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Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of northwestern Argentina
Journal of South American Earth Sciences, VoL 5, No. I, pp. 21-32. 1992 Printed in Great Britain
0895-9811/92 $5.00+.00 © 1992 Pergamon Press Ltd & Earth Sciences & Resources Institute
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of northwestern Argentina J . SAAVEDRA .1, E . PELLITERO-PASCUAL 2, J . N . ROSSI s,
and A. J. TOSELLI s
ICSIC/Institute of Natural Resources and Agrobiology, Apartado 257, Salamanca, Spain; 2Department of Geology, University of Salamanca, Spain; SHigher Institute for Geological Correlations, National University of Tucum(m, Miguel Lillo 205, 4000 San Miguel de Tucum~in, Argentina
(ReceivedDecember 1989; Revision Accepted March 1990) A b l t r a c t - - T h e Cerro Toro granitoids range in composition from granite to gabbro and enclose abundant igneous and metamorphic enclaves that are assimilated in varying degree. The host rocks were intensely heated by intrusion of the Cerro Toro pluton, with local formation of anatexites. Geochronologic determinations point to an initial Sr ratio of 0.70967 + 0.00043 and an age of 456 + 14 Ma (Ordovician). The geochemical and field relationships between the acidic and basic members point to strong interactions between different unconsolidated magmas, with an extreme local fluidity that resulted in a gneiss-like aspect-although without cataclasis. Geochemical evidence indicates that the Cerro Toro granite was emplaced in a magmatic arc, at relatively deep crustal levels, under conditions close to anatexia. Regumen--Los granitoides de Cerro Toro presentan un amplio range de variaciSn, desde granitos a gabros, con abundantes enclaves tgneos y metamSrficos, m~s o menos asimilados. E1 encajante ha sufrido fuertemente la acci6n t6rmica de1 intrnsivo y aparece frecuentemente como una verdadera anatexita. Una determinaciSn geocronol6gica indica una relaci6n inicial de Sr de 0.70967 + 0.00043 y una edad de 456 + 14 Ma (Ordovtcice). Las relaciones geoquimicas y de campo entre los t6rminos ~cidos y b~isicos indican fuertes interrelaciones entre magmas distintos no consolidados, con fiuidaridad muy extrema que hace adquirir un nspecto gn6isico, pero sin cataclnsis. Evidencias geoqutmicas indican que los granitoides de Cerro Toro fueron emplazados en un arco magm~tico en niveles corticales relativamente profundos, en condiciones pr6ximas a la anatexia.
INTRODUCTION PLUTONS within the Andean basement consist of rocks of varying acidity, ranging from true gabbro to tonalite-granodiorite and even granite. They exhibit characteristics of magma interactions and contain enclaves of microgranitoids, of varying morphology, that axe more or less elongated and have both clearcut and diffuse borders. The mineralogy of the enclaves is compositionally related to that of the encompassing granitoid, which is generally more acidic. These are common features of plutons in many localities, as often noted in the literature. We studied one such characteristic pluton, the Cerro Toro granite--an excellent example of the complex process of mingling with some true mixing. This pluton, in which features typical of magma interaction and assimilation are spectacular, is located in the pre-Mesozoic basement near Villa Castelli (in the western zone ofLa Rioja Province). The Cerro Toro granite was chosen for its accessibility and good exposure and for the broad lithologic variations it displays (Fig. 1). The study area is located on the western flank of the Famatina System (Bodenbender, 1916), where the metamorphic basement is comparable to that of the western Sierras Pampeanas (schists, amphibolites, gneisses, marbles, and migmatites). The Cerro *Address all correspondence and reprint requests to: Dr. Julio Saavedra: telephone [34] (23) 219606; telefax [34] (23) 219609.
Toro zone is unlike most of the Famatina System, the metamorphic rocks of which are of low metamorphic grade, as represented by the Negro Peinado Formation (Turner, 1960). Hausen (1921) referred to processes whereby blocks of schist and amphibolite were assimilated by granitic magma with the result that the magma became tonalitic in composition. Toselli et al. (1988a) summarized previously published data, presented new petrological and geochemical data, and considered possible links with other plutons in the area. According to them, biotite-amphibolite tonalites (from syn- to late tectonic) that are metaluminous to weakly peraluminous predominate and intrude micaceous schists and migmatitic gneisses alternating with ortho-amphibolites--typical of a magmatic arc associated with a continental border active during the early Paleozoic. Enclaves within the Cerro Torro granite are dispersed and appear in the form of synplutonic mafic dikes, dismemberment of which resulted in hybridization accompanied by remobilization and assimilation of crustal material. The outcrops studied exhibit abundant mafic metamorphic enclaves that vary considerably in size and stand out from the granitic landscape (Fig. 2). The metamorphic host rocks show frequent intercalations of amphibolite~ essentially composed of amphibole and plagioclase, with very little or no quartz and with a common ortho-derived character. This ensemble shows the effects of a regional metamorphism that led to the development of micas and a few aluminum silicates (andalusite and sillimanite). Numerous leucocratic segregations 21
22
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Fig. 1. Geologic setting and study area: A)position on the general geologic map of Argentina (oblique hatching = study zone); B) geologic 9cheme of Villa Castelli and its environs, partly based on data from the authors.
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of N W
Argentina
23
Fig. 2. Western flank of the Sierra de Famatina seen from Villa Castelli. Darker masses are more ferromagnesian rocks encompassed withindifferent,more leucocraticgranitoids.The drawing was made from a panoramicphotograph.
and injections are apparent at the contact with the granitoids, conferring on them the aspect of a true anatexite (Fig. 3). The granitoids commonly show a degree of foliation but lack evidence of cataclasis or solid-state deformation (except for a few regional faults). It is difficult to establish a sequence of events because of the paucity of fossils in low-grade metamorphic rocks of the eastern part of the mountain system--ichno-
fossils such as Nereites and algae that can be assigned a late Precambrian-Early Cambrian age (Ace5olaza, pers. commun., 1988)--and because the age of the metamorphic host rocks of the pluton.is unknown. A Rb-Sr isochron obtained for five samples of the Cerro Toro granite yielded an initial 87Sr/SeSr ratio of 70967__.0.00043 and an age of 456+_14 M a (Teledyne Isotopes, USA, 1989). This age is the basis for suggesting Ordovician plu-
Fig.3. Metamorphic hostrockofthe CerroToro granite.
24
J. SAAVEDRA,F~.PELLITERO-PASCUAL,J. N. ROSSI, and A.J. TOSELLI
tonization and associated host-rock metamorphism, in agreement with the regional plutonic scheme proposed by Toselli et al. (1988b). ANALYSES AND RESULTS Chemical analyses were carried out in the laboratories of the Higher Institute for Geological Correlations, National University of Tucum~n, and the Institute of Natural Resources and Agrobiology, CSIC, in Salamanca, using techniques described by Saavedra and Medina (1983) and Garcfa S~uchez and Saavedra (1983). Oxides were determined by atomic absorption spectrophotometrywwith the exception of SiO2 and P205, which were evaluated by spectrophotometry of molybdenum and vanado-molybdenum complexes, respectively. Trace elements were determined by X-ray fluorescence. Modal analyses of quartz and feldspars of coarse- and fine-grained rocks were performed directly on thin sections; those with greater granularity were subjected to the X-ray diffraction technique reported by Maniar and Cooke (1987).
