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Earth and Planetary Science Letters 213 (2003) 235^247 www.elsevier.com/locate/epsl

Yarrabubba - a large, deeply eroded impact structure in the Yilgarn Craton, Western Australia Francis A. Macdonald a;
, John A. Bunting b , Sara E. Cina a a

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA b Helix Resources Ltd., Level 3, 24 Kings Park Road, West Perth, WA 6005, Australia Received 19 February 2003; received in revised form 27 May 2003; accepted 3 June 2003

Abstract Yarrabubba is a newly discovered impact structure situated within the complex granite^greenstone terrain of the Yilgarn Craton. Shock-metamorphic effects including shatter cones, planar deformation features in quartz grains, and pseudotachylites, were found in deeply eroded Archean granites near Yarrabubba station, southeast of Meekatharra, Western Australia. Aeromagnetic images reveal arcuate demagnetization features at diameters between 11 and 25 km, which roughly correspond to the outcropping of the Yarrabubba Granite, and are centered on a magnetic-high halo around the Barlangi Granophyre (119‡50PE, 27‡10PS). Rapid-quench textures in the Barlangi Granophyre and its inter-fingering relationships with pseudotachylites suggest that it is an impact melt that was injected into the Yarrabubba Granite and spread along fault discontinuities. Both the potassic Yarrabubba Granite and the felsic Barlangi Granophyre are atypical in the northern Yilgarn, as are the abundant fracturing and frictional melting within the local granitoids. These anomalous geological features associated with shock-metamorphic effects are indicative of a hypervelocity impact origin. The age of the impact is uncertain, but geological and geophysical relationships suggest that the Yarrabubba structure was formed during the early Proterozoic. 8 2003 Elsevier B.V. All rights reserved. Keywords: impact melt; impacts; pseudotachylites; shatter cones; shock metamorphism; Yilgarn Craton

1. Introduction Because of crustal recycling, the terrestrial impact record is strongly biased towards more recent impact events [1]. Few Proterozoic structures have been discovered; the V2.023 Ga Vredefort structure and the V2.4 Ga Suavjarvi structure are the

* Corresponding author. E-mail address: [email protected] (F.A. Macdonald).

oldest impact structures that have been reported [2^4], although there is evidence of earlier impacts in the form of Paleoproterozoic and Archean vapor condensate spherules (e.g. [5]). Unlike other well-studied Archean terrains, such as the Fennoscandian shield [6], few impact structures have been identi¢ed in the Yilgarn Craton, despite its large area (V1,000,000 km2 ) [7,8]. Nearly all of the V30 impact structures in Australia are situated in relatively undisturbed sedimentary basins [7,8]. Conventional methods of searching for impact structures, such as the identi¢cation of circu-

0012-821X / 03 / $ ^ see front matter 8 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0012-821X(03)00322-4

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Fig. 1. Location map and sketch map of regional geology near the Yarrabubba homestead. The structure is centered at impact melt outcrop at Barlangi Rock. Of particular interest are the greenstone belts and lineaments (ma¢c dikes and faults) that show up very distinctly in the aeromagnetic images. Moreover, notice the Tertiary Paleodrainage that forms an arcuate feature at V20 km diameter to the northwest of Barlangi Rock.

lar magnetic, topographic, and gravitational anomalies, are most e¡ective in £at-lying sedimentary terrains, where such anomalies present a noticeable contrast to the surrounding environment [1,9]. Although Archean granite^greenstone terrains may contain a rich record of extraterrestrial

bombardment, the presence of circular intrusions and complex magnetic fabric makes the detection of impact structures by these methods more di⁄cult, and discoveries may rely, as in this case, on the recognition of unusual geological relations and petrographic features.

