TRACE-ELEMENT DISTRIBUTION BETWEEN CLINOPYROXENE AND GARNET IN GABBROIC ROCKS OF THE DEEP CRUST - AN ION MICROPROBE STUDY

Clinopyroxenes and garnets from gabbroic rocks of the Ivrea Verbano mafic complex have been analyzed by electron microprobe for major elements and by ion microprobe for REE, Sc, Cr, Ti, V, Zr, Na, and Sr content. The samples represent two petrographic types: in the first, garnet is formed by subsolidus reaction and occurs in coronas (c-type); in the other, garnet occurs as large porphyroblasts (p-type) and may have been a phase on the liquidus. Clinopyroxenes and garnets are unzoned (with one exception) for major and trace elements, suggesting that, in general, equilibrium has been attained under granulite facies conditions as indicated by the geothermometers. Clinopyroxene, although affected in its HREE and Sc content by the coexistence with garnet, has REE patterns which vary, along with the bulk rock patterns, stratigraphically upwards from LREE-depleted to LREE-enriched. Trace element distribution coefficients (D) between clinopyroxene and garnet, as measured in the p-type assemblages, vary systematically with major-element compositional parameters such as FeO, MgO, FeO/MgO, Al2O3, Na2O, and apparent equilibration temperature. In addition, the overall pattern of REE partitioning, D(Ce) to D(Yb), is significantly steeper than those found in previously published estimates, except when these were determined on exceptionally carefully prepared mineral separates. The D values determined on c-type assemblages are comparatively erratic and appear to depend on the modal gnt/cpx ratio. This feature is tentatively attributed to failure to achieve complete equilibrium during slow cooling when the corona structures were formed. Subsolidus reequilibration between phases has generally obliterated the igneous phase chemistry of the rocks sufficiently so that the composition of the parent liquid cannot be determined from those of the constituent minerals even when these represent original "phenocrysts."