TSK 11 Göttingen 2006 Steenken et al. Syn-kinematic magma as- cent and batholith in- flation (Sierra de San Luis/Argentina) Vortrag André Steenken1 Siegfried Siegesmund2 Mónica G. López de Luchi1 Augusto Rapalini3 Klaus Wemmer2 The measurement of the anisotropy of the magnetic susceptibility (AMS) is now routinely used since more than four decades in the analyses of rock fabrics in granitic rocks (e.g. Stacy 1960, Henry 1975, Gleizes et al. 1993). Even though the intensity of fabrics in granitoids is often weakly developed the significance of orientation and shape of crystals is the same like in other deformed rock types. By revealing the distribution of fabrics in plutonic rocks one of the still ongoing discussions in granite tecton- ics may be addressed: How did those sometimes voluminous batholiths were inflated in the middle crust? We are pre- senting magnetic fabric data on a series of Devonian batholiths that intruded the polyphase deformed metaclastites of the Sierra de San Luis (32°10′– 33°20′ S / 65°15′ – 66°20′ W) in central Ar- gentina. Regional considerations on the tectonic regime during the emplacement of the batholiths are inferred from com- bined field, microstructural and AMS observations. 1 Instituto de Geocronología y Geología Isotópica (INGEIS), Ciudad Universitaria, 1428 Buenos Aires, Argentina 2 Geoscience Centre of the University of Göttingen (GZG), Goldschmidtstr. 3, 37077 Göttingen, Germany 3 Instituto de Geofísica Daniel Valencio (IN- GEODAV), Departamento de Ciencias Geológ- icas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, CONICET, Pa- bellón 2, Ciudad Universitaria, 1428 Buenos Aires, Argentina Geological background The proto-Andean basement outcrop- ping in the Sierras Pampeanas experi- enced a polyphase magmatometamor- phic history during the Ediacaran/Early Palaeozoic resulting from the accretion of different crustal fragment to Gond- wanas southwestern margin (Ramos 1988). The final manifestation of this history is the emplacement of voluminous, elliptical granodioritic to syenogranitic composite batholiths in the Sierra de San Luis and the Sierras de Córdoba during the Early/Middle Devo- nian (Fig. 1). There exists discordance on the regional stress field that enabled the magma as- cent during this period. According to Llambías et al. (1998) the Devonian magmatism is the result of the collapse of the Famatinian orogen whereas Sims et al. (1998) emphasised the relation of the Devonian batholiths with the con- tinual subduction followed by the col- lision of the Chilenia Terrane with the western outboard of Gondwana. The Devonian batholiths Detailed fabric studies on the largest batholiths in the Sierra de San Luis, i.e. the La Totora, Renca and Las Chacras-Potrerillos batholiths were car- ried out. The studies comprised system- atic field surveys, microstructural obser- vations and anisotropy of magnetic sus- ceptibility (AMS) measurements. Mi- crostructural studies indicate that the batholith rocks are mainly characterised by magmatic microstructures with lim- ited sub-magmatic to high temperature sub-solidus deformation. Magmatic fo- liations are defined by the planar ar- rangement of tabular feldspar and bi- otite. The local appearance of intracrys- talline fractures in the feldspar filled 1 Steenken et al. TSK 11 Göttingen 2006 Figure 1: Schematic map of the southern section of the Sierras Pampeanas. The Devonian magmatism is restricted to the basement complexes of the Sierra de San Luis and the Sierras de Córdoba with fine-grained granitic melt point to the syn-kinematic emplacement of the batholiths. Continued deformation be- low the solidus is accommodated by feldspar recrystallisation as indicated by sub-grain boundaries. Recrystalli- sation of quartz at low temperatures, leading to the formation of quartz rib- bons is observed in the SE sector of the Las Chacras-Potrerillos batholith only. Shear indicators point to sinis- tral as well as dextral sub-horizontal dis- placement. All three batholiths possess concentric foliation patterns that along the margins show a steep inclination. The average magnetic foliation patterns in the studied