The geochemistry of high-pressure gneisses from Cabo Ortegal (NW Spain): residues of deep anatexis

Abstract

Garnet-hornblende gneisses from the eclogite to high-pressure granulite facies complex of Cabo Ortegal, NW Spain, show three main geochemical characteristics: (1) low K, Rb, Th and Rb/Sr ratios, and high K/Rb ratios compared with upper crustal rocks; (2) high Ca, low Sr and Ba, low Ba/Sr ratios and high CalSr ratios compared with low - to intermediate - pressure granulite facies gneisses; (3) low REE and absence of positive Eu anomalies compared with granulite-facies gneisses. These geochemical peculiarities reflect the gneisses being residues of partial melting at depths in excess of 35 km, which may have been induced by rise of hot ultramafic diapirs from the underlying mantle.