Paleoproterozoic mafic dyke swarms of the Belomorian Province,eastern Fennoscandian Shield

Paleoproterozoic mafic igneous rocks (2450–1970 Ma) are exposed in the form of layered intrusions, dykes, and volcanic rocks in the Karelian, Kola and Murmansk provinces and in the form of dykes and small intrusions in the Belomorian Province, Eastern Fennoscandian Shield. The age and sequence of mafic dyke emplacement during the Paleoproterozoic are very similar in these regions. Further comparisons of geochemical characteristics of mafic dyke swarms in the Belomorian Province and neighboring cratons show considerable similarities.     

1891–1883 Ma Southern Bastar–Cuddapah mafic igneous events, India: A newly recognized large igneous province

A newly recognized remnant of a Paleoproterozoic Large Igneous Province has been identified in the southern Bastar craton and nearby Cuddapah basin from the adjacent Dharwar craton, India. High precision U–Pb dates of 1891.1 ± 0.9 Ma (baddeleyite) and 1883.0 ± 1.4 Ma (baddeleyite and zircon) for two SE-trending mafic dykes from the BD2 dyke swarm, southern Bastar craton, and 1885.4 ± 3.1 Ma (baddeleyite) for a mafic sill from the Cuddapah basin, indicate the existence of 1891–1883 Ma mafic magmatism that spans an area of at least 90,000 km² in the south Indian shield.This record of 1.9 Ga mafic/ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south-eastern margin of the south Indian shield is a widespread geologic phenomenon on Earth. Similar periods of intraplate mafic/ultramafic magmatism occur along the margin of the Superior craton in North America (1.88 Ga Molson large igneous province) and in southern Africa along the northern margin of the Kaapvaal craton (1.88–1.87 Ga dolerite sills intruding the Waterberg Group). Existing paleomagnetic data for the Molson and Waterberg 1.88 Ga large igneous provinces indicate that the Superior and Kalahari cratons were at similar paleolatitudes at 1.88 Ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a Limpopo-like 2.0 Ga deformation zone in the Superior Province. The widespread occurrence of 1.88 Ga intraplate and plate margin mafic magmatism and basin development in numerous Archean cratons worldwide likely reflects a period of global-scale mantle upwelling or enhanced mantle plume activity at this time.     

Precambrian mafic magmatism in the Bastar craton,central India

The Bastar craton has experienced many episodes of mafic magmatism during the Precambrian. This is evidenced from a variety of Precambrian mafic rocks exposed in all parts of the Bastar craton in the form of volcanics and dykes. They include (i) three distinct mafic dyke swarms and a variety of mafic volcanic rocks of Precambrian age in the southern Bastar region; two sets of mafic dyke swarms are sub-alkaline tholeiitic in nature, whereas the third dyke swarm is high-Si, low-Ti and high-Mg in nature and documented as boninite-norite mafic rocks, (ii) mafic dykes of varying composition exposed in Bhanupratappur-Keskal area having dominantly high-Mg and high-Fe quartz tholeiitic compositions and rarely olivine and nepheline normative nature, (iii) four suites of Paleoproterozoic mafic dykes are recognized in and around the Chattisgarh basin comprising metadolerite, metagabbro, and metapyroxenite, Neoarchaean amphibolite dykes, Neoproterozoic younger fine-grained dolerite dykes, and Early Precambrian boninite dykes, and (iv) Dongargarh mafic volcanics, which are classified into three groups, viz. early Pitepani mafic volcanic rocks, later Sitagota and Mangikhuta mafic volcanics, and Pitepani siliceous high-magnesium basalts (SHMB). Available petrological and geochemical data on these distinct mafic rocks of the Bastar craton are summarized in this paper. Recently high precision U-Pb dates of 1891.1±0.9 Ma and 1883.0±1.4 Ma for two SE-trending mafic dykes from the BD2 (subalkaline) dyke swarm, from the southern Bastar craton have been reported. But more precise radiometric age determinations for a number of litho-units are required to establish discrete mafic magmatic episodes experienced by the craton. It is also important to note that very close geochemical similarity exist between boninite-norite suite exposed in the Bastar craton and many parts of the world. Spatial and temporal correlation suggests that such magmatism occurred globally during the Neoarchaean-Paleoproterozoic boundary. Many Archaean terrains were united as a supercontinent as Expanded Ur and Arctica at that time, and its rifting gave rise to numerous mafic dyke swarms, including boninitenorite, world-wide.     

