Precambrian Metamorphic Terrain Research Articles

Filtering of hydraulically significant lineaments from lineament map of precambrian metamorphic terrain in NE India using set theory

Identification of hydraulically potential lineaments directly from lineament map required for studies of groundwater resource development in crystalline metamorphosed rock terrain, which is considered as a poor groundwater province area, is a big challenge. Lineament study from satellite image gives all available lineaments present in the location including the hydraulically potential lineaments. But there are no existing methods present at our disposal which can detect only the hydraulically potential lineaments. This study attempts to delineate the hydraulically significant lineaments those are actively promoting groundwater flow in Purulia district, West Bengal, NE India, where the exposed rock-types are porphyritic granite gneisses, quartzite, mica schist, garnetiferous-sillimanite-biotite schist and phyllite, using Set theory. For the study, standard FCC image setup has been used to view the satellite imagery on computer screen within Erdas Imagine software and lineaments are examined visually on-screen, without taking any print out of the map. The azimuth directions of the lineaments are diagrammed using GEOrient software. Azimuth rose diagrams of the trend of foliation planes and fracture planes, obtained from direct measurement at rock exposures, are compared with the lineament trends to yield hydraulically significant lineaments, concordant to the trends of hydraulically significant fractures. Hydraulically significant lineaments are filtered from the total lineament map by using the laws of set theory, like universal set, sub-set, union, intersection, subtraction and equality. Validation of the obtained result with ground truth reveals that intense deformation of brittle rocks, that allow fluid movement, have a significant role in the development of hydraulically significant lineaments. This study successfully distinguishes the hydraulically significant lineaments, exhibiting a directional concurrence to hydraulically significant fractures, implying that set theory can be successfully used to extract hydraulically significant lineaments directly from lineament map.

Radial anisotropy technique to target fractured aquifer in Precambrian Metamorphics: An exemplar from Eastern India

Prospecting of groundwater in sub-surface horizons is a major challenge in Precambrian metamorphic terrains, e.g. the Purulia district of West Bengal, providing little scope for locating groundwater. However, detailed study of the Fracture-Correlated Lineaments (FCL) in variably fractured metamorphic terrains can give vital hydrological clue in selecting sites for tube wells with promising discharge of groundwater. This is an attempt to identify the hydraulically significant FCL using twin approach of surface geological investigation and radial anisotropic method. An integrated approach was adopted based on study of bedrock fractures, weathered-zone profiles, hydrological data with use of radial anisotropy method by Georient and Surfer softwares; revealing directional dominance of hydraulically significant FCL along 100–140°, concordant to regional foliation-trend of granite gneiss, and also along 15–40°, perpendicular to regional foliation-trend of phyllite, mica schist and epidiorite. The litho-stratigraphic preference of hydraulically significant FCL was found maximum for phyllite (38%) followed by mica schist (26%), epidiorite (21%) and granite gneiss (15%). The FCL pattern shows concurrence with thick weathered mantle, and high groundwater productive zones. The findings validate the usefulness of radial anisotropy as an important technique for identification of hydraulically significant FCL, and encourage their applicability in groundwater development in case of Precambrian metamorphic terrains.

Lithostratigraphic contact – a significant site for hydrogeological investigation in crystalline fractured-rock terrains

Estimating the hydrogeologic control of fractured aquifers in hard crystalline and metamorphosed rocks is challenging due to complexity in the development of secondary porosity. The present study in the Precambrian metamorphic terrain in and around the Balarampur of Purulia district, West Bengal, India, aims to estimate the hydrogeologic significance of lithostratigraphic contacts using fracture characteristics obtained from surface bedrock exposures supported by hydrological data from the existing dugwells. This study involves the domain-wise analysis of the frequencies of fractures that control the fracture-porosity. The domain-wise study reveals higher fracture-frequencies adjacent to the lithostratigraphic contacts. The concurrence of lithostratigraphic contacts with the occurrences of high-discharging wells and also with the deep weathered zone in low-lying areas is clearly established, thus assigning the lithostratigraphic contact as hydrogeologically significant. An increase in frequencies of the fractures within the ‘influence zone’ of the lithocontact, is clearly visible. Among those fractures, particularly, which make the angle greater than the ‘limiting angle’ with the lithocontact are characterised by increased frequencies. However, brittle rocks like quartz biotite granite gneisses, phyllite and epidiorite show high porosity of fracture, within the ‘influence zone’ of the lithostratigraphic contact. Enhanced deepening of the weathered-zone at lower topographic region may perhaps be a plausible explanation for this increased fracture-porosity at lithocontact to assign it as a hydrogeologically significant transmissive zone within fractured rocks.

