Nearly circular plutons emplaced by stoping at shallow crustal levels, Cerro Aspero batholith, Sierras Pampeanas de Córdoba, Argentina

Journal of South American Earth Sciences 15 (2002) 251±265

www.elsevier.com/locate/jsames

Nearly circular plutons emplaced by stoping at shallow crustal levels, Cerro Aspero batholith, Sierras Pampeanas de CoÂrdoba, Argentina L.P. Pinotti a,b,*, J.E. Coniglio b, A.M. Esparza b, F.J. D'Eramo a,b, E.J. LlambõÂas a,c a

Consejo Nacional de Investigaciones Cientõ®cas y Tecnicas (CONICET), Rivadavia 1917, CP C1083AAJ, Buenos Aires, Argentina b Departamento de GeologõÂa, Universidad Nacional de RõÂo Cuarto, Agencia Postal No. 3, 5800 RõÂo Cuarto, CoÂrdoba, Argentina c Centro de Investigaciones GeoloÂgicas (CIG), Calle 1 No. 644, 1900 La Plata, Argentina Received 1 September 2001; accepted 1 December 2001

Abstract The Cerro Aspero batholith (CAB) (440 km 2) is one of many discordant granites emplaced during the Middle to Late Devonian, marking the closure of igneous activity in the southern part of the Sierras Pampeanas de CoÂrdoba, Argentina. The main feature of the CAB is the conspicuous circular shape of some of its plutons and internal structures, which were developed entirely under pressures lower than 2 kb. The aim of this work is to study the processes responsible for the emplacement of these plutons. The CAB is composed of a succession of three major plutons aligned in a NNW±SSE trend that intrude mainly into mylonites corresponding to an Early Paleozoic shear zone. The emplacement of the plutons was controlled by a set of early master fractures, with magma ascent by means of fracture propagation. The space for magma was created by a combination of tectonically created cavities, internal magma-related buoyancy, and heating and thermal fracture of the host rocks. Field and structural evidence suggests that stoping mechanisms were the most effective for the ®nal emplacement of magma. Thermal fracture and stoping were favored by the thermal contrast between granite and host rock and by the magma viscosity as reduced by the presence of ¯uorite. The apparent circular shape of the plutons is polygonal in detail and was developed from several linear and curviplanar segments as a result of brittle fracturing of the host rocks, as well as by changes in the stress ®eld, from vertical to horizontal, related to magma arrival at shallow crustal levels. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Plutons; Cerro Aspero batholith; Magma

Resumen El batolito Cerro Aspero (440 km 2) es uno de los principales cuerpos graniticos discordantes del magmatismo DevoÂnico de las Sierras Pampeanas de CoÂrdoba, Argentina. La caracterõÂstica principal es la conspicua circularidad de sus plutones y estructuras internas, desarrolladas completamente bajo condiciones de presioÂn menores a 2 kb. El objetivo de este trabajo fue estudiar los procesos responsables del emplazamiento de estos plutones. El batolito esta constituido por una sucesioÂn de tres plutones principales, alineados en sentido NNW±SSE, que intruyen principalmente milonitas en una faja de deformacioÂn de edad Paleozoica inferior. El emplazamiento de los plutones fue controlado por un conjunto de fracturas maestras que posibilitaron el ascenso del magma mediante propagacioÂn de fracturas. El espacio fue generado tectoÂnicamente, en combinacioÂn con la fuerza boyante del magma y el fracturamiento teÂrmico. Las evidencias de campo y estructurales sugieren que los mecanismos de stoping predominaron durante el emplazamiento del magma. La fracturacioÂn teÂrmica y stoping fueron favorecidos por el alto contraste teÂrmico entre granito/roca de caja y una viscosidad del magma disminuida por la presencia de ¯uÂor. La forma circular de los plutones es poligonal en detalle y se desarrollo a partir de varios segmentos lineales y curviplanares como resultado de la fracturation fraÂgil de las rocas encajantes y de cambios en el campo de esfuerzos, de vertical a horizontal, relacionados con el emplazamiento del magma en la corteza superior. q 2002 Elsevier Science Ltd. All rights reserved. Keywords: Plutones; Cerro Aspero batolito; magma

1. Introduction

* Corresponding author. Address: Departamento de GeologõÂa, Universidad Nacional de RõÂo Cuarto, Agencia Postal No. 3, 5800 RõÂo Cuarto, CoÂrdoba, Argentina. Tel.: 154-358-467-6198; fax: 154-358-468-0280. E-mail address: [email protected] (L.P. Pinotti).

The most extensive granitic bodies of the Sierras de CoÂrdoba and San Luis (eastern Sierras Pampeanas of Argentina) are large batholiths that were emplaced after metamorphic and deformational episodes that took place from Late Precambrian until Late Silurian times (Bonorino, 1950;

0895-9811/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(02)00033-0

252

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

Fig. 1. Simpli®ed geologic map of Sierras de CoÂrdoba and San Luis, showing the location of the major postorogenic granitic intrusions (Modi®ed from Pinotti et al., 1996). AB: Achala batholith; CAB: Cerro Aspero batholith; IHP: Inti-Huasi plutoÂn; LNP: Los Nogales plutoÂn; CBP: Cerro Blanco plutoÂn; LCH±PCB: Las Chacras±Piedras Coloradas batholith; RB: Renca batholith; and EMP: El Morro pluton.

Gordillo and Lencinas, 1979; Ortiz SuaÂrez et al., 1992; Rapela et al., 1998). This magmatism is considered signi®cant not only because of its large volume, but also because it represents the closure of the Paleozoic magmatic activity in the Sierras Pampeanas of Argentina. Most batholiths have conspicuous discordant relationships and are made up of several plutons characterized by nearly circular geometry. In addition, they have associated thermal contact aureoles that have not been deformed, with the exception of rare shear belts. Excellent examples of these batholiths are Achala (AB) and Cerro Aspero (CAB) in Sierras de CoÂrdoba and Las Chacras±Piedras Coloradas (LCH±PCB) and Renca (RB) in Sierras de San Luis (Fig. 1). Because of these features, the plutons have been

described as postorogenic relative to the metamorphic peaks in the Cambrian for the Sierras de CoÂrdoba (Pampean cycle) and in the Ordovician for the Sierras de San Luis (Famatinian cycle) (AcenÄolaza and Toselli, 1976; LlambõÂas et al., 1984; GonzaÂlez Bonorino and LlambõÂas, 1996; Rapela et al., 1999). For the Sierras de CoÂrdoba, the postorogenic batholiths have been related to an igneous cycle called Achaliano (Stuart-Smith et al., 1999), which would be unrelated to previous orogenies. Therefore, though these granites frequently are described as postorogenic, postcinematic, or Famatinian, an unambiguous geological approach for their denomination and regional correlation does not exist. Despite the great size of these batholiths and their importance in the structure of the Paleozoic basement,

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

253

Fig. 2. Geological map of the Cerro Aspero batholith (CAB) and surrounding host rocks. ACP: Alpa Corral plutoÂn; ETP: El Talita plutoÂn; and LCP: Los Cerros pluton.

