Crustal contribution in the genesis of the bimodal Triassic
volcanism from the Coastal Range, central Chile

Diego Morata
Luis Aguirre	Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile dmorata@cec.uchile.cl
Marcela Oyarzún
Mario Vergara

ABSTRACT

Triassic volcanism in the Coastal Range of central Chile is represented by the Pichidangui Formation (31°54' to 32°15'S) composed of acid and basic lava flows and sub-volcanic intrusives with some sedimentary intercalations. Both rock types have been affected by non-deformational very low- to low-grade metamorphism. Their geochemical characteristics indicate a tholeiitic affinity, with Nb/Y ratios ranging from 0.20-0.17. Low [mg] values in the basic rocks are indicative of non-primitive liquids. Differences in immobile trace element and REE (LREE/HREE, Eu/Eu*) between basic and acid rocks are consistent with their isotopic signature, with initial ratios of (⁸⁷Sr/⁸⁶Sr)_o and (¹⁴³Nd/¹⁴⁴Nd)_o ranging from 0.70397 to 0.70597 and from 0.512515 to 0.512608 respectively in the basic rocks, and from 0.70514-0.71161 to 0.512108-0.512526 respectively in the acid rocks. These chemical and isotopic differences between both types of magmatism are interpreted as a consequence of different degrees of crustal contamination from an enriched mantle source with a lithospheric component in an extensional intracontinental geodynamic setting followed by crystal fractionation to generate acid and basic melts. Geochemical and isotopic similarities between the Triassic basic rocks and the Lower Jurassic gabbros from the Limarí Complex in the Coastal Range in central Chile are interpreted as a consequence of a cogenetic event, related to the Mesozoic evolution of this portion of the Gondwana pacific margin.

Key words: Geochemistry, Petrogenesis, Crustal contamination,Triassic volcanism, Coastal Range, central Chile.

RESUMEN

Aporte cortical en la génesis del volcanismo bimodal Triásico de la Cordillera de la Costa, Chile central. El volcanismo Triásico en la Cordillera de la Costa de Chile central está representado por la Formación Pichidangui (31°54'-32°15'S), constituida por flujos de lavas ácidas y básicas con intercalaciones de cuerpos subvolcánicos y rocas sedimentarias. Ambos tipos de lavas presentan un metamorfismo no deformativo de muy bajo a bajo grado. Las características geoquímicas de estas lavas corresponden a las de la serie toleítica, con valores de Nb/Y entre 0,20 y 0,17. Los bajos valores de [mg] en las rocas básicas son indicativos de líquidos no primarios. Las diferencias observadas en elementos traza inmóviles y tierras raras (LREE/HREE, Eu/Eu*) entre las rocas ácidas y básicas son consistentes con sus signaturas isotópicas, cuyos valores iniciales son: (⁸⁷Sr/⁸⁶Sr)_o 0,70397-0,70597 y (¹⁴³Nd/¹⁴⁴Nd)_o 0,512515-0,512608 para las rocas básicas y 0,70514-0,71161 a 0,512108-0,512526 en las ácidas. Estas diferencias geoquímicas e isotópicas entre ambos tipos de lavas se interpretan como consecuencia de diferentes tasas de contaminación cortical a partir de una fuente mantélica enriquecida, con un componente litosférico, en un ambiente extensional intracontinental, seguido por cristalización fraccionada para generar los líquidos ácidos y básicos. Las similitudes geoquímicas e isotópicas observadas entre las rocas básicas del Triásico y los gabros del Jurásico Inferior del Complejo Limarí de la Cordillera de la Costa de Chile central se interpretan como la consecuencia de una génesis común para ambos magmatismos, que estaría relacionada con la evolución mesozoica del margen pacífico de Gondwana.

Palabras claves: Geoquímica, Petrogénesis, Contaminación cortical, Volcanismo Triásico, Cordillera de la Costa, Chile central.

INTRODUCTION

The Coastal Range in central Chile, located between the Pacific Ocean and the Central Valley, is mainly composed of Mesozoic-Cenozoic igneous and sedimentary rocks deposited on a pre-Mesozoic crystalline basement. Exposures of the Triassic rock units in central Chile are poor and scattered (Charrier, 1979; Aguirre, 1985) and for this reason, a new study on these rocks would help to the understanding of the geotectonic and petrological evolution of the Chilean Pacific margin during the beginning of the Mesozoic. These Triassic units correspond to El Quereo, Pichidangui and Los Molles Formations from which only the Pichidangui Formation contains significant volumes of volcanic and subvolcanic rocks.