Study of fractures in Precambrian crystalline rocks using field technique in and around Balarampur, Purulia district, West Bengal, India

Location of recharge zone in Precambrian crystalline rock is still unclear. The present study attempts to perform a detailed analysis of the joints/fractures developed in a Precambrian metamorphic terrain in and around Balarampur in Purulia district of West Bengal, India using bedrock data. The analysis shows that the orientations of major fracture trends are variable along with varying lithological units and structural affinities. The application of lithology-based analysis technique identifies highly predominant fracture frequency and fracture aperture in mica schist and phyllite in the area. This property is not evident in the granite gneiss and epidiorite. The moderate to high fracture permeability value is also associated with the fractures occurring in the shear zone. Mica schist and phyllite associated with the shear zone may represent a permeable recharge zone in the region.

Analysis of lineament swarms in a Precambrian metamorphic rocks in India

Addressing the geologic significance of lineaments and their correlation with joints/fractures is still unclear. The present study attempts to analyse the lineament swarms developed in a Precambrian metamorphic terrain in India using both unfiltered and filtered techniques. The unfiltered analysis technique shows that the major lineament and fracture trends are oriented along EW and NS directions respectively, thus failing to provide any correlation between them. The application of domain-based filtering techniques identifies a highly predominant fracture-correlated lineaments in mica schist constituting the EW trending shear zone in the area. This correlation is not evident in the areas north and south of the shear zone, where the lineaments are consistently oriented along the foliation planes of the rocks and are designated as ‘foliation correlated’. The present analysis indicates that the fracture frequency and the strain history may have played significant roles for the formation of fracture-correlated lineaments in the metamorphic terrain.

Áreas Fontes dos Minerais Pesados e sua Distribuição sobre a Plataforma Continental Sul-brasileira, Uruguaia e Norte-argentina

The heavy mineral fraction of sedimentary rocks and sediments can provide important information about provenance in sedimentary basins studies because some minerals or group of minerals can be traced to a source rock or terrain of specific composition. This work is a provenance study based on heavy mineral analyses of samples collected on the Rio Grande do Sul (southern Brazil), Uruguay and north of Argentine continental shelf (between 29º30´ and 37º 30´ south latitude and 48º 30´ and 56º 30´ west longitude). The fine and very fine sand fractions (2-4 Φ) were selected for heavy mineral analysis employing bromoform (S.G.= 2.65). The quantitative analysis was made using a microscope for the mineral identification and the counting of 300 grains from each sample, in a total of 106 samples. Multivariate data analysis was applied for mineralogical analysis, principally the RQmode vector. Four principal assemblages were defined applying this technique revealing the main source as well as the dispersal patterns for the sediments. The first component was dominated by augite indicating a panpean-patagonic source. The second component was dominated by hornblende and hypersthene minerals, indicating a provenance from Precambrian terrains with basaltic influence. The third component was marked by the dominance of tourmaline, staurolite, epidote and kyanite reflecting a source from the Precambrian metamorphic terrains. Finally, the four components with apatite, zircon and sillimanite minerals were dispersed by the Rio de La Plata system.