254

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

their internal anatomy, relations with the regional structures, and mechanisms of emplacement have received scant attention. This study describes nearly circular plutons emplaced at shallow crustal levels on the basis of Landsat TM imagery, standard aerial photographs, and ®eld and petrographic data. The term emplacement is used here to refer to the processes that occur after magma has ascended during the growth, assembly, and crystallization of the pluton. The CAB was selected as a case study because it provides an excellent example of a granitic intrusive complex composed of several nearly circular plutons. The epizonal character of the CAB emplacement was detailed by Coniglio and Esparza (1988) and Pinotti et al. (1996), and the geometry of the plutons was described by Pinotti (1998). In Sierras de San Luis, one of the best known batholith examples is the LCH±PCB, for which Brogioni (1992) concludes that a close relationship exists between the deformation of the host rocks and the internal structures of the circular plutons and suggests ballooning as a main mechanism of emplacement. Most studies related to circular pluton emplacement generally invoke diapiric or ballooning mechanisms as plausible explanations (Marsh, 1982; Weinberg and Podladchikov, 1994; Bateman, 1985; Ramsay, 1989; Hutton, 1997). However, in the CAB, though the plutons have a nearly circular section, such mechanisms do not seem viable. The circular shape of the plutons is apparent, and it is more appropriate to describe it as a polygonal shape that was generated under the brittle conditions of the host rocks. 2. Geological setting The Sierras de CoÂrdoba are mainly composed of Late Precambrian and Paleozoic igneous±metamorphic rocks that are locally covered by continental deposits and ma®c volcanic rocks of Cretaceous age (Gordillo and Lencinas, 1979; Schmidt et al., 1995). The Sierras Pampeanas were uplifted as north±south-trending blocks as a result of the Andean orogeny (Jordan and Allmendinger, 1986). The CAB outcrops in the Sierra de Comechingones (Fig. 1) constitute much of the central and southern Sierra de CoÂrdoba, which is a major morphological unit of the Sierras Pampeanas (Dalla Salda, 1987). The Sierra de Comechingones consists of medium- to high-grade amphibolite to granulite facies metasedimentary rocks, among which aluminous greywackes and limestones are the most widespread protoliths (Otamendi et al., 1999). An Early Cambrian age was suggested for the metamorphic peak (Rapela et al., 1995; Stuart-Smith et al., 1996). In the Sierras de CoÂrdoba, the postorogenic granites present several common geological characteristics, including large-volume granite intrusions, development of

porphyritic texture with large K-feldspar crystals, lithological homogeneity, a conspicuous circular shape of the plutons, and crystallization of magma at high levels within the upper crust. The postorogenic batholiths were interpreted, from the geochemical signature, as intraplate granites (Lira and Kirschbaum, 1990) formed in an inner-arc magmatic setting (Rapela et al., 1992). 3. Metamorphic host rocks of the CAB The host rocks of the CAB are composed of migmatites and gneisses, intercalated with ma®c and ultrama®c bodies, marbles, and amphibolites recrystallized to upper amphibolite facies. These rocks have been locally affected by retrograde metamorphism within shear zones. Four phases of deformation were recognized in the Sierra de Comechingones (Martino et al., 1995). The D1 and D2 phases either predated or were coeval with the thermal peak, and they reached medium- to high-grade in the host rock in the CAB area. The D3 and D4 phases occurred after the thermal peak and produced the dominant planar fabrics in the metamorphic rocks. Associated with D2 and D3, conspicuous S2 and S3 foliations were progressively developed and affected the entire region (Guereschi and Martino, 1998). The S2 foliation is a composite planar fabric de®ned by compositional banding in gneisses and migmatites. In the studied area, S3 foliation is the more penetrative and produces a pervasive transposition foliation, which develops subparallel to S2 foliation. Both S2 and S3 foliations de®ne a NNW-trending planar fabric. The late D4 deformation phase is characterized by an important event of dynamic retrograde metamorphism that produced a submeridional mylonitic foliation (S4) with a strong reworking of the S2 ±S3 fabrics. D4 occurs mainly along the Guacha Corral shear zone (GCSZ) (Fig. 1). In the studied area, the GCSZ has a N±S to NNE±SSW trend, dipping at high angle to the east (Pinotti et al., 1997; Fagiano et al., 1997), and is located on the eastern border of the CAB. In some sectors, the GCSZ is composed of protomylonites to utramylonites in which kinematic indicators suggest a ductile reverse shear with westward tectonic transport (Fagiano et al., 1997). At the eastern limit, the GCSZ is characterized by a conspicuous narrow belt (,2 km) of phyllonites, as de®ned by the presence of low-grade mineral assemblages (Stuart-Smith and Skirrow, 1997). In the CAB area, the GCSZ strongly affected the structural evolution of the Sierra de Comechingones. Thus, east of this shear zone, the NNW S2 ±S3 foliations predominate, whereas toward the west, a penetrative N±S to NNE±SSW foliation predominates (Fig. 2). The CAB truncates all these structures, including the GCSZ, which indicates that it predates the intrusion, which began in the Early Devonian.

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

255

Fig. 3. Modal composition of the Cerro Aspero batholith in terms of quartz (Q), alkali feldspar (A), and plagioclase (P). Based on point counts in thin sections combined with modal determinations in slab and outcrops. The same abbreviations for plutons are used as in Fig. 2.

4. The CAB

4.1. Alpa Corral pluton

The CAB is a composite intrusion of 440 km 2 constituted by three major overlapping plutons, arranged in a NNW±SSE trend, named Alpa Corral, El Talita, and Los Cerros (ACP, ETP, and LCP, respectively) (Fig. 2). The emplacement of the CAB is interpreted to have taken place during the Middle to Late Devonian. A Rb/Sr age of 369 ^ 9 Ma and a K/Ar age of 389 ^ 19 Ma probably re¯ect cooling ages rather than the timing of magma crystallization (Pinotti, 1998). On the basis of Landsat TM imagery analysis and ®eld and petrographic data, these granitic plutons can be subdivided into internal units. The internal units possess a narrow compositional variation that ranges from biotite monzogranite to leucogranodiorite. The modal composition is given in diagram QAP (Fig. 3), and the main geological and mineralogical characteristics of the plutons are summarized in Tables 1 and 2. The dominant rock type is a porphyritic to coarsegrained equigranular biotite monzogranite characterized by large crystals of microcline that are interpreted as phenocrysts (Pinotti et al., 1992). The granites of the CAB belong to a high K, calc alkaline series, with high amounts of large-ion lithophile elements (LILE), as well as P and Ti (Porta, 1992; Pinotti, 1998). Several signi®cant quartz±wolframite (molybdenite), quartz, and ¯uorite vein deposits occur in the CAB area, spatially and/or temporally related to the granitic rocks (FernaÂndez Lima et al., 1963; GonzaÂlez DõÂaz, 1972; Coniglio et al., 2000). As a result of this profuse hydrothermal activity, pervasive hydrothermal alterations frequently can be observed in the granites and metamorphic rocks.