A petrogenetic relationship between 2.37 Ga boninitic dyke swarms of the Indian Shield: Evidence from the Central Bastar Craton and the NE Dharwar Craton

The Indian Shield is cross-cut by a number of distinct Paleoproterozoic mafic dyke swarms. The density of dykes in the Dharwar and Bastar Cratons is amongst the highest on Earth. Globally, boninitic dyke swarms are rare compared to tholeiitic dyke swarms and yet they are common within the Southern Indian Shield. Geochronology and geochemistry are used to constrain the petrogenesis and relationship of the boninitic dykes (SiO₂ = 51.5 to 55.7 wt%, MgO = 5.8 to 18.7 wt%, and TiO₂ = 0.30 wt% to 0.77 wt%) from the central Bastar Craton (Bhanupratappur) and the NE Dharwar Craton (Karimnagar). A single U-Pb baddeleyite age from a boninitic dyke near Bhanupratappur yielded a weighted-mean ²⁰⁷Pb/²⁰⁶Pb age of 2365.6 ± 0.9 Ma that is within error of boninitic dykes from the Dharwar Craton near Karimnagar (2368.5 ± 2.6 Ma) and farther south near Bangalore (2365.4 ± 1.0 Ma to 2368.6 ± 1.3 Ma). Rhyolite-MELTS modeling indicates that fractional crystallization is the likely cause of major element variability of the boninitic dykes from Bhanupratappur whereas trace element modeling indicates that the primary melt may be derived from a pyroxenite mantle source near the spinel-garnet transition zone. The Nd isotopes (ε_Nd(t) = −6.4 to +4.5) of the Bhanupratappur dykes are more variable than the Karimnagar dykes (ε_Nd(t) = −0.7 to +0.6) but they overlap. The variability of Sr-Nd isotopes may be related to crustal contamination during emplacement or is indicative of an isotopically heterogeneous mantle source. The chemical and temporal similarities of the Bhanupratappur dykes with the dykes of the Dharwar Craton (Karimnagar, Penukonda, Chennekottapalle) indicate they are members of the same giant radiating dyke swarm. Moreover, our results suggest that the Bastar and Dharwar Cratons were adjacent but likely had a different configuration at 2.37 Ga than the present day. It is possible that the 2.37Ga dyke swarm was related to a mantle plume that assisted in the break-up of an unknown or poorly constrained supercontinent.     

Qualitative analysis of mafic dyke swarms and kimberlites from morphological and geophysical signatures,NW of Proterozoic Cuddapah basin,Eastern Dharwar craton

Widespread distribution of mafic dykes and scanty occurrence of ultrabasic intrusives of kimberlitic affinity around Proterozoic Cuddapah basin, parts of Eastern Dharwar craton of south India has been the focus of attention since their discovery, to understand the structural fabric in relation to their emplacement in geological time. Satellite Imagery, geomorphological, geophysical and radiometric age data of Narayanpet area, northwest of Cuddappah basin, have clearly displayed the alignments and structures of geological significance, such as deep seated fault / fracture / shear zones, stratigraphic / lithological contacts, basic / ultrabasic intrusives and younger granites etc,. Based on the field observations such as emplacement of mafic dykes, their cross cutting relationship, study of morphological and geophysical signatures, inferred linears drawn from satellite imagery, aeromagnetic and gravity maps are arranged in a chronological order. A system of long, narrow and widely spaced dykes trending NW-SE direction conformable to gneissic foliation, typically associated with migmatites in the southwestern part of the study area are the oldest. Followed by E-W dykes, cut across by the sparsely distributed dykes associated with NW-SE and N-S features and in turn off set by dykes of NE-SW trends are the youngest. Kimberlites of Narayanpet area, belongs to hypabysal facies, which are essentially controlled by E-W to ENE-WSW deep seated fault / fracture zone, their intersection with NW-SE, NE-SW to N-S trends, which may have been reactivated during Proterozoic period as indicated by the intrusion of mafic dykes (～2270 to 1701 Ma) and emplacement of kimberlitic magmatism (～1300 to 1100 Ma) suggesting different intrusive episodes. Kimberlite pipes of Narayanpet field, falls in an ellipsoid form trending WNW-ESE direction in the northern part of the area, associated with radial drainage / topographic high and a gravity low. In addition, physical properties such as density and magnetic susceptibilities of mafic dykes and kimberlites, their geophysical signatures, emplacement of kimberlites at the close vicinity of mafic dykes or at their intersections have also been discussed.     

