Eastern Dharwar Craton Research Articles

Crustal structure across the Deccan Volcanic Province and Eastern Dharwar craton in south Indian shield using receiver function modelling

The south Indian shield, primarily consisting of Archean cratons and Cretaceous-Tertiary Deccan Volcanic Province (DVP), has undergone several major tectonic episodes during its evolution. The Deccan volcanism at Cretaceous-Tertiary (~65 Ma) is the last major tectono-thermal event, which influenced a substantial part of the south Indian shield. To understand the influence of the Deccan volcanism on the evolution of the south Indian shield, we study the crustal seismic structure of the ~65 Ma Deccan Volcanic Province and the adjacent ~2.6 Ga Eastern Dharwar Craton (EDC), which forms the basement of the volcanic terrain. We calculate teleseismic receiver functions for 18 broadband seismic stations along a ~1000 km long seismological profile that cut across both the EDC and DVP. The analysis and modelling, using H-Vp/Vs stacking and generalized neighbourhood algorithm inversion of the receiver functions show distinct crustal structure (crustal thickness, average composition, shear wave velocity variation, nature of crust-mantle boundary, etc.) across the EDC and DVP. The results clearly indicate that the crustal structure is heterogeneous beneath the DVP compared to a relatively uniform structure below the EDC. Using results from this study along with earlier results, we infer that the present Eastern Dharwar Craton terrain is not affected by any tectono-thermal event for a long geological time, including the Deccan volcanism. Whereas, the present Deccan Volcanic Province is highly affected by the Reunion mantle plume-crust interaction.

Geochemistry data of the nadergul granodiorite of the late archean hyderabad granite batholith, part of eastern dharwar craton, india; implications for composition of the lower continental crust.

The Nadergul granodiorite (NG), is a part of Hyderabad Granite Batholith (HGB), exposed at ground level covering an area of 6 km2. Field observational data suggests that the NG is involved in the magma mixing mingling process. Petrographical data shows holocrystaline, phaneritic, coarse grained, inequigranular, mesocratic characteristics, essentially consists of quartz, orthoclase, plagioclase and microcline. The common accessory minerals are hornblende, biotite, clinopyroxene, zircon, fluorite, apatite and opaque. The myrmekitic texture is the most common intergrowth texture which seems to be magmatic origin. Geochemical data suggest that the NG is metaluminous and show calc-alkaline trend. It is charecterised by SiO2 content ranging from 63.55 % to 64.54%, moderate alkali content (Na2O + K2O = 5.6%), relatively high MgO content ranging from 2.8 % to 3.5 %. The total CaO-Na2O-K2O content ranges from 9.5 % to10.4 %. The normalised REE patterns show LREEs enrichment with slight negative europium anomaly. The High Field Strength Elements such as Zr (67.3 to 114.3 ppm), Y (21.1 to 30.1 ppm), Nb (6.3 to 8.5 ppm) and Th (8.4 to 13.8 ppm) are relatively enriched. The granodiorite is I type, falls in the Volcanic Arc Granitoid (VAG) field in tectonic discrimination diagram.

Evolution of geodynamic processes in Neoarchean Kadiri greenstone belt, eastern Dharwar Craton, India: Implications on the migrating arc magmatism

Dharwar Craton of southern peninsular India is one of the significant Archean cratons in the world with well developed greenstone belt sequences representing plume-arc accretionary tectonics. The eastern Dharwar Craton (EDC) is endowed with excellent gold mineralization which is being mined and the greenstone belt lithounits among which majority of them represent the convergent margin processes. The Neoarchean Kadiri greenstone belt of EDC consists of an array of mafic, intermediate and felsic volcanic rocks among which the mafic rocks exhibit a wide range of silica (49.74–65.19 wt.%), high LILE, HFSE including the Nb contents (7−15 ppm) that are classified as Nb-enriched basaltic andesites (NEBA). Moderate composition of SiO2 (57−63 wt.%) along with higher MgO (3.96–7.58 wt.%), Cr (44−890 ppm) and Ni (22−491 ppm) contents distinguish some of the intermediate rocks as Mg-andesites. The petrographic characteristics along with higher silica (63−75 wt.%), Na2O (2.21–4.01 wt.%) and low K2O (0.7–3.35) classify the associated felsic volcanic rocks as sodic adakites. Abundance of K-feldspar phenocrysts and higher K2O content (1.19–7.0 wt.%) in some adakites identify them as potassic adakites. The geochemical characteristics of the calc-alkaline andesite-dacite-rhyolites (ADR) reflect on typical island arc environment involving the processes of fractional crystallization and crustal contamination in an active continental margin environment. The NEBA and Mg-andesites are the products of hybridized mantle wedge under the influence of slab fluids/melts whereas the Na-adakites are the slab melts erupted within the oceanic domain and the K-adakites were emplaced under the influence of continental crust during the migration of the island arc towards the continental margin. Beside the generation of various types of volcanic rocks, these convergent margin processes have played a significant role in the generation and mobilization of gold mineralized fluids, which are being mined from some greenstone belts of EDC.

