Overview of Along-arc Variation of Magmatism
Tertiary orogenic activity shows important compositional variations along the Alpine chain, highlighting strong heterogeneities of mantle sources at a regional scale as well as various modalities of magma emplacement and evolution, as a consequence of variable structural setting along the Alps. The main compositional variations can be summarised as follows:
1 – Calcalkaline to shoshonitic magmatism occurs in all the sectors of the Alpine chain (Fig. 15a). Ultrapotassic alkaline rocks, however, are restricted to the Western Alps. In the Eastern Alps, some Na-alkaline rocks are found in association with calcalkaline and shoshonitic rocks. Moreover, CaO of mafic rocks increases from western to eastern Alps (Fig. 15b). Al2O3 also shows the same behaviour (Fig. 15d). Since major element abundances of mafic magmas basically depend on the composition and proportions of mineral phases that enter into the melt during magma formation, major element variation along the Alps point to mineralogically heterogeneous mantle sources and/or different conditions of melting.
2 – Incompatible element ratios for mafic rocks (MgO > 4 wt%) also show large variations along the Alps (Fig. 15c,d), implying again important lateral variations of mantle compositions. Some element ratios (e.g., Rb/Ba) show different trends for Western and Eastern Alps (Fig. 15d), indicating different types of metasomatic modification in the upper mantle along the Alps. Finally, the mafic rocks from Eastern Alps exhibit incompatible element ratios (especially LILE/HFSE, such as Rb/Nb) that are similar or overlap with those of the intraplate rocks of the Veneto Volcanic Province (VVP). This supports a role of OIB-type components in the origin of orogenic magmatism of Eastern Alps. These components may represent composition of the popre-metasomatic mantle wedge or, alternatively, may derive from deep mantle components that are also responsible for the origin of VVP.
3 – Crustal anatectic melts apparently are more abundant in the Central and Eastern Alps than in the Western Alps. Moreover, magma volumes and, to a lesser extent, degrees of magma evolution are larger in the east. This suggests different thermal regimes, structural settings and/or type of intruded crust.
4 – There is a wide range of Sr-Nd isotopic compositions for the Alpine orogenic magmatism. The negative correlation in the Sr-Nd space suggests that both mantle and crustal end-members contributed to magmatism (Fig. 16a). Some of this interaction occurred by contamination of mantle-derived melts during magma ascent within the crust, as discussed earlier. However, if only the mafic rocks are considered one can see that there is still a very wide variation of Sr-Nd isotopes. Quantitative modelling clearly indicate that these variations cannot be due to intra-crustal processes and require mantle contamination by upper crust during subduction. Since the highest Sr and lowest Nd isotopic values are encountered in the Western Alps (Figs. 15c,16), it can be concluded that mantle contamination by subducted upper crust was stronger in the west.
5 – In spite of the wide Sr-Nd isotopic variations summarised above, Pb isotopic compositions of orogenic rocks are remarkably similar all over the Alpine arc (Fig. 16b). In particular, 206Pb/204Pb values are mostly comprised between 18.6 and 18.8, a range that is much more narrow that crustal compositions (Fig. 16b). Variation of 206Pb/204Pb vs. 87Sr/86Sr shows that Alpine magmas define a vertical trend, which points towards upper continental crust. This supports the idea that the magmatism was the products of mantle-crust interaction, as concluded on the basis of Sr-Nd isotopic evidence. However, the narrow range of Pb isotope ratios would require that a single type of crustal composition, rather than different crustal rocks, was involved in the interaction with mantle. This corresponds roughly to an average of the crustal values reported in Fig. 16b. Alternatively, the narrow range of Pb isotope signatures may reveal a homogenisation of Pb isotopes in the crustal material during mantle contamination. Such a process could have not affected Sr-Nd isotopes, possible because of the lower mobility of these elements.
Figure 15. Compositional variations along the Alps
Variation of key major and trace element abundances and ratios, and 87Sr/86Sr for Tertiary mafic igneous rocks (MgO > 4%) from the Alps. The field of the anorogenic Veneto Volcanic Province (VVP) is also reported.
Figure 16. Radiogenic isotope variations, Alps
A - 143Nd/144Nd vs. 87Sr/86Sr diagram for Tertiary mafic igneous rocks (MgO > 3%) from the Alps. B - 87Sr/86Sr vs. 206Pb/204Pb for Tertiary igneous rocks from the Alps. The composition of depleted MORB mantle (DMM; Workman & Hart, 2005) and fields of continental upper-crustal rocks from the Alpine area (UCC) and anorogenic volcanic rocks from the Veneto Province (VVP) are also reported (Lustrino M. & Wilson M., 2007; Macera et al., 2008 with references).