentina bLaboratorio de Paleomagnetismo, ‘D.A. Valencio’, Departamento de Cs. Geolo´gicas, Univ. de Buenos Aires, Pabello´n II, Ciudad Universitaria, 1428 Buenos Aires, Argentina

Received 1 February 2002; accepted 1 June 2002

Abstract Middle Jurassic volcanism is a major magmatic event in the evolution of . In the southwestern corner of the North Patagonian Massif at Sierra de Mamil Choique, basic to intermediate, ca. 170 Ma dykes are widespread. In this article, new chemical data from the dykes are presented, analyzed, and compared with regional information about Jurassic volcanism. The dyke swarms are composed of an alkaline and a subalkaline series. Fractionating phases that controlled the magmatic evolution were plagioclase (andesine) þ clinopyroxene with clinoamphibole (for compositions of SiO2 . 60%) in the subalkaline series and clinoamphibole þ plagioclase (oligoclase) ^ magnetite in the alkaline series. The source of both series could be an enriched lithospheric mantle or lower crust. Although crustal contamination or enrichment by subduction-derived fluids cannot be demonstrated from the available data, a subduction-enrichment process of the magma source is suggested for the subalkaline rocks. Different volatile composition, water content and perhaps temperature at the melting site would control the higher Zr content of the alkaline series and would allow low melt fractions to leave their sources. Absolute values for Ba/Nb (60–160), La/Nb (4–5), and K/Nb (.2000) in both series indicate a significantly LIL-enriched crustal component. Nd/Th of approximately 4 for the alkaline rocks argues against a primitive mantle, supported by the low MgO, Cr, and Ni and high LILE contents. In contrast, Nd/Th between 15 and 27 in the subalkaline rocks is similar to a mantle source. Crustal components are indicated by the higher La/Yb and Na2O and lower CaO in the alkaline rocks. Regional, structural, and, to a certain extent, petrological evidence for the Middle Jurassic dyke swarms of the Sierra de Mamil Choique point to an extensional intracontinental tectonic setting in which older structures controlled the development of the volcanism. q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Dyke swarms; Petrology of basic to intermediate volcanism; Middle Jurassic; Patagonia

1. Introduction Chon-Aike Province (Kay et al., 1989; Pankhurst et al., 1998), a general designation that includes the extra-Andean Jurassic volcanism represents a major magmatic event in Patagonia, the Marifil, Chon-Aike, Lonco Trapial, and Bajo the evolution of Patagonia (Lesta and Ferello, 1972; Page Pobre Formations. In central and eastern Patagonia, two and Page, 1993; Pankhurst and Rapela, 1995; Pankhurst units of silicic rocks have been distinguished: the (185–168/ et al., 1998, 2000; Bertrand et al., 1999). The volcanic rocks 175 Ma) Marifil Formation in the eastern sector of the North are predominantly rhyolitic and form one of the world’s Patagonian Massif (NPM) (Rapela and Pankhurst, 1993; most voluminous silicic provinces (Pankhurst and Rapela, Pankhurst and Rapela, 1995; Alric et al., 1996) and the 1995), which extends from the Atlantic margin to Chile (167–165 Ma) Chon-Aike Formation in the eastern sector (Fig. 1). As a whole, the rocks have been designated the of the (DM) (Pankhurst et al., 1993). Basic to intermediate volcanism, or the Central Volcanic Belt of Page and Page (1993), develops to the west of these areas * þ Corresponding author. Tel.: 54-11-4-783-3021; fax: 54-11-4-783- and includes the 175–159? Ma Lonco Trapial Formation in 3024. E-mail addresses: [email protected] (M.G. Lo´pez de Luchi), the NPM, a general term that refers to the previously [email protected] (A.E. Rapalini). designated Lonco Trapial, Can˜adon Asfalto, and Taquetren

0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0895-9811(02)00083-4 626 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641

Fig. 1. Regional distribution of the Chon-Aike province (Kay et al., 1989; Pankhurst et al., 1998) redrawn from Pankhurst and Rapela (1995). Star: SMC dyke swarms (Lo´pez de Luchi and Rapalini, 1997, 1999). NPM: North Patagonian Massif; SJB: San Jorge Basin; DM: Deseado Massif.

Formations (Pankhurst et al., 1998). It also includes the Bajo involvement of subduction activity along the Pacific margin Pobre Formation in the DM, previously regarded as of (Fig. 2). contemporaneous with the Chon-Aike Formation (Alric Basic to intermediate dykes are widespread in the NPM, et al., 1996) or younger (Pankhurst et al., 2000). Arago´n but their geochemistry, petrology, and ages are poorly et al. (2000) describe the 161.4 ^ 7.3 Ma alkaline Alvar constrained. In the southwestern corner of the NPM, andesites as flows of a sodic alkaline series located in between Sierra de Mamil Choique (SMC; SW Rio Negro the Piedra Parada-Paso del Sapo area in Chubut. Province) and Sierra del Medio (NW Chubut Province), the North–south migration of acidic volcanism associated dykes appear as two parallel WNW belts of dyke swarms. with a western migration of volcanic activity with younger A preliminary geochemical characterization, together with more basic compositions has been proposed (Pankhurst and K/Ar ages, an AMS study, and paleomagnetism, has been Rapela, 1995). New shrimp data of zircons from Jurassic provided only for the dyke swarms of the SMC that make up silicic rocks enabled Pankhurst et al. (1999, 2000) to the northern belt and have an almost 40 km along-strike recognize three distinct magmatic events: V1, V2, and V3. extension (Lo´pez de Luchi and Rapalini, 1997, 1999; V1 is dated at 184 ^ 2 Ma and considered related to a Rapalini and Lo´pez de Luchi, 2000). predominantly extensional period, whereas V2 On the basis of the available information (Lo´pez de (168 ^ 3 Ma) and V3 (153 Ma) reflect the increasing Luchi and Rapalini, 1997, 1999), dyke swarms belong to the M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 627

Fig. 2. Synthesis of the different ages assigned to the formations that have been included within the Chon-Aike Province.

