Mapping trace-element zoning in garnet from mylonitic micaschist of NE Sardinia, Italy

Marcello Franceschelli

First

;Gabriele Cruciani

Second

;Dario Fancello

Penultimate

;Daniela Rubatto

Last

2023-01-01

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Abstract

The relative slow diffusivity of trace elements in garnet is particularly suited to preserve garnet growth zones that results from complex metamorphic evolutions. As for major elements, the complexity of trace element distribution is best investigated by 2D mapping. This approach is applied to a garnet porphyroblast from a mylonitic micaschist along the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan chain. The micaschist consists of quartz, white mica, biotite, garnet, staurolite, chlorite, plagioclase and chloritoid that reflect amphibolite-facies metamorphism. Accessory phases are ilmenite, rutile, zircon, monazite, apatite and tourmaline. Garnet porphyroblasts are enveloped by the S2 schistosity that is marked by the alternation of quartz-feldspathic and micaceous layers. They show a typical texture with distinct core and rim. The garnet cores contain numerous inclusions of quartz, rutile, apatite, monazite and zircon that define a rotated foliation with “snowball garnet” microstructure. In order to assess the relative behaviour of major and trace elements and gain insight into the garnet growth process, a large garnet crystal (ca. 6 mm in diameter) was investigated by LA-ICPMS mapping. Major element zoning evidences a wide core (ca. 4 mm; Alm45; Prp1;Grs25; Sps29) characterized by bell shaped zoning. The grossular and spessartine components progressively decrease, whereas almandine and pyrope increase, towards the 2 mm thick rim (Alm86; Prp11;Grs3; Sps1). Despite the relatively simple major element zoning, trace element distribution is more complex. The boundary between core and rim is marked by a thin and sharp annulus enriched in Y and HREE (Tb, Dy, Ho, Er, Tm, Yb and Lu). The annular enrichment supports a history of the garnet with partial resorption after the core growth. The garnet core consists of an inner and an outer zone where the maximum concentration of elements from Lu to Tb progressively moves outwards with decreasing atomic number. This trend continues in the rim outside the annulus, where a further distinction between a Sm-, Eu-, Tb-rich inner rim and a REE-poor outer rim is revealed by the maps. The REE total content (ΣREE) of the inner core is significantly higher (200-400 ppm) than that observed in the outer rim (10 ppm). The strong REE fractionation between garnet core and rim results in distinct REE patterns with chondrite-normalised abundances up to 1000 for HREE in the core, whereas the outer rim show less fractionated REE patterns with HREE chondrite-normalised abundances up to 10. The progressive zoning of HREE to LREE from core to rim is in line with diffusion limited uptake of REE, as previously described in amphibolite and eclogite facies garnet. Superimposed on this core-rim pattern are additional complexities and fluctuations in trace element concentrations that are best explained by local availability of elements and variability in transport mechanisms (e.g. Konrad-Schmolke et al. 2022, J. metamorphic Geol., 10.1111/jmg.12703).