Search

Variably oriented dolerite intrusions outcrop in the Albany–Fraser Orogen along the south coast of Western Australia with previously unknown ages but where previous studies interpreted Mesoproterozoic to Cretaceous emplacement. Here, we place temporal constraints on seven mafic intrusions across ∼150 km of coast using zircon U–Pb, apatite U–Pb, and plagioclase 40Ar/39Ar geochronology, coupled with whole-rock major and trace-element geochemistry, that reveal late Mesoproterozoic to potentially Early Cretaceous crystallisation ages. Three intrusions metamorphosed to greenschist facies are likely associated with either the emplacement of the ca 1210 Ma Marnda Moorn large igneous province or Stage II Albany–Fraser Orogeny, both of which were associated with the assembly of Rodinia. Three unmetamorphosed dykes have (probable) Neoproterozoic to lower Cambrian emplacement ages, likely associated with the ca 550–500 Ma Kuunga Orogeny during Gondwana assembly. The final sill, also unmetamorphosed, strikes perpendicular to the other six intrusions, shows unusual Pb anomalies and contains inherited zircon that has been reset by a Permian or younger event, pointing towards magmatism in southwestern Australia during the breakup of Gondwana. The new results provide hitherto unrecognised mafic intrusive evidence for modification of Proterozoic crust, potentially associated with Rodinia assembly, Gondwana assembly and Gondwana breakup in southwestern Australia. Variably oriented mafic dykes in southwest Australia are dated by zircon U–Pb, apatite U–Pb and plagioclase 40Ar/39Ar methods. The dykes are related to Rodinia assembly (ca 1200 Ma), Gondwana assembly (ca 550 Ma) and, probably, Gondwana breakup (ca 135 Ma). These new ages provide evidence for mafic activity clearly linked to the supercontinent cycle. [ABSTRACT FROM AUTHOR]

Meteoritic matrices are commonly classified by their modal mineralogy, alteration, and shock levels. Other "textural" characteristics are not generally considered in classification schemes, yet could carry important information about their genesis and evolution. Terrestrial rocks are routinely described by grain morphology, which has led to morphology‐driven classifications, and identification of controlling processes. This paper investigates three CV chondrites—Allende (CV3.2oxA), Kaba (CV3.0oxB), and Vigarano (CV3.3red)—to determine the morphologic signature of olivine matrix grains. 2D grain size and shape, and crystallographic preferred orientations (CPOs) are quantified via electron backscatter diffraction mapping. Allende contains the largest and most elongate olivine grains, while Vigarano contains the least elongate, and Kaba contains the smallest grains. Weak but notable CPOs exist in some regions proximal to chondrules and one region distal to chondrules, and CPO geometries reveal a weak flattening of the matrix grains against the edge of chondrules within Allende. Kaba contains the least plastically deformed grains, and Allende contains the most plastically deformed grains. We tentatively infer that morphology is controlled by the characteristics of the available population of accreting grains, and aqueous and thermal alteration of the parent body. The extent of overall finite deformation is likely dictated by the location of the sample with respect to compression, the localized environment of the matrix with respect to surrounding material, and the post deformation temperature to induce grain annealing. Our systematic, quantitative process for characterizing meteorite matrices has the potential to provide a framework for comparison within and across meteorite classes, to help resolve how parent body processing differed across and between chondritic asteroids. [ABSTRACT FROM AUTHOR]

In situ extraterrestrial samples returned for study (e.g., from the Moon) are crucial in understanding the origin and evolution of the Solar System as, contrary to meteorites, they provide a known geological context for the samples and their analyses. Asteroid 25143 Itokawa is a rubble-pile asteroid consisting of reaccumulated fragments from a catastrophically disrupted monolithic parent asteroid, and from which regolith dust particles have been recovered by the Hayabusa space probe (Japan Aerospace Exploration Agency). We analyzed two dust particles using electron backscatter diffraction and 40Ar/39Ar dating techniques. One of the grains, showing signs of 15-25 GPa impact shock pressure, yielded a 40Ar/39Ar plateau age of 2.3 ± 0.1 Ga. We developed a novel temperature-pressure-porosity model, coupled with diffusion models to show that the relatively low pressure and high temperature involved in the impact process can be reconciled only if the asteroid was already made of porous material at ca. 2.3 Ga and, thus, if asteroid Itokawa was already formed, thereby providing a minimum age for catastrophic asteroid breakup. A second particle shows no sign of deformation, indicating shock pressure of <10 GPa and a calculated maximum temperature of ~200 °C. This low temperature estimate is compatible with a lack of isotopic resetting for this particle. This suggests that the breakup of Itokawa's parent was a relatively low-temperature process at the scale of the asteroid, and occurred on a pre-shattered parent body. [ABSTRACT FROM AUTHOR]