Lonco Trapial Formation, which comprises a sequence of Previous regional studies of volcanic rocks from the Lonco , volcaniclastic rocks, and sedimentary intercalations Trapial Formation recognize a subalkaline group for (cf. Can˜ado´n Asfalto Formation) that extends along the the basic lithologies (Page and Page, 1993) and a sodic western and southern margin of the NPM (Fig. 1). alkaline series for the lava flows that make up the Alvar Intermediate dykes associated with small zones of carbo- andesites (Arago´n et al., 2000). A similar geochemical nitic and siliceous hydrothermal alteration have been noted distinction in the two series was defined for the of in Sierra de Taquetre´n in Chubut (Pankhurst et al., 1998), the Marifil Formation near Las Plumas (Demichelis et al., and trachyandesites–trachybasalts have been described as 1996). small domes in Las Plumas (Haller, 1981). The Alvar Country rock mainly consists of the intrusive Early andesites are andesites–trachyandesites and trachytes that Mamil Choique granitoids (MCG) (Lo´pez de made up the lava flows in the Paso del Sapo area. This Luchi et al., 1999) that are ductile deformed biotite ^ article presents a petrological study of the SMC dyke hornblende granitoids with a predominant WNW–NW swarms (Fig. 3) to characterize their chemistry, magmatic foliation. The pre-MCG evolution is related to medium- evolution, source, and tectonic setting in relation to the pressure regional metamorphism of a siliciclastic sequence, evolution of northern Patagonia in the Middle Jurassic. with some acidic and basic volcanic thin interlayers. The MCG were emplaced after regional metamorphism. Regional relationships among metamorphism, deformation, 2. Geology and magmatism correspond to a collisional setting; peak metamorphic conditions are synchronous or shortly after the 2.1. General description second deformation, and magmatism postdates regional metamorphism (Cerredo and Lo´pez de Luchi, 1998). The The Middle Jurassic dyke swarms of the SMC (418400 – MCG were intruded by undeformed granitoids 418550S and 708–708330W) are located in the northernmost (Cerredo and Lo´pez de Luchi, 1998). sector of to the central volcanic belt (Page and Page, 1993) There is no stratigraphic control to define a precise in extra-Andean Patagonia and included in the Lonco temporal location in the Sierra because younger Permian Trapial Formation (Lo´pez de Luchi and Rapalini, 1999). granitoids (Cerredo and Lo´pez de Luchi, 1998)are They are exceptionally well exposed along a WNW–NW restricted outcrops separated from the area covered by the belt that cross-cuts the Sierra from its NW corner to Pto. dyke swarms. Antinau and continues toward the SE and in the smaller K/Ar data of 168.4 ^ 3.5 and 172.7 ^ 4.5 Ma for the satellite hills located at the northern border of the range (e.g. dykes indicate a Bajocian-Bathonian magmatic event Loma Guacha hill) (Fig. 3). Dykes are made up of dark coeval with the Lonco Trapial Formation (Lo´pez de Luchi green to greenish gray basaltic and andesitic rocks and some and Rapalini, 1997). The paleomagnetic pole for these reddish gray . According to their mineralogy and dykes is coincident with the paleomagnetic pole of the geochemistry, the dykes constitute two series: subalkaline Chon-Aike Formation in Estancia La Reconquista, Santa and alkaline (Lo´pez de Luchi and Rapalini, 1997, 1999). Cruz. Therefore, these poles may indicate a 170 Ma pole for 628 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641

Fig. 3. Geological sketch of the SMC showing the distribution of the Jurassic dyke swarms. Sampling areas include field surveys for petrology, radiometric age determinations, paleomagnetism, magnetic susceptibility, and anisotropy of magnetic susceptibility. Modified after Lo´pez de Luchi and Rapalini (1997).

South America (Lo´pez de Luchi and Rapalini, 1997, 1999; in outcrops with a median segment length of 100 m (Fig. 5). Rapalini and Lo´pez de Luchi, 2000). Some en echelon arrangement of the dyke segments is recognized, especially in the SE sector of the Sierra (Fig. 3). 2.2. Lithology and structure of the dyke swarms The dykes are emplaced in three systems of extensional

Dykes are dark green to greenish gray basalts, basaltic andesites for the subalkaline series and basaltic trachyande- sites and trachyandesites for the alkaline series. Reddish gray dacites also are part of the subalkaline series. The rocks are fine grained with affanitic to very fine-grained crystal- line granular groundmass. Textures vary from porphyritic to aphyric types with a broadly defined zonal pattern and increasing phenocryst percentages (15–60%) and grain sizes toward the central part of the widest dykes. The highest phenocryst percentages are linked to andesitic compositions, with scarce mafites as phenocrysts. Inter- mediate and basic dyke xenoliths appear as enclaves in the dacites (Fig. 4). Thickness of the individual dykes varies from 1 to 6 m; Fig. 4. Dacitic subalkaline dyke with xenoliths of andesitic composition the dykes may be up to 3 km in length though discontinuous (medium gray). Location is the same as Fig. 5. M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 629

restricted to the groundmass in the alkaline series. The groundmass also varies; whereas in the clinopyroxene- bearing basalts, it is intergranular or coarse pilotaxic, in the trachyandesites, it is either pilotaxic with plagioclase microlites, opaque minerals, and altered mafites or micro- granular, though both contain interstitial quartz, alkali feldspar, apatite, and ceolites. The interstitial quartz content increases as the amphibole phenocryst percentages increase. Magnetite content varies from 15 to 5% in the groundmass and constitutes no more than 3% as a bigger subhedral crystal associated with altered mafites in basalts. Higher opaque content is restricted to the southern sector, especially in the basaltic trachyandesites. Fig. 5. Cross-cutting relationship between two dyke systems (dark gray). Plagioclase is the predominant phase in the majority of Country rock (light gray) is the monzogranitic facies of the Mamil Choique the studied dykes and appears as both phenocrysts and Granitoids. Location: southeast sector of the Sierra, near Pto. Antinau smaller laths in the groundmass. Zonal euhedral to (Fig. 3). Xenoliths of the granites are seen inside the dykes. subhedral plagioclase is typical, rarely as glomerules with corroded borders. Phenocrysts are square shaped in the fractures, 80–908, 100–1108, and 120–1308 with a medium subalkaline basalts. Zonation is defined by slightly corroded azimuth of 100, that define the WNW belt that parallels labradorite cores and andesine to calcic oligoclase rims with regional lineaments. These lineaments were active during well-defined internal borders in the subalkaline series. In the Paleozoic and Early Mesozoic and controlled the contrast, andesine cores and sodic oligoclase rims with more subsidence of the post-Jurassic basins (Fig. 3). diffuse internal borders are typical in the alkaline rocks. Even though no systematic relation has been found Oscillatory zoning, in either reversal of the alkaline or the between dyke attitude and composition or location in the normal trend, in the subalkaline is present, which indicates belt, there is a predominance of dykes emplaced in the 100– variable pressure, temperature, and composition during 1108 and 120–1308 systems in the NW corner of the Sierra crystallization. Colorless subcalcic augite occurs as euhe- and 80–908 in the SE. Dacites are located in a NW system dral to subhedral phenocrysts and smaller subhedral grains and more continuous along their strike (Fig. 3). Rotation of in the groundmass, together with brownish-green to green the least compressive stress direction during dyke emplace- hornblende phenocrysts in basalts of the subalkaline series. ment could be responsible for the variable attitude of the Green hornblende is the only ferromagnesian mineral that dykes, but geochronological information is scarce to appears as phenocrysts in the alkaline rocks, and it occurs in constrain this possible rotation (Lo´pez de Luchi and the mesostasis with some biotite in the trachyandesites. Two Rapalini, 1997). clinoamphibole generations appear in trachyandesite samples where plagioclase phenocrysts are scarce. Apatite 2.3. Petrography is conspicuous in the alkaline basic rocks, where it appears as both a long prism in the groundmass and needles in the Although at the outcrops, there is a general lithological outer rims of the plagioclase. similarity among the rocks in the dyke swarms, regardless of Propylization is not generalized. Clinopyroxene and their compositional differences, plagioclase percentage and hornblende crystals are variably altered to chlorite ^ flow textures of the groundmass differ in the two series. epidote and fine-grained opaque mineral aggregates that, In the porphyritic facies, phenocryst associations have in extreme cases, are pseudomorphs of clinoamphibole. In been recognized: clinopyroxene (subcalcic augite)–plagio- addition, some argillaceous minerals, ceolites, calcite, clase (An50-30) ^ clinoamphibole s.l. for the subalkaline epidote, and opaque minerals associated with the phenocryst basalts, with modal plagioclase and phenocrystic percen- are locally present toward the borders of the thicker dykes. tages higher for the basaltic andesites, and plagioclase (An35-20)–clinoamphibole (Ca-rich to actinolitic) ^ biotite for the alkaline basaltic trachyandesites and 2.4. Chemical characterization trachyandesites, with higher modal values of plagioclase for the latter and plagioclase in both the basaltic andesites Representative samples were analyzed for major and and the trachyandesites. In one basaltic andesite, orthopyr- trace elements; of these, 10 have rare earth element (REE) oxene has been recognized. When clinoamphibole dom- determinations. Analyses were performed at ACTlab, inates the phenocrystic association in the alkaline series, Canada, using a combination of ICP, INAA, ICP/MS, and rocks are coarser and textures tend to serrate. Accessory XRF with a research grade. The locations of the analyzed minerals include interstitial quartz and alkali feldspar in samples are shown in Fig. 3. Major and minor element some trachyandesites, as well as apatite, which is mainly contents appear in Table 1. Analyses were recalculated 630