Geological mapping and petrographic studies of the ACP have been carried out by Coniglio and Esparza (1988) and Pinotti et al. (1992, 1996). The ACP (50 km 2) is a nearly circular, concentrically zoned pluton 8 km in diameter, and t forms the southeastern part of the CAB. It comprises a central and an external unit. The former represents approximately 93% of the outcrops of the ACP, and the contacts between both units are sharp and generally well exposed (Fig. 4). Modal analysis shows that the central unit is generally enriched in ma®c minerals as compared with the external unit. The central unit consists of a pink porphyritic to coarsegrained biotite granite, with perthitic microcline phenocrysts up to 5 cm across and many biotite-schlieren and small, ®ne-grained ma®c enclaves. The grain size in this unit decreases and becomes texturally more heterogeneous near the border of the pluton, in a zone of up to tens of meters wide, thereby acquiring a minor porphyritic character. Large rock pendants are rare, but ovoid enclaves of ®negrained ma®c minerals are common. These enclaves are small (10 cm) and widely disseminated and consist mainly of biotite with variable amounts of plagioclase and accessory minerals. The central unit typically has biotite as the predominant ma®c phase in medium-grained clusters in the matrix and within the feldspars. Accessory minerals include apatite, zircon, and ¯uorite. The latter is present as inclusions from 5 to 120 mm in size inside biotite or, less frequently, as interstitial crystals. The external unit appears either as roughly tabular, subhorizontal bodies in the upper portions of the pluton or as a prominent ring emplaced between the metamorphic rocks and the central unit. The subhorizontal

256

Pluton

Granitic units

Description

Enclaves

Host-rock xenoliths

Surface (km 2)

Bigger axis, smaller axis

Felsic Dikes

Late-magmatic alteration

Alpa Corral

Central

Porphyritic to coarse grained biotite monzogranite

Abundant

Rare

50

8 km of diameter

Rare

External

Inequigranular monzoleucogranite

Rare

Scarce

Annular dikes (scarce); radial dikes (abundant)

Central

Porphyritic biotite monzogranite

Abundant

Rare

Rare

External

Coarse grained to porphyritic biotite monzogranite Inequigranular monzoleucogranite

Scarce

Scarce

Annular dikes (abundant); radial dikes (scarce)

Rare

±

Coarse grained to porphyritic biotite monzogranite

Rare

Scarce

El Talita

Top

Los Cerros

Top

385

25, 16 km

Sericitization± greisenization (pervasive)

Rare Sericitization± greisenization (pervasive)

5

5, 1 km

Thin dikes (abundant) inside the pluton and host rocks

Greisenization (pervasive); albitization (scarce)

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

Table 1 Syntheses of the granitic units of the Cerro Aspero batholith

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

257

Table 2 Mineralogical composition of the Cerro Aspero batholith. Mineral abbreviations are from Kretz (1983). X: abundant; x: scarce; (x): rare Plutons

Units

Pl

Kfs

Qtz

Bt

Ms

Ap

Aln

Ttn

Zr

Fl

Alpa Corral

Central External

X X

X X

X X

X X

x x

x x

(x) (x)

(x) (x)

x x

(x) x

El Talita

Central External Top

X X X

X X X

X X X

X X x

x x X

x x x

X x ±

X x ±

x x x

(x) (x) x

Los Cerros

Top

X

X

X

x

X

x

±

±

x

x

Fig. 4. Geological map of the Alpa Corral pluton showing the arrangement of the circular granitic units, the contact between ETP and ACP, radial and annular structures, and the polygonal geometry of the contact with the host rocks. (A) Polygonal geometry of the contact with the metamorphic rocks showing the sharp, stepped contact associated with radial fractures. (B) Polygonal geometry of the contact with the metamorphic rocks and associated thermal aureole.

258

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

Fig. 5. Landsat TM imagery (overlay bands 4 1 7) enchasing internal structures of the CAB.

structures of the external unit are ®lled by a set of pegmatite and aplite dikes. The external unit is composed of a inequigranular leucogranite, rich in late magmatic volatile ¯uids, as evidenced by miarolitic cavities and greisen bearing ¯uorite and molybdenite. Chilled borders are common and appear as a change in the texture, which acquires a dike-like aspect. The best exposures of the external unit are Cerro La Mesada and Cerro La Cocha, the latter of which forms a scarp along the northern boundary of the external unit. Most outcrops consist of intensely greisenized granitic rocks ranging in color from pink to red. Grain size is medium

to coarse, with pink alkali feldspar phenocrysts in a matrix of felsic minerals and biotite. Zircon and apatite are the most frequent accessories. Alkali feldspars are commonly replaced by muscovite and sericite. Primary muscovite could not be identi®ed. The more prominent internal structures of the ACP are radial and annular fractures that affect the metamorphic host rocks, delimiting a well-de®ned aureole of brittle fracturing that extends more than 5 km away from the contact (Fig. 4a and b). Some radial fractures reach 7 km in length, but the annular fractures vary from meter to decameter scale with a discontinuous

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

pattern. These structures have irregular walls and display no evidence for shear. 4.2. El Talita pluton

259

to delimit in the ®eld, air photographs and Landsat TM imagery indicate that the ETP crosscuts the northern sector of the ACP (Fig. 5). 4.3. Los Cerros pluton

