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1.
Abstract— The Offset Dikes of the 1.85 Ga Sudbury Igneous Complex (SIC) constitute a key topic in understanding the chemical evolution of the impact melt, its mineralization, and the interplay between melt migration and impact‐induced deformation. The origin of the melt rocks in Offset Dikes as well as mode and timing of their emplacement are still a matter of debate. Like many other offset dikes, the Worthington is composed of an early emplaced texturally rather homogeneous quartz‐diorite (QD) phase at the dike margin, and an inclusion‐ and sulfide‐rich quartz‐diorite (IQD) phase emplaced later and mostly in the centre of the dike. The chemical heterogeneity within and between QD and IQD is mainly attributed to variable assimilation of host rocks at the base of the SIC, prior to emplacement of the melt into the dike. Petrological data suggest that the parental magma of the Worthington Dike mainly developed during the pre‐liquidus temperature interval of the thermal evolution of the impact melt sheet (>1200 °C). Based on thermal models of the cooling history of the SIC, the two‐stage emplacement of the Worthington Dike occurred likely thousands to about ten thousand years after impact. Structural analysis indicates that an alignment of minerals and host rock fragments within the Worthington Dike was caused by ductile deformation under greenschist‐facies metamorphic conditions rather than flow during melt emplacement. It is concluded that the Worthington Offset Dike resulted from crater floor fracturing, possibly driven by late‐stage isostatic readjustment of crust underlying the impact structure.  相似文献   

2.
Daniel Lieger  Ulrich Riller 《Icarus》2012,219(1):168-180
The central Vredefort Impact Structure is characterised by impact melt rocks, known as the Vredefort Granophyre dikes, the mode of emplacement of which is not well known. Whole-rock and petrographic analyses of two dikes were conducted and compared to published geochemical data to elucidate the mode and timing of dike formation. The dikes are characterised by compositional and textural heterogeneity between, and within, individual dikes. Specifically, central dike portions are felsic and rich in wall rock fragments, whereas marginal dike phases are more mafic and fragment-poor. Collectively, this suggests that melt was derived from compositionally different parental melts and emplaced in at least two pulses. In addition, the chemical heterogeneity between fragment-rich and fragment-poor dike zones can be explained by variable assimilation of a mafic component, notably Ventersdorp basalt, at the base of the impact melt sheet, from which melt of the Granophyre dikes is derived. This scenario accounts for the mafic and fragment-poor character of melt emplaced first in the dikes and the more felsic and fragment-rich nature of melts of the following emplacement pulse, i.e., when the impact melt was less hot and thus less capable of digesting large quantities of (mafic) wall rock fragments. Differences in geometrical, textural, chemical and fragment characteristics between the Granophyre dikes and pseudotachylite bodies can be explained by the same process, i.e., impact melt drainage, but operating at different times after impact.  相似文献   

3.
Abstract To investigate the origin of Offset Dikes and their age relationships to major impact generated lithologies in the Sudbury multi-ring impact structure, such as the Main Mass of the Sudbury “Igneous” Complex, zircon and baddeleyite were dated by the U-Pb chronometer. The rocks analysed are one diorite and two quartz diorites from inside the Foy Offset, one quartz diorite from the contact zone, and two country rock samples collected at 10 and 30 m distances from the contact within the Levack Gneiss Complex. The 21 analysed zircon and baddeleyite fractions yield a crystallization age of 1852 +4/-3 (2σ) Ma for the accessory minerals in the Foy Offset Dike and an age of 2635 ± 5 Ma for the shocked Levack country rock, in which zircons show significant shock effects (multiple sets of planar fractures), in contrast to the totally unshocked zircons of the Offset Dike. Within given errors, the new age of 1852 Ma is identical to the pooled 1850 ± 1 Ma U-Pb age determined by Krogh et al. (1984) as the crystallization age of accessory phases in different lithologies of the Sudbury “Igneous” Complex, which has been interpreted to represent the coherent impact melt sheet of the Sudbury Structure. This excellent agreement of the ages substantiates that emplacement of the Offset Dikes occurred coevally with the formation of the impact melt sheet. Total absence of inherited zircons in the central part of the Foy Offset indicates melting of the precursor material at temperatures well above 1700 °C, which emphasizes the origin of the dike lithologies by impact melting.  相似文献   