Geochemical studies and petrogenesis of ~2.21–2.22 Ga Kunigal mafic dyke swarm (trending N-S to NNW-SSE) from eastern Dharwar craton,India: implications for Paleoproterozoic large igneous provinces and supercraton superia

The Archean eastern Dharwar craton is transacted by at least four major Proterozoic mafic dyke swarms. We present geochemical data for the ~2.21–2.22 Ga N-S to NNW-SSE trending Kunigal mafic dyke swarm of the eastern Dharwar craton to address its petrogenesis and formation of large igneous province as well as spatial link to supercontinent history. It has a strike span of about 200 km; one dyke of this swarm runs ~300 km along the western margin of the Closepet granite. Texture and mineral compositions classify them as dolerite and olivine dolerite. They show compositions of high-iron tholeiites, high-magnesian tholeiites or picrites. Geochemical characteristics of the sampled dykes suggest their co-genetic nature and show variation from primitive (Mg#; as high as ~76) to evolved (differentiated) nature. Although geochemical characteristics indicate possibility of minor crustal contamination, they show their derivation from an uncontaminated mantle melt. These mafic dykes are probably evolved from a sub-alkaline basaltic magma generated by ~20 % batch melting of a depleted lherzolite mantle source and about 15–30 % olivine fractionation. Paleoproterozoic (~2.21–2.22 Ga) mafic magmatism is recognized globally as dyke swarms or gabbroic sill complexes in the Superior, Slave, North Atlantic, Fennoscandian and Pilbara cratons. Possible Paleoproterozoic Dharwar–Superior–North-Atlantic–Slave correlations are constrained with implications for the configuration of supercraton Superia.     

Analyses of Aeromagnetic Data to Delineate Basement Structures and Reveal Buried Igneous Bodies in Kaladgi Basin,Karnataka

Aeromagnetic data overcome constrain of inadequate exposures and provide signatures of bodies beneath sediment cover. Present work on analysis of aeromagnetic data over western part of Kaladgi basin provided insight into the basement structures and their role in basin evolution. In the study area, the NW-SE and NE-SW are the major trends of magnetic lineament followed by E-W and N-S trends.Archean to Paleoproterozoic basement is manifested by two structural zones, NW-SE trends related to major lineaments within the basement and the NE-SW trends presumed intra-basinal fault systems which controlled the local depressions. The basin configuration deduced from depth to basement show that the Kaladgi basin is an open deep basin and divided into several sub-basins, separated by fault-controlled NE-SW and NW-SE oriented basement ridges. An intriguing find in the western part are the numerous scattered smaller-scale, circular or semicircular, distinct magnetic anomalies of moderate to strong magnetic signal with strong remenance. Analyses coupled with 3D inversions in combination with sub-surface probing reveal in-homogeneities within basement gneisses and supracrustal rocks of the Kaladgi basin, Dharwar craton. 3D inversions of these circular bodies, suggest that they are apophyses of the intrusions or alternatively as younger intrusive stocks. Sub-surface probing by boreholes over circular bodies revealed leucocratic granite with porphyritic texture emplaced as intrusive within the Chitradurga metasediments. This implies that these intrusives are post-Chitradurga schist and pre Badami sediments as they have not affected the latter. However, they can be presumed to be coeval to potassic granites, which intrude the eastern part of the western Dharwar craton in southern India, until geochronological data are available.     

Precambrian crustal evolution of Peninsular India: A 3.0 billion year odyssey

The Precambrian geologic history of Peninsular India covers nearly 3.0 billion years of time. India is presently attached to the Eurasian continent although it remains (for now) a separate plate. It comprises several cratonic nuclei namely, Aravalli–Bundelkhand, Eastern Dharwar, Western Dharwar, Bastar and Singhbhum Cratons along with the Southern Granulite Province. Cratonization of India was polyphase, but a stable configuration between the major elements was largely complete by 2.5 Ga. Each of the major cratons was intruded by various age granitoids, mafic dykes and ultramafic bodies throughout the Proterozoic. The Vindhyan, Chhattisgarh, Cuddapah, Pranhita–Godavari, Indravati, Bhima–Kaladgi, Kurnool and Marwar basins are the major Meso to Neoproterozoic sedimentary repositories. In this paper we review the major tectonic and igneous events that led to the formation of Peninsular India and provide an up to date geochronologic summary of the Precambrian. India is thought to have played a role in a number of supercontinental cycles including (from oldest to youngest) Ur, Columbia, Rodinia, Gondwana and Pangea. This paper gives an overview of the deep history of Peninsular India as an introduction to this special TOIS volume.     

