Late-Proterozoic tectonics in northwest Scotland: one contractional orogeny or several?
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Keywords:
Mylonite
Titanite
Sillimanite
Orogeny
Greenschist
Ultramylonites and margarite-bearing quartz-feldspar S-C mylonites, containing amphibolite lenses with symplectitic texture, were encountered in a borehole (Bajánsenye-B-M-I) close to the west of the Transdanubian Central Range Unit. These rocks demonstrate a ductile, horizontal extensional shear zone attaining a thickness of 300 m. Microstructural data, mineral parageneses and mineral chemistry of these rocks indicate a multistage metamorphic evolution, which is consistent with that of the Koralm-Pohorje basement. The youngest mylonitic event (Early Tertiary) took place in the Bajánsenye mylonites at 430-450oC (greenschist-facies); it rejuvenated coarse-grained muscovite crystals of eo-Alpine age (Early-Middle Cretaceous). The radiometric data presented in this paper demonstrate for the first time an important Early Tertiary tectonic zone in this area.
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Greenschist
Muscovite
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Detrital rutile is observed in trace amounts in the sub-greenschist facies, graywacke, and argillite lithologies of the Torlesse Terrane-derived Otago Schist, New Zealand. Trace element analyses reveal that mean compositions of detrital rutile grains are characterized by high concentrations (500–3600 ppm) of V, Cr, Fe, Zr, Nb, and W, moderate amounts (100–500 ppm) of Sn and Ta, and minor amounts (50–100 ppm) of Mn and Hf. Recrystallization of detrital rutile to metamorphic titanite is first observed in the prehnite-pumpellyite facies rocks, with recrystallization largely complete by upper chlorite greenschist facies. Dissolution, material transport, and precipitation reactions were crucial in the progression of this recrystallization reaction, with pore spaces generated allowing the transportation of Ca, Si, and O to the titanite-rutile interface. Redistribution of trace elements during the mineral transition of detrital rutile to metamorphic titanite was assessed using mass-balance techniques. These calculations indicate that V, Cr, Zr, Nb, Ta, and W were released during the prograde mineral reaction. The liberation of this suite of trace elements potentially provides a source for the W and Cr enrichment observed in the orogenic deposits of the Otago Schist.
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Greenschist
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Mylonites in the Dabie orogen are fault and shear zone related rocks formed under metamorphic conditions.They can be divided into greenschist,amphibolite,granulite,and eclogite facies mylonites.The main criterion for identifying the mylonites in the Dabie Mountain is the ductile(plastic)deformation of index minerals of the constituent metamorphic facies.This is clearly different from the former definition and description of mylonite.Formation epoch of the mylonites in the Dabie Mountain was approximately the same as(or a little later than)that of peak metamorphism of their host rocks.From older to younger,the series follows the order eclo-gite facies mylonite→granulite facies mylonite →amphibolite facies mylonite→greenschist facies mylonite.According to lithology and texture,the mylonites can be further divided into second and third subclasses if necessary.Former mylonites of eclogite and granulite facies are always superimposed by the later facies and show the appearance of amphibolite or even greenschist facies mylonites;the earlier formed mylonites are thus preserved as relics in their retrograded products.Mylonites in the low-grade metamorphic rocks in southern and northern Dabie Mountain only experienced the metamorphic history of greenschist and amphibolite facies mylonitization during the Paleozoic-Triassic period.
