Gneiss Fragments Research Articles

Позднепалеолитический комплекс культурного горизонта 2Г стоянки Коврижка IV на р. Витим (Байкало-Патомское нагорье)

In 2016—2018 a living complex dated to ca. 18,6 kyr cal BP was excavated at Kovrizhka IV site, level 2G. This is an ocher colored area of 5.0 m in diameter, with a semi-circular hearth lined by four slabs, and a massive slab surrounded with large boulders and gneiss fragments. Slabs and boulders forming an arc were recorded on the periphery of the complex, and two pits (post-holes?) of 20 cm in diameter were found on the northern and southern outskirts. Based on use-wear analysis, 17 tools were identified as meat knives, scrapers and carvers for bone, antler and skins. Their number and composition indicate a short-term occupation. About 60% of artifacts in the collection are of quartz, hence it is assumed that access to pebble raw materials was seasonally limited. The three central stones in the hearth were heated to the temperature ranging from 250 to 380—440 °C. The source of ocher is hematite of two types: quartz-hematite association and rock with a significant content of calcite, chlorite, feldspar and mica. A comparison with the ore compositions of the known iron locations and archaeological sites showed that ocher was delivered from the north of Western and Eastern Baikal regions, situated more than 500 km away from Kovrizhka. The available evidence points to the existence of a winter dwelling. The closest analogies are known at the Transbaikalian sites of Studenoye–1 and 2, Ust-Menza–1 and 2, Sukhotino–4, etc.

Nb–Ta–Ti Oxides in Topaz Granites of the Geyersberg Granite Stock (Erzgebirge Mts., Germany)

Oxide minerals (Nb–Ta-rich rutile, columbite-group minerals and W-bearing ixiolite) represent the most common host for Nb, Ta and Ti in high-F, high-P2O5 Li-mica granites and related rocks from the Geyersberg granite stock in the Krušné Hory/Erzgebirge Mts. batholith. This body forms a pipe like granite stock composed of fine- to middle-grained, porphyritic to equigranular high-F, high-P2O5 Li-mica granites, which contain up to 6 vol. % of topaz. Intrusive breccia’s on the NW margin of the granite stock are composed of mica schists and muscovite gneiss fragments enclosed in fine-grained aplitic and also topaz- and Li-mica-bearing granite. Columbite group minerals occur usually as euhedral to subhedral grains that display irregular or patched zoning. These minerals are represented by columbite-(Fe) with Mn/(Mn + Fe) ratio ranging from 0.07 to 0.15. The rare Fe-rich W-bearing ixiolite occurs as small needle-like crystals. The ixiolite is Fe-rich with relatively low Mn/(Mn + Fe) and Ta/(Ta + Nb) values (0.10–0.15 and 0.06–0.20, respectively). Owing to the high W content (19.8–34.9 wt. % WO3, 0.11–0.20 apfu), the sum of Nb + Ta in the ixiolite does not exceed 0.43 apfu. The Ti content is 1.7–5.7 wt. % TiO2 and Sn content is relatively low (0.3–4.1 wt. % SnO2).

Shock-metamorphic features in amphiboles from the Xiuyan crater of China

Amphibole-bearing gneiss fragments are common in the impact breccias of the Xiuyan crater, China. Three kinds of amphibole-bearing gneiss fragments with different shock-metamorphic levels have been identified. Shock-metamorphic features of amphiboles in these gneisses were investigated in situ by optical microscope, electron microprobe, Raman spectroscopy, and X-ray diffraction. Amphiboles in the weakly shocked gneiss (shock pressure less than 10 GPa) basically remain intact. Amphiboles in the moderately shocked gneiss (shock pressure range between 35 and 45 GPa) show strong deformation, reduced optical interference color, and partial loss of OH−. In the strongly shocked gneiss (shock pressure above 50 GPa), amphiboles are completely melted and dendritic pyroxenes crystallize from the melt. The formation of dendritic pyroxenes shows nearly complete loss of water in the amphibole melt at shock-induced high temperature above 1,500 °C. The occurrence of both diopside and pigeonite dendrites crystallized in the same amphibole melt shows inhomogenous melt composition and rapid cooling of the melt.

