Feldspar Clasts Research Articles

Zn-clays in the Kihabe and Nxuu prospects (Aha Hills, Botswana): A XRD and TEM study

Abstract Zinc clays are commonly found in oxidized Zn deposits and, even though they rarely represent the main target of the ore exploitation, they can be used as a proxy to restore the genetic conditions during ore-forming processes. This work sheds light on the micro- to nano-mineralogy and on the genesis of Zn-clays in the Kihabe and Nxuu prospects (located in the Aha Hills district, Northern Botswana), through an integrated XRD and TEM study of the mineralized facies occurring in the ore system. The Kihabe and Nxuu ores are hosted in a Neoproterozoic metamorphozed quartzwacke unconformably covered by the recent sedimentary rocks, also containing calcretes, of the Kalahari Group. In the analyzed samples, four distinct mineralogical facies have been recognized: (1) vanadate-calcrete facies, poor of Zn-clays; (2) low Zn-clay facies, characterized mostly by clays showing low Zn concentrations; (3) Zn-clay facies, containing proper Zn clay minerals; and (4) sulfide facies, devoid of Zn-clays. In all the facies detrital dioctahedral mica (muscovite and illite) is interstratified with smectite in the form of random (R0) to short-range ordered (R1) I/S, which locally shows significant Zn concentrations. In the sulfide facies kaolinite overgrowing onto mica packets has been detected. The low Zn-clay facies is dominated by Zn-bearing beidellite, with minor kaolinite and fraipontite. The Zn-clay facies consists mostly of a random (R0) interstratified clay between a 7 Å phase corresponding to fraipontite, and a 2:1 swelling clay component identifiable with the dioctahedral smectite, with minor sauconite. The micro- to nano-scale paragenetic study performed by TEM indicates that the above-mentioned clays formed through a multistage process, eventually ending with the genesis of Zn-bearing phyllosilicates assemblages. The main steps were: (1) alteration of detrital mica and dissolution of feldspar clasts, which led to the formation of epitaxial kaolinite and replacive beidellite; (2) fertilization of barren clays and formation of replacive to epitaxial fraipontite/smectite and of Zn-bearing mica, through input of Zn2+ deriving from sphalerite or willemite dissolution by mixed meteoric-hydrothermal fluids; (3) formation of low tetrahedral charge sauconite, either in pores or as replacement of K-feldspars under surficial hypersaline conditions, possibly also linked to the establishment of the arid climate in region. These processes provide insights into genesis of Zn-Pb-V ore deposits in northwest Botswana. Furthermore, the identification of a Zn-smectite species having a stevensite-like stoichiometry is valuable for future studies dealing with the systematics of clay minerals.

Luminescence Sensitivity of Rhine Valley Loess: Indicators of Source Variability?

Loess provides a valuable terrestrial record of past environmental conditions, including the dynamics and trajectories of air mass circulation responsible for dust transport. Here we explore variations in the luminescence sensitivity characteristics of sedimentary quartz and feldspar as possible tools for identifying changes in source down a loess-palaeosol sequence (LPS). Luminescence sensitivity is a rapidly measurable index which is the product of interplay between source lithology and the history of the quartz or feldspar clasts. Variations in sensitivity of down profile may therefore reflect changes in sediment provenance as well as other factors such as weathering through pedogenesis. We undertake an empirical investigation of the luminescence sensitivity of quartz and feldspar from different grain-size fractions from the Schwalbenberg LPS in the German Rhine valley. We compare samples from a 30 m core spanning the last full glacial cycle with samples of oxygen isotope stage (OIS) 3–2 age exposed within nearby profile. We find an overall inverse relationship between quartz and feldspar sensitivity, as well as variability in sensitivity between different quartz grain sizes. Statistical analyses yield a significant correlation between IR50 sensitivity from unprocessed sediments and clay content, and feldspar sensitivity and Si/Al ratios down the core. Since Si/Al ratios may indicate changes in provenance, the latter correlation suggests that IR50 measurements on unprocessed samples may be used to provide a reliable, rapid scan of source variability over millennial timescales.

