Search

A climatic transition of humid to arid conditions from west to east on the Western Dharwar Craton, southern India provided a suitable setting to understand the relative role of chemical vs. physical weathering of amphibolites. Weathering pattern of amphibolites was studied from two locations in humid climatic zone and two locations in semi-arid zone. The purpose of the present study is to evaluate the relative importance of structural features in rocks and rainfall in the extent of chemical weathering based on mobilization of elements and mineralogical variations. Under conditions of humid climate with good vegetation and relief bedrock physical features (all interconnected complex system) become less important and biota becomes more important. Biological activity and relief together made weathering solutions acidic under humid conditions. Under semi-arid conditions, weathering solutions normally remained alkaline; however, microbially mediated acidity prevailed in the microzones of weathering profile. In the humid zones, the immobile or less mobile chemical elements, Ce and Ti, and to a less extent Fe and Ba show high degree coherence by strongly enriching in the most weathered products, whereas the REEs are mobile in both conditions. There is significant loss of HREE resulting in fractionated patterns, relative to the parent rock, only under humid conditions, in the weathered products. The present study of weathering has shown that local geological features in a rock become important to the nature and extent of weathering under semi-arid condition by creating zones of aridity and humidity on the outcrop scale.

Fil: Singh, Pradip K.. Instituto Potosino de Investigacion Cientifica y Tecnologica; Mexico

This paper presents brief sedimentological inventory of the Misaki and the overlying Hasse Formation (constituting the Miura Group) based on observations made at Jogashima Island, Miura Peninsula and infers the nature of the stratigraphic transition from tectonic viewpoint. The lower part of the Misaki Formation is made of very coarse to coarse-grained scoriaceous, normally graded sandstones, fine-grained sandstones/siltstones, and mudstones and indicates a deep sea submarine canyon depositional setting. There is no evidence of storm reworking (shallow marine processes) in the lower parts of the Misaki Formation at least in the Jogashima Island as suggested by Stow et al. (1998). However, the upper part of the Misaki succession formed in a shallow marine depositional setting. The overlying Hasse Formation characterized by brownish coloured, very coarse to coarse-grained large-scale trough cross stratified, very poorly sorted sandstones, very poorly sorted polymictic and matrix dominated mass flow conglomerates with siltstone/fine-grained sandstone clasts derived from the underlying Misaki Formation and occasional reverse grading. In significant contrast to the underlying Misaki Formation the Hasse Formation lacks very fine-grained sandstone/siltstone/mudstone facies. Unlike the tholeiitic basalt composition of the lapilli and pebbles of the Misaki sandstone beds, the scoriaceous lapilli and pumiceous beds of the Hasse Formation are of calc-alkaline composition. The Hasse sedimentation probably took place in much shallower paleogeography compared to the underlying Misaki Formation, most probably in an alluvial fan-braided fluvial setting. The Misaki-Hasse stratigraphic contact is very sharp and erosional, and represents a subaerial unconformity (i.e., depositional sequence boundary). Our sedimentological data from the Jogashima Island clearly indicate progressive shallowing of the depositional setting during the terminal phase of the Misaki sedimentation and much abrupt shallowing across the Misaki-Hasse transition possibly as a consequence of ongoing collision between the Izu-Bonin and Honshu arcs.