High-pressure polymorph of TiO2-II from the Xiuyan crater of China

Abundant TiO2-II, a high-pressure polymorph of titanium dioxide, was found in the gneiss fragments of impact-produced breccias from the Xiuyan crater. Rutile in the gneiss was severely fragmented and fine-grained clasts less than 2 μm in size had been transformed to TiO2-II. Irregular thin layered TiO2-II is also observed in coarse-grained rutile fragments, where the TiO2-II layers distributes along fractures and cracks in rutile. About 30 percent of rutile in the gneiss had been transformed to TiO2-II. Fine grains of TiO2-II display light bluish grey to light yellow brown in plane-polarized reflected light. Crystallographic investigation shows that TiO2-II has an orthorhombic structure with space group Pbcn. The cell parameters are a=4.543(1) A, b=5.491(9) A and c=4.895(2) A. Its empirical formula calculated on the basis of two oxygen atoms can be written as (Ti0.985Fe0.008Nb0.006-Si0.003Zr0.001)1.003O2, or simply formula TiO2. According to the shock effects of quartz and feldspars, the peak shock pressure and post-shock temperature in the TiO2-II-bearing gneiss are estimated to be between 35 and 43 GPa and 300–900°C, respectively. The finding of TiO2-II in the shock-metamorphosed gneiss provides another mineral physics evidence for shock origin of the Xiuyan crater.

The Paleoproterozoic Waterberg Group, South Africa: Provenance and its relation to the timing of the Limpopo orogeny

Four Paleoproterozoic formations of the Waterberg Group in South Africa are composed of coarse clastic detritus derived from erosion of the Limpopo Belt. Timing of the Limpopo orogeny, an event involving the collision of the Kaapvaal and Zimbabwe cratons, has long been a contentious issue. The results of point counting, major and trace element geochemistry, and U–Pb detrital zircon geochronology indicate that the Waterberg sedimentary formations in the study area were primarily sourced by siliceous (rifted margin) sedimentary and minor mafic volcanic rocks of the Archean Beit Bridge Complex, Limpopo Central Zone.The volumetrically predominant beige/brown sandstones in the four studied formations are quartz-rich with average QFR ratios of 80:7:13 (Blouberg), 70:19:11 (Setlaole), 88:5:7 (Makgabeng), and 89:3:8 (Mogalakwena). Chert and arenite account for >90% of lithic fragments in all formations, with minor siliceous gneiss fragments. Although all formations are silica enriched, the Makgabeng dune samples produce extremely high SiO2 abundances (92–99wt%), which are attributed to the presence of silica cement and quartz within rock fragments. Geochemically, the stratigraphically highest Mogalakwena Formation is unique with elevated Ti and Zr values, and intra-formational differences in REE patterns; the latter feature is consistent with a mixed provenance. Volumetrically minor green and purple sandstones in the Waterberg formations contain the greatest Cr, Ni, Ti, and V abundances, which supports localized derivation from a mafic or ultramafic source. Chemical index of alteration (CIA) values range from 57 to 89, which could indicate significant chemical weathering of the source rocks, but a plot of Th/Sc versus Zr/Sc illustrates that the sandstones have undergone recycling, which was probably responsible for enrichments in Al2O3 relative to Na2O3, CaO and K2O. Combinations of well rounded and subangular quartz grains support a recycled origin. Detrital zircons in the lowermost Blouberg Formation produced a wide array of ages ranging from ca. 3379 to 2043Ma. The youngest peak at 2046Ma is also the largest, and represents the maximum age of deposition for the formation. Additional peaks at 3281Ma, 3330Ma, and 3379Ma are consistent with ages previously determined from the Beit Bridge Complex, whereas peaks at 2578Ma and 2649Ma coincide with ages determined from gneisses of the Limpopo Central Zone.Derivation of detritus from the Beit Bridge Complex is directly indicated by sedimentary and siliceous gneiss fragments in the sandstones, subrounded quartz grains suggestive of relatively short transport distances, green and purple sandstone drapes derived from mafic volcanic units, and paleocurrent patterns consistent with south to southwest flow directions. Therefore the timing of deposition of the Blouberg Formation (2046Ma) equates to the end stages of the Limpopo orogeny. This negates previous suggestions that the Limpopo orogeny occurred only during the Neoarchean.

