THE EFFECT OF CRUSTAL CONTAMINATION ON ULTRAPOTASSIC MAGMAS WITH LAMPROITIC AFFINITY - MINERALOGICAL, GEOCHEMICAL AND ISOTOPE DATA FROM THETORRE-ALFINA LAVAS AND XENOLITHS, CENTRAL ITALY

Torre Alfina, a Quaternary volcano located in the northernmost part of the Roman Province, is characterized by two main types of mafic ultra potassic lavas with slightly different chemical, isotopic, and mineral ogic characteristics. Mica-rich enclaves, peridotitic and granulitic x enoliths, and upper crustal xenoliths are hosted by the lavas. Both of the lavas have the assemblage olivine + clinopyroxene + sanidine + ph logopite with mafic phases showing insufficient Si + Al cations to com pletely fill the tetrahedral sites, as is typical of lamproitic minera ls. 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 K2O (5.7-7.5 wt.%) and incompatible trac e Sr-87/Sr-86 (0.7159-0.7165) and low initial Nd-143/Nd-144 (0.51211- element abundances. Furthermore, they have high initial 0.51212) value s. Passing from the most primitive through the more evolved lavas, the Sr isotopic ratios, (Tb/Yb)(N) ratios, and SiO2 and Al2O3 contents in crease, 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 ha ve variable chemical compositions with Sr-87/Sr-87 and Nd-143/Nd-144 v alues, calculated at the time they were trapped by the magma, in the r anges 0.7136-0.7176 and 0.51204-0.51209, respectively. Digestion of cr ustal rocks, such as those included in the lavas, is not able to drive the magmas from Roman-type to lamproitic compositions. Mass-balance c alculations show that extensive crustal contamination of lamproitic pa rental magma can produce the observed chemical, isotopic, and mineralo gical characteristics of the Torre Alfina rocks. Since no crustal rock s with incompatible trace element abundances as high as those of lampr oitic rocks are common at the earth's surface, lamproitic magmas assim ilating crustal rocks become diluted in incompatible trace elements, a nd enriched in silica and alumina. (C) 1998 Elsevier Science B.V. All rights reserved.