Petrology and geochemistry (including Sr and Nd isotopes) of two lamprophyre dykes, intruding the Archaean granitic gneisses at Sivarampeta in the diamondiferous Wajrakarur kimberlite field (WKF), eastern Dharwar craton, southern India, are presented. The Sivarampeta lamprophyres display porphyritic–panidiomorphic texture comprising macrocrysts/phenocrysts of olivine, clinopyroxene (augite), and mica set in a groundmass dominated by feldspar and comprising minor amounts of ilmenite, chlorite, carbonates, epidote, and sulphides. Amphibole (actinolite–tremolite) is essentially secondary in nature and derived from the alteration of clinopyroxene. Mica is compositionally biotite and occurs as a scattered phase throughout. Mineralogy suggests that these lamprophyres belong to calc–alkaline variety whereas their bulk–rock geochemistry portrays mixed signals of both alkaline as well as calc–alkaline (shoshonitic) variety of lamprophyres and suggest their derivation from the recently identified Domain II (orogenic–anorogenic transitional source type mantle source) from eastern Dharwar craton. Trace element ratios imply melt–derivation from an essentially the garnet bearing–enriched lithospheric mantle source region; this is further supported by their 87Sr/86Srinitial (0.708213 and 0.708507) and ‘enriched’ εNdinitial (−19.1 and −24.2) values. The calculated TDM ages (2.7–2.9 Ga) implies that such enrichment occurred prior to or during Neoarchean, contrary to that of the co–spatial and co–eval kimberlites which originated from an isotopically depleted mantle source which was metasomatized during Mesoproterozoic. The close association of calc–alkaline shoshonitic lamprophyres, sampling distinct mantle sources, viz., Domain I (e.g., Udiripikonda) and Domain II (Sivarampeta), and kimberlites in the WKF provide further evidence highly for heterogeneous nature of the sub–continental lithospheric mantle beneath the eastern Dharwar craton.
Serpentine mineralogy controls fault rheology in the ocean and continental rift settings to subduction settings and hence can be used to discern the paleo deformational conditions. The Rakhabdev lineament from Rajasthan, India, provides a unique opportunity to understand its tectonic evolution inferred from the deformation microstructures. However, the complexity of surrounding calc–silicate rocks had resulted in a long–driven debate on the origins of these serpentinite rocks. The source rocks of the serpentinites also cannot be determined previously due to complete serpentinization and metasomatism rendering complete alteration of the source rocks. In this study, the serpentinite mineral was analyzed using Raman spectroscopy to accurately characterize its molecular structure. The presence of the antigorite–variety of serpentine mineral indicate towards the origin of Rakhabdev serpentinites in the upper mantle condition. The antigorite serpentinite of Rakhabdev is a hydration product of mantle materials showing high Mg# values obtained from EPMA data. The microstructural and EBSD analysis also indicates two stages of deformation, with deformation of antigorite at upper mantle conditions, followed by their shallow crustal carbonate metasomatism and subsequent deformation of the carbonates, with later stage calcite vein intrusion. This resulted in the appearance of antigorite in contact with calcite, dolomite, talc, tremolite, and chlorite. The exhumation of mantle wedge antigorite serpentinite is, therefore, indicating a paleo–subduction zone culminating in a crustal–scale collision boundary expressed as arcuate discontinuous bodies forming the Rakhabdev lineament.
Detailed structural mapping in the southern part of Chitradurga Schist Belt (3.0–2.5 Ga) (CSB) distributed around the Chikkanayakanahalli–Kibbanahalli area was carried out. Sargur Group, Basement Gneiss, Bababudan Group, Chitradurga Group, and Hiriyur Group of rocks are well preserved in the investigated area. Unconformable relation between Basement Gneiss–Sargur Group and Bababudan Group is defined by oligomict conglomerate with quartzite clast and occasionally preserve granite clast. A polymictic conglomerate separates Bababudan and Chitradurga Groups; similarly, Chitradurga and Hiriyur Groups are also separated by a polymictic conglomerate. A new zone, Akkanahalli Zone, in the eastern margin of the study area is proposed which is belonging to Sargur Group. Zircon grains in the metatuff sample from this zone provide an age of 3313 ± 6 Ma. Six stages of deformation events are recognized in the study area. General trend and megascopic structures in the mapped area have resulted from the earlier two stages of deformations (D2 and D3). The D2 stage structure is distinctly characterized by a fold–and–thrust belt consisting of a NNW–SSE trending fold zone sandwiched between a pair of NNW–SSE trending thrust faults dipping east. Deformation during the D3 stage resulted in regional–scale sinistral shear zones, such as N–S striking Gadag–Mandya Shear Zone, and narrow N–S and NW trending sinistral ‘echelon’ shear zones. Based on our structural and field relationship it is proposed that CSB developed in an immature or failed rift setting where shallow marine sequence and shelf deposits are predominant. Sediments and volcanic rocks were unconformably deposited horizontally above Basement Gneiss and later got deformed together in a sinistral transpression setting.