4.
Abstract— The Hess Offset is a steeply dipping dyke located 12–15 km north of the 1.85 Ga Sudbury igneous complex (SIC) within the 200–250 km diameter Sudbury impact structure. It is up to 60 m wide and strikes subconcentrically to the SIC for at least 23 km. The main phase of the dyke is granodioritic, but it conforms with what is locally referred to as Quartz Diorite: a term used for all the Offset Dykes of the Sudbury impact structure. Rare earth element data shows that the Hess Offset is genetically related to the SIC. Hess is most closely affiliated with an evolved Felsic Norite component of SIC and not bulk impact melt. This indicates that Hess was emplaced during fractionation of the impact melt sheet, rather than immediately following impact. The main Quartz Diorite phase of the dyke comprises a quartz + plagioclase + hornblende + biotite ± clinopyroxene ± orthopyroxene assemblage. Critically, the Hess Offset occupies a concentric fault system that marks the northern limit of a pseudotachylyte-rich, shatter cone-bearing annulus about the SIC. This fault system was active during the modification stage of the impact process.  相似文献   

5.
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6.
The offset dykes of the Sudbury Igneous Complex comprise two distinct main magmatic facies, a high-temperature inclusion-free quartz diorite (QD), and a subsequently intruded lower temperature, mineralized, and inclusion-rich quartz diorite (MIQD). The MIQD facies was emplaced after QD dykes had solidified. Key controlling factors of the two injection phases were (1) the development of a coherent roof, which confined the melt sheet; and (2) the periodic increase of melt and fluid pressure within the melt sheet. For the injection of QD melt, the melt pressure exceeded the normal stress acting on fracture surfaces. For the later refracturing of QD dykes and the injection of MIQD melt, the melt pressure increased further, exceeding the tensile strength of, and the normal stress acting on, QD dykes. We associate the melt pressure increase required for both injection episodes with degassing and devolatilization of cooling melt close to the roof. Within the hydraulically connected melt column, the related pressure increase was transmitted to the base of the melt sheet where QD and MIQD melt was extracted into dykes. Residual core to rim thermal gradients in the QD dykes produced tensile strength gradients, accounting for the typically central location of MIQD dykes within QD dykes.  相似文献   

7.
Models of the emplacement of lateral dikes from magma chambers under constant (buffered) driving pressure conditions and declining (unbuffered) driving pressure conditions indicate that the two pressure scenarios lead to distinctly different styles of dike emplacement. In the unbuffered case, the lengths and widths of laterally emplaced dikes will be severely limited and the dike lengths will be highly dependent on chamber size; this dependence suggests that average dike length can be used to infer the dimensions of the source magma reservoir. Probable examples on Earth of the unbuffered case are flanking rift zones on shield volcanoes such as the Hawaiian Kilauea East Rift Zone, in which the dikes of average widths of less than a meter extend for several km from the central part of the edifice. In contrast, emplacement of lateral dikes in the constant driving pressure (buffered) case can produce dikes which have sizes and widths which are very large, and are independent of chamber size. For relatively shallow magma chambers, buffered emplacement is expected to produce graben of relatively fixed length which are associated with eruptive fissures and long, large volume lava flows. A decline in magma supply rate and loss of pressure buffering during the later stages of such eruptions may give rise to caldera formation/collapse events. Deeper dikes are not likely to erupt but will produce surface graben of variable length. Therefore, edifices or dike swarms which show an extremely wide variation in fracture or dike lengths are likely to have been formed in buffered conditions. On Earth, the characteristics of many mafic-dike swarms suggest that they were emplaced in buffered conditions (e.g., the Mackenzie dike swarm in Canada and some dikes within the Scottish Tertiary). On Venus, the distinctive radial fractures and graben surrounding circular to oval features and edifices on many size scales and extending for hundreds to over a thousand km are candidates for dike emplacement in buffered conditions.  相似文献   