The history of rifting and breakup of eastern Gondwana is recorded by the development and fill of the Perth Basin in Western Australia. Knowledge of the structural architecture and stratigraphic geometry of the Perth Basin is essential to understand the evolution of the Western Australian margin and its applications to hydrocarbon and geothermal prospects, and effective aquifer management. This study integrates existing, publicly available geological, gravity, magnetic and digital elevation data to develop the first refined, regional structural and stratigraphic interpretation of the entire onshore and offshore Perth Basin, Western Australia. This new 3D model offers formation depth and thickness predictions in areas of sparse or no data. The model shows significant heterogeneity in the preserved formation thicknesses and depths at both local and regional scales. These differences may have resulted from differential subsidence and/or differential exhumation, but the formation geometries alone cannot distinguish between these two models if any erosion has occurred. Only the Lower to Middle Jurassic stratigraphy has been minimally eroded and thereby records the net subsidence. This stratigraphic interval shows that subsidence was broadly hinged from south to north, with a greater subsidence rate in the southern and central Perth Basin. Localised differences in thicknesses across adjacent sub-basins were likely controlled by differential displacement along sub-basin bounding faults during subsidence and, subsequently, during exhumation episodes. This new 3D model of the entire Perth Basin provides a framework for numerical simulations of fluid and heat flow and large-scale tectonic analysis, such as stratigraphic forward modelling of the southwestern Australian margin. [ABSTRACT FROM PUBLISHER]

The evolution of faults and paleodrainage patterns on the southwestern Australian passive margin during and after the breakup of Gondwana in the Early Cretaceous remains poorly understood. This contribution investigates the fault and paleodrainage evolution in the southern Perth Basin with the use of the ‘Bunbury Basalt’, the only lava flows known to be synchronous with continental breakup. New aeromagnetic data have been integrated with well intersections and outcrop constraints to establish the first 3D model of the Bunbury Basalt. The model reveals that flows are up to 100 m thick and are predominantly confined to two north–south-trending paleovalleys and their tributaries situated in the Bunbury Trough in the southern Perth Basin. The Donnybrook Paleovalley flow ponded in a paleovalley proximal to the Darling Fault and is truncated by the two later flows within the Bunbury Paleovalley, which is positioned centrally in the Bunbury Trough. Offsets of the Bunbury Basalt have been used to identify new northeast- and northwest-trending faults in the southern Perth Basin, and broad folding is interpreted as a consequence of drag into the Darling and Busselton faults. The model has been used to determine post-basalt net displacements for the Darling and Busselton faults of 370 and 210 m, respectively, and <175 m for the northeast and northwest-trending faults. The source vents for the Bunbury Basalt were probably located at extensional jogs at intersections between the Darling Fault and subordinate oblique faults. These results challenge the views on longstanding quiescence of the post-breakup western Australian passive margin. [ABSTRACT FROM PUBLISHER]

