Variscan intracrustal recycling by melting of Carboniferous arc-like igneous protoliths (Evora Massif, Iberian Variscan Belt)
Variscan intracrustal recycling by melting of Carboniferous arc-like igneous protoliths (Evora Massif, Iberian Variscan Belt)
Geological Society of America Bulletin (September 2021) 134 (5-6): 1549-1570
- absolute age
- Carboniferous
- chemical composition
- Europe
- Evora Portugal
- granites
- Iberian Peninsula
- igneous rocks
- lithostratigraphy
- magmatism
- melting
- metals
- metamorphic rocks
- migmatites
- nesosilicates
- orthosilicates
- Paleozoic
- petrography
- plutonic rocks
- Portugal
- protoliths
- rare earths
- silicates
- Southern Europe
- tectonics
- U/Pb
- Variscides
- zircon
- zircon group
- Evora Massif
Bulk rock geochemistry and sensitive high-resolution ion microprobe zircon geochronology of igneous and metaigneous rocks of the Evora gneiss dome, located to the north of the reworked Rheic Ocean suture zone in the southwest Iberian Variscan belt, reveal a succession of magmatic and melting events lasting approximately 30 m.y. between ca. 341-314 Ma. The study of detailed field relationships of orthomigmatites (i.e., migmatites from igneous protoliths) and host granitic rocks proved to be crucial to reconstruct the complex sequence of tectono-thermal events of the Evora gneiss dome. The older igneous protoliths, with marked geochemical arc-like signatures, are represented by 338+ or -3 Ma tonalites and 336+ or -3 Ma diorites. These tonalites and diorites appear as mesosomes of igneous orthomigmatites containing new melts (leucosomes) of monzogranite composition and silica-poor trondhjemites formed in a melting episode at 329+ or -4/6 to 327+ or -3 Ma. The absence of peritectic phases (e.g., pyroxene), together with shearing associated with migmatization, imply the existence of water-rich fluids during melting of the older igneous rocks of the Evora gneiss dome. This melting event is coeval with the second magmatic event of the Evora gneiss dome represented by the neighboring Pavia pluton. A porphyritic monzogranite dated at 314+ or -4 Ma defines a later magmatic event. The porphyritic monzogranite encloses large blocks of the orthomigmatites and contains magmatic mafic enclaves (autoliths) dated at 337+ or -4 Ma that are approximately 23 m.y. older than the host rock. All studied rocks of the Evora gneiss dome show arc-like, calc-alkaline geochemical signatures. Our results support recycling of intermediate-mafic plutonic rocks, representing the root of an early magmatic arc that formed at the time of Gondwana-Laurussia convergence (after the closure of the Rheic Ocean) and coeval subduction of the Paleotethys. A geodynamic model involving ridge subduction is proposed to explain the Early Carboniferous intra-orogenic crustal extension, dome formation, exhumation of high-grade rocks, compositional variations of magmatism and formation of new granitic magmatism in which, arc-like signatures were inherited from the crustal source.