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The Yarrabubba structure is located on the Yilgarn Craton in Western Australia (lat. 27‡ 11PS, long. 118‡ 50PE), approximately 100 km southeast of the township of Meekatharra (Fig. 1). The area was mapped in 1979 by the Geological Survey of Western Australia (GSWA). Samples collected by S.J. Williams were described by W.G. Libby [10], who attributed certain features to shock metamorphism. The description in the Explanatory Notes [11] included phrases such as ‘bent plagioclase twin lamellae, inclusion trains in quartz crystals, and unusual parting in muscovites’, features that were interpreted as evidence of severe mechanical shock, possibly related to the emplacement of the soda rhyolite plugs (Barlangi Granophyre). These observations were also noted by Bunting et al. [12] as ‘‘a second possible shock metamorphic structure 260 km to the southwest [of the Lake Teague/Shoemaker structure]’’. In 2001, aeromagnetic images of the area were processed at Geoscience Australia in Canberra, showing a semi-circular 15^25 km diameter magnetic low centered on a ring of magnetic highs at Barlangi Rock. Re-examination of the original GSWA thin section (sampled from Yarrabubba Granite, V3.5 km north of the center of the structure) con¢rmed the presence of planar deformation features (PDFs) in quartz, indicative of shock metamorphism. During our recent ¢eld work (2002) we identi¢ed shatter cones and other shock-metamorphic e¡ects attributable to hypervelocity impact.

2. Geology

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3.0^2.9 Ga, with a younger succession at about 2.7 Ga [13]. Granitic events in the Murchison Province have been dated at 2.65^2.92 Ga [13,14]. Metamorphism of the greenstone belts and early granodiorite reached greenschist to lower amphibolite facies. The voluminous monzogranites were recrystallized at greenschist facies, and this event produced retrograde greenschist facies in the adjacent greenstones [11,13]. The most common rock type in the region is biotite monzogranite. It is a medium- to coarsegrained rock, with ¢ne-grained and pegmatitic varieties. Cutting the Archean rocks are ma¢c dikes of various Proterozoic suites that are present over most of the Yilgarn [15,16]. The dikes are visible as linear aeromagnetic anomalies, but most are exposed either intermittently or not at all. Their age and signi¢cance are discussed in later sections. Eighty kilometers to the north of Yarrabubba the Yilgarn Craton is overlain by mildly deformed sedimentary rocks of the Paleoproterozoic Yerrida Basin [17], with small outliers only 25 km to the north of Yarrabubba. These sediments were laid down on a peneplained surface, and may have extended as far south as Yarrabubba before being removed by erosion. During the late Mesozoic and early Tertiary the region was subjected to deep weathering that resulted in the kaolinization of much of the granite, and the development of silcrete and lateritic duricrust. Subsequent erosion has produced ‘breakaway’ scarps, and revealed the fresh bedrock beneath the deep weathering. Also during the Tertiary the mature river valleys were ¢lled with sediment. These ancient trunk drainages are now evident as systems of playa lakes and calcrete (Fig. 1) [11].

2.1. Regional geology 2.2. The Yarrabubba structure The Yarrabubba structure is situated in the late Archean granite^greenstone terrain of the Murchison Province, northern Yilgarn Craton (Fig. 1) [13]. The structure is developed primarily within granitoids, and is centered on outcrops of granophyre at Barlangi Rock. The nearby Meekatharra and Barrambie greenstone belts consist of metamorphosed ma¢c, ultrama¢c and felsic volcanic rocks, related intrusions, and various sedimentary rocks [11]. Early greenstones are dated at about

The highly eroded central region of the Yarrabubba structure was mapped using 1:50,000 scale black and white aerial photographs (Fig. 2). No topographical crater forms can be discerned either from the ground, on aerial photographs or from satellite images. Considering the lack of topography, some of the granites in the central region of the Yarrabubba structure are remarkably well exposed; however, like most of the Yilgarn, outside

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Fig. 2. Geological map of the Yarrabubba Impact Structure mapped on 1:50,000 air photos. The large faults to the north of Barlangi Rock are also pseudotachylites that are more than 1 m thick. Rodding occurs at the intersection of these two pseudotachylites (faults). The contacts of the Barlangi Granophyre at its western outcrops are shallow, as shown with the 5‡ dip of contact, with the Yarrabubba Granite resting above.