Gravity–magnetic appraisal at the interface of Cuddapah Basin and Nellore Schist Belt (NSB) for shallow crustal architecture and tectonic settings

Cuddapah Basin (CB) is an intracontinental, Proterozoic basin flanked by Eastern Dharwar Craton (EDC) in the west, Nellore Schist Belt (NSB) and Eastern Ghat Mobile Belt (EGMB) in the east, represents second largest Proterozoic basin of India. Gravity and magnetic surveys were carried out at the interface of Cuddapah Basin (CB) and Nellore Schist Belt (NSB) covering ~2880 km2 area. Gravity map has brought out some distinct zones. The thrusted contact of NSB and Cuddapah sediments has been well delineated from the gravity map by NE–SW trending steep gradient of contours. Relatively high gravity values are observed over NSB in the southeastern part, moderately high values are observed over Cumbum Formation, but distinct low is observed over Baironkonda Formation. These gravity highs and lows are mainly the manifestation of basement characteristics and intrusives. The magnetic map shows two distinct domains, viz., moderate to low zone in the southern part, and moderate to high zone in the northern part. Regional gravity map suggests a change in basement characteristics from felsic to mafic from NW to SE. Presence of mafic basement may be representing EGMB group of rocks underneath the Cuddapah sediments at the eastern part of the study area. The joint gravity and magnetic modelling reveal varied nature of sedimentary units in terms of density and susceptibility and change in basement characteristic.

Bundelkhand, Aravalli and Dharwar cratons Indian Shield: comparison of Archean crustal evolution and location in the Kenorland Supercontinent structure

Comparison of the Meso-Neoarchean crustal evolution of the Bundelkhand, Aravalli, Western and Eastern Dharwar Cratons shows that geodynamic mechanisms, similar to modern plate-tectonic and mantle-plume mechanisms, were active in that period. The Mesoarchean crustal evolution of the Bundelkhand Craton is considerably different from that of other cratons discussed here in that it comprises subduction-accretion processes, while in other cratons plume processes were more active in this period. In the Neoarchean (2.7-2.5 Ga), all the cratons display subduction-accretion processes. Each of the cratons displays its own crust formation pattern. It seems that in Mesoarchean time the Bundelkhand, Aravalli and Western and Eastern Dharwar Cratons were parts of the Kenorland Supercontinent, rather than one block.

Evidence of Archaean metamorphism from the Yerrabali schist belt of Eastern Dharwar craton from EPMA dating of monazite

The Dharwar craton is divided in to the Western and Eastern Dharwar craton (WDC and EDC) by the N–S trending Chitradurga Shear zone. The EDC mainly consists of late Archean granites interspersed with N–S trending linear schist belts. The Yerrabali Schst belt (YSB) is located at the north-eastern corner of the EDC, near the Karimnagar granulite belt. The YSB is metamorphosed in middle-upper amphibolite facies and consists of Banded Magnetite Quartzite (BMQ), ferruginous quartzite, ultramafics, quartz–cordierite–gedrite ± garnet bearing quartzite, dolomite, amphibolite, deformed meta-pyroxenite, tremolite schist, sillimanite–garnet–cordierite schist, and dolerite dykes. We have dated monazites in the sillimanite–garnet–cordierite schist, exposed at the SW part of the YSB by EPMA chemical method. So far no dates are available from the YSB. The monazite chemical ages with three distinct populations, viz., 2658 ± 42, 2817 ± 19 and 3097 ± 34 Ma, probably correspond to three distinct metamorphic phases. The peak metamorphism at around 2800 Ma corresponds with the garnet and cordierite growth. A temperature of 673°C and a pressure of 4.7 kb have been estimated for peak metamorphism.