Differences in growth and nutrient uptake from a coarse-soil substrate by ectomycorrhizal- and fungicide-treated Picea abies seedlings

In order to quantify the importance of ectomycorrhizal fungi on nutrient uptake from the coarse-soil fraction of a haplic Cambisol (alumic), a microcosm study that allowed for nutrient budgets was designed. Ectomycorrhizal- and fungicide-treated spruce seedlings were grown on isolated and cleaned gneiss fragments (6.3 mm > O > 2 mm) from 90 cm soil depth. The substrate was the only source of Ca, K and Mg. Ectomycorrhizal seedlings showed no signs of nutrient deficiencies and biomass increased significantly compared to initial seedling biomass. Ectomycorrhizal seedlings seemed well adapted to survive on the coarse-soil substrate and acquired Ca, K and Mg from the coarse-soil substrate. Gneiss fragments of the ectomycorrhizal treatment were covered by fine roots and ectomycorrhizal hyphae, as observed microscopically. Fungicide-treated seedlings’ root development was retarded, and shoot biomass only increased from stored nutrient reserves of the seedlings. The suppression of EcM colonization by the fungicide Topas® apparently caused a root growth inhibiting effect. Furthermore, the extinction of mycorrhiza initiated an increased nitrification and acidification and a consequent nutrient cation release triggered by nitrate in the drainage.

IBERIAN CERAMIC PRODUCTION FROM BASTI (BAZA, SPAIN): FIRST GEOCHEMICAL, MINERALOGICAL AND TEXTURAL CHARACTERIZATION

Ceramic fragments from the archaeological excavation of the Iberian–Roman city of Basti (Spain) were studied from a geochemical point of view and by applying a statistical tool to X‐ray fluorescence data to discover similarities between ceramic materials. The analysis of these samples was completed by performing a mineralogical analysis, textural observation, and by characterizing the porous system and the colour of the pieces. Our results enabled us to identify the source area of the clayey raw material in the surroundings of Basti and to estimate the firing temperature of the ceramics. Differences in the chemistry were confirmed by characteristics of the pastes and the mineralogical composition of the pieces. Some samples show black cores, which would suggest the presence of organic matter in the raw material and fast firing of the ceramics. The main types of temper were quartz grains and gneiss fragments, although carbonate grains were also identified. Our evidence suggests that most of the samples were well fired. New silicate phases were found to be present in several samples. The mercury intrusion porosimetry verified and confirmed the firing temperature of non‐carbonated samples. Colorimetry showed that the colour of the ceramics varied according to the amount of CaO that they contained.

Effects of weathering state of coarse‐soil fragments on tree‐seedling nutrient uptake

AbstractFine earth accumulated within the weathering fissures of the coarse‐soil fraction (particles > 2 mm), so called “stone‐protected fine earth”, can provide a high short‐term nutrient release by cation exchange. It is thus hypothesized that unweathered gneiss particles cannot provide plants with exchangeable‐cation nutrients and that biological weathering is needed to include silicate‐bound nutrients into biochemical cycles. In a microcosm experiment, ectomycorrhizal Norway spruce (Picea abies) seedlings were grown on either weathered or unweathered paragneiss coarse‐soil fragments under natural hydraulic and climatic boundary conditions. A nutrient solution containing N, P, and K was added, however Mg and Ca could only be taken up from the coarse‐soil substrate. Solutes in drainage were analyzed during the experiment; plant nutrient uptake was determined after the experiment ended. Solute dynamics depended on the weathering state of the substrates: unweathered gneiss showed high initial Mg and Ca fluxes that diminished strongly afterwards, whereas weathered gneiss showed a much more gradual and sustainable release of these cations. Patterns in dissolved organic C and sulfate drainage indicated that the internal pores of weathered gneiss fragments contained organic material most likely as a result of living spaces from microorganisms. Plant biomass did not differ between treatments, however Mg content was higher in seedlings grown on weathered gneiss. Nutrient budgets demonstrated that the “stonesphere” of weathered gneiss can act as a quasi‐constant nutrient source whereas unweathered gneiss only provided high initial nutrients fluxes. In nutrient‐depleted, acidified fine‐earth environments, the coarse‐soil fraction may therefore act as a retreat for nutrient‐adsorbing tissues and as a buffer for nutrient shortages.