The Bastar Craton is one of the oldest cratonic nuclei of the Indian shield, comprising Paleoarchean to Mesoproterozoic crust that preserves the record of protracted crustal evolution and metallogeny. This study describes whole-rock geochemistry, U-Th-Pb ages of zircon and monazite from Neoarchean TTGs, sanukitoids, and Paleo- to Mesoproterozoic granites, and Sm-Nd isotope data from mafic enclave within TTGs of the Western Bastar Craton (WBC). The TTGs and sanukitoids represent Neoarchean crust formed by collision-accretion processes. The TTGs are of two types: low-HREE and high-HREE with both groups derived from low-K mafic sources. The sanukitoids have moderate SiO2 (55.1–65.1 wt%, average = 61.7 wt%), Mg# (20–36, average = 24.7), Ni (10–40 ppm; average = 14 ppm), Sr (339–528; average = 418 ppm) and moderate to high concentrations of incompatible elements like Rb (26–112 ppm, average = 48), Ba (482–2300 ppm, average = 1542), Zr (69–593 ppm, average = 334 ppm), Nb (3.8–14.8 ppm, average = 8.7 ppm), Y (13.2–24.5 ppm, average = 19.5 ppm), and REE (91–301 ppm, average = 212)]. They post-date TTG emplacement and formed by mixing between metabasalt-derived and mantle wedge-derived melts in an arc environment. The Mul granite represents a younger Paleo- to Mesoproterozoic suite of granites that was derived by reworking of pre-existing crust. U–Pb ages of zircon constraints the TTG magmatism to 2544–2496 Ma, while Nd-model ages (3259–3142 Ma) of mafic enclaves within the TTG suggest the presence of Paleoarchean crust in the WBC. Zircon and monazite ages indicate that the emplacement of the Mul granite was synchronous with 1666–1547 Ma regional tectonothermal event. These granites were produced by reworking of older granitoid crust. The Neoarchean TTGs/sanukitoids were affected by 1666–1547 Ma tectonothermal event which constitutes a widely documented Paleo- to Mesoproterozoic orogeny in the Central Indian Tectonic Zone and Bhopalpatnam granulite belt to the south of the WBC. Copper and gold mineralization in the Thanewasna belt along the craton's western margin is linked to this tectono-magmatic event. In the global supercontinent outlook, the WBC preserves the imprints of Neoarchean events related to the Ur supercontinent as well as Paleo-Mesoproterozoic and Neoproterozoic (Grenville-age) events associated with the Columbia and Rodinia supercontinent

In this contribution we present field relations, U-Pb zircon geochronology, in-situ Lu-Hf isotopes, bulk-rock Sm-Nd isotopes and geochemistry of the tonalite-trondhjemite-granodiorite (TTG) gneiss, sanukitoid and anatectic granites from the Central Bundelkhand granite-greenstone terrane (CBGGT). The present study provides a comprehensive insight into the crust-mantle evolution, reworking, and cratonization of the Bundelkhand Craton (BC) during the Archean Eon. We report two discrete episodes of Neoarchean TTG magmatism at ~ 2.71 and ~ 2.68 Ga from the BC for the first time. Additionally, we identify a TTG gneiss that is significantly older (~3.34 Ga) than thus far assumed in the eastern part of the area. Furthermore, zircon U-Pb ages of the sanukitoids and the anatectic granites suggest synchronous emplacement at 2.58–2.50 Ga. Zircon of Paleoarchean TTG, Neoarchean TTG, sanukitoid, and anatectic granites show eHf(t) values in the following ranges: +1.7 to −0.9, +4.1 to −10.7, −3.6 to −6.2, and −5.9 to −7.8, respectively, which are indicative of significant crustal reworking. Neoarchean TTG gneiss shows eNd(t) and eHf(t) value of + 4.5 and + 10.0 to −6.8, respectively, indicating a juvenile crustal source and probably formed by the partial melting of deep-seated mafic crust in the garnet stability field. Neoarchean TTGs formed in arc creation/collision to the Paleoarchean nucleus of the BC. The amalgamation of diverse micro-blocks occurred by the arc-continent collision and probable breakoff of the descending slab between crustal blocks of the BC during Neoarchean. This event generated anatectic granites by intense partial melting of the existing crust, which resulted as closing of subduction and marked as final stabilization and cratonization of the BC. This event also shows evidence that the BC did not amalgamate until ~2.50 Ga.