Mineralogical variation in platinum group element within altered chromitite of the Kondapalli layered igneous complex (Southern India): Implication on magmatic evolution and its petrogenetic significance

The Kondapalli Layered Igneous Complex (KLIC) is deformed and dismembered stratiform type of mafic-ultramafic complex emplaced into high-grade metamorphic rocks of Eastern Ghat Belt (EGB). Which contains chromite with relatively oxidized compositions that occur as bands, pockets, pods, stringer, and dissemination. This chromite hosts traces of PGM eg. laurite, irarsite [(Ir, Ru, Rh, Pt)AsS], Os-Ir-Ru alloys and Fe–Ni–As–S minerals. The present study helps to understand the changes and/or reworking of chromite and PGM due to the effect of alteration. Petrographic observations and EPMA study indicate that the chromite core is still consistent with magmatic fractionation processes, whereas sequential alteration of the chromite rim suggests upper greenschist to lower amphibolite facies of metamorphism (500° to 550 °C). The platinum-group minerals (PGM), however, display evidence of partial hydrothermal/metamorphic reworking. Especially, laurite and irarsite which occur near the chromite grain boundary. In contrast, the primary ones which probably formed at high-T and low fs2 and occur in the chromite cores are relatively less affected by alteration.The calculated parental magma composition was similar to that of modern primitive boninitic/tholeiitic basalt formed by a high-degree of mantle melting. Present study ascribes the formation of the subduction-generated arc mafic-ultramafic magmatism in the Kondapalli to the closure of the ocean that led to the formation of the suture between Eastern Dharwar Craton (EDC) and EGB. The parent magma of the KLIC is interpreted as mantle-derived arc-related boninites with S-undersaturated nature like other layered intrusions. This is indicating that an initial percolation of sulphur-poor melts for long-period without significant contamination for the genesis of KLIC. The mineral chemistry supports the very limited crustal contamination and non-availability of the sufficient meta-sediments during assimilation precludes the sulphur saturation in the KLIC, which is mainly constrained for the development of potential Ni-Cu-PGE mineralization. As a result, the KLIC fever the deposition of only primary orthomagmatic chromite-PGE mineralization. These findings determined that the investigated KLIC are low-priority Ni-Cu-PGE targets. The compositions of the chromites can be used to determine the petrologic history of the intrusions and may prove to be a useful exploration tool in such mineralised belt.

Petrogenesis of low-Ti and high-Ti basalt, adakite and rhyolite association in the Peddavuru greenstone belt, eastern Dharwar craton, India: A Neoarchean analogue of Phanerozoic-type back-arc magmatism

The petrogenetic record, available in the literature, illustrating Neoarchean arc magmatic processes in a back-arc setting is rare. In this contribution, we present a comprehensive account of field, petrography and geochemistry of the Neoarchean bimodal mafic – felsic metavolcanic rock association in the Peddavuru greenstone belt, eastern Dharwar craton, India.Basalt is the predominant rock type. Felsic volcanic rocks are interleaved with the basalts, in the central part. The basalts are fine grained, aphyric and essentially composed of amphibole and plagioclase with rutile, magnetite and ilmenite as accessory Fe-Ti oxide phases. The felsic volcanic rocks exhibit porphyritic texture. Based on the composition of the phenocryst type i.e. feldspar or quartz, two variants have been recognized. Biotite is present in subordinate amounts while, apatite and magnetite are the accessory phases.Basalts are tholeiitic in composition, whereas the felsic volcanic rocks are calc-alkaline in nature. On the basis of TiO2 contents, the basalts can be classified into low-Ti (< 1 wt. % TiO2) and high-Ti (> 1 wt. % TiO2) geochemical subgroups. The two subgroups, however, are consanguineous in nature. They display slightly depleted (La/Sm ∼ 0.89) and mildly enriched (La/Sm ∼ 1.44) chondrite normalized light rare earth element patterns (REE), and slightly depleted heavy REE (Gd/Yb ∼ 1.2). On a primitive mantle normalized trace element variation diagram, irrespective of low- or high-Ti, the basalts display negative Nb and Ti anomalies, and zero to negative Zr-Hf anomalies relative to the neighbouring REE.The felsic volcanic rocks are characterized by contrasting geochemical compositions. On the basis of high field strength element systematics, they are classified as dacites and rhyolites. Compared to the rhyolites, the dacites are characterized by high Mg# (39 ± 9 vs. 24 ± 5), low Nb (≤ 5 ppm vs. 14 ppm), Y (5.9 ± 1.8 ppm vs. 19 ppm) and Yb (0.47 ± 0.17 ppm vs. 1.72 ppm) contents. Further, the dacites exhibit comparatively steep chondrite normalized REE patterns (LaN/YbN ∼ 30 vs. 10) with negligible to slightly positive Eu anomaly (Eu/Eu* = 0.9–1.2 vs. 0.6). On a primitive mantle normalized trace element variation diagram they exhibit negative Nb and Ti anomalies, similar to the rhyolites, but contrasting positive Zr-Hf peaks and high Zr/Sm ratio (73 ± 18 vs. 24). The trace and rare earth element attributes of these dacites are identical to the adakitic rocks that have been recognized from the Phanerozoic intraoceanic arcs. On the contrary, rhyolites are the partial melt products involving minor plagioclase fractionation that are generated beneath the arc crust under extension.Overall, the chemical compositions of the mafic and felsic volcanic rocks in the Peddavuru belt indicate that interaction with the Archean upper continental crust, magma mixing and/or assimilation and fractional crystallization processes cannot be the cause of these geochemical patterns. The attributes instead reflect primary mantle source characteristics. MORB-like trace element signatures in combination with arc-like geochemical affinity in the Peddavuru basalts, provides compelling evidence of their origin in an intraoceanic back-arc setting. Accordingly, the Peddavuru greenstone belt presents a Neoarchean analogue of Phanerozoic-type back-arc magmatism.