8.
Abstract— The South Range Breccia Belt (SRBB) is an arcuate, 45 km long zone of Sudbury Breccia in the South Range of the 1.85 Ga Sudbury Impact Structure. The belt varies in thickness between tens of meters to hundreds of meters and is composed of a polymict assemblage of Huronian Supergroup (2.49–2.20 Ga), Nipissing Diabase (2.2 Ga), and Proterozoic granitoid breccia fragments ranging in size from a few millimeters to tens of meters. The SRBB matrix is composed of a fine‐grained (～100 μm) assemblage of biotite, quartz, and ilmenite, with trace amounts of plagioclase, zircon, titanite, epidote, pyrite, chalcopyrite, pyrrhotite, and occasionally chlorite. The SRBB hosts the Frood‐Stobie, Vermilion, and Kirkwood quartz diorite offset dykes, the former being associated with one of the largest Ni‐Cu‐PGE sulphide deposits in the world. Optical petrography and whole‐rock geochemistry concur with previous studies that have suggested that the matrix of the SRBB is derived from comminution and at least partial frictional melting of the wall rock Huronian Supergroup lithologies. Rare earth element (REE) data from all sampled lithologies associated with the SRBB exhibit crustal signatures when normalized to C1 chondrite values. Additionally, REE data from the quartz diorites, disseminated sulphides in Sudbury Breccia, and a sample of an aphanitic biotite‐hornblende tonalite dyke exhibit flat slopes when compared to the mafic and felsic norites, quartz gabbro, and granophyre units of the Sudbury Igneous Complex (SIC), which suggests that these lithologies are representative of bulk SIC melt. We suggest that the SRBB was formed by high strain‐rate (>1 m/s), gravity‐driven seismogenic slip of the inner ring of the Sudbury Impact Structure during postimpact crustal readjustment (crater modification stage). Failure of the hanging wall may have facilitated the injection of bulk SIC melt into the SRBB, along with the Ni‐Cu‐PGE sulphides of the Frood‐Stobie deposit. Postimpact Penokean (1.9–1.7 Ga) tectonism, particularly northwest‐directed shearing along the South Range Shear Zone and associated thrust faulting, could account for the present subvertical orientation of the SRBB, and the apparent lack of a connection at depth with the SIC.  相似文献   

9.
Abstract– The twin Arkenu circular structures (ACS), located in the al‐Kufrah basin in southeastern Libya, were previously considered as double impact craters (the “Arkenu craters”). The ACS consist of a NE (Arkenu 1) and a SW structure (Arkenu 2), with approximate diameters of about 10 km. They are characterized by two shallow depressions surrounded by concentric circular ridges and silica‐impregnated sedimentary dikes cut by local faults. Our field, petrographic, and textural observations exclude that the ACS have an impact origin. In fact, we did not observe any evidence of shock metamorphism, such as planar deformation features in the quartz grains of the collected samples, and the previously reported “shatter cones” are wind‐erosion features in sandstones (ventifacts). Conversely, the ACS should be regarded as a “paired” intrusion of porphyritic stocks of syenitic composition that inject the Nubia Formation and form a rather simple and eroded ring dike complex. Stock emplacement was followed by hydrothermal activity that involved the deposition of massive magnetite–hematite horizons (typical of iron oxide copper‐gold deposits). Their origin was nearly coeval with the development of silicified dikes in the surroundings. Plugs of tephritic‐phonolitic rocks and lamprophyres (monchiquites) inject the Nubian sandstone along conjugate fracture zones, trending NNW–SSE and NE–SW, that crosscut the structural axis of the basin.  相似文献   

10.
Magellan radar data from western Vinmara Planitia on Venus reveal a system of radiating lineaments extending 450 km from a small central annulus. Spatial variations in lineament density, orientation, and morphology, as well as structural and volcanic correlations, provide strong evidence that formation of the lineaments was related to subsurface dike emplacement. We infer from the observed surface deformation that the dikes were emplaced laterally, at shallow depth, from a large central magma reservoir. This configuration is analogous to that of radiating dike swarms found on Earth. Because dikes inject normal to the least compressive stress direction, swarm plan view geometry will reveal the greatest horizontal compressive stress trajectories. We interpret strongly radial orientations near the swarm center to represent radial stresses linked to pressurization of the magma reservoir. Increasingly non-radial behavior dominating at greater distances is interpreted to reflect a N60E±20° regional maximum horizontal compressive stress. Contrary to previous inferences that a persistent E–W compressive stress dominated throughout, analysis of the arachnoid indicates that a N60E compressive stress must have existed across western Vinmara Planitia during a portion of its deformation. This and the absence of distributed shear within the adjacent deformation belts indicates that the regional maximum horizontal compression orientation has varied over time. Comparison between the regional stress orientations inferred from the arachnoid and several nearby ridge belts illustrates that stress orientations may potentially be useful for determining relative belt ages in areas where the timing of ridge belt formation is difficult to assess by more direct means. This demonstrates one way that identification and analysis of giant radiating dike swarms can provide new information critical for regional stress interpretations on Venus.  相似文献   

