Viramonte Et Al., 1999. Cretaceous Rift Related Magmatism in Central-western South America

Cretaceous rift related magmatism in central-western SouthAmerica

J.G. Viramontea,*, S.M. Kayb, R. Becchioa, M. Escayolaa, I. Novitski c

aUniversidad Nacional de Salta, GEONORTE and CONICET, Buenos Aires, 177, 4400, Salta, ArgentinabCornell University, Dept. Earth Sciences, Snee Hall, Ithaca, New York, NY, USA

cInstitute of the Lithosphere, Staromonetney per, 22, Moscow, l 109180, Russia

Abstract

The Cretaceous±Paleocene Andean basin system of central-western South America, comprises northwestern Argentina andsouthwestern Bolivia. It is situated between 628±688W and 188±278S, but extends westward to northern Chile and northward toBolivia and Peru. These basins have been interpreted as an aborted foreland rift. In a general sense, it may be possible to relate

this rift to the opening of the South Atlantic Ocean, however it was directly associated, in a backarc position, with thesubduction of the Nazca Plate below the South American Plate. Three main magmatic episodes were recognized: the pre-riftstage (130±120 Ma) which is characterized by an early phase of anorogenic plutonism, with subalkaline and alkaline graniticintrusives; the syn-rift volcanic episode which started with a mainly alkaline volcanic activity (110±100) in which alkaline rocks

prevail; a second more voluminous volcanic episode (80±75 Ma) which is characterized by an alkaline suite represented bybasanites and tephriphonolites; and the last volcanic episode (65±60 Ma) which consists of lamproitic sills and basic K rich lava¯ows. Petrography, chemistry and chronology of the Cretaceous plutonic bodies indicate anorogenic pre-rift related A-type

granite complexes closely related to the further evolution of the Cretaceous rift basin. The petrology and geochemistry of theCretaceous volcanic rocks show strong alkaline a�nities and suggest a similar rift-related origin. The geochemical and isotopiccharacteristics of the alkaline basalts suggest that they originated through low degrees of partial melting of a depleted mantle

subcontinental lithosphere which was previously enriched by processes such as the introduction of veins rich in amphibole, highTi phlogopite, and apatite. Cretaceous plutonic and volcanic rocks from central-southwestern South America are related to anintracontinental rift environment and although their ages are correlative with those of the ParanaÂ volcanic province, their

petrology, geochemistry and isotopic compositions reveal di�erent source regions and petrogenetic processes. # 1999 ElsevierScience Ltd. All rights reserved.

Resumen

El Sistema de cuencas cretaÂ cicas±paleocenas de la zona centro-oeste de SudameÂ rica comprende el noroeste de Argentina y lascuencas andinas del sur de Bolivia. Este sistema estaÂ situado entre los 628 y 688 W y los 288 y 278S y se extiende hacia el norte

hasta Bolivia y PeruÂ y hacia el oeste hasta las cuencas cretaÂ cicas del norte de Chile. Estas cuencas han sido interpretadas comoun rift de trasarco abortado. En sentido general es posible relacionar el reÂ gimen extensional de rift a la apertura del oceÂ anoAtlaÂ ntico Sur, sin embargo se asocia su evolucioÂ n a una extensioÂ n de trasarco debido a la subduccioÂ n de la Placa de Nazca en el

maÂ rgen PacõÂ ®co de SudameÂ rica. Fueron reconocidos tres episodios magmaÂ ticos diferentes relacionados a distintas etapas deevolucioÂ n del rift. El estadio de prerift (130±120 Ma) caracterizado por una fase de plutonismo anorogeÂ nico de composicioÂ ngranõÂ tica alcalina a subalcalina. El estadio sin-rift comenzoÂ con actividad volcaÂ nica alcalina entre los 110 y 100 Ma. y siguioÂ conun segundo pulso volcaÂ nico (80±75 Ma) que estaÂ representado por una suite alcalina de basanitas y tefrifonolitas. La uÂ ltima fase

volcaÂ nica (65±60 Ma) consiste de sills lamproõÂ ticos y ¯ujos de lavas baÂ sicas ricas en potaÂ sio. Las caracterõÂ sticas geoquõÂmicas e

Journal of South American Earth Sciences 12 (1999) 109±121

0895-9811/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0895-9811(99 )00009 -7

* Corresponding author.

E-mail address: [email protected] (J.G. Viramonte)

isotoÂ picas de los basaltos alcalinos sugieren que los mismos fueron originados por bajos grados de fusioÂ n parcial de un manto

litosfeÂ rico empobrecido, enriquecido en LREE por diferentes procesos entre ellos el metsomatismo producido por la intrusioÂ n devenas ricas en anfõÂ bol, ¯ogopita y apatita. Si bien las rocas plutoÂ nicas y volcaÂ nicas cretaÂ cicas del centro-sudoeste de SudameÂ ricaestaÂ n relacionadas a un ambiente geoteÂ ctoÂ nico de rift de antepaõÂ s y sus edades son coincidentes con las efusiones de la Provincia

volcaÂ nica de ParanaÂ , su geoquõÂmica sugiere una diferente fuente de generacioÂ n de los magmas y distintos procesospetrogeneÂ ticos. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

1.1. General geological setting

The Cretaceous±Paleocene Andean basin system ofcentral-southwestern South America comprises thenorthwestern Argentina basins and the Andean basinsof central-south Bolivia. It is situated between 628±688W and 188±278 S. It extends northward to thenorth of Bolivia and PeruÂ . The western boundary isconnected to the Cretaceous basin of northern Chile

(Antofagasta or Purilactis basin) (Reyes and Sal®ty,1973; Schwab, 1984; Marquillas and Sal®ty, 1988).