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Greenschist
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Mylonite
Greenschist
Orthoclase
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Mylonite
Greenschist
Grain Boundary Sliding
Deformation mechanism
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<p>Carbonate rocks can be thick, mineralogically-homogeneous packages, which accomodate strain in orogenic belts. Despite its contribution to rock strength, the deformation of dolomite as a major rock forming mineral is understudied in comparison to calcite, quartz, and feldspar. We use field, petrographic, and electron back scatter diffraction (EBSD) analyses of dolomitic and calcitic marbles to investigate the response of these rocks to different degrees of strain under greenschist facies. Mt. Hymittos, Attica, Greece, preserves a pair of Miocene top-SSW ductile-then-brittle low-angle normal faults dividing a tripartite tectonostratigraphy. The bedrock of the massif comprises sub-greenschist facies phyllites and marbles in the uppermost hanging wall unit, and high-pressure greenschist facies schists and marbles of the Cycladic Blueschist Unit in the lower two packages. Ductile mylonites in the footwalls of both detachments grade into brittle-ductile mylonites and finally into cataclastic fault cores. The dolomitic and calcitic marbles of the lower units deformed under greenschist facies conditions and their fabrics reflect the relative differences in strengths between these two minerals. In the middle tectonostratigraphic unit, dolomitic rocks are brittlely deformed and calcitic marbles are mylonitic to ultramylonitic with recrystallized grain sizes ranging from 55 to 8 &#956;m. Within the lower package, dolomitic and calcitic rocks are both mylonitic to ultramylonitic with previous P-T data suggesting metamorphism at ~470 &#176;C and 0.8 GPa. EBSD analysis of six dolomitic marbles of the lower unit reveals a progressive fabric evolution from mylonites to ultramylonites reflecting the magnitude of strain and decreasing temperature of deformation. In mylonitic domains, average grain diameters range from 70 to 25 &#956;m. The mylonitic dolomite exhibits low-angle grain boundaries, internal misorientation zones and textures suggestive of subgrain-rotation recrystallization. This mylonitic fabric is crosscut by ultramylonite bands of dolomite with grain diameters of 15 to 5 &#956;m, which overlaps with the dominant grain size of the subgrains formed within the mylonitic domains. In samples closer to the fault core, the ultramylonite fabric is predominant though boudinaged veins, and relict mylonite zones with coarser grains may still be observed. Uniformly ultramylonitic dolomitic marbles exhibit grain diameters of 40 to 5 &#956;m; the majority of grain diameters are less than 15 &#956;m. The ultramylonite bands have low degrees of internal misorientation and an absence of low-angle grain boundaries that, along with correlated misorientation diagrams, suggest the ultramylonitic dolomite grains are randomly oriented and deforming via grain-boundary sliding. Interstitial calcite grains within these samples may reflect creep-cavitation processes interpreted to have occurred syn-kinematically with grain-boundary sliding. The change from subgrain-rotation recrystallization to grain-boundary sliding is interpreted to reflect the interplay of grain-size sensitive and insensitive processes. Following grain size reduction, subsequent deformation was dominantly accommodated by grain boundary sliding. The dolomitic marbles of the lower unit deformed on the retrograde path from the high-pressure, mid-temperature portion of the greenschist facies. The position of the dolomitic ultramylonites immediately below the cataclastic detachment fault suggest these ultramylonites were deforming very close to the brittle-ductile transition suggesting ductile deformation at lower temperatures than might be predicted by deformation experiments.</p>
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Greenschist
Cataclastic rock
Blueschist
Protolith
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Garnet glaucophanite and greenschist facies assemblages were formed by the recrystallization of barroisite‐bearing eclogite facies metabasites in northern New Caledonia. The mineralogical evolution can be modelled by calculated P–T and P–X H2O diagrams for appropriate bulk rock compositions in the model system CaO–Na 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O. The eclogites, having developed in a clockwise P–T path that reached P ≈19 kbar and T ≈590 °C, underwent decompression with the consumption of free H 2 O as the volume of hydrous minerals increased. Eclogite is preserved in domains that experienced no fluid influx following the loss of this fluid. Garnet glaucophanite formed at P ≈16 kbar during semi‐pervasive fluid influx. Fluid influx, after further isothermal decompression, was focused in shear zones, and resulted in chlorite–albite‐bearing greenschist facies mineral assemblages that reflect P ≈9 kbar.
Greenschist
Omphacite
Blueschist
Recrystallization (geology)
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Titanite
Omphacite
Coesite
Migmatite
Rutile
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