Introduction: The recognition that zircon can undergo a variety of transformations during shock deformation has resulted in a rapidly evolving state of understanding for how this mineral responds to the extreme thermodynamic changes generated during meteorite impact [1]. Among the pantheon of different shocked zircon textures, FRIGN zircon is the most unusual type of zircon known [2]; it is formed by initial transformation of zircon to reidite [3], and the subsequent reversion of reidite back to neoblastic zircon [1] at temperatures >1200 °C [4]. A key requirement to identify a given granular zircon as FRIGN is to demonstrate the presence of systematic disorientation relationships of approximately 90°/<110> among zircon neoblasts, which are crystallographically inherited from the zircon-reidite transformation. What is now recognized as FRIGN zircon was first described at Meteor Crater [5], Acraman [1], Ries [3], and in Australasian tektites [6], prior to formalization of the FRIGN acronym [2]. FRIGN zircon has been documented from 20 published sites, most recently at the Gosses Bluff impact structure [7], and has been demonstrated to be a useful geochronometer for dating impact events [eg, 8, 9]. FRIGN without reidite? Reidite is not predicted to be preserved in FRIGN zircon [2], given its thermal instability in impact melt rocks [4]. The absence of reidite in FRIGN zircon has led to a proposal that formation of FRIGN zircon does not involve transformations to reidite [10]. The alternative model to form FRIGN zircon involves wholesale melting of zircon at T >2700 °C based on the inferred presence of an 'yttria' rim on zircon and <100 nm zirconia inclusions in zircon neoblasts. Most critically, the model proposed by [10] does not involve formation of reidite; the systematic orientation relations among zircon neoblasts are attributed to 'synneusis'. Here we refute the model of [10] that claims FRIGN zircon forms by melting of zircon and without involvement of reidite. The non-FRIGN model is flawed for the following reasons: The T estimate of >2700 °C is based on an erroneous identification of yttria; the thermal metastability of zirconia polymorphs at grain sizes <100 nm is unaccounted for; zircon does not 'melt' (it dissociates); and no adequate explanation is given for systematic orientation relations among zircon neoblasts that is founded in crystallography. FRIGN with reidite: (F)RIGN. Over the last year, multiple reports have been published of FRIGN zircon grains that contain preserved reidite [7, 11, 12, 13, 14]. All of these occurrences are in various impact melt rocks from the Gosses Bluff [7], Mistastin [11], Rochechouart [12], Haughton [13] and Ries [14] impact structures. The preservation of reidite in FRIGN zircon, here denoted as (F)RIGN, given evidence of both former and present reidite, provides incontrovertible evidence for the role of reidite in formation of FRIGN zircon. In these cases reidite has been effectively 'frozen in', and thus did not complete the transformation to neoblastic zircon. These examples include a variety of textural observations of the progressive recrystallization of reidite to neoblastic zircon. Examples from Haughton include reidite-bearing zircon grains in the same sample as fully FRIGN zircon grains, demonstrating a close spatial association [13]. Examples from Rochechouart include non-granular zircon grains dominated by reidite lamellae and reidite 'wedges' that contain fractures filled with granular zircon in FRIGN orientation [12]. Examples from Mistastin include grains with cores of coherent reidite that appear to have fully transformed to reidite; tendrils of granular zircon with FRIGN orientations encroach from the grain exterior, overprinting reidite [11]. Examples from Gosses Bluff include fully granular FRIGN zircon grains that contain scattered 'islands' of reidite domains throughout [7]. Summary: The FRIGN zircon model offers a phase-equilibria and crystallography-founded explanation for the systematic orientation relations among zircon neoblasts found in granular zircon from impact melt rocks. FRIGN zircon has thus far only been reported from impact settings, and has been described at 20 of the ~200 confirmed terrestrial impact sites (~10%). Recent descriptions of reidite preserved within FRIGN zircon, here termed (F)RIGN zircon, provide the critical 'missing link' for the FRIGN zircon model, and uniquely identifies the role of reidite in its formation. [ABSTRACT FROM AUTHOR]