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Fig. 3. Photomicrographs of: (a) Quartz grain in the Yarrabubba Granite (crossed polars) from thin section Y34, sampled V1.5 km NW of Barlangi rock, with multiple sets of decorated PDFs. (b) Close up of at least three sets of PDFs (crossed polars) in a quartz grain from thin section Y34, showing £uid inclusions along the planes and close spacing between the planes. (c) GSWA thin section 60388 of the Barlangi Granophyre (crossed polars) sampled from Barlangi Rock with skeletal intergrowths of K-feldspar and quartz lathes in a nucleation texture. (d) Silica spherule in GSWA sample 60388 (uncrossed polars), sampled from Barlangi Rock, with radiating quartz needles.

of the central area, most of the structure is covered with silcrete, calcrete, playa lake sediment and Quaternary sands [11]. The playa lakes and calcrete in the paleodrainage system form an arcuate ‘moat’ to the north and east of Yarrabubba (Fig. 1). The Yarrabubba Granite [14] is a muscovite^ biotite^albite granite that outcrops east of the Yarrabubba homestead, surrounding the Barlangi Granophyre. It does not appear to outcrop elsewhere in the Sandstone 1:250,000 sheet [11], or anywhere else in the region. The rock is a pale pink, medium- to coarse-grained monzogranite, consisting of quartz, albitic plagioclase and microcline, with subordinate muscovite and biotite.

Near Barlangi Rock, quartz grains in the Yarrabubba Granite display multiple sets of planar elements (Fig. 3a,b). Plagioclase twin lamellae and cleavages in mica are often disrupted or bent. Biotite is commonly altered to a mixture of chlorite and iron oxides, and it also appears that muscovite was formed from hydrothermal alteration of biotite. Although brecciation in the Yarrabubba Granite is rare, in some outcrops the granite is so highly fractured that it could be classi¢ed as monomict breccia. The Barlangi Granophyre was originally described as a sodium-rhyolite hypabyssal granophyre, with bladed silica rather than cuneiform intergrowths [10,11]. Our ¢eld work revealed

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that there was substantially more of the Barlangi Granophyre in the area than previously mapped, that is, outcrops of the granophyre can be found V3 km to the north and west of Barlangi Rock (Fig. 2). Where contacts are exposed, the granophyre has a shallow intrusive relationship with the Yarrabubba Granite. Mapping and aeromagnetic data suggest that the center of the granophyre body is at Barlangi Rock, with radiating sill-like apophyses. In the ¢eld the granophyre is a pink ¢negrained rock, with scattered coarser grains of quartz and feldspar. Also common in places are xenoliths of coarse-grained granite that range from a few centimeters to 0.4 m across. The granophyre appears relatively fresh and undisturbed, and, unlike the Yarrabubba Granite, contains no quartz veining, ma¢c diking, or pseudotachylites. Fracturing (crude columnar jointing?) and southwest-dipping terraces are developed on Barlangi Rock, but not in the surrounding granites. The granophyric matrix typically has no discernible megascopic textures ; however, outcrops of the granophyre near the granite contact display weak £ow banding. In thin section, the coarser grains in the granophyre appear to be xenocrysts. The quartz xenocrysts are rounded, in part resorbed, and consist of a ¢ne mosaic of granulated quartz (Fig. 3d). Similar granulated textures appear in quartz in the granite xenoliths and in the granite near the granophyre contact, but not in the main Yarrabubba Granite. Quartz needles in the Barlangi Granophyre form skeletal textures, indicating rapid quenching (Fig. 3c,d) [18]. These needles often radiate from quartz xenocrysts or around siliceous spherules (Fig. 3c,d), similar to nucleation textures described in the Vredefort impact melt (for comparison see [18]). Shock features have not been observed in the granulated quartz, the granophyre, or the xenoliths; such is the case with most of the granular quartz of the Vredefort impact melt [19,20]. 2.3. Faults, breccias, and pseudotachylites The Yarrabubba Granite contains many dikelike bodies, which range in thickness from less

