Lensoid Bodies Research Articles

Geochemical Signatures of Platinum Group Elements in Ultramafic Rocks of Chotanagpur Gneissic Terrain, Eastern India and Their Genetic Control

Ultramafic rocks occur as intrusives in the form of lensoid bodies in the northwestern part of Chotanagpur Gneissic Terrain and consist mainly of olivine, orthopyroxene and clinopyroxene and are devoid of plagioclase. Based on distinct geochemical characteristics, the rocks have been identified as komatiites. The rocks are geochemically analogous to Al-undepleted Munro type (Al2O3/ TiO2=17.88-54.73) with distinctly high MgO (26.2-35.62 wt%), Ni (958-1902 ppm) and Cr (21.32-3320 ppm) contents. These rocks are characterised by low CaO/Al2O3, (Gd/Yb)n, (La/Yb)n with positive Zr, Hf, Ti anomalies suggesting high degree partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. These high MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for Platinum Group Elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. These rocks have low ΣPGE (29-269.02ppb) content with Ir (0.1-0.8ppb) and Ru (1.05-5.78ppb) among Iridium group PGE (IPGE); and Pt (5.04-18.72ppb), Pd(3.5-18.0ppb), Rh (0.22-0.84ppb) among Platinum group PGE (PPGE). The PGE abundances in komatiites were controlled by olivine fractionation. The Major, trace, REE and PGE composition of the rock suggest melting under anhydrous condition at shallow depth above the garnet stability field under S-undersaturated condition. Anhydrous melting associated with mantle plume activity gave rise to the rock which subsequently contaminated by lower crustal materials during magma ascent and emplacement. Keywords: Chotanagpur Gneissic Terrain, Komatiite, Platinum Group Elements, Sulphur Undersaturation, Plume

Application of Microgravity Survey in the Exploration of Rare-metal Bearing Pegmatites – A Case Study from Marlagalla, Mandya District, Karnataka

Abstract Geologically, the Marlagalla area represents the southern part of NNE-SSW trending Nagamangala schist belt, which is a member of ancient supracrustals of Sargur Group and situated in the southern part of Western Dharwar Craton. The area is mostly soil cover with limited exposure of amphibolite/hornblende schist and intrusive pegmatite. Pegmatites occur as lensoidal bodies within the amphibolite.Microgravity surveys are attempted for the first time to detect pegmatites within the amphibolite. The density between pegmatite (Density= 2.58-2.61 gm/cc) and host rock amphibolite (Density=3.09-3.14 gm/cc) gives detectable contrast to identify pegmatite within amphibolite. Gravity data are acquired over a grid of 5m x 30m using CG-5 Autograv gravimeter which has 1 μGal sensitivity. Elevation data are acquired using SP80 post processing DGPS instrument which has 5 mm elevation accuracy. Analysis of residual gravity and first vertical derivative (FVD) filters facilitated in delineating the pegmatite bodies. The pegmatite bodies show low gravity anomalies with an amplitude range of -5 to -200 μGal. FVD map of the residual gravity indicated four major N100E trending linear anomalies of -36 to 3.0 μGal/m corresponding to pegmatite bodies of varying strike (100-200m) and width (10-15m).2D-Forward modeling of microgravity data helped in deriving the subsurface geometry of the pegmatites. Borehole drilled subsequently based on the results of microgravity surveys intercepted pegmatites within the amphibolite. Thus, the technique has proved for exploration of pegmatite in contrasting environment of amphibolite. Pegmatite are known for rare metals and lithium minerals. Indirectly the model proposed here will help in exploration of rare metals and Li bearing pegmatite.

Chromite Composition and Platinum-Group Element Distribution in the Proterozoic Chimalpahad Anorthosite Complex, South India: Implications for Magmatic Processes and Discrimination of Tectonic Setting