Crustal growth of the Eastern Dharwar Craton: a Neoarchean collisional orogeny?

Abstract The Eastern Dharwar Craton (EDC) is predominantly made of Neoarchean potassic granitoids with subordinate linear greenstone belts. Available geochemical and isotopic systematics of these granitoids suggest variations in the source and petrogenetic mechanisms. By compiling the available geochemical data, these granitoids can be classified into four groups, namely: TTGs (tonalite–trondhjemite–granodiorite); sanukitoids; biotite and two-mica granites; and hybrid granites. This classification scheme is in line with the global classification of Neoarchean granites, and enables the sources and petrogenetic mechanisms of these variants to be distinguished. Available geochemical, isotopic and geochronological datasets of these granitoids are integrated and the existing tectonic models for the Neoarchean EDC are reviewed. The variability of the EDC granitoids is ascribed to crustal reworking associated with the collision of two continental blocks. The tectonomagmatic evolution of the EDC is analogous to the development of the Himalayan Orogeny. Based on the evolutionary history of the Dharwar Craton, it can be concluded that convergent margin tectonics were operational in the Indian Shield from at least c. 3.3 Ga and continued into the Phanerozoic. However, the nature and style of plate tectonics could be different with time.

Mineralogy and petrology of shoshonitic lamprophyre dykes from the Sivarampeta area, diamondiferous Wajrakarur kimberlite field, Eastern Dharwar craton, southern India

Mineralogy and petrology of shoshonitic lamprophyre dykes from the Sivarampeta area, diamondiferous Wajrakarur kimberlite field, Eastern Dharwar craton, southern India

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

Paleoproterozoic mafic magmatism in the Dharwar Craton (DC) is evident by ubiquitous dykes of varied orientations and emplacement ages. Eight distinct mafic dyke events have been identified in the Eastern Dharwar Craton (EDC), while studies in the Western Dharwar Craton (WDC) were restricted to a previously dated Dandeli dyke. Precise baddeleyite TE-TIMS Pb-Pb ages for five mafic dykes across the WDC reveal emplacement ages of 2212.3 ± 0.8 Ma, 2212.9 ± 0.4 Ma, 2208.1 ± 0.8 Ma, 2205.2 ± 0.8 Ma for NW-SE to NNW-SSE trending dykes and 1957.3 ± 0.7 Ma for ENE-WSW dyke. The ca. 1.96 Ga Mundargi dyke establishes a new episode of mafic magmatism in the DC, whereas ca. 2.21 Ga mafic dykes suggests widespread mafic magmatism across the DC. The collective trend of ca. 2.21–2.20 Ga radiating dykes vary from N100°E in EDC to N165°E in WDC, converging towards the north-western margin of DC (fanning angle ≃65°), where a mantle plume appears to be located. It is postulated that ca. 2.18 Ga Mahbubnagar-Dandeli dyke swarm of the DC also originated from a similar plume source, but as a distinct pulse within a time span of 35–40 Myr. Present study depicts barcode of the WDC and represents an epitome of correlations with the EDC, Superior, Slave and Greenland Cratons. Widespread ca. 2.21–2.18 Ga LIP event indicates that these cratons were located at proximity and suggests breakup of supercraton (Sclavia/Superia). The single ≃1.96 Ga Mundargi dyke is coeval with the dykes and sills of Karelia, North China and North Atlantic Cratons.