Source of K-Feldspars and Kaolinitization in Arkoses of West and Central Bohemian Continental Permo–Carboniferous

Abstract The examined K-feldspars in arkoses of the continental Upper Carboniferous – Lower Permian of the West and Central Bohemian basins originated in the whole bed sequence (Duckmantian to Autunian; 314.2–297.1 Ma) from one source, which were so-called mountain granites of the older intrusive complex (OIC) of the Krušné hory Mts, as confirmed by the X-ray and geochemical analyses. The presence of feldspar clasts from other granitoids (Merklín and Louny massifs) was found only in the straight transgreding basal Carboniferous rocks. While other arkoses underwent synsedimentary and/or post-sedimentary kaolinization, in the deposits of Kaznějov and Horní Bříza (Nýřany Member of the Kladno Formation) pre-sedimentary kaolinization (sedimentation of sandstones and conglomerates with kaolinitic cement) occurred.

Exploring the relationships between shear zones and granites: field and microstructural data for contrasting case studies of the Borborema Province (NE Brazil)

We discuss meso- and microstructural features of granites closely related to strike-slip shear zones in the Borborema Province, NE Brazil. The Riacho do Icó stock is an en-cornue intrusion aged at ca. 607 Ma. Magmatic fabric is recorded in the core of the granite, whilst increasing deformation is marked by the development of mylonitic fabrics towards the Afogados da Ingazeira shear zone, including magmatic foliation and lineation rotation. Early recrystallization of quartz and K-feldspar crystals is widespread as a fabric with well-developed granoblastic polygonal textures and lobate subgrain boundaries, heterogeneously deformed lenses and ameboid quartz ribbons, typical of igneous rocks submitted to deformation in deep crustal levels. On the other hand, the Espinho Branco-Santa Luzia leucogranitic belt is hosted along the Patos Lineament, aged between the ca. 575 – 565 Ma interval. These rocks show discordant relationships with the host migmatites and the main deformational fabric is characterized by a dominant magmatic foliation that is locally overprinted by structures that are typical of solid-state flow. Quartz melt pockets and interstitial quartz grains filling fractures in feldspar clasts are common. Such characteristics are compatible with granites that were injected in the continental crust along planar anisotropies (i.e., shear zones) formed during the late-stage partial melting events that originated the migmatites of the area. The case studies are proxies in the understanding of different episodes of magma emplacement along shear zones in this part of West Gondwana.

Resolving the age of the Haughton impact structure using coupled 40Ar/39Ar and U-Pb geochronology

The Haughton Dome located on Devon Island, in the Canadian Archipelago represents a well-preserved, moderate-sized, complex impact crater. Previous age constraints for the 24 km-diameter impact structure have ranged from ca. 21 Ma to ca. 39 Ma. Herein, we present a coordinated microstructural and in situ U-Pb study of zircon and monazite coupled with 40Ar/39Ar laser step heating of shock-melted K-feldspar clasts from shock metamorphosed gneissic fragments collected from the allochthonous impact breccia at Haughton. Moderately shocked zircon and monazite grains yield an age distribution consistent with an Archean protolith metamorphosed at ca. 1.9 Ga, whereas shock recrystallized zircon and monazite yield a lower intercept age of 31.8 ± 1.7 Ma (n = 48, MSWD = 0.58, P = 0.99). Four inverse isochron 40Ar/39Ar ages of shocked feldspar clasts yield a weighted mean age of 31.04 ± 0.37 Ma (MSWD = 0.98, P = 0.40), within uncertainty of the U-Pb lower concordia intercept. Ar diffusion modelling supports the interpretation of the impact age and helps resolve impact-driven age resetting. These results highlight the power of coupling multiple geochronologic techniques for determining impact ages, especially from targets with complex geologic histories. Furthermore, they resolve previous discrepancies in the age of the Haughton Dome and the interpretation of the post impact stratigraphy of the crater fill.

Deformation mechanisms, mineral chemistry and zircon U-Pb geochronological constraints in the south Patos shear zone: implications for the crustal evolution of the Borborema Province, NE Brazil

The southern boundary of the Patos Shear Zone (PSZ) is characterized by mylonites resulting from the deformation of Paleoproterozoic basement gneisses, Neoproterozoic metavolcanic and metasedimentary rocks, granitic and granodioritic intrusions. Among the latter, the Santa Terezinha and Catingueira plutons show stretched shapes in agreement with the the shear zone’s regional trend. Recrystallized quartz grains in these plutons accommodate deformation by dislocation creep. The host banded gneisses show similar deformation structures to the plutons. Quartz crystallographic fabrics mainly record the activity of basal and prism slip planes with local contribution of rhomb planes, whilst feldspar clasts show evidence of intracrystalline deformation in the larger porphyroclasts. These characteristics suggest that shearing occurred at lower-amphibolite facies conditions. U-Pb zircon data for the Santa Terezinha pluton yields an age of 625 ± 7 Ma, which, coupled with the pluton elongated shape and microstructure, suggests that its emplacement was pre-kinematic in relation to the Patos shear zone. Post-emplacement elongation of the granite body occurred along the strike-slip trend. Such structural and geochronological constraints highlight the close association between magma emplacement and shear zone deformation during distinct stages of the Brasiliano-Pan-African orogeny in the Borborema Province.