Abstract The Proterozoic Chimalpahad Anorthosite Complex (CAC) is a deformed and dismembered complex in South India comprising of anorthosite-leucogabbro-gabbro-ultramafic rocks. It is located in the Nellore-Khammam Schist Belt (NKSB) within the contact zone between the Eastern Dharwar Craton (EDC) and the Eastern Ghats Belt (EGB). The chromitites occur as small dismembered lensoidal bodies within the ultramafic rocks. Presence of micrometric inclusions of iridium-like platinum-group minerals (IPGM), such as laurite (RuS2), erlichmanite (OsS2), irarsite (IrAsS), and Ir-Os-Ru alloys, is reported for the first time in the CAC chromites. The chromite cumulates preserve magmatic character (Cr#: 0.61 to 0.77; Mg#: 0.29 to 0.55), while disseminated chromites and rims of some grains (Cr#: 0.59 to 0.89; Mg#: 0.10 to 0.25) show postmagmatic modifications. The chromites are characterised by higher Cr and Al contents and lower Fe and Ti contents. The mineral chemistry of chromites such as Cr# and Mg# contents of Al2O3 (10.05–20.07 wt.%) and TiO2 (0.1 to 0.25 wt.%) typically suggests that the CAC chromitites and their ultramafic hosts were generated from the S-undersaturated boninite melt in a magmatic arc in suprasubduction tectonic setting. A general negative slope of bulk-rock PGE distribution with upward convex pattern is observed for the CAC chromitites in the chondrite-normalized plot. These geochemical and mineral chemical characters in addition to location of the chromitites and the CAC in the tectonic zone between Eastern Dharwar Craton and Eastern Ghats Mobile Belt point towards ophiolitic affinity of the CAC chromitites. Laurite (RuS2) grains are trapped close to the cores of chromites, whereas erlichmanites (OsS2) are trapped away from the cores. Textural relations indicate primary orthomagmatic nature of laurite and Or-Ir-Ru alloys and their precipitation prior to or concomitant with chromite from an S-undersaturated melt at ~1200–1300°C and log fS2 varying between −2 and −1.3. Ru-Os disulfide and erlichmanite crystallized subsequently with a gradual decrease in temperature in the differentiating ultramafic magma. Precipitation of irarsite and base-metal sulfides can be attributed to local and rapid fluctuations of temperature, fS2, and fAs during the formation of the chromitites due to influx of fresh batch of melts or crystallization in an open magmatic system. The compositional variations of laurite-erlichmanite record heterogeneous physicochemical environment during their crystallization. In addition to zoning, preservation of IPGMs in the CAC chromites points towards fast cooling of the magma possibly associated with rapid exhumation of the chromitites and their ultramafic hosts from the roots of the subcontinental lithospheric mantle (SCLM) towards their final emplacement into the crust. Occurrence of the CAC along a linear extension with the Kondapalli Ultramafic Complex and deformed remnants of ophiolites (Kandra and Kanigiri complexes) along the eastern margin of the Eastern Dharwar Craton (EDC) bordering the Eastern Ghats Belt (EGB) indicates this zone to be a Proterozoic convergent margin.

Multi-scale analysis of shallow-water carbonate depositional geometries from a reservoir perspective: Insights from outcrop (Middle Jurassic, Portugal) and subsurface (Lower Cretaceous, U.A.E.) contrasting case studies

Outcrop observations offer valuable insights into depositional architecture as a primary control on facies and reservoir properties variability, in contrast to subsurface cases, where observation points are limited to well locations. Two dominantly high-energy shallow-water carbonate case studies were analysed, including outcrops of a Bathonian-Callovian (Middle Jurassic) carbonate succession in the Maciço Calcário Estremenho (MCE) region, central Lusitanian Basin, Portugal and a Barremian (Lower Cretaceous) subsurface carbonate reservoir from Abu Dhabi (U.A.E.). The MCE succession comprises a range of inner ramp deposits, which consist of oobiointraclastic grainstones, biointraclastic grainstone-rudstones and coral/algal boundstones, mostly defining sandbody and coral biostrome units which represent potential reservoir facies, as suggested by their textural types and locally significant porosity, though now mostly occluded by calcite cements. The succession is exposed in recently-cut quarry faces showing minimal weathering, hence allowing for detailed observations on depositional geometries. The U.A.E. case study is a world-class reservoir characterized by mid-ramp to high-energy inner-ramp facies, ranging from biomicritic wackestones to biointraclastic packstones, bioclastic packstones, grainstones and rudstone-grainstones, interpreted using core and thin section data from four onshore wells.In the analysed outcrops, strong continuity of main major bedding surfaces is observed at larger scales (dozens to hundreds of metres), with strong spatial variability seen at smaller centimetre to metre scales and depositional geometries varying from tabular, wedge-like, lensoid bodies and coral buildups. The subsurface case also shows strong continuity of larger-scale depositional packages (at kilometre scale) and moderate to strong small-scale heterogeneities at centimetre to metre scales. Discontinuity-bounded intervals are observed in the younger section of this reservoir generally showing stronger vuggy porosity and coarser skeletal grains at the base, while the youngest set of these intervals are characterized by an abundance of rudist fragments. The interpreted depositional geometries in this reservoir consist of tabular, wedge-like and lenticular geobodies of limited lateral extension.The multi-scale heterogeneity in both these case studies is strongly influenced by depositional factors related to hydrodynamics and the balance between carbonate factory productivity and accommodation space, affecting lateral facies migration and creating complex depositional patterns. This study illustrates how the perception of heterogeneity, though reflecting an intrinsic property of carbonate rocks, is influenced by the scale of observation and, from a reservoir perspective, both cases show that addressing small-scale heterogeneities and associated geological factors in higher detail is vital in defining conceptual geological models for a better understanding of the reservoir.