The Skaergaard Layered Series, Part VII: Sr and Nd Isotopes

The initial isotopic ratios of strontium and neodymium in the Skaergaard Layered Series vary both vertically and laterally, on every scale from the intrusion as a whole down to coexisting minerals in a single rock. The magma that filled the Skaergaard chamber was contaminated to various degrees with the metamorphic rocks through which it rose and was never completely homogenized after being intruded. The contamination was most pronounced in contact zones and aureoles around rare xenoliths. The greater concentrations of lithophile trace elements in the Upper Border Series was previously attributed to assimilation of buoyant fragments of gneiss that collected under the roof, but most of the rocks of the Upper Border Series are isotopically indistinguishable from those of the Layered Series. It is doubtful, therefore, that this part of the intrusion assimilated much more of the metamorphic basement than did the rest of the magma. Similarly, the marked increase in the concentrations of excluded elements in the upper part of the Layered Series is not matched by a change in the isotopic character of the rocks and cannot be attributed to a later influx of new magma. Analyses of minerals separated from rocks with exceptionally mafic or felsic modal compositions revealed marked inhomogeneities in the isotopic compositions of their constituent minerals. For example, coexisting plagioclase and pyroxene from closely associated anorthosites and pyroxenites have very different initial isotopic ratios of both strontium and neodymium. The same is true of mafic and felsic layers in modally graded gabbros. These differences are unrelated to the low-temperature alteration shown by oxygen isotopes. They must have been introduced when the original gabbro was largely crystallized and underwent local metasomatic replacement by nearly mono-mineralic mafic and felsic assemblages.

Sulfur and oxygen isotopic evidence of country rock contamination in the Voisey's Bay Ni–Cu–Co deposit, Labrador, Canada

The emplacement of basaltic magma into sulfide-bearing country rocks provides a favorable geologic environment for magmatic sulfide ore formation related either directly to assimilation of country rock sulfur or indirectly to a depression of sulfide solubility caused by assimilation-induced changes in magma composition. Pelitic country rocks of the Proterozoic Tasiuyak Gneiss in the area of the Voisey's Bay Ni–Cu–Co deposit contain sulfidic layers that may have provided sulfur to basaltic magmas during emplacement of the Voisey's Bay intrusion. Sulfur isotopic compositions of the Tasiuyak Gneiss range from −0.9 to −17.0‰, values typical for sulfides produced via bacterial sulfate reduction in an open marine environment. Archean gneisses in the area contain low amounts of sulfide and are less likely to have served as a source of externally-derived sulfur. Sulfur isotopic compositions of the sulfide minerals from the Voisey's Bay deposit show consistent variations, both spatially and with rock types. Disseminated and massive sulfides show a decrease in δ 34 S to the west, with values typically between 0 and −2‰ in the Eastern Deeps, Ovoid, and Discovery Hill zone, and between −2 and −4‰ in the Reid Brook zone. δ 34 S values of the Mushua intrusion to the north and the Normal Troctolite in the Eastern Deeps are more positive, ranging between −0.5 and 1.8‰. This range is taken to represent the isotopic composition of primary mantle-derived sulfur in the area because the Mushua intrusion and Normal Troctolite show the least geochemical evidence for contamination by country rocks. Sulfur isotopic data from the Reid Brook zone are consistent with up to a 50% sulfur contribution from the Tasiuyak Gneiss. Correspondingly lower proportions are indicated for the eastern portion of the deposit where country rocks are predominantly low-sulfide enderbitic and quartzofeldspathic gneisses. Oxygen isotopic values of gneiss fragments in the Basal Breccia Sequence and Feeder Breccia suggest that the assimilation process involved a greater proportion of high− 18 O contaminant to the west. δ 18 O values of the Tasiuyak Gneiss (5.9 to 14.0‰), enderbitic gneiss (6.4 to 8.7‰), and Archean quartzofeldspathic gneiss (9.5 to 9.7‰) are consistent with an increased proportion of Tasiuyak Gneiss contaminant to the west. Isotopic data strongly indicate that sulfur from the Tasiuyak Gneiss has been involved in ore deposition at the Voisey's Bay deposit. However, sulfur and oxygen isotopic data also strongly suggest that the addition of externally derived sulfur was not the sole process responsible for mineralization, and that assimilation of both Proterozoic and Archean country rocks played a key role in depressing sulfide solubility prior to sulfide localization via dynamic, physical mechanisms.