Table 1 Major and trace element data for selected samples of the dyke swarms. Major elements expressed in %w/w, trace elements expressed in ppm. See text for details of the methods

Subalkaline series Alkaline series

1-6a 10-2b 14-5a 03-a 02-b 4-1B 11-5A 17-5A 07-a 13-a 6-4a 7-2B 12-5b 19-5a 20-A 3-5b 4-3B 13-3A

SiO2 60.69 49.77 52.11 64.06 64.71 53.23 50.07 49.80 52.02 62.37 54.09 57.38 55.71 52.49 58.78 55.60 54.53 59.48 Lo M.G. TiO2 1.01 1.30 1.25 0.72 0.73 1.12 1.15 1.16 0.94 0.79 1.18 1.07 0.94 1.64 0.93 0.93 1.14 1.12 Al O 18.33 18.66 19.65 16.94 17.47 18.29 18.38 18.26 19.62 15.15 16.60 18.64 17.65 16.75 15.86 18.76 17.56 16.83 2 3 ´ e eLci ..Rpln ora fSuhAeia at cecs1 20)625–641 (2002) 15 Sciences Earth American South of Journal / Rapalini A.E. Luchi, de pez Fe2O3 5.51 9.65 8.32 5.31 4.99 9.21 9.48 9.98 9.81 6.14 8.28 7.48 8.42 11.23 6.00 8.35 8.58 8.04 MnO 0.07 0.13 0.13 0.10 0.10 0.20 0.14 0.63 0.13 0.01 0.16 0.13 0.13 0.15 0.11 0.15 0.14 0.12 MgO 2.64 6.53 5.73 2.03 1.97 6.08 7.01 6.69 3.62 3.79 5.79 2.66 3.74 4.47 4.01 4.22 5.15 2.78 CaO 4.54 8.97 8.09 4.37 3.62 7.97 9.18 9.42 8.76 6.60 5.90 3.79 5.35 6.35 5.88 4.36 6.09 3.59 Na2O 4.88 3.22 3.57 3.36 3.19 3.12 3.35 3.04 2.31 2.83 5.14 5.15 6.00 4.00 4.29 5.24 4.70 5.13 K2O 1.99 1.26 0.90 2.81 2.94 0.55 0.93 0.71 2.18 1.92 2.51 3.47 1.77 2.47 3.42 2.16 1.77 2.61 P2O5 0.33 0.50 0.24 0.31 0.29 0.25 0.30 0.31 0.61 0.41 0.36 0.22 0.28 0.44 0.71 0.23 0.34 0.30 Cr 20 149 64 54 103 97 52 1 22 25 89 9 Ni 1 72 5 35 81 78 129 7 23 35 41 20 Co 52 47 54 36 39 38 46 44 44 52 25 27 22 Sc 28 26 26 17 24 17 V 115 170 197 229 202 200 157 195 164 252 180 198 220 Rb 71 46 35 100 109 21 41 22 51 71 59 112 52 79 110 62 56 76 Cs 0.52 4.11 2.02 0.94 1.36 2.55 Ba 569 767 337 575 542 232 475 422 939 822 508 770 572 592 398 706 Sr 495 618 662 455 468 712 649 661 420 477 494 249 646 516 423 550 654 402 Ga 27 22 24 Ta 1.04 1.03 1.01 1.04 1.04 1.02 Nb 8.5 5.0 3.2 7.1 7.2 5.0 3.0 8.2 7.0 7.4 7.9 6.0 Hf 2.61 2.88 2.73 3.22 3.97 6.23 Zr 206 161 126 155 169 87 119 123 177 167 187 160 183 203 181 145 173 228 Y 242019202118161725162327212726202427 Th 6.71 3.01 3.47 1.77 0.72 1.01 4.84 5.87 4.34 4.63 3.95 4.49 6.74 U 1.29 0.20 1.22 0.94 0.62 0.61 0.69 1.21 0.91 1.58 0.94 0.73 0.72 La 36.13 13.03 16.05 16.16 38.48 28.59 34.80 18.82 25.39 31.48 Ce 72.57 29.19 33.94 33.33 82.53 62.95 78.09 36.40 49.12 63.36 Pr 9.96 7.54 Nd 33.68 17.72 19.54 18.18 38.18 27.59 84.46 17.68 24.04 26.57 Sm 6.74 3.96 4.11 4.04 6.84 5.24 15.85 3.54 4.91 5.52 Eu 1.43 1.36 1.34 1.31 1.77 1.27 3.81 1.04 1.36 1.33 Gd 4.59 5.75 4.38 10.03 Tb 0.52 0.51 0.81 0.00 0.62 0.63 0.61 Dy 4.59 4.31 3.04 5.42 Ho 0.88 0.63 Er 2.35 1.65 Yb 1.74 1.56 1.75 1.72 0.32 1.51 1.71 1.87 2.30 2.45 Lu 0.20 0.24 0.26 0.24 0.23 0.30 0.29 0.36 0.38 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 631 using an anhydrous base. Only the eighteen samples with 53–61% SiO2; total alkalis are higher than CaO at low LOI values under 2% were used. SiO2 amounts (Fig. 6b–d). TAS classification distinguishes two series: a subalkaline Alkaline series basalts and andesites are more fractio- series composed of subalkaline basalts, basaltic andesites, nated than the subalkaline basalts and andesite basalts and and dacites, and an alkaline series composed of alkaline have lower values of the compatible major and trace trachyandesites and basaltic trachyandesites (Fig. 6a). elements. Relatively low MgO, Cr, and Ni (Table 1) indicate Comparison with the volcanic rocks from the Lonco Trapial that the magmas from which the dykes derive are not direct and Bajo Pobre Formations shows that the subalkaline melts from the mantle. Absolute values for MgO, CaO, Cr, basalts are more basic than their less evolved rocks and and Sr are lower and TiO2,Na2O, K2O, Rb, Ba, Zr, Nb, La, located along a hypothetical extension of the trend depicted and Y are higher in the alkaline series. for the Lonco Trapial Formation (Fig. 6a). The alkaline For samples with SiO2 , 60%, K/Rb (180–220), Ba/Nb series rocks cover a SiO2 interval similar to that of the (100–160), Ba/Rb (10–20), Rb/Sr (0.08–0.02), Zr/Nb (24– Lonco Trapial Formation but are higher in total alkalis 40), Nd/Th (15–27), and K/Nb (2000–2400) distinguish the (Fig. 6a). The subalkaline series is composed of medium-K subalkaline series from the alkaline series, which possesses metaluminous calc-alkaline basalts, basaltic andesites with higher K/Rb (260–280), Rb/Sr (0.15–0.4), and K/Nb a somewhat restricted silica range (49–53%), and high-K (2400–4800) and lower Ba/Nb (60–120), Ba/Rb (6–10), mildly peraluminous dacites with silica content of 62–64%. Nd/Th (4–5), and Zr/Nb (22–24). La/Nb is similar in both In addition, Na2O þ K2O increases with SiO2. The alkaline series, between 5 and 4. LREE/HREE is slightly higher in series is composed of high-K alkali-calcic metaluminous the alkaline series, with La/Yb from 10 to 13 for the alkaline trachyandesites and basaltic trachyandesites that span series and 8–10 for the basalts of the subalkaline series.

