This approach resulted in much more reliable P–T paths compared with the previous geothermobarometric methods. Thermodynamic modelling of the phase relations in metamorphic rocks started to be increasingly used in this millennium. In addition, the applied geothermobarometric methods were adversely affected by the selection of mineral compositions that were only assumed to be in equilibrium. An assumed simple clockwise P–T loop through such conditions was usually presented in older petrological works (e.g. Early geothermobarometry studies on eclogites frequently resulted in a single pressure–temperature ( P–T) datum, which was, generally, assigned to (minimal) peak metamorphic conditions. This information can be obtained by detailed studies of texture and compositions of minerals in such rocks, including the subsequent application of geothermobarometric methods and geochronology. Metamorphic rocks of the eclogite facies have attracted geoscientists for a long time because they can provide valuable information about the thermal characteristics of paleo-subduction zones and, possibly, the early stage of continent–continent collision after complete subduction of the intervening oceanic plate. 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. 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. The protoliths of the eclogites with tholeiitic affinity were related to basalt/gabbro of thickened oceanic crust (island arc in the Rheic Ocean). 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. All the mafic rocks are characterized by a Nb anomaly. The tonalitic gneiss is characterized by a subalkaline affinity.
The other two eclogites have a tholeiitic affinity. Major and trace element geochemical features demonstrate that the protoliths of one eclogite and the glaucophanite were calc-alkaline igneous rocks. The peak P of rocks in the eclogite body is much higher than that of the surrounding gneisses (≤13 kbar). (3) The clockwise P–T path of the glaucophanite is characterized by a temperature increase from 610 to 680☌ at nearly constant pressure around 19 kbar. (2) A clockwise P–T loop was derived for an eclogite starting at ∼18 kbar at 580☌ to peak pressures of ∼23 kbar at 620☌.
The retrograde path passed through P–T conditions of ∼17♵ kbar and 650☌. In particular, one of these eclogites recorded an extended prograde path from about 8♵ kbar at 575☌ to peak pressures of ∼24♵ kbar at 630☌.
(1) Two eclogites and the gneiss yielded a similar anticlockwise P–T path. The following three contrasting P–T paths resulted from the application of P–T pseudosections, calculated with PERPLE_X, and Zr-in-rutile geothermometry. 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 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.
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