Key factors in the genesis of Noril'sk, Sudbury, Jinchuan, Voisey's Bay and other world‐class Ni‐Cu‐PGE deposits: Implications for exploration

Magmatic Ni‐Cu sulfide deposits form as the result of the segregation and concentration of droplets of liquid sulfide from mafic or ultramafic magma and the partitioning of chalcophile elements into these from the silicate melt. Sulfide saturation of a magma is not enough in itself to produce a massive concentration of magmatic sulfide. The appropriate physical environment is required to concentrate sulfides from a large mass of magma into one place. The deposits of the Noril'sk region have developed within flat, elongate bodies (15 X 2 X 0.2 km) that intrude argillites, evaporites and coal measures, adjacent to a major, transcrustal fault and immediately below the centre of a 3.5 km‐thick volcanic basin. An anticlinal axis that transects the axis of the basin at a high angle has brought these intrusions to surface to give rise to the two major ore junctions, Noril'sk and Talnakh. Studies of the overlying basalts have shown that basalts forming a 500 m‐thick sequence have lost 75% of their Cu and Ni and more than 90% of their PGE. Basalts above this show a gradual recovery in their chalcophile element concentrations to reach ‘normal’ values 500 m above the highly depleted zone. Two groups of mineralised bodies have been identified as correlative with these basalts: the poorly mineralised Lower Talnakh‐type bodies,which resemble the highly depleted basalts; and the ore‐bearing Noril'sk‐type intrusions which correlate with the overlying, essentially undepleted basalts. The high proportion of sulfide (2–10 wt%) associated with the Noril'sk‐type intrusions, the high PGE content of the ores, the extensive metamorphic aureole (100–400 m around the bodies), and the heavy sulfur isotopic composition of the ores (+ 8 to +12 834S) are explicable if the ore‐bearing bodies are exit conduits from high‐level intrusions, along which magma has flowed en route to surface. The Lower Talnakh bodies are interpreted as intrusions along which magma flow stopped earlier than along those of the Noril'sk type. The first magma to enter the high‐level intrusion reacted with much evaporitic sulfur, at a low ‘R’ value and thus gave rise to sulfides with low metal tenors. Successive pulses of magma through the system progressively enriched the sulfides in the conduits, losing progressively less of their chalcophile metals, and thus accounting for the upward increase in metals in successive lava flows. The flow direction along the conduits is shown by the direction in which the tenor of disseminated sulfides decreases. Sulfides have settled from the moving magma to form separate injections of liquid sulfide, up to 3.5 X 1.5 X 0.05 km in size. Recent reflection seismic studies at Sudbury have shown the Sudbury Igneous complex to have been much more extensive than originally supposed. Nd and Sr isotopic studies on rocks of the Sudbury Igneous Complex and Re‐Os studies on the ores have indicated the incorporation of much country rock gneiss in the complex. Debate has centred around whether the Sudbury Igneous Complex is entirely an impact melt or the consequence of the mixing of primary magma with 50 or more wt% impact melt; the most recent evidence favours the latter hypothesis. The Jinchuan deposit of north‐central China occurs within a 6 km‐long dyke‐like body of peridotite. The compositions of olivine within the dyke, the igneous rocks themselves, and the ore are all inconsistent with derivation of the body from ultramafic magma, as originally supposed, and indicate that the structure forms the keel of a much larger intrusion of magnesian basalt. Flow of magma into the intrusion has resulted in olivine and sulfide being retained within this keel. The Voisey's Bay deposit lies partly within a 30–100 m‐thick sheet of troctolite, which is interpreted as a flat‐lying part of a feeder for an adjacent intrusion, and partly at the base of the intrusion, where the feeder adjoins it. Ore types range from disseminated sulfides in troctolite, which increase downward, grading into massive ore. The latter is underlain by a breccia composed of fragments of gneiss, unmineralised troctolite and peridotite in a troctolitic matrix that, in places, contains appreciable sulfide. When most major Ni‐Cu sulfide deposits, including those at Kambalda, Western Australia, are viewed in the light of studies at Noril'sk, Sudbury, Jinchuan and Voisey's Bay, three factors become apparent: (i) the concentration of sulfides in channels or conduits through which much magma has flowed (feeder conduits for intrusions are much more prospective targets for exploration than the base of the intrusions themselves); (ii) the interaction of the source magma with country rocks, either leading to the incorporation of sulfur, or the felsification of the magma in question; and (iii) fractional crystallisation of sulfide liquid giving rise to Cu‐rich ores which may be far removed from the ‘source’ ore.