Characteristics of Outcrops Two principal groups of plutonic rocks were observed in the field. The most abundant is composed mainly of tenalites, together with granodiorites and granites (Fig. 4). The second group (not included in Fig. 4) comprises more basic rocks such as gabbros (some of them quartz-bearing and others dioritic) and diorites; these rocks display complex relationships described below. Also present are many xenolithic enclaves of the host rock in different states of transition ranging from schists with more-or-less leucocratic veins of segregation, to metasedimentary mafic remains that are not clearly delimited from the encompassing granite in contrast to amphibolites, all of which have clear-cut borders. The contacts in the Cerro Toro grAnlte vary; those between the mafic and felsic members are as likely to be sharp as they are to be diffuse. Most of the grAnitoids have a coarse grain size and are equigranular, with varying relative amounts of the ferromagnesian minerals biotite and hornblende. Occasionally, particularly in proximity to the more basic gr~nitoids, they give way to facies with different granularities that are irregular and exhibit gradational transitions. Schlieren are abundant, beginning irregularly in the heart of the leucocratic facies and ending gradually with diffuse borders in femic enclaves and masses (Fig. 5). Moreover, there is usually a direct relationship between the dimensions of such femic masses and the amplitude of the aureole richest in ferromagnesian minerals separating both lithologies (Fig. 5). Sometimes, several such zones are visible, each slightly more mafic than its predecessor and showing gradational or well defined contacts, although always
smaller in size. This kind of sequence is observed both in proximity to basic rock and in zones distant from it. Unequal veins and masses of leucocratic material that cut the mafic parts, or are in diffuse contact with them, are also typical (Fig. 5). On the megascopic scale, some of the basic members resemble well delimited dikes inside the more acidic granitoids, although their components are unequal (in basicity, in grain size, and sometimes by the presence of metasedimentary enclaves separated by more or less defined limits) and may be bordered by small fine-grained leucocratic masses. All this is typical of a process of mingling by which granites evolved from gabbroids. Despite the presence of zones of homogeneous mixing between both extremes, the degree of such homogenization cannot be ascertained in the field. The most abundant enclaves are those with a typically igneous composition and texture; they appear in all environments, including late aplitic dikes that encompass round mafic masses. The host granitoid is always more acidic and finer grained than the enclaves, even though the mineralogy of the enclaves may be comparable or identical to that of the intrusive body. Although diffuse borders are common, such as those shown in Fig. 5, well defined limits usually predominate, with convex forms toward the host granitoid and with pillows and lobulate or crenulate borders. The following were also observed: mafic enclaves within masses of lower acidity; double enclaves; irregular masses and well delimited contacts of the acidic granitoid with the more basic members; aureoles of quartz and alkaline feldspar around inclusions of gabbros/diorites; and mafic edges in the acidic granitoids. Although not abundant in the tonalites, feldspar phenocrysts exhibit a complete transition to xenoliths in rocks with lower basicity (Fig. 5). In a very few exposures, phenocrysts have been observed on both sides of the tonalitic-basic enclave. A significant observation is that the gabbroids do not have igneous enclaves and are homogeneous in appearance. Quantitatively, the number of igneous enclaves is generally inversely related to the degree of acidity of the host granitoid. Despite considerable variation, textures are unequivocally igneous and orthomagmatic. The orientations of the mineralogic constituents of the granitoids and the most mafic members, enclaves or not, are clear and predominant, with no evidence of contemporary cataclasis. Variable phenomena of solid-state cataclasis are associated with later tectonic events, which may be generalized to a regional scale, although details vary considerable (Fig. 6). At mesoscopic scale, the main tonalitic body usually shows a certain orientation that coincides with the principal dimension of the enclaves. The enclaves are often planar in shape and are mostly finegrained. Prominent angular and polyhedral xenoliths are found in this kind of inclusion, but although showing unequivocal features of amphibolites, they lack the above-mentioned external orientation. This
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of N W Argentina
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Fig. 5. Schematic detail (10 cm scale), of the relationship between basic and acidic magmas at El Infernillo Canyon (Cerro Toro), northeast of Villa Castelli. Mixing (hybridization) is seen in proximity to the more basic part; schlieren and acidic veins cut the basic materials without passing to the more leucocratic part.
26
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suggests a morphologic differentiation between the igneous enclaves (autoliths) and the metamorphic enclaves (xenoliths). Such autoliths appear throughout the pluton, although they are concentrated and elongate (1:40 or more) in certain places, with a fluid aspect and with more or less delimited borders and thus could represent disrupted syn-plutonic dikes.
The ensemble (Fig. 6) has all the characteristics of a zone of mobilization of a heterogeneous magma, with a certain occasional and partial mixing of components and concentrations of components locally elongated in the same direction, like passages or corridors of enclaves; these could also represent disrupted syn-plutonic dikes.
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of N W Argentina
Petrography and Mineralogy The climate of this region is arid; thus, the outcrops are good and sampling is very satisfactory in the main canyons. The large number of excellent observation points permitted adequate sampling for the study of every magmatic process. The general predominance of tonalites shown in Fig. 4 corresponds to members ranging from biotitic to amphibolic. These rocks are essentially equigranular, with coarse to medium-sized crystals--the latter occurring in varieties richer in mafic materials. Despite the predominance of a certain fluid orientation of mica, amphibole, and some quartz crystals, it is sometimes possible to observe in the elongations of the minerals directions at an angle to the flow structure, with no breakage features. Growth seems to have been free and is very reminiscent of the rotation of crystals in a medium with a certain degree of viscosity. Except in the gabbroid members, the plagioclases are fairly uniform (andesines in tonalites,oligoclaseandesine in the more acid granitoids), with minor zoning; some exhibit a partially corroded nucleus with a fresher crown. The habit is generally euhedral to subhedral, with complex twinning and occasional abundant inclusions. The potassium feldspar is usually microcline, with or without perthites and with an albitic border on the more leucocratic facies. Often, a rim of myrmekite encloses the plagioclase. However, it is not uncommon to observe two morphologically different generations--the oldest without the quadrangular twin typical of microcline. Biotite is the most abundant ferromagnesian mineral, followed by hornblende. It is often partially altered to chlorite,whether or not potassium feldspar is involved, and is deformed to a certain degree. Although inclusions of opaque minerals, zircon, aparite, and epidote, are not common in biotite, some crystals show lineations of small grains of iron oxides with the same aspect as those found in the biotite of the enclosing schist. Muscovite is scanty and appears only in the leucocratic facies. Hornblende, sometimes the dominant mafic mineral, is occasionally altered to mixtures of biotite, quartz, and epidote, with or without titanite. This usually takes the form of layers of biotite within hornblende prisms. Less commonly, hornblende crystals exhibit lineations of iron oxides along exfoliation planes, with a morphology identical to that exhibited by some of the biotites;presumably, these would be of relictmetamorphic origin. The development of hornblende after biotite,by inversion of the normal crystallizationsequence, is a general feature. W h e n the hornblende is blastic, it essentially includes titanite, opaques, and biotite. Some hornblende has a heterogeneous aspect, with differing colors due to differing M g content, and the appearance of having been derived from olivine or pyroxene, but only in the more basic members of the pluton.