The Cretaceous basins of central-western SouthAmerica were interpreted as an aborted foreland rift(Galliski and Viramonte, 1988) consisting of intercon-nected grabens meeting at a triple junction and a cen-tral horst (Fig. 1). This phenomenon is related, in ageneral sense, to the opening of the South AtlanticOcean. This rift, however, was directly associated, in atransarc position, to the subduction of the Nazca Platebelow the South American Plate.

Fig. 1. (a) Paleostructural sketchmap of the Cretaceous rift with outcrops of Cretaceous volcanic and plutonic rocks; (b) stratigraphic columns of

the PotosõÂ basin; (c) stratigraphic column of the AlemanõÂ a sub-basin. Modi®ed from Sal®ty and Marquillas (1981).

J.G. Viramonte et al. / Journal of South American Earth Sciences 12 (1999) 109±121110

The Cretaceous rift-related structures are well pre-served in some cases (Bianucci and Homovc, 1982) inother cases they have been obliterated by later Andeanstructures (Cahill, 1990; Bianucci et al., 1993).

The sedimentary in®ll is situated to the south of theSalta±Jujuy horst on the Pampean craton, whereas tothe north the pre-Cretaceous basement is of Paleozoicage. The sedimentary in®ll and associated igneousrocks of the basin, known as the Salta Group inArgentina and Puca Group in Bolivia, exceed 6 km inthickness.

The stratigraphic sequence of the Salta Group iswell known in northwestern Argentina and comprisesthree units: the Pirgua subgroup (La Yesera, LasCurtiembres and Los Blanquitos Formations),Balbuena subgroup (Lecho, Yacoraite and OlmedoFormations) and Santa BaÂ rbara subgroup (Mealla,Maiz Gordo and Lumbrera Formations) (Moreno,1970; Reyes and Sal®ty, 1973; Sal®ty and Marquillas,1981; Sal®ty, 1982; Gomez Omil et al., 1989) (Fig. 1).

The sedimentary rocks characterizing each subgroupare: redbeds (Pirgua), sandstones, limestones, evapor-

ites and claystones (Balbuena), mudstones and evapor-ites (Santa BaÂ rbara). The lithostratigraphic sequenceof the Puca Group (Fig. 1) in the PotosõÂ basin inBolivia, comprises four megasequences with a range ofsubgroups (Sempere et al., 1988): P1 subgroup(Condo, Kosmina, Sucre and the La PuertaFormations); P2 subgroup (Tarapaya and Mira¯oresFormations) mainly marine limestone; P3 subgroup(Aroi®lla, Chaunaca, Coroma and Toro ToroFormations); and P4 subgroup (El Molino limestones,Santa Lucia and Impora Formations) (Russo andRodrigo, 1965; Reyes and Sal®ty, 1973; Cherroni,1977).

The paleogeographic development of the Salta andPuca Groups and their relationship with theCretaceous basins of central South America have beenstudied by Marquillas and Sal®ty (1988) and Sal®tyand Zambrano (1989). The southern outcrops of theSalta Group and the development of the basin duringthe syn and post-rift stages was studied by Bossi andPeral (1992).

The Pirgua subgroup (Argentina) and P1, P2 and P3

Fig. 1 (continued)

J.G. Viramonte et al. / Journal of South American Earth Sciences 12 (1999) 109±121 111

subgroups (Bolivia) are typical continental red bedsin®lling the intracontinental rift, with minor marinetransgressive? episodes (Mira¯ores and LowerChaunaca Formations) in Bolivia. These sequencesrepresent the maximum subsidence of the basin duringthe syn-rift stage development.

The Balbuena and P4 subgroups are the mostcharacteristic lithostratigraphic units of theCretaceous±Paleocene basins in central-western SouthAmerica and represent the culmination of the syn-riftsequences. Because of their white color, relief, andwide distribution, the Yacoraite Formation inArgentina and El Molino Formation in Bolivia, rep-resent a ``marker bed'' along the basins for the Saltaand Puca Groups sediments. The transition fromMaastrichtian to Paleocene lies within these strata butits stratigraphic level remains to be determined. In ouropinion Yacoraite Formation is a product of lime pre-cipitation in a series of interconnected shallow saltlakes.

On the other hand, the Santa BaÂ rbara Subgroupand Santa Lucia Formation represent the culminationof the Cretaceous±Eocene post-rift sequences in cen-tral-southwestern South America when the thermotec-tonic collapse of the rift structures began.