Sikkimite: A New and Rare Suite of Potassic-perpotassic Syenites from South Sikkim, Lesser Himalaya, India

Abstract A new and rare suite of radioactive, plutonic-hypabyssal, nonperalkaline to peralkaline, potassic to perpotassic syenites have been reported as dyke-like and/or lensoid bodies (c. 100 m x 20 m to 5 m x 2 m) within the Buxa and Daling formations (Proterozoic) and Gondwana Group of rocks (Permo-Carboniferous) within the Ranjit tectonic window (RTW) and its peripheral parts in south Sikkim. These are christened in this work as ‘Sikkimites’ considering their unique field occurrences, petrography, mineralogy and geochemistry, unlike any known saturated potassic - perpotassic syenites from the plutonic environment from India or elsewhere. The modal (vol.%) mineralogical variations observed within the potassic syenites include orthoclase (57-65%) with subordinate to minor amounts of brown alkali amphibole (kataphorite, 3-32%), aegirine (1-20%), biotite (13% in one sample), opaque minerals (ilmenite and magnetite, 4.2-24.5%), quartz (traces to 12.9%) and minor to trace amounts (< 3%) of apatite, sphene, zircon, and monazite. Compositionally, the Sikkimite suite range from non-peralkaline to highly peralkaline types (with Mol. Na2O + K2O/Al2O3 varying from<1 to1.2 to 1.5), mildly-perpotassic (Mol. K2O/Al2O3>0.83 to 0.96) and perpotassic (Mol. K2O/Al2O3>1 and Wt.% K2O/Al2O3 = 2.52-23.9) and Wt.% K2O/Na2O(3.05-23.9). The potassic syenites shows wide variations in major, minor and trace elements with significant enrichments in TiO2, Ba, REE, Y, Zr and Nb. The petrogenetic model for these rare potassic syenite suite of rocks (termed here as Sikkimite) invokes a near-primary origin by partial melting of enriched, amphibole- and phlogopite-bearing heterogeneous mantle, having chemical signatures of both non-subduction and subduction-related metasomatism. Ar40-Ar39 dates on them indicate a Cretaceous age (100-90 Ma) suggesting their links to either the widespread igneous manifestation of the Kerguelen or the Marion plume of the Indian Ocean.. These rare potassic syenites were apparently brought to the near surface environments in south Sikkim due to the collision of Greater India with Asia and the ensuing Himalayan thrusts from the north (MCT1 and MCT2) which shifted the crystalline basement and the Gondwana rocks maximum to the south, unlike any other parts of the Himalaya.

Investigation of the Geochemical Composition and Paleo-Depositional Environment of Ubo and Ikpeshi Marble Deposit, Southwestern Nigeria: A Comparative Study

The marble deposits at Ubo and Ikpeshi areas of Edo state, Southwestern Nigeria, were studied in order to determine the major elements and the paleo-depositional environment of the original sediments using standard methods. Results obtained using test of difference between Ubo and Ikpeshi marbles showed that CaO (51.977±0.922 & 54.726±0.23), MgO (3.034±0.829 & 0.499±0.115), Na2O (1.7±0.73 & 0.024±0.008), MgCO3 (6.337±1.734 & 1.034±0.238), Cu (24.589±0.692 & 27.447±0.711), Ni (23.907±0.854 & 30.979±0.494), all for Ubo and Ikpeshi respectively; with Ni showing highest significance with P<0.01. The Ubo marble deposit occurs as a lensoid body within the younger metasedimentary sequence. The major element composition reveal a mean chemical composition of CaO (51.97 and 54.73 %), MgO (3.0 and 0.49 %), SiO2 (0.74 and 0.70%), K2O (0.08 and 0.04%), Na2O (1.70 and 0.02%), Al2O3 (0.75 and 0.25%), Fe2O3 (0.34 and 0.25%), and Loss on Ignition - L.O.I (43.34 and 49.31%) in Ubo and Ikpeshi marbles respectively, which is indicative that the marble samples were all calcitic. The low values of the total alkali content in the marble samples from the two locations indicate that the environment of deposition of the original carbonate materials that metamorphosed into marbles from both locations must have been a shallow, highly saline environment with probably little influx of salty brine water in the basin. Silica was used as an abscissa in these plots because it shows substantial variations among the marbles with most of the linear relationship between silica and the various oxides showing negative correlation, this probably reflects the admixture of the carbonates with chert. The trend of the plots of Na2O + K2O vs. SiO2 for the marbles from both locations show a variation in the salinity.
 Keywords: Ubo, Ikpeshi, Marble, Marine Environment, Metasedimentary