11.
Abstract The pattern of radial and concentric offset dikes at Sudbury strongly resembles fracture patterns in certain volcanically modified craters on the Moon. Since the Sudbury dikes apparently formed shortly after the impact event, this resemblance suggests that early endogenic modification at Sudbury was comparable to deformation in lunar floor-fractured craters. Although regional deformation has obscured many details of the Sudbury Structure, such a comparison of Sudbury with lunar floor-fractured craters provides two alternative models for the original size and surface structures of the Sudbury basin. First, the Sudbury date pattern can be correlated with fractures in the central peak crater Haldane (36 km in diameter). This comparison indicates an initial Sudbury diameter of between 100 and 140 km but requires loss of a central peak complex for which there is little evidence. Alternatively, comparison of the Sudbury dikes with fractures in the two-ring basin Schrödinger indicates an initial Sudbury diameter of at least ～ 180 km, which is in agreement with other recent estimates for the size of the Sudbury Structure. In addition to constraining the size and structure of the original Sudbury crater, these comparisons also suggest that crater modification may reflect different deformation mechanisms at different sizes. Most lunar floor-fractured craters are attributed to deformation over a shallow, crater-centered intrusion; however, there is no evidence for such an intrusion at Sudbury. Instead, melts from the evolving impact melt sheet probably entered fractures formed by isostatically-induced flexure of the crater floor. Since most of the lunar floor-fractured craters are too small (<100-km diameter) to induce significant isostatic adjustment, crater modification by isostatic uplift apparently is limited to only the largest of craters, whereas deformation over igneous intrusions dominates the modification of smaller craters.  相似文献   

12.
Recent observations and geophysical studies at the Vredefort impact structure have indicated that the impact melt dikes in the central uplift of the structure have small depth extents. In this study, we performed magnetic and electrical resistivity tomography (ERT) surveys of the Lesutoskraal granophyre dike (LGD) and trenched to confirm its depth extent. The ERT survey showed that outcrops of the LGD are associated with shallow resistive zones with <3 m depth extent, but such zones do not occur where outcrops are absent. Visual observations in the trench confirmed that the dike has a small depth extent (~0.75 m) at this location. However, the magnetic survey revealed anomalies along the entire strike of the dike, even where no outcrops occur. We suggest that remagnetization of the host rock within a metamorphic contact aureole could explain the presence of magnetic anomalies in the absence of outcrops. Considering the results of the ERT survey, the observations made in the trench, and the surface distribution of outcrops of the LGD, we confirm that this dike has a small depth extent (<3 m) along its entire length and propose that outcrops represent the intersection of the dike terminus with the current erosional surface.  相似文献   

13.
The 1.85 Ga Sudbury impact structure is one of the largest impact structures on Earth. Igneous bodies—the so‐called “Basal Onaping Intrusion”—occur at the contact between the Sudbury Igneous Complex (SIC) and the overlying Onaping Formation and occupy ~50% of this contact zone. The Basal Onaping Intrusion is presently considered part of the Onaping Formation, which is a complex series of breccias. Here, we present petrological and geochemical data from two drill cores and field data from the North Range of the Sudbury structure, which suggests that the Basal Onaping Intrusion is not part of the Onaping Formation. Our observations indicate that the Basal Onaping Intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components. Increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the Basal Onaping Intrusion. Planar deformation features within quartz clasts of the Basal Onaping Intrusion are indicators for shock metamorphism and, together with the melt matrix, point to the Basal Onaping Intrusion as being an impact melt rock, by definition. Importantly, the contact between Granophyre of the SIC and Basal Onaping Intrusion is transitional and we suggest that the Basal Onaping Intrusion is what remains of the roof rocks of the SIC and, thus, is a unit of the SIC and not the Onaping Formation. We suggest henceforth that this unit be referred to as the “Upper Contact Unit” of the SIC.  相似文献   

14.
Abstract— The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil‐1 (Yax‐1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact‐generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax‐1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán‐6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán‐6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax‐1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km‐ thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.  相似文献   