than 1 mm to more than 1 m, and which occur along faults. The thicker bodies are generally of a £inty green aphanitic felsic rock with inclusions of granulated quartz. Fault movement is indicated by displacement of pegmatite veins with partially melted pegmatitic clasts strung out within the melt. In a northwest-trending shear zone, V3 km NNW of Barlangi Rock (Fig. 2), the green material has a mylonitic texture with streaks of black melt, rodding, and potassium-feldspar overgrowths on plagioclase xenocrysts. The green bodies that have developed mylonitic textures are perhaps best described as ‘ultramylonites’ [21], and are most common where two of the faults cross. It appears that the majority of the large dike-like bodies at Yarrabubba are fault melts, or pseudotachylites, which formed during the modi¢cation stage of the cratering process [22]. In the northern granite outcrops, millimeter thick pseudotachylite veins [23] were observed that display ‘cobweb’ patterns [1]. These can be observed in the Yarrabubba Granites to at least 5 km from the center, but their distribution appears to be sporadic. Near the southern granite^ granophyre contact, black, vein-like material can be as thick as 20 cm; however, it only seems to run a few meters. Abundant boulders of similar hard, black veins, over 30 cm thick have been discovered near the western granite-granophyre contact, and although veins of this size were not found in place, smaller black veins were found in the vicinity. Both of these occurrences of black material contain microscopic £ow textures ; however, the £ow textures appear to be largely recrystallized, perhaps because of their proximity to the granophyre. At the northern granite^granophyre contact, the granophyre intrudes along a shear zone, and the relationship between the granophyre and the pseudotachylites is complex, particularly as both are ¢ne-grained melt rocks of the Yarrabubba Granite. Near the contact, the granophyre is £ow banded, intrudes into, and inter-¢ngers with a very pale-colored, annealed pseudotachylite. Several meters northwards of this irregular contact, away from the granophyre, the pseudotachylite begins to take its typical green, £inty appearance.

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A 0.5 m thick, deeply weathered dike of allogenic breccia with a crushed matrix is located V4 km south of Barlangi Rock. The breccia contains angular clasts of granite, granular quartz, feldspars, and glassy material that may be impact melt. The dike runs nearly tangential to the structure, but as exposure is poor, it could only be followed a couple of meters. To the southwest of Nullor Well (Fig. 2), about 7 km south of Barlangi Rock, granite that may be transitional from Yarrabubba Granite into the regional biotite monzogranite is severely fractured in places. Some of the fractures contain black veins up to 2 mm thick, and at one locality the fracturing has produced a monomict granitic breccia.

3. Shock metamorphic features 3.1. Shatter cones Shatter cones were discovered originally near the Yilby well track (Fig. 2), V4 km NNW of Barlangi Rock. In general, shatter cones point upwards and have very divergent striations (Fig. 4a), indicating that the current exposure is far below the original source of the shock wave [24]. At the discovery locality, cones range from up to 1 m in size to small reversed cones that are only about 10 cm in height (Fig. 4b) [25]. Reversed cones have been observed before at many other impact structures and are thought to represent the negative branch of the mathematical cone [25]. Most of the shatter cones at Yarrabubba are located within 1 m from a contact between a pegmatite vein and a medium-grained Yarrabubba Granite. It appears that the shatter cones formed as a shock front interacted with this discontinuity in the rock. A similar concept has been invoked at other impact structures where shatter cones are associated with some inhomogeneity acting as a nucleus or apical ‘seed’ ([25^27], pg. 110^111, ¢gure 35). The diagnostic feature of shatter-fracturing is the presence of hierarchical, diverging striated features [25,28], known as ‘horse-tailing’. Many of the shatter-features at Yarrabubba are developed on segments that are nearly £at and parallel to