Provenance of Late Ordovician sedimentary rocks in the SE Yangtze block: Implications for deposition in an active continental margin

The Jiangnan Orogen separates the Yangtze block from the Cathaysia block to the southeast. Several competing models have been proposed for the evolution of both blocks and the intervening orogen, which mainly focus on aspects of igneous and metamorphic rocks. Limited research has been conducted on the sedimentary rocks in this region. Provenance of the Late Ordovician sedimentary rocks in the northeastern Jiangnan Orogen has been constrained based on systemic analyses of petrology, heavy mineral assemblages, geochemistry, and detrital zircon U-Pb ages, which provide vital clues for reconstructing the evolution of this belt during the early Paleozoic. Late Ordovician deposits in the northeastern Jiangnan Orogen consist of conglomerates, sandstones, siltstones, shales, calcareous mudstones, and limestones. These strata were deposited in a shallow to deep sea environment, showing water depth deepening to the northwest. The sandstone contains lithic graywackes and feldspar-lithic graywackes, with low compositional and textural maturities. They consist, on average, of quartz clasts (27%), feldspar clasts (18%), and lithic fragments (55%). The lithic fragments are angular, including andesites, rhyolites, tuffs, granites, slates, phyllites, quartzites, siltstones, and chert, which also occur in conglomerate form. Minor pyroxene, chromite, magnetite, and garnet grains occur in the sandstone. Geochemical analyses show that the sandstone and siltstone have widely variable Fe2O3T + MgO (4.43–12.05%), Cs, V, Ni, and Ti contents. They also exhibit a distinct negative Nb-Ta trough compared to the average upper continental crust and have light rare earth element (LREE) enrichment patterns with obvious Eu anomalies, similar to subduction-related rocks in an arc setting. Detrital zircon U-Pb ages demonstrate a mixed source from ca. 784 Ma and ca. 455 Ma. Paleocurrent indicators point to a southeastward origin. These results demonstrate that Late Ordovician sedimentary deposition occurred in a northwestward-facing active continental margin along the southeastern Yangtze block.

Brittle–ductile fabrics and P – T conditions of deformation in the East Pernambuco shear zone (Borborema Province, NE Brazil)

Fabrics of the East Pernambuco shear zone (EPSZ) were studied via microstructural analysis, mineral chemistry and isochemical phase diagram modelling to constrain the pressure and temperature conditions of deformation during shearing. Granitic mylonites show fractured feldspar porphyroclasts embedded in a fine-grained, recrystallized quartzo-feldspathic matrix. These mylonites grade laterally into banded ultramylonites characterized by stretched feldspar clasts alternated with recrystallized quartz bands. Fractures in these ultramylonites are filled by phyllosilicates. The mineral chemistry of the feldspars points to systematic changes between porphyroclasts, grains within fractures and fine-grained mixtures. Quartz crystallographic fabrics in the mylonites suggest activation of prism slip, while the ultramylonites show the activation of both rhomb and basal slip systems. Thermodynamic modelling suggests that the mylonites were formed at 4.75 ± 0.25 kbar and 526 ± 9°C, while the ultramylonites yield conditions of 5.9 ± 1 kbar and 437 ± 17°C. These observations suggest that the EPSZ records a heterogeneous path of strain accommodation, marked by decreasing temperature from its western sector to its eastern termination. The differences in metamorphic conditions are consistent with a transitional, brittle–ductile strain regime. Such characteristics indicate that the EPSZ is a Neoproterozoic shear belt nucleated and heterogeneously exhumed at the brittle–ductile transition, possibly in an intracontinental setting. Supplementary Material: EPMA analysis of feldspars in Caruaru and Gravatá domains and T-X(O 2 ) pseudosections are available at https://doi.org/10.6084/m9.figshare.c.5125957

