The effect of crustal contamination on ultrapotassic magmas with lamproitic affinity: mineralogical, geochemical and isotope data from the Torre Alfina lavas and xenoliths, Central Italy

Abstract

Torre Alfina, a Quaternary volcano located in the northernmost part of the Roman Province, is characterized by two main types of mafic ultrapotassic lavas with slightly different chemical, isotopic, and mineralogic characteristics. Mica-rich enclaves, peridotitic and granulitic xenoliths, and upper crustal xenoliths are hosted by the lavas. Both of the lavas have the assemblage olivine+clinopyroxene+sanidine+phlogopite with mafic phases showing insufficient Si+Al cations to completely fill the tetrahedral sites, as is typical of lamproitic minerals. Other features are also transitional between lamproite mineralogy and Roman-type mineralogy. Chemically, the host lavas have high Mg/Mg+Fe values (71–77) and compatible trace element contents (e.g., Ni=245–249 ppm), as well as high K 2O (5.7–7.5 wt.%) and incompatible trace element abundances. Furthermore, they have high initial 87 Sr/ 86 Sr (0.7159–0.7165) and low initial 143 Nd/ 144 Nd (0.51211–0.51212) values. Passing from the most primitive through the more evolved lavas, the Sr isotopic ratios, (Tb/Yb) N ratios, and SiO 2 and Al 2O 3 contents increase, whereas the abundances of both the compatible and incompatible trace elements decreases. Crustal xenoliths are represented by shale, sandstone, gneiss, micaschist, and granulitic rock fragments. They have variable chemical compositions with 87 Sr/ 87 Sr and 143 Nd/ 144 Nd values, calculated at the time they were trapped by the magma, in the ranges 0.7136–0.7176 and 0.51204–0.51209, respectively. Digestion of crustal rocks, such as those included in the lavas, is not able to drive the magmas from Roman-type to lamproitic compositions. Mass-balance calculations show that extensive crustal contamination of lamproitic parental magma can produce the observed chemical, isotopic, and mineralogical characteristics of the Torre Alfina rocks. Since no crustal rocks with incompatible trace element abundances as high as those of lamproitic rocks are common at the earth's surface, lamproitic magmas assimilating crustal rocks become diluted in incompatible trace elements, and enriched in silica and alumina.