Genesis and evolution of Eppawala carbonatites, Sri Lanka

Massive apatite-bearing carbonatites intruded into a Precambrian metamorphic terrain around Eppawala, North-central Sri Lanka, have been investigated using petrographic and geochemical methods. The geochemical study includes mineral compositions, whole rock elemental abundances, and Sr–Nd and C–O isotopes. The carbonatites are generally coarse grained, and are mineralogically and texturally layered. Calcite, dolomite, olivine, and apatite are the major mineral phases, whereas spinel, ilmenite, magnetite, amphiboles, and micas occur as minor phases. Major element compositions show variation from calciocarbonatite to magnesiocarbonatite. The calciocarbonatites are enriched in ΣREE relative to the magnesiocarbonatites, but all samples show broadly comparable REE patterns. Extreme depletion of Ni, Ti, Cs, Rb, Nb, Ta, Zr, and Hf are characteristic of the Eppawala carbonatites. Stable C–O isotope ratios vary from −1.9 to −3.5 δ 13C (PDB) and from 7.7 to 16.2 δ 18O (SMOW). Sr and Nd isotope data ( 87Sr/ 86Sr 0.7049–0.7052 and 143Nd/ 144Nd 0.5019–0.5020) are compatible with an enriched mantle source lying between EM-I and EM-II mantle components. The mineral chemistry and overall geochemical signatures are consistent with an igneous origin, in which primary carbonatite magma was derived from a low degree partial melting of carbonated eclogites.

Geothermal gradient and heat flow in the state of Rio de Janeiro

Results of geothermal studies carried out at 72 localities have been used in evaluation of temperature gradient and heat flow values of the upper crust in the state of Rio de Janeiro. The investigations included temperature logs in boreholes and wells, calculation of geothermal gradients, measurements of thermal conductivity and determination of heat flow density. In addition, estimates of temperature gradients and heat flow were also made for areas of thermo-mineral springs, based on the so-called geochemical methods. Analysis of these data sets, after incorporation of appropriate corrections (for the perturbing effects of drilling operations, topography and climate changes) has allowed for the first time a better understanding of the regional distribution of thermal gradients and heat flow within the study area. The results obtained indicate that geothermal gradient values are in the ranges of 14 to 26oC/km in Precambrian metamorphic terrain and 19 to 33oC/km in areas of Phanerozoic sedimentary basins. Most of the rock formations are characterized by thermal conductivity values varying from 2.2 to 3.6 Wm-1K-1. Consequently regionally averaged mean heat flow values are found to fall in the interval of 40 to 70 mW/m2. Computer generated contour maps reveal that geothermal gradients and heat flow are systematically high in the western compared to the eastern parts of the state of Rio de Janeiro. There are indications that this geothermal anomaly is probably associated with the belt of Tertiary alkaline intrusives, between Itatiaia and Cabo Frio. Residual heat of large scale magma intrusions in the later part of the Tertiary period may be one of the possible mechanisms responsible for this thermal anomaly.

