Geochemistry of middle Cambrian to early Ordovician metasedimentary rocks of the Sierra de Los Llanos (Sierras Pampeanas, Argentina): Source of sediments, correlations and geotectonic setting.

Abstract

The metamorphic complex of Sierra de Los Llanos is composed of low- to high-grade metasedimentary rocks, namely phyllites, schists, gneisses and migmatites that out crop like large septa within lower Ordovician magmatic units. Cornubianite associated with the intrusion of basic composition bodies are locally recognized. Chemical compositions of these metasedimentary rocks are equivalent to pelite and arenite (sublitharenite-subarkose). Chemical ratios of Ti/Zr (5.4-8.4), La/Sc (5.2-8.5), Th/Sc >1 (2.2-3.4), La/Th (2.0-2.5) and Zr/Sc (60-103), along with contents of Hf (7.9 a 12.4 ppm) and Sc (4.4-5.2 ppm) in meta-psammites, suggest that the source of the sediments was enriched in felsic components or recycled material and limited participation mafic components. A similar composition of source areas for meta-psammites of this complex and Chepes, La Cébila and Pringles metamorphic complexes is suggested from geochemical data. Ti, Zr, Hf, La, Th and Sc concentrations are variable and do not allow to clearly define a specific geotectonic context and suggest that the composition of the protoliths was largely controlled by sedimentary processes rather than compositional heterogeneity of the sources. The integration of geochemical and known geochronological data and the regional correlation of Ordovician metamorphic complexes strengthen the interpretation that defines igneous and metamorphic rocks from Pampean orogenic belt and Río de La Plata craton as the mayor sources of these complexes. Furthermore, this information is consistent with the existence of an extensive continental shelf in the southwestern margin of Gondwana during the Middle Cambrian to Early Ordovician.