Structural mapping based on potential field and remote sensing data,South Rewa Gondwana Basin,India

Intracratonic South Rewa Gondwana Basin occupies the northern part of NW–SE trending Son–Mahanadi rift basin of India. The new gravity data acquired over the northern part of the basin depicts WNW–ESE and ENE–WSW anomaly trends in the southern and northern part of the study area respectively. 3D inversion of residual gravity anomalies has brought out undulations in the basement delineating two major depressions (i) near Tihki in the north and (ii) near Shahdol in the south, which divided into two sub-basins by an ENE–WSW trending basement ridge near Sidi. Maximum depth to the basement is about 5.5 km within the northern depression. The new magnetic data acquired over the basin has brought out ENE–WSW to E–W trending short wavelength magnetic anomalies which are attributed to volcanic dykes and intrusive having remanent magnetization corresponding to upper normal and reverse polarity (29N and 29R) of the Deccan basalt magnetostratigrahy. Analysis of remote sensing and geological data also reveals the predominance of ENE–WSW structural faults. Integration of remote sensing, geological and potential field data suggest reactivation of ENE–WSW trending basement faults during Deccan volcanism through emplacement of mafic dykes and sills. Therefore, it is suggested that South Rewa Gondwana basin has witnessed post rift tectonic event due to Deccan volcanism.     

Precise U–Pb dating of Paleoproterozoic mafic dyke swarms of the Dharwar craton,India: Implications for the existence of the Neoarchean supercraton Sclavia

We report seven high precision U–Pb age determinations for mafic dykes from a number of major Precambrian swarms located in the Dharwar craton, south India. These new age results define two previously unrecognized widespread Paleoproterozoic dyking events at 2221–2209 and 2181–2177 Ma, and confirm a third at 2369–2365 Ma. Three parallel E–W trending mafic dykes from the petrographically and geochemically variable Bangalore dyke swarm, the most prominent swarm in the Dharwar craton, yield indistinguishable U–Pb baddeleyite ages of 2365.4 ± 1.0, 2365.9 ± 1.5 and 2368.6 ± 1.3 Ma, indicating rapid emplacement in less than five million years. A compilation of Paleoproterozoic U–Pb ages for mafic magmatic events worldwide indicates that the 2369–2365 Ma Bangalore dyke swarm represents a previously unrecognized pulse of mafic magmatism on Earth.     

Pb–Pb baddeleyite ages of mafic dyke swarms from the Dharwar Craton: Implications for Paleoproterozoic LIPs and diamond potential of mantle keel

The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs. Here we report ten precise Pb–Pb TE-TIMS age determinations on baddeleyite grains separated from seven mafic dykes and three sills, intruding into Archean basement rocks and Proterozoic sedimentary formations of the Eastern Dharwar Craton respectively. The crystallization age of the baddeleyite shows 2366.3 ​± ​1.1 ​Ma, and 2369.2 ​± ​0.8 ​Ma for the NE–SW trending dykes, 2368.1 ​± ​0.6 ​Ma, 2366.4 ​± ​0.8 ​Ma, 2207.2 ​± ​0.7 ​Ma and 1887.3 ​± ​1.0 ​Ma for the ENE–WNW to E–W striking dykes, 1880.6 ​± ​1.0 ​Ma, 1864.3 ​± ​0.6 ​Ma and 1863.6 ​± ​0.9 ​Ma for Cuddapah sills, and 1861.8 ​± ​1.4 ​Ma for the N–S trending dyke. Our results in conjunction with those from previous studies identify eight distinct stages of widespread Paleoproterozoic magmatism in the Dharwar craton. The mantle plume centres of the four radiating dyke swarms with ages of ~2367 ​Ma, ~2210 ​Ma, ~2082 ​Ma, and ~1886 ​Ma were traced to establish their proximity to the EDC kimberlite province. Though the ~2367 ​Ma and ~1886 ​Ma plume centres are inferred to be located to the west and east of the present day Dharwar craton respectively away from the kimberlite province, location of plume heads of the other two swarms with ages of ~2207 ​Ma and ~2082 ​Ma are in close proximity. In spite of the ubiquitous occurrence of dyke intrusions of all the seven generations in the kimberlite province, only few of these kimberlites are diamondiferous. Kimberlite occurrences elsewhere in the vicinity of older Large Igneous Provinces (LIPs) like the Mackenzie, Karoo, Parana-Etendeka and Yakutsk-Vilui are also non-diamondiferous. This has been attributed to the destruction of the lithospheric mantle keel (that hosts the diamonds) by the respective mantle plumes. The diamondiferous nature of the EDC kimberlites therefore suggests that plume activity does not always result in the destruction of the mantle keel.     

