Precambrian continental strain and shear zone patterns: South Indian case

Pb-Pb baddeleyite ages of mafic dykes from the Western Dharwar Craton, Southern India: a window into 2.21–2.18 Ga global mafic magmatism

Archaean cratons provide a critical window into early Earth dynamics, preserving a record of crustal evolution processes that include the start of ‘modern’ plate tectonics and the development of supercontinents. However, these same processes can also destroy or rework substantial volumes of crust, and the oldest extant Archaean terrane is only ~3.9 Ga. Where Archaean rocks are exposed, bulk techniques such as Pb and Nd isotopes can provide considerable information about continental formation, but these may have been altered by later metamorphic events. Another method is to examine sedimentary units, which can preserve fragments of the crust that are no longer exposed at the Earth’s surface. Resistant detrital minerals such as zircon have particular use in these studies, as they incorporate a range of isotopic and geochemical tracers and can survive multiple crystallisation and/or sedimentary events. In this manner, a more complete record of a craton’s evolution may be obtained, with the benefit of contributions from contrasting isotopic systems. The Dharwar craton of southern India is one such Archaean block, comprising >2.7 Ga trondjhemite-tonalitegranodiorite (TTG) gneisses, volcano-sedimentary belts (>3.0 and 2.9–2.6 Ga) and 2.7–2.5 Ga calc-alkaline to potassic granitoids. These rocks preserve evidence for several cycles of supracrustal formation, deformation, meta-morphism and granitic activity during the Precambrian. New in situ U–Pb-Hf analyses of detrital zircons from across the craton indicate significant juvenile crustal extraction events at ~3.3 and 2.7 Ga, and continuous extraction from 3.7–3.3 Ga. Reworking in the older western block at ~3.0 Ga marks the onset of cratonisation, most likely due to ‘modern’ plate tectonic processes, while reworking in both the western and younger eastern block at 2.55–2.50 Ga indicates accretion of the two terranes and final cratonisation much later than in most other Archaean terranes (~2.7 Ga). Different patterns of disturbance to the zircon U–Pb systematics reflect variations in both the U content of parent rocks and later metamorphic conditions. Tectonic links are observed between the Kaapvaal and western Dharwar cratons, and between the north China and eastern Dharwar cratons, though none of these links necessarily requires a consanguineous origin.

Contrasting crustal evolution processes in the Dharwar craton: Insights from detrital zircon U–Pb and Hf isotopes

The Variscan continental collision has led to the development of large strike-slip shear zones in western Europe. Our study focuses on the regional deformation and shear zone patterns in the Massif Armoricain and the French Massif Central. The synthesis of granite emplacement ages associated to granite deformation fields, allow us to propose a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma (Late Devonian – Namurian).After the first steps of the continental subduction-collision, leading to high temperature and anatexis associated with N-S shortening at 380-370 Ma (Frasnian to Famennian), the southern part of the Massif Armoricain and western part of French Massif Central underwent large dextral shearing along N100-N130 trending shear zones up to early Visean time. These large-scale displacements progressively decreased at around 350-340 Ma, during the first emplacements of biotite bearing granites (Moulins-les Aubiers-Gourgé massif and Guéret massif intrusions).During middle Visean times, the shortening axis direction rotated towards a NNE-SSW direction implying changes in the regional deformation field. The occurrence of N070-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that time. Finally, new N150-N160 dextral shear zones appeared in middle to late Visean times: as for examples, the Parthenay and the Pradines shear zones in the SE Massif Armoricain and the Millevaches massif, respectively. These shear zones were conjugated to the sinistral N020 Sillon Houiller in the French Massif Central. They reflect large scale brittle continental indentation in the French Variscan belt during the middle to late Visean.

Subduction-related origin of eclogite xenoliths from the Wajrakarur kimberlite field, Eastern Dharwar craton, Southern India: Constraints from petrology and geochemistry

Networking of shear zones at the brittle-to-viscous transition (Cap de Creus, NE Spain)

<p>The area of Cap de Creus, at the eastern termination of the Axial Zone of the Pyrenean Belt, exposes some of the most famous outcrops of ductile shear zones and shear zone networks (Carreras, 2001). Recent studies proposed that the nucleation and growth of such shear zones may have taken place by brittle processes (Fusseis et al., 2006; Fusseis and Handy, 2008).</p><p>The present study investigates the geometrical relationships between fracture systems and some shear zones, the deformation temperature of these shear zones, and the processes leading to the nucleation and growth of shear zones along fracture planes. We selected two areas of the Cap de Creus, the Cala d’Agulles, and the Punta de Cap de Creus, because they are most intensely dissected by subparallel sets of shear zones and fractures. The orientation of the average shear zone planes is sub-parallel to the orientation of the major set of fractures, and the great extent and close spacing of some shear zones that we characterized by aerial photos from a drone, is similar to the distribution and extent of the fracture planes. These observations, in addition to those of Fusseis et al. (2006) suggest that the shear zones nucleated on previous fracture planes. </p><p>These fractures are surrounded by haloes of nearly 1 cm thickness affecting the fabric of the country rock, an amphibolite-facies, biotite-andalusite bearing schist. Microscopic observations show that the haloes correspond to the wide-spread presence of thin (less than 2µm thickness) phosphate seams coating the grain boundaries, preferentially those oriented at low angle to the fracture plane, and to the alteration of plagioclase to white mica and sericite, and to the growth of tourmaline, also related to grain boundaries and micro-fractures.</p><p>Deformation temperature in the shear zones is assessed by white mica thermometry and pseudosections. The calculated T of at least 350-400° C is consistent with qualitative observations showing the presence of stable biotite within very fine-grained (<< 10 µm) shear bands and the recrystallization of quartz by rotation of sub-grain boundaries.</p><p>In summary, fractures formed at high temperature, possibly associated with the intrusion of tourmaline-bearing pegmatites and fluids, which predate the ductile mylonitic event (Druguet, 2001; Van Lichtervelde et al., 2017). Fluids altered and weakened a volume of approximately 2 cm thickness all along the fracture planes, whose extent may reach > 100 m. The inferred, relatively high T of ca.  400° C indicates that fracturing is not due to the proximity of the brittle-ductile transition. In addition, no significant micro-fracturing of the mylonites is observed in thin sections. Therefore, fracturing precedes the ductile shear zones, which nucleate on some of the “inherited” sets of thin, planar, weakened structures, the large majority of which remains undeformed. These observations raise the question on whether nucleation and propagation of ductile shear zones is mechanically unrelated to brittle fracturing. Their weakening of planar structures would originate from fluid migration along fracture planes, but fracturing would no longer be active during ductile deformation.</p>

