Patterns of rare earth and other trace elements in Paleogene and Miocene clayey sediments from the Mondego platform (Central Portugal)
Introduction
The sediments’ chemical composition result from a combination of factors such as: the composition of the source area, intensity of alteration, calibration and sediment diagenesis, which will determine its final characteristics (Piper, 1974, McLennan, 1989). This paper focuses on the geochemical characterization of silt-clay facies that constitute important clay resources for construction ceramics and integrate deposits occurring in inland basins, west of Coimbra, in the designated Mondego platform. These facies mostly belong to two formations, assigned respectively to the Paleogene and Neogene: Coja and Campelo formations (Cunha, 1992), which have been studied before, from a stratigraphic and sedimentological approach (Carvalho, 1960, Soares et al., 1983, Cunha, 1992, Cunha, 1999). More recently these formations’ silt-clay facies were assessed regarding the relationship between the composition and most relevant ceramic properties (Lisboa, 2009, Lisboa et al., 2013) as both facies provide the main supply of clays for ceramics in the Mondego planation.
The composition and particularly the presence of lithic clasts suggest a metasedimentary and granitic provenance; the latter especially concerning the Coja formation (Reis et al., 1991), but so far geochemical techniques have not been applied to the provenance assessment or the tectonic setting of these sediments. Other studies within the study area broaching mineralogy and geochemistry of Cretaceous sediments were carried out by Prudêncio et al., 1988, Prudêncio et al., 1989.
This study complements the preliminary geochemical study performed on the major elements (Lisboa, 2009) of the silt-clay facies, towards a better knowledge on the provenance, tectonic setting and weathering in the source area, using trace elements and REE, on which studies regarding these sediments are not known. As these facies present similar compositional characteristics, particularly from a textural point of view, hindering its distinction, the geochemical characteristics of both facies are examined in an attempt to reveal relationships or, on the opposite, distinctive geochemical signatures if any, among the sediments. Given that both facies constitute the exploited raw material for ceramics, the knowledge of their genetic inter-relationships is essential to create raw material stocks with controlled mean quality (constant quality) and also to discriminate clay fillings derived from different sources.
Section snippets
Study area and geological setting
The study area is located in the central zone of Portugal (Fig. 1I), east of Coimbra and west of the Portuguese central mountain range, corresponding to a large polygenic planation surface (Mondego planation) that encompasses Precambrian and Palaeozoic age metasediments (Beiras Group) and Variscan granitoids. Continental fluvial and alluvial deposits (Cretaceous and Cenozoic age) were partly preserved in tectonically depressed areas, which are confined by two major faults: Miranda do Corvo and
Methods
Twenty eight representative samples of clayey facies, 12 from the Monteira Member (M1 to M12) and 16 from the Arroça Member (A1 to A16), were collected mostly in clay pits along the Mondego planation. Apart from trace element chemical analysis by ICP-MS undertaken at Activation Laboratories (Ontario, Canada), the analytical tests on samples were performed in the LNEG laboratory.
The grain size distribution analysis procedure involves determination of: (a) >354 μm size fraction using wet sieving,
Texture and mineralogy
The samples show a marked homogeneity in grain size distribution. According to Shepard (1954) classification the samples fall in the silt field, apart from 4 samples belonging to the monteira member, which fall in the clayey silt field (Fig. 2).
Average mineralogical compositions (Table 1) indicate quartz as the major mineral in both groups of samples and tending to higher concentration in the coarser ones. Feldspar and hematite occur in minor amounts, though the feldspar content is higher in
Weathering, sorting and recycling
In the feldspar diagram Al2O3 − (CaO + Na2O) − K2O (A-CN-K) (Nesbitt and Young, 1982, Nesbitt and Young, 1984, Nesbitt and Young, 1989, Nesbitt and Young, 1996, Nesbitt et al., 1996), all the samples display an intense weathering record (Fig. 4). In analogy the Chemical Index of Alteration (CIA, e.g. Nesbitt and Young, 1984, Nesbitt et al., 1996) average value, for both sets of samples is between 75 and 87. This CIA value is slightly higher compared to PAAS, UCC and average shales (70–75, Nesbitt and
Conclusions
The MM (Coja Fm.) and AM (Campelo Fm.) sediments show accentuated compositional homogeneity. On a mineralogical level they are distinguished by significant amounts of smectite and interstratified clay minerals in MM, which is reflected by the chemical composition with slightly higher content of alkaline and alkali earth metals. The trends observed in SiO2, Al2O3 and K2O concentration reflect energy loss in sedimentary loads from the NE to NW and increase in sedimentation with clayey illitic
Acknowledgements
Fernando Rocha is grateful for financial support from Geobiotec through projects PEst-C/CTE/UI4035/2011 and PEst-C/CTE/UI4035/2014. The authors are grateful for the constructive review by J. Farinha Ramos. Additionally, the authors would like to thank a panel of two anonymous reviewers and the editor, Martin Dietzel for especially pertinent comments and suggestions that have greatly improved the quality of the manuscript.
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