Early Variscan P-T evolution of an eclogite body and adjacent orthogneiss from the northern Malpica-Tuy shear-zone in NW Spain

Abstract

An eclogite and its surrounding gneiss from the Malpica-Tuy zone, northwestern Spain, were studied. In addition to garnet and omphacite, the eclogite contains barroisitic amphibole, quartz, phengite with Si contents between 3.27 and 3.45 per formula unit (pfu), rutile, and accessory talc and (clino)zoisite–epidote. Garnet exhibits a chemical zonation with Alm 59 Prp 13 Grs 24 Sps 4 , Alm 57 Prp 13 Grs 27 Sps 3 , Alm 56.6 Prp 12.5 Grs 30.2 Sps 0.7 , and Alm 54 Prp 20 Grs 25 Sps 1 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 Alm 26.9 Prp 0.2 Grs 70 Sps 2.9 , Alm 33.8 Prp 0.7 Grs 64 Sps 1.5 , and Alm 42.6 Prp 1.4 Grs 54 Sps 2 , respectively. A series of P–T pseudosections, calculated with PERPLE_X for the bulk-rock compositions of the studied eclogite and gneiss, were contoured by isopleths of various parameters, including molar fractions of garnet components. Using mainly the variable garnet and phengite compositions and these isopleths, P–T paths were derived. After an isothermal burial path the eclogite reached peak pressures 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 suggest that the protolith of the eclogite, a basic rock of an island-arc setting, underwent high-pressure metamorphism in a subduction zone. During its exhumation it came in contact with the buried metagranodiorite, representing the tip of a descending continental plate, at depths of 45–50 km probably 350–355 Ma ago.