Previous works concerning the Triassic rocks in the Coastal Range of central Chile are mainly focused on stratigraphic considerations (see, among others, Vicente, 1976; Charrier, 1979 and Rivano and Sepúlveda, 1991, and references therein) or to the plutonic rocks (Gana, 1991; Parada et al., 1991, 1999). Major element data were presented in Vicente (1976), and additional geochemical (major and trace elements) data for the Triassic volcanic rocks are found in Levi et al. (1988), Vergara et al. (1991) and Cancino (1992).

The results and conclusions of a study on the geochemistry of volcanic rocks from the Pichidangui Formation (31°54'-32°15'S) are reported in this paper. A bimodal (acid and basic) magmatism has already been described for this unit (Vicente, 1976; Vergara et al., 1991; Cancino, 1992). The aim of the present work is to propose a new petrogenetic model for this bimodal magmatism on the basis of their trace element geochemistry, including isotopic (Rb-Sr and Sm-Nd) data. The characterization of this bimodal magmatism, as well as its petrogenetic implications, will be outlined. With this paper, the authors contribute to the understanding of the magmatic evolution of the central Chilean Pacific margin during the beginning of the Mesozoic.

GEOLOGICAL CONTEXT

The Pichidangui Formation (Fig. 1), a 4,000 to 5,000 m thick isoclinal sequence, dipping 15 to 25° to the east-southeast, was deposited in a subaquatic environment. Marine sedimentary rocks predominate in its lower section whereas rocks characteristic of paralic conditions of sedimentation are found at the top where temporary continental conditions are reported (Muñoz Cristi, 1942, 1973; Cecioni and Westerman, 1968; Vicente, 1976; Charrier, 1979; Vergara et al., 1991). Acid and basic lava flows, with intercalations of shales containing a Middle to Late Triassic marine fauna, are typically represented in the Pichidangui Formation; some basic dykes and sills intrude this unit (Vicente, 1976, for a more detailed geographical and stratigraphical information). Basic lavas are mostly restricted to the bottom of this formation, alternating with the acid lavas, the volume of volcanic rocks (mostly acid lavas) decreasing upward, where turbiditic sedimentary rocks and shales with a Late Triassic to Early Jurassic (Hettangian) fauna become predominant. This upper part of the Pichidangui Formation has been assigned by Fuenzalida (1938) and Cecioni and Westermann (1968) to a separate formation (Los Molles Formation). The Pichidangui Formation overlies conformably the marine El Quereo Formation (graywackes and shales) of Middle Triassic age (Cecioni and Westermann, 1986) which, in turn, rests unconformably on the Arrayán Formation (Fig. 1), a strongly folded rhythmic sequence of slates and sandstones with a Devonian-Late Carboniferous flora (Rivano and Sepúlveda, 1991).

The main interpretative studies of the Pichidangui Formation (see Vergara et al., 1991) are those of Vicente (1976), Charrier (1979), Forsythe et al. (1987) and Mpodozis et al. (1988). Vicente (1976) considered it as part of a 'Western Euliminar Series' of the 'Andean Orogenic Cycle'. Charrier (1979) invoked deposition in a Triassic graben, probably caused by shearing between the Gondwana continent and a continental or oceanic block located to the west. On the basis of paleomagnetic data, Forsythe et al. (1987) concluded that the coastal Triassic rocks of central-northern Chile (including the Pichidangui Formation) could represent an allochthonous terrane accreted during the Middle to Late Jurassic. Mpodozis et al. (1992) included all the upper Paleozoic-Triassic magmatic rocks of central-northern Chile in a Pacific margin magmatic belt of Gondwanaland. Finally, Vergara et al. (1991), using major element discriminating diagrams and chondrite normalized rare earth element patterns, interpreted the volcanic rocks of the Pichidangui Formation as: i- belonging to a volcanic arc floored by a quasioceanic crust, compatible with an origin at the rim of Gondwanaland before its disintegration; ii- a continental/oceanic block located to the west as in Charrier (1979); or iii- an allochthonous terrane formed south of its present position as proposed by Forsythe et al. (1987).

