Correlation of growth and breakdown of major and accessory minerals in metapelites from Campolungo, Central Alps

Regionally metamorphosed pelitic rocks at Campolungo, Central Alps, contain biotite, muscovite, garnet, staurolite, kyanite, and quartz, and the minor minerals tourmaline, plagioclase, chlorite, rutile, and ilmenite. Accessory allanite, apatite, monazite, potassium feldspar, xenotime, and zircon have also been identified. The bulk-rock chemical composition is similar to that of shales, and indicates that the protolith was deposited in an active continental margin setting. Element distribution maps, electron microprobe analyses and in situ UV–laser ablation inductively coupled plasma mass spectrometry data document a pronounced zoning in garnet and tourmaline porphyroblasts. Garnet displays a typical bell-shaped MnO zoning profile, with a maximum (∼3 wt %) in the euhedral core. Cores are also rich in Y and heavy rare earth elements (HREE; e.g. 2150 ppm Y). In their broad rim, all garnet crystals display a subhedral annulus (10–15 µm wide), which is distinctly enriched in Ca, Sr, Y, and HREE, and which probably resulted from the breakdown of allanite (at ∼550°C, ∼6·4 kbar). Another characteristic feature of garnet rims is their sinusoidal chondrite-normalized REE pattern, which may represent partial equilibration with a light REE-enriched medium, probably generated through the breakdown of metamorphic allanite. Similar REE patterns are exhibited by a Ca-poor internal zone (inside the annulus), which may represent an earlier partial equilibration following the breakdown of detrital monazite. The large tourmaline crystals exhibit an optically visible three-stage zoning, which comprises: a euhedral core; a continuously zoned inner rim with a prominent euhedral Ca-rich annulus; and an outer rim, which also displays a distinct Ca-rich annulus and is separated from the inner rim by a sutured boundary. This boundary represents a marked chemical discontinuity, characterized for example by a decrease in the Zn concentration from 250 ppm (inner rim) to 20 ppm (outer rim). This change in Zn content reflects staurolite growth, which started after resorption of the inner rim of tourmaline and after a major deformation event. This chemical and textural discontinuity coincides with a marked shift in δ18O, which increases by ∼0·8‰ across the inner rim–outer rim boundary. Our thermodynamic models suggest that resorption of the inner rim of tourmaline may be associated with small amounts (5–7 vol. %) of melt formed at ∼650°C and 8·5 kbar. By using detailed textural observations, major and trace element zoning patterns and thermodynamic data, it was possible to model the metamorphic evolution of these rocks in considerable detail and, specifically, to correlate the growth and breakdown of major and accessory minerals.