27
Epidote, both pistacite and clinozoicite,often exhibits features of secondary minerals and is also idiomorphic; it usually has inclusions of nuclei of allanite and opaque minerals and is more abundant in the basic members. Allanite is also very common; it is idiomorphic and its crystals are zoned. Zircon and apatite crystals are elongate (1:10 or greater), as is typical of faster cooling (Wyllie et al., 1962; Didier, 1964; Williams et al.,1983), and are also present in short normal prisms. Titanite appears in isolated grains, developing over epidote, biotite, and plagioclase and forming crowns around a nucleus of ilmenite, with clear evidence of interaction of the mineral grain with the surrounding milieu; this is especially evident in the less leucocraticfacies. Sillimanite and cordieritewere found in only two of the 80 samples studied; these samples, which contained no andalusite, came from a complex zone with an abundance of autoliths, metamorphic xenoliths, and very evident mingling. Sillimanite, as fibrolite or in prisms, is associated with mica. The cordierite is partially altered and, together with the sillimanire,is included within feldspars or in grains. The appearance of the host granitoid is very similar to that of the other igneous facies,with mafic, intermediate, or acidic varieties present in different parts of the pluton.
Geochemistry The geochemistry of these plutonic rocks is complex. On an A F M diagram (Irvine and Baragar, 1971), gabbroid samples plot in the tholeiitic field and diorites in the calc-alkaline field (Fig. 7). This contrast is also clear on an Fe/Mg variation diagram (Fig. 8). The diagram shows that only the less magnesian tholeiiticmembers and the most ferromagnesian members of the calc-alkaline trend overlap. Considering both trends, it is thus highly significant that only the extremes of highest and lowest M g content correspond to facies exhibiting the least evidence of mingling, whereas this phenomenon is typical and pronounced in the trends of the less magnesian gabbroids and, above all,in the calc-alkaline rocks with higher Fe and M g values. The lack of discontinuity therefore suggests chemical links. Relative variations in element pairs of the rocks corresponding to the tholeiitic and calc-alkaline groups are shown in the Harker diagrams (Fig. 9), which display the following: •
The gabbroids show unique variations in both major elements and in minor and trace elements. The most acidic rocks, those with a calc-alkaline trend, show no geochemical links with the gabbroids.
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In general, there is no direct and significant relationship between the element pairs.
J. SAAVEDRA, E. PELLITERO-PASCUAL,J. N. ROSSI, and A.J. TOSELLI
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Fig. 7. Position on the AFM diagram of Irvine and Baragar (1971) of the Cerro Toro granites. Circles correspond to tonalites, granodiorites, and granites. Points correspond to more basic rocks.
F i g . 8. V a r i a t i o n i n F e 2 0 3 (T) a n d M g O c o n t e n t . C i r c l e s correspond to tenalites, granodiorites, and granites. Points corresp o n d to m o r e b a s i c r o c k s .
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The dispersions in both types of rocks are noteworthy in most cases. I N T E R P R E T A T I O N AND DISCUSSION
Intrusion of the Cerro Toro granite must have occurred at an appreciable depth. The schists of the intrusive contact (see Fig. 3), in which quartz-feldspar segregations are abundant near the granitoid, suggest temperatures and pressures not very different from those of anatexia. The very generalized presence of muscovite and scarcer sillimanite or cordierite reinforces this impression. There is a complete grading between xenolithic enclaves, from those that are identical to the metamorphic rocks of the immediate contact to those that are confused with the granitic host rock (advanced assimilation), the case in which the main change must have been temperature. This is only possible in domains of higher pressure where muscovite, always present, is stable. In the granitoid, the biotite and muscovite of the assimilated schists were subjected to new conditions of the milieu, implying a different kind of stability. However, amphibolites intercalated among the schists were not assimilated; even the most extreme cases show amphibolic xenoliths without quartzmicaceous material. Thus, in materials with a gabbroic composition, upper temperature limits cannot have been above the melting point of hornblende (Wyllie, 1971), which may reach more than 1000°C. In summary, as pointed out by Whitney (1988), the destabilization of muscovite is followed by that of biotite to give pyroxene, whose possible relicts, mentioned before, are now converted into amphibole. These occur in the 750-850°C temperature range, depending on chemical composition, indicating emplacement at fairly high temperatures.
Mafic enclaves are common in granitoids, and their origin has been the subject of numerous studies (e.g., Didier, 1987; Vernon, 1983; among others). In general, two groups of enclaves are recognized and classified according to whether they exhibit clear and unequivocal metamorphic features, such as those referred to above, or whether they have typically igneous textures. Although a residual origin has been proposed for typically igneous microgranltoids in other regions (Chappell and White, 1974: demixing of restitic rocks from a melt upon ascension of the granitic mush), a model which assumes variable amounts of the same melt (White and Chappell, 1977) is not applicable to the Cerro Torro granite. The reasons are as follows: Although microscopic observations, as reported above, clearly point to the existence of some relict metamorphic minerals (biotite and others), the textures, the morphologies of the enclaves, the presence of double enclaves, relationships with the host rock, etc., also clearly suggest that all were once in a plastic state--the most mafic rocks dispersing in the form of globules into the more acidic rock, accompanied by a certain mingling. The restitic mechanism, a phenomenon that has not been observed, requires linear variations among pairs of elements (Fig. 9). The separation of melt and restite that must have occurred under similar conditions of viscosity, temperature, and composition (not very variable factors in this case) is not supported by the experimental data, which indicate exactly the opposite (a more favorable hybridization) as
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the efficacy of diffusion processes at the relatively high temperature deduced here and by the presence of volatiles, as attested to by significant amounts of A n alternative hypothesis for the origin of the hydroxylated minerals. This chemical exchange is enclaves is the immiscibility of acidic and basic com- well documented (Alibert and Carr6n, 1980; Lesher, ponents during cooling of an intermediate melt, as 1986; Jambon, 1980; Watson and Jurewicz, 1984; and proposed by Bender et al. (1982) and Mezger et al. many others) and is manifested by the following: (1985). At Cerro Torro, the morphologic field evi- replacement of biotite by hornblende, rims of sphene dence does not support such an origin: pillow-shaped around grains of ilmenite, relicts of previous mag(even multiple) enclaves are present, and there are nesian minerals (now hornblende in its different considerable variations in texture and limits with varieties), and biotization of amphibole (much rarer the host granite. This is confirmed by the broad than the inverse phenomenon)--all of which reflect range of quantitative mineralogic variations in the the migration of K, Ca, Ti, and other elements. maflc material and hence of the chemistry. Despite such clear signs of a previous mixing, the It thus seems quite clear that different m a g m a s results were of limited significance, for the history of interacted. However, the disproportional chemical mingling of different magmas (many of them of not variations shown in Fig. 9 indicate that the variety of very variable composition) is patent. lithologic types does not derive from a simple mixAccordingly, cooling at temperatures lower than ture. Furthermore, the heterogeneity of the enclaves those at which the process could have operated must suggests several magmatic pulses, and the minera- have occurred relatively quickly. The different delogic similarity of