2

The ETP, with a surface area of 385 km , is the largest of the CAB. Three main units have been distinguished, designated central, external, and top units according to their relative position in the pluton. The central unit is the most extensive and represents more than 50% of the outcrops. It forms the core of the CAB and consists of two coalescent circular intrusions (Figs. 2 and 7). Although the contact between them is dif®cult to recognize in the ®eld, it is well de®ned on Landsat TM imagery. The central unit consists of pink porphyritic biotite granite characterized by K-feldspar phenocrysts up to 14 cm long that constitute 15±30% of the volume of the rocks. Accessory minerals include titanite, allanite, apatite, zircon, and ¯uorite. Finegrained ma®c enclaves are commonly hosted by this granite. The alignment of euhedral K-feldspar phenocrysts together with the ®ne-grained ma®c enclaves de®nes a conspicuous magmatic orientation in the granite, which is interpreted as product of the magmatic ¯ow generated during the emplacement of the magma (Pinotti et al., 1992). In some cases, enclaves and feldspars are cut by syn-magmatic dikes. The external unit forms a roughly circular belt 6±8 km wide between the central unit and the metamorphic host rock. It is composed of pale pink coarse biotite granite in which porphyritic texture is only rarely developed. The contact between the external and central units is gradational over a distance of 50±100 m, and the distinction between the two units depends on the development of the typical porphyritic texture. The granitic rocks in the external unit of the ETP display the strongest foliations of the CAB. In outcrops, these foliations are marked by a better de®ned elongation of ®ne-grained ma®c enclaves and biotite schlieren. Within the enclaves, feldspars, quartz, and biotite show weak effects of deformation. Additional microscopic evidence shows that the plagioclase of the granite has locally suffered a weak elongation, according to the aligned twins or loss of twin lamella but no fractures. Similar to the plagioclase, microcline appears with deformed twins. Rarely, quartz appears recrystallized, but generally it displays undulatory extinction. As shown in Fig. 4, these foliations are parallel to the internal pluton contact but discordant with foliations of the metamorphic host rocks. The top unit of the ETP occurs as discontinuous outcrops formed by a shallow dipping cap overlying the central and external units with sharp contacts. This unit consists of inequigranular leucogranite and has structural and petrographic characteristics similar to the external unit of the ACP. The circularity of the internal structures in the ETP is notably de®ned by the nearly rounded pattern of contact between the central and external units and the contact between the ACP and ETP. Although this contact is dif®cult

The LCP is the smallest intrusion of the CAB (5 km 2), consisting mainly of a single top unit. It comprises a texturally heterogeneous porphyritic biotite granite, which varies from strongly porphyritic to locally coarse-grained. The most abundant phenocryst is alkali feldspar with subordinate plagioclase; both feldspars form crystals up to 2.5 cm long. This pluton is pervasively affected by subsolid alterations, mainly greisenization, K-feldspathization, and, to a lesser degree, albitization. The greisenization is strongly related to the areas that bear wolframite±quartz vein deposits. Swarms of quartz veinlets crosscut the altered zones in the granite and the metamorphic host rocks. 4.4. Dikes A conspicuous feature of the CAB is the occurrence of dikes. On the basis of texture and geometric relationships, syn-magmatic, later-magmatic, and postmagmatic types were identi®ed. The postmagmatic dikes are more extensive, broadly distributed inside the CAB and host rocks (mostly near the contacts), and emplaced in annular and radial fractures. Chilled borders are common at contact sites with the granite or metamorphic host rocks. Each pluton of the CAB is associated with a particular set of dikes, and therefore, different relative ages for the dikes have been identi®ed. Monzogranite compositions are dominant, muscovite exceeds biotite, and scarce garnet occurs in the leucocratic types. The groundmass is medium-grained to aplitic with a phenocryst of microcline that ranges in size from 2±10 mm. Locally, the dikes are strongly enriched with volatiles that produce extreme variations in texture with pegmatitic tendencies. The radial dikes are best developed in the ACP; they converge toward the center of the pluton. They occupy steeply dipping fractures that extend up to 7 km, though with discontinuous outcrops, and intrude into the metamorphic rocks. These fractures are ®lled by ®ne-grained granite or, locally, by quartz veins with abundant open-space-®lling textures. The annular dikes in the ACP are scarce, discontinuous, and generally less than 2 m thick. Both radial and annular dikes de®ne a prominent aureole of brittle fracturing that extends for more than 5 km from the contact. The most notable internal structure of the CAB is a wide swarm of annular dikes that are up to 4 km wide and 5 km long. The thickness of these annular dikes varies from a few centimeters to 5 m, and they are normally less than 100 m in length. They are located along steeply dipping, well-developed annular fracture systems in a pattern that is strongly conditioned by the structural anisotropy along the contact between the ACP and ETP. In this sector, the dikes intrude the metamorphic host rocks locally (Figs. 2 and 4). Radial

260

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

dikes are present in the ETP, but they are scarce. Field evidence shows that both radial and annular dikes of the ETP crosscut those of the ACP. Pluton-derived dikes are locally abundant in the host rocks near the northern contact of the LCP and show both unconformable and conformable relationships with the S3 foliation. In the eastern pluton contact, an aplitic dike reaches ,200 m into the metamorphic rocks. The top units of the plutons generally consist of a profuse intrusion of gently dipping leucocratic dikes associated with pegmatitic lenses and pods bearing beryl and tourmaline. Frequently, these dikes form a subhorizontally strati®ed intrusion structure. 5. Pluton/host rock contact relationships The granite/host rock contact is well exposed throughout the CAB, except at the western border of the ETP, where its circular structures have been cut by the Cenozoic shear zone responsible for the uplift of the Sierra de Comechingones. The host rock has not been preserved on the pluton roofs. Some contact is sharp and always intrusive. The granite cuts all the previous foliations and structures of the metamorphic rocks discordantly, particularly the mylonitic S4 foliation, which is the most affected because of its widespread development in this sector. The discordant relationship between the granite and its host rocks is remarkable where the contact surface is oriented nearly E±W. In this case, it truncates the host rock foliation at high angles, and polygonal geometries are developed. A good example of this is provided by the ACP, though the northern contacts of ETP and LCP also present polygonal shapes. In contrast, where the contact is subparallel to the metamorphic foliation, as in the eastern sector of the CAB, the contact acquires a rectilinear geometry controlled by the anisotropy of the GCSZ. In this case, the internal structures of the GCSZ (D4) are truncated at low angles by the granite. Detailed mapping indicates that the granite/host rock contact is not a continuous surface but is formed by a set of fractures with irregular walls, as in the example of the ACP. Fig. 4 shows that the circular shape of the contact is more apparent than real because it consists of several linear or curviplanar segments of 200±900 m in length that join at sharp edges, thus conforming to a noticeable polygonal design. Such polygonal geometry is not easily identi®ed at a regional scale of observation and is therefore easily confused with a circular shape. The segments of the polygon have offsets of a few meters along the radial fractures (Fig. 4a), and the granite/host rock relationship displays sharp, stepped contacts with the S4 mylonitic foliation and/or the fold axes of the host rocks, cutting them at great angles. 5.1. Thermal contact aureole A discontinuous thermal aureole was locally developed and overprinted the retrograde metamorphism in the GCSZ.