Petrogenesis of an alkaline lamprophyre (camptonite) with ocean island basalt (OIB)-affinity at the NW margin of the Cuddapah basin, eastern Dharwar craton, southern India

We report petrology and geochemistry (including Sr and Nd isotopes) of a fresh lamprophyre at Ankiraopalli area at the north-western margin of Paleo-Mesoproterozoic Cuddapah basin, eastern Dharwar craton, southern India. Ankiraopalli samples possess a typical lamprophyre porphyritic-panidiomorphic texture with phenocrysts of kaersutite and diopside set in a plagioclase dominant groundmass. Combined mineralogy and geochemistry classify it as alkaline lampro- phyre in general and camptonite in particular. Contrary to the calc-alkaline and/or shoshonitic orogenic nature portrayed by lamprophyres occurring towards the western margin of the Cuddapah basin, the Ankiraopalli samples display trace element composition revealing striking similarity with those of ocean island basalts, Italian alkaline lamprophyres and highlights an anorogenic character. However, the87 Sr/86 Srinitial (0.710316 to 0.720016) and εNdinitial (– 9.54 to – 9.61) of the Ankiraopalli lamprophyre show derivation from an 'enriched' mantle source showing long term enrichment of incompatible trace elements and contrast from those of (i) OIB, and (ii) nearby Mahbubnagar alkaline mafic dykes of OIB affinity. Combining results of this study and recent advances made, multiple mantle domains are identified in the Eastern Dharwar craton which generated distinct Mesoproterozoic lamprophyre varieties. These include (i) Domain I, involving sub-continental lithospheric mantle source essentially metasomatized by subduction-derived melts/fluids (represented by orogenic calcalkaline and/or shoshonitic lamprophyres at the Mudigubba, the Udiripikonda and the Kadiri); (ii) Domain II, comprising a mixed sub-continental lithospheric and asthenospheric source (represented by orogenic-anorogenic, alkaline to calc-alkaline transitional lamprophyres at the Korakkodu), and (iii) Domain III, representing a sub-continental lithospheric source with a dominant overprint of an asthenospheric (plume) component (represented by essentially alkaline lamprophyres at the Ankiraopalli). Our study highlights the varied mantle source heterogeneities and complexity of geodynamic processes involved in the Neoarchean-Paleo/Mesoproterozoic evolution of the Eastern Dharwar craton.

Constraining paleoproterozoic (∼1.7 Ga) collisional orogenesis between the Eastern Dharwar and Bastar cratons: New Sm–Nd garnet isochron and Th–U-total Pb monazite chemical ages from the Bhopalpatnam orogen, central India

The timing of assembly into India by amalgamation of cratonic fragments, is controversial with estimates ranging from Tonian, earliest Mesoproterozoic to Neoarchean. All models have as the nucleus the South Indian Block of Dharwar, Bastar and Singhbhum cratons. Although the time of assembly of the Bastar and Singhbhum cratons is uncertain, the implication of a proposed 1891–1883 Ma-aged Large Igneous Province, straddling the Bastar and Eastern Dharwar cratons is that their amalgamation occurred by Paleoproterozic time (≥1.9 Ga ago) and is at variance with the ages reported from the Bhopalpatnam granulite belt between 1.7-1.6 Ga ago. To test whether this belt formed by convergent collisional tectonics along the margins of these two cratons, petrochronologic analysis was conducted on high-grade rocks of this belt using garnet Sm–Nd and texturally-controlled Th–U-total Pb monazite chronometry.The whole rock-garnet Sm–Nd isochron from garnet-clinopyroxene-amphibolite in migmatite (M1a-stage), yielded an isochron age of 1692±13Ma with Ndi of 0.51042±0.00006 (2σ, MSWD = 2.5 n = 4) whereas a whole rock-garnet Sm–Nd isochron from interlayered garnet-sillimanite-rutile metapelite (M1b-stage), yielded an isochron age of 1675±7 Ma with Ndi of 0.50997±0.00003 (2σ, MSWD = 1.9 n = 3) Monazite inclusions within garnet porphyroblasts in this metapelite have a weighted mean age of 1703±28 Ma (2σ, MSWD = 0.95, n = 8), within error of the age of monazite inclusions within matrix biotite at 1665±16 Ma (2σ, MSWD = 0.56, n = 17). Post M1b stage lower temperature hydration-retrogression events likely occurred by late Mesoproterozoic time. The metamorphism followed a clockwise P-T path (high-pressure amphibolite-facies stage M1a P ≥ 12 kbar at ∼750 °C, followed by M1b granulite-facies stage with T ≥ 850 °C at lower pressures), interpreted as a response to crustal thickening during Paleoproterozoic continental collision. We propose that the Bhopalpatnam orogen is the exhumed locus of the leading edge of the Archean Bastar craton that was thrust under the EDC craton.