plutons agree well with the macroscopic fabrics measured in the field indicating that the AMS-data can be used to study the orientation of fabric elements. However, a bulk susceptibility (Kvol) of up to 8000× 106 in almost all granitic sub-units indicates a predomi- nance of ferromagnetic contributions to the bulk susceptibility. Therefore bi- otite fabrics of selected samples were used to calculate a theoretical AMS ten- sor (Siegesmund et al. 1995) that was 2 TSK 11 Göttingen 2006 Steenken et al. Figure 2: Orientation of magnetic lineations for the La Totora, Renca and Las Chacras- Potrerillos batholiths. Note that except for the marginal part of the Renca batholith, most lineations tend to be sub-horizontal or shallowly plunging. compared to the measured AMS fabric, indicating acceptable accordance in the directional data but do not necessarily support the shape of the magnetic fabric ellipsoid. Most foliations and lineations reflect magmatic flow and their attitude is linked to the interference between re- gional deformation and batholith infla- tion, i.e. fabrics may be due to regional strain in combination with the internal dynamics of the magma bodies. Mag- netic lineations either are sub-vertical or follow the NNE–SSW trend that is also documented by the linear fabrics of their hosts (Fig. 2). Results It is proposed that the opening of transtensional pull-apart structures controls those shallow dipping lin- eations during batholith inflation. K/Ar and Ar/Ar muscovite data from regional shear zones at ∼360Ma (Sims et al. 1998, Steenken et al. 2004) support the idea of a syn-kinematic magma ascent and batholith inflation following the direction of extension in a transpressional tectonic framework (Fig. 3). It turns out that the Devo- nian batholiths intruded the basement syn-kinematically with respect to the Achalian deformational cycle. References Henry, B (1975) Microtectonique et anisotropie de susceptibilité et magnétique du massif tonalitique des Riesenferner-Vedrette di Ries (Frontiere Italo-Autrichienne). Tectono- physics 27: 155-165 Gleizes, G, Nédélec, A, Bouchez, JL, Autran, A, Rochette, P (1993) Magnetic susceptibil- ity of the Mont-Louis Andorra Ilmenite-Type Granite (Pyrenees): A new tool for the pet- rographic characterisation and regional map- ping of zone granite plutons. Journal of Geo- physical Research 98: 4317–4331 3 Steenken et al. TSK 11 Göttingen 2006 Figure 3: Comic strip illustrating the syn-tectonic emplacement of the major Devonian granitoids in the Sierra de San Luis. Schematic depiction of the major tectonic strike slip faults connected with the ‘space-creation’ for the magma ascent. The development of a secondary set of NNW trending sinistral strike slip faults leads to the counter clockwise step over of the sinistral displacement along the Río Guzmán and related NNE–SSW trending shear zones. NNE directed crustal extension would allow the formation of magma conduits and subsequent magma accommodation in a transtensional setting. Llambías EJ, Sato A, Ortiz Suárez A, Prozzi C (1998) The granitoids of the Sierra de San Luis. Geological Society of London, Special Publication 142: 325–341 Ramos VA (1988) Late Proterozoic-Early Pa- leozoic of South America — a collisional his- tory. Episodes 11: 168–174 Siegesmund S, Ullemeyer K, Dahms M (1995) Control of magnetic rock fabrics by mica pre- ferred orientation: a quantitative approach. Journal of Structural Geology 17: 1601–1613 Sims JP, Ireland TR, Camacho A, Lyons P, Skirrow RG, Stuart-Smith PG, Miró R (1998) U–Pb, Th–Pb and Ar–Ar geochronol- ogy from southern Sierras Pampeanas, Ar- gentina: Implications for the Palaeozoic tec- tonic evolution of the western Gondwana margin. Geological Society of London, Spe- cial Publication 142: 235–258 Stacey FD (1960) Magnetic anisotropy of ig- neous rocks. Journal of Geophysical Re- search 65: 2429–2442 Steenken A, López de Luchi MG, Siegesmund S, Wemmer K, Pawlig S (2004) Crustal provenance and cooling of the basement com- plexes of the Sierra de San Luis: An insight into the tectonic history of the proto-Andean margin of Gondwana. Gondwana Research 7: 1171–1195 4