Mineral Chemistry and Reaction Textures of Calc-silicate Rocks of the Lunavada Region, SAMB, NE Gujarat

Abstract Calc-silicate rocks occurring in and around Lunavada town belong to the Kadana Formation of the Lunavada Group of the Aravalli Supergroup. These rocks are embedded within pelitic schists in the form of sporadically distributed lensoidal bodies and are surrounded by quartzitic ridges. Rocks of the Kadana Formation and its surrounding area have experienced intrusive event named as ‘Godhra granite’ which occupy the SW part of this area. Some prominent field characteristics shown by these calc-silicates include fine to medium grained size, dark grey colour, unoriented/star shaped amphibole needles and maculose structure. These rocks have contact metamorphic textural features and the typical mineral assemblage viz., Act +Di + Qtz + Ttn+ Cal ± Mc ± Bt ± Pl ± Ep ± Scp ± Chl with minor proportion of apatite, zircon and opaques. EPMA studies revealed that the Ca-amphibole composition of these rocks ranges from magnesio-hornblende to actinolite whereas the clinopyroxene is salitic to diopsidic and the mica is found to be phlogopitic biotite. Certain prograde and retrograde reactions textures present within these rocks have been interpreted, for e.g. the development of diopside (salite) from ankeritic composition which is then retrogressed and actinolite appeared, prograde reaction leading to the formation of titanite from ilmenite and calcite and breakdown of scapolite into calcite and quartz with plagioclase lacking co-existence with these scapolites, indicating retrogression. Mineral assemblage and mineral chemistry data interpretation points towards the calcareous sandstone or marl as a probable protolith having impure calcareous composition, moreover, field characteristics and reaction textures observed give indication that the protolith might had passed through the contact metamorphic event to give rise to the present day calc-silicates.

Internal Stratigraphy of the Mesoarchean Keonjhar Siliciclastics, Singhbhum Craton, Eastern India: Paleogeographic Implications.

Abstract In comparison to the greenstone belt volcano-sedimentary successions, mature arenite dominated sedimentary deposits are very rarely preserved in Paleo-Mesoarchean rock record. Such arenitic deposits may be important for the recognition of extensive development of continental freeboard and onset of Phanerozoic style plate tectonics. The Keonjhar siliciclastics are ~715-m thick mature arenite-dominated Mesoarchean succession that unconformably overlies Paleo-Mesoarchean granite-greenstone core of the Singhbhum craton. The succession is mapped on 1:50,000 scale, and is designated as the Keonjhar Quartzite. It is subdivided into conglomerate dominated lower unit as the Asurkhol Member and mature arenite dominated upper unit as the Kamalpur Member. A lensoid body of iron ore clast bearing conglomerate unconformably overlies the Keonjhar Quartzite, and is designated here as the Chamakpur Conglomerate. The succession is repeated by three westerly dipping reverse faults that have upthrown the deeper parts of the basinal transects against shallower parts. The Asurkhol Member represents subaerial part of a coastal alluvial fan. The Kamalpur Member represents shallow-shelf depositional setting. Detailed mapping and recognition of the fault slices led to the reconstruction of the paleogeographic setting. The present study also highlights the importance for the stratigraphic correlation of the Keonjhar Quartzite with underlying metabasics of Malangtaoli- Jagannathpur lava and the overlying western IOG succession. The internal stratigraphic classification proposed here is also significant from economic point of view with regard to the occurrences of uraniferous QPCs in the Asurkhol Member and detrital iron ore deposits in the Chamakpur Conglomerate.

Geochemistry of calc-silicate rocks around Lunavada region, NE Gujarat: Implications for their protolith, provenance and tectonic setting

In this work, the calc-silicate rocks affiliated to the ‘Kadana Formation’, a youngest formation of the Lunavada Group have been investigated. These rocks are found to be embedded within associated rock types, viz., quartzites and metapelites in the form of isolated lensoidal bodies. Contact metamorphic textures and the typical mineral assemblage, viz., Act +Di + Cal + Qtz + Ttn ± Mc ± Pl ± Bt ± Ep ± Scp ± Chl with minor proportion of apatite, zircon and opaques can be observed in these calc-silicates. Major oxides, trace and rare earth elements were analysed to investigate the protolith type, provenance and tectonic setting of these rocks. Protolith must be calcareous sandstone with varied proportion of clay and deposited in shallow water environment as revealed by CaO, Al2O3 and FeO+MgO and Al–Zr–Ti ternary diagram, respectively. Low to moderate weathering of source rocks has been indicated by A–CN–K ternary diagram. Th/Sc vs. Zr/Sc and Th/Co vs. La/Sc plots confirm the continental source with felsic nature for these rocks and the calc-silicate samples fall within the active continental margin region of Sc–Th–Zr/10 diagram which also justify the kind of provenance for primary sediments.

