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.     

Petrology, geochemistry and genesis of rift-related carbonatites of Ambadungar, India

Summary The Ambadungar (Amba Dongar) alkaline carbonatite complex is emplaced in the Deccan traps igneous province. A wide range of carbonatites and alkaline rocks are exposed around Ambadungar. The alkaline rocks have been classified as tinguaite, phonolite and/or phononephelinite, melanephelinite, and syenite and/or nepheline syenite whereas carbonatites vary from calcio-carbonatites to ferro- and silicocarbonatites. The enrichment in large-ion lithophile elements (LILE), P, and rare-earth elements (REE) in carbonatites is considered to result from fractionation of a mantle derived magmatic liquid, i.e. nephelinitic magma, by liquid immiscibility which also produced melanephelinite and/or phononephelinite with high field strength elements (HFSE) such as Ca, Mg, Fe, and Mn in the alkaline silicate liquid fraction. The La:Lu ratios of the carbonatites are typical of igneous rocks and vary between 590 and 1945, similar to many known magmatic carbonatites. The ¹³C concentration varies between –2 and –8 whereas ¹⁸O-values vary between 7.7 and 26.8. The ¹³C concentration is typical of primary igneous carbonatites but ¹⁸O enrichment is thought to be the result of post-magmatic processes such as interaction with meteoric water and re-equilibration with hydrous fluids at low temperatures.

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Petrologie, Geochemie und Genese der riftgebundenen Karbonatite von Ambadungar, Indien

Zusammenfassung Der Ambadungar (Amba Dongar) Alkalikarbonatit-Komplex liegt in der magmatischen Deccan Provinz. Er umfaßt eine Vielzahl von karbonatitischen und alkalischen Gesteinen, die in der Umgebung von Ambadungar aufgeschlossen sind. Die Alkaligesteine sind als Tinguaite, Phonolite und/oder Phononephelinite, Melanephelinite, Syenite und/oder Nephelinsyenite zu klassifizieren, die Karbonatite als Calcio-, bis Ferro- and Silicokarbonatite. Die Anreicherung an LIL-Elementen und Seltenen Erden in den Karbonatiten werden als das Ergebnis der Fraktionierung von Mantelschmelzen, i.e. eines nephelinitisches Magmas, infolge von Nichtmischbarkeit interpretiert. Melanephelinite und/oder Phononephelinite and hohe Gehalte an HFS-Elementen (Ca, Mg, Fe and Mn) in der alkalisch-silikatischen Schmelzfraktion sind ebenfalls das Ergebnis dieser Prozesse. Die La/Lu-Verhältnisse sind typisch für magmatische Karbonatite and variieren zwischen 590 and 1945. Die ¹³C Konzentrationen variieren zwischen -2 and -8 %o, die ¹⁸O Werte zwischen 7.7 and 26.8 %0. Während die ¹³C Konzentration typisch für primär magmatische Karbonatite ist, ist die ¹⁸O-Anreicherung mit postmagmatischen Prozessen, wie etwa die Interaktion mit meteorischen Wässern and die Reequilibration mit niedrig temperierten wäßrigen Fluiden, erklärbar.

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with 11 figures     

Sargipali galenas: Unusual lead-isotope data from eastern India

The Sargipali sulphide deposit, hosted by a sequence of metasediments of the Precambrian Gangpur Series, represents the stratiform and stratabound Pb-Cu mineralization. Lead-isotope analyses obtained from six galena samples of different forms and stratigraphical levels, yielded a model Pb age of 1665 Ma, which predates the episodes of deformation and metamorphism ( 850 Ma) as well as syn-to late-tectonic magmatism. The model Pb age, on the contrary, shows a relationship to the closing phase of the Gangpur sedimentation (1700 Ma) in a shallow-water marine environment. The notable paucity of the interlayered volcanic rock in the deposit and its Pb-rich nature possibly point to its being the sedimentary exhalative (SEDEX) type. Documentary evidence for the Pb component from the geological environment of the upper continental crust is revealed by an exceptionally high (= ²³⁸U/²⁰⁴Pb) and the slightly above average W(= ²³²Th/²³⁸U) values. These indicate that the source rock was initially rich in U-Th-Pb concentration. For the Sargipali ore Pb, the parent is assumed to be late Archean-lower Proterozoic soda granite.     

