Resumo

DI PAOLA, Elda  e  GONZALEZ, Mirta. Minerales pesados: asociaciones terciario-cuaternarias y provincias actuales. Sierra de San Luis: Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 1995, vol.2, n.1-2, pp.91-104. ISSN 1853-6360.

A grant of the National Council of Scientific and Technical Research (CONICET) allowed to perform geologic studies and recollection of samples both from Tertiary-Quaternary sequences and from present sediments of the alluvial plains of the austral portion of the Sierra de San Luis, at Province of San Luis, Argentina (Fig. 1). Heavy minerals of 30-125 microns grade of size were studied in order to characterize the heavy mineral suites of present provinces and the associations of the older sequences. Heavy minerals were chosen because they are sensitive indicators of provenance and other processes as tectonism, weathering, etc., proving to be a very useful tool to distinguish stratigraphic units of continental origin, when they are barren of marker fossils (Sozzi & Ojeda, 1992). Four heavy mineral provinces were defined. PI, in the surroundings of Nogolí locality; PII which includes an area with several ephemeral creeks, located at the south of Nogolí River; PIII, that includes the Río Quinto basin and Río Chorrillos depression, covering more than 2000 km2 and PIV, located to the southeast of the Sierra in the vicinity of Cerro El Morro (Fig. 1). PI heavy mineral association is dominated by green hornblende, derived from the erosion of amphibolites and gneisses, and by subordinate quantities of epidote, apatite, zircon, garnet and turmaline. The most frequent mineral in PII is epidote derived from La Escalerilla Granite, and minor quantities of apatite, homblende, zircon, garnet and orthopyroxene; the last coming from mafic or ultramafic complex (Fig. 2). PII has a greater number of species such as green, blue, brown and basaltic horblende, as well as important amounts of orthopyroxenes, clinopyroxenes (augite) and sillimanite (Fig. 3). The garnet-sillimanite ratio in this province is about 2:1-3:1 when it is provided by areas composed of quartz-micaceous schists and gneisses, while the proportion between orthoand clinopyroxene is about 2:1-1:1 when ultramafic rocks were eroded. PIV has been subdivided into two subprovinces, the northern one with a dominant suite of homblende and epidote, derived from basement rocks, whereas the southern part is composed of a high percentage of volcanic clinopyroxene (Fig.4). Tertiary-Quaternary heavy mineral suites were determined in several localities, such as Nogolí, on the west side of the Sierra, and in Potrero de los Funes, Estancia La Petra, Las Cañitas and Río Quinto, in the south (Fig. l). In general there is a difference between the composition of Tertiary and Quaternary heavy mineral assemblages, due to the presence of different species or to different percentages of the minerals (Fig.5). The performed studies clearly show that heavy mineral suites do accurately reflect the source area, while other factors, which might modify the heavy minerals suites, like weathering and diagenesis, can be excluded for this area. Older tertiary sediments are devoid of volcanic minerals, restricting the deposition of the strata to a time before the volcanic events took place. Block faulting episodes are considered to be the major process controlling erosion and deposition, by modifying exposed geology of the source areas and the topography of the source and the deposition area. A recent example is the Río Chorrillos which at present is separated from Río Quinto basin by a subsoil structure that operates as a local water divisory (Fig. l). Heavy minerals seem to preserve a "memory" of primitive provenance areas, prior to assemblage modifying neotectonic episodes. By means of this "memory" it could be deduced that Río Chorrillos was once related to Río Quinto and that La Petra creek once eroded mafic-ultramafic rocks, whose outcrops at present are far away from its headwaters. Climate, which is another factor in mineral stability, has been arid and semiarid since Los Andes elevation. Good indicators for this are the unaltered detritus, the abundance of calcareous cements and of calcretes of Paso de las Carretas Formation. Another conclusion is that the ZTR index (Hubert, 1962) should not be used in active tectonic environments, while the concepts of "maturity" and "recycling" should be analyzed; in Sierra de San Luis "recycling" of immature sedimentites provides immatures detritus. An attempt has been made to correlate heavy mineral suites with tectonic settings derived from modes of sandstones, sensu Dickinson (1984). The PI and PII heavy mineral assemblages can be related with basement uplift of continental blocks, while PIII and PIV may be considered to be the result of the erosion of areas of composite provenance, which include recycled orogen, magmatic arc and basement uplift.

Palavras-chave : Heavy mineral assemblages; Present mineralogical provinces; Tertiary and Quaternary suites; Sierra de San Luis; Provenance; Tectonic setting; ZTR Index.

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