PETROLOGY

The bimodal character of the Pichidangui Formation was first described by Muñoz-Cristi (1942, 1973) who used the terms keratophyre and quartz-keratophyre to name the silica-rich rocks (lavas, ignimbrite-like breccias and dykes) and the terms oligoclase-augite and labradorite-amphibole diabase for the basic components (lavas, sills and dykes). Cecioni and Westermann (1968) referred to these rocks as keratophyres and andesites, respectively; whereas Vicente (1976) named them keratophyres (albitophyres), spilites and dolerites.

About fifty samples were collected from outcrops of Triassic rocks belonging to the Pichidangui Formation along the Coastal Range between Los Vilos and Punta Molles (Fig.1). According to their field relationships and petrographic characteristics these rocks classify as lithic breccias, lapilli tuffs, ash tuffs, rhyolitic flows with porphyritic seriate textures, basalts, porphyritic basalts, pillow-breccias and related doleritic sills and dykes. In the silica-rich types, quartz, albite and minor K-feldspar appear as phenocrysts in a spherulitic to microcrystalline groundmass. The basic rocks have inequigranular to seriate textures with albitized calcic plagioclase¹ (An₂₁Ab₇₇Or₂ to An₁Ab₉₉) and clinopyroxene¹ (Wo₃₆En₄₉Fs₁₅ to Wo₃₈En₄₂Fs₂₀) as the main phenocrysts. Titanomagnetite, biotite, hornblende and scarce modal quartz (as microcrysts or intergrown with Na-plagioclase and K-feldspar) are the accessory phases. The field relationships, showing an alternation between acid and basic rocks, suggest that they are coeval.

Both, acid and basic rocks, have been affected by non deformational very low- to low-grade metamorphism, giving a prehnite-pumpellyite-epidote association which has been interpreted by Oyarzún et al. (1997) as indicative of the transition between the pumpellyite-actinolite, the prehnite-actinolite and the greenschist metamorphic facies.

GEOCHEMISTRY

Analytical methods

Thirteen of the less altered rocks (five basic and eight acid rocks) with loss on ignition values (LOI) under 3% were selected (Table 1). Major and some trace element analyses were performed at the Departamento de Geología (J.M. Martínez, analyst), Universidad de Chile using a Perkin-Elmer P-430 inductively coupled plasma atomic emission spectrometer (ICP-AES). The elements Rb, Nb, Pb, Ta, Hf, U, Tb and Tm were also analyzed in eight of the same samples (110, 111, 113, 114, 118, 120, 124 and 142, see Table 1) at the Centro de Instrumentación Científica, Universidad de Granada, Spain using a Perkin-Elmer Sciex Elan-5000 inductively coupled plasma mass-spectrometer (ICP-MS). The results of the Hf, Th (and Nb only in the acid rocks) ICP-MS analyses for these eight rocks were similar to those obtained using the ICP-AES. Sr and Nd isotope data (Table 2) were obtained for the same eight samples (four acid and four basic rocks) at the Centro de Instrumentación Científica, Universidad de Granada, Spain, using a Finnigan MAT 262 thermal ionization mass spectrometer (TIMS) with variable multicollector and RPQ. Normalization value for ⁸⁷Sr/⁸⁶Sr was ⁸⁸Sr/⁸⁶Sr = 8.375209 and the reproducibility under successive determinations of the NBS-987 dissolved standard was better than 0.0007% (2 s). For the Nd determinations, the normalization value for ¹⁴³Nd/¹⁴⁴Nd was ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, with a precision better than 0.0016% (2 s) calculated under successive measures of the WSE power standard. The reproducibility under successive measures of the La Jolla dissolution standard was better than 0.0014% (2 s). Initial isotopic ratios were calculated based on assumed Triassic ages of 220 Ma.

 

Results

The chemical analyses corroborate the bi-modality petrographycally described by Vicente (1976) and Vergara et al. (1991) for these rocks. The basic rocks plot into the trachybasalt-traquiandesitic basalt and andesitic basalt fields, whereas the acid rocks plot into the rhyolite field (Fig. 2).