Mesoarchaean (2820 Ma) high-pressure mafic granulite at Uauá, São Francisco Craton, Brazil, and its potential significance for the assembly of Archaean supercratons

High pressure (HP) granulites of regional scale form as a result of tectonic events that lead to crustal thickening or subduction of the crust into the mantle. Most HP granulites are Phanerozoic, a few are Proterozoic, and Archaean HP granulites are even scarcer. Here we present field relationships, mineral chemistry and zircon U-Pb ages, Hf isotope data and trace elements data for the mafic granulite and associated rocks of the Uauá terrane, São Francisco Craton, Brazil, as evidence for the likely existence of a thick continental crust in the Mesoarchaean/Neoarchaean transition. The HP mafic granulite occurs as lensoid bodies within shallow dipping diorite to leucodiorite gneisses. Small scale layering between mafic granulite and diorite gneiss indicate these rocks are cogenetic. Garnet-clinopyroxene pairs with quartz, zircon, ilmenite, plagioclase, and clinopyroxene inclusions in garnet characterize the HP assemblage. Garnet porphyroblasts also show opx-cpx-plag symplectite coronas, which coupled with hornblende and plagioclase define PT conditions to lower grade granulite and amphibolite facies. Microprobe data combined with phase equilibria modelling (pseudosections) indicate 16–18 kbar and 930–960 °C for the peak HP assemblage, and 6.2–7.0 kbar and 660–760 °C for lower pressure granulite to amphibolite facies symplectite coronas. Metamorphic zircon rims in equilibrium with garnet have SHRIMP ages of 2819 ± 14, and igneous zircon cores of 3127 ± 14 Ma. The cores of zircon in associated gneiss samples have U-Pb ages between 3090 ± 13 Ma (LA-ICP-MS) and 3125 ± 15 Ma (SHRIMP) with age cluster at 3120 ± 6 Ma. εHf(t) values on igneous zircon of the HP mafic granulite are slightly positive whereas metamorphic zircon rims are negative. The associated diorite gneiss invariably yielded negative zircon εHf(t) values. Trondhjemite sheets intrusive in the mafic granulite are ca. 20 m.y. younger (2794 ± 13 Ma) than the host granulite. We interpret the igneous protoliths of the mafic granulite and leucodiorite gneiss as a single igneous complex emplaced in the continental crust, later deformed and metamorphosed by contraction and crustal thickening during collision of blocks/cratons to form Earth's first supercratons by the end of the Mesoarchaean.

Gravity and Magnetic Survey in Southwestern Part of Cuddapah Basin, India and its Implication for Shallow Crustal Architecture and Mineralization

ABSTRACT Gravity and magnetic surveys were carried out in southwestern part of Cuddapah basin (CB) covering an area of ~3660 km2. Southwestern part of CB gained lot of attention after discovery of second most important uranium province in India. Some non-metallic industrial minerals and base metal occurrence are also reported in the study area. Present study with high spatial resolution gravity-magnetic survey aims to decipher detail basin geometry, nature of sediments, along with possible mineral deposits in SW part of CB. The gravity survey comprising short and long wavelength anomalies brought out sedimentary characteristics and basement architecture underneath the sediments. The long wavelength features of the gravity map shows gneissic basement, which is characterized by both basic and acid magmatic intrusive. Residual gravity anomaly map shows good correspondence with the exposed high density basaltic rock units and also brought out occurrence of concealed high density litho-units, which have significance for mineral prospecting. The magnetic map shows that both sediments and underneath basement are non-magnetic in nature except SW part of the study area, where study suggests occurrence of concealed mafic lensoid body. Euler solutions and combined gravity-magnetic modelling further facilitated for understanding of structural feature and basement geometry. Based on the integrated gravity and magnetic study mineral prospecting zones have been delineated for further detailed study.