Fig. 6. Classification and chemical characterization diagrams. (a) TAS (Le Maitre et al., 1989). For comparison, the compositional field of the basaltic and andesitic rocks of Lonco Trapial and Bajo Pobre Formations are indicated. The subalkaline series is included in the Bajo Pobre Formation, whereas the alkaline series is located in the compositional field of Lonco Trapial; (b) K2O versus SiO2; subdivisions by Le Maitre et al. (1989) for subalkaline rocks and Rickwood (1989) for shoshonite series. Note that the alkaline rocks of (a) are located mainly in the high-K field; (c) Shand index diagram; the subalkaline series is metaluminous to slightly peraluminous in the more evolved lithologies, whereas the alkaline series is metaluminous but richer in total alkalis; and (d) Peacock index diagrams to illustrate the calc-alkaline versus alkali-calcic compositions of the subalkaline series compared with the alkaline series. The dotted line is the subalkaline series, the full line represents the alkaline series. Symbols: unfilled circles are the subalkaline series, black circles are the alkaline series. 632 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641

Trends for major elements, TiO2,Fe2O3t, MgO, CaO, separately, a characteristic linked with the higher propyliza- and P2O5, are generally continuous negative slopes in tion that might be recognized in thin sections. Harker diagrams and steeper in the alkaline series, except Trace element variation diagrams (Fig. 8) show positive for P2O5.Al2O3 shows a scattered pattern, and both Na2O trends for Rb and Th and negative trends for Zr, but and K2O correlate positively with SiO2. Although the P2O5 generally, patterns for the alkaline series are scattered. If content is low, values for the alkaline series resemble those both basalts and dacites of the subalkaline series are for the subalkaline series in the more basic samples (Fig. 7). analyzed, slopes are negative for Sr and positive for Rb, In the subalkaline series, sample D-7 systematically plots Th, Y, Zr, Nb, and La. The Rb and Ba patterns parallel

Fig. 7. Major element variation diagrams versus SiO2 for representative samples of both series. Symbols are as in Fig. 6. M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 633

Fig. 8. Selected trace-element variation diagrams versus SiO2 for representative samples of both series. Symbols are as in Fig. 6.

trends depicted by K2O. The Sr slope is negative but steeper plotted versus SiO2. However, in the alkaline rocks, the for the alkaline series; subalkaline dacites have high Sr basaltic trachyandesites show decreased Hf. contents, which might indicate recycled plagioclase. The Rb versus Sr correlation is negative, but slopes Absolute values for Nb are lower than 10 ppm, but trends are steeper for the basalts of the subalkaline series, depicted by Nb are parallel those of Zr. Although Hf data which suggests that no K-feldspar or acidic plagioclase are scarce, values for subalkaline samples are lower (mean fractionation controlled the crystallization sequence in the value 3), and no variation is recognized when they are less evolved rocks (Fig. 9). 634 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641

Primordial mantle-normalized spider diagrams for samples from both series with equivalent SiO2 content exhibit a steeper pattern from Cs to Y, which becomes steeper from K to Cs and thereby indicates enrichment in LILE. Subalkaline series show notorious positive peaks at U and Sr and a negative peak at Th and Zr. Nb troughs are common for both series (Fig. 10a). Primordial mantle- normalized REE patterns show relatively smooth slopes (Fig. 10b) with moderate total REE contents and LREE/ HREE . 1. The La/Yb varies between 8 and 13 with higher values for the alkaline rocks. In addition, LREE slopes are steeper than HREE, the Tb/Lu ratio is higher for the subalkaline rocks, and there is no Eu anomaly (Fig. 10b).

2.5. Age Fig. 9. Rb versus Sr logarithmic plot for samples of both series. Note the different slopes depicted. Symbols are as in Fig. 6. Field data cannot appropriately constrain the age of the dykes. The first surveys in SMC treated the dykes as if they stemmed from the widespread Tertiary volcanism that characterizes the area (Ravazzoli and Sesana, 1977). Country rocks are Early Carboniferous and intruded by Permian undeformed granites emplaced at higher crustal levels. Because the dykes are emplaced in a brittle crust, they must be younger than Permian. Whole rock K/Ar ages are 168.4 ^ 3.5 Ma for the subalkaline series basalts and 172.7 ^ 4.5 Ma for the alkaline series (Table 2). The sodic alkaline Alvar andesites yield a K/Ar age for plagioclase of 161.4 ^ 7.3 Ma, which

Table 2 Medium values and compositional ranges for the analysed samples of both series. Selected inter-element ratios are also presented. Major elements expressed in %w/w, trace elements expressed in ppm

Element/ratios Subalkaline series Alkaline series

Basalts Dacites Trachybasalts Trachyandesites

SiO2 49–53 62–64 53–56 57–61 Al2O3 19–18 18.5–17.2 18.8–16 MgO 7.2–6 3–2.4 5.6–3.0 CaO 9.5–8 4.4–3.6 6.5–3.6