1.2. Development of the rift and related igneous activity

The subdivision of the northwestern Argentina andcentral-south Bolivia Cretaceous magmatism into threemajor phases was previously recognized by Galliskiand Viramonte (1988), Avila Salinas (1986), Sal®tyand Marquillas (1981), and Omarini et al. (1989).These igneous phases were associated with the develop-ment of the rift by Galliski and Viramonte (1988): theearliest phase (130±100 Ma) is characterized by theintrusion of anorogenic alkaline and subalkaline pluto-nic bodies (Rangel, Hornillos, Abra Laite, Aguilar,Tusaquillas, Fundiciones in Argentina and RejaraÂ inBolivia, Fig. 1). These may have been emplaced duringthe pre-rifting stage of epirogenic doming and theextrusion of the ®rst lava ¯ows (Alto de las Salinas-ElCadillal-Isonza in Argentina and Maragua in Bolivia)predominantly within the borders of the grabens at thebeginning of the syn-rift stage. This ®rst stage can berelated to the initial doming produced by the rise ofhot asthenospheric masses with associated crustal thin-ning.

The second phase (80±75 Ma) includes basanitesand mugearites of the Las Conchas Basalt (Argentina)and the Basalto Betanzos, Ravelo and Tarabuco(Bolivia). It represents the syn-rift stage, withmagmatic products located in the center of the struc-tures.

The last igneous phase (65±60 Ma) comprises lam-proitic sills intruded into Los Blanquitos Formation

(Omarini et al., 1989), the basanites interbedded in theBalbuena and Santa BaÂ rbara subgroups in Argentina(Schlagintweit, 1937; Lyons, 1951; Bercowski, 1982;MaÈ del, 1984; Rubiolo, 1992), the basanites and pho-nothephrites from Tupiza (Vargas Gil, 1965; Galliskiand Viramonte, 1988) and lava ¯ows fromMandeyapecuaÂ , TacuarendõÂ , and Abaroa (CastanÄ osand Rodrigo, 1978) in Bolivia. It marks the start ofthe thermal subsidence and collapse of the basin inthe post-rift stage, just before the Cretaceous±Paleocene rift basins were closed by a dramatic changein the geodynamic style of the region caused by theIncaic compressive diastrophic phase in the UpperPaleocene.

Galliski and Viramonte (1988), proposed that theforeland rift was developed by a viscous drag of themantle which caused a foreland extension. Thisphenomenon was related to three changes in the geo-metry of the subduction in the Paci®c margin duringthree periods of low spreading rates in the Paci®cridge. Each is preceded by a stage of faster spreadingassociated with compressive tectonic phases (PiraceÂ s,1979; Frutos, 1981).

Fig. 2. Normative QAPF classi®cation of alkaline and subalkaline

Cretaceous plutonic rocks from northwestern Argentina and south

Bolivia.


2. The magmatism

2.1. Cretaceous plutonism

Some small alkaline and subalkaline complexes cropout in the Eastern Andes from Southern Peru tonorthwestern Argentina. They comprise the complexesof Macusani in PeruÂ , and Ayopaya, Velazco,Candelaria, Khoallaqui, Vichacla, Tacsara, and RejaraÂin Bolivia. Outcrops of Cretaceous granitoid rocks arerestricted to the northern sections of the Salta Groupbasins (Fig. 1). The alkaline plutonic complexes ofPeruÂ and Bolivia except for the RejaraÂ magmatic com-plex (Avila Salinas, 1986) are not considered in thisstudy.

The northwestern Argentina complexes are locatedon both sides of the Tres Cruces sub-basin. On thewestern side, they include the Rangel, Tusaquillas,Abra Laite, Aguilar, and Fundiciones plutons, all ofwhich intrude slates, graywackes and minor quartzitesof Cambrian±Ordovician age. These bodies developeda limited contact aureole. At Sierra de Santa Victoria(the eastern side of the branch) the Hornillos, RejaraÂand Maiguasi intrusive granitoid and lamprophyrecomplexes intrude Cambrian quartzites, upperProterozoic greenschist facies rocks and Ordovicianmarine sequences. The rocks are alkaline and subalka-line granitoid plutons and ma®c to ultrama®c dikes(Rubiolo et al., 1994).

2.1.1. Alkaline plutonismAlkaline plutons include the Rangel, Fundiciones,

Hornillos, RejaraÂ and Maiguasi complexes. The alka-line granite of the Rangel pluton is situated in theCobres range and consists of a partially exposed com-plex of about 30 km2 described by Toselli and Rosside Toselli (1977) and recently by Menegatti et al.(1997). By using Rb/Sr whole rock methods theserocks were dated at 12326 and 12928 Ma (Halpernand Latorre, 1973). New data on biotite give ages of14621.6 Ma and 12221.5 Ma (Menegatti et al.,1997). The major rocks are alkali feldspar granite,alkali feldspar quartz syenite and syenite cut by dikesof similar composition. They plot in the granite, alkali-feldspar granite and syenite ®eld (Fig. 2).

The granites are medium- to ®ne-grained rocks. TheK-feldspars are microperthitic microcline or ortho-clase. According to Toselli and Rossi de Toselli (1977),the plagioclase is albite (An 10). The ma®c mineralsare riebeckitic amphibole and annite-rich biotite. Theserocks have undergone secondary alteration includingalbitization with aegirine development, as well as thor-ite or pyrochlore mineralization. The alkali feldsparquartz syenite has microperthitic K-feldspar, augitesurrounded by hastingsite, and late biotite. The di�er-entiated dikes comprise rare nepheline syenite with

aegirine and arfvedsonite, aegirine granite, porphyriticsyenite, and some lamprophyres.