Metacarbonatite rocks from Amesmessa area (In Ouzzal Terrane), Hoggar shield, Algeria

The Hoggar Shield (South Algeria) belongs to the 3,000 km-long Pan-African Trans-Saharan Belt which formed in the Neoproterozoic and Cambrian, between 750 and 545 Ma. At the western part of this shield, carbonate rocks crop out to the north and south of Amesmessa gold deposit. They occur as lensoid bodies and dikes cross-cutting Archean granulitic rocks, and are consistently associated with mafic rocks and syenite. The petrogenesis of the Amesmessa metacarbonate rocks, which have previously been interpreted as marbles of Archean age, are reconsidered in the present study. Within the Amesmessa area, a small volume of carbonate rocks consisting of calcite, biotite, clinopyroxene, hornblende, olivine and apatite, and are spatially associated with ultramafic rocks, are surrounded by granosyenite, pyroxenite and rhyolite. The cross-cutting relationship and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Archean rocks. The average total REE concentration in the metacarbonate is up to 5,700 ppm. However the associated rocks; syenite and pyroxenite contain only 60 and 12 ppm, respectively. Sr concentration is also high (>1,390 ppm) which is normally high in carbonatitic rocks and low in limestones. On the basis of available chemical analyses and outcrop evidence, it becomes clear that the Amesmessa carbonate rocks consist of metamorphosed carbonatite.

Anticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: Geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block

In the Migmatite Complex from NE Sardinia, a large lensoid body of coarse-grained, dark-green amphibolite with a schistose to weakly massive aspect crops out. Within this amphibolite centimetre-sized layers locally occur which contain millimetric porphyroblastic garnet. We investigated the amphibolite and the layers applying microstructural analyses and thermodynamic modelling in the NCKFMASH+Ti + Mn system in order to reconstruct the pressure-temperature (P-T) metamorphic evolution. The amphibolite underwent a burial path, recorded by the compositional zoning of garnet, that started at pressures of 0.8 GPa and showed only a slight increase in temperature leading to peak P-T conditions. The garnet rim records peak P-T conditions of 1.3–1.4 GPa at 690–740 °C. As the early exhumation of the amphibolites occurred already at lower temperatures than the burial, an anticlockwise P-T path results which is in contrast to the typical clockwise P-T paths reported for several high-pressure metamorphic rocks from NE Sardinia. We interpret the anti-clockwise path by the location of the studied rocks in the lowermost part of the upper plate and their burial to depths of around 45 km during the Variscan continental collision between Laurussia and Gondwana. This process could have affected some rock slices of the upper plate only owing to tectonic erosion by the downgoing plate. The subsequent uplift occurred in an exhumation channel where these slices were continuously cooled by the upper portion of the lower continental plate.

Contrasting Textural and Chemical Signatures of Chromitites in the Mesoarchaean Ulamertoq Peridotite Body, Southern West Greenland

Peridotites occur as lensoid bodies within the Mesoarchaean orthogneiss in the Akia terrane of Southern West Greenland. The Ulamertoq peridotite body is the largest of these peridotites hosted within the regional orthogneiss. It consists mainly of olivine, orthopyroxene, and amphibole-rich ultramafic rocks exhibiting metamorphic textural and chemical features. Chromitite layers from different localities in Ulamertoq show contrasting characteristics. In one locality, zoned chromites are hosted in orthopyroxene-amphibole peridotites. Compositional zonation in chromites is evident with decreasing Cr and Fe content from core to rim, while Al and Mg increase. Homogeneous chromites from another locality are fairly uniform and Fe-rich. The mineral chemistry of the major and accessory phases shows metamorphic signatures. Inferred temperature conditions suggest that the zoned chromites, homogeneous chromites, and their hosts are equilibrated at different metamorphic conditions. In this paper, various mechanisms during the cumulus to subsolidus stages are explored in order to understand the origin of the two contrasting types of chromites.

The formation of giant clastic extrusions at the end of the Messinian Salinity Crisis

This paper documents the discovery of five multi-km scale lensoid bodies that directly overlie the upper surface of the thick (>1 km) Messinian Evaporite sequence. They were identified through the analysis of 3D seismic data from the western Nile Cone. The convergence of the upper and lower bounding reflections of these lensoid bodies, their external and internal reflection configuration, the positive ‘depositional’ relief at their upper surface, and the stratal relationship with underlying and overlying deposits supports the interpretation that these are giant clastic extrusions. The interpretations combined with the stratal position of these clastic extrusions demonstrate a prior unsuspected link between periods of major environment change and basin hydrodynamics on a plate scale.All five lensoid bodies were extruded onto a single, seismically resolvable marker horizon correlatable with the end of the Messinian Salinity Crisis (Horizon M). It is argued that the source of these clastic extrusions is pre-Messinian in origin, which implies massive sediment remobilisation at depth in the pre-evaporitic succession and intrusion through the thick evaporite layer. We propose that the scale and timing of this dramatic event was primed and triggered by near-lithostatic overpressure in the pre-evaporitic sediments generated through (1) their rapid burial and loading during the Messinian Salinity Crisis and (2) catastrophic re-flooding during its immediate aftermath.The largest of these clastic extrusions has a volume of over c. 116 km3, making it amongst the largest extruded sedimentary bodies described on Earth. The findings extend the understanding of the upper scale of other analogous clastic extrusions such as mud volcanoes and sediment-hosted hydrothermal systems. Following the 2006 eruption of the Lusi sediment-hosted hydrothermal system in Indonesia, an understanding of the upper scale limit of clastic extrusions has even greater societal relevance, in order to increase awareness of the risk posed by the potential size and longevity of future giant clastic extrusions.