15.
Abstract— Orogenic deformation, both preceding and following the impact event at Sudbury, strongly hinders a straightforward assessment of impact‐induced geological processes that generated the Sudbury impact structure. Central to understanding these processes is the state of strain of the Sudbury Igneous Complex, the solidified impact melt sheet, its underlying target rocks, overlying impact breccias and post‐impact sedimentary rocks. This review addresses (1) major structural, metamorphic and magmatic characteristics of the impact melt sheet and associated dikes, (2) attempts that have been made to constrain the primary geometry of the igneous complex, (3) modes of impact‐induced deformation as well as (4) mechanisms of pre‐ and post‐impact orogenic deformation. The latter have important consequences for estimating parameters such as magnitude of structural uplift, tilting of pre‐impact (Huronian) strata and displacement on major discontinuities which, collectively, have not yet been considered in impact models. In this regard, a mechanism for the emplacement of Offset Dikes is suggested, that accounts for the geometry of the dikes and magmatic characteristics, as well as the occurrence of sulfides in the dikes. Moreover, re‐interpretation of published paleomagnetic data suggests that orogenic folding of the solidified melt sheet commenced shortly after the impact. Uncertainties still exist as to whether the Sudbury impact structure was a peak‐ring or a multi‐ring basin and the deformation mechanisms of rock flow during transient cavity formation and crater modification.  相似文献   

16.
Genesis and emplacement of Vredefort Granophyre, the impact melt rock exposed on the Vredefort Dome, the erosional remnant of the central uplift of the Vredefort impact structure, South Africa, have long been debated. This debate was recently reinvigorated by the discovery that besides the previously known felsic variety of >66 wt% SiO2, a second, somewhat more mafic phase of <66 wt% SiO2 occurs along a Granophyre dike on farms Kopjeskraal and Eldorado in the northwest sector of the dome. Two hypotheses have been put forward to explain the genesis and emplacement of this second phase: (1) successive injections of impact melt into extensional fractures opened in the course of central uplift formation/crater modification, with melts of distinct compositions derived from a differentiating impact melt body in the crater, and (2) generation of the more mafic phase as a product of admixture/assimilation of a mafic country rock component, either the so-called epidiorite of possible Ventersdorp Supergroup affiliation or the Dominion Group meta-lava (DGL), to Felsic Granophyre. In the latter model, contamination with mafic country rock would have occurred during downward intrusion and stoping into and below the crater floor. The so-called Mafic Granophyre has previously only ever been sampled on a single site (Farm Kopjeskraal). In this study, samples of Granophyre occurring along the southerly extension of this dike on farm Rensburgdrif, and from a second dike on the Rietkuil property further southwest were investigated by field work, and petrographic, geochemical, and isotopic analysis. The mafic phase indeed occurs in the interior of the dike at Rensburgdrif, and also on Rietkuil. New geochemical and Sr-Nd isotope data support the hypothesis that the Mafic Granophyre composition represents a mixture between Felsic Granophyre and a mafic country rock. A 20% admixture of epidiorite or DGL to Felsic Granophyre provides an excellent match for the chemical composition of the Mafic Granophyre. The Sr-Nd isotope data indicate that this admixture likely involved the epidiorite component rather than DGL. Together with earlier Sr-Nd-Os-Se isotopic data, and other geochemical data, these results further support formation of the Mafic Granophyre by local assimilation/admixture of epidiorite to Felsic Granophyre.  相似文献   

17.
Abstract— A large impact event 500 Ma ago shocked and melted portions of the L‐chondrite parent body. Chico is an impact melt breccia produced by this event. Sawn surfaces of this 105 kg meteorite reveal a dike of fine‐grained, clast‐poor impact melt cutting shocked host chondrite. Coarse (1–2 cm diameter) globules of FeNi metal + sulfide are concentrated along the axis of the dike from metal‐poor regions toward the margins. Refractory lithophile element abundance patterns in the melt rock are parallel to average L chondrites, demonstrating near‐total fusion of the L‐chondrite target by the impact and negligible crystal‐liquid fractionation during emplacement and cooling of the dike. Significant geochemical effects of the impact melting event include fractionation of siderophile and chalcophile elements with increasing metal‐silicate heterogeneity, and mobilization of moderately to highly volatile elements. Siderophile and chalcophile elements ratios such as Ni/Co, Cu/Ga, and Ir/Au vary systematically with decreasing metal content of the melt. Surprisingly small (?102) effective metal/silicate‐melt distribution coefficients for highly siderophile elements probably reflect inefficient segregation of metal despite the large degrees of melting. Moderately volatile lithophile elements such K and Rb were mobilized and heterogeneously distributed in the L‐chondrite impact breccias whereas highly volatile elements such as Cs and Pb were profoundly depleted in the region of the parent body sampled by Chico. Volatile element variations in Chico and other L chondrites are more consistent with a mechanism related to impact heating rather than condensation from a solar nebula. Impact processing can significantly alter the primary distributions of siderophile and volatile elements in chondritic planetesimals.  相似文献   

