Petrology of an eclogite-metarodingite suite at Cima di Gagnone, Ticino, Switzerland

Mafic rocks composed essentially of garnet + clinopyroxene occur in association with metamorphic ultramafic rocks in the Penninic Adula-Cima Lunga Nappe around the summit of Cima di Gagnone, Ticino, Switzerland. The mafic rocks are interpreted as basaltic volcan-ics and minor intrusives and their mildly and strongly rodingitized products, later metamorphosed under eclogite facies conditions. They show continuous variation in their bulk chemical, mineralogical, and textural properties from eclogite (metabasaltic, Cpx with Jd > 10 percent) to metarodingite (CaO ≈ 24 weight percent, Na₂0 < 0.) weight percent). Garnets in this series vary continuously from 84 percent pyralspite, 16 percent ugrandite (in eclogite), to 42 percent pyralspite, 58 percent ugrandite; some highly calcic metarodingites contain garnet with 80 percent ugrandite. Clinopyroxenes vary correspondingly from omphacite (45 percent Jd) to diopside (<1 percent Jd) and eventually to fassaite (23 percent Cats). The metarodingites differ from grospydites and kyanite eclogites in kimberlites by the absence of kyanite and presence of diopside rather than omphacite; they also differ in bulk composition.

Abundances of some trace and minor elements appear to have been little affected by the process of rodingitization or by the succeeding episodes of eclogite and amphibolite facies metamorphism. They suggest sea-floor tholeiitic basalt affinities and a possible oceanic origin for the ultramafic-mafic rock suite at Cima di Gagnone. An eclogite facies garnet Iherzolite assemblage is preserved in one of the ultramafic bodies.

The K_D for Fe_total/Mg partitioning between garnet and clinopyroxene averages 6 in the eclogites, and rises, with increase in grossular component in the garnet, to 15 in the more calcic metarodingites. The Råheim and Green temperature calibration applied to eclogites with garnets containing up to 25 percent grossular component, and a new calibration by Ganguly suitable for low-Na pyroxene-calcic garnet pairs applied to the metarodingites, give comparable values and suggest conditions for the eclogite facies metamorphism were likely to have been 800°C, P ≈ 25 kbar. Similar values have previously been obtained for the Gagnone garnet Iherzolite. These conditions do not overlap those estimated for the Tertiary Central Alpine Barrovian-style metamorphism in the area (600±100°C, P < 10 kbar).

The Central Alpine metamorphic overprint did not reequilibrate Fe and Mg between garnet and pyroxene. However, it partially amphibolitized the eclogites and was responsible for hornblende and epidote growth in the metarodingites. Contact reaction zones (probably polymetamorphic) between metarodingite boudins and ultramafic rock contain hornblende, diopside, epidote, sphene, and chlorite; contact zones against eclogite contain the same minerals, except for garnet instead of diopside. Phlogopite and staurolite have been found in amphibolitized eclogite.

Assuming metasomatic rodingitization to be an exclusive accompaniment of serpentiniza-tion, the mafic-ultramafic rock suite at Cima di Gagnone must, at an early stage in its metamorphic history, have been in a low T, low P environment. This was followed by subduction and eclogite facies metamorphism, then tectonic uprise to shallower depths, and regional Barrovian-style metamorphism.