Extension Related Magmatism and Basin Evolution in Northwestern Part of the Cuddapah Rift, India

Spatial distribution and field relationship of igneous intrusives viz. basic dykes, pink granite-leucogranite-tourmaline pegmatites and quartz reefs across interface between the Neoarchean Gadwal schist belt (Basement) and the Proterozoic Cuddapah basin have been documented. The Cuddapah basin is a half graben (rift) developed at right angle to the basement and displays two sets of faults: (1) NE-SW longitudinal and (2) NW-SE transverse. The longitudinal faults oriented parallel to basin margin have controlled sedimentation, subsidence and uplift. The transverse faults extending from rift shoulder (basement) right into central part of the basin are post depositional faults and imparted en-echelon pattern to the basement-basin interface. Sedimentary facies, depositional environments and basin setting indicate that the basin fill was deposited in a rift setting over the northwestern part of the epeiric sea embayment that flooded on to the Dharwar batholith of the Eastern Dharwar craton. Basin evolution stages have been outlined. Basin uplift/inversion is controlled by reverse faults and is viewed as a cause and effect to the post Cuddapah extension related magmatism which is attributed to magma underplating in the environs of the northwestern part of the Cuddapah rift at crustal levels.

Neoarchaean magmatism through arc and lithosphere melting: Evidence from eastern Dharwar Craton

The Neoarchaean era is characterized by rapid crustal growth corresponding to some fundamental global changes in geodynamic processes. However, the nature of crustal growth including the mechanism and tectonic setting of the Neoarchaean are controversial issues. The eastern Dharwar Craton (EDC) exposes widespread Neoarchaean granite‐greenstone belts, which provide an opportunity to evaluate the various models proposed for Neoarchaean crustal growth. In this study, we present field, petrographic, and geochemical data and discuss the petrogenesis and significance for crustal evolution for a suite of previously undescribed banded gneisses, TTG (tonalite–trondhjemite–granodiorite), biotite granites, alkali feldspar granite and gabbro. These rocks are associated with Neoarchaean metavolcanic and metapelites rocks of the Tsundupalle greenstone belt, in the eastern fringe of the EDC. Whole‐rock major and trace element geochemical data are consistent with diverse sources, including both crust and enriched mantle in an evolving subduction zone. A convergent orogenic setting is proposed for interpreting the association of various granitoids in the Tsundupalle area. Finally, intrusion of crustally derived, highly silicic, alkali‐rich granite, and mantle‐derived gabbro emplaced in a post‐subduction regime is proposed. Lithospheric delamination and attendant mantle melting are suggested as possible mechanisms for generation of these rocks. The understanding of generation of the different granitoid types along with gabbro provides significant insights into the mechanism of Neoarchaean crustal growth.

Proterozoic Magnesian Feldspathic Magmatism in the Nallamalai Fold Belt, Southern India – Petrogenesis of Vellaturu-Ipuru-Nakerikallu (VIN) Arc Magmatic Granites, Eastern Dharwar Craton

