Original paper

A possible new UHP unit in the Western Alps as revealed by ancient Roman quern-stones from Costigliole Saluzzo, Italy

Groppo, Chiara; Ferrando, Simona; Castelli, Daniele; Elia, Diego; Meirano, Valeria; Facchinetti, Luca

European Journal of Mineralogy Volume 28 Number 6 (2016), p. 1215 - 1232

published: Dec 1, 2016

DOI: 10.1127/ejm/2016/0028-2531

BibTeX file

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Abstract

Peculiar coesite + chloritoid + garnet ± glaucophane talcschists have been used to make at least four quern-stones, unearthed in the ruins of a villa rustica belonging to the Roman imperial period and located at Costigliole Saluzzo, Western Alps. The site of the villa rustica and the presence of coesite relics could suggest a possible provenance of these rocks from one of the already known ultra-high pressure ( UHP ) units of the Western Alps, i. e ., either the Brossasco-Isasca Unit (BIU) of the southern Dora-Maira Massif or the Lago di Cignana Unit (LCU) of the Piemonte Zermatt-Saas Zone. However, similar talcschists have never been reported from these units. Two samples of coesite-bearing, chloritoid + garnet ± glaucophane talcschist collected from two different specimens of quernstones have been petrologically investigated with the aim of defining their peak pressure – temperature ( P-T ) conditions. The stability field of the coesite + garnet + talc + chloritoid + glaucophane assemblage has been constrained using isochemical phase diagrams modelled in the MnNCFMASHO system; prograde P-T conditions have been additionally constrained using the yttrium-in-garnet (YAG) geothermometer. Thermodynamic modelling tightly constrains peak P-T conditions at 480 – 510°C, 27 – 31 kbar. The unusual Mg-rich composition of the talcschists suggests that they originated by Mg-metasomatism of either a continental crustal protolith (granodiorite or Fe-rich metapelite) or an oceanic crustal protolith (altered oceanic crust). A mechanism similar to that proposed for the well-known pyrope-bearing whiteschists of the UHP BIU, i. e . in fl ux of antigorite-derived fl uids along shear zones during subduction, can be envisaged. Although the field occurrence of these coesite + chloritoid + garnet ± glaucophane talcschists is still unknown, the obtained results clearly show that these rocks cannot belong to the UHP BIU, whose peak P-T conditions are at significantly higher T and P (730°C, 40 – 43 kbar). Therefore, thisfinding opens the challenging hypothesis of the existence of a further, still unmapped, UHP Unit in the Western Alps that also experienced UHP metamorphism and fluid influx from underlying serpentinites during subduction. A detailed mapping and petrologic investigation of the tectono-metamorphic continental ( i. e ., southern Dora-Maira Massif) and oceanic ( i. e ., Monviso meta-ophiolite Complex) units in the proximities of Costigliole Saluzzo is required in order to further constrain the location and the dimension of this new UHP unit.

Keywords

thermodynamic modellinguhp metamorphismwestern alpsmg-metasomatismcoesite + chloritoid + garnet + glaucophane talcschistmicro-raman spectroscopy