Fig. 4. Shatter cones at discovery locality, developed in medium grained Yarrabubba Granite near a contact with a pegmatite vein (rock hammer for scale): (a) Large shatter cone fractured into three pieces with the basal piece in place and pointing upward. (b) Small, reversed shatter cones found approximately 1 m from the large cone shown in (a) with the vertices pointing downward.

one another. These would best be described as shatter surfaces, which are a transitional state between shatter coning and shatter cleavage [28]. Sets of curved cleavage were also observed near the shatter cone outcrops and on rocks northwest and southwest of Barlangi Rock. Shatter-cleaved rocks appear to be cut by close-set cleavage whose planes are V5 cm long and spaced as close as V1 mm apart. 3.2. Shocked mineral grains Quartz grains in the Yarrabubba Granite display spectacular PDFs that are typically decorated (Fig. 3a,b). Multiple sets of PDFs are developed in quartz grains within the Yarrabubba

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Granite to at least an 8 km diameter, and within the impact breccia, but no PDFs have been found in the granophyre or the pseudotachylites. Spacing between individual laminae is typically less than 10 Wm (Fig. 3b). Crystallographic orientations of PDFs were measured on an optical microscope ¢tted with a four-axis universal stage. The orientations of 100 planes were measured within 44 grains from thin sections Y34 and YB34 of the Yarrabubba Granite, sampled V1.5 km NE of Barlangi Rock. Fig. 5 shows a frequency histogram of the angle between the poles of PDF planes and the c-axis of individual quartz grains, and displays clear maxima at shock-characteristic angles of g {1013}, Z {1012}, h {1122}, and {2241} [29]. The presence of multiple sets of closely spaced PDFs at these angles con¢rms the impact origin of the Yarrabubba structure and is consistent with a crystalline target [30]. The appearance of the Z plane suggests that these samples experienced shock-pressures exceeding 16 Gpa [1,29,30].

4. Geophysical signature The current resolution of gravity surveys in the Yarrabubba area is not detailed enough to resolve its structural features. No seismic lines have been run in the area, and ASTER satellite images do not show any circular features around Barlangi Rock. Aeromagnetic data along with the extent of shock metamorphism at least give hints of the structural form. 4.1. Aeromagnetics In 2001, high-resolution digital aeromagnetic data for the Sandstone sheet was released by the GSWA. The surveys were £own at 400-m line spacing along 90‡ true constant latitudes, and the altitude speci¢cation was 60-m continuous ground clearance. Aeromagnetic images were processed at Geoscience Australia in Canberra, and then later at the Western Australian Geological Survey using ER Mapper0 software. Best results were obtained using a grayscale image with a NE sun angle and a histogram modi¢cation that ac-

Fig. 5. Frequency histogram plot of PDF orientations in thin sections Y34 and YB34 of the Yarrabubba Granite, sampled V1.5 km NW of Barlangi Rock. The measurements are binned in 5‡ increments of angle between the c-axis and poles of the PDF planes. Dominant PDF orientations are indicated. 100 planes were measured on 44 quartz grains, and V80% of the planes were indexed on these dominant orientations.