ABSTRACT Through extensive field studies substantiated by detailed petrographic, geochemical and mineral chemistry studies, a significant event of “Proterozoic magnesian feldspathic magmatism” is reported from the SE part of the Indian subcontinent. Three major granitoids i.e. Vellaturu, Ipuru and Nakerikallu (VIN) are emplaced along an NNE-SSW axis in the Proterozoic Nallamalai fold belt (NFB) in the Eastern Dharwar craton (EDC). The axis of emplacement of VIN granites represents a tectonic contact that is juxtaposed along the NE fringes of NFB and adjoining Precambrian accretionary terrain comprising of gneisses and amphibolites with intrusive granites in the northern part of the Nellore schist belt (NSB). Petrographically the VIN granitoids are biotite ± hornblende bearing with apatite and zircon as accessory phases. Exsolution texture is observed in the form of microcline microperthite intergrowth. In IUGS, QAP diagram the VIN granitoids essentially fall in granite - granodiorite field. Presence of mafic rich microgranitoid enclaves (MMEs) are prominently noticed in the VIN granitoids indicating magma mingling. These granitoids are deformed along their margins. Enclaves of quartzite of NFB and older amphibolites indicate the intrusive nature of VIN granitoids. Major oxide analyses indicate that the Mg# ranges from 43 to 56. Rare earth element studies reveal a general enrichment of LREE relative to HREE. Incompatible HFSE element studies indicate a significant Nb and Ti negative anomaly; characteristic of subduction zone magmatism. In Y + Nb vs Rb trace element tectonic discrimination diagram the granitic rocks of Nakerikallu, Ipuru and Vellaturu essentially fall in post-collisional granites (post-COLG) field. Based on the remarkable magnesian feldspathic character coupled with their emplacement in an arc magmatic tectonic setting, it is concluded that the VIN granitoids in the Proterozoic NFB, SE India; mark a significant event of “Proterozoic magnesian feldspathic magmatism” in the EDC.

Geochemistry, petrogenesis, and geodynamic implications of NE–SW to ENE–WSW trending Palaeoproterozoic mafic dyke swarms from southern region of the western Dharwar Craton

A number of NE–SW to ENE–WSW trending Palaeoproterozoic mafic dykes, intruded within the Archean basement rocks and more conspicuous in the southern parts of the western Dharwar Craton (WDC), was studied for their whole‐rock geochemistry to understand their petrogenetic and geodynamic aspects. Observed mineralogical and textural characteristics classify them either as meta‐dolerites or dolerites/olivine‐dolerites. They show basaltic to basaltic–andesitic compositions and bear sub‐alkaline tholeiitic nature. Three geochemically distinct groups of mafic dykes have been identified. Group 1 samples show flat REE patterns (LaN/LuN = ~1), whereas the other two groups have LaN/LuN = ~2–3 (Group 2; enriched LREE and flat HREE patterns) and LaN/LuN = ~4 (Group 3; inclined REE patterns). Chemistry is not straightforward to support any significant role of crustal contamination and probably reflect their source characteristics. However, their derivation from melts originated from a previously modified metasomatized lithospheric mantle due to some ancient subduction event cannot be ignored. Most likely different mantle melts were responsible for derivation of these distinct sets of mafic dykes. The Group 2 dykes are derived from a melt generated within spinel stability field by ~10% batch melting of a lithospheric mantle source, whereas the Group 3 dykes have their derivation from a melt originated within the spinel–garnet transition zone and were fed from slightly higher (~12–15%) batch melting of a similar source. The Group 1 samples were also crystallized from a melt generated at the transition zone of spinel–garnet stability field by higher degrees (~20%) of melting of a primitive mantle source. Geochemistry of the studied samples is typical of Palaeoproterozoic mafic dykes emplaced within the intracratonic setting, reported elsewhere globally as well as neighbouring cratons. Geochemistry of the studied mafic dyke samples is also compared with the mafic dykes of the eastern Dharwar Craton (EDC). Except the Group 3 samples, which have good correlation with the 1.88–1.89 Ga Hampi swarm, no other group shows similarity with the EDC mafic dykes. There is an ample possibility to have some different mafic magmatic events in the WDC, which could be different from the EDC. However, it can only be confirmed after precise age determinations.

Magma mixing in Neoarchean granite from Nalgonda region, Eastern Dharwar Craton, India: Morphological, mineralogical and geochemical evidences