Both, acid and basic rocks, plot along the alkaline-subalkaline boundary. The scattering of the alkalis observed in the acid rocks may be a consequence of the known mobility of these elements during low-grade metamorphic processes. Consequently, the discrimination between alkaline and subalkaline affinities becomes obscured due to the alteration phenomena. In this respect, in the SiO₂ versus Nb/Y diagram (not shown), the basic rocks plot into the subalkaline basalt and andesite fields, and the acid rocks between those of rhyolite-rhyodacite fields. In this diagram, both types of rocks show similar Nb/Y ratios (mean of 0.20 for the acid rocks and 0.17 for the basic rocks), both indicating a subalkaline affinity. The projection of the clinopyroxene composition into the Leterrier et al. (1982) diagrams also indicates a subalkaline affinity for them, plotting along the tholeiitic and calc-alkaline fields boundary.

The Al₂O₃ values range between 13.83 and 15.52% in the basic rocks and 11.20 to 15.19% in the acid rocks. The MgO values are in the interval 4.17-5.28% in the basic rocks, and between 0.07 and 2.07% in the acid rocks (Table 1). The [mg] values (=Mg/(Mg+Fe²⁺)) are in the range 0.52-0.53 in the basic and 0.05-0.48 in the acid rocks with the highest [mg] values in the silica-rich rocks corresponding to the rhyolites. Dispersion in these [mg] values in rhyolites is a consequence of the mobility of Mg (and Fe²⁺) in this type of rocks during the secondary alteration. CaO in the basic rocks range from 7.30 to 8.25% indicating that, in spite of the albitization of the primary Ca-rich plagioclase, these rocks behaved as a 'closed-system' to the CaO mobility during very-low grade metamorphism. On the contrary, in the acid rocks, the low CaO values (0.04-1.68%) are due to the higher albitic component of the plagioclase phenocrysts. The K₂O contents vary from 0.62 to 1.17% in the basic rocks and from 0.06 to 3.32% in the acid ones. TiO₂ and P₂O₅ are in the ranges 1.65-3.24% and 0.25-0.73%, respectively for the basic rocks and 0.15-0.50% and 0.01-0.13%, respectively, for the acid rocks. The last two oxides show a negative correlation with the SiO₂ content. The Al₂O₃ content of the basic rocks as well as the projection of both, basic and acid rocks, on the FeO/MgO versus SiO₂ diagram (Fig. 3) and the AFM and Jensen (1976, in Rollinson, 1993, not shown) diagrams suggest a tholeiitic rather than a calc-alkaline affinity for these Triassic rocks.

Differences between acid and basic rocks are also reflected in their trace element contents. Major differences between these two types of rocks can be observed in a N-MORB normalized spidergram. In the basic rocks (Fig. 4a) almost all incompatible elements have normalized values between 1 to 10 times the N-MORB and the overall pattern shows an enrichment in K₂O, Rb, Ba and Th (with RbžTh) and a decrease in enrichment from Ta to Y, with a characteristic Ta-Nb trough. Normalized values for Yb and Sc are close to N-MORB, whereas Cr is depleted with respect to it. In the acid rocks (Fig. 4b), the N-MORB normalized spidergrams have, in general, similar patterns characterized by a strong enrichment (> 10 times) in K₂O, Rb, Ba and Th (Rbª Th). With the exception of Sr, P₂O₅, TiO₂ and Sc, the rest of the trace elements are also enriched with respect to the N-MORB (1 to 10 times). The strong negative anomalies shown by P₂O₅ and TiO₂ and the depletion in Sc and Cr (values for this last element plot below the 0.1 line in Fig. 4b) are typical features in these acid rocks which also show a negative Ta-Nb anomaly. The scattering in the K₂O, Rb and Ba values (Fig. 4b) are mainly related to the mobility of these elements during low-grade metamorphism. With the exception of the strong P₂O₅ and TiO₂ anomalies in the acid rocks, the spidergrams of these and those of the basic types tend to be rather similar, with higher values for the acid rocks.

The chondrite-normalized diagrams for the rare earth elements (REE) show similarities and differences between the basic and acid types (Figs. 5a and 5b). The basic rocks (Fig. 5a) have a rather flat pattern, with 30-50 times the chondrite values for the light rare earth elements (LREE) and about 20 times that value for the heavy rare earth elements (HREE). No significant Eu anomaly is present in these basic rocks, which are characterized by average (La/Lu)_N, (La/Sm)_N and (La/Ce)_N ratios of 2.2, 1.2 and 1.0, respectively. The acid rocks (Fig. 5b) have higher total REE values, 60 to 140 times the chondritic values for the LREE and 20 to 45 times for the HREE. A conspicuous negative Eu anomaly is present in these rocks (Eu/Eu* = 0.51) and their average (La/Lu)_N, (La/Sm)_N and (La/Ce)_N ratios are 3.7, 2.3 and 1.1, respectively, showing rather parallel patterns with respect to those of the basic lavas.