centuated the weakly magnetic granitic rocks (Fig. 6). The e¡ect of hypervelocity impacts on the magnetic signature of target rocks varies widely from structure to structure, and is dependent upon the lithology of the target rocks, the magnitude of the impact, the level of erosion, and the e¡ect of hydrothermal alteration, among other variables [9]. As such, the magnetic e¡ect of impact structures is not systematically understood [1]. Nevertheless, aeromagnetics can provide clues to the existence, size and shape of impact structures. The most common magnetic anomaly associated with meteoritic impacts is a magnetic low, due to brecciation and shock-demagnetization of the target rocks [9,31]. At Yarrabubba, there is an elliptical non-magnetic zone, about 11 km east^west and 15 km north^south, within which the typical complex magnetic patterns of the regional granites are replaced by a uniform, amorphous low magnetic signature (Fig. 6). There is a suggestion of a larger zone, V25 km across, in which the regional patterns are partly subdued, particularly north, south, and east of Barlangi Rock. At the center of the non-magnetic zone is an ovoid ring, about 2 km north^south and 4 km east^west, of small positive magnetic anomalies that partially coin-

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Fig. 6. Regional total aeromagnetic intensity (TMI) image with northeast illumination showing the central ring and demagnetized Yarrabubba granite. Data supplied by Geosciences Australia and processed using ER Mapper0 . Notice the Barrambie Greenstone Belt to the northeast; east-west lineaments (ma¢c dikes) cutting the structure to the north; the broad demagnetization zone of the Yarrabubba Granite at an 11 km diameter to the west and a 15^25 km diameter to the north; and arcuate features at a 25 km diameter to the east and at a 40 km diameter to the southwest.

cide with the contact of the Barlangi Granophyre with the Yarrabubba Granite. In the middle of the eastern and widest part of this inner ring is Barlangi Rock. We interpret the 11U15 km low zone as being due to impact-related demagnetization of the regional biotite monzogranite. The demagnetization, along with other impact-related e¡ects such as brecciation and metasomatism, distinguishes the Yarrabubba Granite from the biotite monzogranite. The inner ring of magnetic highs re£ects remagnetization along the contact metamorphosed boundaries between the Barlangi Granophyre and the Yarrabubba Granite. Strongly

magnetic linear anomalies trending NNE occur over greenstone belts on the eastern and northeastern sides of the structure (Fig. 6). The magnetic fabric of the greenstone belts in the area do not appear to be markedly a¡ected by the impact; however, the Morokoweg structure in southern Africa is also situated in a granite^greenstone terrain, and the greenstone belts within the structure are also little changed in magnetic images [32]. The linear east^west-trending magnetic features that cut the northern half of the demagnetized zone are almost certainly ma¢c dikes, but no exposures of these dikes have been found. The undisturbed nature of the magnetic signature of the

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east^west-trending dike, approximately 3 km north of the Barlangi Rock, suggests that the dikes postdate the impact. East^west dolerite dikes elsewhere in the Yilgarn Craton have been dated at 2.4 Ga [15,16,33], although there are east^west dikes in the Paleoproterozoic Yerrida Basin, about 100 km north of Yarrabubba, that are likely younger than 2.1 Ga, and other dikes in the Yilgarn have been dated at 1.07 and 1.2 Ga ([34], Michael Wingate, pers. comm.). The aeromagnetic image of Yarrabubba is also cut by a NNW trending lineament V7 km east of Barlangi Rock that runs parallel to the Barrambie Greenstone Belt. Although this region is not exposed, this magnetic feature has the appearance of a fault. North-trending faults in the Northern Yilgarn have been previously attributed to the tectonics of the Paleoproterozoic [11,13].

tures worldwide [1,37^39], then we can begin to make rough estimates of the original size of the Yarrabubba impact structure. In terrestrial observations with a basement core a scaling relation of 0.33 Ds ^ 1.02 = Du has been suggested [37,39], where Ds is the original outer diameter of the structure and Du equals the diameter of the central uplift. Applying this equation with Du equal to a range between 11 and 25 km yields an outer diameter between 30 and 70 km. In such a highly eroded impact structure, it may be impossible to determine the original size with any certainty; however, the presence of large bodies of structurally penetrating impact melt, the wide distribution of PDFs and shatter cones, and the existence of meter-sized pseudotachylites imply a large original diameter.