We present field, petrographic, mineralogical and whole-rock geochemical data for part of the Neoarchean granite and associated microgranular enclaves (MEs) occur in the Nalgonda region, NE part of Eastern Dharwar Craton (EDC), and demonstrate the end-member magma mixing processes in the petrogenesis of the host granite. Extensive occurrence of ME and uni-directional flow band structures (N–S trend) in all the studied outcrops exposed over about 20 km depict that intensive magma mixing–mingling occurred at magma chamber scale. MEs are the portions of intermediate mafic magma that had interacted at two stages with acidic host granite. Mixing at the initial stage promoted efficient thorough mixing which resulted in rapakivi texture and mesocratic enclaves. Wispy filament structures around these enclaves indicate that mafic magma globules were mechanically diluted in the host granite magma by chaotic advection. These filamental magmas were further linearly diluted along with convection-related flow of the host magma. Smaller mafic globules also got linearly stretched along with this flow. Upon solidification of the host magma, these wispy mafic filaments were preserved as flow structures. The second stage of mafic magma mixing did not promote thorough mixing due to the large viscosity contrast with the host granite magma and preserved their physical entity. These enclaves are melanocratic with sharp boundary and devoid of gradational contact and filament structures around it. They characteristically show network of granitic vein injection which often show crenulation folding. These evidences indicate that during the second-stage interaction, the enclave magma was rigid to plastic nature with respect to the host granite magma. Coherent linear Harker variation trend of CaO, MgO, \(\hbox {TiO}_{2}\), \(\hbox {K}_{2}\hbox {O/Na}_{2}\)O, V, Y and Sc indicates that mixing has promoted to develop chemical gradient between ME and host granite magmas. Mechanical dilution by chaotic advection must have enhanced the chemical diffusion of both magmas. Smooth decreasing Harker trend of Y content in both the magmas indicates that theyhad undergone certain degree of fractional crystallisation. In contrast, elements like Rb, Ba and Sr with large value of diffusion coefficient (D) show scattered behaviour in element–element plots suggesting that diffusive fractionation was active during chaotic advection mixing. It is concluded that heterogeneity in compositional variation of Nalgonda granite can be attributed to difference in degree of mechanical dilution of mafic magma, fractional crystallisation and diffusive fractionation. The geochemical evidences indicate the role of subduction in the evolution of these rocks.

Complex upper mantle deformation beneath the Dharwar craton inferred from high density splitting measurements: Distinct lateral variation from west to east

Upper mantle anisotropy investigated using 172 core-refracted (SKS, SKKS) seismic phases along a ~660 km long profile at 10 to 20 km intervals from the west to the east coast of South India reveals significant lateral variations in its magnitude and direction. This profile, with 38 broadband seismic stations, covers mid-Archean Western Dharwar craton (WDC), late-Archean Eastern Dharwar Craton (EDC), Proterozoic Cuddapah Basin (CB) and the passive continental margins along the west and east coast. The observed fast polarization directions (FPDs) show lateral variability: NW50o to NW5o beneath the WDC, NW40o to NE30o beneath the EDC and N5o to N85o beneath the CB and further east. The delay time varies between 0.4 and 2.0 s with an average of 1 s. However, we are unable to fit a two layers anisotropy model for the region due to sparse azimuth coverage. Beneath the WDC, the direction of the fast axis follows trends of shear zones and faults, suggesting “frozen-in” anisotropy in the lithosphere, possibly established during the lithospheric evolution in mid-late Archean. In the EDC, the fast axis does not only follow the plate motion direction but it deviates, manifesting late Archean to Proterozoic deformation may still be present as fossil mantle anisotropy. The splitting trend beneath the CB and Eastern Ghat (EG) follows the strike of the rift along with plate motion direction, indicating anisotropy is influenced by the combination of “frozen” anisotropy due to continental rifting along the eastern margin of Indian plate and active asthenospheric flow.

Coupled Assimilation and Fractional Crystallization (AFC) and Mantle Plume Source(s) Contribution in the Generation of Paleoproterozoic Mafic Dykes of the Eastern Dharwar Craton, Southern India

Abstract The eastern Dharwar craton (EDC) of the southern Indian Shield hosts five geochronologically distinct Paleoproterozoic mafic dyke swarms emplaced at 2.37, 2.21, 2.18, 2.08 and 1.89 Ga. Trace element geochemical data available for these dykes display the ‘arc signals’ viz., negative Nb-Ta anomalies and elevated Zr/Nb, Th/Yb and Th/Ta values, which are conventionally interpreted to represent involvement of subduction in their genesis. It is shown that these ‘arc signals’ resulted from coupled assimilation and fractional crystallization (AFC) processes that modified these mantle-derived melts. Since, mafic dykes under study are highly evolved, an attempt has been made to estimate (using PRIMELTS2.xls software) the composition of the primary magma from the most primitive sample available from the 2.21 and 2.37 Ga swarms. The mantle potential temperature derived from the estimated primary magma compositions revealed anomalously hot mantle source regions compared to the known ambient upper mantle temperatures during Paleoproterozoic, thus implying the possible involvement of thermal plumes in their genesis.