the enclaves and host granitoid grees of turbulence and the magmatic fluidities suggests that crystallization occurred under the indicate the influence of the higher temperature same conditions in both, after the mafic m a g m a had mafic members, which prevented formation of an been incorporated and had been dispersed into the ordered, concentric zoning to be reached. As a result, felsic magma. The melt content in the mafic m a g m a fractionated crystallization, if in fact it did take must have been very high, judging from the plasti- place, must have been relatively minor, as also sugcity implied by very elongated enclaves (see Fig. 6), gested in Fig. 9, which does not give the regular and which confer the aspect of a gneiss, and the numer- dominant variations among pairs of elements typical ous diffuse and irregular contacts between felsicand of this process but rather shows a dispersion reflecmafic rocks. A fluid phase was not quite reached, ting several mechanisms of emplacement. however, as hybridization is not extensive. According to Sparks and Marshall (1986), this can only occur when m a g m a s remain liquid after equili- Position in the Geotectonic Environment brating thermally. The frequency of pillowed enclaves, with pointed The dominant trend of the granitoids, as shown borders convex toward the host granite, and of acicu- in Fig. 4, indicates a low K content typical of the lax minerals in the basic members suggests some early internal stage of active continental borders cooling. O n the other hand, the absence of chilled (Lameyre and Bowden, 1982), although with a margins, together with the high percentage of melt significantbut subordinate potassic component that in the mafic magmas, suggests relatively high tem- prevents the differentiation of I and S types. In peratures. Under such circumstances, the viscosity principle, this is consistent with the evidence cited of of the acid host magma, already decreased by injec- assimilation of the host rock: a high STSr/SeSr ratio tion of the basic magma, would be lowered and without a general aluminosity, the presence of magapproach that of the basic m a g m a (in turn increased mas with a tholeiitic and calc-alkaline trend, high during its dispersion in the acid magma), and the temperatures, and evidence of superposition of diffusion rate (exponentially controlled by tempera- several mechanisms that would have been acting ture) would increase. All this favors mixing (Camp- simultaneously (see Fig. 9). bell and Turner, 1985), evidence of which is found in The distribution of elements considered of most the form of acicular and prismatic apatites occurring significance (Pearce et al., 1984) is illustrated in Figs. together, mineral relicts, crowns of ferromagnesian 10 and 11. The data displayed in Fig. 10 rule out the minerals around quartz, and other signs of dis- possibility of intraplate granites or granites from an equilibrium previously described. oceanic ridge (geologically improbable). Figure 11 The experiments of Koyagushi (1985, 1986) and indicates that the granites are associated with an Kouchi and Sunagawa (1985) reveal the importance arc, with no possibility of collision, because--as menof additional factors that strongly affect homogeniza- tioned above--they have been intruded in rocks in tion phenomena. Examples of these are turbulence which an enrichment in Rb by diffusion is feasible, originated by phenocrysts, mechanical agitation, together with others in which this phenomenon has limited cooling of the mafic magma in the felsic mag- not been observed, thus constituting a homogeneous ma, and convection in the magma chamber. Figures group. Viewed in this light, the Rb content is clearly 5 and 6 illustrate the importance here of turbulent significant. dynamic phenomena within the granitic fluids. Interaction between the magmas is also favored by most likely (Campbell and Turner, 1986; Kouchi and Sunagawa, 1985).
Magmatic evolution ofthe Cerro Toro granite,a complex Ordovician pluton ofN W Argentina
31
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-
CONCLUSIONS The Cerro Toro granite, a constituent of the Andean basement, is a complex Ordovician pluton that was emplaced in a magmatic arc at relatively deep crustal levelsunder conditions close to anatexia and influenced by the presence of mafic magma. Manifestations of more or less complex mixing of m a g m a s are spectacular. Also manifest are features indicative of convection and turbulence, as well as other evidence of the fluid dynamics that governed the chemistry of the batholith prior to its consolidation.
Acknowledgments--The authors would like to thank Drs. C. E. Macellari, H. Miller, and W. Pitcher for their invaluable comments on the style and content of this paper and for their suggestions concerning the development of the scientific ideas expressed. This work was a contribution to IGCP Project 249: Andean Magmatism and Its Tectonic Setting (IUGS-UNESCO).
REFERENCES Alibert, C., and Carton, J. P., 1980. Doun6es exp~rimentales sur la diffusion des 616ments msjeurs entre verres ou liquides de compositions basaltique, rhyolitique et phonolitique entre 900°C et 1300°C &pression ordinaire. Earth and Planetary Science Letters 47,294-306.
0
®
0
lO
I
lO
!
I
I
I
I
5O
Y+ Nb,
ppm
Fig. 11. Variation in (Y+Nb)-Rb (Pearce et al.,1984). Circles correspond to tonalitos,granodiorites,and granites. Points correspond to more basic rocks.
Bender, J. F., Hanson, B. H., and Bence, A. E., 1982. The Cortland Complex: Evidence for large-scale liquid immiscibility involving granodiorite and diorite magmas. Earth and Planetary Science Letters 58, 330-344. Bodenbender, G., 1916. El Nevado de Famatina. Boletin de la Academia Nacional de Cienciasde Cdrdoba 21,100-182. Bowden, P., Batchelor, R. A., Chappell, B. W., Didier, J., and Lameyre, J., 1984. Petrological, geochemical and source criteria for classification of granitic rocks: A discussion. Physics of the
Earth andPlanetary Interiors 35,1-11. Campbell, I. H., and Turner, J. S., 1985. Turbulent mixing between fluidswith differentviscosities.Nature 313, 39-42. Campbell, I. H., and Turner, J. S., 1986. The influence of viscosity on fountains in magma chambers. Journal of Petrology 27, 1-30.
32
J. SAAVEDRA, E. PELLITERO-PASCUAL,J. N. ROSSI, and A.J. TOSELLI
Chappell, B. W., and White, A. J. R., 1974. Two contrasting granite types. PacificGeology 8,173-174. Didier, J., 1964. Etude P~trographique des Enclaves de Quelques Granites du Massif Central Franfais. Universit6 Clermont-Ferrand, Annales Facult6 de Sciences 23, 254 p. Garcia Sfinchez, A., and Saavedra, J., 1983. Dates analiticos sobre cuatro patrones geoquimicos de Salamanca (granitos s.1.) y t~cnicas utilizadas. Anales Centro de Edafologia y Biologia Aplicada, Salamanca 9, 321-331. Hausen, H., 1921. On the Lithology and Geologicul Structure of the Sierra de Umango Area, Province of La Rioja. Argentine Republic [Acta Academiae Aboensis, Mathematica et Physica, Vol. I, Part 4]. Abo Akademi, Finland, 147 p. Irvine, T. H., and Baragar, W. R., 1971. A guide to the chemical classification of the common volcanic recks. Canadian Journal of Earth Sciences 8, 524-548.
Pearce, J. A., Harris, N. B., and tindle, A. G., 1984. Trace elements discrimination diagrams for the tectonic interpretation of granitic rocks. JournaI of Petrology 2§, 956-983. Saavedra, J., and Medina, M. E., 1983. Esquema para el anfilisis r~pido de elementos mayores y menores en rocas. Revista de ia Asociacidn de Minerulogia, Petrologia y Sedimentologia (Buenos Aires) 14 (1-2), 7-14. Sparks, R. S. J., and Marshall, L. A., 1986. Thermal and mechanical constraints on mixing between mafic and silicicmagmas. Journal of Volcanology and Geothermal Research 29, 99-124. Toselli, A. J., Rossi de Toselli, J., Saavedra, J., Pellitero, E., and Medina, M. E., 1988a. Aspectos petrolSgicos y geoquimicos de los granitoides del enterno de Villa Castelli, Sierras Pampeanas Occidentales-Sistema de Famatina, Argentina. Actas, V Congreso Geoldgico Chileno, Santiago 3, I17-I28.
Jambon, A., 1980. Diffusion of water in granitic melt: An experimental study. Carnegie Institution of Washington, Year Book 79, 352-355.
Toselli, A. J., Saavedra, J., and Rossi de Toselli, J. N., 1988b. Estudio preliminar de granitoides del Sistema de Famatina, La Rioja, Argentina: Sus relaciones con otros granites Pampeanos. Reuista de la Asociacidn de Mineralogia, Petrologia y 8edimentoIogia(Buenos Aires), 19 (1-4), 1-12.