Crustal Magnetic Studies over Krishna-Godavari Basin in Eastern Continental Margin of India

Total field magnetic data were collected over the Krishna-Godavari basin covering 20, 000 sq.km with an average spacing of 8.5 km. This was mainly to study the long wavelength features related with the deep structures. Aeromagnetic map of the region compared well with the ground maps. The anomaly maps show a combination of NE-SW, NS/NNE-SSW and NW-SE trends. The anomalies of ground data are transformed to isolate the sources at different depths. The second vertical derivative and downward continuation maps bring out clearly the NE-SW and NS/NNE-SSW trends related to the coastal basin and Eastern Ghats implying that they are shallow. These are probably superposed on much deeper NW-SE trending structural features of Pre-Gondwana breakup as evidenced in the Horizontal Gradient of Pseudogravity and upward continuation maps. From the offshore magnetic data it appears that these trends extend up to the Ocean Continent Boundary. It is inferred that the deeper features are associated with rifting of Dharwar and Bastar cratons within the Indian plate, prior to the rifting of India from Gondwanaland. The superposed horst and graben structures are related to the formation of the pull-apart Krishna-Godavari basin as a result of rifting and drifting of India from Gondwanaland. These two structural features are associated with two different tectonic events.     

Remnants of Early Continental Crust in the Amgaon Gneisses,Central India: Geochemical Evidence

The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than ∼2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (∼2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean.     

Remnants of Early Continental Crust in the Amgaon Gneisses, Central India: Geochemical Evidence

The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than 2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean.     

Reassembly of the Dharwar and Bastar cratons at ca. 1 Ga: Evidence from multiple tectonothermal events along the Karimnagar granulite belt and Khammam schist belt,southern India

The northern part of the Nellore–Khammam schist belt and the Karimnagar granulite belt, which are juxtaposed at high angle to each other have unique U–Pb zircon age records suggesting distinctive tectonothermal histories. Plate accretion and rifting in the eastern part of the Dharwar craton and between the Dharwar and Bastar craton indicate multiple and complex events from 2600 to 500 Ma. The Khammam schist belt, the Dharwar and the Bastar craton were joined together by the end of the Archaean. The Khammam schist belt had experienced additional tectonic events at \(\sim \)1900 and \(\sim \)1600 Ma. The Dharwar and Bastar cratons separated during development of the Pranhita–Godavari (P–G) valley basin at \(\sim \)1600 Ma, potentially linked to the breakup of the Columbia supercontinent and were reassembled during the Mesoproterozoic at about 1000 Ma. This amalgamation process in southern India could be associated with the formation of the Rodinia supercontinent. The Khammam schist belt and the Eastern Ghats mobile belt also show evidence for accretionary processes at around 500 Ma, which is interpreted as a record of Pan-African collisions during the Gondwana assembly. From then on, southern India, as is known today, formed an integral part of the Indian continent.     

The drift history of the Dharwar Craton and India from 2.37 Ga to 1.01 Ga with refinements for an initial Rodinia configuration

