The geochemistry of outcropping rocks as the starting point for the knowledge of a territory (the Valchiavenna area)

Knowledge of rock chemical composition is very important for the evaluation of soil chemistry, particularly as for the trace element content of residual soil corresponds to the characteristics of the parental rock. These elemental concentrations are known as "background values" and their contents can vary by some orders of magnitude in relation to rock chemistry composition. The evaluation of the most recent literature has allowed us to look at the geochemistry of rocks regarding those trace elements which are particularly significant from the environmental point of view, as Cr, V, Ni, Zn , Pb. The distribution of these elements in some zones of the Valchiavenna area clearly shows the relationship with the different rock types. In particular, the ophiolitic complex of Chiavenna, characterised by mafic and ultramafic rocks, shows high values of Cr (>900 ppm) and Ni (>600). On the contrary, in the Truzzo granitic complex and Roffna rhyolites, the main metals are Zn and Pb. Moreover, the comparison between the abundance sequences of the above mentioned elements clearly highlights that : 1) the various rock types have different amounts of trace elements; 2) the sequence of abundance is different as regards rock chemistry; 3) the absolute value of all trace element sums is very different in the various rock types (e.i.>2000 ppm in ophiolites and < 200 ppm in granites). During the development of "Progetto Valchiavenna" performed by Dipartimento di Scienze della Terra "A.Desio", some new samplings of the outcropping rock types were made and the geochemical analyses of major and trace elements have increased the chemical knowledge of all the main rock types in the Valchiavenna area. Some maps of trace element distribution are presented, which are useful in making provision for the elemental amounts in soils. A brief illustration of a case study regarding a restrict zone of the neighbourhood of Chiavenna was reported as an example of the integration of chemical knowledge on rocks, soils and vegetation. The high Mg content of ophiolite outcrops (average 16 wt% MgO, with some values reaching up to 38 wt%) correspond to as many Cr and Ni concentrations ( up to 2000 ppm). Chromium and nickel determined in some soil sections sampled near Chiavenna, Prosto di Piuro and Uschione have shown contents between 450 e 1600 ppm and 400 and 1800 ppm, respectively. These values exceed the mean contents for soils assigned for different uses. The presence of high levels of metals in soils exerts a pressure on plant species leading to the selection of a specific flora; in particular, the ultramafic sites have an unusual flora related to the effects of Mg and Ni toxicities. On the soils of the Chiavenna area, endemic plant species and Ni-hyperaccumulators (e.i. Alyssum) have not been found. Nevertheless, some species like Hypericum perforatum , Brachypodium pinnatum , Vincetoxicum hirundinaria and Silene vulgaris typical of ofiolithic outcrops are present. Moreover, the metal content determined in some species shows higher values in roots than in aboveground part. This is a strategy that the plant displays to defend itself from the effects of element toxicities.