特集 火山活動と地殻応力場 中生代玄武岩岩脈の岩脈連結断層と火山性地震の関係 群発地震の化石か？

In the coastal region of the State of Rio de Janeiro, Brazil, Jurassic (120 Ma) basaltic dykes are present intruding in late Precambrian (600 Ma) gneissic rocks. They are typically 10 m wide, constituted by thoreiitic gabbro, and frequently have sinistral en-echelon setting. Intrusion depth of the present surface exposure is estimated to be 4 km based on apatite fission track dating and regional uplift history. The en-echelon dyke terminals are often linked by a short oblique path, showing extended “Z” -shaped dyke appearance on outcrops. Some of the link paths of wide dykes (>4 m) are accompanied by well-consolidated narrow (<1 m) brecciated zone on both sides, composed uniquely of angular host gneiss fragments. The fragment size is relatively fine (1 cm) on dyke link path side (inner), and gross (10 cm) on host rock side (outer). Large fragments are very angular, and show auto-brecciated texture. The lithologic characteristics of the breccia correspond to those of fault breccia.Geometric setting and kinematic relation between these fault breccia zones and the dykes are similar to those of mid-ocean ridges and transform faults. Dyke dilatation due to magma injection may cause shear stress at the dyke junction zones. If the stress is sufficiently great, the host rock will rupture to form fault fracture zones, and they are successively filled by magma to result the link paths. If the stress is insufficient, there small cracks may be created. Such type of faults, namely dyke linkage faults, are primary and monogenic ones, which are generated in uncracked host rock body with no repeated stick slips.The movement mechanism of dyke linkage faults is similar to the Hill's model, but differs a little from it especially in case of the faults with link path filled by magma. Total displacement of dyke linkage fault is defined by dyke width, Even after the magma fill on the fault plane, fault displacement will continue aseismically. The basaltic dykes in this region commonly have well-developed horizontal columnar joints, but the cooling joints at link paths are pertubated and platy-like joints take place, indicating that the link paths were submitted to shear deformation during magma cooling. In this sense, the earthquakes are more likely to occur only at the top of the intruding dykes. The fault length is defined by en-echelon dyke off-set at junction zones, being several times more than the dyke width. In comparison with tectonic faults and earthquakes, the diplacement is very large relative to the length.The earthquakes generated by dyke linkage fault formation are expected to have strike-slip focal mechanism, linear epicentre trend, nodal plain oblique to the trend, and a high frequency spectrum. Some Japanese volcanic earthquake swarms have such characteristics.

Impact-induced phase transformations at 50–60 GPa in continental crust: an EPMA and ATEM study

Strongly shocked gneiss fragments from the allochthonous polymict breccia of the 23 Ma old Haughton impact crater were studied by optical microscopy, electron probe microanalysis (EPMA) and analytical transmission electron microscopy (ATEM). The study is focused on the substantiation of shock-induced phase transformations in rock-forming minerals, and their implications for post-shock thermal regimes and impact age dating. In the gneiss fragments, the original layering is largerly preserved, although all the minerals have experienced dramatic phase transformations. Chemically, the various shock-generated decomposition regions still reflect the pre-shock mineralogy. However, significant heterogeneities exist on the 10–100 μm scale. Four different regions of decomposition products can be distinguished (EPMA): (1) shocked biotite-rich regions with strong variations in Si, Al, K, Fe and Mg on the 10 μm scale, (2) shocked quartz with about 3% Al and K and traces of Fe and Mg, (3) a region with Al/>Si ≈ 1, which does not correspond to any pre-shock mineral, and (4) shocked K-feldspar showing a significant deficit in K, and traces of Fe and Mg. On the nanometer scale, a uniform, bimodal reaction pattern emerges: all minerals are transformed to new crystals embedded in a silica-rich glass. The shock-generated crystals are: (1) spinel in shocked biotite, (2) α-quartz, cristobalite, and coesite in shocked quartz, (3) an unknown polymorph of corundum (Al 2O 3) in feldspar and in the Al/Si ≈ 1 region, and (4) mullite in association with SiO 2 glass in shocked sillimanite grains. All crystals range in size between 0.05 and 1.0 μm. Taking into account estimated durations and pressure-temperature conditions for shock-wave passage and post-shock temperatures, the reactions observed can be ascribed to high temperatures during and after pressure release. The different mineral-glass parageneses suggest temperatures in excess of 1400°C. Moreover, given the small size of the newly formed minerals, very rapid cooling down to 1200°C is required.