The Mio-Pliocene Misaki Formation of the Miura Group (Miura Peninsula, Japan) shows an extremely wide variety of soft-sediment deformation structures. The most common deformation structures are load casts and associated flame structures, dish-and-pillar structures, synsedimentary faults, multilobated convolutions, chaotic deformation structures, sedimentary veins and dykes, and large-scale slides and slump scars. The formation, which accumulated in a deep-sea environment (2000–3000 m), is well exposed in and around Jogashima; it consists of relative thin (commonly dm-scale) alternations of deep-marine fine-grained sediments and volcanic ejecta that are, as a rule, coarse-grained. Since the formation represents fore-arc deposits of the Izu-Bonin and the Honsu arc collision zone, it might be expected that tectonic activity also played a role as a trigger of the soft-sediment deformation structures that abound in these sediments. This is indicated, indeed, by the abundance of soft-sediment deformations over large lateral distances that occur in numerous beds that are sandwiched between undeformed beds. On the basis of their characteristics and the geological context, these layers can be explained satisfactorily only by assuming deformation triggered by seismicity, which must be related to the Izu-Bonin and Honsu arc collision. The layers thus form deep-marine seismites.

Geochemical data from Early Cretaceous volcanic rocks (ultra-basic, basic, intermediate) from the Fresnillo area (Chilitos Formation), Zacatecas, are presented in this study. These rocks are characterized by variable SiO2 = 38.1–61.7 wt %, Al2O3 = 17.5–27.2 wt %, and MgO = 0.21–6.4 wt % with the Mg# = 20.6–78.9. They are influenced by variable degrees of hydrothermal alteration, being the propylitic more dominant than argillic alteration. The geochemistry displays strong enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs). Moreover, a depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and nearly flat heavy rare earth element (HREE) patterns are observed. Negligible or absence negative Eu anomalies indicate that fractional crystallization of plagioclase played an insignificant role during the magma evolution. The geochemical features suggest that volcanic rocks are derived from partial melting of an enriched mantle source that has experienced assimilation (low degree) and fractional crystallization. The low La/Nb (0.68–2.23) and Nb/Y (0.19–0.69) ratios, relatively high Zr/Y (3.68–10.56) and Th/Ta (1.03–3.65) ratios, and progressively enriched normalized patterns clue an island arc and most likely a back-arc basin environment for the Chilitos Formation in the Fresnillo area. New multidimensional discrimination diagrams indicate a mid-ocean ridge to island arc setting. A new tectono-magmatic model combined with previous geological observations indicate that the Chilitos Formation was formed in a back-arc tectonic setting and was part of the Guerrero terrane during the Late Jurassic to Early Cretaceous time.

A meteorite shower occurred in Katol (21° 15′ 30″ N; 78° 35′ 00″ E; at an elevation of 415 m above msl), Nagpur district of Maharashtra state, India on May 22, 2012 between 14:00 to 14:30 hrs (Indian Standard Time) with a presently observed strewn field of ∼5.0 sq km area. The event was experienced by the hundreds of inhabitants with a loud noise and fire ball between Akola in the west to Nagpur in the east. The Geological Survey of India has so far collected 27 meteorite pieces with a total weight of 3500 gm. The locations, size, shape and surface features of the individual meteorite pieces have been recorded. Based on their physical properties, mineralogy, mineral chemical mapping and REE chemistry, the Katol meteorites have been classified as olivine-rich H5 type differentiated stony meteorite with reconstituted chondrules. The preliminary evidence suggests the presence of pre-solar grains in Katol meteorite.

In the upper catchment of the Kaveri river in the Sahyadri mountains of southern India, middle to lower crustal Archean granulite grade mafic and felsic rocks with similar structures and textures are exposed under the conditions of active tectonics, high rainfall and thick tropical vegetation. Occurrence of the two major rock types in close association under identical geological, geographical and biological conditions provides an uncommon situation for the study of weathering, elemental mobilization and sediment generation processes. Field observations, mineralogical and geochemical data including major, trace and rare earth elements (REE) of fresh rocks and variably weathered saprolite samples suggest that close association of mafic and felsic rocks accelerates the denudational processes by early weathering of mafic minerals in felsic rocks and mafic rocks in the terrain. Due to differential weathering of rocks, unweathered to less weathered felsic grains are likely transferred to the coarser fraction of fluvial sediments deposited on the floodplains of the river imposing an upper continental crust (UCC) geochemical signature. It is found that during chemical weathering, in addition to other factors, weatherability of host minerals of REE control the mobility of REE in the weathering profile. It is suggested from the observations on the weathering process and on the geochemistry of derivative sediments, that in a tectonically active system with a climate maximum, as in Sahyadris, an equilibrium could be dynamically maintained between weathering and erosional regimes. Also we infer from our study that there exist certain commonalities between surface denudational and mantle-magmatic geochemical differentiation processes. Similarity of these processes, therefore, may have implication to common UCC-like geochemistry of Post Archean sediments.

