Tectono-Thermal History of the Neoarchean Balehonnur Shear Zone, Western Dharwar Craton (Southern India)

AbstractA widely spaced Neoarchean shear zone network traverses the granite-greenstone terrains of the Western Dharwar craton (WDC). The NNW-SSE trending Balehonnur shear zone traverses the largest part of the preserved tilted Archean crustal ensemble in the Western Dharwar craton (WDC) from the amphibolite-granulite transition in the south to greenschist facies in the north and eventually concealed under Deccan lava flows. Published tectonic fabrics data and kinematic analysis, with our data reveal a sinistral sense of shearing that effectuate greenstone sequences, Tonalite-Trondhjemite-Granodiorite Gneisses (TTG), and Koppa granite as reflected in variable deformation and strain localization. A profound increase of strain towards the core of the shear zone in the ca. 2610 Ma Koppa granite is marked by a transition from weak foliation outside the shear zone through the development of C-S structures and C-prime fabrics, mylonite to ultramylonite. The mineral assemblages in the Koppa granite and adjoining greenstone indicate near peak P-T conditions of 1.2 Gpa, 775-800°C following a slow cooling path of 1.0 GPa and 650°C. Field-based tectonic fabrics data together with U-Pb zircon ages reveal that the Koppa granite emplaced along the contact zone of Shimoga-Bababudan basin ca. 2610 Ma, coinciding with the emplacement of ca. 2600 Ma Arsikere-Banavara, Pandavpura, and Chitradurga granites further east which mark the stabilization of WDC. Significant variation in major element oxide (SiO2 = 56-69 wt.%) together with high content of incompatible elements (REE, Nb, Zr, and Y) and high zircon crystallization temperatures (~1000°C) of Koppa granite suggests derivation by partial melting of composite sources involving enriched uppermost mantle and lower crust. The development of widely spaced shear zones is probably linked to the assembly of eastern and western blocks through westward convergence of hot oceanic lithosphere against already cratonized thick colder western block leading to the development of strain heterogeneities between greenstone and TTGs due to their different mineral assemblages leading to rheological contrast in the cratonic lithologies

Formation of Archean (3600-2500 Ma) continental crust in the Dharwar Craton, southern India

The generation, preservation and destruction of continental crust on Earth is of wide interest in understanding the formation of continents, cratons and supercontinents as well as related mineral deposits. In this contribution, we integrate the available field, petrographic, geochronologic, elemental Nd-Hf-Pb isotope data for greenstones, TTG gneisses, sanukitoids and anatectic granites from the Dharwar Craton (southern India). This review allows us to evaluate the accretionary processes of juvenile crust, mechanisms of continental growth, and secular evolution of geodynamic processes through the 3600–2500 Ma window, hence providing important insights into building of continents in the Early Earth. The Dharwar Craton formed by assembly of micro-blocks with independent thermal records and accretionary histories. The craton can be divided into three crustal blocks (western, central and eastern) separated by major shear zones. The western block contains some of the oldest basement rocks with two generations of volcano-sedimentary greenstone sequences and discrete potassic plutons whereas the central block consist of older migmatitic TTGs, abundant younger transitional TTGs, remnants of ancient high grade supracrustal rocks, linear volcanic-dominated greenstone belts, voluminous calc-alkaline granitoids of sanukitoid affinity and anatectic granites. In contrast, the eastern block comprises younger transitional TTGs, abundant diatexites, thin volcanic-sedimentary greenstone belts and calc-alkaline plutons. Published geochronologic data show five major periods of felsic crust formation at ca. 3450–3300 Ma, 3230–3150 Ma, 3000–2960 Ma, 2700–2600 Ma, and 2560–2520 Ma which are sub-contemporaneous with the episodes of greenstone volcanism. U-Pb ages of inherited zircons in TTGs, as well as detrital zircons together with Nd-Pb-Hf isotope data, reveal continental records of 3800–3600 Ma. The U-Pb zircon data suggest at least four major reworking events during ca. 3200 Ma, 3000 Ma, 2620–2600 Ma, and 2530–2500 Ma corresponding to lower crustal melting and spatially linked high grade metamorphic events. The TTGs are sub-divided into the older (3450–3000 Ma) TTGs and the younger (2700–2600 Ma) transitional TTGs. The older TTGs can be further sub-divided into low-Al and high-Al groups. Elemental and isotopic data suggest that the low-Al type formed by melting of oceanic island arc crust within plagioclase stability field. In contrast, the elemental and isotopic features for the high-Al group suggest derivation of their magmatic precursor by melting of oceanic arc crust at deeper levels (55–65 km) with variable garnet and ilmenite in residue. The transitional TTGs likely formed by melting of composite sources involving both enriched oceanic arc crust and sub-arc mantle with minor contamination of ancient crustal components. The geochemical and isotopic compositions of granitoids with sanukitoid affinity suggest derivation from enriched mantle reservoirs. Finally, anatectic granites were produced by reworking of crustal sources with different histories. In the light of the data reviewed in this contribution, we propose the following scenario for the tectonic evolution of the Dharwar Craton. During 3450–3000 Ma, TTGs sources (oceanic arc crust) formed by melting of down going slabs and subsequent melting of such newly formed crust at different depths resulted in TTG magmas. On the contrary, by 2700 Ma the depth of slab melting increased. Melting of slab at greater depth alongside the detritus results in enriched melts partly modified the overlying mantle wedge. Subsequent melting of such newly formed enriched oceanic arc crust and surrounding arc-mantle generated the magmatic precursor to transitional TTGs. Finally at ca. 2600–2560 Ma, eventual breakoff of down going slab caused mantle upwelling which induced low degree (10–15%) melting of overlying enriched mantle at different depths, thereby, generating the sanukitoid magmas which upon emplacement into the crust caused high temperature metamorphism, reworking and final cratonization. The crustal accretion patterns in the Dharwar Craton share similarities with those in other Archean cratons such as the Bundelkhand Craton in Central India, Pilbara-Yilgarn Craton in Western Australia, Southern Africa (Swaziland and Limpopo belt), North China Craton, Tanzania Craton, Antongil Craton, NE Madagascar.Jayananda M., Santosh M., Aadhiseshan K.R