Obtaining geologically meaningful 40Ar– 39Ar ages from altered biotite

Biotite is the most used 40Ar– 39Ar geochronometer yet two significant problems arise from Ar–Ar step-heating. Dating altered biotite can be problematic, producing disturbed age spectra that reflect 39Ar recoil. However, unaltered biotite can yield disturbed ages with apparently meaningful plateau ages as a result of mineral breakdown during stepped heating. Obtaining meaningful ages from such spectra is very difficult. Because alteration of biotite is common and widespread in nature, and because many biotites affected by alteration are nonetheless unique geologically and/or representative of key localities, the capability to obtain reliable ages from altered material is extremely important. In this study, the effects of alteration progress on biotite age spectra were tested using both IR laser step-heating and UV laser microprobe 40Ar– 39Ar dating techniques. Our aims were to extract geologically meaningful ages from altered biotite and to identify cases where the ages had been influenced by alteration. Three variably altered biotites from the Precambrian metamorphic terrain of southwestern Montana were selected for argon isotopic analysis. Sample A is an unaltered rock containing pristine biotite, sample B is a highly altered rock with chlorite and prehnite interlayers within biotite, and sample C contains biotite with only incipient alteration. For each sample, the biotite ages obtained with IR and UV laser techniques were compared and the validity of the apparent ages was assessed. IR step-heating analysis of biotite from sample A yielded a well-defined plateau of 1776±6 Ma. UV laser microprobe analysis of the same biotite yielded a concordant weighted mean age of 1771±8 Ma, based upon 65 spot analyses of eight grains that ranged in age from 1819±54 to 1722±62 Ma. In contrast, IR step-heating of biotite from sample B resulted in disturbed spectra, with two separate fragments yielding total gas ages of 1505±12 and 1540±16 Ma, whereas UV laser microprobe analysis yielded 14 spot ages ranging from 1806±72 to 1565±78 Ma. UV laser analysis of sample C produced apparent ages ranging from 1772±52 to 1359±200 Ma. Detailed analysis of age variations occurring perpendicular to (001) cleavage planes of biotite in sample B was accomplished by depth profiling using the UV laser microprobe. Depth profiling of single grains revealed considerable age variation among the biotite layers, with profiles 1 and 2 yielding apparent ages ranging from 1730±226 to 1511±186 Ma, and from 1741±290 to 956±230 Ma, respectively. In both samples B and C, younger apparent ages correspond to higher 36Ar/ 39Ar ratios. The youngest ages, therefore, are believed to have derived from altered layers, whereas the oldest ages are related to layers with relatively little or no alteration. In summary, incremental step-heating of altered biotite single grains yields 40Ar– 39Ar age spectra that are compromised by recoil and variable release patterns. However, extraction of small samples via the UV laser microprobe provides a simpler pattern of apparent ages, all younger than the “true” cooling age yet inversely correlated with atmospheric argon. This result suggests that true biotite ages can be recovered from areas of pristine material if they are sufficiently large to be ablated by the UV laser.

Geochemistry and Origin of Megacrystic Granitoid Rocks from Eastern Ghats Granulite Belt

The Eastern Ghats Granulite Belt (EGGB) forms part of a continuous Precambrian metamorphic terrain in Gondwana. It is characterised by widespread development of an Archaean khondalite suite of metasedimentary rocks, Archaean to Late-Proterozoic charnockites and Late Proterozoic anorthositic, granitic and syenitic emplacements. A 1900 Ma megacrystic granitoid suite, containing varying proportions of charnockites and granites, forms an important and widely distributed litho-unit in the central khondalite and eastern migmatite zones of the EGGB. It contains metasedimentary enclaves, megacrystic K-feldspar, quartz, plagioclase ovoids, biotite, garnet (porphyroblasts and coronas), apatite, zircon, ilmenite, magnetite, etc. Hypersthene is present in the charnockite phase. Monazite is present in some garnet-free granites. It is characterised by low Na 2O/K 2O ratios, high alumina saturation index, CaO, MgO, and ÝREE, negative correlation of TiO 2, Al 2O 3, Fe 2O 3 t, MgO, MnO, CaO, P 2O 5, Ba, Sr, Zr and V with SiO 2, positive correlation of K 2O, REE, Th and Rb with SiO 2, fractionated LREE, relatively flat HREE and negative Eu anomalies. The data suggest S-type nature of the suite. Fractionation of the granitic magma and local variations in pH 2O and fCO 2 caused the formation of megacrystic charnockites. Formation of the corona garnet is related to the reworking of the suite during late Proterozoic (ca. 1250 Ma) isothermal decompression associated with channelised CO 2-rich fluid flux along narrow shear zones.

Petrography and metamorphism of the lower Gondwana (Permian) coal of Rangit Valley, eastern Himalays, India

A triangular outlier of coal-bearing lower Gondwana rocks comprises a tectonic window within the Precambrian metamorphic terrain of the Rangit Valley, in the eastern Himalays of India. Due to a series of tectonic events which took place during the Mesozoic and Tertiary the succession has become reversed and the coal has undergone severe physical, chemical, microstructural and optical changes. Studies reveal that (1) the rank of the coal prior to tectonism was low, (2) the coal underwent oxidation in the peat forming stage and during subsequent orogenic stages but effect of oxidation on the properties of coal was insignificant, (3) the coal reached its present abnormally high rank mainly due to heating under tectonic pressure and (4) the heat which was generated due to tectonic pressure, affected all the coal seams of this belt.