Zirconium in rutile thermometry data from the garnet granulites of the Jijal Complex of Kohistan arc, NW Himalaya are presented in this study. The garnet granulites are composed of garnet, clinopyroxene, plagioclase, quartz, symplectic augite/amphibole, rutile, ilmenite, zircon, and magnetite. Rutile grains range in size from 50 to 350 µm, occur as inclusion in garnet, clinopyroxene, and in plagioclase as well as along the grain boundaries. In total 19 rutile grains were analyzed for Zr contents using an X–ray Analytical Microscope (XGT–5000) by HORIBA. The Zr contents among the analyzed grains ranged between 450 and 920 ppm, where the analyzed spots with lower Zr contents (containing SiO2 or Fe2O3), indicating some influence of host silicate or ilmenite, were removed from results. At the individual grain scale, most of the rutile grains exhibited homogeneous chemical compositions, regardless of their textural affinity. Temperature values, based on zirconium in rutile thermometry, ranged between 792 and 849 °C for rutile enclosed in garnet, 771 and 851 °C for rutile in clinopyroxene, and 784 and 862 °C for rutile in plagioclase whereas matrix rutile grains showed T values between 820 and 847 °C. Using the pressure–dependent zirconium in rutile thermometry, the T values were slightly lower (±50 to 100 °C). The maximum temperature values were consistent with the temperature data obtained from the conventional thermobarometry results (P; 1.2 ± 0.2 GPa and T; 818 ± 80 °C) whereas the lower values, likely, reflect chemical resetting of the analyzed grains during later stages of retrogression.
The central Indian shield area is characterized by earthquakes with dominantly oblique–reverse movements on pre–existing faults along paleo–rift zones. These earthquakes, occurring hundreds of kilometers away from the plate boundary, are considered as Stable Continental Region (SCR) earthquakes. In this contribution, we have attempted to analyze the nature of intraplate earthquakes of the central Indian shield with special reference to the Son–Narmada–Tapti (SONATA) zone which is known as a paleo–rift with Precambrian ancestry and a proven geological history of repeated tectonic rejuvenation throughout the Phanerozoic. The cause of the neotectonic fault movements, the paleo–seismic records and the seismic history of the SONATA zone are discussed in detail. Geological, geophysical and heat flow measurement data suggest stress concentration in the SONATA zone in response to the far–field plate boundary forces arising out of the Indo–Tibetan plate collision in the north (Himalayas), which causes intraplate earthquakes in this tectonically stable region. A review of the present state of knowledge on the tectonically active areas in the central Indian cratonic area is presented here. We have also attempted to give an appraisal of the tectonic reactivation of pre–existing rift–related faults under the neotectonic stress field, using new data from our analogue experimental work.
After a brief review on tectonic settings of the Himalayas and Tibetan Plateau, we survey the literature on characteristics of earthquakes occurring in Tibetan Plateau and the surrounding regions. Shallow events (focal depths <50 km) show remarkable correlation with surface fault systems in both spatial locations and focal mechanisms. Some shallow events appear to be triggered by remote earthquakes and seasonal variations of ground water storage, suggesting stress levels nearing critical threshold. Intermediate–depth earthquakes (IDEQs; i.e., earthquakes with focal depths between 50 and 300 km) are concentrated beneath southern Tibet, the Hindu Kush–Pamir region, and the Burmese subductions zones. Underneath southern Tibet, the subducted Indian plate extends northward to at least the Bangong–Nujiang suture zone, with IDEQs occurring in the lower crust and the adjacent upper mantle. Possible mechanisms for IDEQs are also reviewed.
The disposition of the Indian Gondwana basins along some linear zones coinciding with the boundaries of the Precambrian protocontinental components of the peninsular shield mosaic primarily indicates a major tectonic control on intracratonic rifting. These rift systems along with the late Paleozoic fault systems developed within the reconstructed Gondwanaland define broad concentric patterns with some radial strike–slip varieties about ‘centers’ tentatively located along the south–eastern margin of Africa. The reconstructed directions of ice movement of the late Paleozoic ice–sheet also define broad radial patterns about ‘centers’ located roughly between Madagascar and India, and western part of Antarctica. The radial pattern of ice movement and development of concentric fault zones can be attributed to some regional domal uplift, due to penetration of plume heads into the upper mantle. These ‘centers’ were located very close to the linear zone along which a rift system developed during Sakmarian (~ 280 Ma) and the Gondwana fragmentation was initiated with the separation of East and West Gondwana at around 170 Ma. The geochemical signatures indicate that this plume was possibly responsible for the earlier phases of emplacement of the potassium–rich lamproites within the Gondwana basin–fill succession and was finally led to the emplacement of Panjal volcanics. Therefore, it appears that the process of Gondwanaland break–up was initiated with the formation of the Gondwana rift system in India and its equivalents in the rest of the supercontinent. Acquisition of systematic geochemical and geochronological data from the intrusive bodies from the Gondwana basins can help in tracing back the complete history.
