Sukanta Dey

Neoarchaean crustal growth by combined arc–plume action: evidence from the Kadiri Greenstone Belt, eastern Dharwar craton, India

Abstract Field and geochemical studies combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating set important constraints on the timing and petrogenesis of volcanic rocks of the Neoarchaean Kadiri greenstone belt and the mechanism of crust formation in the eastern Dharwar craton (EDC). The volcanic rocks are divided into three suites: tholeiitic basalts, calc-alkaline high-Mg# andesites and dominant dacites–rhyolites. The basalts (pillowed in places) show flat rare earth element (REE) and primordial mantle-normalized trace element patterns, but have minor negative Nb and Ta anomalies. They are interpreted as mantle plume-related oceanic plateau basalts whose source contained minor continental crustal input. The andesites are characterized by high Mg# (0.66–0.52), Cr and Ni, with depletion of high-field strength elements (HFSE) and enrichment of light REE (LREE) and large-ion lithophile elements (LILE). They were probably derived from a metasomatized mantle wedge overlying a subducted slab in a continental margin subduction zone. The dacites–rhyolites are silicic rocks (SiO2=61–72 wt%) with low Cr and Ni, K2O/Na2O mostly 0.5–1.1, highly fractionated REE patterns, enrichments of LILE and distinctly negative HFSE anomalies. One rhyolite sample yielded a zircon U–Pb age of 2353±32 Ma. This suite is similar to potassic adakites and is explained as the product of deep melting of thickened crust in the arc with a significant older crustal component. Collision between a continental margin arc with an oceanic plateau followed by slab break-off, upwelling of hot asthenosphere and extensive crustal reworking in a sustained compressional regime is proposed for the geodynamic evolution of the area. This is in corroboration with the scenario of EDC as a Neoarchaean hot orogen as suggested recently by some workers. Supplementary material: Details of whole-rock major and trace element determination, Nd isotope analysis and zircon U–Pb dating and trace element analysis, the geographical coordinates of the samples and the values of the international rock standards analysed are available at http://www.geolsoc.org.uk/SUP18660

Geochronology of Neoarchaean granitoids of the NW eastern Dharwar craton: implications for crust formation

Abstract The Neoarchaean Era is characterized by large preserved record of continental crust formation. Yet the actual mechanism(s) of Neoarchaean crustal growth remains controversial. In the northwestern part of the eastern Dharwar craton (EDC) granitoid magmatism started at 2.68 Ga with gneissic granodiorites showing intermediate character between sanukitoid and tonalite–trondhjemite–granodiorite (TTG). This was followed by intrusion of transitional (large-ion lithophile element-enriched) TTGs at 2.58 Ga. Finally 2.53–2.52 Ga sanukitoid and Closepet-type magmatism and intrusion of K-rich leucogranites mark the cratonization in the area. These granitoids mostly display initial negative ϵNd and Mesoarchaean depleted mantle model ages, suggesting presence of older crust in the area. Available data show that most of the Neoarchaean sodic granitoids in the EDC are transitional TTGs demonstrating the importance of reworking of older crust. It is suggested that the various c. 2.7 Ga greenstone mafic–ultramafic volcanic rocks of EDC formed in oceanic arcs and plateaus which accreted to form continental margin environment. Subsequent 2.7–2.51 Ga granitoid magmatism involved juvenile addition of crust as well as reworking of felsic crust forming transitional TTGs, sanukitoids and K-rich leucogranites. Microcratons were possibly the source of older crustal signatures and their accretion appears to be one of the important processes of Neoarchaean crustal growth globally. Supplementary material: Analytical techniques are available at https://doi.org/10.6084/m9.figshare.c.3470724

Chapter 19 Geological history of the Kaladgi–Badami and Bhima basins, south India: sedimentation in a Proterozoic intracratonic setup

Abstract The Proterozoic Kaladgi–Badami and Bhima basins are intracratonic basins occurring over the Archaean Dharwar craton. The Kaladgi–Badami Basin contains arenites, shales and carbonates with minor cherts and conglomerates deposited in continental, transitional and shallow-marine environments presumably during the late Palaeoproterozoic/Mesoproterozoic to Neoproterozoic. The lower part of the succession (Bagalkot Group) is deformed into east–west-trending elongated doubly plunging synclines and anticlines. The upper part of the succession (Badami Group) is undeformed and unconformably overlies the lower part. The evolution of the Kaladgi–Badami Basin was controlled by movements along east–west-trending normal faults under an extensional stress regime. The Bhima Basin hosts mainly limestones with subordinate arenites and shales deposited in fluvial, deltaic and tidal flat environments possibly during the Neoproterozoic. These sediments are undeformed except along faults with significant strike-slip components. The basin is exposed in narrow strips arranged in an en echelon pattern and appears to be a pull-apart basin. Inadequate data exist on the age of the basin fills, the deep basinal architecture, subsidence history and tectonic controls for both of the basins. Future research may be directed towards these aspects which will have wide implications for understanding intracratonic basin formation, reconstructing Proterozoic supercontinents and studying the evolution of the atmosphere and primitive life forms.