Type and Distribution of Gold Deposits along the Northern Margin of the North China Craton, People's Republic of China

Gold deposits and prospects are widely distributed along the northern margin of the North China craton. Up to the present, about 900 individual gold deposits, which constitute one of the most important gold metallogenic belts and the second largest gold-producing base in China, have been discovered, explored, and mined. About one-third of China's endogenic gold deposits and 45% of its gold reserve come from this belt, particularly in the Early Precambrian metamorphic terrains. All these gold deposits can be classified into five principal types according to their host rocks: (1) gold-bearing quartz or albite-quartz veins hosted by Archean amphibolite, gneiss, and migmatite; (2) gold-quartz veins hosted by Proterozoic schist, slate, sandstone, and marble; (3) gold deposits hosted by or related to intrusive complexes, including (3a) calc-alkaline granitoids, (3b) alkaline syenite and monzonite, and (3c) ultramafic intrusions; (4) gold deposits occurring in Phanerozoic volcanics; and (5) a group containing placer gold. The tectonic settings, geological features, and temporal and spatial distribution of these different types of gold occurrences indicate that gold mineralization reflects to a great extent the evolution of tectonic environments at the northern margin of the North China craton throughout geological time. Type-1 and -2 deposits generally are hosted by Archean and Proterozoic volcano-sedimentary rocks with variable degrees of metamorphism. The ore-forming materials of these two categories were derived from their wall rocks and were enriched by late metamorphism and magmatism. Types 3 and 4 are recognized in different stages and in various intrusive rocks and volcanics that were formed during Hercynian and Mesozoic Yanshanian orogenies. Gold and base metals were deposited from aqueous fluids with relatively low CO2 content (<20 mol%) and low to moderate salinity (5 to 16 wt% NaCl equiv.) that were related to a mixed fluid of magmatic and meteoric water. Type 5 results from recent weathering of existing gold-bearing volcano-sedimentary, metamorphic, and intrusive rocks as well as gold ores or protores.

REE geochemical evolution and its significance of early precambrian metamorphic terrain, Wuyang, Henan

The supracrustal rocks of the Wuyang metamorphic terrain are divided into the Zhao ’anzhuang, Tieshanmiao and Yangshuwan Formations. These three Formations were dated at 3000–2550 Ma, 2550–2300 Ma and 2300–2200 Ma, respectively. ΣREE and La/Yb)n of the Zhao ’anzhuang Formation volcanic rocks are obviously higher than those of the Tieshanmiao Formation equivalents, suggesting a sedimentary gap (2550 Ma boundary) between these two formations. The Zhao ’anzhuang Formation is older than the Tieshanmiao Formation. The sediments of these two Formations show no obvious differences in REE and are generally characterized by low ΣREE and positive Eu anomalies. On the contrary, the sediments of the Yangshuwan Formation are characterized by high ΣREE and negative Eu anomalies. Detailed discussions demonstrate that the Yangshuwan Formation was deposited in an oxidizing environment whereas the other two formations were formed in a reducing environment. At the end of the evolution of the Tieshanmiao Formation about 2300 Ma ago, the sedimentary environment was transformed from reducing to oxidizing. On the basis of the SHAB (soft/hard acid and base) theory, an oxidation-reduction model for sedimentary REE evolution has been established. It is proposed that the mantle tends to become gradually depleted in REE, especially in LREE, and the indices ΣREE and La/Yb)n of mantle-derived volcanic rocks also tend to become lower and lower.

