HTLP metamorphism and fluid‐fluxed melting during multistage anatexis of continental crust (N Sardinia, Italy)

Abstract

The Variscan high‐grade metamorphic basement of northern Sardinia and southern Corsica record lower Carboniferous anatexis related to post‐collisional decompression of the orogen. Migmatites exposed in the Punta Bianca locality (Italy) consist of quartz + biotite + plagioclase + K‐feldspar orthogneisses, garnet and cordierite‐bearing diatexite and metatexites, derived from metasediments. Field evidence, petrographic observations, ELA‐ICP‐MS zircon and monazite dating and pseudosection modelling suggest that anatexis was apparently episodic involving two main stages of partial melting. Using pseudosection modelling, we infer that the first stage of partial melting is in the upper amphibolite facies (~0.45 GPa at ~740°C). Cordierite overgrowths replacing sillimanite, combined with the composition of plagioclase and K‐feldspar, suggest decompression followed cooling below the solidus at low pressures of ~0.3 GPa. The age of the first anatectic event is not precisely constrained because of extensive resetting of the isotopic systems during the second melting stage, yet few zircons preserve a lower Carboniferous age which is consistent with the regional dataset. This lower Carboniferous migmatitic fabric is offset by a network of pseudotachylyte‐bearing faults suggestive of cooling to greenschist facies conditions. Garnet/cordierite‐bearing diatexites incorporate fragments of pseudotachylite‐bearing orthogneiss and metatexites. Pseudosection modelling indicates nearly isobaric re‐heating up to ~750°C, followed by further cooling below the solidus. The inferred P–T path is consistent with decompression and cooling of the Variscan crust through post‐collisional extension and collapse of the thickened orogenic crust, followed by nearly isobaric re‐heating at low pressures (~0.3 GPa) yielding to a second melting stage under LP‐HT conditions. U/Th‐Pb monazite ages for diatexite migmatites indicate an upper bound of 310–316 Ma for the second melting stage, suggesting that the second melting stage is coincident with the regional phase of crustal shearing. The cause of the high geothermal gradient required for re‐heating during the second melting stage is unknown but likely requires some heat source that was probably related to dissipation of mechanical work within crustal‐scale shear zones. According to this interpretation, some upper Carboniferous peraluminous granite precursors of the Corsica–Sardinia Batholith could be the outcome rather than the cause of the late‐Variscan high‐T metamorphism.

Late‐Orogenic Evolution of the Southern European Variscan Belt Constrained by Fabric Analysis and Dating of the Camarat Granitic Complex and Coeval Felsic Dykes (Maures–Tanneron Massif, SE France)

“…The 206 Pb/ 238 U zircon date of 482 ± 16 Ma (sample CA17) is slightly older than zircon and monazite U-(Th-)Pb ages of ∼455 to ∼405 Ma obtained on a metabasite and calc-alkaline orthogneisses from the MTM Internal Zone, which date emplacement of oceanic subduction-related magmatic rocks, possibly followed by eclogitic metamorphism (M 0 ) (Demoux et al, 2008;Moussavou, 1998;Oliot et al, 2015). It falls, however, within the time span known for a widespread Ordovician arc-related magmatic event recognized in the Corso-Sardinian Block (∼485-440 Ma; U-Pb zircon ages of the protoliths of calc-alkaline meta-igneous rocks; Casini et al, 2023; and references therein), which was followed by an early Devonian subduction-related HP metamorphism dated at ∼420-400 Ma on mafic eclogites whose protoliths are penecontemporaneous with the calc-alkaline rocks (U-Pb zircon dating; references in Oliot et al, 2015). It falls also between ∼505 and ∼460 Ma, an interval of U-Pb dates obtained on overgrow rims of zircons from the Bormes orthogneiss, probably related to an Ordovician tectono-metamorphic event (Tabaud et al, 2022).…”

Post-Collisional Tectonomagmatic Evolution, Crustal Reworking and Ore Genesis along a Section of the Southern Variscan Belt: The Variscan Mineral System of Sardinia (Italy)

“…From the inner zone to the external domain, syn-collisional polyphase ductile deformation is associated with Barrovian-type metamorphism [51], showing a regional gradient characterized by a decreasing metamorphic grade from NNE to SSW. Geochronological constraints for the collision-related metamorphic peak range from about 360 to 320 Ma [52][53][54][55][56][57][58]. This large timespan indicates that shortening was likely diachronous at a regional scale.…”

“…In detail, the inner zone consists of a heterogeneous crystalline nappe made of migmatites, ortho-and para-gneisses, mafic granulites, and eclogites, interpreted as the Variscan orogenic roots [56,58]. In the northern sectors, both Ar/Ar ages on post-collisional metamorphic muscovite and amphibole and U/Pb dating of zircon, monazite, and xenotime indicate that the Variscan crust recorded a regional HT-LP metamorphic event in upper Carboniferous times, between about 320 and 305 Ma [55][56][57][58][59].…”

“…In detail, the inner zone consists of a heterogeneous crystalline nappe made of migmatites, ortho-and para-gneisses, mafic granulites, and eclogites, interpreted as the Variscan orogenic roots [56,58]. In the northern sectors, both Ar/Ar ages on post-collisional metamorphic muscovite and amphibole and U/Pb dating of zircon, monazite, and xenotime indicate that the Variscan crust recorded a regional HT-LP metamorphic event in upper Carboniferous times, between about 320 and 305 Ma [55][56][57][58][59]. Extensive crustal reworking and post-collisional HT deformation transposed all of the previous fabric, preventing any stratigraphic correlation with the sedimentary and volcanic sequences of the low-grade tectonic units exposed in the greenschist zone [51,58].…”

“…Accordingly, the final voluminous felsic activity corresponding to YMP was related to a post-collisional phase of crustal heating triggered by lithospheric delamination and the intrusion of mafic magmas into the lower crust [76]. The contribution of shear and radiogenic heating is also claimed as an important heat source, particularly for OMP-related magmas [58,78,[104][105][106]. Remarkably, the emplacement time span documented by YMP overlaps the HT-LP granulitic event in the deep Variscan crust exhumed along the thinned "European" margin of the Alpine Tethys in Corsica and Calabria, which in fact is in the range of 285 Ma (U/Pb age on zircon) [75].…”