Clockwise and Anticlockwise P–T Paths of High-pressure Rocks from the ‘La Pioza’ Eclogite Body of the Malpica–Tuy Complex, NW Spain

Abstract

The Malpica–Tuy complex (MTC) in northwestern Spain is a key area for the understanding of geodynamic processes related to the early collision of Gondwana and Laurussia. To better understand this collisional situation in Variscan times, we deciphered mass-flow paths in terms of pressure (P) and temperature (T) for five samples (three eclogites, a glaucophanite and a tonalitic gneiss) occurring in a large eclogite body at the 'La Pioza' site in the central MTC. The following three contrasting P–T paths resulted from the application of P–T pseudosections, calculated with PERPLE_X, and Zr-in-rutile geothermometry. (1) Two eclogites and the gneiss yielded a similar anticlockwise P–T path. In particular, one of these eclogites recorded an extended prograde path from about 8·5 kbar at 575°C to peak pressures of ∼24·5 kbar at 630°C. The retrograde path passed through P–T conditions of ∼17·5 kbar and 650°C. (2) A clockwise P–T loop was derived for an eclogite starting at ∼18 kbar at 580°C to peak pressures of ∼23 kbar at 620°C. (3) The clockwise P–T path of the glaucophanite is characterized by a temperature increase from 610 to 680°C at nearly constant pressure around 19 kbar. The peak P of rocks in the eclogite body is much higher than that of the surrounding gneisses (≤13 kbar). Major and trace element geochemical features demonstrate that the protoliths of one eclogite and the glaucophanite were calc-alkaline igneous rocks. The other two eclogites have a tholeiitic affinity. The tonalitic gneiss is characterized by a subalkaline affinity. All the mafic rocks are characterized by a Nb anomaly. The protoliths of the eclogite with calc-alkaline affinity and the gneiss can be assigned to a continental magmatic arc formed in Late Cambrian times according to previous age dating results. The protoliths of the eclogites with tholeiitic affinity were related to basalt/gabbro of thickened oceanic crust (island arc in the Rheic Ocean). The contrasting P–T paths and different nature of the protoliths are explained by different upwards-directed mass flows and, thus, mixing of various types of rocks in a subduction channel. Our findings also support the hypothesis that the MTC represents subduction-related rocks embedded in high-pressure orthogneisses from the downgoing tip of a continental plate during initial continent–continent collision.