Gold-bearing iron duricrust in Central Brazil

In the Cuiabá region—State of Mato Grosso, Central Brazil—primary gold mineralization is hosted by two generations of quartz veins in Precambrian metamorphic terrains of the Cuiabá Group. Gold is mined from the veins and mainly from the eluvial horizons that cover the deeply altered basement. In the lodes gold occurs as small particles (less than 1 mm) associated with pyrite and contains up to 5% Ag. Larger particles and nuggets of almost pure gold are found in the iron duricrust which caps the upper levels of the weathering profile. It is difficult to determine the average grade of this kind of deposit but some prospects in the Cuiabá region produce up to 2 g gold per ton of ore. Lateritization is responsible for both the formation of the iron crust and the concentration of gold within the regolith. Under a tropical climate, the supergene alteration of phyllites of the Cuiabá Group has led to the formation of a weathering profile consisting typically of saprolite, mottled clay zone and duricrust, from bottom to top. The duricrust is directly derived form the in situ weathering of phyllites. Geochemical balance calculations indicate that in the transition from the saprolite to the duricrust lateritization has promoted a progressive loss of Si, Al and K, and more than 500% of absolute Fe enrichment. Gold underwent a supergene evolution related to the development of the weathering profile. In the saprolite and mottled clay zone, associated with quartz and oxidized sulfides, gold dissolves as demonstrated by corrosion features at the surface of the particles. The formation of secondary gold in the duricrust is indicated by the larger size of the nuggets, their higher fineness and the close relationship between gold and the neoformed iron oxy-hydroxides.

Use of Landsat multispectral scanner data for detection and reconnaissance mapping of iron oxide staining in mineral exploration, central East Greenland

Test area; color anomalous hydrothermal alteration associated with stockwork molybdenum mineralization. Tertiary Malmbjerg molybdenum occurrence. Interactive digital image processing system combined with computerized color plots. Special ratio and factor analysis techniques. The best results with ratio techniques are obtained when band 4/band 5 is displayed as cyan (red), band 5/band 7 as magenta (green), and band 6/band 7 as yellow (blue). A characteristic pink to orange color corresponds to areas of limonitic rocks. Using factor analysis, color composites displaying F2 as magenta (green), F3 as cyan (red), and F4 as yellow (blue) with a visually determined color stretch discriminates very effectively with yellow-orange to red-orange colors representing limonitic rocks. More than 50 significant rust zones located. Ratio and factor analysis techniques were applied to reconnaissance mapping of Precambrian metamorphic and Devonian volcanic terrains covering more than 30,000 km 2 of ice-free area in a province of 500 km long by 300 km wide, resulting in the localization of 38 major (surface area > 0.5 km 2 ) rust zones in Precambrian metamorphic terrain.--Modified journal abstract.

Chemical clues to the origin of basic fragments embedded in a basaltic suite of the precambrian metamorphic terrain of Singhbhum, Eastern India

Pyroclastic fragments of basaltic composition are obtained in a thick metabasaltic layer from the Singhbhum orogenic belt of Eastern India. Field relations indicate the basalts to be submarine lava flows. Chemically, the metabasalts represent an original dry basaltic magma, showing a differentiation trend of Fe-enrichment much like the abyssal tholeiites and the Skaergaard rocks. The fragments are sharply angular in shape and distinctly richer in FeO, MgO, and MnO relative to the enclosing matrices. It is shown that the fragments are least likely to represent solidified lavas. These are best explained as cumulates which were fragmented and erupted with the fractionated melts.

Modern sediments and sedimentary processes in Lake Rudolf (Lake Turkana) eastern Rift Valley, Kenya

ABSTRACTTertiary sediments around Lake Rudolf (now Lake Turkana) in the East African Rift Valley have yielded abundant palaeontological and palaeoanthropological remains. The present study provides a basis for interpreting the ancient lake environment and furthering our knowledge of rift valley lacustrine deposits.Bottom sediments in Lake Rudolf are fine‐grained (average 71% clay) well laminated and have montmorillonite, kaolinite and illite as the principal clay minerals. The sediments are relatively poor in silica (40–45%) but rich in Fe2O3 (10%). Both mineral proportions and chemical composition change systematically over the area of the lake and delineate four sedimentological provinces: (1) iron‐rich, silty kaolinitic muds (Omo Delta); (2) iron‐rich, fine‐grained montmorillonite muds (North Basin); (3) silty montmorillonite muds rich in Na2O and K2O (Central Delta); and (4) argillaceous calcite silts (South Basin). Omo Delta and North Basin sediments are derived from the volcanics of the Ethiopian plateau; the source of Central Delta sediments is the Precambrian metamorphic terrain of the rift valley margin; the South Basin has a restricted detrital input.The water in the lake is alkaline (pH 9.2) and moderately saline (TDS = 2500 p.p.m.). Comparisons with influent water from the Omo River indicate a 200‐fold concentration for the lake water. Models based on equilibrium between sediments and water column account for most of the non‐conservative chemical components in the lake water.Sedimentation rates are high (about 1 m per 1000 years) and the dominance of detrital sediments makes Lake Rudolf unusual in comparison with other closed‐basin lakes in the African Rift Valley although some similarities with ancient rift valley deposits are suggested.

