Mantle xenoliths in Triassic camptonite dykes of the Predazzo Area (Dolomites, Northern Italy) petrography, mineral chemistry and geothermobarometry

Abstract

A suite of spinel lherzolite xenoliths from the Triassic Igneous Complex of Predazzo (Dolomites, Northern Italy) was investigated for petrography and mineral composition. The xenoliths are hosted by camptonitic dykes and have protogranular to weakly porphyroclastic texture with the following mineral assemblages: forsteritic olivine, enstatitic orthopyroxene, diopsidic clinopyroxene and Cr-bearing spinel. Rare xenoliths also contain pargasitic amphibole. Reasonably consistent values were obtained by applying various thermometric calibrations to the rim compositions of the lherzolite minerals. For most xenoliths, the four phases (clinopyroxene, orthopyroxene, olivine and spinel) reached equilibrium at temperatures between 800 and 950°C. An earlier stage, at higher temperature (> 1000°C) and pressure (> 2.5 GPa), is suggested by clusters of Cpx+Opx+Spl after probable garnet precursors, the presence of exsolution lamellae in some clinopyroxenes and the core composition of some pyroxenes. Ca-in-olivine barometry yields values (1.2–1.6 GPa) consistent with the maximum pressures (2.4 GPa) indicated by spinel composition and the pargasitic amphibole stability field (up to 3.0 GPa). The pressure field defined by the lherzolite minerals corresponds to a depth of about 45 km within the upper mantle. During host magma ascent, most xenolith minerals did not undergo appreciable re-equilibration.