Very-high-pressure metamorphism and uplift of coesite-bearing metasediments from the Zermatt-Saas zone, Western Alps

The metapelitic assemblage of a very-high-pressure metamorphic manganiferous quartzite occurring at Lago di Cignana, Valtournanche, Western Alps, Italy, on top of eclogite-facies metaophiolites, provides an outstanding record of the P-T evolution during subduction and subsequent uplift in a segment of the oceanic Zermatt-Saas zone. At peak metamorphic conditions of 590-630 degrees C, 26-28 kbar, rim compositions of pyrope-rich spessartine containing up to 41 mol % pyrope coexisted with coesite, talc, kyanite, phengite, paragonite, braunite, piemontite, haematite, rutile, dravitic tourmaline, Mg-rich ardennite, apatite and zircon under high H (sub 2) O-activity. Coesite is exclusively present as rare inclusions in tourmaline and pyrope-rich garnet and has partially or completely inverted to poly crystalline quartz. Texturally distinguishable inclusions of unstrained and typically monocrystalline quartz which occur in less Mg-rich garnet cores containing 14-27 % pyrope component were apparently formed on the prograde highpressure path in the stability field of alpha -quartz. Sealing in garnet allowed them to persist in the stability field of coesite on a further increase of P and T towards peak metamorphic conditions. The continuous decrease of the pyrope content in the outer rim of zoned garnet coexisting with phengite, talc and quartz/coesite suggests a noticeable decrease in temperature at an initial stage of the decompressional uplift. Further uplift is constrained by two stages of partial re-equilibration at about 500 degrees C, 13-15 kbar and at 360-430 degrees C, 4-6 kbar in the stability fields of talc + phengite and of phlogopite + clinochlore + quartz, respectively. The established P-T path provides evidence that pelagic sediments forming part of the lithosphere of the former Piemonte-Ligurian ocean were subducted to depths of about 90 km. Preservation of the very-high pressure metamorphic assemblage formed on an apparent geotherm of about 7 degrees C/km may have been favoured by the limited access of hydrous fluids and by a retrograde P-T path on which decompression was associated with major cooling.