Hercynian subduction‐related processes within the metamorphic continental crust in Calabria (southern Italy)
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Abstract Linking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation ( P–T–d ) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a D n ‐1 under‐thrusting event. The following exhumation during the D n mylonitic event, and contact metamorphism during D n +1 and D n +2 folding events, have also been modelled because they are essential to restore the previous tectono‐metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid‐deficient conditions, as well as the final late Carboniferous contact metamorphism up to T max of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating‐dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P / T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T / P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during D n ‐1 under‐thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.Keywords:
Mylonite
Massif
Greenschist
The subject of the present contribution is the analysis of deformation in the Fellos ductile shear zone, which crops out in the island of Andros. Fellos shear zone (FSZ) is an outcrop scale NNE –striking structure with a total structural thickness of, approximately, 200 m. The central main domain of the shear zone is defined by mylonitized metapelitic rocks, bounded by strongly deformed meta-basic and meta–ultrabasic rocks both on top and bottom. Mylonitic rocks in the shear zone can be classified as S to S – L tectonites. Structural mapping revealed that the Fellos shear zone resulted from the complete transposition of a gently inclined synform the axial plane of which is orientated sub – parallel to the mylonitic foliation. The rheological contrasts between the core of the shear zone and its margins in conjunction with the structural framework of a transposed synform are first order influences to the localization of deformation. The map scale pattern of the stretching lineations in the shear zone shows that the lineation swings from a NNE orientation, trending parallel to the strike of the shear zone, to an ESE orientation. This variation shows that transport orientation in the shear zone is spatially partitioned into strike parallel and strike normal movements. This kinematic partitioning in the Fellos shear zone is a characteristic feature of transpressional high strain shear zones. The recognized partitioning was further investigated by examining the pattern of quartz [c]-axes fabrics in quartz veins oriented parallel to the mylonitic foliation as well as in quartz –rich mylonites. Quartz [c] axes fabric diagrams shows variations in their topology at different structural levels of the shear zone and distinct differences with the [c] axes patterns of monoclinic strain symmetry. Distinct feature, especially at the uppermost structural levels of the shear zone, is the point maxima clustering of the [c] axes in peripheral position of the crystallographic diagrams. At the base of the shear zone, in the rheological interface of metapelites with metabasites, the fabric diagrams from neighboring quartz vein specimens shows consistently small circle girdle pattern. This observation denotes localization in the geometry of deformation (flattening) at this lithological contact. Finite strain analysis shows that Rxz varies from 2.1 to 6.45, while the Flinn parameter range between 0.01 and 0.93, supporting quantitatively the flattening geometry of strain. Flattening strain is another diagnostic criterion of transpressional shear zones.
The means to constrain the compositional features of the mylonitic rocks was mineral chemistry and petrologic analysis. Petrographic examination, specifically, of a metabasite outcrop from the base of the shear zone revealed, in thin section scale, two lithologic types : (a) Garnet blueschist and (b) Epidote blueschist. The mineral chemistry of zoned garnets and amphiboles from these petrographic types has been examined using SEM/EDS. The microchemical analysis revealed three different zoning patterns in the amphiboles which are characterized by (a) dark green Ferro – hornblende cores with crossite rims (b) Amphiboles with compositional zoning and alternations of blue (Ferro – glaucophane) and green amphiboles (Ferro – barroisite) and (c) Isolated grains with crossite cores and riebeckite rims. Compositional mapping of zoned garnets witnessed fractionation of manganese to the cores, which is the Prima facie evidence of growth zoning. In addition, the almandine rims of the zoned garnets are in textural equilibrium with blue amphiboles, observation which implies that these garnets nucleated during the HP event.
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Tectonite
Finite strain theory
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Abstract Late Gothian (c. 1.58 Ga) and Sveconorwegian (1.1–0.9 Ga) structures outline a 35 km long, NNE‐oriented, open gneiss synform in the Varberg‐Horred region of SW Sweden. This is a region of the Southwest Scandinavian Domain, within which a major shear zone and tectonic boundary, the Mylonite Zone, forms a branching shear zone system which converges in the eastern part of the synform. A subdivision between the Gothian and Sveconorwegian events is made by using the intervening anorogenic intrusions as structural markers. This, and the non‐recognition of a previously assumed orogenic event, results in a geodynamic model which is similar for the crustal segments on both sides of the largely N‐S trending Mylonite Zone, except for the higher grade Sveconorwegian metamorphism to the east. The evolution is characterised by one or more major Gothian gneiss‐forming events, followed by intermittent anorogenic magmatism and a later Sveconorwegian development that, outside discrete shear zones, gave rise to moderate fabric‐forming deformation and only localised formation of migmatitic leucosomes. The final Gothian orogenic episode at c. 1.58 Ga and three distinct anorogenic events between 1.51 and 1.20 Ga are correlated across the Mylonite Zone, thus supporting models where the Mylonite Zone constitutes an intracratonic Sveconorwegian shear zone. The Sveconorwegian development is interpreted to include eastward thrusting on the Mylonite Zone, followed by dominantly static metamorphism prior to 0.98 Ga, due to the thickened crust. Subsequent uplift and rapid cooling preserved granulite‐facies assemblages in the southern Eastern Segment. Late Sveconorwegian extensional movements occurred until c. 0.92 Ga along the largely west‐dipping Mylonite Zone system. Åhäll, K.‐L, 1995: Crustal units and role of the Mylonite Zone system in the Varberg‐Horred region, SW Sweden. GFF, Vol. 117 (Pt. 4, December), pp. 185–198. Stockholm. ISSN 1103–5897.
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Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.
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The near NS ductile shear zone was first discovered in Proterozoic Bendong granodiorite pluton. It dips toward 240°～280° at the dip angle of 40°～63°. The lineations, which represent the shear direction, plunge to 216°～226° at the plunge angle of 39°～46°. The kinematics of Bendong ductile shear zone is characterized by sinistral-normal shearing, and shows a sliding from NE to SW. Typical granitic mylonites were developed in the ductile shear zone and clearly show a zoning from phyllonite through mylonite to initial mylonite from center to both walls. The discovery of Bendong ductile shear zone and mylonite indicates an important structural event superimposed on Proterozoic Bendong pluton.
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The eastern margin of the Chitradurga schist belt is marked by a NNW-SSE trending sub vertical crustal scale ductile shear zone. The kinematic indicators indicate a predominant sinistral sense of strike-lip movement along the shear zone. Syntectonically emplaced granitic rocks are converted to mylonites and ultramylonites as a result of crystalplastic deformation in the shear zone. In contrast, there are localized zones of brittle failure with attendant functional heat generation exemplified by the development of thin but conspicuous bands and veins of pseudotachylytes, which are emplaced either subparallel with or transgressing the C-planes of the mylonites. From our field and petrographic studies it is interpreted that these two coexisting rock types, namely the mylonite and pseudotachylyte, which are the results of contrasting deformational mechanisms, have generated near synchronously in a progressively developed ductile shear zone. The pseudotachylytes represent the brief interlude of sudden increase in strain rate in an overall ductile regime.
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