K2O 1.6–0.8 2.0–3.2 3.0–2.0 3.8–3.0 Na2O 2.8–3.6 3.8–5.2 Cr 160–60 ,20 60–5 Ni 80–5 n.d. 40–10 Co 48–36 50 50–46 24–20 V 150–220 90 150–210 Hf 3.0 n.d. 4.0–6.0 Zr 140–80 155 210–170 130–180 Y 16–18 15–20 27–20 Rb 50–20 60–120 60–120 Sr 630–730 760 670–400 Fig. 10. (a) Morb-normalized spider diagrams for the most primitive Ba 800–240 530 360–900 samples of each series. Nb depletion is present in both series, but no Ti K/Rb 220 260–280 trough is recognized. Norm values are from NEWPET version 94. (b) Rb/Ba 0.05–0.1 0.12–0.09 0.1 Chondrite (Sun, 1980) normalized diagram, La/Yb varies from 7 for the N La/Yb 9–8 20 12–10 subalkaline series to 8 for the alkaline series. The differences are related to La/Ta 16–13 n.d. 30–18 increasing LREE but lower Ho/Lu for the alkaline series basaltic Ba/Ta 420–220 n.d. 690–460 trachyandesites. Although field relationships are not conclusive, K/Ar La/Sm 4–3 2 6–5 ages indicate that the alkaline series is slightly older than the subalkaline Zr/Y 8–4.8 8–10.4 8.8–6.0 series. M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 635 reflects a partial overlap, within error, with the ages arises from the total alkali content represented by a more calculated for both series. The volcanic event, located in sodic plagioclase in the alkaline series, together with the Bajocian-Bathonian, is included in the time span of both interstitial alkali-feldspar. Additional mineralogical the Lonco Trapial and the Marifil Formations in the NPM evidences of the alkaline tendency include the predomi- and that of the Chon-Aike Formation in the DM, according nance of two amphibole generations in some samples in to radiometric data reported by Pankhurst and Rapela (1995) which plagioclase phenocrysts are scarce, the prismatic and Alric et al. (1996). Recent new Ar/Ar dating (Bertrand apatite in the groundmass, and the acicular apatite in the et al., 1999; Pankhurst et al., 2000) of the Jurassic volcanism external rims of the oligoclase phenocrysts of the basaltic of Patagonia confirms that Bajo Pobre andesites are younger trachyandesites and trachyandesites. That apatite can bear than the Chon-Aike acid rocks. Therefore, though there both Cl and F, and halogen content higher than that of could be chemical similarities between these andesites and the subalkaline series can be inferred for the trachyba- the subalkaline series, the two magmatic events are not salts and trachyandesites. Chemically, the alkaline rocks coeval. The paleomagnetic pole for the dykes under study is are characterized by lower MgO, CaO, Cr, and Sr and particularly coincident with the paleomagnetic pole from higher TiO2,Na2O, K2O, Rb, Ba, Zr, Nb, La, and Y. The the Chon-Aike lavas at Estancia La Reconquista, Santa Cruz higher Zr content of the alkaline series rocks could be (Vilas, 1974), which suggests a temporal correlation because zircon stability in a melt decreases with between the intrusion of the SMC dykes and the extrusion increasing alkalinity, temperature, and water content of the Chon-Aike lavas. These poles may indicate the (Watson and Harrison, 1983). In addition, F may form <170 Ma pole for South America (Lo´pez de Luchi and stable fluorocomplexes with Zr (Alderton et al., 1980). Rapalini, 1997, 1999). Therefore, increasing alkalinity and halogen-rich melting Dyke swarms would be included in the V2 period of processes would favor the melting of zircon, which Pankhurst et al. (1999, 2000), defined for acid rocks of both would result in higher Zr content in the melt. In addition, Patagonia and Antarctica. The authors linked this volcanism halogen complexes are preferentially partitioned into to destructive plate margins. volatile phases and would induce less polymerization of In a regional context, though different ages have been the melts, which thus could become separated from their calculated (Fig. 2) at broadly the same latitude, basic to source, even at low degrees of partial melting. intermediate volcanism is younger than acidic and located Higher oxygen fugacity may be suggested for the higher to the west. This could suggest less involvement of the modal amount of primary magnetite in the basaltic continental crust to the west (a thinner crust?). However, trachyandesites. volcanism started earlier in the NPM than in the DM. Major element trends within each series are controlled by Therefore, basic to intermediate volcanism in the central the fractionating phases. CaO patterns compared with SiO and western sectors of the NPM is contemporaneous with 2 indicate that a calcic plagioclase might be the main predominantly acid magmatism in the DM. Further age determinations in these dyke swarms and paleomagnetic fractionating phase in the subalkaline series. Normal zoning studies in the Middle Jurassic volcanism are needed to with some oscillatory zoning is indicated by the CaO constrain the temporal evolution and their relation with depletion without NaO2 variation in this series. In the major rotations of Gondwana evolution. alkaline series, scattering in Al2O3 contrasts with the steep negative slope for CaO. Reverse or oscillatory zoning might be reflected in the scattered pattern for Al2O3 (Fig. 7). In the 3. Discussion alkaline series, because plagioclase is more sodic, the steep CaO slope would imply the fractionation of another Ca- The main points refer to the factors that affected the bearing phase. Arago´n et al. (2000) note that the reverse chemical evolution of melts, including open or closed zoning that characterized the plagioclases, amphiboles, and system evolution, crustal contamination, the relationship clinopyroxenes of the Alvar andesites evidence a mixing between the subalkaline and alkaline rocks, and the source process and theorize incomplete mixing between two of the melts that gave rise to the dykes (lower crust versus similarly evolved magmas with different Fe, Mg, and Ca. enriched lithospheric mantle). The chemistry of the dyke Although it is highly speculative, the two magmatic series swarms is analyzed from two points of view: fractionating that the dykes represent fit the proposed end members that phases and less evolved rocks of each series, which provides Arago´n et al. (2000) consider. a more general approach to constrain the source and the The negative slopes of CaO, coupled with the steeper tectonic setting. slopes of Rb versus Sr (Fig. 9), point to clinopyroxene ^ a more calcic plagioclase as fractionating phases in the rocks 3.1. Chemistry and magmatic evolution with less than 60% SiO2 in the subalkaline series, as well as clinoamphibole for rocks with SiO2 higher than 60%, A combination of petrographic features, mineralogy, and together with a more acidic plagioclase and a lack of chemical characterization shows that the alkaline character clinopyroxene. Smooth slopes in the alkaline series indicate 636 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 that plagioclase is the more acidic plagioclase for alkaline series with higher Kd compared with Sr. Analyzing Fe–Mg silicates as fractionating phases, FeO/MgO is lower for alkaline series silicate with relative iron enrichment in the trachyandesites; in the subalkaline series, this relative enrichment appears in the dacites. Therefore, more oxidizing conditions again can be inferred for the alkaline series (Fig. 7). The general scattered patterns for HFS elements and REE might reflect some cumulatic minerals or contami- nation. Y, Zr, and Zr/Y show positive slopes for subalkaline series basalts, in accordance with clinopyroxene and calcic plagioclase controlling liquid evolution. However, the Y correlation is negative for rocks with SiO2 . 60%, which indicates that clinoamphibole joins the fractionating assem- blage. In contrast, Y and Zr decrease for the alkaline series, thus providing evidence of control by clinoamphibole. An Nb decrease that almost parallels TiO2 could indicate that sphene is present as a fractionating phase. An La increase alongwithSiO2 in each series results from either clinopyroxene or clinoamphibole fractionation (Fig. 8). La/Yb varies from 8–10 for the subalkaline series to 10– 13 for the alkaline series. The differences between the two series are related to increasing LREE but lower Tb/Lu for alkaline series basaltic trachyandesites. The highest La/Yb values for the alkaline rocks are associated with the highest La/Sm (Fig. 11). No explanation is proposed because lower degrees of partial melting of the same source, crustal contamination, or clinoamphibole fractionation could affect Fig. 11. (a) Ba/Ta vs. La/Ta ratios (Kay et al., 1991) for selected samples of LREE. the more basic rocks of both series. Although two trends with slightly Within each series, fractional crystallization is respon- different slopes are defined, both are included in the interval of Ba/La ratios of 20–30. See text for discussion; (b) La/Sm vs. La/Yb ratios (Kay et al., sible for gradually steeper slopes of REE. The higher La/Yb 1991) for selected samples of both series. Note the higher La/Sm values for for the dacitic rocks correlates with variable La/Sm and a the alkaline rocks. See text for discussion; Symbols as in Fig. 6. steep HREE trend. This implies that clinoamphibole þ plagioclase are the fractionating phases. Absolute values of Ba/La vary between 20 and 30 and conditions at the melting site, different degrees of partial decrease as SiO2 increases for the subalkaline rocks, in line melting, or crustal thickness. with clinopyroxene þ clinoamphibole ^ plagioclase frac- The two series are not related by crystal fractionation, tionation. An increase of this ratio in the alkaline rocks because at the same evolutionary stage, they differ in would suggest magnetite fractionation (Fig. 11). absolute major elements, LIL, REE, HFSE, and interele- The Alvar andesites (Arago´n et al., 2000) share some ment ratios (Figs. 7, 8, 10 and 11; Tables 1 and 2). common chemical characteristics with the alkaline series for Higher Th/Yb (Fig. 12), La/Ta (30–18), and Ba/Ta (420) the same SiO2 content, such as La/Yb (11) and absolute (Fig.11), coupled with lower Ba/Th for the alkaline rocks, values for REE, though they are poorer in K2O, Fe2O3, reflects either a subduction input or a crustal component, as MgO, Zr, Y, Th, and U. well as different degrees of partial melting or crystal fractionation. The higher Na2O and lower CaO exhibited in 3.2. Processes at the melting site and source the alkaline rocks indicate a thicker crust (Plank and Langmuir, 1988).