Dynamics of intensification of the boreal summer monsoon flow during IOD events

The present study focuses on understanding the dynamics of intensification of the boreal summer monsoon cross-equatorial flow generally observed during positive-Indian Ocean Dipole (IOD) events, by taking 1994 as a case study. In particular, the influence of the anomalous divergent motions during 1994 (i.e., east–west circulation over equatorial Indian Ocean and the monsoon Hadley-type circulation) on the intensification of summer monsoon cross-equatorial flow is investigated. This problem is examined using diagnostic analyses and simulation experiments from a multi-level global atmospheric model forced with observed diabatic heating. The results suggest that the transfer of kinetic energy (KE) from the divergent motions to the rotational flows can be very effective during IOD periods like 1994; and provides a plausible explanation for the increase of KE of the monsoon cross-equatorial flow over Bay-of-Bengal and adjoining areas. The study also investigates the enhanced activity of westward propagating disturbances observed during 1994 over the monsoon region.     

Electrical resistivity changes in rocks during frictional sliding and fracture

Significant changes of electrical resistivity of saturated rocks and water pressure along sliding surface occurred during stick-slips in our direct shear experiment. Two types of changes of electrical resistivity occurred. In the first, resistivity decreased with increasing shear stress, reached minimum together with a sudden release of shear stress and returned to a higher value immediately afterwards. In the second, resistivity again decreased with increasing stress but, in contrast to the first type of changes, it decreased further upon the sudden drop of shear stress. The magnitude and the direction of the changes of water pressure on the sliding surface during stick-slip were not uniform, indicating local variations of surface deformation.     

The mode of association of alkanes with coals

The yield of alkanes released from three coals by liquefaction in tetralin at 400°C is 6–8 times greater than the yield obtained by Soxhlet extraction with the azeotropic micture of benzene and ethanol. The alkanes are dominated by a series of n alkanes, in most cases in the range C₁₄-C₃₄, together with major amounts of pristane and phytane. Homologous series of pentacyclic triterpanes are also present, according to GC/MS analyses. These consist almost entirely of hydrocarbons of the hopane and moretane series (17αH, 21βH and 17βH, 21αH), in the range C₂₇-C₃₄ (C₂₈ being absent). Several members of the series are found in S and R epimeric pairs. Differences in several aspects of alkane distribution between extract and liquefaction products were carefully examined. taking an overall view, the distributions in extract and product oil from any one coal were quite closely similar. It is concluded that the additional alkanes yielded by liquefaction had most probably been physically trapped inside the macromolecular network of the coals, and releasable only on disruption of that network.     

Age of granitic activity associated with copper–molybdenum mineralization at Malanjkhand, Central India

The Malanjkhand copper–molybdenum deposit in the Bhandara Craton, Central India, is hosted by a granite complex which consists of regionally dominant grey granitoid and pink granitoid confined to the mineralized zone. New SHRIMP RG data on zircons from both granite types are inferred to have crystallized during the same magmatic pulse at ca 2.48 Ga. The discrepancy between zircon age and earlier obtained Rb–Sr whole-rock age is attributed to modification of the Rb–Sr system by hydrothermal overprint. Similarity in petrographic features and chemical affinity in combination with identical age strongly indicate that the pink granite is the hydrothermally altered variety (microclinization and silicification) of the grey granite. The spatially associated, main Cu–Mo mineralization event at Malankhand appears to be broadly contemporaneous with and genetically related to the emplacement of the host granitoids at about 2.48 Ga.     

Sediment hosted lead–zinc deposits of the Neoproterozoic Bambuí Group and correlative sequences, São Francisco Craton, Brazil: A review and a possible metallogenic evolution model

The Proterozoic sediment-hosted Zn–(Pb) sulfide and non-sulfide deposits of the São Francisco Craton, Brazil, are partially syn-diagenetic and epigenetic and were probably formed during extensional events. The majority of the deposits occur within shallow water dolomites. The Pb isotopic data of sulfides are relatively homogeneous for individual deposits and plot above the upper crust evolution curve of the Plumbotectonic model. Some of the deposits are characterized by highly radiogenic lead (²⁰⁶Pb/²⁰⁴Pb ≥ 21) originating from the highly radioactive crust of the São Francisco Craton. Pb and S isotopic data suggest the sources of metal and sulfur for the deposits to be the basement rocks and seawater sulfates in the sediments, respectively. The relatively high temperatures of formation (100 to 250 °C) and moderate salinity (3% to 20% NaCl equiv.) of the primary fluid inclusions in the sphalerite crystals suggest the participation of basinal mineralizing fluids in ore formation. The steep paleo-geothermal gradient generated by the radioactively enriched basement rocks probably assisted in heating up the circulating mineralizing fluids.