Introduction: Gosses Bluff is a ~24 km diameter complex impact structure in central Australia, located 160 km WSW of Alice Springs. The impact age is poorly constrained at between 383-165 Ma [1,2,3]. Target rocks are near horizontal Late Proterozoic to Devonian sedimentary units of the Amadeus Basin [4]. An eroded expression of the ~4.5 km diameter central uplift forms well-preserved ridges that stand ~180-240 m above the surrounding plains [4,5]. The impact structure is well represented with gravity data, showing a ~3 km diameter low over the central uplift, with a gradient corresponding to the ~24 km diameter crater rim [6,7]. While shocked quartz, shatter cones, impact melt, impact breccias and impact glass have all been reported at Gosses Bluff [5], no accessory mineral microstructural studies have previously been done. An anomalous topographic high, called Mt Pyroclast, is located ~3 km south of the central uplift. Mt Pyroclast predominantly consists of tan and pale grey with black-coloured polymict lithic impact breccia and impact melt-bearing breccia [4]. Fieldwork in 2017 identified dark red polymict lithic breccia dykes ranging from 2 cm to 3 m in width that cross-cut the tan-coloured polymict lithic impact breccia; the red dykes are the focus of this study. Methods: A microscopic analysis of thin sections of the largest identified (3 m-wide) dark red polymict breccia dyke, which cross-cuts the polymict lithic impact breccia at Mt Pyroclast, was analysed in this study. Scanning electron microscopy (SEM) with backscatter electron (BSE), energy dispersive spectroscopy (EDS) and electron backscatter diffraction (EBSD) were used. Analysis was done using the Tescan Mira3 FESEM with Oxford Instruments Symmetry EBSD detector and AZtec Symmetry software, and a Tescan integrated mineral analyzer (TIMA) with four EDS detectors at the John de Laeter Centre (JDLC), Curtin University. Results and discussion: The red breccia is matrix supported (60% matrix, 40% clasts) and includes clasts of varying sedimentary compositions, the largest of which (up to 1 cm) is predominantly a porous carbonate which contains a mixture of calcite, dolomite, lechatelierite and other amorphous silica. The clasts are very angular and show very little evidence of reworking. Other than lechatelierite no conspicuous melt particles were observed. TIMA analysis of one thin section identified 85 zircon grains larger than 10 µm. These grains display a multitude of different microstructures, including: granular neoblastic zircons, many of which preserve crystallographic orientation microstructures that indicate former reidite in granular neoblastic (FRIGN) zircon; grains with evidence of localised dissociation of zircon to zirconia (ZrO2) and silica (SiO2) within granular domains along grain margins; grains with {112} deformation twins; and grains with planar deformation bands (PDB). Reidite was identified in several zircon grains, both as lamellae within subhedral non-granular grains, and as granular neoblasts coexisting with zircon within FRIGN zircon. This presence of reidite in non-granular grains, FRIGN zircon, FRIGN zircon with preserved reidite domains, {112} twins, PDBs, and dissociation textures, all within this section, represents nearly the full spectrum of pressure and temperature conditions reported for zircon from impact environments [8]. Such a wide variety of zircon microstructural responses suggests that the red breccia dykes contain material sampled from a variety of environments within the impact structure, and thus are not necessarily a remobilized/modified version of the dyke host rocks. Ongoing work is focused on determining the nature of the enigmatic dykes, including if the matrix contains evidence of fusion or is entirely clastic. FRIGN zircon grains in the red dyke are appropriate for U-Pb dating and will be targeted for analysis to further constrain the age of Gosses Bluff impact. [ABSTRACT FROM AUTHOR]

Introduction: The Ilkurlka structure is a ~12 km diameter circular aeromagnetic and gravity anomaly located ~400 km NW of Eucla, Western Australia, within the Officer Basin. The structure has been previously identified as a possible impact structure based on the geophysical anomalies [1,2,3]. The lack of diagnostic evidence of shock metamorphism has led to a range of origins being postulated for Ilkurlka, including a salt dome, an igneous intrusion, or a meteorite impact crater. The Officer Basin is regionally undeformed, except for local salt tectonics. We report the presence of shocked quartz grains with multiple sets of planar deformation features (PDFs) parallel to the {1013}-orientation that confirms an impact origin for the buried Ilkurlka structure. Methods: This study aimed to investigate and document the presence of deformation features at various scales in drill cores from two boreholes (BH01 and BH02) drilled by Maria Resources Pty Ltd. in 2019, which targeted the circular gravity anomaly. A survey of quartz from samples spanning the two boreholes was conducted to search for microstructural evidence of deformation uniquely produced by impact cratering (e.g., PDFs). Twenty samples from sandstone intervals were selected for investigation. Ten representative samples were cut and cast into 1-inch round epoxy mounts and polished. Scanning electron microscopy (SEM), including backscattered electron (BSE), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD), was performed at Curtin University. Nineteen samples were prepared into polished thin sections. Petrography and a search for planar fractures (PFs) and PDFs in quartz in thin section was conducted at Curtin University. Crystallographic orientations of PDFs in shocked quartz were determined using a universal stage at Lund University. Results and Discussion: Samples from BH01 and BH02 generally consist of poorly to moderately sorted fine to coarse grained sandstone interlayered with siltstone and mudstone, and are locally brecciated. These rock units have been provisionally assigned to the lower Cambrian Marla Group based on lithological similarities to that succession in other drill cores in the eastern Officer Basin. The sandstones consist of medium to coarse, sub-rounded to rounded quartz, lithics, and feldspar grains, set in matrix and/or cements (~10-25%). The matrix components consist of micas and carbonates, silts and clays, and very fine to fine, angular to subangular quartz grains that appear to be crushed. Original pore space is locally cemented with calcite, gypsum, and minor anhydrite. Petrographic analysis and SEM imaging reveal variably deformed quartz grains. Two adjacent grains of shocked quartz were found, each with multiple sets of PDFs parallel to the {1013}-orientation, in a sample from 415.9 m (downhole depth) in BH02. Other deformation features observed in quartz include discrete zones of crushed angular grains, sub-planar fractures, grains with high fracture density in random orientations including shattered grains, concussion fractures at the boundary of multiple grains, and fractures that offset grains, indicating shear displacement. The concussion fractures are Hertzian fractures that emanate from grain-to-grain contacts and cut across up to six adjacent grains. Concussion fractures in quartz have been reported previously in shock-metamorphosed sandstone samples from several impact craters [e.g. 4-7] and in shock experiments [e.g. 8]. Kink bands in biotite are also present. The presence of PDFs parallel to the {1013}-orientation in quartz suggests that the target underwent shock pressures corresponding to shock stage 3 [9]. However, the apparent preservation of porosity and lack of diaplectic glass or high pressure SiO2 phases are characteristic of shock stage 1a [9]. The timing of cementation of the porosity with respect to impact-related deformation is difficult to ascertain. It is possible that impedance contrasts between cements and quartz grains created discrete zones of deformation which could have allowed spatially variable shock effects to occur. Regardless of shock stage assignment, PDFs allow confirmation of an impact origin. Acknowledgements: Financial support was provided by the Barringer Family Fund Research Grant (Quintero, 2020), The Institute for Geoscience Research (TIGeR) Curtin University, and an Australian Research Training Program Scholarship (Quintero, 2019). Core samples were provided by Strategic Elements/Maria Resources. PWH publishes with the permission of the Executive Director of GSWA. [ABSTRACT FROM AUTHOR]