Initial isotopic data (Table 2) confirm the existence of a difference between the basic and acid rocks. The (⁸⁷Sr/⁸⁶Sr)_o values for the former are in the range 0.70397-0.70597, whereas in the latter a wider interval (0.70514-0.71161) exists. Concerning the (¹⁴³Nd/¹⁴⁴Nd)_o ratio, smaller differences are found between the two types of rocks, with values ranging from 0.512515-0.512608 in the basic rocks and from 0.512108-0.512526 in the acid types. Initial

⁸⁷Sr/⁸⁶Sr ratios (about 0.7034) have been reported for a basic lava flow from the Pichidangui Formation (Nyström, unpublished information, in Vergara et al., 1995). Among the acid rocks, the highest (⁸⁷Sr/⁸⁶Sr)_o and lowest (¹⁴³Nd/¹⁴⁴Nd)_o values correspond to the rhyolite (sample 110) with the highest SiO₂ content (76.70%) which is the only specimen with a negative value of eNd^t_CHUR (-4.81, Table 2).

PETROGENESIS

The relatively moderate alteration of the rocks selected for chemical analysis (reflected in the restricted dispersion of the immobile trace elements) means that they are close representatives of their original chemical compositions. Bimodal magmatism, without significant volumes of magma with an intermediate composition, is common in many anorogenic continental extensional settings (e.g., Doe et al., 1982; Garland et al., 1995) and in continental margins of back-arc extensional basin environments (McCulloch et al., 1994; Gibbons and Young, 1999). In the Upper Triassic-Lower Jurassic plutonic complexes of the Coastal Range in central Chile, a bimodal magmatism also exists with the presence of related leucogranites and gabbros. A crustal component has been proposed for the genesis of the leucogranites (Gana, 1991; Parada et al., 1991, 1999), whereas, for the genesis of the Mesozoic gabbros, an asthenospheric dominated source is invoked (Parada et al., 1999). According to Huppert and Sparks (1988), crustal melting by intrusion of basaltic magmas could permit the generation of silicic magmas and, consequently, the development of a bimodal suite like the one which characterizes the Triassic volcanism in the Coastal Range of central Chile. In order to explain the petrogenesis of this bimodal magmatism, two main aspects must be investigated: one concerns the nature of the basic magma source and the other refers to the genesis of the related acid rocks and the probable crustal participation in their composition. Three different questions can be proposed: 1- are both basic and acid contaminated magmas derived from different mantle source?; 2- are both cogenetic magmas only related by fractionation?; and 3- are both cogenetic magmas with different crustal contamination degrees?

A major restriction in the characterization of the basic magma source in this case is the absence of primitive mantle-derived liquids as indicated by the low [mg], Cr and Ni values in the analyzed basic rocks, which suggest olivine (and/or clinopyroxene?) fractionation. Nevertheless, trace element ratios, together with isotopic values, can help to obtain some information regarding the genesis of this basic magma.

Concerning the variations of trace elements and specially the REE (Figs. 4 and 5), the almost parallel patterns of acid and basic rocks could be interpreted as due to a genetic link between these two rock types. In fact, the acid rocks could derive from the basic ones by fractionation of clinopyroxene, Ca-rich plagioclase, Ti-magnetite and apatite as suggested by the depletion in Sc, V, Cr (elements compatible with clinopyroxene), TiO₂ and P₂O₅ and the strong negative Eu anomaly shown by the acid rocks. In fact, simple mass balance calculations point to a probable evolution from the basaltic melts to the acid liquids by a fractionation of 30% clinopyroxene, 20% plagioclase, 10% Ti-magnetite and ª 1% apatite. However, the gap and differences which exist in the isotopic initial ratios (Table 2) between acid and basic rocks must be interpreted on the light of another petrogenetic process such as crustal contamination.

