CORRELATION OF GROWTH AND BREAKDOWN OF MAJOR AND ACCESSORY MINERALS IN METAPELITES FROM THE 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. Additionally, accessory allanite, apatite, monazite, potassium feldspar, xenotime, and zircon have been identified. The bulk-rock chemical composition is typical for 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 ICP-MS data document a pronounced zoning in two types of porphyroblasts: Garnet displays a typical bell-shaped zoning profile for MnO, with a maximum (~3 wt%) in the euhedral core, which is rich in Y and HREE (e.g., 2150 ppm Y). In their broad rim, all garnet crystals display a subhedral annulus (10-15 micrometers wide), which is distinctly enriched in Ca, Sr, Y, and HREE, and which probably resulted from the breakdown of allanite (at ~550 ºC, ~6.2kb). Another characteristic feature of the garnet rims is their sinusoidal chondrite-normalized REE pattern, which may represent partial equilibration between garnet and an LREE-enriched medium (produced by allanite breakdown?). The large tourmaline crystals exhibit an optically visible three-stage zoning, which comprises: a euhedral core; a continuously zoned inner rim (IR) with a prominent euhedral Ca-rich annulus; and an outer rim (OR), which also displays a distinct Ca-rich annulus and which is separated from the IR by a sutured boundary. This boundary represents a marked chemical and oxygen isotopic discontinuity, characterized, for example, by a decrease in the Zn concentration from 250 ppm (IR) to 20 ppm (OR). This change in Zn content is due to the growth of staurolite, which started after resorption of the IR of tourmaline and after a major deformation event. Our thermodynamic model suggests that resorption of the IR of tourmaline may be associated with the small amounts of melt formed at ~650 ºC and 8.5 kb.

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.