Evolution of stretching lineations in granulite-hosted ductile shear zones, Eastern Ghats Province, India: Role of temperature, strain rate and pre-existing stretching lineations

Shear zone kinematics are interpreted from sections (X-Z) parallel to the stretching lineation (X-direction) and perpendicular to the mylonitic foliation (X–Y). The stretching lineation is a product of finite strain, defined by minerals or mineral aggregates that deform plastically. In quartzofeldspathic rocks deformed below 600 °C, rheological contrasts between quartz and feldspar are amplified, and the mesoscopic stretching lineation in the shears may be indistinct in outcrop as quartz ribbons anastomose around relatively competent feldspar clasts. In high strain-rate zones, rheological contrasts between these minerals are reduced and a stretching lineation suitable for kinematic interpretation develops. In late amphibolite and greenschist facies shear zones transecting pre-lineated quartzofeldspathic granulites of the Eastern Ghats Province, India, the stretching lineation represents the resultant of pre-existing stretch and superposed increments of later shearing. Until the stretch accumulated in the X-direction of the later deformation exceeds the earlier finite stretch, the pre-existing stretching lineation progressively rotates towards the later X-direction with decreasing aspect ratio. These intermediate lineation orientations, have no kinematic relevance to either earlier or later deformations. This example highlights the importance of temperature, strain rate and orientation of pre-existing stretching lineations in the appearance and kinematic interpretation of,stretching lineations in ductile shear zones.

U–Pb geochronology of apatite and zircon from the Brent impact structure, Canada: a Late Ordovician Sandbian–Katian boundary event associated with L-Chondrite parent body disruption

In situ LA-ICP-MS U–Pb geochronology has been performed on apatite and zircon within thermally recrystallized clast-laden and clast-poor impact melt rocks from the Brent impact structure. A total of 377 laser analyses on 120 impact melt-grown (n = 9) and impact-recrystallized zircon grains (n = 111) were obtained, from which a concordia age of 452.8 ± 2.7 Ma (MSWD 0.57, n = 11), and a weighted average mean 206Pb/238U age of 453.2 ± 2.9 Ma (MSWD 0.60) (n = 11) are calculated. A total of 300 laser analyses from 100 relict apatite grains were obtained, with an unanchored regression through all data yielding a lower intercept age of 453.2 ± 6.0 Ma (MSWD 5.8, n = 300), that overlaps within error of zircon. 207Pb/206Pb ratios obtained from feldspar clasts within clast-laden impact melt retain the same initial Pb composition as the target rocks from which they are derived, while feldspars that crystallized from impact melt have 207Pb/206Pb ratios indicative of isotopic re-equilibration between basement lithologies of two different ages. A similar variability in 207Pb/206Pb is recorded by apatite. This provides evidence for the involvement of Neoproterozoic Lake Nipissing alkaline suite, as well as Mesoproterozoic Grenville gneisses in the production of impact melt at Brent. Recrystallized apatite grains exhibit enrichments in light rare earth elements (LREEs) along neoblast grain boundaries, indicative of trace element substitution and phase precipitation during impact-induced recrystallization. An age of 452.8 ± 2.7 Ma from zircon and 453.2 ± 6.0 Ma from apatite places the impact event in the Late Ordovician, at or near the Sandbian–Katian boundary, confirming Brent’s involvement in the Middle to Late Ordovician crater cluster event—a period of enhanced impactor flux to Earth related to the L-Chondrite parent body disruption.

Ferricretes at Burringurrah (Mount Augustus), Western Australia: Proof of lateral derivation