Implications for cratering mechanics from a study of the Footwall Breccia of the Sudbury impact structure, Canada

Abstract— The Footwall Breccia layer in the North Range of the Sudbury impact structure is up to 150 m thick. It has been analyzed for several aspects: shock metamorphism of clasts, matrix texture, mineralogy, and geochemistry with respect to major and trace element compositions. The matrix of this heterolithic breccia contains mineral and lithic fragments, which have suffered shock pressures exceeding 10 GPa, along with clasts of breccia dikes originating from the crater basement. The matrix in a zone near the upper contact of the breccia layer is dominated by a dioritic composition with intersertal textures, whereas beneath this zone the matrix is characterized by poikilitic to granular textures and a tonalitic to granitic composition. Major and trace element analyses of adjacent slices of a thin‐slab profile from the breccia show that the matrix is chemically inhomogeneous within a range of 3 mm. The breccia layer has been thermally annealed by the overlying Sudbury Igneous Complex, which is interpreted as a coherent impact melt sheet. The Rb‐Sr isochron age of 1.825 ± 0.021 Ga for the matrix is a cooling age after partial melting of fine grained clastic material by the melt system. Two‐pyroxene thermometry calculations give temperatures in excess of 1000 °C for this thermal overprinting. Clasts were affected by recrystallization, melting, and reactions with the surrounding matrix at that time. The crystallization of the molten matrix resulted in the observed variety of igneous textures. Results of clast population statistics for the Footwall Breccia along with both geochemical considerations and the Sr‐Nd isotopic signature of the matrix indicate that the breccia constituents exclusively derived from the Levack gneiss complex, which forms the local country rock to the breccia layer in the Levack area. K‐feldspar‐rich domains, which tend to replace parts of matrix and felsic gneiss fragments have been formed due to metasomatic activities during the Penokean orogeny, ∼ 1.7 Ga ago.The available observations suggest that the Sudbury structure represents the remnant of a multi‐ring basin with an apparent diameter between 180 and 200 km and a diameter of the transient cavity of about 100 km. For a crater of the size of the Sudbury basin a maximum depth of excavation of ∼21 km and a depth of shock‐melted target rocks of ∼27 km are obtained. In the Sudbury crater, the Footwall Breccia layer represents a part of the uplifted crater floor directly underlying the thick coherent impact melt sheet.

Recent Advances in Meteorite Crater Research at the Dominion Observatory, Ottawa, Canada

AbstractThe Dominion Observatory's meteorite crater research program is reviewed, with brief outlines of the methods of investigation, the progress to date, and plans for the future. The results and implications of recent investigations at the Deep Bay crater, the New Quebec crater and the Carswell Lake structure are presented.At Deep Bay diamond drilling near its center has confirmed that Cretaceous sedimentary rock lies within the crater, indicating a minimum crater age of 140 million years. Analysis of deformed rock and gneiss breccia encountered in a drill hole one mile from shore suggests that the original crater diameter is about 31,000 feet, considerably less than first estimated.Preliminary results of a gravity survey and a new shaded topographical map of the New Quebec Crater are presented. After correcting for terrain and regional effects a residual negative gravity anomaly of 6 milligals is indicated. Integration of topographical irregularities shows that the volume of the rim of the New Quebec Crater is about double the volume of the crater below the surrounding plain.Gravity and magnetic results over the Carswell Lake structure are consistent with an impact crater some 19 miles in diameter, taking into consideration that its original deformed floor has been uplifted in response to isostatic forces. Fragments of gneiss having radiating strations characteristic of shatter cones, discovered in the area of central uplift, support the meteoritic hypothesis of origin for this structure.