The peak of this thermal event produced black, nonfoliated, ®ne-grained, typical hornfels, particularly in the areas of interference between the ETP and ACP (Fig. 4). However, the contact aureole was mainly produced as a result of ETP intrusion because of its size (8.5 times larger than the ACP). Neither textural nor mineralogical changes in the metamorphic host rocks related to the ACP and LCP intrusions were observed. On the southern border of the ETP, the contact aureole is approximately 500 m wide and 9.5 km long and can be easily recognized on satellite images and standard aerial photographs. Textural features and mineral assemblages of the thermal aureole rocks vary gradually from the contact toward the host rock. Rocks that occur next to granite have a hornfelslooking, spotted, massive texture and are characterized by the coexistence of Sil±Bt±Crd±And, which indicates that these rocks reached the highest contact temperatures at the hornblende hornfels facies. Aureole rocks that outcrop between 70±200 m away from the granite bear a relict gneissic fabric, due to regional metamorphism that is partly recrystallized to a ®ne-grained semi-hornfels porphyroblastic texture. The matrix of these semi-hornfels consists of Bt 1 Chl coexisting in textural equilibrium. Therefore, the stable mineral assemblage in the external part of the aureole is diagnostic of albite±epidote hornfels facies. To the east of the interference zone between the ACP and ETP, the thermal aureole was developed entirely on the GCSZ, where it is narrower than at the southern border (50±70 m). The foliation of these rocks decreases away from the contact, and the new minerals are cordierite and sillimanite. A spotted texture was generated by the growth of cordierite from biotite. The cordierite is poikiloblastic with minute biotite inclusions and both prismatic and ®brolitic sillimanite. Small blasts (,1 mm) of subhedral andalusite appear with little frequency. Evidence for ductile strains in the host rocks of the CAB are rare, restricted exclusively to some sectors of the inner zone of the thermal aureole. They are marked by meter- to centimeter-scale de¯ection of the penetrative S3 gneisic foliation and the S4 mylonitic foliation. The static growth of the mineral assemblage during the episode of thermal metamorphism does not indicate later ductile deformation. In this context, thermal metamorphism is signi®cant because it highlights the last event in the metamorphic evolution of the crystalline basement of the Sierra de Comechingones. 5.2. Host rock xenoliths Few blocks of host rocks (in less than 1% of the granite outcrops) have been preserved. These are generally distributed throughout the border zone of the CAB, close to the contact. Gneisses and amphibolite are the dominant lithologies. The size of the blocks ranges from 0.1±5 m, though

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

261

6. Discussion

Fig. 6. Sketch of stoped host rock blocks preserved at several stages of partition. (a) Swarm of polygonal stoped blocks, fractured and rotated. (b) Partition surface of a small block as a result of the thermal fracture and subsequent magma intrusion.

size and abundance decrease toward the interior of the plutons. Most xenoliths have been preserved at the northwestern border of the ETP, in the external unit, and are characterized by large blocks of host rock up to 30 m in length. It is important to note that, in this part of the pluton, a chilled border of previously crystallized granite was also observed as rock pendants. The internal structures of the larger blocks have been rotated in relation to those of the adjacent metamorphic host rocks. Furthermore, different blocks have been preserved in several stages of partition, and some of them display an almost complete separation from the walls. The partitions of these blocks began from planar surfaces and internal fractures at high angles from one another, induced by a profuse injection of granitic and aplo-pegmatite dikes (Fig. 6a and b). Similar examples are also common at the northern contact of the LCP.

To assess the processes involved in the emplacement of large volume batholiths, as is the case of the CAB and several other Devonian granitic bodies from Sierras de CoÂrdoba and San Luis, conspicuous features such as discordant relationships with the host rocks, the occurrence of a contact aureole that does not display any deformation, and the nearly circular geometry of the plutons must be taken into account. Field evidence shows that brittle behavior of the host rocks was broadly dominant during the emplacement of the plutons that compose the CAB. According to the mineral assemblage observed in the thermal contact aureole (Crd± Bt±And±Sil) and using the KFMSH petrogenetic grid for pelitic metasediments, it is possible to estimate a temperature close to 800 8C and a pressure lower than 2 kb for magma crystallization, which is equivalent to an estimated depth of emplacement #7.5 km (Pinotti, 1998). Consequently, the rheologic behavior of the host rock can be considered equivalent to that of a material with elastic behavior (Macosko, 1994; Atkinson, 1987; FernaÂndez and Castro, 1999). These considerations indicate that the polygonal shape of the plutons and their circular internal structures were developed at shallow crustal levels, and in this context, the mechanisms that involve ductile deformation, such as diapirism (Marsh, 1982; Weinberg and Podladchikov, 1994), are not applicable. Although a circular shape is generally typical of diapiric bodies, the lack of harmony and ductile deformation of the host rocks does not support a model of diapiric intrusion. However, the unmodi®ed structures of the host rock and the absence of an aureole with ductile deformation also rules out the possibility of a ballooning style of emplacement (Bateman, 1985; Ramsay, 1989; Hutton, 1997). In the few areas where some ductile behavior was observed in the host rock, it was related to the contact aureole. However, these sectors are regionally insigni®cant because they represent less than 0.5% of the overall volume of the CAB. Mechanisms that involve high strain rates, such as accretion deformation (Castro and FernaÂndez, 1998) or translation and rotation (Tikoff et al., 1999), did not play a dominant role in the emplacement process of the CAB. The presence of discordant contacts, numerous roof pendants, and annular and radial dykes can be explained only by brittle behavior, that is, stoping (Daly, 1903, 1933). In addition, the conspicuous, nearly circular zoned arrangement of the plutons indicates that magmatic stress prevailed over the regional stresses. However, in the brittle upper portions of the crust, the internal forces of the magma alone are not enough to explain the intrusion, and additional favorable structural conditions are required, as was suggested by Vigneresse (1995). The CAB is a notable example of multistage emplacement of a granitic body that grew mainly from three major

262

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

Fig. 7. Schematic block diagrams showing the relationships of the main intrusion episodes of the Cerro Aspero batholith during the Middle to Late Devonian. (A) Later stage of the evolution of the GCSZ. Distension after main compressive episode, granitic melt ascent. (B) Beginning of the Devonian granitic emplacement in the Sierra de Comechingones: Intrusion of the ACP (in dark grey) structurally controlled by the GCSZ. (C) Main stage of magma intrusion. Emplacement of the ETP (in light gray), coeval intrusion of coalescent batches of melt during the later stage of the ACP intrusion. The arrows show the expansion of the ETP related to the intrusion of its central unit. (D) Later stage of the CAB emplacement, intrusion of the LCP (in black) showing the arrangement of the three plutons in relation to the inferred NNW±SSE master fractures.