Kouchi, A., and Sunagawa, I., 1985. A model for mixing basaltic and dacitic magmas as deduced from experimental data. Contributions to Mineralogy and Petrology 89,17-23.
Turner, J. C., 1960. Estratigrafiadel tramo medio de la Sierra del Famatina y adyacencias (La Rioja). Boletinde ia Academia Nacional de Cieneiz~sde Cdrdoba 42, 77-126.
Koyaguchi, T., 1985. Magma mixing in a conduit. Journal of Vol. canology and GeothermaI Reseurch 25, 365-369.
Vernon, R. H., 1983. Restite,xenoliths and microgranitoid enclaves in granites. Journal and Proceedings of the Royal Societyof New South Wales 116, 77-103.
Koyaguchi, T., 1986. Evidence for two stage mixing in magmatic inclusions and tholeiitic lava domes on N ~ i m a Island, Japan. Journal of Geological and Geothermal Research 29, 71-98.
Watson, E. GB., and Jurewicz, S. R., 1984. Behavior of alkalies during diffusive interaction of granitic xenoliths with basaltic magma. Journal ofGeology 92, 121-131.
Lameyre, J., and Bowden, P., 1982. Plutonic rock type series: Discrimination ofvarions granitoid series and related rocks. Journal of Vocanology and GeothermaI Research 14,169-186.
White, A. J. R., and Chappell, B. W., 1977. Ultrametamorphism and granitoidgenesis. Tectonophysics 43, 7-22.
Lesher, C. A., 1986. Effects of silicate liquid composition on mineral-liquid element partitioning from Soret diffusion studies. JournaIofGeophysicaIResearch 91, 6123-6141.
Whitney, J. A., 1988. The origin of granite: The role and source of water in the evolution of granitic magmas. Bulletinof the Geo. logical Society of America 100, 1886-1897.
Maniar, P. D., and Cooke, G. A., 1987. Modal analyses of granifolds by quantitative X-ray diffraction. American Mineralogist
Williams, I. S., Compston, W., and Chappell, B. W., 1983. Zircon and monazite U-Pb systems and the histories of Y-type magmas, Berridale Batholith, Australia. Journal of Petrology 24, 76-97.
72,433-437. Mezger, K., Altherr, R., Okruseh, M., Henjes-Kunst, F., and Kreuzer, H., 1985. Genesis of acid/basicrock associations: A case study of the Kallithea intrusive complex, Samos, Greece. Contri. butions to Mineralogy and Petrology 90, 353-366.
Wyllie, P. J., 1971. The role of water in magma generation and initiationof diapiricuprise in the mantle. Journal of Geophysical Research 76,1328-1338. Wyllie, P. J.,Cox, K. G., and Bigger, G. M., 1962. The habit ofapatire in synthetic systems and igneous rocks. Journal of Petrology 3, 238-243.
0895-9811/92 $5.00+.00 © 1992 Pergamon Press Ltd & Earth Sciences & Resources Institute
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of northwestern Argentina J . SAAVEDRA .1, E . PELLITERO-PASCUAL 2, J . N . ROSSI s,
and A. J. TOSELLI s
ICSIC/Institute of Natural Resources and Agrobiology, Apartado 257, Salamanca, Spain; 2Department of Geology, University of Salamanca, Spain; SHigher Institute for Geological Correlations, National University of Tucum(m, Miguel Lillo 205, 4000 San Miguel de Tucum~in, Argentina
(ReceivedDecember 1989; Revision Accepted March 1990) A b l t r a c t - - T h e Cerro Toro granitoids range in composition from granite to gabbro and enclose abundant igneous and metamorphic enclaves that are assimilated in varying degree. The host rocks were intensely heated by intrusion of the Cerro Toro pluton, with local formation of anatexites. Geochronologic determinations point to an initial Sr ratio of 0.70967 + 0.00043 and an age of 456 + 14 Ma (Ordovician). The geochemical and field relationships between the acidic and basic members point to strong interactions between different unconsolidated magmas, with an extreme local fluidity that resulted in a gneiss-like aspect-although without cataclasis. Geochemical evidence indicates that the Cerro Toro granite was emplaced in a magmatic arc, at relatively deep crustal levels, under conditions close to anatexia. Regumen--Los granitoides de Cerro Toro presentan un amplio range de variaciSn, desde granitos a gabros, con abundantes enclaves tgneos y metamSrficos, m~s o menos asimilados. E1 encajante ha sufrido fuertemente la acci6n t6rmica de1 intrnsivo y aparece frecuentemente como una verdadera anatexita. Una determinaciSn geocronol6gica indica una relaci6n inicial de Sr de 0.70967 + 0.00043 y una edad de 456 + 14 Ma (Ordovtcice). Las relaciones geoquimicas y de campo entre los t6rminos ~cidos y b~isicos indican fuertes interrelaciones entre magmas distintos no consolidados, con fiuidaridad muy extrema que hace adquirir un nspecto gn6isico, pero sin cataclnsis. Evidencias geoqutmicas indican que los granitoides de Cerro Toro fueron emplazados en un arco magm~tico en niveles corticales relativamente profundos, en condiciones pr6ximas a la anatexia.
INTRODUCTION PLUTONS within the Andean basement consist of rocks of varying acidity, ranging from true gabbro to tonalite-granodiorite and even granite. They exhibit characteristics of magma interactions and contain enclaves of microgranitoids, of varying morphology, that axe more or less elongated and have both clearcut and diffuse borders. The mineralogy of the enclaves is compositionally related to that of the encompassing granitoid, which is generally more acidic. These are common features of plutons in many localities, as often noted in the literature. We studied one such characteristic pluton, the Cerro Toro granite--an excellent example of the complex process of mingling with some true mixing. This pluton, in which features typical of magma interaction and assimilation are spectacular, is located in the pre-Mesozoic basement near Villa Castelli (in the western zone ofLa Rioja Province). The Cerro Toro granite was chosen for its accessibility and good exposure and for the broad lithologic variations it displays (Fig. 1). The study area is located on the western flank of the Famatina System (Bodenbender, 1916), where the metamorphic basement is comparable to that of the western Sierras Pampeanas (schists, amphibolites, gneisses, marbles, and migmatites). The Cerro *Address all correspondence and reprint requests to: Dr. Julio Saavedra: telephone [34] (23) 219606; telefax [34] (23) 219609.
Toro zone is unlike most of the Famatina System, the metamorphic rocks of which are of low metamorphic grade, as represented by the Negro Peinado Formation (Turner, 1960). Hausen (1921) referred to processes whereby blocks of schist and amphibolite were assimilated by granitic magma with the result that the magma became tonalitic in composition. Toselli et al. (1988a) summarized previously published data, presented new petrological and geochemical data, and considered possible links with other plutons in the area. According to them, biotite-amphibolite tonalites (from syn- to late tectonic) that are metaluminous to weakly peraluminous predominate and intrude micaceous schists and migmatitic gneisses alternating with ortho-amphibolites--typical of a magmatic arc associated with a continental border active during the early Paleozoic. Enclaves within the Cerro Torro granite are dispersed and appear in the form of synplutonic mafic dikes, dismemberment of which resulted in hybridization accompanied by remobilization and assimilation of crustal material. The outcrops studied exhibit abundant mafic metamorphic enclaves that vary considerably in size and stand out from the granitic landscape (Fig. 2). The metamorphic host rocks show frequent intercalations of amphibolite~ essentially composed of amphibole and plagioclase, with very little or no quartz and with a common ortho-derived character. This ensemble shows the effects of a regional metamorphism that led to the development of micas and a few aluminum silicates (andalusite and sillimanite). Numerous leucocratic segregations 21
22
J. SAAVEDRA, E. PELLITERO-PASCUAL, J. N. RossI, and A. J. TOSELLI
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Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of N W
Argentina
23
Fig. 2. Western flank of the Sierra de Famatina seen from Villa Castelli. Darker masses are more ferromagnesian rocks encompassed withindifferent,more leucocraticgranitoids.The drawing was made from a panoramicphotograph.