4.2. Dimensions of the Yarrabubba impact structure

5. Age

Several lines of evidence indicate that the current exposure at Yarrabubba is well below the original crater £oor. There are no obvious topographic circular features, the granites are highly shocked but there is very little impact breccia, and the Barlangi Granophyre is not a crater-¢lling melt sheet, but instead intrudes into the surrounding granites. Denudation rates of the northern Yilgarn since the Paleoproterozoic are poorly constrained [35], and consequently it is di⁄cult to determine the current level of exposure relative to the original crater form. However, denudation rates in other Precambrian areas in Australia (SW Australia, Kimberley Block, Gawler Craton and Mt Isa Inlier) have been calculated to average about 10 m per 1,000,000 yr over the last 300 1,000,000 yr [36]. If we assume this rate through to the Paleoproterozoic, the current exposure could be as much as 20 km below the original crater £oor. The demagnetized area corresponds roughly to the distribution of the Yarrabubba Granite, which may be a rock unique to the highly eroded central core. If this aeromagnetic anomaly can be used as an estimate of the diameter of the central uplift, as has been done at several complex impact struc-

An older age limit of about 2.6 Ga is set by age determinations on the regional monzogranites [13]. U^Pb SHRIMP (Super High Resolution Ion Micro Probe) data on samples collected by Geoscience Australia gave a 2.65 Ga emplacement age and a 2.75 Ga xenocryst age for the Yarrabubba Granite [14]. Zircons in the Barlangi Granophyre were dated at 2.715 Ga (xenocrysts?), and there was an additional 2.6^2.7 Ga suite [14]. One zircon from the granophyre gave a concordant Paleoproterozoic age [14], which is unusual for igneous rocks from the northern Yilgarn. It is tempting to suggest that this is the age of impact, but more zircons need to be analyzed before the signi¢cance of the Paleoproterozoic age can be established. A Proterozoic age of Yarrabubba is supported by the magnetic image of the central region, which appears to be cut by dolerite dikes and faults, both of which are considered to be Proterozoic features of the northern Yilgarn Craton [11,13,15,16].

6. Discussion Several features of the Yarrabubba structure are unique. Aside from perhaps being one of the

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oldest impact structures on Earth [2^4], Yarrabubba is the ¢rst large structure to be discovered within the Archean granite^greenstone terrains of Australia [4,7,8]. Moreover, the exposures of meter-wide pseudotachylite veins at Yarrabubba are some of the largest in the world. Also, the nature of the Barlangi Granophyre as an injected substructural impact melt with radiating sills is quite rare amongst terrestrial impact structures [1]. As the Yarrabubba Granite does not appear anywhere else in the area, it also may represent a prime example of impact-related metasomatism. 6.1. Impact melt We believe that the Barlangi Granophyre is an impact melt that was injected along fault discontinuities. Similar bodies of impact melt have been reported at other large impact structures [1,18,40]. Skeletal and nucleation textures of bladed quartz in the granophyre (Fig. 3c,d) indicate that it was formed by shock melting of the Yarrabubba Granite [18]. Chemical analyses [14] show that the Barlangi Granophyre and Yarrabubba Granite have almost identical abundances and patterns in chondrite-normalized REE and primordialmantle-normalized multi-element plots (K. Cassidy, pers. comm.). The granophyre’s complex, inter-¢ngering relationships with the pseudotachylites at the northern granite^granophyre contact suggest that the granophyre intruded along impact generated faults directly after the formation of the pseudotachylite. An alternative interpretation of the nature of the Barlangi Granophyre is that it is a true granophyre that intruded upwards on a weakness created by the impact, perhaps as a late-stage di¡erentiation of a magma associated with younger ma¢c dykes. This possibility cannot be entirely dismissed without either drilling or more detailed geophysical surveys to provide a better three-dimensional picture of the Barlangi Granophyre. The authors, however, ¢nd this latter explanation unlikely, as the geology and petrology of the granophyre are very di¡erent from the endogenic granophyres in the Yilgarn Craton. Other granophyres in the region tend to have a closer relationship with large ma¢c bodies, and in thin section,