Chemographic relationships in sapphirine-bearing rocks from Sonapahar, Assam, India

Silica-deficient rocks in which sapphirine coexists with cordierite, gedrite, chondrodite, spinel, corundum and phlogopite in various parageneses, occur in association with cordierite-garnet-sillimanitebearing granulitic gneisses in a Precambrian metamorphic terrain displaying amphibolite-granulite transition facies. Brucite coexists with spinel and corundum and its unusual stability in this environment has been attributed to its significant fluorine content. Kornerupine occurs as armoured relics within cordierite in one sample of the sapphirine-spinel-cordierite-phlogopite-bearing rock. Sapphirines (X Mg 0.77 to 0.92) plot near the 7∶9∶3 composition.X Mg varies in the order: chondrodite and brucite>gedrite>cordierite >phlogopite>sapphirine>spinel.

Genesis of the melt rocks at Tenoumer Crater, Mauritania

A common problem encountered at probable meteorite impact craters is the occurrence of associated melt rocks, presumably produced by the impact event, that cannot be matched chemically to the target rocks. A similar situation is found at Tenoumer crater, Mauritania. Chemical analyses of 11 melt rocks range from felsic country rock values to much more mafic compositions in two well-defined chemical sequences. Only one of the melt rocks is essentially isochemical with the adjacent granodioritic country rock. This specimen consists of a heterogeneous glassy to cryptocrystalline groundmass commonly containing shocked country rock fragments and lechatelierite. It is clearly an impactite. Conversely, some of the more mafic melt rocks display reasonably normal igneous textures and may show no shock features whatsoever. Without the certain knowledge that all these rocks had a common genesis, some of these mafic rocks could be mistaken for products of endogenous magmatic activity. Neither meteoritic contamination nor vapor fractionation can explain the chemical sequences exhibited by these melt rocks. They most likely result from the combination of felsic country rocks with various amounts of an amphibolite vein that appears to strike through the center of the crater. The hypothesis is advanced that the amphibolite was preferentially incorporated in the impact-generated melt because of favorable kinetic and viscosity characteristics. Since mafic segregations are common in Precambrian metamorphic terrains, this mechanism may also explain other similar occurrences. The clear-cut evidence from Tenoumer makes it difficult to deny an impact origin for certain craters solely on the basis of associated melt rocks that are not isochemical with the country rocks.

Variation in the Composition of Plagioclase and Epidote in some Metamorphic Rocks near Bancroft, Ontario

A Precambrian metamorphic terrain near Bancroft displays a variation in metamorphic grade, as indicated by the presence of the following metamorphic zones: garnet, staurolite–biotite, kyanite–biotite, sillimanite, and sillimanite–garnet–biotite.In rocks that contain plagioclase and epidote, there is a general increase in the anorthite content of plagioclase and a decrease in the Al content of epidote with increasing metamorphic grade. Plagioclase crystals are commonly zoned, becoming richer in anorthite toward the rims, and epidote crystals may also be zoned, becoming enriched in iron toward the rims. Various chemical reactions are proposed to account for these changes.In rocks that do not contain epidote, plagioclase is typically albitic in composition, regardless of metamorphic grade.In rocks containing microcline, some albite has evidently exsolved from the microcline, resulting in the development of sodic rims about plagioclase crystals and normal zoning in these crystals.The distribution of albite between plagioclase and microcline is in agreement with the results from other metamorphic terrains of similar grade. Observed irregularities in this distribution may be attributed to exsolution of albite from microcline.Mineralogical and chemical data from the study area provide an opportunity to propose various chemical reactions, some of which may closely approach those that have taken place within the rocks.