Tectono-Thermal History of the Neoarchean Balehonnur Shear Zone, Western Dharwar Craton (Southern India)

Abstract A widely spaced Neoarchean shear zone network traverses the granite-greenstone terrains of the Western Dharwar craton (WDC). The NNW-SSE trending Balehonnur shear zone traverses the largest part of the preserved tilted Archean crustal ensemble in the Western Dharwar craton (WDC) from the amphibolite-granulite transition in the south to greenschist facies in the north and eventually concealed under Deccan lava flows. Published tectonic fabrics data and kinematic analysis, with our data reveal a sinistral sense of shearing that effectuate greenstone sequences, Tonalite-Trondhjemite-Granodiorite Gneisses (TTG), and Koppa granite as reflected in variable deformation and strain localization. A profound increase of strain towards the core of the shear zone in the ca. 2610 Ma Koppa granite is marked by a transition from weak foliation outside the shear zone through the development of C-S structures and C-prime fabrics, mylonite to ultramylonite. The mineral assemblages in the Koppa granite and adjoining greenstone indicate near peak P-T conditions of 1.2 Gpa, 775-800°C following a slow cooling path of 1.0 GPa and 650°C. Field-based tectonic fabrics data together with U-Pb zircon ages reveal that the Koppa granite emplaced along the contact zone of Shimoga-Bababudan basin ca. 2610 Ma, coinciding with the emplacement of ca. 2600 Ma Arsikere-Banavara, Pandavpura, and Chitradurga granites further east which mark the stabilization of WDC. Significant variation in major element oxide (SiO2 = 56-69 wt.%) together with high content of incompatible elements (REE, Nb, Zr, and Y) and high zircon crystallization temperatures (~1000°C) of Koppa granite suggests derivation by partial melting of composite sources involving enriched uppermost mantle and lower crust. The development of widely spaced shear zones is probably linked to the assembly of eastern and western blocks through westward convergence of hot oceanic lithosphere against already cratonized thick colder western block leading to the development of strain heterogeneities between greenstone and TTGs due to their different mineral assemblages leading to rheological contrast in the cratonic lithologies

Geochronology and geochemistry of Meso- to Neoarchean magmatic epidote-bearing potassic granites, western Dharwar Craton (Bellur–Nagamangala–Pandavpura corridor), southern India: implications for the successive stages of crustal reworking and cratonization

International audienceWe present field and petrographical characteristics, zircon U–Pb ages, Nd isotopes, and major and trace element data for the magmatic epidote-bearing granitic plutons in the Bellur–Nagamangala–Pandavpura corridor, and address successive reworking and cratonization events in the western Dharwar Craton (WDC). U–Pb zircon ages reveal three stages of plutonism including: (i) sparse 3.2 Ga granodiorite plutons intruding the TTG (tonalite–trondhjemite–granodiorite) basement away from the western boundary of the Nagamangala greenstone belt; (ii) 3.0 Ga monzogranite to quartz monzonite plutons adjoining the Nagamangala greenstone belt; and (iii) 2.6 Ga monzogranite plutons in the Pandavpura region. Elemental data of the 3.2 Ga granodiorite indicate their origin through the melting of mafic protoliths without any significant residual garnet. Moderate to poorly fractionated REE patterns of 3.0 Ga plutons with negative Eu anomalies and Nd isotope data with εNd(T) = 3.0 Ga ranging from −1.7 to +0.5 indicate the involvement of a major crustal source with minor mantle input. Melts derived from those two components interacted through mixing and mingling processes. Poorly fractionated REE patterns with negative Eu anomalies of 2.6 Ga plutons suggest plagioclase in residue. The presence of magmatic epidote in all of the plutons points to their rapid emplacement and crystallization at about 5 kbars. The 3.2 Ga intrusions could correspond to reworking associated with a major juvenile crust-forming episode, whilst 3.0 Ga potassic granites correspond to cratonization linked to melting of the deep crust. The 2.6 Ga Pandavpura granite could represent lower-crustal melting and final cratonization, as 2.5 Ga plutons are absent in the WDC