18.
Abstract— We present a textural comparison of localized shock melt pockets in Martian meteorites and glass pockets in terrestrial, mantle‐derived peridotites. Specific textures such as the development of sieve texture on spinel and pyroxene, and melt migration and reaction with the host rock are identical between these two apparently disparate sample sets. Based on petrographic and compositional observations it is concluded that void collapse/variable shock impedance is able to account for the occurrence of pre‐terrestrial sulfate‐bearing secondary minerals in the melts, high gas emplacement efficiencies, and S, Al, Ca, and Na enrichments and Fe and Mg depletion of shock melt compositions compared to the host rock; previously used as arguments against such a formation mechanism. Recent experimental studies of xenoliths are also reviewed to show how these data further our understanding of texture development and can be used to shed light on the petrogenesis of shock melts in Martian meteorites.  相似文献   

19.
Ejecta from the Connors Creek site in Michigan (500 km from the Sudbury Igneous Complex [SIC]), the Pine River site in western Ontario (650 km from the SIC), and the Coleraine site in Minnesota (980 km from the SIC) were petrographically and geochemically analyzed. Connors Creek was found to have approximately 2 m of ejecta, including shocked quartz, melt droplets, and accretionary lapilli; Pine River has similar deposits about 1 m in thickness, although with smaller lapilli; Coleraine contains only impact spherules in a 20 cm‐thick layer (impact spherules being similar to microkrystites or microtektites). The ejecta transition from chaotic deposits of massively bedded impactoclastic material with locally derived detritus at Connors Creek to a deposit with apparently very little detrital material that is primarily composed of melt droplets at Pine River to a deposit that is almost entirely composed of melt spherules at Coleraine. The major and trace element compositions of the ejecta confirm the previously observed similarity of the ejecta deposits to the Onaping Formation in the SIC. Platinum‐group element (PGE) concentrations from each of the sites were also measured, revealing significantly elevated PGE contents in the spherule samples compared with background values. PGE abundances in samples from the Pine River site can be reproduced by addition of approximately 0.2 wt% CI chondrite to the background composition of the underlying sediments in the core. PGE interelement ratios indicate that the Sudbury impact event was probably caused by a chondritic impactor.  相似文献   

20.
Abstract— The Vredefort Granophyre represents impact melt that was injected downward into fractures in the floor of the Vredefort impact structure, South Africa. This unit contains inclusions of country rock that were derived from different locations within the impact structure and are predominantly composed of quartzite, feldspathic quartzite, arkose, and granitic material with minor proportions of shale and epidiorite. Two of the least recrystallized inclusions contain quartz with single or multiple sets of planar deformation features. Quartz grains in other inclusions display a vermicular texture, which is reminiscent of checkerboard feldspar. Feldspars range from large, twinned crystals in some inclusions to fine‐grained aggregates that apparently are the product of decomposition of larger primary crystals. In rare inclusions, a mafic mineral, probably biotite or amphibole, has been transformed to very fine‐grained aggregates of secondary phases that include small euhedral crystals of Fe‐rich spinel. These data indicate that inclusions within the Vredefort Granophyre were exposed to shock pressures ranging from <5 to 8–30 GPa. Many of these inclusions contain small, rounded melt pockets composed of a groundmass of devitrified or metamorphosed glass containing microlites of a variety of minerals, including K‐feldspar, quartz, augite, low‐Ca pyroxene, and magnetite. The composition of this devitrified glass varies from inclusion to inclusion, but is generally consistent with a mixture of quartz and feldspar with minor proportions of mafic minerals. In the case of granitoid inclusions, melt pockets commonly occur at the boundaries between feldspar and quartz grains. In metasedimentary inclusions, some of these melt pockets contain remnants of partially melted feldspar grains. These melt pockets may have formed by eutectic melting caused by inclusion of these fragments in the hot (650 to 1610 °C) impact melt that crystallized to form the Vredefort Granophyre.  相似文献   

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