An approach of understanding acid volcanics and tuffaceous volcaniclastics from field studies: A case from Tadpatri Formation, Proterozoic Cuddapah basin, Andhra Pradesh, India

The lower stratigraphic part of the Cuddapah basin is marked by mafic and felsic volcanism. Tadpatri Formation consists of a greater variety of rock types due to bimodal volcanism in the upper part. Presence of bimodal volcanism is an indication of continental rift setting. Various genetic processes involved in the formation of such volcanic sequence result in original textures which are classified into volcaniclastic and coherent categories. Detailed and systematic field works in Tadpatri–Tonduru transect of SW Cuddapah basin have provided information on the physical processes producing this diversity of rock types. Felsic volcanism is manifested here with features as finger print of past rhyolite-dacite eruptions. Acid volcanics, tuffs and associated shale of Tadpatri Formation are studied and mapped in the field. With supporting subordinate studies on geochemistry, mineralogy and petrogenesis of the volcanics to validate field features accurately, it is understood that volcanism was associated with rifting and shallow marine environmental condition. Four facies (i.e., surge, flow, fall and resedimented volcaniclastic) are demarcated to describe stratigraphic units and volcanic history of the mapped area. The present contribution focuses on the fundamental characterization and categorization of field-based features diagnostic of silica-rich volcanic activities in the Tadpatri Formation.

Petrogenesis of Two Types of Archean TTGs in the North China Craton: A Case Study of Intercalated TTGs in Lushan and Non‐intercalated TTGs in Hengshan

TTG (Tonalite-Trondhjemite-Granodiorite) gneisses, a major component of Precambrian continental crust, play a significant role in understanding the process and mechanism of the crustal evolution in the early periods of the Earth. In terms of field occurrence, there are two kinds of Archean TTGs in the NCC (North China Craton): intercalated and non-intercalated TTGs. In this contribution, we make a comprehensive comparison of these two types of TTGs from the typical areas (Lushan and Hengshan) in the NCC with an aim to constrain their petrogenesis. The results suggest that they have similar mineral assemblages of Pl + Qtz + Bt ± Amp ± Kfs but different field appearances and geochemical compositions, thus probably reflecting different source materials and tectonic settings. Differences in the contents of characteristic elements, such as Sr, REE and HFSE, suggest that the non-intercalated TTGs in Hengshan were generated at deeper levels than those of intercalated TTGs in Lushan. Constraints from element contents and geochemical modeling results are consistent with derivation from dual sources involving both garnet amphibolite and rutile-bearing eclogite residues for the non-intercalated TTGs in Hengshan, whereas the compositions of intercalated TTGs in Lushan indicate that they were formed by partial melting with amphibolite to garnet-amphibolite residues. Moreover, accumulation of plagioclase is also required in the petrogenesis of intercalated TTGs in Lushan, at least for part of them. In addition, the non-intercalated TTGs in Hengshan display distinctly higher MgO, Mg#, Cr and Ni values and lower SiO2 average contents compared to the intercalated TTGs in Lushan. These features suggest that the former magma, at least a part, might have interacted with the mantle wedge during ascent. Considering all the above factors and in combination with the whole-rock Nd and zircon Hf isotopic data, it is suggested that the non-intercalated TTGs in Hengshan were produced by partial melting of subducted slab contaminated by the overlying mantle wedge at deeper levels and high pressures, whereas the intercalated TTGs in Lushan were generated by melting of the thickened lower crust at lower pressures and shallower depths. The tectonic settings of the two types of TTGs shed new light on the growth of the NCC.

Early Neoarchaean A-type granitic magmatism by crustal reworking in Singhbhum craton: Evidence from Pala Lahara area, Orissa

Several volumetrically minor

Contrasting crustal evolution processes in the Dharwar craton

\sim

Proterozoic platform sequences of Peninsular India: Implications towards basin evolution and supercontinent assembly

∼2.8 Ga anorogenic granites and rhyolites occur along the marginal part of the Singhbhum craton whose origin and role in crustal evolution are poorly constrained. This contribution presents petrographic, geochemical, zircon U–Pb and trace element, and mineral chemical data on such granites exposed in the Pala Lahara area to understand their petrogenesis and tectonic setting. The Pala Lahara granites are calc-alkaline, high-silica rocks and define a zircon U–Pb age of 2.79 Ga. These granites are ferroan, weakly metaluminous, depleted in Al, Ca and Mg and rich in LILE and HFSE. They are classified as A2-type granites with high Y/Nb ratios. Geochemical characteristics (high