Surficial iron-rich deposits (ferricretes) occur on and around Burringurrah (Mount Augustus), a huge, remote inselberg in the Gascoyne district of Western Australia. Oriented NW-SE, Burringurrah is approximately 14 km long, 5 km wide, rises to 1105 m above sea level and stands some 700 m above the surrounding plain. The majority of the ferricretes occupy bedrock valley bottom floors on the southern side of the inselberg at Edney Springs, Flintstone Creek, and on the Edney Trail where they have formed on unweathered quartzose Mount Augustus Sandstone, which only contains a few percent Fe2O3. At ‘The Pound’, a boulder ferricrete overlies deeply weathered igneous bedrock, unroofed by erosion of the overlying Mount Augustus Sandstone, exposing the unconformity between the weathered granitic bedrock and the Mount Augustus Sandstone in the core of a broad, asymmetrical and doubly plunging anticline. On the northern, steeper, side of the mountain, vermiform and conglomeratic ferricretes are sparsely exposed in drainage lines. Ferricrete mineralogy is dominated by goethite with only small amounts of hematite. Oxygen isotope inferred ages of weathered monzogranite produced a top-down weathering front extending up to the Neogene at an elevation coincident with the ferricretes within bedrock valley bottoms. Beyond the mountain, the age of ferricreted sandy alluvium was established by thermoluminescence polymineral fine grain dating. These produced a minimum depositional age of >37.6 ± 2.6 ka and a second finite age of 139 ± 44 ka, suggesting ferricrete formation here may have begun during the last interglacial period and continued throughout the late Pleistocene. Formation of ferricretes on top of iron-poor quartzites is unusual. It reflects not only the absolute accumulation of iron oxides derived from the Mount Augustus Sandstone but possibly the additional contributions of iron-bearing aeolian dust from the surrounding plains subsequently washed down drainage lines to accumulate in swampy depressions at the foot of the inselberg. Ferricrete formation was initially simple, with iron in solution impregnating sandy sediment and organic host materials, sporadically impacted by bioturbation by termites. Thin-section petrography reveals an interesting form of in-situ, self-brecciation of quartz and feldspar clasts, which are fractured and infilled with iron oxides and newly precipitated clay which further promotes fragmentation. The mode of ferricrete formation described from Burringurrah is relevant to the laterite-ferricrete debate, for here it is most unlikely that the zones of iron enrichment developed in situ and vertically above the iron-poor quartzite basement as in the sense of the regional ‘classic laterite profile’.

Roundness of survivor clasts as a discriminator for melting and crushing origin of fault rocks: A reappraisal

Roundness of survivor clasts (mineral/rock fragments) in fault rocks (e.g., pseudotachylyte and cataclasite/gouge) has been used by some workers to distinguish melt-origin from crush-origin of such rocks. Keeping in view the large overlap in the published data on the roundness of fault rocks, the reliability of such a discriminator appears somewhat uncertain. The present study attempts to reappraise the aforesaid criterion through roundness analysis of quartz and feldspar clasts in melting-dominated pseudotachylyte (M-Pt), crushing-dominated pseudotachylyte (C-Pt) and fault-related cataclasite (F-Ct) collected from Sarwar–Junia Fault Zone in Rajasthan and from Gavilgarh–Tan Shear Zone in central India. Our analysis shows that roundness of clasts cannot reliably distinguish between fault rocks of melt-origin and crush-origin (especially M-Pt and F-Ct) as the roundness values overlap and a distinct limit of roundness value for each rock type cannot be established. While the roundness of clasts in M-Pt and C-Pt may be enhanced due to melt-induced rounding off of the initially angular clasts, rounding of clasts can also occur by abrasion during rolling of crushed material in F-Ct. Furthermore, anomalous thermal expansion of clasts in melt-origin pseudotachylyte may cause disintegration of larger clasts into smaller angular fragments, thereby increasing the percentage of angular clasts in melt-origin fault rocks. Therefore, roundness of survivor clasts cannot be solely used as a discriminator between melt-origin and crush-origin fault rocks.

Mineralogical constraints on the thermal history of martian regolith breccia Northwest Africa 8114