successive intrusions. The magmatic event began with the emplacement of a small-volume intrusion (ACP 50 km2), followed by the larger (ETP 390 km2), and ending with LCP. This interpretation is based on the relative disposition of magmatic structures disrupted by the contacts between granitic plutons. The alignment of the pluton's cores, together with the distribution of their internal structures, indicate a NNW±SSE trend. This suggests that granite emplacement was probably controlled by a set of early master fractures (Fig. 8), which probably were a consequence of a Paleozoic system of local extension that facilitated the granitic magma ascent by means of fracture propagation. The orientation of such fractures would have been induced by the host rock anisotropy and therefore would be favored by the most penetrative structures (S3 and S4 foliations) of the Sierra de Comechingones. The formation of a thermal contact aureole without the deformation overprinting the retrograde metamorphism of the GCSZ indicates that, during the Middle Devonian period during which the intrusion of the CAB began, the shear zone was already extinct. However, the location of the plutons in relation to the GCSZ suggests that it controlled the position

of the master fractures and magma intrusion. The ACP clearly intruded after the main compression episode of the GCSZ ceased and a relaxation stage had begun. Therefore, it is possible to interpret these granites as associated with postorogenic tectonic processes related to a rise and progressive cooling of the crust (Pinotti et al., 1996). The discordant relationship of the plutons and the metamorphic foliations, together with the polygonal geometry of the contact, indicate that the emplacement occurred along brittle fractures and the host material was not displaced sideways. Two alternatives could explain how accommodation was created: (1) host rocks were moved toward the lower part of the magmatic chamber or (2) the overlying roof rocks were domed upward by the magmatic intrusion making space for itself and then eroded away. However, because of the depth inferred for the pluton emplacement (#7 km), a host rock doming process appears unlikely. Paterson et al. (1996) refer to the irregular contacts in the Chita Pluton, Argentina, as `irregular steep-sided contacts' and argue that they were produced by lateral stoping. As the generator of polygonal granitic geometry in the CAB, stoping is very important herein. During the intrusion of the ACP, some of the space was ®rst created by the driving pressure of the magma, which was responsible for the formation of the radial and annular fractures of the host rocks. This mechanism produced a profuse fracture set, which allowed the invasion of dikes and favored the stoping process. The consequent downward removal of small host rock blocks may have been broadly facilitated by heating and thermal fracturing (Marsh, 1982; Fowler et al., 1995). Evidence for such processes is provided by the swarms of small host rock xenoliths (stoped blocks), as shown in Fig. 6a and b. In the case of the CAB, the processes of weakening and fracturing the small blocks were facilitated by an important thermal contrast between the granite and the host rocks. The Devonian granites of the Sierras de CoÂrdoba are rich in ¯uorite (Dorais et al., 1997; Coniglio et al., 2000). The viscosity therefore may have been low because of the high ¯uorine content (Dingwell, 1987). The thermal fracture mechanism requires a low viscosity of the magma, consistent with the inferred magma properties. At shallow crustal levels, when a decrease in the lithostatic pressure occurs, the buoyancy force is neutralized, and the principal stress orientation changes from vertical to horizontal. Thus, the magma no longer ascends and tends to expand laterally with isotropic stress in the horizontal plane. At this point, lateral stoping, together with propagation of fractures, reaches its maximum importance, which results in plutons with internal radial and circular structuration and external polygonal shape (Paterson et al., 1996; Pinotti, 1998). From the variations in the section size of the CAB plutons, it is possible to infer that the energy and quantity of available magma were widely variable during its emplacement and reached a maximum with the ETP intrusion.

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

263

associated circular structures (Fig. 8), as was suggested for other zoned granites (Vigneresse, 1995). The small size of the LCP suggests a low volume and decline in magma production, probably due to a decrease in the ascent energy related to the generalized cooling of the crust. In these conditions of low energy input, when the magma arrives at levels at which the buoyancy capacity is neutralized, it does not seem to be able to expand laterally. The maximum axis of the LCP is nearly N±S, thereby indicating the orientation of the feeder fracture. These considerations regarding CAB emplacement are summarized in Figs. 7 and 8, which provide simpli®ed schemes that indicate the main stages of intrusion. 7. Conclusions

Fig. 8. Sketch of the aligned plutons showing their internal expansion and possible root of the plutons.

This could be related to different rates of opening of the channels that feed the intrusions, due to variable activity of the master and transverse fractures. The two contemporary circular intrusions that overlap in the core of the central unit of the ETP re¯ect a multi-pulse intrusion with root paths that coalesce at the exposed level of the intrusion. In the central unit, the alignment of euhedral minerals, enclaves, and schlieren indicate that the foliations are magmatic (see Paterson et al., 1989). However, the presence of foliation and mineral deformation in the ETP external unit suggests that they were the result of solid-state deformation at high temperatures, and no evidence of a tectonic origin was found. These foliations are interpreted as formed by the expansion of the pluton during the intrusion of the central unit, which could have occurred as solid-state ballooning or, more likely, as magmatic ¯ow. This second possibility is more probable because the occurrence of a generalized magmatic ¯ow is well established, and it likely was responsible for the generation of large internal convection cells, as well as for the high compositional homogeneity shown by the granitic rocks (Pinotti et al., 1992). In addition, the gradual contacts between the central and external units of the ETP suggest that the intrusion expansion took place by continuous magma emplacement. Evidence of lateral expansion also is provided by the internal zoning of the plutons and its

On the basis of ®eld observations and structural and petrographic data, it is possible to state that the well-developed circular geometry of the Devonian plutons of the Sierras de CoÂrdoba is apparent and that it would be more appropriate to describe it as a polygonal geometry. This shape consists of several linear and curviplanar segments developed under the brittle behavior of the host rocks without substantial ductile deformation. The polygonal shape was formed as a result of changes in the stress ®eld, from vertical to horizontal, related to magma arrival at shallow crustal levels. The CAB is composed of three major successive and continuous intrusions. The magmatic event began with the emplacement of the ACP, continued with the synchronic emplacement of two coalescent circular intrusions of the ETP, and ended with the LCP emplacement. Each pluton of the CAB is associated with a particular set of dikes, and different relative ages for each set of dikes have been identi®ed. Pervasive subsolid alterations are mainly related to the crystallization of the top or external units of each pluton. Despite the uncertainty that exists on the kinematic history of the GCSZ and the relatively scarce volume of rock pendants in the granite, the best explanation for the emplacement of the CAB is to invoke fracture propagation in conjunction with stoping mechanisms. Explanations based on diapirism or ballooning are not suitable in this case. The emplacement process was controlled by the movement of master fractures developed in a extensional tectonic regime, as is inferred for the CAB. The stoping mechanisms acquired importance in the later stages of CAB emplacement and were widely favored by the development of the master fractures, the thermal contrast between granite and host rock, and a diminished viscosity of the magma due to its ¯uorine content. Acknowledgements All ®eld and laboratory expenses were supported by