This intrusive complex is crosscut by genetically re-lated ¯uorite-thorite-bearing veins and hydrothermalcarbonate dykes (Santomero, 1958; Lurgo andZappettini, 1989). It is surrounded by thorite-alkalifeldspar-quartz veins and dykes (Sureda et al., 1987).The chemistry of major elements demonstrates that thesuite is alkaline with both non-peralkaline and peralka-line granitic facies.

Fundiciones is a small plutonic body located nearthe Quebrada de Humahuaca (Fig. 1). The pluton isintruded into Eopaleozoic host rocks. There are nopublished geochemical or geochronological data, butpetrographic studies suggest alkaline a�nities consist-ent with the Cretaceous alkaline magmatism observedin other outcrops of the basin.

The Hornillos complex is a small (nearly 6 km2)high-level, almost sub-volcanic, stock surrounded bynumerous, mainly north±south aligned dikes (Rubiolo,1992). The rocks of this complex do not have pub-lished ages, but genetically related mineralizations haveU/Pb ages of 6525 Ma and 169210 Ma (Sureda etal., 1987; Linares, 1961). The principal rock types arenordmarkites, syenites, monzonites, monzodiorites,pulaskites and trachytes (Rubiolo, 1992) (Fig. 2). Twodi�erent types of rocks are found: one with silicaundersaturated (foyaitic trend) and another with silicaoversaturated character (nordmarkitic range). In thelocality of Maiguasi (Iruya) a few kilometers to thesouth from Hornillos Complex, several sannaite andtinguaite dykes cut the upper Precambrian to lowerCambrian sediments (Rubiolo et al., 1994).

The RejaraÂ complex is located on the southern partof the Tarija Province in Bolivia. The rocks are predo-minantly granodiorites and biotite+hornblende+ae-girine+riebeckite-bearing granites, with some fenitizedzones (Avila Salinas, 1986; Rubiolo, 1992). The age of

Fig. 3. SiO2 vs Na2O+K2O diagram after Irvine and Baragar (1971)

for plutonic Cretaceous rocks.


the RejaraÂ complex is actually under discussion. A K/Ar age of 141 Ma was reported by Linares (inCastanÄ os and Rodrigo, 1978). These rocks were con-sidered Cretaceous by Avila Salinas (1986), althoughTroeng et al. (1993) consider an Upper Proterozoicage for them. Rubiolo (1992) indicates a Vendian-lower Cambrian age for the intrusion and aCretaceous age for the fenitization processes related tothe stock.

2.1.2. Sub-alkaline plutonismSubalkaline plutons include the Tusaquillas, Abra

Laite and Aguilar plutons and some rocks fromRangel. The Tusaquillas intrusive body is located tothe northeast of the Rangel complex and may be apart of it (Fig. 1). It is the largest Cretaceous pluton,about 24 km wide, with its principal axis orientednorth±south. K/Ar isotopic ages range between147210 and 9625 Ma (Turner et al., 1979).Petrographically, the rocks of this pluton consist ofsubalkaline biotite and muscovite granites cut by apli-tic dykes and ¯uorite veins. The Tusaquillas plutonalso includes diorites, monzodiorites and minor alka-line granite facies (Galliski and Viramonte, 1988). Thintephrite dykes crosscut the plutonic body.

The Abra Laite stock is situated northeast ofTusaquillas granite. There are large discrepancies in re-lation to its geochronological data, with K/Ar ages of200210 Ma and 8025 Ma (Turner et al., 1979). Thepetrographic features are similar to those of theTusaquillas, with subalkaline biotite-muscovite faciesand ¯uorite plus quartz-tourmaline veins.

The Aguilar pluton is an epizonal intrusive of40 km2 located immediately to the northeast of AbraLaite (Brodtkorb et al., 1978). Petrographically, itsrocks consist of hornblende biotite granodiorite orgranite facies with restricted minor metasomatizedrapakivi granites, monzonites, diorites and alkalicgranites. It is cross-cut by lamprophyric and apliticdikes that carry ¯uorite and tourmaline. Rb/Sr agesrange between 11025 and 13328 Ma (Halpern andLatorre, 1973). On the basis of K/Ar and Rb/Sr dat-ing, Linares and Latorre (1975) assigned a main age of118215 Ma for these rocks. Chemical analysis of theprincipal rock types show that the pluton has a sub-alkaline a�nity (Fig. 3).

2.2. Cretaceous volcanism

Cretaceous volcanism is widespread in the Salta andPuca Groups (see Reyes et al., 1976; Avila Salinas,1986; Galliski and Viramonte, 1988). The best out-crops are found on the southwestern and eastern sidesof the AlemanõÂ a sub-basin in Argentina and theBetanzos area in the Bolivian basin (Fig. 1). Theybelong mainly to the second volcanic phase. We

speci®cally refer to them because we consider them tobe the best representatives of Cretaceous rift-relatedmagmatism in the region.

In northwestern Argentina, the volcanic sequencebegins with the Alto de las Salinas Complex, ElCadillal Formation and the Isonza Basalt (Fig. 1). Theformer was described by Bossi (1969) and is composedof lava ¯ows, volcanic necks, dykes and minor pyro-clastic ¯ows of trachytic, and nephelinite compositionintruded by basanitic dykes and sills (Galliski andViramonte, 1988). Numerous xenoliths of spinel peri-dotites and partially digested crustal rocks can befound in the basanitic lava ¯ows.

