Original paper

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

Reinecke, Thomas

European Journal of Mineralogy Volume 3 Number 1 (1991), p. 7 - 18

33 references

published: Feb 21, 1991
manuscript accepted: Sep 19, 1990
manuscript received: Apr 23, 1990

DOI: 10.1127/ejm/3/1/0007

BibTeX file

ArtNo. ESP147050301009, Price: 29.00 €

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Abstract

Abstract 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°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 H2O-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 α-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°C, 13-15 kbar and at 360-430°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-highpressure metamorphic assemblage formed on an apparent geotherm of about 7°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

Keywords

coesiteeclogite-faciesmanganiferous quartziteP-T pathZermatt-Saas zoneValtournanche - Western Alps - Italy