PETROLOGY AND MINERALOGY OF GRANULITE-FACIES MAFIC XENOLITHS (SARDINIA, ITALY): EVIDENCE FOR KCL METASOMATISM IN THE LOWER CRUST

Abstract

Here new mineralogical data is presented on the occurrence of K-feldspar in granulite-facies metagabbronorite xenoliths found in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of variably evolved subduction-related basaltic liquids, which underwent cooling and recrystallisation in the deep crust (T = 850-900 °C, P = 0.8-1.0 GPa). They consist of orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An = 50-66 mol%) in a granoblastic, recrystallised, quartz-free matrix composed of pyroxene + plagioclase (An = 56-72 mol%) + Fe-Ti oxides ± K-feldspar ± biotite ± fluorapatite ± Ti-biotite. Texturally, the K-feldspar occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of plagioclase grains in the form of antiperthite; (2) micro-veins along plagioclase-plagioclase and plagioclase-pyroxene grain rims; (3) myrmekite-like intergrowths with Ca-rich plagioclase along plagioclase-plagioclase grain boundaries; and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of K-feldspar is characterized by relatively high albite contents (16-33 mol%). An increasing anorthite content in the plagioclase towards the contact with the K-feldspar micro-vein and myrmekite-like intergrowths into the K-feldspar along the plagioclase-K-feldspar grain boundary are also observed. Small amounts of biotite (TiO2 = 4.7-6.5 wt.%; F = 0.24-1.19 wt.%; Cl = 0.04-0.20 wt.%) in textural equilibrium with the granulite-facies assemblage is present in both K-feldspar-bearing and K-feldspar-free xenoliths. These K-feldspar textures suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the fluorapatite in these xenoliths, which is enriched in Cl (Cl = 6-50% of the total F + Cl + OH). These lines of evidence suggest that formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during and after in-situ high-T recrystallisation of relatively dry rocks. © 2006 Elsevier B.V. All rights reserved.