Introduction: Dynamic shock recovery experiments constrain conditions of mineral transformations during shock metamorphism. For zircon, existing dynamic experiments involve either single crystals or powder [1, 2]; no dynamic experiments have been conducted on zircon in a host rock (porous or non-porous). We report a microstructural study of 597 zircon crystals in sandstone samples experimentally shock-deformed under the auspices of the MEMIN program [3]. The samples are Seeberger sandstone with 25-30% initial porosity, dynamically shocked in 2.5 GPa increments from 2.5 to 17.5 GPa. They are the same thin sections analyzed by Kowitz et al. [3, 4] to characterize shock deformation of quartz. A total of 414 zircons were mapped by electron backscatter diffraction. Reidite formed in samples from 10 to 17.5 GPa, including irregular lenses and coherent domains up to 10 μm across. The abundance of grains with reidite (10 to 40 %) increases with pressure. Deformation Twins along {112} planes formed in samples from 10 to 17.5 GPa. Twins occur as sets of parallel lamellae with widths <1 μm in one {112} orientation. Planar Fractures formed in samples from 2.5 to 17.5 GPa; their abundance did not increase with pressure. Planar Deformation Bands formed in samples from 2.5 to 17.5 GPa; their abundance did not increase with pressure. Vesicular melt films along zircon margins formed in samples from 7.5 to 17.5 GPa. Reidite and {112} deformation twins formed in samples shocked from 10 to 17.5 GPa. Zircon {112} twins have not previously been reported from dynamic experiments (c.f. [2]). The twins demonstrate shear stresses sufficient for twinning [5] occur at and above 10 GPa in porous targets. Previous dynamic experiments have not formed reidite below 30 GPa [1, 2]. We attribute reidite formation at 10 GPa in Seeberger sandstone to heterogeneous pressure excursions at the pore-scale, which have been shown to result in localized amplification of shock pressures up to 4x within these porous sandstones [4, 6]. Based on [4], we present a shock stage classification system for progressive deformation of both zircon and quartz in dry porous sandstone (25-30 % porosity) shocked between 0 and 17.5 GPa. [ABSTRACT FROM AUTHOR]