The most primitive samples (,60% SiO2) of each series The increase in La/Yb that the alkaline rocks exhibit were analyzed to constrain both the processes that gave rise (Figs. 10 and 11) could arise from different factors. If La/Yb to the two series and the nature of the source. Dyke swarms variation is considered an indicator of crustal contamination represent two chemically distinctive magmatic series that and compared with Rb/Cs (an indicator of subduction zone are almost coeval in space and time. The differences input), it correlates negatively in both series. However, in between the alkaline and the subalkaline basalts and the alkaline rocks, higher La/Yb is associated with higher andesites could arise from crystal fractionation, variable Rb/Cs (Table 2). Rb/Cs decreases with increasing slab M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 637

fluxing (Morris and Hart, 1983). Therefore, the lower Rb/Cs that the subalkaline rocks exhibit could point to higher degrees of early partial melting, because it would give rise to melt compositions more similar to their source. In addition, increasing slab-derived fluids would favor melting processes. La/Ta is higher for the alkaline rocks, which could indicate larger amounts of crustal components or the retention of residual phases associated with small degrees of partial melting (Kay et al., 1991) In spite of the higher Zr content of the alkaline rocks (Table 1), there is no difference in Zr/Y between the two series. Unless garnet is a residual phase (which is not valid for these rocks), the degree of partial melting would affect the Zr content, with higher absolute values of Zr for lower Fig. 12. Th/Yb versus Ta/Yb (Wilson, 1989, p. 219) that show enriched degrees of partial melting (Pearce and Norry, 1979). A versus depleted mantle sources. The studied rocks plot in the field next to similar panorama of increasing Zr could derive from enriched mantle. Samples from the subalkaline series exhibit variations in fractional crystallization of non-Zr-bearing phases, but Th/Yb ratio, which reflects either a subduction or a crustal component. Samples of the alkaline series are next to the active continental margin field. clinoamphibole with a Kd to Zr ratio higher than 1 is a The simultaneous increase of both Th/Yb and Ta/Yb would indicate an dominating phase in alkaline rock evolution. Halogen (as intraplate enrichment. Symbols are as in Fig. 6. indicated by apatite in the alkaline series) also contributes to the higher Zr content of the originating melts and the possibility of migration of low melt fractions of the same source for the alkaline Alvar andesites might be the partial source. melting of mafic rocks of either the lower crust or the An La/Sm increase in the alkaline rocks is associated lithospheric mantle. Pankhurst and Rapela (1995) propose with lower Sr content; therefore, it could be linked to that the Jurassic silicic volcanic rocks could be remelts of amphibole fractionation at the source (together with andesite magmas crystallized in the lower crust. Kay and plagioclase), which would be favored at low pressure in Gorring (1999) suggest a hydrated and enriched Jurassic water-rich conditions. Different water and volatiles at the lithospheric mantle. Jurassic volcanism is characterized by melting site would control the generation of more alkaline 87Sr/86Sr initial ratio .0.706 and negative 1Nd values compositions, because low melt fractions with higher LREE independent of their location and composition (Pankhurst and Zr content could leave their source. et al., 1998). In the Andes, variations of ratio with both Although we lack isotopic data that would enable us to constrain the source more accurately, some insights into space and time during the Tertiary have been ascribed to their nature could be inferred from trace elements. The increasing crustal thickness as a result of the compression source for both series could be an enriched mantle source or along the active margin (Kay et al., 1991). If these ratios the lower crust. An La/Yb (Figs. 10 and 11) lower than 13 is correlate with the low La/Yb of the dyke rocks, an early an indication of a thin crust. Kay et al. (1991) note that conclusion might be that, in this area, they are independent values of this ratio lower than 18 correspond to crustal of a process of crustal thickening. thickness less than 40 km. Fractionation of the plagioclase N–S/E–W migration of younger units to the south and at that controls the evolutionary trends of both series also the same latitude to the west has been proposed for the indicates low pressures in the crust. Jurassic volcanism of the NPM and DM. In this connection, Absolute values for Ba/Nb (60–160), La/Nb (4–5), and if we accept the idea of the melting of the lower crust as the K/Nb (.2000) in both series indicate a significantly LIL- source for the protoliths of the mainly acid Marifil enriched crustal component. Nd/Th of approximately 4 for Formation, it would leave a refractory lower crust. Renewed the alkaline rocks argues against a primitive mantle, which volcanism that migrates to the west would need a thermal is also evidenced by the low MgO, Cr, and Ni and high LIL. input to melt this restitic crust; alternatively, as the crust However, Nd/Th between 15 and 27 indicates a mantle gets thinner to the west, decompression melting of a source. Therefore, though mantle-like sources are inferred previously enriched lithospheric mantle could give rise to for the subalkaline rocks, the crustal signature exists. the basic magmatism of the Lonco Trapial Formation. In the Th/Yb versus Ta/Yb diagram, the rocks plot next Comparison with the chemistry of the Tertiary basalts to the enriched mantle field. The subalkaline rocks are and andesites (Fig. 11a,b) shows that the fields depicted by located closer to the plate basalts, and the alkaline series is both series are close to OIB and partially included in the next to the active continental margin field, on account of its area defined by the ,65 SiO2 Late Oligocene–Early higher Th/Yb (Fig. 12). Miocene volcanics emplaced in a crust thinner than that of Different magma sources have been proposed for the the Middle–Late Miocene (Fig. 11a). Data provided by the Jurassic volcanic rocks. Arago´n et al. (1996) suggest that the La/Sm versus La/Yb plot are located closer to the fields of 638 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 the southern volcanic zone of the Andes, for which a thin conditions at the melting site are coupled with different crust is proposed (Fig. 11b). The most evolved rocks with degrees of relatively shallow decompressional (?) partial dacitic composition plot in a sector that corresponds to melting of either an enriched mantellic source or the lower increased crustal thickness, but in this case, fractionation crust for compositions of less than 60% SiO2. This model is might be responsible for the higher La/Yb. based on the following aspects: In Patagonia, Pankhurst and Rapela (1995) propose a genetic link between the Jurassic magmas and the LIL- 1. Limited and different compositional range in each series depleted lower crust xenoliths and suggest that LIL-depleted with higher LILE content for the alkaline series; basaltic andesites and andesites (e.g. Bajo Pobre Formation) 2. Plagioclase fractionation, which indicates low pressure; could be the source of LIL-enriched intermediate and 3. Low and relatively homogeneous La/Yb in both series; acid rocks through partial melting. Andesitic rocks of the 4. Absolute values of incompatible elements that are not Bajo Pobre Formation have 1Nd values of 21.9 and related by crystal fractionation and, in some cases, are 23.9 and a 87Sr/86Sr ratio of 0.7067 ^ 0.0006, both higher than those of the dacites. Apatite suggests unrepresentative of a normal mantle and similar to the halogens may be in the source, which would favor less 87Sr/86Sr ratio of 0.7065 ^ 0.0004 and negative 1Nd of polymerized melts that could be easily removed from the majority of the rhyolites of the Chon-Aike and their source, even at low melt fraction; and Marifil Formations and the Dique Ameghino andesitic 5. Smoother slopes of the REE for the subalkaline series dyke (Marifil Formation). According to Pankhurst and and higher LREE/HREE for the alkaline series. Rapela (1995), the two potential source regions are compatible with the isotopic and trace-element charac- 3.3. Tectonic setting teristics of an enriched lithospheric upper mantle and a juvenile lower crust, and the isotopic uniformity and Geochemical results, as applied in a tectonic diagram, are REE modeling do not distinguish fractional crystal- not conclusive for the dykes. Absolute values for interele- lization from variable partial melting. Therefore, both ment ratios such as Ba/La and Ba/Th are common for arc mechanisms could be involved. volcanic rock; however, Zr/Y (6–11) values (Table 1) are Kay and Gorring (1999) suggest that the isotopic ratios of typical for intraplate basalts. In a Zr–Ti–Y diagram Jurassic volcanic rocks show that the Patagonian Jurassic (Fig. 13), samples from the subalkaline series plot in the lithospheric mantle was different than the modern mantle. intraplate tholeiitic field, whereas the alkaline samples are Evidence comes from initial 87Sr/86Sr (,0.707), 1Nd (23 located in the area for calc-alkaline basalts but displaced to 29), and Pb isotopic ratios of andesitic and dacitic flows toward the Zr apex. In a Th–Hf–Ta diagram (Wood, 1980), and rhyolitic in the Jurassic Chon-Aike subalkaline rocks are in the limit between alkaline and Formation, as well as from the granulite xenoliths that are intraplate tholeiites, with Hf/Ta ¼ 2.5, whereas trachyba- interpreted as the complementary lower crustal residue. The salts plot in the volcanic arc field. Migration toward the Th concept of a hydrated and enriched Jurassic mantle litho- apex could be related to upper crust contamination, crystal sphere in Patagonia is based on the premise that Chon-Aike fractionation, or partial melting. High LILE contents could silicic magmas, like Parana´ rhyolites, were largely produced imply either an enrichment process at the source or crustal by the melting of contemporaneously underplated mafic contamination (Fig. 9). magmas in the lower crust. This Jurassic melting of such an Either an active subduction margin or a lithospheric enriched, hydrated Patagonian lithospheric mantle would extension has been suggested as the tectonic scenario for the have left a generally dehydrated lithospheric mantle largely Jurassic volcanism (Pankhurst and Rapela, 1995). The first stripped of enriched components. hypothesis was proposed for the central volcanic belt (Page Bertrand et al. (1999) suggest, on the basis of Nd and Page, 1993). Extensive lower crustal melting was isotopes, that the basalts and andesites originate from a related to rifting and incipient break up of the Gondwana mantle enriched by a subduction component and that this supercontinent (Storey et al., 1996). Pankhurst and Rapela contamination process could have risen during Grenvillian (1995) assign the extensive lower crustal melting event events. represented by the Jurassic volcanic complexes located east Arago´n et al. (2000) propose that the olivine–plagioclase of 688W to an extensional tectonic setting associated with xenoliths mafic microgranular enclaves of the Alvar the rifting and incipient break up of Gondwana. This setting andesites would result from the partial melting of a mafic can be assigned mainly to the V1 (and partiallyV2) periods source. On the basis of textural evidence of the olivine of Pankhurst et al. (2000). On the basis of geochemistry, from the lavas, they assign a deep origin for those enclaves. Pankhurst et al. (2000) assign a subduction-related setting The partial melting event might have taken place at for the rhyolites to their V2 and V3 periods, though zircon depths greater than 15 km, according to geobarometric inheritance, isotopic, and geochemical data suggest a calculations. primary origin by anatexis of the continental crust. Summarizing the chemical information provided by the Petrogenesis of the basic volcanism is less constrained. rocks under study, a model is proposed in which variable The intermediate to basic volcanic rocks of the central M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641 639

significant decrease in La/Lu from north to south and from NPM intermediate rocks to the DM Bajo Pobre Formation (Pankhurst and Rapela, 1995, Fig. 5). If the dykes under study are compared to the compositionally similar 186.2 ^ 2 Ma Ameghino dyke in Marifil, the 161.4 ^ 7.3 Ma Alvar andesites, and the 150.6 ^ 2.0/ 156.7 ^ 4.6 Ma andesites of Bajo Pobre, the following points, which could bear on thinning of the crust, higher degrees of partial melting, or the involvement of subduction, arise:

† La/Yb is approximately 18 for the Ameghino dyke, whose plot in the field is defined by the subalkaline series rocks (Pankhurst and Rapela, 1995, Fig. 3). These rocks are associated with the Marifil Formation, with which they share similar isotopic compositions, and therefore must be associated with the initial stages of rifting and thicker crust; † La/Yb is approximately 11 for the Alvar andesites, but melting is believed to have taken place at a depth greater than 15 km in an intracratonic extensional setting; † La/Yb is approximately 9.2 for the Bajo Pobre Formation, which plots in the subalkaline area in the TAS diagram (Fig. 6a), though with more evolved compositions. Following Pankhurst et al. (2000),V3 volcanism would correspond with an increase in the subduction component; and † La/Yb 8–13 in the dykes with SiO2 , 60% could be linked to an extensional intracratonic setting, because more evolved rocks show an increase in this ratio. The subduction component that can be inferred from the lower Rb/Cs ratio of the subalkaline rocks does discriminate the timing of the potential subduction- related enrichment.

