Late Palaeozoic tectonics in Central Mediterranean: a reappraisal

A revision of late Palaeozoic tectonics recorded in Tuscany, Calabria and Corsica is here presented. We propose that, in Tuscany, upper Carboniferous-Permian shallow-marine to continental sedimentary basins, characterized by unconformities and abrupt changes in sedimentary facies, coal-measures, red fanglomerate deposits and felsic magmatism, may be related with a transtensional setting where upper-crustal splay faults are linked with a mid-crustal shear zone. The remnants of the latter can be found in the deep-well logs of Pontremoli and Larderello-Travale in northern and southern Tuscany respectively. In Calabria (Sila, Serre and Aspromonte), a continuous pre-Mesozoic crustal section is exposed, where the lower-crustal portion mainly includes granulites and migmatitic paragneisses, together with subordinate marbles and metabasites. The mid-crustal section, up to 13 km-thick, includes granitoids, tonalitic to granitic in composition, emplaced between 306 and 295 Ma. They were progressively deformed during retrograde extensional shearing, with a final magmatic activity, between 295 ± 1 and 277 ± 1 Ma, when shallower dykes were emplaced in a transtensional regime. The section is completed by an upper crustal portion, mainly formed by a Palaeozoic sedimentary succession deformed as a low-grade fold and thrust belt, and locally overlaying mediumgrade paragneiss units. As a whole, these features are reminiscent of the nappe zone domains of the Sardinia Variscan Orogen. In Corsica, besides the well-known effusive and intrusive Permian magmatism of the “Autochthonous” domain, the Alpine Santa Lucia Nappe exposes a kilometer-scale portion of the Permian lower to mid-crust, exhibiting many similarities to the Ivrea Zone. The distinct Mafic and Granitic complexes characterizing this crustal domain are juxtaposed through an oblique-slip shear zone named Santa Lucia Shear Zone. Structural and petrological data witness the interaction between magmatism, metamorphism and retrograde shearing during Permian, in a temperature range of c. 800–400 °C. We frame the outlined paleotectonic domains within a regional-scale, strain–partitioned, tectonic setting controlled by a first-order transcurrent/transtensional fault network that includes a westernmost fault (Santa Lucia Fault) and an easternmost one (East Tuscan Fault), with intervening crustal domains affected by extensional to transtensional deformation. As a whole, our revision allows new suggestions for a better understanding of the tectonic framework and evolution of the Central Mediterranean during the late Palaeozoic

Greenstone burial–exhumation cycles at the late Archean transition to plate tectonics

Converging lines of evidence suggest that, during the late Archean, Earth completed its transition from a stagnant-lid to a plate tectonics regime,although how and when this transition occurred is debated. The geological record indicates that some form of subduction, a key component of plate tectonics—has operated since the Mesoarchean, even though the tectonic style and timescales of burial and exhumation cycles within ancient convergent margins are poorly constrained.Here, we present a Neoarchean pressure–temperature–time (P–T–t) path from supracrustal rocks of the transpressional Yilgarn orogen (Western Australia), which documents how sea-floor-altered rocks underwent deep burial then exhumation during shortening that was unrelated to the episode of burial. Archean subduction, even if generally short-lived, was capable of producing eclogites along converging lithosphere boundaries, although exhumation processes in those environments were likely less efficient than today, such that return of high-pressure rocks to the surface was rare

Incremental pluton emplacement during inclined transpression

Multidisciplinary investigations suggest that many igneous plutons were assembled incrementally, through emplacement of discrete magma batches. Internal pluton contacts between different units can be remarkably cryptic and/or complex in nature. This presents a major challenge in understanding both how repeated intrusions were emplaced relative to each other, and in their relationships with regional deformation.Here we present the results of a multidisciplinary study from a Neoarchean pluton exposed in the Yilgarn Craton of Western Australia. Our new structural, geophysical, geochemical and geochronology datasets indicate that the pluton was emplaced incrementally along an active, inclined transpressional shear zone. Structural relationships indicate that magmatic to high-temperature solid-state fabrics locally postdate a mid- to upper-greenschist tectonic fabric in an adjacent older unit. We consider these relationships as diagnostic of incremental emplacement, indicating that the shearzone recorded several cycles of strain softening and hardening, in relation to the syntectonic emplacement of discrete magma pulses, followed by syndeformational cooling down to upper-greenschist facies conditions. Our data also suggest that pluton growth occurred through over-accretion of progressively more evolved magma pulses

Refertilization of subcontinental mantle recorded by the lherzolite-websteritehorneblendite association from St. Lucia (Corsica)