Introduction: The geological record of the formation and differentiation of our planet has been destroyed by its subsequent evolution, but extremely rare clues may be obtained from other terrestrial planets. Mars provides a unique and accessible example of an early evolutionary path corresponding to that, inaccessible, of our own world. We can investigate it with spacecraft, and samples are available for in-depth analysis on Earth in the form of martian meteorites. So far, the only available martian samples that appear to have recorded the early conditions and the evolution of the planet until the present time are Northwest Africa (NWA) 7034 and its paired stones, nicknamed "Black Beauty". This regolith breccia has been ejected a few million years ago by the formation of a large impact crater, and contains the oldest martian igneous material ever dated: ~4.5 Ga old [1-4]. However, its source and geological context have so far remained unknown. and with it, a region where the earliest geological records of the planet [1-4] are exposed on the surface. Knowing this source region would provide insights into early Mars geological history and crustal extraction [1-4]. This source region may therefore become a high-priority target for detailed orbital analyses and in-situ exploration [5]. Constraints on the meteorite launch site: Following a hypervelocity impact, ejecta materials moving faster than the escape velocity (5 km/s) may get through the martian atmosphere and continue their course into interplanetary space to become martian meteorites [6]. Slower debris fall back on the surface in a radial pattern or ray around the primary crater, forming secondary craters. The presence of 100 meter-size secondaries attests to the freshness of their associated primary craters [7]. Using the size and spatial distribution of more than 90 million impact craters >50 m (both primaries and secondaries) detected using a Crater Detection Algorithm (CDA) [7-8] on the whole surface of Mars from the global Context Camera (CTX) mosaic [9], a previous work [7] identified ray systems of secondary craters <150 m associated with 19 large primary craters, potential source of the ejection of martian meteorites. Here we compare the abundance of K and Th [10-11] as well as the magnetic field intensity and the magnetization of the surface of Mars derived from orbital measurements [12] at the immediate vicinity of each crater candidate with those of the breccia. We also compare the geological context of each of the crater candidates with the chronology and the lithology of the meteorite [e.g. 1-3, 13-14]. The ejection site for Black Beauty: Among the 19 crater candidates investigated, we found that only one match with the characteristics of the meteorite: a 10 km crater located in the north-east of the Terra Cimeria - Sirenum (TCTS) region, between Hesperia Planum and the Tharsis bulge. Our work suggests that clasts contained in the regolith breccia are representative of the TCTS province, making this region a relic of the early crustal processes on Mars [e.g. 15], and thus, a region of high interest for future missions. Details on the identification of the crater source of this unique meteorite, as well a its geological context and broader implications for early crustal processes on Mars will be presented at the conference. [ABSTRACT FROM AUTHOR]

The Perth Basin (PB) hosts important aquifers within the Yarragadee Formation and adjacent geological formations with potential for economic exploitation by both geothermal energy and carbon capture and sequestration. Published studies on the reservoir quality of the sedimentary units of the PB are very few. This study reports some petrophysical and lithological characteristics of the sedimentary units of interest for geothermal and geosequestration scenarios and help interpolation toward non-sampled intervals. A new fluvial-dominated lithofacies scheme was developed for the Mesozoic stratigraphy from four wells drilled in the central PB (Pinjarra-1, Cockburn-1, Gingin-1 and Gingin-2) based on grainsize, sorting, sedimentary structures and colour that relate to the environment of deposition. Systematic laboratory measurements of permeability, porosity, and thermal conductivity were conducted on core samples to investigate a variety of lithofacies and depths from these wells. Empirical correlations are established among the different physical properties, indicating encouraging relationships for full PB basin interpolation such as between porosity and permeability, when the samples are grouped into ‘hydraulic units’ defined by a ‘flow zone indicator’ parameter. The common principal controls on the PB thermal conductivity are the pore space arrangement and mineralogical content, which are strongly lithofacies-specific. Therefore, the lithofacies type could be a good first-order discriminator for describing spatial variations of thermal conductivity and then estimate their flow zone indicator. [ABSTRACT FROM PUBLISHER]