264

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265

SECYT-Universidad Nacional de RõÂo Cuarto (Proj. 18/ C097) and CONICOR-Agencia CoÂrdoba Ciencia (Proj. 18/C058). This work represents a portion of the doctoral research of L. Pinotti, who was supported by CONICET, Argentina. We thank G. Villalba and D. Ducart for assistance during the ®eld work and the ITGD for logistical support. We also acknowledge F. CanÄas and M. Fagiano for constructive comments and suggestions that helped improve the text and I. Martinez for helpful language assistance. Observations and suggestions of the reviewers (C. Miller and A. Toselli) contributed greatly to improving the interpretations. We also appreciate very much the editorial suggestions provided by V. Ramos. References AcenÄolaza, F.G., Toselli, A.J., 1976. Consideraciones estratigra®cas y tectoÂnicas sobre el Paleozoico inferior del Noroeste Argentino. Memoria II, Congreso Latinoamericano de GeologõÂa Actas 2, 755±764. Atkinson, B.K., 1987. Fracture Mechanics of Rocks. Academic Press, London. Bateman, R., 1985. Aureole deformation by ¯attening around a diapiri during in situ ballooning: Canibal Creek granite. Journal of Geology 93, 293±310. Bonorino, F., 1950. Algunos problemas geoloÂgicos de la Sierras Pampeanas. Revista de la AsociacioÂn GeoloÂgica Argentina 3, 81±110. Brogioni, N., 1992. GeologõÂa del batolito de Las Chacras-Piedras Coloradas. Provincia de San Luis. Revista del Museo de la Plata, Nueva Serie, SeccioÂn GeologõÂa 11, 1±16. Castro, A., FernaÂndez, C., 1998. Granite intrusion by externally-induced growth and deformation of the magma reservoir, the example of the Plasenzuela pluton, Spain. Journal of Structural Geology 20, 1219± 1228. Coniglio, J., Esparza, A., 1988. GeologõÂa del sector sur del Batolito del Cerro Aspero-Alpa Corral, CoÂrdoba, Argentina. V Congreso GeoloÂgico Chileno II, 1±15. Coniglio, J., Xavier, R.P., Pinotti, L., D'Eramo, F., 2000. Ore-forming ¯uids of vein-type ¯uorite deposits of the Cerro Aspero batholith, Southern Cordoba Province, Argentina. International Geology Review 42 (4), 368±383. Dalla Salda, L., 1987. Basement tectonic of the southern Pampean ranges. Tectonics 6 (3), 249±260. Daly, R.A., 1903. The mechanics of igneous intrusion. American Journal of Science 16, 107±126. Daly, R.A., 1933. Igneous Rocks and the Depths of the Earth. McGrawHill, New York. Dingwell, D.B., 1987. Melt viscosities in the system NaAlSi3O8±H2O± F2O-1. Magmatic Processes: Physicochemical Principles, Mysen, B.O. (Ed.). Geochemical Society Special Publication 1, 423±433. Dorais, M.J., Lira, R., Chen, Y., Tingey, D., 1997. Origin of biotite±apatiterich enclaves, Achala batholith, Argentina. Contributions to Mineralogy and Petrology 130, 31±46. Fagiano, M., Pinotti, L., Esparza, A.M., 1997. Estudio geoloÂgico de las rocas metamoÂr®cas en el sector centro-sur de la Sierra de Comechingones,CoÂrdoba, R. Argentina. VIII Congreso GeoloÂgico Chileno III, E1627±E1630. FernaÂndez, C., Castro, A., 1999. Brittle behaviour of granitic magma: The example of Puente del Congosto, Iberian Massif, Spain. Understanding Granites: Integrating New and Classical Techniques, Castro, A., Fernandez, C., Vigneresse, J.L. (Eds.). Geological Society of London, Special Publications 158, 191±206. FernaÂndez Lima, J.C., Jurotan, A., Kroger, J., Aspilcueta, J., 1963. Informe

preliminar de los grupos wolframõÂferos Cerro Aspero, LambareÂ, Constancia y Fischer. Informe TeÂcnico N8 18, Ministerio de EconomõÂa de la NacioÂn. SecretarõÂa de Industria y MinerõÂa, 89 pp. Fowler Jr, T.K., Yoshinobu, A., Paterson, S.R., Tickyj, H., LlambõÂas, E.J., Sato, A.M., 1995. Chita pluton, San Juan Province, Argentina. 3D constraints on pluton emplacement by magmatic stoping. Geological Society of America, Abstracts with Programs 27, 7±125. GonzaÂlez Bonorino, G., LlambõÂas, E.J., 1996. Geologic and paleogeographic development of southern South America (excluding Brazil) in the late Proterozoic and early Paleozoic. In: Moullade, M., Nairn, A.E.M. (Eds.). The Fanerozoic Geology of the World I, The Paleozoic. Elsevier, Amsterdam, pp. 265±338. GonzaÂlez DõÂaz, E., 1972. Estudio geoloÂgico del distrito minero Cerro Aspero. Servicio Nacional de MinerõÂa y GeologõÂa, Buenos Aires, Anales XVI 55 pp. Gordillo, C., Lencinas, A., 1979. Sierras Pampeanas de CoÂrdoba y San Luis. Segundo Simposio de GeologõÂa Regional Argentina, Academia Nacional de Ciencias, CoÂrdoba I, 577±650. Guereschi, A.B., Martino, R.D., 1998. Las migmatitas estromatõÂticas de Huerta Vieja, Sierra de Comechingones, CoÂrdoba, Argentina. Revista de la AsociacioÂn GeoloÂgica Argentina 53 (1), 101±116. Hutton, D.H.W., 1997. Sintectonic granites and the principle of effective stress: a general solution to the space problem? In: Bouchez, J.L., Hutton, D.H.W., Stephens, W.E. (Eds.). Granites from Segregation of Melt to Emplacement Fabrics Ascent. Kluwer Academic Publishers, Amsterdam. Jordan, T., Allmendinger, R.W., 1986. The Sierras Pampeanas of Argentina; a modern analogue of Rocky Mountain foreland deformation. American Journal of Science 286, 737±764. Kretz, R., 1983. Symbols for rocks forming minerals. American Mineralogist 68, 277±279. Lira, R., Kirschbaum, A., 1990. Geochemical evolution of granites from the Achala batholith of the Sierras Pampeanas, Argentina. Plutonism from Antarctica to Alaska, Kay, S., Rapela, C. (Eds.). Geological Society of America Special Paper 241, 67±76. LlambõÂas, E., Caminos, R., Rapela, C.W., 1984. Las plutonitas y vulcanitas del ciclo eruptivo GondwaÂnico. IX Congreso GeoloÂgico Argentino, Relatorio, 85±117. Macosko, C.W., 1994. Reology: Principles, Measurements and Applications. VCH, New York. Marsh, B.D., 1982. On the mechanics of igneous diapirism, stoping and zone melting. American Journal of Science 282, 808±855. Martino, R., Kramer, P., Escayola, M., Giambastiani, M., Arnosio, M., 1995. Transecta de las Sierras Pampeanas de CoÂrdoba a los 328S. Revista de la AsociacioÂn GeoloÂgica Argentina 50 (1±4), 60±77. Ortiz SuaÂrez, O.A., Prozzi, C., Llambias, E.J., 1992. GeologõÂa de la parte sur de la Sierra de San Luis y granitoides asociados. Revista Estudios GeoloÂgicos 48, 269±277. Otamendi, J.E., PatinÄo Douce, A.E., Demichelis, A.H., 1999. Anphibolite to granulite transition in aluminous greywackes from the Sierra de Comechingones, CoÂrdoba, Argentina. Journal of Metamorphic Geology 17, 415±434. Paterson, S.R., Vernon, R.H., Tobisch, O.T., 1989. A review of criteria for the identi®cation of magmatic and tectonic foliations in granitoids. Journal of Structural Geology 11, 349±363. Paterson, S.R., Fowler Jr, T.K., Miller, R.B., 1996. PlutoÂn emplacement in arcs: a crustal-scale exchange process. Transactions of the Royal Society Edinburgh: Earth Sciences 87, 115±123. Pinotti, L.P., 1998. El batolito Cerro Aspero, Provincia de CoÂrdoba. Modelo de intrusioÂn y su relacioÂn con la evolucioÂn de las Sierras Pampeanas. Unpublished Ph.D. thesis, Universidad Nacional de RõÂo Cuarto, Argentina, 203 pp. Pinotti, L.P., Esparza, A.M., Coniglio, J.E., 1992. FormacioÂn de megacris spero-Alpa tales de feldespato potaÂsico en el sector sur del batolito C8 A Corral (32840'S y 64850'O), Sierras Pampeanas, CoÂrdoba, Argentina. Revista Estudios GeoloÂgicos 48, 211±219. Pinotti, L.P., Coniglio, J.E., LlambõÂas, E.J., 1996. CaracterõÂsticas