and injections are apparent at the contact with the granitoids, conferring on them the aspect of a true anatexite (Fig. 3). The granitoids commonly show a degree of foliation but lack evidence of cataclasis or solid-state deformation (except for a few regional faults). It is difficult to establish a sequence of events because of the paucity of fossils in low-grade metamorphic rocks of the eastern part of the mountain system--ichno-
fossils such as Nereites and algae that can be assigned a late Precambrian-Early Cambrian age (Ace5olaza, pers. commun., 1988)--and because the age of the metamorphic host rocks of the pluton.is unknown. A Rb-Sr isochron obtained for five samples of the Cerro Toro granite yielded an initial 87Sr/SeSr ratio of 70967__.0.00043 and an age of 456+_14 M a (Teledyne Isotopes, USA, 1989). This age is the basis for suggesting Ordovician plu-
Fig.3. Metamorphic hostrockofthe CerroToro granite.
24
J. SAAVEDRA,F~.PELLITERO-PASCUAL,J. N. ROSSI, and A.J. TOSELLI
tonization and associated host-rock metamorphism, in agreement with the regional plutonic scheme proposed by Toselli et al. (1988b). ANALYSES AND RESULTS Chemical analyses were carried out in the laboratories of the Higher Institute for Geological Correlations, National University of Tucum~n, and the Institute of Natural Resources and Agrobiology, CSIC, in Salamanca, using techniques described by Saavedra and Medina (1983) and Garcfa S~uchez and Saavedra (1983). Oxides were determined by atomic absorption spectrophotometrywwith the exception of SiO2 and P205, which were evaluated by spectrophotometry of molybdenum and vanado-molybdenum complexes, respectively. Trace elements were determined by X-ray fluorescence. Modal analyses of quartz and feldspars of coarse- and fine-grained rocks were performed directly on thin sections; those with greater granularity were subjected to the X-ray diffraction technique reported by Maniar and Cooke (1987).
Characteristics of Outcrops Two principal groups of plutonic rocks were observed in the field. The most abundant is composed mainly of tenalites, together with granodiorites and granites (Fig. 4). The second group (not included in Fig. 4) comprises more basic rocks such as gabbros (some of them quartz-bearing and others dioritic) and diorites; these rocks display complex relationships described below. Also present are many xenolithic enclaves of the host rock in different states of transition ranging from schists with more-or-less leucocratic veins of segregation, to metasedimentary mafic remains that are not clearly delimited from the encompassing granite in contrast to amphibolites, all of which have clear-cut borders. The contacts in the Cerro Toro grAnlte vary; those between the mafic and felsic members are as likely to be sharp as they are to be diffuse. Most of the grAnitoids have a coarse grain size and are equigranular, with varying relative amounts of the ferromagnesian minerals biotite and hornblende. Occasionally, particularly in proximity to the more basic gr~nitoids, they give way to facies with different granularities that are irregular and exhibit gradational transitions. Schlieren are abundant, beginning irregularly in the heart of the leucocratic facies and ending gradually with diffuse borders in femic enclaves and masses (Fig. 5). Moreover, there is usually a direct relationship between the dimensions of such femic masses and the amplitude of the aureole richest in ferromagnesian minerals separating both lithologies (Fig. 5). Sometimes, several such zones are visible, each slightly more mafic than its predecessor and showing gradational or well defined contacts, although always
smaller in size. This kind of sequence is observed both in proximity to basic rock and in zones distant from it. Unequal veins and masses of leucocratic material that cut the mafic parts, or are in diffuse contact with them, are also typical (Fig. 5). On the megascopic scale, some of the basic members resemble well delimited dikes inside the more acidic granitoids, although their components are unequal (in basicity, in grain size, and sometimes by the presence of metasedimentary enclaves separated by more or less defined limits) and may be bordered by small fine-grained leucocratic masses. All this is typical of a process of mingling by which granites evolved from gabbroids. Despite the presence of zones of homogeneous mixing between both extremes, the degree of such homogenization cannot be ascertained in the field. The most abundant enclaves are those with a typically igneous composition and texture; they appear in all environments, including late aplitic dikes that encompass round mafic masses. The host granitoid is always more acidic and finer grained than the enclaves, even though the mineralogy of the enclaves may be comparable or identical to that of the intrusive body. Although diffuse borders are common, such as those shown in Fig. 5, well defined limits usually predominate, with convex forms toward the host granitoid and with pillows and lobulate or crenulate borders. The following were also observed: mafic enclaves within masses of lower acidity; double enclaves; irregular masses and well delimited contacts of the acidic granitoid with the more basic members; aureoles of quartz and alkaline feldspar around inclusions of gabbros/diorites; and mafic edges in the acidic granitoids. Although not abundant in the tonalites, feldspar phenocrysts exhibit a complete transition to xenoliths in rocks with lower basicity (Fig. 5). In a very few exposures, phenocrysts have been observed on both sides of the tonalitic-basic enclave. A significant observation is that the gabbroids do not have igneous enclaves and are homogeneous in appearance. Quantitatively, the number of igneous enclaves is generally inversely related to the degree of acidity of the host granitoid. Despite considerable variation, textures are unequivocally igneous and orthomagmatic. The orientations of the mineralogic constituents of the granitoids and the most mafic members, enclaves or not, are clear and predominant, with no evidence of contemporary cataclasis. Variable phenomena of solid-state cataclasis are associated with later tectonic events, which may be generalized to a regional scale, although details vary considerable (Fig. 6). At mesoscopic scale, the main tonalitic body usually shows a certain orientation that coincides with the principal dimension of the enclaves. The enclaves are often planar in shape and are mostly finegrained. Prominent angular and polyhedral xenoliths are found in this kind of inclusion, but although showing unequivocal features of amphibolites, they lack the above-mentioned external orientation. This
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of N W Argentina
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Fig. 5. Schematic detail (10 cm scale), of the relationship between basic and acidic magmas at El Infernillo Canyon (Cerro Toro), northeast of Villa Castelli. Mixing (hybridization) is seen in proximity to the more basic part; schlieren and acidic veins cut the basic materials without passing to the more leucocratic part.
26
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suggests a morphologic differentiation between the igneous enclaves (autoliths) and the metamorphic enclaves (xenoliths). Such autoliths appear throughout the pluton, although they are concentrated and elongate (1:40 or more) in certain places, with a fluid aspect and with more or less delimited borders and thus could represent disrupted syn-plutonic dikes.
The ensemble (Fig. 6) has all the characteristics of a zone of mobilization of a heterogeneous magma, with a certain occasional and partial mixing of components and concentrations of components locally elongated in the same direction, like passages or corridors of enclaves; these could also represent disrupted syn-plutonic dikes.
