Petrology of jadeite metagranite and associated orthogneiss from the Malpica-Tuy allochthon (Northwest Spain)
Ibarguchi, José I. Gil
European Journal of Mineralogy Volume 7 Number 2 (1995), p. 403 - 416
published: Mar 29, 1995
manuscript accepted: Nov 4, 1994
manuscript received: Oct 30, 1993
ArtNo. ESP147050702015, Price: 29.00 €
Abstract Jadeite occurs in plagioclase pseudomorphs of decimeter size granitic veins associated with corona granodioritic orthogneiss of the Malpica-Tuy allochthon. Igneous textures and the igneous composition of plagioclase and biotite, as well as high-pressure assemblages with sodic pyroxene, Ca-rich corona garnets, Si-rich phengite and rutile, are preserved in the least deformed zones of jadeite-bearing and related rocks. Reaction textures include formation of garnet coronas around biotite and opaques, replacement of biotite by Si-rich phengite + rutile, and development of pseudomorphic aggregates of zoisite + albite + jadeite + phengite after primary plagioclase phenocrysts. Almandine- and grossular-rich coronitic garnets show conspicuous asymmetric zoning (e.g., Alm76GrsnPrp13 to Alm54Grs41Prp5 across a 1.6 GPa, are consistent with those deduced from eclogites within the same structural unit. Limited extent of metamorphic reactions suggests biotite breakdown as the main source of water in weakly deformed jadeite-bearing rocks and orthogneiss. Enhanced deformation and water circulation resulted in lower pressure associations of sheared rocks with epidote, albite and magnetite, together with biotite and Si-poor phengite neoblasts. Mineral assemblages of eclogite-facies granitoids studied are evidence for continental crust of the Iberian margin (Gondwana) being metamorphosed at depths >50km in a subduction zone related to the Hercynian collision.