Whole rock K/Ar dating of the felsic rocks gaveages of 12825 and 112210 Ma for the lower levelsand 103210 to 9725 Ma for the upper levels (Bossiand Wampler, 1969).

The Isonza Basalt is located on the northwesternborder of the AlemanõÂ a sub-basin and is interbededwith the lower sequences of the La Yesera Formation(Fig. 1). K/Ar dating gave an age of 11425 Ma forthe lower lava ¯ow and 9925 to 9625 Ma for theupper lava ¯ow (Valencio et al., 1976). Both lava ¯owsare basanitic in composition with olivine and augite.In Bolivia, the volcanic sequence begins with theBasalto Maragua which is interbedded in La PuertaFormation (Avila Salinas, 1986).

The second volcanic pulse is mainly represented byLas Conchas Basalt in Argentina and BasaltoBetanzos in Bolivia. The former comprises the volcan-ism associated with the upper levels of the LasCurtiembres Formation. The best outcrops of the unitare widely distributed on both sides of the LasConchas River. K/Ar ages of 7825 and 76.423.5Ma are considered for this unit (Valencio et al., 1976;Reyes et al., 1976).

The lithology of the sequence is indicative of astrombolian volcanism with frequent hydromagmaticpulses. The products of the volcanism are lava ¯owsinterbedded with wet surge deposits, lahars, explosionbreccias, air fall strombolian deposits, and minor ¯u-vial sedimentary beds.

In the lower part of the Las Curtiembres Formationit is possible to see the roots of this volcanism, withdykes and volcanic necks. This hypabyssal volcanismis present in the central-western sector of the LasConchas River (Santa BaÂ rbara and El Sapo area).

In several places (Las Curtiembres, El Sunchal, ``km49'', El Sapo, Cerro Amarillo) the rocks contain, par-ticularly the necks and dykes, abundant xenoliths.Both mantle and crustal-derived xenoliths have beenidenti®ed (Galliski et al., 1990; Risso and Viramonte,1992; Lewerenz, 1977) including spinel lherzolithes,websterites, wehrlites, harzburgites, dunites, ma®c andacid granulites, granitoids, gneisses, schists, and quart-zites. Likewise, megacrysts (olivine, pyroxene, feldspar)


are common in the basanites. They possibly comefrom broken peridotitic xenoliths, pyroxenitic cumu-lates and granitoids, with reaction rims from the hostbasanites in some samples.

Basalto Betanzos (Ravelo, Tarabuco, etc.) crops outmainly in the Chuquisaca Valley interbedded with sedi-ments of the middle levels of the Puca Group. K/Arages are 83 and 82.5 Ma (Evernden et al., 1977). Inthe Betanzos area, lava ¯ows and sills of basanites,mugearites and trachytes crop out. The petrography issimilar to that of rocks from Las Conchas volcanicsuite. Rare tiny spinel lherzolite xenoliths can befound.

The third volcanic event is represented by the lam-prophyre sill intruded into Los Blanquitos Formationand the lava ¯ows and tu�s interbedded with theBalbuena and Santa Barbara subgroup. K/Ar ages are65 and 6022 Ma (Omarini et al., 1988; FernaÂ ndez,1975). In the eastern branch of the rift, near theArgentina±Paraguay border (Palmar Largo well), lava¯ows and volcanic pyroclastics related to YacoraiteFormation are found (MaÈ del, 1984).

The compositions of the volcanic rocks are, in gen-eral, basanites, trachytes and tephriphonolites.Likewise, in the Bolivian basin, lava ¯ows interbeddedin the Tocloca Formation, equivalent to LosBlanquitos-Yacoraite Formation, are present nearTupiza (Vargas Gil, 1965, Fig. 1). The rocks representan alkaline suite, with basanites and minor mugearites,trachytes and tephriphonolites (Fig. 7).

2.3. Petrography and mineralogy

The basanites are dark colored with idiomorphic oli-vine (Fo80) and salitic pyroxene in an aphaniticgroundmass. The groundmass contains clinopyroxene,biotite, opaque minerals, interstitial analcime2sanidi-ne2nepheline and scarce plagioclase microliths. Themugearites are greyish rocks with plagioclase (An40)and altered olivine phenocrysts in a plagioclase micro-lith (An20±30) groundmass. Secondary deuteric calciteand calcite-analcime veinlets and nodules are frequent.The tephriphonolites are greyish rocks, with mega-crysts of pyroxene and K feldspar, interstitial nephe-line and Ti-rich biotites. The trachytes are greyish-pink, occasionally coarse grained rocks. In the ElCadillal area the trachytes have sanidine+pyroxe-ne+olivine2anorthoclase, whereas in Betanzos theyhave phenocrysts of sanidine+plagioclase+biotite.

In the ``km 49'' locality in the AlemanõÂ a sub-basin,numerous fresh, ®ne- and coarse-grained monzogabbroclasts come from the upper parts of La BilleteraCreek. An aereal photographic survey reveals a smallintrusive gabbro body that intruded into the LasCurtiembres sediments.