Geochemical evidence of crustal contamination in the genesis of these Triassic rocks is the enrichment in elements from the crust such as K, Ba, Rb, Th and LREE and the decrease in Ta and Nb. The trace element patterns normalized to N-MORB show enrichment in K, Rb, Ba and Th and a slight negative Ta anomaly, which is more marked in the acid rocks (Figs. 4a and b). Crustal components are, therefore, more evident in the acid rocks. Ratios of trace elements with similar incompatibilities are different for the basic and acid rock types (e.g., mean values in basic versus acid rocks of La/Nb = 1.95 versus 2.91; Zr/Nb = 26.9 versus 40.0; Ti/V = 47.0 versus 141.8; La/Yb = 3.3 versus 5.4; Zr/Yb = 46.3 versus 75.7). Moreover, the characteristically low TiO₂ and P₂O₅ contents in the rhyolites with respect to N-MORB (Fig. 4b) can also point to a crustal component in their genesis considering the low values of these two elements in the upper crustal rocks (e.g., Wedepohl, 1995).

In the geotectonic discrimination diagram by Cabanis and Lecolle (1989), the basic rocks plot in the intracontinental and post-orogenic domain corresponding to the field of basalts generated in a back-arc basin context (Fig. 6a). Projection of the chemistry of these same basic rocks in other geotectonic discriminant diagrams (e.g., the Zr/4:2Nb:Y diagram of Meschede, 1986, Ti/100:Zr:Y*3, Pearce and Cann, 1973, not shown here) confirm the within plate extensional character of these rocks. In the Nb versus Y diagram by Pearce et al. (1984), the acid rocks plot mainly in the within-plate field close to the triple joint (Fig 6b), whereas according to the Eby (1992) diagrams for granitic rocks, these rocks plot in the A₂-type, which correspond to acid rocks generated in the region of continental margin and island-arc basalts. The projection of both types of rocks in the same 'within-plate extensional field' setting is consistent with the presence of a geodynamic extensional regime in the Coastal Range of central Chile (Charrier, 1979) during the genesis of these Triassic rocks.

With respect to the source of this magmatism, values of the ratio La/Nb > 1.5 in basaltic rocks have been interpreted as indicative of a lithospheric component in the genesis of these magmas (Thompson and Morrison, 1988). In this sense, the enrichment in high field strength elements (Nb to Yb in Fig. 4a) with respect to the N-MORB in the basic rocks is indicative of a mantle source more enriched than N-MORB. Also, the marked Ta-Nb trough (Fig. 4a, b) is indicative of a crustal (subduction-related?) component in their genesis. The rocks from the Pichidangui Formation have been also represented in the Ta/Yb versus Th/Yb diagram (Fig. 7). In this diagram, normalization of Th and Ta with respect to Yb minimizes the effect of partial melting as well as effects related to the fractional crystallization (Pearce, 1982). Due to the enrichment of Th with respect to Ta in the continental crust, processes related to crustal contamination would be represented by compositional trends along the C vector and those related with a subduction component along the S vector in figure 7. In this case, both types of rocks plot out of the within-plate (W) or fractional (f) trends proposed by Pearce (1982, 1983) and are mainly aligned following the crustal-subduction C-S vectors. The authors interpret this trend as a consequence of an increase in the Th content in the basic and acid rocks, probably due to crustal contamination from an enriched mantle source. Crustal contamination would also explain the fact that both, basic and acid rocks, plot in the calc-alkaline field in the 'active continental margins' of figure 7.

FIG. 6. Tectonic discrimination diagrams for the volcanic rocks of the Triassic Pichidangui Formation, Coastal Range of central Chile. a- basic rocks plot in the Cabanis and Lecolle (1989) diagram; 1- orogenic domains; 2-intracontinental and post-orogenic domains; 3- anorogenic domains. CA- calc-alkaline basalts; VAT- volcanic arc tholeiites; CB- continental tholeiites; BAB- basalts generated in fore- and back-arc basins; E-MORB- enriched mid-ocean ridge basalts; N-MORB- normal mid-ocean ridge basalts. Symbols as in figure 2. b- Nb versus Y discrimination diagram for acid rocks (after Pearce et al., 1984). WPG-within plate granites; ORG- orogenic granites; VAG- volcanic arc granites; syn-COLG- syn-collisional granites. The broken line is the field boundary for ORG from anomalous ridges. Symbols as in figure 2.

