Provenance of the Balcarce Formation: indicator for Early Palaeozoic volcanism in eastern Argentina?

ZIMMERMANN, U.; SPALLETTI, L.

Mendoza

Simposio; Gondwana 12 Conference; 2005

Injternational Association for Gondwana Research

The Balcarce Formation is interpreted as a Lower Palaeozoic succession made of quartz-arenites, kaolinitic fine-grained siliciclastics and conglomerates. Based on trace fossils, a Cambrian - Ordovician age was assigned by Poiré et al. (2003). The rocks are exposed in the Tandilia System, a northwest-southeast striking mountain belt located 300 km south of Buenos Aires city. Palaeoproterozoic crystalline rocks are so far interpreted as the basement (Pankhurst et al., 2003), covered by Neoproterozoic rocks of the Sierras Bayas Group. These rocks are, in turn, followed by a mainly fine-grained heterolithic succession (Cerro Negro Formation, Las Águilas Formation, Punta Mogotes Formation and Sierra del Volcán Diamictites). The Balcarce Formation was unconformably deposited on the basement or the mentioned siliciclastic formations. The sedimentary rocks of the Balcarce Formation are interpreted as developed in a nearshore and inner shelf environment on a tide dominated platform, affected by storm events, and it is suggested that the marine system was open to the south based on progradational clinoforms (Poiré et al., 2003). Geochemical data show that the Balcarce Formation was deposited on a rifted margin composed mainly of sub-rounded to angular quartz. Underlying rocks of the Sierras Bayas Group, the Cerro Negro Formation and the tillite from the Sierra Volcán are unlikely source rocks for the Balcarce Formation, based on petrographic and geochemical characteristics. Interesting is the occurrence of rutile in the eastern part of the basin, absent in the underlying rocks. However, the provenance of the rutile is so far unknown. The source terrane to the north suggested by Poiré et al. (2003) was preferably composed of (fine-grained sedimentary) upper crustal rocks and high-grade quartz-rich metamorphic sequences as well as schists. Palaeocurrent trends (Teruggi, 1964; Poiré et al., 2003) were assumed to be dominated by east-west oriented coast-parallel streams. Therefore detrital provenance from other areas far east cannot be disregarded. The kaolinite-rich mudstones and claystones, interbedded with quartz-arenite in the eastern part of the basin, are composed of fine-grained quartz and partly larger crystals, which show resorption embayments, caused by aggressive fluids or a rapid cooling. The kaolinites are usually flaky, others can be interpreted as products of replacement of feldspar grains, and a third group appear to be probably recrystallised vitric fragments as some grains show “shadows” of glass shards. The geochemical signature, based on rare-earth elements (REE), high-field strength elements, of the kaolinite-rich rocks and some fine-grained clastic rocks points to a volcanic arc or oceanic source. Moreover, the REE patterns of the kaolinite-rich rocks show enrichments in middle REE caused by the addition of Ti-rich phases or/and the existence of plagioclase, now weathered to kaolinite. Assuming the kaolinite-rich rocks represent altered volcanic deposits, they could have derived (i) from the active continental margin of south-west Gondwana: the Famatinian arc active during the Lower Ordovician (Fanning et al., 2004), or (ii) from the Cambrian rhyolitic volcanism identified at the Sierra de la Ventana Fold Belt (Rapela et al., 2003). However, other potential sources, such as (iii) from active continental or oceanic terranes related to the Adamaster Ocean, or (iv) from an active oceanic environment to the east and south-east of the Balcarce basin cannot be disregarded. Further studies are in progress to resolve the genesis of the kaolinite-rich rocks, to decipher the origin of the rutiles and to date the detrital zircons. This will lead, hopefully, to a better understanding of the provenance and palaeotectonic setting of the Balcarce Formation in southwest Gondwana.