Martian meteorite Northwest Africa (NWA) 8114 – a paired stone to NWA 7034 – provides an opportunity to examine the thermal history of a martian regolith and study near-surface processes and ancient environmental conditions near an impact crater on Mars. Our study reports petrographic and alteration textures and focuses on pyroxene and iron oxide grains. Some of the pyroxene clasts show exsolution lamellae, indicating a high temperature magmatic origin and slow cooling. However, transmission electron microscopy reveals that other predominantly pyroxene clasts are porous and have partially re-crystallised to form magnetite and a K-bearing feldspathic glassy material, together with relict pyroxene. This breakdown event was associated with oxidation, with up to 25% Fe3+/ΣFe in the relict pyroxene measured using Fe-K XANES. By comparison with previous studies, this breakdown and oxidation of pyroxene is most likely to be a result of impact shock heating, being held at a temperature above 700 °C for at least 7 days in an oxidising regolith environment.We report an approximate 40Ar-39Ar maximum age of 1.13–1.25 Ga for an individual, separated, augite clast. The disturbed nature of the spectra precludes precise age determination. In section, this clast is porous and contains iron oxide grains. This shows that it has undergone the high temperature partial breakdown seen in other relict pyroxene clasts, and has up to 25% Fe3+/ΣFe. We infer that the age corresponds to the impact shock heating event that led to the high temperature breakdown of many of the pyroxenes, after consolidation of the impact ejecta blanket.High temperatures, above 700 °C, may have been maintained for long enough to remobilise and congruently partially melt some of the alkali feldspar clasts to produce the feldspar veins and aureoles that crosscut, and in some cases surround, the oxidised pyroxene. However, the veins could alternatively be the result of a hydrothermal event in the impact regolith. A simple Fourier cooling model suggests that a regolith of at least five metres depth would be sufficient to maintain temperatures associated with the pyroxene breakdown for over seven days.Low temperature hydrous alteration took place forming goethite, identified via XRD, XANES and FTIR. Comparing with previous studies, the goethite is likely to be terrestrial alteration pseudomorphing martian pyrite.

Size distribution and roundness of clasts within pseudotachylytes of the Gangavalli Shear Zone, Salem, Tamil Nadu: An insight into its origin and tectonic significance

Gangavalli (Brittle) Shear Zone (Fault) near Attur, Tamil Nadu exposes nearly 50 km long and 1–3 km wide NNE–SSW trending linear belt of cataclasites and pseudotachylyte produced on charnockites of the Southern Granulite Terrane. Pseudotachylytes, as well as the country rock, bear the evidence of conjugate strike slip shearing along NNE–SSW and NW–SE directions, suggesting an N–S compression. The Gangavalli Shear Zone represents the NNE–SSW fault of the conjugate system along which a right lateral shear has produced seismic slip motion giving rise to cataclasites and pseudotachylytes. Pseudotachylytes occur as veins of varying width extending from hairline fracture fills to tens of meters in length. They carry quartz as well as feldspar clasts with sizes of few mm in diameter; the clast sizes show a modified Power law distribution with finer ones (<1000 \({\upmu }\)m\(^{2})\) deviating from linearity. The shape of the clasts shows a high degree of roundness (>0.4) due to thermal decrepitation. In a large instance, devitrification has occurred producing albitic microlites that suggest the temperature of the pseudotachylyte melt was >1000\(^{\circ }\hbox {C}\). Thus, pseudotachylyte veins act as a proxy to understand the genetic process involved in the evolution of the shear zone and its tectonic settings.

Melting behaviour of feldspar clasts in high-fired Sue ware

This contribution examines the melting behaviour of feldspar clasts in 9 th century CE Sue ware from the Nakadake Sanroku kiln site, Kagoshima prefecture (Kyūshū), southern Japan. Sue ware is stoneware of typical grey to brownish-grey appearance fired at high temperature; its surfaces may be untreated or covered by natural ash glaze. Firing was performed in sloping single-chamber tunnel kilns at reducing conditions and temperatures exceeding 1000 °C. Set in a vitrified matrix (glass, mullite, minor spinel and cristobalite), quartz and feldspar constitute ubiquitous clast components. The varying degree of vitrification (15–60 wt.% glass phase) and reversely correlated modal proportions of quartz and feldspar clasts likely reflect superposed effects of varying exposure time to peak-firing temperature and temperature gradients in the kilns. Sodic plagioclase clasts (An 18-25 ) melted congruently, producing sieve-textured domains of vitreous phase and residual calcic plagioclase, while calcic plagioclase clasts (An 35-63 ) were dissolved incongruently by reaction with matrix melt, generating narrow vitreous rims embedding residual calcic plagioclase laths at the immediate contact with the host plagioclase and acicular mullite towards the ceramic matrix. Elevated Fe, Mg, and K contents of the melt phase with development of concentration gradients indicate bi-directional diffusive element transfer between reaction domains and the ceramic matrix. Melting phenomena of alkali feldspar clasts, depending on the intensity of vitrification, range from few micrometre-thin vitreous margins to completely melted blistered clasts. With advanced melting, pronounced chemical zonations are developed with continuous gradation from potassic core compositions (Or 90-80 Ab 10-20 An ) to sodic compositions (Or 68-51 Ab 30-46 An 2-3 ) in domains adjoining the vitreous domain. The chemically homogeneous vitreous phase retains the 1:3 Al:Si-stoichiometry of the feldspar framework, whereas K and Na are depleted and Ca, Fe and Mg enriched relative to the precursor feldspar. These findings demonstrate that (1) alkali feldspar clasts did not melt through eutectic reaction with silica/quartz nor via the incongruent reaction K-feldspar → leucite + liquid; (2) melting was initiated by diffusive K-Na exchange with the matrix, thereby shifting the pristine clast compositions towards the thermal minimum (Or 40 Ab 60 ) in the Ab-Or-An ternary system; (3) while sodic marginal domains (Or 68-51 Ab 30-46 An 2-3 ) melted congruently, residual core domains (Or 75-66 Ab 24-33 An 1 ) melted incongruently producing micrometre-scale intergrowths of skeletal plagioclase and melt; (4) melting involved diffusive cation exchange through a quasi-stationary aluminosilicate framework, viz . influx of Ca + Fe + Mg to the feldspar melt domains and concomitant release of the fluxing components K + Na to the ceramic matrix. The evaluation of feldspar melting phenomena suggests firing temperatures did not exceed ca . 1150 °C.