In a more regional context, some insights regarding the tectonic setting could be obtained from Jurassic basin evolution. Figari and Courtade (1993) and Cortin˜as (1996) propose that volcanism and sedimentation were essentially coeval during the Middle Jurassic in an intracratonic setting, the Somoncura-Can˜ado´n Asfalto basin. The initial stages of the basin development Fig. 13. (a) Ti–Zr–Y discrimination diagram for basalts (after Pearce and began during the Middle to Late , and different Cann, 1973). Samples of the subalkaline series are in the intraplate basalt rift-postrift evolution processes were controlled by exten- field, whereas samples of the alkaline rocks are in the calc-alkaline basalts sional processes. The Lonco Trapial group represents field but displaced toward the Zr apex. (b) Th–Hf–Ta discrimination volcanism related to early stages of rift development diagram (after Wood, 1980). Samples of the subalkaline series are in the (Cortin˜as, 1996). Arago´n et al. (1996, 2000), Lo´pez de limit between alkaline intraplate basalt and E-Morb and tholeiitic intraplate basalt fields, with Hf/Ta ¼ 2.5. In contrast, samples of the alkaline series Luchi and Rapalini (1997), and Rapalini and Lo´pez de trachybasalts plot in the calc-alkaline volcanic-arc field. (c) Zr/Y versus Zr Luchi (2000) indicate that old faulting systems controlled diagram that shows the location of samples from both series. All the the development of the Jurassic basin and the dyke swarms. samples plot in the intraplate basalt field with similar Zr/Y but higher Zr for Furthermore, Arago´n et al. (2000) suggest that the tectonic the alkaline rocks. Symbols are as in Fig. 6. scenario for the alkaline Alvar andesites would be an volcanic belt (180–136 Ma) have been assigned to either an intracratonic extensional setting. Pankhurst et al. (2000) eastern branch of the Jurassic volcanic arc (Page and Page, indicate that the large amount of crustal extension and 1993) or an extensional setting for the Bajo Pobre thinning during the Jurassic could have been a cause of Formation (Pankhurst and Rapela, 1995). There is a the silicic magmatism through decompression melting. 640 M.G. Lo´pez de Luchi, A.E. Rapalini / Journal of South American Earth Sciences 15 (2002) 625–641

Therefore, models that consider Middle Jurassic basic to with regional data indicating that initial rifting of intermediate volcanism as related exclusively to geo- Gondwana has controlled for preexisting crustal disconti- chemical active margin evolution along the western margin nuities. Progressive rifting of Gondwana could have of Gondwana must be revised. allowed the ascent of primitive magmas that might provide the heat source for the extensive lower crustal melting event that originated the more silicic eastern 4. Conclusions volcanism. A model is proposed in which the alkaline and subalka- The dykes represent two magmatic series, alkaline and line series could be related to the melting of the same subalkaline. Fractionating phases that control the magmatic shallow enriched mantle source at different water content, evolution are, for the subalkaline series, clinopyroxene ^ halogen content, and temperature (different degrees of plagioclase (andesine) with clinoamphibole for composition partial melting [?]) conditions. The crustal signature is with SiO2 . 60%, and, for the alkaline series, present in both series. Age and paleomagnetic data indicate clinoamphibole þ plagioclase (oligoclase) ^ magnetite. that basic to intermediate magmatism is coeval with the Within each series, fractional crystallization ^ crustal silicic magmatism in the DM. However, the relationship contamination is responsible for the gradually steeper between this intracratonic extensional setting and the slopes of the REE. initiation of the active Pacific margin of Gondwana remains The most primitive samples of each series could be unclear. related by higher degrees of partial melting to a source undergoing decompression, which would be in accord with the smooth slopes of the REE. Although the Acknowledgments geochemical results for the dykes do not point conclus- ively to one tectonic setting, Zr/Y between 6 and 11 is This work was partially financed by a grant from typical of intraplate basalts or continental arcs that share Fundacio´n Antorchas: Estudio paleomagne´tico y petrogra´- an enriched source. This source could be either an fico–petrolo´gico de la Formacio´n Mamil Choique, Rı´o enriched lithospheric mantle or the lower crust. Although Negro y Chubut, directed by A. Rapalini. A. Gonza´lez and crustal contamination plus enrichment for subduction- Gabriel Giordanengo helped with the illustrations. This derived fluids is not likely to be clearly distinguished by paper is a contribution to IGCP Project 436. the available data, because Rb/Cs is lower for the subalkaline rocks, these rocks favor a subduction- enrichment processes hypothesis. In contrast, the alkaline References rocks reflect more crustal components or lower degrees of partial melting that affect their magmatic evolution, as Alderton, D.M., Pearce, J., Potts, P.J., 1980. Rare earth element mobility is indicated by their higher La/Yb, LILE, and Zr and during granite alteration: evidence from southwest England. Earth and lower CaO, MgO, Sr, and Nd/Th. In addition, an Planetary Sciences Letters 49, 149–165. enriched mantle source is favored by the Nd/Th and Alric, V.I., Haller, M.J., Fe´raud, G., Bertrand, H., Zubia, M., 1996. 40 39 Th/Yb versus Ta/Yb values for the subalkaline rocks. Cronologı´a Ar/ Ar del Volcanismo Jura´sico de la Patagonia Enrichment evidenced by the source could have been Extrandina. Actas 13th Congreso Geolo´gico Argentino 5, 243–250. Arago´n, E., In˜´ıguez Rodrı´guez, A., Benialgo, A., 1996. A caldera field at attained from a subduction component, but the possibility the Marifil formation, new volcanogenic interpretation, North Patago- of a crustal component cannot be discarded. Low nian Massif. Journal of South America Earth Sciences 9 (5–6), absolute value contents of both Ni and Cr, together 321–328. with relatively low MgO, indicate that the dyke magmas Arago´n, E., Gonza´lez, P., Aguilera, Y., Cavarozzi, C., Llambı´as, E., 2000. are not primary melts but have undergone olivine Andesitas Alvar: volcanismo alcalino jura´sico en el a´rea de Paso del Sapo, provincia de Chubut. Revista de la Asociacio´n Geolo´gica fractionation prior to their emplacement. 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