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of N W Argentina
Petrography and Mineralogy The climate of this region is arid; thus, the outcrops are good and sampling is very satisfactory in the main canyons. The large number of excellent observation points permitted adequate sampling for the study of every magmatic process. The general predominance of tonalites shown in Fig. 4 corresponds to members ranging from biotitic to amphibolic. These rocks are essentially equigranular, with coarse to medium-sized crystals--the latter occurring in varieties richer in mafic materials. Despite the predominance of a certain fluid orientation of mica, amphibole, and some quartz crystals, it is sometimes possible to observe in the elongations of the minerals directions at an angle to the flow structure, with no breakage features. Growth seems to have been free and is very reminiscent of the rotation of crystals in a medium with a certain degree of viscosity. Except in the gabbroid members, the plagioclases are fairly uniform (andesines in tonalites,oligoclaseandesine in the more acid granitoids), with minor zoning; some exhibit a partially corroded nucleus with a fresher crown. The habit is generally euhedral to subhedral, with complex twinning and occasional abundant inclusions. The potassium feldspar is usually microcline, with or without perthites and with an albitic border on the more leucocratic facies. Often, a rim of myrmekite encloses the plagioclase. However, it is not uncommon to observe two morphologically different generations--the oldest without the quadrangular twin typical of microcline. Biotite is the most abundant ferromagnesian mineral, followed by hornblende. It is often partially altered to chlorite,whether or not potassium feldspar is involved, and is deformed to a certain degree. Although inclusions of opaque minerals, zircon, aparite, and epidote, are not common in biotite, some crystals show lineations of small grains of iron oxides with the same aspect as those found in the biotite of the enclosing schist. Muscovite is scanty and appears only in the leucocratic facies. Hornblende, sometimes the dominant mafic mineral, is occasionally altered to mixtures of biotite, quartz, and epidote, with or without titanite. This usually takes the form of layers of biotite within hornblende prisms. Less commonly, hornblende crystals exhibit lineations of iron oxides along exfoliation planes, with a morphology identical to that exhibited by some of the biotites;presumably, these would be of relictmetamorphic origin. The development of hornblende after biotite,by inversion of the normal crystallizationsequence, is a general feature. W h e n the hornblende is blastic, it essentially includes titanite, opaques, and biotite. Some hornblende has a heterogeneous aspect, with differing colors due to differing M g content, and the appearance of having been derived from olivine or pyroxene, but only in the more basic members of the pluton.
27
Epidote, both pistacite and clinozoicite,often exhibits features of secondary minerals and is also idiomorphic; it usually has inclusions of nuclei of allanite and opaque minerals and is more abundant in the basic members. Allanite is also very common; it is idiomorphic and its crystals are zoned. Zircon and apatite crystals are elongate (1:10 or greater), as is typical of faster cooling (Wyllie et al., 1962; Didier, 1964; Williams et al.,1983), and are also present in short normal prisms. Titanite appears in isolated grains, developing over epidote, biotite, and plagioclase and forming crowns around a nucleus of ilmenite, with clear evidence of interaction of the mineral grain with the surrounding milieu; this is especially evident in the less leucocraticfacies. Sillimanite and cordieritewere found in only two of the 80 samples studied; these samples, which contained no andalusite, came from a complex zone with an abundance of autoliths, metamorphic xenoliths, and very evident mingling. Sillimanite, as fibrolite or in prisms, is associated with mica. The cordierite is partially altered and, together with the sillimanire,is included within feldspars or in grains. The appearance of the host granitoid is very similar to that of the other igneous facies,with mafic, intermediate, or acidic varieties present in different parts of the pluton.
Geochemistry The geochemistry of these plutonic rocks is complex. On an A F M diagram (Irvine and Baragar, 1971), gabbroid samples plot in the tholeiitic field and diorites in the calc-alkaline field (Fig. 7). This contrast is also clear on an Fe/Mg variation diagram (Fig. 8). The diagram shows that only the less magnesian tholeiiticmembers and the most ferromagnesian members of the calc-alkaline trend overlap. Considering both trends, it is thus highly significant that only the extremes of highest and lowest M g content correspond to facies exhibiting the least evidence of mingling, whereas this phenomenon is typical and pronounced in the trends of the less magnesian gabbroids and, above all,in the calc-alkaline rocks with higher Fe and M g values. The lack of discontinuity therefore suggests chemical links. Relative variations in element pairs of the rocks corresponding to the tholeiitic and calc-alkaline groups are shown in the Harker diagrams (Fig. 9), which display the following: •
The gabbroids show unique variations in both major elements and in minor and trace elements. The most acidic rocks, those with a calc-alkaline trend, show no geochemical links with the gabbroids.
•
In general, there is no direct and significant relationship between the element pairs.
J. SAAVEDRA, E. PELLITERO-PASCUAL,J. N. ROSSI, and A.J. TOSELLI
28
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F i g . 8. V a r i a t i o n i n F e 2 0 3 (T) a n d M g O c o n t e n t . C i r c l e s correspond to tenalites, granodiorites, and granites. Points corresp o n d to m o r e b a s i c r o c k s .
•
Magmatic Heterogeneity
The dispersions in both types of rocks are noteworthy in most cases. I N T E R P R E T A T I O N AND DISCUSSION
Intrusion of the Cerro Toro granite must have occurred at an appreciable depth. The schists of the intrusive contact (see Fig. 3), in which quartz-feldspar segregations are abundant near the granitoid, suggest temperatures and pressures not very different from those of anatexia. The very generalized presence of muscovite and scarcer sillimanite or cordierite reinforces this impression. There is a complete grading between xenolithic enclaves, from those that are identical to the metamorphic rocks of the immediate contact to those that are confused with the granitic host rock (advanced assimilation), the case in which the main change must have been temperature. This is only possible in domains of higher pressure where muscovite, always present, is stable. In the granitoid, the biotite and muscovite of the assimilated schists were subjected to new conditions of the milieu, implying a different kind of stability. However, amphibolites intercalated among the schists were not assimilated; even the most extreme cases show amphibolic xenoliths without quartzmicaceous material. Thus, in materials with a gabbroic composition, upper temperature limits cannot have been above the melting point of hornblende (Wyllie, 1971), which may reach more than 1000°C. In summary, as pointed out by Whitney (1988), the destabilization of muscovite is followed by that of biotite to give pyroxene, whose possible relicts, mentioned before, are now converted into amphibole. These occur in the 750-850°C temperature range, depending on chemical composition, indicating emplacement at fairly high temperatures.