Olivine is the dominant phenocryst phase in Las

Conchas basanites, except in some mugearites andtephryphonolites. Crystals are euhedral to subhedraland range in size from 0.05 to 1.5 mm. Compositionsof the olivine phenocrysts range from Fo90 to Fo70and there is no distinction in composition between theolivines in the basanites and the other rocks.

Clinopyroxene phenocrysts are ubiquitous in LasConchas lavas. They are subhedral to euhedral andrange in size from 0.02 to 1.2 mm. Clinopyroxene isusually colorless to pale green but in some cases theyare pale to pinkish brown. The latter is attributed to ahigher TiO2 content. Clinopyroxenes are often of tita-noaugite composition with TiO2 ranging between 0.55and 2.10 wt%. On the Wo-En-Fs composition trianglethey project predominantly in the salite-augite ®eld.Al2O3 contents are generally high (Galliski et al.,1990).

Feldspars vary in composition between labradorite(An59) in basanites through oligoclase (An20) intephryphonolites. Plagioclase occurs as microlites inbasanites and as phenocrysts in mugearites.Anorthoclase or sanidine are also found in the ground-mass but are di�cult to distinguish. They also occuras phenocrysts in the trachytic rocks. Normal zoning iscommon in plagioclase with rims more sodic than thecores. Apatite commonly occurs as inclusions.

Nepheline has been positively identi®ed by XRDand occurs as interstitial patches in the groundmass.In the phonotephrytes of the Cerro Amarillo, somenepheline crystals are fresh but most of them arealtered.

Opaque minerals occur in all lavas as small anhedralgrains in the groundmass, and some titanomagnetitephenocrysts (1.5 mm) are present as well.Titanomagnetite is the only Fe±Ti oxide identi®ed inthe lavas (Risso, 1990; Galliski et al., 1990).

Biotite with amphibole is important in the ground-mass of the tephryphonolites. Red biotite (Ti rich?)also occurs in the groundmass of the basanites andmugearites. Interstitial analcime, natrolite and calciteare common in all rocks.

2.4. Xenoliths

Xenoliths from Las Conchas alkaline basalts(Galliski et al., 1990) are predominantly peridotitesand pyroxenites. Peridotitic xenoliths are 0.5±10 cm indiameter. They are predominantly spinel lherzolite, spi-nel harzburgite and scarce dunite. Lherzolite xenolithsare medium to ®ne-grained, with protogranular to pro-toclastic textures with equilibrium grain boundariesand vermicular spinel in a recrystallized metamorphictexture. Olivine is sometimes serpentinized in the rimsof the crystals. Spinel is included in clinopyroxene andin orthopyroxene. Along the contact between clinopyr-oxene and orthopyroxene reaction rims are developed.


Orthopyroxene shows clinopyroxene exsolution lamel-

lae textures. Isolated crystals of phlogopite were

observed. Harzburgite is similar in texture and compo-

sition to lherzolite although without clinopyroxene

which is observed only as exsolution lamellae inside

orthopyroxene.

Pyroxenitic xenoliths, up to 30 cm in diameter were

found as nodules hosted in basanites. These xenoliths

consist mainly of clinopyroxene and secondary min-

erals such as natrolite. Some opaque minerals were

determined as chromite, magnetite, pyrrotite, pentlan-

dite and chalcopyrite.

Granulite xenoliths were found at Quebrada Don

Javier, they are 15 cm in diameter and are composed

of acidic and basic granulites. Acidic granulites are

mainly composed of plagioclase, quartz, garnet and

pyroxene. Basic granulites are composed of clinopyr-

oxene, garnet, olivine and scarce interstitial plagioclase

(Risso and Viramonte, 1992; Lewerenz, 1997; Lucassen

et al., 1999).

2.5. Geochemistry

Plutonic rocks exhibit both alkaline and subalkaline

a�nities. Fig. 3 shows that the Hornillos complex

rocks fall in the alkaline ®eld of the SiO2 vs

Na2O+K2O diagram of Irvine and Baragar (1971);

Rangel complex rocks fall in the alkaline and subalka-

line ®elds of the diagram. Aguilar and RejaraÂ com-

plexes plot in the subalkaline ®eld. Hornillos and

RejaraÂ complexes are geochemically characterized by

LREE enrichment (light rare earth elements, Figs. 5

and 6). The Hornillos complex was interpreted by

Rubiolo (1992) as an alkaline complex while the

RejaraÂ is interpreted as granite with a strong crustal S-

type a�nity. The tectonic discriminant diagram of

Pearce et al. (1984) shows that both the Hornillos and

Rangel complexes plot in the ``within plate granite

®eld'' (Fig. 4).

The volcanic rocks of Las Conchas are represented

by a suite of basanite to hawaiite and alkaline basalts

Fig. 4. Tectonic discriminant diagram after Pearce et al. (1984), for

Hornillos and Rangel plutonic rocks.

Fig. 5. REE chondrite normalized patterns of selected samples from

Hornillos plutonic complex (normalization after Taylor and

McLennan, 1985).

Fig. 6. REE chondrite normalized patterns of selected samples from

RejaraÂ plutonic complex (after Taylor and McLennan, 1985).

Fig. 7. Normative classi®cation of volcanic Cretaceous rocks from

northwestern Argentina and south Bolivia after Cox et al. (1979).


