Trace elements and Sr–Nd–Pb isotopes of K-rich, shoshonitic, and calc-alkaline magmatism of the Western Mediterranean Region: Genesis of ultrapotassic to calc-alkaline magmatic associations in a post-collisional geodynamic setting

Abstract

High-MgO ultrapotassic rocks are found in four different areas of the Western Mediterranean basin associated in space and time with shoshonitic and calc-alkaline rocks. They represent different magmatic events at the active continental plate margin from Oligocene to Pleistocene. These rocks are found within the Western Alps (Northern Italy), in Corsica (France), in Murcia-Almeria (South-Eastern Spain), and in Southern Tuscany (Central Italy). Ultrapotassic terms are mostly lamprophyres, but olivine latitic lavas with a clear lamproitic affinity are also found. Lamproite-like rocks range from slightly silica under-saturated to silica over-saturated, and they are characterised by low Al 2 O 3 , CaO, and Na 2 O contents. They are plagioclase-free rocks, but K-feldspar is abundant beside other K-bearing phases. Shoshonitic and calc-alkaline rocks are invariably space associated to lamproites, and they either precede or follow them. High-Mg ultrapotassic rocks are characterised by strong enrichment of incompatible elements, which prevent further enrichment due to shallow level crustal contamination. K 2 O and incompatible element contents decrease passing from high-Mg ultrapotassic to high-Mg shoshonitic and calc-alkaline rocks suggesting that K and incompatible trace elements enrichments are a primary characteristic. Ultrapotassic to calc-alkaline rocks from Western Mediterranean regions, in spite of their different age of emplacement, are characterised by similar incompatible trace elements distribution. Depletion of High Field Strength elements with respect to Large Ion Lithophile elements is observed. Positive spikes at Th, U, and Pb, with negative spikes at Ba, Nb, Ta, Sr, P, and Ti, are common characteristics of ultrapotassic (lamproitic) to high-K calc-alkaline rocks. Ultrapotassic rocks are extremely enriched in radiogenic Sr and unradiogenic Nd with respect to the associated shoshonitic and calc-alkaline rocks. Different isotopic values are distinctive of the different magmatic provinces irrespective of magmatic affinities. 87 Sr/ 86 Sr i ranges between 0.71645 and 0.71759 for Western Alps lamproites, between 0.71226 and 0.71230 for Corsica lamproite, between 0.71642 and 0.72259 for Murcia-Almeria lamproites, and between 0.71578 and 0.71672 for Tuscany lamproites. Radiogenic Sr decreases along with K 2 O through shoshonitic to calc-alkaline rocks. Conversely 143 Nd/ 144 Nd i values increase with decreasing K 2 O, with the highest value of 0.51243 found for the one samples from Murcia-Almeria. Contrasting trends are observed among initial values of lead isotopes, but all falling well within the field of upper crustal rocks. Different trends of 207 Pb/ 204 Pb i and 208 Pb/ 204 Pb i vs. 206 Pb/ 204 Pb i for samples from the different provinces are observed. Several evidences indicate that most of the magmas of the different provinces have been generated in a depleted upper mantle (i.e., lithospheric) modified by metasomatism, but an asthenospheric component is also recognised in Corsica. At least two different s...

Post-collisional polycyclic plutonism from the Zagros hinterland: the Shaivar Dagh plutonic complex, Alborz belt, Iran

“…All together, the Cycle 2 rocks define a straight linear trend in the MALI-SiO 2 diagram (Fig. 3b) crosscutting the field boundaries and almost coincident with typical potassic series of the Mediterranean province (Conticelli et al 2009). …”

Cenozoic Mg-rich potassic rocks in the Tibetan Plateau: Geochemical variations, heterogeneity of subcontinental lithospheric mantle and tectonic implications

“…MPR from southern Tibet show a negative correlation between P 2 O 5 and SiO 2 , whereas the northern samples do not. MPR from southern Tibet have similar characteristics to those of Mg-rich potassic rocks in the Western Mediterranean basin (WMS; Conticelli et al, 2009).…”

Geochronology, geochemistry and geodynamic implications of the Late Mesozoic volcanic rocks in the southern Great Xing’an Mountains, NE China

“…shoshonitic and calc-alkaline rocks Conticelli et al, 2009aConticelli et al, , 2009b). This genetic model has been applied to shoshonitic series rocks associated with ultrapotassic rocks, high-K calc-alkaline rocks, or even to calc-alkaline rocks (e.g., Duggen et al, 2005;Altherr et al, 2008;Avanzinelli et al, 2009;Conticelli et al, 2009a;Yang et al, 2012).…”

“…This genetic model has been applied to shoshonitic series rocks associated with ultrapotassic rocks, high-K calc-alkaline rocks, or even to calc-alkaline rocks (e.g., Duggen et al, 2005;Altherr et al, 2008;Avanzinelli et al, 2009;Conticelli et al, 2009a;Yang et al, 2012). On the other hand, K-rich shoshonitic magma also can be derived from partial melting of the thickened lower crust, following underplating by mantlederived magma in a post-collisional extension setting (Kuster and Harms, 1998;Jiang et al, 2002;Campbell et al, 2014;Liu et al, 2014).…”