Mafic enclaves are common in granitoids, and their origin has been the subject of numerous studies (e.g., Didier, 1987; Vernon, 1983; among others). In general, two groups of enclaves are recognized and classified according to whether they exhibit clear and unequivocal metamorphic features, such as those referred to above, or whether they have typically igneous textures. Although a residual origin has been proposed for typically igneous microgranltoids in other regions (Chappell and White, 1974: demixing of restitic rocks from a melt upon ascension of the granitic mush), a model which assumes variable amounts of the same melt (White and Chappell, 1977) is not applicable to the Cerro Torro granite. The reasons are as follows: Although microscopic observations, as reported above, clearly point to the existence of some relict metamorphic minerals (biotite and others), the textures, the morphologies of the enclaves, the presence of double enclaves, relationships with the host rock, etc., also clearly suggest that all were once in a plastic state--the most mafic rocks dispersing in the form of globules into the more acidic rock, accompanied by a certain mingling. The restitic mechanism, a phenomenon that has not been observed, requires linear variations among pairs of elements (Fig. 9). The separation of melt and restite that must have occurred under similar conditions of viscosity, temperature, and composition (not very variable factors in this case) is not supported by the experimental data, which indicate exactly the opposite (a more favorable hybridization) as
Magmatic evolution of the Cerro Toro granite, a complex Ordovician pluton of NW Argentina
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SAAVEDRA,E. PELLITERO-PASCUAL,J. N. RossI, and A. J. TOSELLI
the efficacy of diffusion processes at the relatively high temperature deduced here and by the presence of volatiles, as attested to by significant amounts of A n alternative hypothesis for the origin of the hydroxylated minerals. This chemical exchange is enclaves is the immiscibility of acidic and basic com- well documented (Alibert and Carr6n, 1980; Lesher, ponents during cooling of an intermediate melt, as 1986; Jambon, 1980; Watson and Jurewicz, 1984; and proposed by Bender et al. (1982) and Mezger et al. many others) and is manifested by the following: (1985). At Cerro Torro, the morphologic field evi- replacement of biotite by hornblende, rims of sphene dence does not support such an origin: pillow-shaped around grains of ilmenite, relicts of previous mag(even multiple) enclaves are present, and there are nesian minerals (now hornblende in its different considerable variations in texture and limits with varieties), and biotization of amphibole (much rarer the host granite. This is confirmed by the broad than the inverse phenomenon)--all of which reflect range of quantitative mineralogic variations in the the migration of K, Ca, Ti, and other elements. maflc material and hence of the chemistry. Despite such clear signs of a previous mixing, the It thus seems quite clear that different m a g m a s results were of limited significance, for the history of interacted. However, the disproportional chemical mingling of different magmas (many of them of not variations shown in Fig. 9 indicate that the variety of very variable composition) is patent. lithologic types does not derive from a simple mixAccordingly, cooling at temperatures lower than ture. Furthermore, the heterogeneity of the enclaves those at which the process could have operated must suggests several magmatic pulses, and the minera- have occurred relatively quickly. The different delogic similarity of the enclaves and host granitoid grees of turbulence and the magmatic fluidities suggests that crystallization occurred under the indicate the influence of the higher temperature same conditions in both, after the mafic m a g m a had mafic members, which prevented formation of an been incorporated and had been dispersed into the ordered, concentric zoning to be reached. As a result, felsic magma. The melt content in the mafic m a g m a fractionated crystallization, if in fact it did take must have been very high, judging from the plasti- place, must have been relatively minor, as also sugcity implied by very elongated enclaves (see Fig. 6), gested in Fig. 9, which does not give the regular and which confer the aspect of a gneiss, and the numer- dominant variations among pairs of elements typical ous diffuse and irregular contacts between felsicand of this process but rather shows a dispersion reflecmafic rocks. A fluid phase was not quite reached, ting several mechanisms of emplacement. however, as hybridization is not extensive. According to Sparks and Marshall (1986), this can only occur when m a g m a s remain liquid after equili- Position in the Geotectonic Environment brating thermally. The frequency of pillowed enclaves, with pointed The dominant trend of the granitoids, as shown borders convex toward the host granite, and of acicu- in Fig. 4, indicates a low K content typical of the lax minerals in the basic members suggests some early internal stage of active continental borders cooling. O n the other hand, the absence of chilled (Lameyre and Bowden, 1982), although with a margins, together with the high percentage of melt significantbut subordinate potassic component that in the mafic magmas, suggests relatively high tem- prevents the differentiation of I and S types. In peratures. Under such circumstances, the viscosity principle, this is consistent with the evidence cited of of the acid host magma, already decreased by injec- assimilation of the host rock: a high STSr/SeSr ratio tion of the basic magma, would be lowered and without a general aluminosity, the presence of magapproach that of the basic m a g m a (in turn increased mas with a tholeiitic and calc-alkaline trend, high during its dispersion in the acid magma), and the temperatures, and evidence of superposition of diffusion rate (exponentially controlled by tempera- several mechanisms that would have been acting ture) would increase. All this favors mixing (Camp- simultaneously (see Fig. 9). bell and Turner, 1985), evidence of which is found in The distribution of elements considered of most the form of acicular and prismatic apatites occurring significance (Pearce et al., 1984) is illustrated in Figs. together, mineral relicts, crowns of ferromagnesian 10 and 11. The data displayed in Fig. 10 rule out the minerals around quartz, and other signs of dis- possibility of intraplate granites or granites from an equilibrium previously described. oceanic ridge (geologically improbable). Figure 11 The experiments of Koyagushi (1985, 1986) and indicates that the granites are associated with an Kouchi and Sunagawa (1985) reveal the importance arc, with no possibility of collision, because--as menof additional factors that strongly affect homogeniza- tioned above--they have been intruded in rocks in tion phenomena. Examples of these are turbulence which an enrichment in Rb by diffusion is feasible, originated by phenocrysts, mechanical agitation, together with others in which this phenomenon has limited cooling of the mafic magma in the felsic mag- not been observed, thus constituting a homogeneous ma, and convection in the magma chamber. Figures group. Viewed in this light, the Rb content is clearly 5 and 6 illustrate the importance here of turbulent significant. dynamic phenomena within the granitic fluids. Interaction between the magmas is also favored by most likely (Campbell and Turner, 1986; Kouchi and Sunagawa, 1985).
Magmatic evolution ofthe Cerro Toro granite,a complex Ordovician pluton ofN W Argentina
31
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-
CONCLUSIONS The Cerro Toro granite, a constituent of the Andean basement, is a complex Ordovician pluton that was emplaced in a magmatic arc at relatively deep crustal levelsunder conditions close to anatexia and influenced by the presence of mafic magma. Manifestations of more or less complex mixing of m a g m a s are spectacular. Also manifest are features indicative of convection and turbulence, as well as other evidence of the fluid dynamics that governed the chemistry of the batholith prior to its consolidation.
Acknowledgments--The authors would like to thank Drs. C. E. Macellari, H. Miller, and W. Pitcher for their invaluable comments on the style and content of this paper and for their suggestions concerning the development of the scientific ideas expressed. This work was a contribution to IGCP Project 249: Andean Magmatism and Its Tectonic Setting (IUGS-UNESCO).
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0
®
0
lO
I
lO
!
I
I
I
I
5O
Y+ Nb,
ppm
Fig. 11. Variation in (Y+Nb)-Rb (Pearce et al.,1984). Circles correspond to tonalitos,granodiorites,and granites. Points correspond to more basic rocks.
Bender, J. F., Hanson, B. H., and Bence, A. E., 1982. The Cortland Complex: Evidence for large-scale liquid immiscibility involving granodiorite and diorite magmas. Earth and Planetary Science Letters 58, 330-344. Bodenbender, G., 1916. El Nevado de Famatina. Boletin de la Academia Nacional de Cienciasde Cdrdoba 21,100-182. Bowden, P., Batchelor, R. A., Chappell, B. W., Didier, J., and Lameyre, J., 1984. Petrological, geochemical and source criteria for classification of granitic rocks: A discussion. Physics of the
Earth andPlanetary Interiors 35,1-11. Campbell, I. H., and Turner, J. S., 1985. Turbulent mixing between fluidswith differentviscosities.Nature 313, 39-42. Campbell, I. H., and Turner, J. S., 1986. The influence of viscosity on fountains in magma chambers. Journal of Petrology 27, 1-30.
32
J. SAAVEDRA, E. PELLITERO-PASCUAL,J. N. ROSSI, and A.J. TOSELLI
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Wyllie, P. J., 1971. The role of water in magma generation and initiationof diapiricuprise in the mantle. Journal of Geophysical Research 76,1328-1338. Wyllie, P. J.,Cox, K. G., and Bigger, G. M., 1962. The habit ofapatire in synthetic systems and igneous rocks. Journal of Petrology 3, 238-243.