Variscan intracrustal recycling by melting of Carboniferous arc-like igneous protoliths (Évora Massif, Iberian Variscan belt)

Bulk rock geochemistry and sensitive high-resolution ion microprobe zircon geochronology of igneous and metaigneous rocks of the Évora 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 ∼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 Évora gneiss dome. The older igneous protoliths, with marked geochemical arc-like signatures, are represented by 338 ± 3 Ma tonalites and 336 ± 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 ± 4/6 to 327 ± 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 Évora gneiss dome. This melting event is coeval with the second magmatic event of the Évora gneiss dome represented by the neighboring Pavia pluton. A porphyritic monzogranite dated at 314 ± 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 ± 4 Ma that are ∼23 m.y. older than the host rock. All studied rocks of the Évora 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.

Episodic Precipitation of Wolframite during An Orogen: The Echassières District, Variscan Belt of France

Monazite and rutile occurring in hydrothermally altered W mineralizations, in the Echassières district of the French Massif Central (FMC), were dated by U-Pb isotopic systematics using in-situ Laser ablation-inductively coupled plasma–quadrupole mass spectrometry (LA-ICP-MS). The resulting dates record superimposed evidence for multiple percolation of mineralizing fluids in the same area. Cross-referencing these ages with cross-cutting relationships and published geochronological data reveals a long history of more than 50 Ma of W mineralization in the district. These data, integrated in the context of the Variscan belt evolution and compared to other major W provinces in the world, point to an original geodynamic-metallogenic scenario. The formation, probably during the Devonian, of a quartz-vein stockwork (1st generation of wolframite, called wolframite “a”; >360 Ma) of porphyry magmatic arc affinity is analogous to the Sn-W belts of the Andes and the Nanling range in China. This stockwork was affected by Barrovian metamorphism, induced by tectonic accretion and crustal thickening, during the middle Carboniferous (360 to 350 Ma). Intrusion of a concealed post-collisional peraluminous Visean granite, at 333 Ma, was closely followed by precipitation of a second generation of wolframite (termed “b”), from greisen fluids in the stockwork and host schist. This W-fertile magmatic episode has been widely recorded in the Variscan belt of central Europe, e.g. in the Erzgebirge, but with a time lag of 10–15 Ma. During orogenic collapse, a third magmatic episode was characterized by the intrusion of numerous rare-metal granites (RMG), which crystallized at ~310 Ma in the FMC and in Iberia. One of these, the Beauvoir granite in the Echassières district, led to the formation of the wolframite “c” generation during greisen alteration.

Supplemental Material: Variscan intracrustal recycling by melting of Carboniferous arc-like igneous protoliths (Évora Massif, Iberian Variscan belt)

Table S1: SHRIMP U-Pb-Th zircon data of samples from Alto de São Bento (Évora Massif); Table S2: Amphibole thermobarometry results on the microprobe analyses from plutonic rocks of Hospistais intrusion.

Supplemental Material: Variscan intracrustal recycling by melting of Carboniferous arc-like igneous protoliths (Évora Massif, Iberian Variscan belt)

Table S1: SHRIMP U-Pb-Th zircon data of samples from Alto de São Bento (Évora Massif); Table S2: Amphibole thermobarometry results on the microprobe analyses from plutonic rocks of Hospistais intrusion.

Characterization of heavy mineral concentrates and detrital gold particles from the Bigorne granite-hosted gold deposit in the Iberian Variscan Belt

The Bigorne gold deposit, located in the Iberian Variscan belt, is a gold-bearing vein system, which crosscuts Variscan granites. The hypogene mineralization corresponds to sheet-veins parallel to the late-Variscan Penacova-Régua-Verín fault.A combined study was undertaken of gold particles from hypogene mineralization and locally derived eluvial material, as well as gold from alluvial deposits in local drainages, to assess possible source(s) of the alluvial gold.The geochemistry and mineralogy of heavy mineral concentrates revealed a similar signature as the hosted granites and hypogene mineralization, which indicates a local source for the surficial materials, with limited contribution from surrounding rocks.Hypogene and detrital gold particles at Bigorne area are entirely primary in origin and correspond essentially a Au-Ag alloy (< 22 wt.% Ag), locally with minor amounts of Cu. The compositional range of Ag and Cu is a consequence of variation in parameters formation of the deposit, such as temperature. Our study highlights the first results on gold detrital particles chemistry for granite-hosted gold deposits in Iberian Variscan belt and has provided a platform for further work to characterize gold from this environment such that it may be used as a generic indicator mineral together with other resistant ore minerals.