1 INSUGEO – CONICET. Miguel Lillo 205. San Miguel de Tucumán. E–mail: insugeo@unt.edu.ar

Key words: Magmatism. Deformation. Granitoids. Velasco. Sierras Pampeanas

The Sierra de Velasco is located in the central region of La Rioja province, Argentina. It is the largest batholith of the Sierras Pampeanas geologic province. Toselli et al. (1986) indicate that this region is composed mainly by porphyric two mica type monzogranites and more rarely granodiorites, occasionally cordieritic. The geochemical signature proved these granitoids were late to post–collisional, peraluminous, calk–alkaline and of S–type affinity.

It is thought that they intruded during the Famatinian Cycle as a consequence of the magmatism originated by the Ocloyic Orogeny (Upper Ordovician–Lower Silurian). López and Toselli (1993) suggest that the range is formed by granitic rocks, deformed on its western flank, being this region part of the TIPA (Tinogasta – Pituil – Antinaco) mylonitic belt. The relative age of deformation is pre–Carboniferous based on stratigraphic relationships.

The central, northern and eastern sectors of the Velasco batholith are constituted mainly by granitic rocks of varying textures (porphyritic and equigranular syeno and monzogranites) that lack deformation (Huaco Granite, HG, and San Blas Granite, SBG, Figure 1). The HG is characterized by large (up to 8–10 cm long) oriented microcline phenocrysts in abundances of nearly 40%. The SBG is also porphyritic although the phenocrysts are less abundant and of smaller size. The western flank of the range consists of granitoids with different degrees of deformation (Antinaco Pluton, AP). Gneissic textures predominate. They are part of the TIPA mylonitic belt that extends from the Fiambalá and Copacabana ranges in the north to the Sierra de Velasco (López and Toselli, 1993). Additionally, in all the range smaller deformation belts formed by protomylonites and mylonites with approximate NNW–SSE strike are present.

On the eastern flank and in a few sectors in the south of the range, low–grade metamorphic outcrops of reduced size are present. Rossi et al. (1997) described hornfels in the NE tip of the range. These rocks occur as roof pendants within the granitoids, with paragenesis that indicate high temperature and low pressure. The presence of these rocks suggests that the granitic bodies were emplaced at shallow levels of the crust.

Figure 1. Geological sketch map of the granitoid units of the Sierra de Velasco.

The southern portion of the range presents different petrological and geochemical characteristics. Granodiorite and tonalite associations dominate. They are related with mafic rocks and have metaluminous to weak peraluminous tendencies that suggest a mafic lower crust and/or upper mantle participation in their genesis.

The relationships observed in the field between the different plutonic bodies permits us to identify the possible sequence of intrusive and deformational events that affected the Sierra de Velasco. The first intrusive event corresponded to the emplacement of granitoids that generated the AP. These later suffered an intense deformative event concentrated along deformation belts, which produced the gneissic and mylonitic foliation of the TIPA belt. The relative age of these rocks is pre–Carboniferous since the Paganzo Group sedimentites lie in discordance on the deformed granitic rocks.

Rapela et al. (2001) have obtained two U–Pb SHRIMP ages in the Sierra de Velasco. They obtained a 481.4 ± 2.4 Ma crystallization age and a 469.0 ± 3.9 Ma age for a deformational event. Both age determinations were done on rocks that we consider to be part of the AP. Höckenreiner et al. (2001) obtained a Sm–Nd on garnet deformation age of 420 to 409 Ma on rocks of the TIPA belt in the Sierra de Copacabana. Recently, Höckenreiner et al. (pers. com.) obtained a crystallization age of 487,5 ± 4,3 Ma by U–Pb dating on zircon of a meta–granodiorite in the Sierra de Copacabana. They also obtained a mylonitization age of 402 ± 2 Ma using Sm–Nd on garnet.These ages indicate that already in the Lower – Mid Ordovician the AP was in an advanced state of crystallization, and that it suffered deformation until the Lower Devonian.

The lack of recent geochronology and the still scant geologic information about the relationship between the AP and the HG prevents to establish with certainty the age of the HG. Until now it has been considered that the AP and the HG are a same body that was emplaced during a sole magmatic event, being both pre–deformational. However, due to the absence of extensive deformation of the HG and the petrographic differences between the HG and the less deformed areas of the AP, we suggest that the HG could be a product of a magmatic event that took place after the emplacement and deformation of the AP.

Although the HG has thin mylonitic zones, product of the reactivation of older faults, these are of much lesser intensity and are widely spaced, compared to the conspicuous deformation of the AP. They can be considered as a second, less intense deformational event. McBride et al. (1976) dated mylonites in Puesto Asha, in the NE of the range, and obtained an age of 328–330 Ma that could correspond to the age of this second deformational event.

The southern section of the range lacks geochronological studies. A thick mylonitic zone of NNW–SSE strike, similar to those found in the AP, affect the granitoids of the region (Bellos et al., 2002). These granitic rocks are petrographically and geochemically similar to those described in the Paganzo and Paimán ranges. Rb–Sr ages obtained by Saal (1993) for granitic rocks of the Sierra de Paganzo are of 450–456 Ma, while in the Sierra de Paimán Perez and Kawashita (1992) obtained an age of 450 Ma, using the same method.

López and Toselli (1993), Le Corre and Rosello (1994), and Aceñolaza et al. (1990), demonstrated that they correspond to granitic rocks that suffered different degrees of deformation and were classified variably as gneissic granites, cataclasites, protomylonites, mylonitic schists and mylonitic gneisses. The deformation in the NW region occurred at deep levels producing mineral paragenesis of the barrowian amphibolite facies (kyanite–sillimanite–garnet), whose metamorphic peak would have been 650 – 700 ºC, with pressures of 6,5 – 8 kbars (Rossi et al., 2000).

They are thick–grained rocks, foliated due to the preferred orientation of micaceous minerals. They generally contain microcline phenocrysts of 4 to 5 cm long in variable amounts between 5 and 17%. Inequigranular, sub idiomorphic to xenomorphic textures predominate. The main accessories are biotite, muscovite and garnet.

According to López (1998), the main structures generated by deformation are mylonitic foliation planes with NNW–SSE strike and sub vertical dips to the East. The statistical average is 350º N / 50º E. Studying cinematic indicators, this author determined that inverse movements produced the overthrust of blocks towards the west.

Geochronological ages, field studies and the distribution of the deformed zones allow us to state that until now only the granitic rocks of the AP posses a certain Ordovician age. We suggest that the Velasco batholith is composed, in addition, by other more recent plutons. The plutons intruded during various stages separated by more than one deformational event. The younger plutons could have an age that reaches the Carboniferous.

Accepted: June 15, 2003