The St. Lucia nappe from Alpine Corsica consists of late-Variscan lower crust exhumed along the European rifted margin of the Ligurian Tethys and Mesozoic sediments (Beltrando et al., 2013). The basement includes a High Grade Mafic Complex and a Granitoid Complex of Early Permian age. The base of the Mafic Complex is associated with mantle slices consisting of mylonitic spinel-bearing lherzolites and mm- to cm-thick websterite and hornblendite layers concordant with the foliation of the host rocks. The mylonite microstructure in the peridotites is characterized by aligned porphyroclasts of pyroxene (± spinel) in a fine-grained polyphase matrix composed of olivine + pyroxenes + spinel. Large exsolved orthopyroxene (Opx) porphyroclasts mantled by neoblastic Cpx + Opx + spinel testify a lowstrain spinel tectonite stage predating the mylonite deformation. Major element compositions of olivine, Cpx and spinel porphyroclasts of the lherzolites indicate a fertile geochemical signature. The websterites are Opx-poor rocks containing Al-Ti-rich Cpx porphyroclasts, Cr-poor spinel, disseminated kaersutite and accessory Fe-Ni sulphides. Spinel-facies mylonite recrystallization in both lherzolites and websterites occurred at ~850-900°C. The hornblendites are composed of K2O-rich kaersutite + Ti-rich phlogopite + ilmenite. Cpx porphyroclasts from the lherzolites have heterogeneous trace element compositions pointing to four different geochemical signatures. Type 1 Cpx is strongly LREE-depleted (CeN/SmN down to 0.05) with nearly flat MREE-HREE at 8-10 times chondrite. Type 2 Cpx is moderately LREEdepleted (CeN/SmN = 0.31-0.49), whereas Type 3 has nearly flat LREE. Type 4 Cpxs are weakly LREE depleted, peak at MREE and have variable HREE (YbN = 7-12). The websterite Cpxs display convex-upward REE patterns and higher concentrations of incompatible trace element. The kaersutite from the hornblendites are enriched in Rb, Ba, U, Nb, Ta, LREE and depleted in HREE with respect to the websterite ones. The peridotite protholiths are consistent with residual rocks after low degrees of near-fractional melting of spinel facies DM. Type 4 Cpx from the lherzolites likely reflect equilibrium with percolating MORB-type melts. Putative liquids in equilibrium with Cpx and kaersutite from the websterites and the hornblendites have transitional to alkaline affinity. Our study has evidenced that the St. Lucia mantle slices underwent injections of melts with MORB to alkaline affinity forming different kinds of magmatic layers, which were associated with metasomatic haloes in the ambient peridotite. Beltrando M., Zibra I., Montanini A. & Tribuzio R. 2013. Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: a hot rift to drift transition, Lithos, 168-169, 99-112

Shearing of magma along a high-grade shear zone: Evolution of microstructures during the transition from magmatic to solid-state flow

Syntectonic plutons may record short-lived geological events related to crustal melting and deformation of the continental crust. Therefore, interpretation of microstructure and orientation of fabrics is critical in order to constrain space/time/temperature/deformation relationships during pluton crystallization. Here we describe the transition from magmatic to solid-state deformation in the late-Variscan Diorite-Granite Suite (DGS) emplaced along the Santa Lucia Shear Zone. The systematic collection of meso-, microstructural and quartz < c > axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics. We identify three magmatic microfabric types, thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization. Our data suggest that diffusion creep, dislocation creep and grain-scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate. We suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions ~0.40, and that most of the hypersolidus fabrics formed during horizontal flow, reflecting the stress field imposed by the shear zone, and preserving no memory of the ascent stage

Crustal thinning and exhumation along a fossil magma-poor distal margin preserved in Corsica: A hot rift to drift transition?

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Shearing of magma along a high-grade shear zone: evolution of microstructures during the transition from magmatic to solid state flow

Syntectonic plutons may record short-lived geological events related to crustal melting and deformation of the continental crust. Therefore, interpretation of microstructure and orientation of fabrics is critical in order to constrain space/time/temperature/deformation relationships during pluton crystallization. Here we describe the transition from magmatic to solid-state deformation in the late-Variscan Diorite-Granite Suite (DGS) emplaced along the Santa Lucia Shear Zone. The systematic collection of meso-, microstructural and quartz axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics. We identify three magmatic microfabric types, thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization. Our data suggest that diffusion creep, dislocation creep and grain-scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate. We suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions gt; w0.40, and that most of the hypersolidus fabrics formed during horizontal flow, reflecting the stress field imposed by the shear zone, and preserving no memory of the ascent stage