Chapter 16 Natural resources

Abstract The natural resources of the Andaman–Nicobar Islands and surrounding seas are discussed. Rocks and minerals found on the Andaman Islands include magnetite, pyrite-chalcopyrite, chromite, gold, nickel, sulphur, coal and limestone, but in most cases their concentrations are sub-economic. Lensoid bodies of algal-biohermal limestones of Palaeogene age on Middle Andaman Island are abundant, accessible and of good grade for cement or foundry work. The Neogene limestones are also suitable for this purpose. Basalts and strongly indurated sedimentary rocks, without any overburden, are quarried for construction work. Limited offshore exploration for oil and natural gas shows great promise, but has yet to be developed.

Geophysical, Geochemical and Structural Features of Chromite Ore in Kuradawe Village/ Mawat Ophiolite Complex, Kurdistan Region, NE Iraq.

Small lensoidal bodies of massive and disseminated chromitites have been examined inassociation with ultramafic rocks of the Mawat Ophiolite Complex (MOC), Iraqi ZagrosThrust Zone, Northeastern Iraq. The Chromites ore have variable thickness enveloped bydunite and of, exhibiting transitional boundaries to harzburgitic host rocks.The primary Chromite composition exhibits high Cr varieties; the average Cr# ofChromite is 0.714, with < 0.3 % TiO2 content, which may reflect the crystallization ofChromite from boninitic magma. The result of the gravity survey leads to lateraldelineation of the ore body and estimates an area of about 2800 m2. The thicknesses of different Chromite pods were estimated to be in the range of 10 – 35 m. Structural feature show that such ore bodies have internal structure making angle of 10–12 degrees with dunite wall and surrounding peridotite which indicates that Chromite ore subconcordant type.

Structure roles for the localization of metasomatite uranium deposit type at Wadi Belih area, Northern Eastern Desert, Egypt

Wadi (W.) Belih area is one of the most important uranium occurrences in the northern parts of Eastern Desert of Egypt. The major mineralizations are structurally controlled especially along the contact between the Hammamat sedimentary rocks and younger granites record a complex history of deformations where, the uranium mineralization are located along a footwall of shear zone striking ENE–WSW to NE–SW direction. This shear zone was reactivated during its tectonic history starting from compression trending NW–SE and NE–SW respectively to younger extensional event trending NW–SE.Due to the resulting younger extension NW–SE event the hydrothermal solution gradually migrates upward forming alkali metasomatite, contemporaneous with uranium mineralization. They are developed along that shear zone where structure contact and the low-stress regions in the vicinity of the shear zone are favorable locations for fluid flow focusing and hence U mineralizations occur in the highly fractured and mylonitized zones along the contact as lensoidal bodies.

Lithofacies palaeogeography and biostratigraphy of the lowermost horizons of the Middle Triassic Hallstatt Limestones (Argolis Peninsula, Greece)