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these granophyres contain quartz and feldspar with cuneiform intergrowths, not the skeletal textures of the Barlangi Granophyre [11,13]. 6.2. Metasomatism of the Yarrabubba Granite The Yarrabubba Granite is unusual in the context of the regional geology of the northern Yilgarn. In addition to all the features described above that can be attributed to impact, the dominant mica is muscovite, the dominant plagioclase is albite, and the dominant K-feldspar is microcline. Its composition is more potassic and less calcic than the typical biotite monzogranite. There is evidence in thin sections that biotite may have been replaced by muscovite, and in places by a chlorite and iron-oxide intergrowth. These features indicate that the granite has been a¡ected by potassium metasomatism, or at least potassium redistribution without necessarily adding signi¢cant potassium to the system. Potassium metasomatism is also indicated by the blastomylonitic overgrowths of potassic feldspar on xenocrysts where the granophyre grades northwards into the mylonitic shear zone north of Barlangi Rock. The Yarrabubba Granite appears to be restricted to the central portion of the impact structure, coincident with the V15 km demagnetization zone, possibly indicating that the impact event or its aftermath caused the metasomatism of the regional biotite monzogranite to form the Yarrabubba Granite. Similar syenitic granites have been described at the Shoemaker structure [8,12,36,41] and Vredefort [42], and have previously been used as arguments against an impact origin of these structures [12,42]. More recently, the enrichment of sodium and potassium and depletion of silica in Archean granites at the Shoemaker structure has been attributed to impact-induced hydrothermal activity [36].

7. Conclusion This paper is a brief and very preliminary investigation of a newly identi¢ed impact structure. We have documented evidence such as PDFs in quartz, shatter cones, impact breccias, pseudo-

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tachylite veins, and impact melts, that demonstrate that the Yarrabubba structure is the core of a large, highly eroded impact structure in Archean rocks of the Yilgarn Craton. As very few impact structures have been described in Archean granite^greenstone terrains, the characteristics of Yarrabubba may help us home in on the signatures of impact that survive in deeply eroded, complex igneous settings, and re¢ne the terrestrial cratering record in ancient times.

Acknowledgements We are grateful to the GSWA, and in particular to Franco Piranjo and Sergey Sebchenko for the use of old thin sections, petrographic microscopes, and geophysical images. We are grateful to Kevin Cassidy and Peter Milligan of Geoscience Australia for geochemical data and the use of aeromagnetic databases and terminals with ER Mapper0 , and to Ian Fletcher of the University of Western Australia for the use of SHRIMP data on zircons. We thank Joanne Giberson at the Caltech GIS lab for her aid in digitizing maps. We are also thankful to Helix Resources who helped support our work. We thank Andrew Glikson, Christian Koeberl, and John Spray for their comments on this paper. We thank the owners of Yarrabubba station for the use of their facilities. And lastly, this work would not have been possible without a Fellowship (to F.A.M.) from the Thomas J. Watson Foundation.[KF] References [1] B.M. French, Traces of Catastrophe. Lunar and Planetary Institute, (Contribution No. 954), Houston, TX, 1998, 120 pp. [2] J.G. Spray, S.P. Kelley, W.U. Reimold, Laser Probe argon-40/argon-39 dating of coesite- and stishovite-bearing pseudotachylites and the age of the Vredefort impact event, Meteorit. Planet. Sci. 30 (1995) 335^343. [3] M.S. Mashchak, M.V. Naumov, The Suavjarvi structure: An Early Proterozoic impact site on the Fennoscandian Shield [abs.], Lunar Planet.Sci. 27 (1996) 825^826. [4] J. Whitehead, J.G. Spray Earth Impact Database, http:// www.unb.ca/passc/ImpactDatabase/CINameSort.html, updated 19/08/02.

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