Petrological Evidences of Impact-induced Shock Metamorphism in the Basement Granitoids and Rhyolitic Melt Breccia of Mohar Area, Shivpuri District, Madhya Pradesh

Abstract The circular structure at Mohar (Dhala structure) in the western part of Bundelkhand Gneissic Complex, is marked by a prominent outlier of Kaimur sediments surrounded by low lying concentric sequence of sediments of Dhala Formation and basement granite breccia. This has been interpreted as a volcanic eruption related cauldron structure and meteoritic impact crater structure by various authors, on the basis of absence or presence of shock indicators in the clasts of a rhyolite-like rock that crops out scantily in the north western part of the structure. During the course of extensive sub-surface uranium exploration in this structure, the geoscientists of Atomic Minerals Directorate for Exploration and Research observed unequivocal and rampant evidences of shock metamorphic features for the first time in drill core samples of basement granitoids which constitute the bed rock for the rhyolite-like melt breccia, which overlies it. Published data of shock metamorphic features from this area are largely confined to the surface samples of the rhyolite-like melt rock, exposed in sparse outcrops. The shock metamorphic features recorded in the sub-surface granitoid bed rock samples during the present study, comprise planar deformation features (PDF) in quartz, feldspar, apatite and zircon, toasted, diaplectic, ladder-textured feldspars, selectively shock-melted feldspars and melt-veined quartz. The shock metamorphic features recorded in surface and sub-surface samples of the melt rock include ballen quartz, PDF in quartz clasts, toasted and diaplectic feldspar clasts shocked basic rock fragments with isotropised feldspars. Both the shocked bedrock granitoid and the melt rock bear uncharacteristic geochemical signatures with elevated K2O, MgO and depleted CaO. The study also observes that the melt breccia overlying the granitoid bedrock also occurs as pocket-like patches at various depths within the granitoids. Thus, the present findings have helped in understanding the attributes of the basement granitoid and associated melt breccia, thereby linking the genesis of the latter by selective melting of the former, due to the process of impact. It reinforces the already propounded theory of impact as the likely cause for the development of the structure in the basement Bundelkhand granitoid that was later filled by sediments standing out presently as a mesa.

Microstructural and geochemical analysis of Paleoproterozoic pseudotachylytes in Río de la Plata craton, Tandilia belt, Argentina

The current study focuses on the analysis of pseudotachylytes from the Azul Megashear Zone, located in the Rio de la Plata craton, central Argentina. This shear zone is a strip of mylonitic rocks of more than 40km in length and with a maximum width of 2.5km. Glassy and microcrystalline pseudotachylytes are hosted by Paleoproterozoic gneiss mylonite, granitoid mylonite, granulite mylonite and striped gneisses. The pseudotachylytes occur as fault and injection veins characterized by sharp contacts with no gradation into the mylonites and show meltorigin features such as partially melted quartz and feldspar clasts with embayed rims and flow structures. Their composition is similar to that of the rocks in which they are injected. In those cases involving banded host rocks, slight chemical differences can be observed in the pseudotachylytes mainly in terms of their fabric and mineralogy. Most pseudotachylytes have textures deriving from post-solidification ductile deformation, but others bearing mylonite-like features are related to viscous flow prior to complete solidification. There are indications that some of these pseudotachylytes were formed during the ductile regime, others being generated in the brittle regime during shear zone reactivation following exhumation or uplift. The occurrence of several pseudotachylytegenerating events indicates intermittent aseismic/seismic slip at different crustal levels throughout this shear zone.

