Provenance and metamorphic evolution of high-pressure rocks from the Malpica-Tuy zone (MTZ) in NW Spain

An eclogite and its surrounding gneiss from the Malpica-Tuy zone, northwestern Spain, were studied. The eclogite contains besides garnet and omphacite, barroisitic amphibole, quartz, phengite with Si contents between 3.27 and 3.45 per formula unit (pfu), rutile, and accessory phases such as talc and (clino)zoisite-epidote. Garnet exhibits a chemical zonation with Alm59Prp13Grs24Sps4, Alm57Prp13Grs27Sps3, Alm56.6Prp12.5Grs30.2Sps0.7, and Alm54Prp20Grs25Sps1 as inner core, outer core, inner rim, and outermost rim compositions, respectively. The gneiss is a former granodiorite now mainly composed of quartz, plagioclase, K-feldspar, biotite, garnet, (clino)zoisite-epidote, and phengite with Si contents between 3.27 and 3.38 pfu. Garnet shows inner core, outer core, and rim compositions of Alm26.9Prp0.2Grs70Sps2.9, Alm33.8Prp0.7Grs64Sps1.5, and Alm42.6Prp1.4Grs54Sps2, respectively. P-T pseudosections, which were calculated with PERPLE_X for the bulk-rock compositions of the studied eclogite and gneiss, were contoured by isopleths of various parameters such as molar fractions of garnet components. Mainly based on the variable garnet and phengite compositions and these isopleths, P-T paths were derived. After an isothermal burial path the eclogite reached peak pressure conditions of 22.5 kbar at 540 °C. The subsequent exhumation path passed through P-T conditions of 21 kbar and 575 °C and 13.5 kbar and 625 °C. The latter data represent the peak P-T conditions of the gneiss. These P-T data