(Fig. 7). None of the samples are Qz normative, andnormative En varies from 4 to greater than 25%.Some samples are leucite normative. The lavas arealkalic in composition, and they range in SiO2 from41.83 to 49.99%, except the El Cadillal trachytes thatcontain 62.51%, but most of them contain around43% SiO2. They exhibit high abundance of TiO2 (2.3±4.2%), Na2O, K2O and P2O5. Their MgO content isvariable between 10.61 and 11.34% for Betanzos basa-nites; El Cadillal mugearites and basanites content isof 8.92 and 14.43% respectively. Las Conchas hawaiiteand basanite content is between 5.35 and 10.1% MgO.Mg number [Mg] range is between 47 and 71, exceptfor El Cadillal trachytes which is 9.

Cr content is between 311 and 349 ppm in rocks ofbasanitic composition; hawaiites and mugerites arebetween 86 and 159 ppm. Ni major values are 140ppm for El Cadillal basanites, and 238 ppm for LasConchas basanites. No data are available for Betanzosbasanites.

Trace element contents show an enrichment inincompatible elements relative to MORB (Fig. 8) es-

pecially Rb, Ba, Sr, K, Nb and Ce. Zr/Nb ratio is verylow, less than 10 and the K/Nb ratio is between 583and 702. Fig. 9 shows an OIB/rock normalized dia-gram (data after Wilson, 1989) that shows that therocks perhaps have a crustal contamination showingpositive peaks in Rb, Ba and La±Ce, although Ba con-tent is similar to other continental rift basalts (around622 ppm for Mt Kenia basanites, Price et al., 1985).

High enrichment of LREE is shown by the steepslope of the REE patterns. Enrichment of LREE issimilar for both Betanzos and Las Conchas lavas(Figs. 10 and 11 ). Sm/Yb ratios are between 3.11 and4.58, Ce/Yb ratio is 37.57 and 48.5, and Ba/Yb isbetween 250 and 292.9 for Las Conchas basanites. ForBetanzos these values are 6.19 to 6.66 Sm/Yb and 52.8

Fig. 8. Trace elements spiderdiagram of selected samples from Las

Conchas basanites, MORB normalized (Wilson, 1989).

Fig. 9. Trace elements spiderdiagrams of selected samples from Las

Conchas basanites, OIB normalized (Wilson, 1989).

Fig. 10. REE patterns for selected samples from Las Cochas basa-

nites (after Taylor and McLennan, 1985).

Fig. 11. REE patterns for selected samples from Betanzos basanites

(after Taylor and McLennan, 1985).


and 62 Ce/Yb. A Eu negative anomaly is not presentin basanites or mugearites but is present in trachytes,possibly caused by the removal of plagioclase.

2.6. Isotopic studies

Preliminary isotopic studies in volcanic rocks andxenoliths from Basalto Las Conchas were carried out(Figs. 12 and 13). A basanite sample from Km 49 hasa 87Sr/86Sr ratio of 0.70354420.0009; a 143Nd/144Ndratio of 0.51279420.0007 and a 206Pb/204Pb of 19.347;a 207Pb/204Pb of 15.632; and a 208Pb/204Pb of 39.352.Basanite samples from El Sunchal, have a 87Sr/86Sr

ratio of 0.703330; 143Nd/144Nd ratio of 0.512862; a206Pb/204Pb of 19.72; a 207Pb/204Pb of 15.66; and a208Pb/204Pb of 39.45.

The Sr and Nd isotopic compositions of a spinel

lherzolite xenolith from Cerro Amarillo are similar

(87Sr/86Sr=0.703416; 143Nd/144Nd=0.512885) to those

of the host alkaline basalt. Likewise, a clinopyroxenite

block, 30 cm in diameter, (87Sr/86Sr=0.703301 and143Nd/144Nd=0.512883) from El Sapo area, gave

values similar to that of the host basanites. They may

be crystal cumulates formed in magma conduits from

previous magmatic events (Galliski et al., 1990).

The 87Sr/86Sr ratios of the ma®c granulites are much

higher (87Sr/86Sr=0.705279) than the previous ones

and the acid granulites show typical crustal values

(87Sr/86Sr=0.7091410 to 0.714136).

Rare lherzolite xenoliths, crossed by amphibole+Ti-

rich-phlogopite+apatite veins, are found in El Sapo

area, which has the lowest Sr isotopic values

(87Sr/86Sr=0.703074) of the Las Conchas Basalts suite.

Volcanic rocks from Basalto Las Conchas have Pb

isotopic compositions which plot in the high206Pb/204Pb end of the MORB array (Fig. 12). They

plot outside the DUPAL OIB anomaly, which pro-

vides a source component for the basalts of certain

oceanic islands basalts at latitudes south of the

Equator (e.g., Tristan da Cunha, Wilson, 1989). This

is in agreement with the Nd±Sr isotopic data (Fig. 13)

which appears to indicate enriched MORB source

characteristics.

Fig. 12. 208Pb/206Pb vs 206Pb/204Pb isotopic diagram for basanites

and xenoliths from Las Conchas outcrops.

Fig. 13. 87Sr/86Sr vs 144Nd/143Nd isotopic diagram for basanites and xenoliths from Las Conchas outcrops.