L.P. Pinotti et al. / Journal of South American Earth Sciences 15 (2002) 251±265 geoloÂgico-estructurales del PlutoÂn Alpa Corral, 32838 0 ±32847 0 S y 64855 0 ±64845 0 W. Sierras Pampeanas de CoÂrdoba, Argentina. XIII Congreso GeoloÂgico Argentino y III Congreso de ExploracioÂn de Hidrocarburos Actas III, 477±483. Pinotti, L.P., LlambõÂas, E.J., Coniglio, J.E., 1997. Stoping as a main mechanism of intrusion in post-orogenic granite from the southern part of the Sierra de Comechingones, Sierras Pampeanas de CoÂrdoba, Argentina. Second International Symposium on Granites and Associated Mineralizations. Abstracts with Programs, 323±325 Salvador, Bahia, Brazil. Porta, G.R., 1992. CaracterizacioÂn petrogra®ca y geoquõÂmica del batolito Cerro Aspero-Alpa Corral, (32834 0 ±32842 0 LS y 64843 0 ±64852 0 LO), Provincia de CoÂrdoba, Argentina. Revista Estudios GeoloÂgicos 48, 237±245. Ramsay, J.G., 1989. Emplacement kinematics of the granite diapir: The Chindamora batholith, Zimbabwe. Journal of Structural Geology 11, 191±209. Rapela, C.W., Coira, B., Toselli, A., Saavedra, J., 1992. The lower Paleozoic magmatism of southwestern Gondwana: End the evolution of the Famatinian orogen. International Geology Review 34 (11), 1081±1142. Rapela, C.W., Pankhurst, R.J., Baldo, E.A.G., Saavedra, J., 1995. Cordieritites in S-type granites: Restites following low pressure, high degree partial melting of metapelites. The Origin of Granites and Related Rocks, Brown, M., Piccoli, P.M. (Eds.). US Geological Survey, III Hutton Symposium Circular 1129, 120±121. Rapela, C.W., Pankhurst, R.J., Casquet, C., Baldo, E., Saavedra, J., Galindo, C., 1998. Early evolution of the proto-Andean margin of South America. Geology 26 (8), 707±710. Rapela, C.W., Coira, B., Toselli, A., LlambõÂas, E.J., 1999. Sistema Famatiniano de las Sierras Pampeanas y magmatismo eopaleozoico de las

265

Sierras Pampeanas, de la Cordillera Oriental y Puna. GeologõÂa Argentina, Caminos, R. (Ed.). Anales 29, 145±158. Schmidt, C.J., Astini, R.A., Costa, C.H., Gardini, C.E., Kraemer, P.E., 1995. Cretaceous rifting, alluvial fan sedimentation and Neogene inversion Southern Sierras Pampeanas, Argentina. Petroleum Basins of South America, Tankard, A.J., SuaÂrez, R.S., Welsnik, H.J. (Eds.). American Association of Petroleum Geologists, Memoirs 62, 341±358. Stuart-Smith, P.G., Skirrow, R.G., 1997. Report on 1:1.000.000 scale geological and metallogenic maps sheet 3366-18. Province of San Luis and CoÂrdoba. Argentine±Australian Cooperative Project, Australian Geological Organization, unpublished report, 41 pp. Stuart-Smith, P.G., Miro, R., Pieters, P.E., Lions, P., Sims, J.P., Camacho, A., 1996. Tectonic framework of the southern Sierras Pampeanas, Argentina. XIII Congreso de GeologõÂa Argentina y III Congreso de ExploracioÂn de Hidrocarburos Actas II, 147. Stuart-Smith, P.G., MiroÂ, R., Sims, J.P., Pieters, P.E., Lyons, P., Camacho, A., Ireland, T., Skirrow, R.G., Black, L.P., 1999. Uranium±lead dating of felsic magmatic cycles in the southern Sierras Pampeanas, Argentina: Implications for a tectonic development of the proto-Andean Gondwana margin. Laurentia±Gondwana Connections Before Pangea, Ramos, V.A., Keppie, J.D. (Eds.). Geological Society of America Special Paper 336, 87±114 Boulder, Colorado. Tikoff, B., Desaint Blanquat, M., Teyssier, C., 1999. Translation and resolution of the pluton space problem. Journal of Structural Geology 21, 1109±1117. Vigneresse, J.L., 1995. Control of granite emplacement by regional deformation. Tectonophysics 249, 173±186. Weinberg, R.F., Podladchikov, Y., 1994. Diapiric ascent of magma through power-law crust and mantle. Journal Geophysical Research 99, 9543± 9560.