Detailed 3-D analysis of inclusion trails in garnet porphyroblasts and matrix foliations preserved around a hand-sample scale, tight, upright fold has revealed a complex deformation history. The fold, dominated by interlayered quartz–mica schist and quartz-rich veins, preserves a crenulation cleavage that has a synthetic bulk shear sense to that of the macroscopic fold and transects the axis in mica-rich layers. Garnet porphyroblasts with asymmetric inclusion trails occur on both limbs of the fold and display two stages of growth shown by textural discontinuities. Garnet porphyroblast cores and rims pre-date the macroscopic fold and preserve successive foliation inflection/intersection axes (FIAs), which have the same trend but opposing plunges on each limb of the fold, and trend NNE–SSW and NE–SW, respectively. The FIAs are oblique to the main fold, which plunges gently to the WSW. Inclusion trail surfaces in the cores of idioblastic porphyroblasts within mica-rich layers define an apparent fold with an axis oblique to the macroscopic fold axis by 32°, whereas equivalent surfaces in tabular garnet adjacent to quartz-rich layers define a tighter apparent fold with an axis oblique to the main fold axis by 17°. This potentially could be explained by garnet porphyroblasts that grew over a pre-existing gentle fold and did not rotate during fold formation, but is more easily explained by rotation of the porphyroblasts during folding. Tabular porphyroblasts adjacent to quartz-rich layers rotated more relative to the fold axis than those within mica-rich layers due to less effective deformation partitioning around the porphyroblasts and through quartz-rich layers. This work highlights the importance of 3-D geometry and relative timing relationships in studies of inclusion trails in porphyroblasts and microstructures in the matrix. [ABSTRACT FROM AUTHOR]

The Allende meteorite, a relatively unaltered member of the CV carbonaceous chondrite group, contains primitive crystallographic textures that can inform our understanding of early Solar System planetary compaction. To test between models of porosity reduction on the CV parent body, complex microstructures within ~0.5-mm-diameter chondrules and ~10-µm-long matrix olivine grains were analyzed by electron backscatter diffraction (EBSD) techniques. The large area map presented is one of the most extensive EBSD maps to have been collected in application to extraterrestrial materials. Chondrule margins preferentially exhibit limited intragrain crystallographic misorientation due to localized crystal-plastic deformation. Crystallographic preferred orientations (CPOs) preserved by matrix olivine grains are strongly coupled to grain shape, most pronounced in shortest dimension , yet are locally variable in orientation and strength. Lithostatic pressure within plausible chondritic model asteroids is not sufficient to drive compaction or create the observed microstructures if the aggregate was cold. Significant local variability in the orientation and intensity of compaction is also inconsistent with a global process. Detailed microstructures indicative of crystal-plastic deformation are consistent with brief heating events that were small in magnitude. When combined with a lack of sintered grains and the spatially heterogeneous CPO, ubiquitous hot isostatic pressing is unlikely to be responsible. Furthermore, Allende is the most metamorphosed CV chondrite, so if sintering occurred at all on the CV parent body it would be evident here. We conclude that the crystallographic textures observed reflect impact compaction and indicate shock-wave directionality. We therefore present some of the first significant evidence for shock compaction of the CV parent body. [ABSTRACT FROM AUTHOR]

The article presents the results of a study on how deformed monazite from granulite facies rocks of the Sandamata Complex, India yields high-temperature tectonic ages. Topics covered include how the granulite host rocks in the Sandamata monazite contain an anhydrous mineral assemblage without any textural evidence of fluid-related retrogression and how the neoblasts have preferentially nucleated and grown along several types of crystal defects.

The deformation of monazite in the polymetamorphic Sandmata granulite complex in India has been investigated by electron backscattered diffraction and sensitive high-resolution ion microprobe (SHRIMP). Quantitative microstructural analyses document the development of deformation twins in {100}, {001}, and {12...} orientations; low-angle (<10°) boundary development associated with dislocation creep; and the development of new grains due to dynamic recrystallization. These data represent the first quantitative evidence of crystal-plastic deformation of natural monazite. SHRIMP U-Th-Pb analysis shows that the host monazite preserves discordant ages as old as 1666 ± 28 Ma, along a trend from ca. 1720 Ma to ca. 1000 Ma, with increasingly discordant ages recorded in zones of higher lattice distortion. Domains of recrystallized new grains within the monazite record a tightly clustered concordia age of 970 ± 14 Ma. This age is interpreted to represent the timing of monazite dynamic recrystallization associated with deformation of the host protolith, and is consistent with partial resetting and Pb loss from domains deforming by dislocation creep. The complex, but systematic, relationship between microstructure and age data in monazite provide the first direct evidence of Pb isotope resetting during deformation. The approach illustrates a new methodology for the dating of deformation events in high-grade metamorphic rocks, which are typically difficult to constrain. [ABSTRACT FROM AUTHOR]