The Sr-Nd isotopic data represented in figure 8 show the basic rocks plotting close or into the mantle array and near to the prevalent mantle reservoir component (PREMA) end member. This mantle reservoir, defined by the values ¹⁴³Nd/¹⁴⁴Nd = 0.5130 and ⁸⁷Sr/⁸⁶Sr = 0.7033, was proposed by Zindler and Hart (1986) and represents the most common isotopic composition of basalts from oceanic islands, intra-oceanic island arcs and continental basalt suites. E-type MORB (as representative of a enriched mantle source) has isotopic values of ¹⁴³Nd/¹⁴⁴Nd = 0.51299-0.51300 and ⁸⁷Sr/⁸⁶Sr = 0.70280-0.70334 (Rollinson, 1993), plotting close to the PREMA end member in figure 8. In the same figure, the acid rocks from the Pichidangui Formation show a high (⁸⁷Sr/⁸⁶Sr)_o dispersion, possibly related to the mobility of Sr during very-low grade metamorphism, limiting the applicability of their isotopic petrogenetic implications only to the less altered rocks.

On the light of the isotopic data, positive eNd and Sr isotope ratios around 0.704 for the basic rocks can be interpreted as the consequence of an enriched (lithospheric dominated) component in their source. In this sense, the representation of the Lower Jurassic bimodal plutonism from the Coastal Range of central Chile plotted in figure 8 reveals the isotopic similarities between the gabbros of the Limarí Complex (Rb/Sr radiometric ages from 220 to 200 Ma, in Gana, 1991) and the basic rocks of the Pichidangui Formation, both with a eNd value ca. 4. Also, both Jurassic gabbroic rocks and the basic lavas of the Pichidangui Formation, have similar model Nd ages (ca. 1 Ga, Table 2), which would point out to a similar (or common?) petrogenetic setting. In accordance with the origin proposed by Parada et al. (1999) for the Lower Jurassic gabbros, an enriched source component is necessary to explain the isotopic signature of the basic Triassic volcanic rocks of the Pichidangui Formation.

The acid volcanic rocks need a more Sr radiogenic and a lower eNd component for their genesis. Their trace element characteristics point to the influence of a crustal component at their origin. A comparison with the coeval plutonic rocks of the Coastal Range of central Chile may help to find genetic relationships with the Si-rich Triassic lavas. The composition of the Lower Jurassic granites of the Coastal Range of central Chile represented in figure 8 suggests a crustal source or a major participation of a crustal component in their genesis. The projection of the more silica-rich granitoids from the Coastal Range Carboniferous Santo Domingo Complex (Parada et al., 1999) allows to estimate the crustal component in the genesis of the Triassic bimodal volcanism of the Pichidangui Formation. A mixing curve is plotted in figure 8 applying the mixing equation in Faure (1986):

 

where X_A , X_B are the concentrations of the element X and eA^x and eB^x are the relevant isotope parameters of element X in components A and B for values of f ranging from f = 1 (pure A) to f = 0 (pure B). The PREMA reservoir is taken as the mantle source. For the contaminant, two end members are considered: the Carboniferous granitoids of the Coastal Range (path A) and a representative value of the upper continental crust (path B). In both model paths, the crustal component in the isotopic signature of the basic rocks is around 10%. A higher crustal component must be necessary to obtain the isotopic signature of the acid rocks. The large mobility of Sr during low-grade metamorphic processes must be also responsible for the relatively high dispersion in the Sr isotopic data which complicates the determination of the amount of crustal component.

With the aim to minimize the influence of Sr mobility, a similar mixing curve has been designed using the Th content (Fig. 9). In this case, the equation for the mixing curve is a simple mass balance:

Cⁱ_M = Cⁱ_Af + Cⁱ_B(1-f)

in which Cⁱ_M is the concentration of the element i in the mixing between the components A and B for values of f ranging from f = 1 (pure A) to f = 0 (pure B). As in figure 8, mixing curves were constructed using the Carboniferous granitoids (path A) and the upper continental crust (path B) as contaminants, whereas the E-type MORB was chosen as an enriched mantle reservoir. In this case, similar conclusions can be obtained with respect to figure 8. A low crustal component is observed in the genesis of the basic Triassic lavas (as well as for the Lower Jurassic gabbros of the Limarí Complex) whereas a higher crustal component (around 20%) is needed to generate the acid rocks. Sample 110, with eNd^t_CHUR = -4.81 plots out of the mixing curves between the mantle reservoir and the Carboniferous granitoids. This rhyolite is the most acid of the studied rocks and its origin must be related to a maximum of crustal melting, as can be interpreted on the light of its isotopic and trace element geochemistry.