Nucleation and initial growth of a shear zone network within compositionally and structurally heterogeneous granitoids under amphibolite facies conditions

Oxygen isotope ratio of quartz veins from the auriferous Ramagiri–Penakacherla schist belt and surrounding granitoids in the Eastern Dharwar craton: A case for a possible link between gold mineralization and granite magmatism

Structure and strain variation at mid-crustal levels in a transpressional orogen: A review of Kaoko Belt structure and the character of West Gondwana amalgamation and dispersal

The Variscan orogeny occurred as a result of the Late Devonian to Late Carboniferous collision and accretion of Gondwana-derived microcontinents and continental masses with those of Laurussia. The irregular boundaries of the colliding continents caused isochronous transpressional and transtensional tectonics, accompanied by a complex pattern of intracontinental shear zones at the scale of the southern European Variscides. These shear zones and their configuration controlled the subsequent evolution of Permian to Middle Triassic paleogeography. The geographic distribution, from Morocco to the Eastern Alps, of the Late Carboniferous–Permian up to Triassic basins, most of which are considered as pull-apart basins, was related with the development of the Late Palaeozoic intracontinental shear network. Our analysis of the stratigraphic, tectonic and volcanic features of the Late Carboniferous/Permian continental basins across the Laurussia/Gondwana boundary reveals the role of the East Variscan Shear Zone during this time as a precursory lineament for the development of the Permian to Triassic rifting of Pangaea and the following opening of oceanic basins (e.g., the Neothetyan Ocean).

Re–Os isotope study of Indian kimberlites and lamproites: Implications for mantle source regions and cratonic evolution

Chromitites from an Archean layered intrusion in the Western Dharwar Craton, southern India

Högbomite, a rare exotic mineral, is found to be associated with the vanadiferous–titaniferous (V‐Ti) bearing magnetite bands at Bhakatarhalli, Nuggihalli greenstone belt, western Dharwar Craton, India. We report on a second occurrence of högbomite from the Dharwar craton in Karnataka, which is the sixth documented occurrence of this mineral from India. We evaluate the chemical characteristics of högbomite and associated Fe‐Ti‐minerals in an attempt to identify its formation as a primary hydrothermal mineral in a metamorphosed magnetite layer. We report here the presence of högbomite as a complex oxide of Fe, Mg, Al and Ti with accessory of Zn, V and Sn. Petrographic studies suggest the (V–Ti) bearing magnetite (Mt) contain spinel, högbomite, chlorite, martite, ilmenite (Il) and minor amounts of diaspore. The högbomite displays euhedral to subhedral textures, and is up to 250 μm along the grain boundaries of magnetite and ilmenite. In the samples studied, högbomite is prismatic, irregular and elongated in shape. The genesis of högbomite in veins between magnetite and ilmenite implies its precipitation from fluids without involving complicated reactions. Several models were proposed for the formation of högbomite; however, the subject is still debatable.

Brittle to semi-brittle shear zones near Gogalgatti, Mincheri and Yerjanti villages in the northern part of Eastern Dharwar craton show sporadic sulfide and gold occurrences. These shear zones are confined to a late Archaean (2.5 Ga) granitoid complex, consisting of TTG gneisses and granite plutons. Intense mylonitization, brecciation and fracturing are conspicuous in all the three shear zones. The mineralization is of disseminated type and the ore minerals are confined to fracture filling quartz veins. The veins are mainly parallel to the shear zones with minor veins showing cross- cutting relationship. The observed sulfide minerals in decreasing order of abundance are pyrite, chalcopyrite, arsenopyrite, tennantite and sphalerite with minor oxide minerals represented by specular hematite. Random chip sampling in Gogalgatti shear zone gave gold values from 0.52 to 0.65 ppm. However, similar sampling from Mincheri has not indicated any gold, though one of the polished sections of the samples from this area showed an exsolved gold globule within arsenopyrite.In the mineralized veins of these shear zones, primary inclusions of both type I (H2O) and type II (H2O-CO2) are more conspicuous. Secondary inclusions, though present, are mostly affected by deformation. Sulfide minerals are associated with moderate to high saline, type I fluids formed in the early stage of mineralization in all the three shear zones, whereas gold precipitated from high saline (28 -30 wt% NaCl eqvi. in Gogalgatti and ~29% wt% NaCl eqvi. in Mincheri) type II fluids in the later stages, in Gogalgatti and Mincheri shear zones. The temperature of fluid ascent has been determined to be 100 - 200° C in epithermal conditions of mineralization at a depth of 150 to 200 m. Granitoids, in which these shear zones are located, form the main source of hydrothermal fluids, which ascented and precipitated in the structural domains.