Amorphous material in SAFOD core samples (San Andreas Fault): Evidence for crush‐origin pseudotachylytes?

Several types of amorphous material in ultracataclastic core samples recovered from 3194 m and 3294 m depth of the main bore hole of the San Andreas Fault Observatory at Depth are identified and described with transmission electron microscopy and scanning electron microscopy. We observed (1) amorphous material on a slickenside surface, (2) glassy bands contained in an ultracataclastic matrix and (3) amorphous rims surrounding quartz or feldspar clasts. Chemical analyses of the amorphous material reveal that silica content is slightly enriched or similar as in the adjacent matrix. We suggest that all amorphous material was formed by comminution of clasts (crush‐origin pseudotachylytes) rather than by melting (melt‐origin pseudotachylytes). The observed amorphous phases may act as lubricating layers that reduce friction in the San Andreas Fault.

Kinematic vorticity analysis and evolving strength of mylonitic shear zones: New data and numerical results

The kinematic vorticity number is an important quantity in structural geology and tecton- ics, giving a nonlinear ratio of simple shear to pure shear deformation. We use natural obser- vations and numerical models to show how rigid clast methods for determining the kinematic vorticity number (W k ) are compromised where strain localization occurs at the matrix-clast interface. Our numerical results show that the critical shape factor cutoff between perma- nently rotating and stable clasts, used to determine W k , is highly sensitive to coupling between the clast and the matrix. This fi nding provides an elegant explanation for the fact that rigid clast methods tend to underestimate W k relative to other methods. We present numerically determined envelopes for clast behavior across a range of kinematic vorticity numbers, clast shape factors, and matrix-clast coupling. Our numerical models show that the shape-pre- ferred orientations of feldspar clasts trend toward the positions of mica fi sh with increasing localization at the clast boundary, suggesting that mica fi sh behave as highly lubricated clasts. Our data and numerical results show that the clast-matrix interface may be several orders of magnitude weaker than the surrounding matrix and that weak interfaces can lead to a marked drop in the bulk shear strength of faults and shear zones.

The Dhala structure, Bundelkhand craton, Central India—Eroded remnant of a large Paleoproterozoic impact structure

Abstract— The newly discovered Dhala structure, Madhya Pradesh State, India, is the eroded remnant of an impact structure with an estimated present‐day apparent diameter of about 11 km. It is located in the northwestern part of the Archean Bundelkhand craton. The pre‐impact country rocks are predominantly granitoids of ˜2.5 Ga age, with minor 2.0–2.15 Ga mafic intrusive rocks, and they are overlain by post‐impact sediments of the presumably &gt;1.7 Ga Vindhyan Supergroup. Thus, the age for this impact event is currently bracketed by these two sequences. The Dhala structure is asymmetrically disposed with respect to a central elevated area (CEA) of Vindhyan sediments. The CEA is surrounded by two prominent morphological rings comprising pre‐Vindhyan arenaceous‐argillaceous and partially rudaceous metasediments and monomict granitoid breccia, respectively. There are also scattered outcrops of impact melt breccia exposed towards the inner edge of the monomict breccia zone, occurring over a nearly 6 km long trend and with a maximum outcrop width of ˜170 m. Many lithic and mineral clasts within the melt breccia exhibit diagnostic shock metamorphic features, including multiple sets of planar deformation features (PDFs) in quartz and feldspar, ballen‐textured quartz, occurrences of coesite, and feldspar with checkerboard texture. In addition, various thermal alteration textures have been found in clasts of initially superheated impact melt. The impact melt breccia also contains numerous fragments composed of partially devitrified impact melt that is mixed with unshocked as well as shock deformed quartz and feldspar clasts. The chemical compositions of the impact melt rock and the regionally occurring granitoids are similar. The Ir contents of various impact melt breccia samples are close to the detection limit (1–1.5 ppb) and do not provide evidence for the presence of a meteoritic component in the melt breccia. The presence of diagnostic shock features in mineral and lithic clasts in impact melt breccia confirm Dhala as an impact structure. At 11 km, Dhala is the largest impact structure currently known in the region between the Mediterranean and southeast Asia.