Coupled paleomagnetic and geochronologic data derived from mafic dykes provide valuable records of continental movement. To reconstruct the Proterozoic paleogeographic history of Peninsular India, we report paleomagnetic directions and U-Pb zircon ages from twenty-nine mafic dykes in the Eastern Dharwar Craton near Hyderabad. Paleomagnetic analysis yielded clusters of directional data that correspond to dyke swarms at 2.37 Ga, 2.22 Ga, 2.08 Ga, 1.89–1.86 Ga, 1.79 Ga, and a previously undated dual polarity magnetization. We report new positive baked contact tests for the 2.08 Ga swarm and the 1.89–1.86 Ga swarm(s), and a new inverse baked contact test for the 2.08 Ga swarm. Our results promote the 2.08 Ga Dharwar Craton paleomagnetic pole (43.1° N, 184.5° E; A95 = 4.3°) to a reliability score of R = 7 and suggest a position for the Dharwar Craton at 1.79 Ga based on a virtual geomagnetic pole (VGP) at 33.0° N, 347.5° E (a95 = 16.9°, k = 221, N = 2). The new VGP for the Dharwar Craton provides support for the union of the Dharwar, Singhbhum, and Bastar Cratons in the Southern India Block by at least 1.79 Ga. Combined new and published northeast-southwest moderate-steep dual polarity directions from Dharwar Craton dykes define a new paleomagnetic pole at 20.6° N, 233.1° E (A95 = 9.2°, N = 18; R = 5). Two dykes from this group yielded 1.05–1.01 Ga ²⁰⁷Pb/²⁰⁶Pb zircon ages and this range is taken as the age of the new paleomagnetic pole. A comparison of the previously published poles with our new 1.05–1.01 Ga pole shows India shifting from equatorial to higher (southerly) latitudes from 1.08 Ga to 1.01 Ga as a component of Rodinia.     

Precambrian mafic magmatism in south Indian granulite terrain

South Indian granulite terrain had witnessed significant part of Precambrian mafic igneous activity in the form of episodic mafic dyke intrusions of the Palaeoproterozoic period. Strike trends of these dykes are not uniform over the region and the dykes are generally fresh, massive, black dolerites except in the Bhavani shear zone bordering the southern fringes of Nilgiri massif. In Agali-Coimbatore area of our study in the western Bhavani shear zone, the dykes appear to be penecontemporaneous with shearing. Isotopic data place age of Agali-Coimbatore dyke intrusions at about 2.1 Ga. The age of these dykes is significant to constrain an early Palaeoproterozoic age for major shearing event in the Bhavani shear zone. Other dyke emplacement ages are placed at about 1.8 Ga and 1.65 Ga based on the Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai areas. Older ages comparable to those of the Dharwar craton are not known and in this respect future isotopic dating is vital. Geochemically, these dykes are quartz/hypersthene normative subalkalic tholeiites. An attempt is made here to provide insights into the general petrogenetic history of the Precambrian dykes. Compositional trends are explained by the fractional crystallization of ferromagnesian phases and plagioclase control is conspicuous at the advanced stages of fractionation. Geochemical characteristics suggest that the dykes have tapped Fe-rich non-pyrolite mantle sources with LIL and LREE enrichment as in many continental basalts. The data suggest that role of crustal contamination is limited in petrogenesis; crustal signatures are noticed in the more mafic end members formed in early stage of evolution suggesting that contamination was temperature controlled with most primitive high temperature magmas being most vulnerable to the process. Nd-Sr isotopic data, at present restricted to Agali-Coimbatore dykes, suggest that Palaeoproterozoic magmas tapped subcontinental lithosphere that may have stabilized in the Archaean times at about 3 Ga during the major crustal building activity in the shield region. Further work coupled with isotopic and mineral chemistry will improve our knowledge on the petrological evolution of the dyke magmas and mafic magmatism in general.     

东准噶尔琼河坝岛弧晚古生代地球动力学环境——来自和尔赛岩体暗色岩墙时空分布的证据

新疆东准噶尔地区地处阿尔泰、准噶尔和东天山等山系和构造单元的结合部位, 是研究中亚造山带(或称北亚造山区)构造演化的关键地区。以往的研究多集中于东准噶尔的岩石组合和地球化学组成, 相对缺乏构造变形尤其是中小尺度构造变形的研究。初步研究发现东准噶尔分布着大量的中基性暗色岩墙, 它们是后期岩浆侵入到前期构造裂隙中的产物, 可以从时空两个方面为构造裂隙的研究提供制约。本文结合地质资料和高分辨率遥感影像解译, 在琼河坝岛弧带中的和尔赛岩体(早泥盆世)中识别出来了874个暗色岩墙(晚石炭世-早二叠世)片段, 它们的走向以北西西-南东东向为主, 另外还有少数北东-南西和北西-南东走向的岩墙。通过岩墙的宏观变形特征可以推测, 北西西-南东东走向的岩墙形成于压张性裂隙之中, 北西-南东走向的岩墙形成于左行张剪裂隙之中, 北东-南西走向的岩墙形成于右行压剪裂隙之中。这些裂隙形成时平面最大主应力为北西西-南东东方向。结合岩体和岩墙的时代, 本文认为在晚泥盆世-早石炭世期间, 和尔赛岩体由于受到北西西-南东东方向的区域挤压作用而产生相应的裂隙, 可能标志着洋盆结束后碰撞作用的发生, 而晚石炭世-早二叠世暗色岩墙的普遍发育可能是后碰撞岩浆活动的标志。     