3. Discussion and conclusion

The petrography, chemistry and chronology of theCretaceous plutonic bodies indicate an anorogenic pre-rift related A-type origin for the granitic complexes,closely associated with the genesis and evolution of theCretaceous rift basin, similar to that described byCollins et al. (1982). These high level plutons showstrong similarities with the granitic ring complexes ofNigeria which have been reviewed by Bowden andTurner (1974).

The petrology and geochemistry of the volcanicCretaceous rocks of central-southwestern SouthAmerica show strong alkaline a�nities and suggest asimilar origin for all of them.

In general, the most basic end members of the basa-nitic suites have rather low concentrations in compati-ble trace elements, such as Ni, suggesting that theyhave experienced some degree of olivine fractionationprior to or en route to the surface. Olivine basaltshows higher values in Ni and MgO content probablybecause it represents more primary magma.

Trace element ratios shows enriched mantle sources,the K/Nb ratio shows an enriched phlogopite bearingsource, and the high Sm/Yb, and Ce/Yb values forBetanzos basalts represent a residual garnet source.Di�erences between these ratios for Las Conchas andBetanzos basalts could either re¯ect that these suitesdo not come from the same mantle sources or that LasConchas basalts have some crustal contamination.

The Sr isotope values fall in the range suggested fora mantle lithosphere depleted in REE content. Pb andNd isotope compositions have P-MORB- (OIB) likesignatures. The variation shown by Pb isotopes of Km49 and El Sunchal basanites could re¯ect a metasoma-tism event also expressed by the K/Nb ratio. Likewise,Pb isotopic compositions and incompatible trace el-ement ratios, such as Ba/Yb, Sm/Yb, Ce/Yb and theREE pattern of these samples, do not contain subduc-tion slab-derived components. In this sense it is poss-ible to suggest that the pre-Cretaceous mantlelithosphere, below the Central Andes, has experiencedan enrichment in REE elements by di�erent processes.One of them would be the introduction of amphibole-high Ti phlogopite-apatite rich veins. Some peridotitexenoliths found in the El Sapo neck show this type ofvein although the same xenolith exhibits lower87Sr/86Sr values than basanites. Comparison with iso-topic data of Nd±Sr of spinel lherzolite xenolithsreveals a considerable overlap, which could re¯ect thatthese basalts are the product of the partial melting ofsubcontinental mantle lithosphere, with a very low par-ticipation of crustal contamination.

Fig. 12 also shows LPT and HPT (low Ti and highTi basalt types) ®elds for the ParanaÂ Basalts andEtendeka Basalts. Rocks from Las Conchas basalts

plot far from these ®elds and show that high crustalcontamination events were not involved in their gen-esis.

Although these are preliminary data, the Nd±Sr iso-topes diagram suggests that the alkaline basalts andxenoliths from Las Conchas are near the P-MORB(OIB) in the mantle array ®eld; their geochemical andisotope features let us propose that these rocks origi-nated by low degrees of partial melting of a depletedsubcontinental mantle lithosphere in which metaso-matic enrichment events occurred. They resemble P-MORB-OIB a�nities, but are di�erent than DUPALOIB sources (Allegre et al., 1981).

The implication of the petrochemistry of xenolithsfound in Las Conchas Basalt is that the pre-Cretaceous subcontinental lithospheric mantle wasmineralogically and chemically heterogeneous, consist-ing of various proportions of spinel lherzolite, wehrlite,harzburgite, and minor dunite and pyroxenite, oc-casionally transected by amphibole-Ti rich phlogopite-apatite veins. The pyroxenite cumulate may haveformed at depths similar to the former and representscumulate bodies originated in previous magmatic epi-sodes, as suggested by both its geothermometry andgeobarometry. On the other hand, ma®c and acidgranulites may be the constituents of the lower andmiddle crust, respectively.

Preliminary data for temperature and pressure equi-libration, based on di�erent geothermometers and theAl content of pyroxenes in di�erent types of peridotitexenoliths, indicate that they were equilibrated atdepths between 30±55 km (Ellam, 1992). Likewise,they suggest a high geothermal gradient of about208C/km, depths between 30±55 km, and a tempera-ture of 11008C at 55 km (Lewerenz, 1997; Luccasen etal., 1999).

The temperature of equilibration of Las Conchasperidotite xenoliths is below the liquidus temperatureof the basalts. These characteristics suggest that thesource of the Cretaceous basins of central-southwes-tern South America alkaline basalts was either in thedeepest parts of the sub-continental lithosphere or atgreater depths than the mantle section that theysampled en route to the surface.

The Cretaceous plutonic and volcanic rocks fromsouth-central western South America are related to anintracontinental rift environment and although theirages are correlative with the ParanaÂ Volcanic Provincemagmatism, the geochemistry of these rocks showdi�erent source regions and petrogenetic processes.

Acknowledgements

This research project was developed with fundingfrom CIUNSa grant 570 (Consejo de Investigaciones


de la Universidad Nacional de Salta) CONICET grant4879 (Consejo Nacional de Investigaciones CientõÂ ®casy TecnoloÂ gicas de la RepuÂ blica Argentina) and theAgencia de PromocioÂ n CientõÂ ®ca y TecnoloÂ gica de laRepuÂ blica Argentina grant 07-00000-02237.

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Documents

Viramonte Et Al., 1999. Cretaceous Rift Related Magmatism in Central-western South America