Trace element and rare earth element (REE) composition of iron formation and carbonate rocks from the Morar Formation, Gwalior Group, central India provides valuable information on the redox condition of late Paleoproterozoic Ocean. Facies types of iron formation suggest deposition in various oceanic environments ranging from shoreface–beach to subtidal shelf settings, whereas carbonates belong to shallow and deep subtidal settings. La/Nd values between 0.57 and 25, MREE enrichment and small negative (0.69) to positive (1.46) Ce anomaly in iron formation suggest a stratified character for the Gwalior Sea with development of shallow transitional redoxcline. Whereas deep sea is interpreted as near anoxic and ferruginous, the shallow sea was not very high in dissolved oxygen (DO2) either. A suboxic to mild oxic shallow sea condition (DO2 ≥ 0.2 µM) is interpreted allowing Mn (II) oxidation and Ce sequestration. Carbonates, however, do not register any geochemical signature of redoxcline possibly because of the depositional setting either close to or below the redoxcline.
The genesis of CLIPPIR diamonds (Cullinan–like, large, inclusion–poor, pure, irregular, and resorbed) have attracted much interest due to their possible crystallization from metal melt in deep horizons of the earth’s mantle. These diamonds usually show a pronounced resorption and irregular morphology. The present paper reports new experimental data on the dissolution of diamond crystals at high P–T parameters in Fe–S melt containing large amounts of silicate components (5–20 wt%). The experiments were performed using a split–sphere multi–anvil apparatus (BARS) at a pressure of 4 GPa and a temperature of 1450 °C. The samples consisted of natural diamond crystals placed in mixtures of Fe, S, and kimberlite. Wide variations in dissolution rates of diamond crystals were obtained. The absence of diamond dissolution in a heterogeneous medium indicates that the amount of solid silicate phases present in metal melt plays a role in the preservation of diamonds. This study demonstrated how diamonds can be stored in natural environments due to the heterogeneity of the medium composition which could insulate diamonds from the metal–sulphide melt. The obtained results improve our understanding of processes that lead to preservation of CLIPPIR diamonds in the deep mantle.
The Eastern Ghats Belt of India remains a focus of intense research by the international geoscience community to understand the Proterozoic crustal evolution. This terrane experienced anomalous thermal perturbation at least two times in Earth’s history and those are linked with the assembly of two Proterozoic supercontinents. It is clearly demonstrated from existing rock records that the terrane experienced multiple events of metamorphism, magmatism, and deformation characterizing separate crustal provinces. Despite this, a clear gap in understanding the evolution of this terrane particularly on its northern and northwestern part still exists. The present review presents an update of these issues which can be taken into consideration for future research of this complex terrane.
The Singhbhum Craton (SC) of eastern India grew and evolved throughout the Precambrian era with a spectacular yet cryptic record of the early Earth processes. An extensive study on geochemistry, metamorphism, deformation, geochronology, and sedimentation of the rocks of the craton produced a large database which is described and synthesized here to build up a deformation and metamorphic history of the craton right from the Eoarchean time. Altogether seven orogenic episodes have been identified from the SC and its margins spanning from Paleoarchean to end–Neoproterozoic. Of these, the earlier two Paleo–Mesoarchean orogenic episodes (~ 3.3 and ~ 3.1 Ga) are confined within the cratonic core and were instrumental in building up of the framework of the Archean nucleus in SC. The Neoarchean event (~ 2.8 Ga) is marked by thrusting of the granulite–grade lower crust of the SC, the Rengali Province (RP), along its southern margin. Later three Proterozoic orogenic events (~ 1.8, ~ 1.6–1.5, and ~ 1.0 Ga) left imprints along the northern margin of the SC. Among these, the Grenvillian (~ 1.0 Ga) event was most pervasive, which remobilized the northern fringe of the Singhbhum Granite massif developing thick–skinned thrust belt–like structures within the narrow northern belt. Active tectonics later shifted again to the southern margin with the last orogenic event (~ 0.5 Ga) being marked within the RP resulting from oblique docking of the Eastern Ghats Belt (EGB) against the RP–SC. Based on this long and winding history, we also discussed the possible tectonic scenarios that eventually shaped the present configuration of the craton.
This study presents a review of the wide spectrum of biotic signatures within the Precambrian Vindhyan Supergroup deposited during the ‘boring billion’ and assesses their biological affinity and age implications. The sedimentation took place in wide–ranging palaeo–environments from fluvial to offshore through shallow marine. While the lower part of the ~ 4500 m thick Vindhyan succession is older than 1650 Ma, the age at its top part is poorly constrained, ranging from 1000 to 650 Ma. Microbial records are abundant in the form of stromatolites in limestone and microbially induced sedimentary structures (MISS) on both siliciclastics and carbonates across the Vindhyan succession. The wide morphological variation of these two features corresponds to depositional processes, early cementation, as well as lithological variations. The stromatolite record, as well as calcified and chertified microbial fossils, attest to the Mesoproterozoic to Neoproterozoic age of the sediments. Although the carbonaceous body fossils do not have age implications, they indicate the proliferation of algal life during the Meso– to Neoproterozoic time. The Ediacaran–like fossils mostly relate either to ‘discoidal microbial colony’ or detached pieces of microbial mat. Wide–ranging putative metazoan fossil reports remain the focal point of attention for many years. Although most of these reports are found to be microbially originated, some of these features have the potential to highlight the evolution of multicellular life during the Precambrian.