THE PRESENT STATUS OF THE THEORY on the ORIGIN OF OIL AND TASKS FOR FURTHER INVESTIGATION

During the Early Proterozoic (2.5 to 2.3 Ga), three types of coeval structural provinces developed in the eastern Baltic Shield—(1) the Karelian and Kola granite-greenstone cratons, (2) the relatively high grade Lapland-Umba granulite belt (LUGB), and (3) the Belomorian (White Sea) mobile belt (BMB). The LUGB represents a compensated compressional zone where synkinematic crustal-derived magmatism of the enderbite-charnockite series predominates. The BMB is a transitional nappe-folded zone between these high- and low-grade terranes, which consists mainly of reworked granite-greenstone lithologies of the adjacent cratons. These cratons were vast extensional areas with mantle-derived, siliceous, high-Mg (boninite-like) series (SHMS) magmatism. This SHMS magmatism occurs in volcano-sedimentary sequences, large layered intrusions, and dike swarms within graben-like structures.

One of the more interesting types of tectonomagmatic activity occurred within the BMB and is expressed as the unique Drusite Complex. It is represented by thousands of small intrusions of mafic and ultramafic rocks, dispersed among the higher-grade BMB host rocks. Geological features of these intrusions show that their formation was synkinematic with deformations within the belt, although they have undergone later, post-solidification deformation and metamorphism. As a result, intrusions often were transformed into lenticular, boudin-like bodies with primary igneous textures preserved only in their central portions. Compositions of the Drusite Complex intrusions, although forming small, individual bodies with associated chill zones, are similar to large layered intrusions in adjacent cratons (plagioclase harzburgites and lherzolites, pyroxenites, troctolites, olivine norites and norites, gabbronorites, anorthosites, and diorites). The areal distribution of the drusite intrusions and their correlation with large layered mafic intrusions in adjacent cratons suggests a vast magma-generation zone beneath western Russia during the Early Proterozoic.

The character and extent of magmatism suggests that during the Early Proterozoic (in Sumian— Sariolian time) the Kola and Karelian cratons were vast extensional areas above spreading plume heads. Within this scenario, the LUGB was an area of intense crustal sagging between these two cratons. The BMB was a transitional zone of tectonic flowage between the LUGB and the cratons, where movements were not as intense; there a nappe-folded structure formed. As a result, the intrusion of new melts occurred under rapidly changing conditions and a specific type of disseminated, intrusive magmatism—The Drusite Complex—emerged instead of the formation of layered intrusions. The petrologic and mineralogic compositions of the Drusite Complex intrusions are indistinguishable from coeval layered mafic intrusions of the adjacent Karelia and Kola cratons, suggesting similar parental magmas and a large zone of magmatism (i.e., large igneous province, or LIP) beneath the eastern Baltic Shield. These magmas were derived either from depleted mantle melts that had assimilated a significant crustal component, or from enriched mantle.     

Gravity anomalies and the Indian lithosphere: Review and analysis of existing gravity data

The upper part of the lithosphere has been actively involved in various exogenic and endogenic processes which have left their imprint on the gravity field on the Indian Peninsula and the Himalaya. Analysis of the gravity field over the Dharwar craton shows that the greenstone belts of this craton have been formed as a result of development of deep fractures in the earth's crust during Archaean times. Precambrian mountain ranges such as the Aravallies, Vindhyans, Satpura and Eastern Ghats are located peripheral to Archaean cratons. Most of these mountain belts are characterized by gravity highs suggesting that the underlying crust is of higher than normal density. These mountain ranges with the exception of the Eastern Ghats do not appear to be locally compensated. Regional compensation seems to prevail over all these areas. Eastern Ghats ranges are also underlain by a crust of higher than normal density relative to the Dharwar and Bastar cratons and exist with a sharp contact with the cratons in the West. Isostatic compensation in the Eastern Ghats appears to have been achieved by thickening of the underlying crust. The Himalaya has attained a fairly high degree of isostatic compensation.