CONCLUSIONS

According to the new trace element and isotopic data presented, the Triassic volcanism of the Pichidangui Formation is characterized by a tholeiitic bimodal suite. The basic magmas, with La/Nb > 1.5, (⁸⁷Sr/⁸⁶Sr)_o in the range 0.70396-0.70597, and eNd^t_CHUR ³4, would be related to an enriched lithospheric dominated source. A weak contamination by a crustal component is apparent in the genesis of these basic lavas. The acid rocks show geochemical evidences of a larger crustal component at their origin (Figs. 4b, 7, 8 and 9). The projection of different elements characterizing the geochemistry of this Triassic magmatism into various discriminant tectonomagmatic diagrams, indicates a continental extensional setting for the genesis of the magmatism, corroborating the extensional geotectonic model proposed by Charrier (1979) and Mpodozis and Kay (1992) for the Triassic pacific Gondwana margin.

Isotopic geochemical similarities between the basic rocks of the Pichidangui Formation and the geographically close Lower Jurassic (220 to 200 Ma, in Gana, 1991) gabbros of the Limarí Complex (Figs. 8 and 9) have been established here. Their similar (⁸⁷Sr/⁸⁶Sr)_o, model t_DM ages and eNd^t_CHUR values, could be interpreted in terms of a common enriched mantle source for both types of basic rocks. The following, simplified petrogenetic model (Fig. 10) is proposed to explain this magmatism. During Late Triassic to Early Jurassic times, an extensional geotectonic setting developed in the pacific Gondwana margin, accompanied by an upwelling of enriched mantle material. Melting of this enriched (lithospheric dominated) mantle would generate an enriched basaltic liquid which was emplaced in a shallow level magma chamber. Contamination by the continental crust (dominated by Carboniferous granitoids) would occur in this shallow magma chamber, with higher assimilation degrees of crustal material, derived from partial melting of the roof rocks, in the upper zone. According to this model two liquids, with different viscosity and slightly different initial isotopic signatures, would be generated, resulting in a less dense, chemically isolated upper level, and a higher density, basaltic liquid in the lower level of this shallow magma chamber. These two levels of the chamber would evolve in a chemically independent way. Olivine fractionation took place in the lower level, originating the evolved basaltic liquids of the Pichidangui Formation, whereas a ca. 67% closed fractionation occurred in the upper level giving rise to the rhyolitic liquids, with slightly contrasted isotopic signatures

with respect to those of the lower basaltic level. The final emplacement of these two differentiated liquids would have originated the bimodal magmatic pattern of the Pichidangui Formation. A similar two-stage petrogenetic model has been proposed by McCulloch et al. (1994) for the genesis of the bimodal (basalts and rhyolites) back-arc basin volcanism in the Taupo Volcanic Zone of New Zealand. The petrogenetic model proposed here explains why the silicic rocks, with isotopic compositions not significantly different from those of the associated basalts, cannot be interpreted as the result of pure melting of the crustal basement (e.g., Carboniferous granitoids). On the other hand, deep crystallization in a different (?) basaltic magma chamber would have generated the Lower Jurassic (220-200 Ma) gabbros of the Limarí Complex which, according to the model proposed, would be cogenetic with the bimodal magmatism of the Triassic Pichidangui Formation.

ACKNOWLEDGEMENTS

This work has been supported by the FONDECYT Project 1961108.The authors thank Dr. F. Bea (Universidad de Granada, Spain) for the analytical facilities with the ICP-MS and TIMS analyses. The authors thank P.T Leat (British Antarctic Survey), B. Levi (Stockholm University) and M.A. Parada (Universidad de Chile) for their comments and suggestions which considerably improved the original manuscript. This paper is a contribution to the IGCP Project 436 'Gondwana Pacific Margin'.

¹ Electron microprobe analysis (CAMECA SX-50, 20n A, 20 Kv, 3 µ, Universidad de Granada, España).

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Manuscript received: November 12, 1999; accepted: May 16, 2000.