The study of lineament pattern based on IRS-IA sub-scene (P24–R50) and Landsat (P143– R42) data combined with the drainage analysis and field observations, two prominent sets of lineament (NE-SW and NNW-SSE), besides less prominent E-W and N-S trending lineaments of tectonic origin have been observed in parts of Allahabad area. Pervasive and penetrative sub-vertical joint sets parallel to the macroscopic linear structures along with collapse structures possibly of neotectonic origin are also noted in the basement rock exposed to the SSW of Allahabad. The development of various meso- and macroscopic deformed structures, presence of collapse structure in the basement strata and the near orthogonal channel shift of axial rivers provide evidence of basement reactivation in parts of Allahabad area, Ganga plain in space and time.

The occurrence of metamorphosed basaltic pillow lava in close association with serpentinized ultramafic rock, metamorphosed basaltic komatiite, volcaniclastic metasediment, and banded iron formation (BIF) in the Mauranipur area is the first explicit evidence for subduction related submarine volcanism in the Archean Bundelkhand craton, Central India. The Mauranipur pillow lava underwent greenschist to amphibolite facies metamorphism while retaining a geochemical signature of its igneous protolith. The pillow lava and associated massive volcanic rock is subalkalic, low-K tholeiitic basalt to basaltic andesite with SiO2 = 51.9-55.9 wt% and Mg/(Mg + Fetotal) × 100 = 63.4-67.2. The rock shows depletion in HFS elements with Nb/Nb* (0.07-0.17), and LIL elements are relatively enriched compared with present-day N-MORB compositions. Chondrite normalized REE patterns for the Mauranipur pillow lava are nearly flat with (La/Sm)N = 0.9-1.5, (Gd/Yb)N = 1.0-1.1, and Eu/Eu* = 0.81-1.10. The geochemical characteristics of the pillow lava are similar to those of low-K tholeiitic basalt reported from modern intra-oceanic arcs. The associated high-Mg andesite is compositionally similar to basaltic komatiite, enriched in LREE, with (La/Sm)N = 2.95-6.44, and depicts a nearly flat chondrite normalized HREE pattern with a low MREE/HREE ratio (Gd/Yb)N = 1.24-1.58. The basaltic komatiite displays remarkably similar geochemical characteristics to modern boninite. The present study, combined with available geological data, suggests that the supracrustal rocks of the Mauranipur area represent an Archean ophiolite sequence formed in a plate convergent setting.

We report here hematite-rich concretions observed in the sandstone of the Mesoproterozoic Vindhyan Supergroup of rocks occurring in parts of Bihariya, Uttar Pradesh, northern India. These concretions are similar to `blueberries' from Mars and their terrestrial analogues reported from the Jurassic Navajo Sandstone in Utah, USA. The presence of diagenetically formed hematite concretions gave the first confirmation of the presence of liquid water in the red planet in the past. We report here the detailed morphology, petrography, mineral chemistry, magnetic susceptibility characteristics and spectral radiometric data of hematite-rich concretions observed in the Vindhyan sandstone. These are compared with `blueberries' from Mars and other similar terrestrial analogues reported from different parts of the world. In spite of similarities, these hematite-rich concretions are strikingly distinct in having a nucleus and alternate iron-rich and iron-poor rims unlike other global occurrences. In addition, we document here outcrop scale evidence of possible fluid pathways considered responsible for the development of the concretions.

Giant quartz veins (GQVs; earlier referred to as 'quartz reefs') occurring in the Archean Bundelkhand Craton (29, 000 km(2)) represent a gigantic Precambrian (similar to 2.15 Ga) silica-rich fluid activity in the central Indian shield. These veins form a striking curvilinear feature with positive relief having a preferred orientation NE-SW to NNE-SSW in the Bundelkhand Craton. Their outcrop widths vary from