Reactivated fracture-controlled uranium mineralization: An example from NNE-SSW Kamaguttapalle–Kammapalle tract, Kadapa district, Andhra Pradesh, India
1
Citation
27
Reference
10
Related Paper
Citation Trend
Keywords:
Orogeny
Mylonite
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.
Mylonite
Cite
Citations (16)
Since 1969, a major geochronological investigation has been carried out by the South Australian Department of Mines and Energy on rocks of the Gawler Craton. Isotopic dates define three broad groupings corresponding to the Sleafordian Orogeny 2500–2300 Ma, the Kimba Orogeny 1820–1580 Ma, and the Wartakan Event 1580–1400 Ma. The oldest rocks in the craton belong to the Mulgathing Complex (in the north) and the Sleaford Complex (in the south). They consist of strongly folded and metamorphosed paragneisses (probably sediments and volcanics) intruded by several granites during the Sleafordian Orogeny. The Sleafordian Orogeny was followed by a 500 Ma period of magmatic and tectonic quiescence prior to the Kimban Orogeny. During that time sediments of the Hutchison Group were deposited. The Kimban Orogeny is characterized by complex deformation, high‐grade metamorphism and multiple synorogenic granite intrusion. It concluded at ca. 1580 Ma, but was followed by postorogenic acidic magmatism, sedimentation and minor deformation (the Wartakan Event). The spectrum of isotopic dates ends at ca. 1400 Ma, the approximate time of cratonization of the Gawler Craton.
Orogeny
Geochronology
Cite
Citations (75)
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.
Mylonite
Protolith
Shearing (physics)
Greenschist
Cite
Citations (2)
Archean cratons-Proterozoie mobile belts mega-structural relationships in Precambrian Gondwanaland can be compared to meso-microstructures found in ductile mylonite and shear zones. The large Archean cratons are comparable to the much smaller scale, porphyroclasts in shear and mylonites. The late Archean-Proterozoie mobile belts resemble large scale equivalents of the fine-grained, ductile matrix of shear and mylonite zones. The relationships are excellent examples of high strain, deformation of vastly different magnitudes (in the order of 106). The processes of deformation within a shear or mylonite zone have been described in some detail and may serve as a deformation scale model for craton-mobile belt relationships. The craton-mobile belts recognized in Precambrian Gondwanaland and Australia are analysed using whole rock strain methods that are usually applied to meso-microstructures. This assumes simple, ideal eonditions of deformation, of originally circular era tons. The particle (or craton)-centre to centre technique applied on the mega-scale suggests that the greatest relative extension in Precambrian Gondwanaland is between the South American and Australian era tons. Within Precambrian Australia a similar exercise results in greatest extension between Pilbara and Gawler cratons. These results lead to the development of a concept that the Archean cratons were once originally part of a larger craton unit, or were close neighbours, and have been subsequently deformed and disrupted in their transform mobile belt matrix, in late Archean-Proterozoic times.
Mylonite
Cite
Citations (2)
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.
Mylonite
Lineation
Shearing (physics)
Cite
Citations (2)
Mylonite
Lineation
Shearing (physics)
Cite
Citations (2)
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.
Mylonite
Lineation
Cite
Citations (4)