Condensed ammonoid beds of the Hallstatt facies (Anisian–Ladinian) are widespread around the Ancient Theatre of Epidaurus, in the locality Theokafta of the Argolis Peninsula (eastern Peloponnesus). The Hallstatt Formation in Argolis appears, generally, in the form of lensoid bodies of variable sizes, inclination and direction and is always found overlying a formation consisting of keratophyric tuffs. In fact, the contact of the keratophyric tuffs with the overlying limestones, specifically evidenced by an in situ brecciated zone, is stratigraphic and constitutes the base of the Hallstatt Limestones. The contact of the Hallstatt Limestones with the overlying radiolarites is stratigraphic as well.Lithofacies and biostratigraphic research has focused on the lowermost horizons of the Hallstatt Limestones of Anisian age (average thickness about 1.30m), where a dense sampling has been performed, followed by detailed facies analysis. The lowermost horizons of the Hallstatt Limestones of Theokafta represent typical hiatus beds/concretions sensuWetzel and Allia (2000), characterized by discontinuous sedimentation and erosion. They consist of red ammonoid-bearing hemipelagic limestones with calcium carbonate nodules floating in an enriched Fe-oxides matrix with dispersed lensoid/prismatic calcium carbonate crystals. This part of the section is characterized by condensed sedimentation, due to significant lowering of the rate of sedimentation and includes omission surfaces, firmgrounds and hardgrounds along certain horizons. Nine lithostratigraphic units have been distinguished in the lowermost horizons of the Hallstatt Limestones, including radiolarian packstones, volcaniclastic facies, packstones/floatstones with ammonoids and lag deposits.Tselepidis (2007) defined nine distinct ammonoid biozones from the Anisian to Ladinian, documenting deposition of the Hallstatt facies during a low depositional rate over nearly 5 million years (using the timescale of Gradstein et al., 2004). The biozones: Japonites/Paracrochordiceras, Hollandites, Procladiscites/Leiophyllites, zoldianus, trinidosus, Reitziites/Parakellnerites and the Nevadites (Anisian) and the biozone curionii (Lower Ladinian). Although sedimentation was very condensed, it didn’t reach the level of mixing fauna.Synsedimentary and early burrowing processes differentiated the primary texture characteristics of the deposited sediments. Multiphase diagenesis occurred not very deep below the sediment surface and includes boring and/or encrustation, burial and cementation. The deposition of the studied Hallstatt Limestones is considered to be due to anaerobic oxidation of organic matter, which provided excess alkalinity, inducing carbonate precipitation. Sedimentation took place on differentially-subsided deep swells. After drowning, the swells were covered by pelagic carbonate deposits. Further slight rotation of blocks, along listric faults, may have led to additional differential subsidence of the blocks. Shelf bathymetry and third-order sea-level changes played a significant role in the formation of the Hallstatt beds. In terms of sequence stratigraphy, the studied hiatus concretions and beds are considered genetically linked to rising or high sea-level, formed at the initiation of transgressions, as well as during the time of maximum rate of transgression, in areas where the sediment input was strongly reduced (“condensed section”).Taking into consideration the present location of the Hallstatt Formation, in the context of the Hellenides, an area suitable for the deposition of the Hallstatt Limestones, should be located between the sub-Pelagonian (western part of the Pelagonian zone) and Pindos geotectonic zones, which during the Triassic corresponded to a platform slope and a deep ocean, respectively. The widespread Middle Triassic Han Bulog Limestones (ammonoid-bearing pelagic limestones) from Triassic successions of the Eastern Alps (Dinarides, Hellenides) may have formed partly in similar slope environments.

A catalogue of Danian gastropods from the Baunekule facies, Faxe Formation, Denmark

This catalogue of 194 gastropod taxa is based on the collection of Danian gastropods from the Baunekule facies, Faxe Formation in eastern Denmark. The gastropod fauna is extremely rich and well preserved. Most of the gastropods (106 species) are referred to genus level only, 9 morphotypes to even higher taxonomical levels and 79 gastropods are referred to species level. The gastropods are classified following Bouchet & Rocroi (2005) as 4 different clades: Vetigastropoda (represented by 26 species and 10 superfamilies), Caenogastropoda (represented by 142 species and 17 superfamilies), Heterobranchia (represented by 23 species and 5 superfamilies) and Opisthobranchia (represented by 1 species and 1 superfamily). The new species Zaclys? nuetzeli n. sp. is introduced.
 The Faxe Formation is recognised as a cold-water coral ecosystem with interfingering smaller bryozoan mounds. The Baunekule facies is found in the upper part of the coral mound complex of the Faxe Formation, where it forms isolated lensoidal bodies in the flanks of some of the coral mounds. It is characterised by a high diversity invertebrate fauna that occurs in weakly consolidated coraldominated floatstone to rudstone. The diagenesis of the Baunekule facies is of special significance because a high proportion of the originally aragonite-shelled fauna is preserved by recrystallization to calcite during early burial diagenesis. Most of the gastropods are not known from other parts of the Faxe Fm. The fauna is very important for comparative evolutionary studies of fossil and modern gastropods on cold-water coral mounds. Many of the genera have not previously been recorded from Danian strata. None of the gastropod species found in the Baunekule facies are known for certain to range below the Cretaceous–Palaeogene boundary. The fauna is comparable to gastropods found on modern cold-water coral mounds in the North Atlantic.
 The gastropod fauna from the Baunekule facies is characterised by a very high diversity of rather small millimetre-sized gastropods with a preference for hard substrates; 63.9% of the taxa belong to the browsing carnivore trophic group, feeding mainly on sedentary animals. Surprisingly, the fauna contains some common occurrences of typically warm-water species. The fauna consists mostly of Cenozoic genera and up to 87% of the species may be endemic to the cold-water coral ecosystem of the Faxe Formation. The diverse and rather unusual gastropod fauna from the Baunekule facies is undoubtedly linked to the evolution of cold-water coral ecosystems.