Dating shear zones with plastically deformed titanite: New insights into the orogenic evolution of the Sudbury impact structure (Ontario, Canada)
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Titanite
Mylonite
Orogeny
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Abstract U–Pb dating of titanite from a mafic lens in the Mylonite Zone in southwestern Sweden yields an almost concordant age of c. 920 Ma. The titanite formed during retrogression of granulite-to amphibolite-facies rocks. Sources for Ti and Ca in titanite were Fe-Ti oxides and relatively grossular-rich garnets, respectively. Deformation is contemporaneous with or post-dates the growth of titanite, which took place at temperatures above c. 550ÅC. The obtained U–Pb age of titanite is close to the Sm-Nd isochron ages of garnet-bearing granulite-facies mineral assemblages in the Varberg area just south of the Mylonite Zone. Since the blocking temperature is higher for Nd diffusion in garnet than for the Pb diffusion in titanite it is suggested that the rocks of the Mylonite Zone were undergoing amphibolite-facies deformation, uplift and cooling while the granulites of the Varberg area were still at a considerably higher temperature. Key Words: GranuliteMylonite Zoneretrogressionsouthwestern SwedenSveconorwegiantitaniteU-Pb dating
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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 earliest deformation of the curved Kongsberg-Bamble Belt in Norway produced a northwestward-directed tectonic wedge, suggesting oblique NW-SE collision during the Labradorian-Gothian Orogeny in southern Scandinavia. Oblique terrane assembly, with the outboard (western) units colliding last, explains the trapping of the Ostfold-Marstrand island arc (formed at c. 1760 Ma) between younger (1700-1600 Ma) terranes, the apparent lack of Svecofennian crust west of the Trans-Scandinavian Igneous Belt, and the apparent lack of Labradorian-Gothian age crust in the British Isles. The Labradorian of North America may have also resulted from oblique terrane assembly.
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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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