Diamondiferous peridotite xenoliths from the Argyle (AK1) lamproite pipe,Western Australia

This paper describes a suite of peridotite xenoliths. some carrying diamonds at high grades, from the richly diamondiferous early Proterozoic (1180 Ma) Argyle (AK1) lamproite pipe, in northwestern Australia. The peridotites are mostly coarse garnet lherzolites but also include garnet harzburgite, chromite — garnet peridotite, a garnet wehrlite, and an altered spinel peridotite with extremely Cr-rich chromite. In all cases the garnet has been replaced by a kelyphite-like, symplectic intergrowth of Alrich pyroxenes, Al-spinel and secondary silicates. The peridotites have refractory compositions characterized by high Mg/(Mg+Fe) and depletion in lithophile elements (Al₂O₃ and CaO < 1%, Na₂O0.03%) and high field strength cations such as Ti, Zr, Y, and Yb. Olivines have high Mg/(Mg+Fe) (Mg ^91–93 ) and, like olivine inclusions in diamonds from the Argyle pipe, contain detectable amounts of Cr₂O₃ (0.03%–0.07%) but have very low CaO contents (typically 0.04%–0.05%). Enstatites (Mg ^92–94 ) have comparatively high Cr₂O₃ (0.2%–0.45%) and Na₂O (up to 0.18%) but very low Al₂O₃ contents (0.5%–0.7%). Diopsides (Mg ^92–94 , Ca/(Ca+Mg+Fe)=0.37–0.43) are Cr-rich (0.7%–1.9% Cr₂O₃) and have low Al₂O₃ (0.7%–2.2%) and Na₂O (0.5%–1.6%) contents. Many have high K₂O contents, typically 0.1%–0.4% but up to 1.3% K₂O in one xenolith. The chromite coexisting with former garnet is Mg-and Cr-rich [Mg/(Mg+Fe²⁺)=0.68–0.72, Cr/(Cr+Al)=0.72–0.79] whereas chromite in the spinel peridotite is even more Cr-rich (65% Cr₂O₃, Cr/(Cr+Al)=0.85, resembling inclusions in diamond. One highly serpentinized former garnet peridotite contains a Cr-rich (up to 13% Cr₂O₃) titanate resembling armalcolite but containing significant K₂O (1%–2.5%), CaO (0.6%–2.2%), ZrO₂ (0.1%–0.8%), SrO (0.1%–0.3%), and BaO (up to 0.58%): this appears to have formed as an overprint of the primary mineralogy. Temperatures and pressures estimated from coexisting pyroxenes and reconstructed garnet compositions indicate that the garnet lherzolites equilibrated at 1140°–1290° C and 5.0–5.9 GPa (160–190 km depth), within the stability field of diamond. Oxygen fugacties within the diamond forming environment are estimated from spinel-bearing assemblages to be reducing, with f _O2 between MW and IW. The presence of significant K in the diopsides from the peridotite xenoliths and in diopsides from heavy mineral concentrate from the Argyle pipe implies metasomatic enrichment of the subcontinental lithosphere within the diamond stability field. The P-T conditions estimated for the Argyle peridotites demonstrate that diamondiferous lamproite magmas incorporate mantle xenoliths from similar depths to kimberlites in cratonic settings, and imply that Proterozoic cratonized orogenic belts can have lithospheric roots of comparable thickness to beneath Archaean cratons. These roots lie at the base of the lithosphere within the stability field of diamond. The xenoliths, the calcic nature of chrome pyropes from heavy mineral concentrate, and the diamond inclusion assemblage indicate that the lighosphere beneath the Western Australian lamproites is mostly depleted lherozolite rather than the harzburgite commonly found beneath Archaean cratons. Nevertheless, the dominance of eclogitic paragenesis inclusions in Argyle diamonds indicates a significant proportion of diamondiferous eclogite is also present. The form, mineral inclusion assemblage, and the C-isotopic composition of diamonds in the peridotite xenoliths suggest that disaggregated diamondiferous peridotites are the source of the planar octahedral diamonds which constitute a minor component of the Argyle production. These diamonds are believed to have formed from mantle carbon in reduced, refractory peridotite (Iherzolite-harzburgite) in contrast to the predominant strongly ¹³C-depleted eclogitic suite diamonds which contain a recycled crustal carbon component. The source region of the lamproites has undergone long-term (2 Ga) enrichment in incompatible elements.     

New insights into volcanic processes from deep mining of the southern diatreme within the Argyle lamproite pipe,Western Australia

Underground mining and deep drilling of the richly diamondiferous ~1.2 Ga Argyle lamproite in Western Australia has prompted a re-evaluation of the geology of the pipe. Argyle is considered to be a composite pipe that formed by the coalescence of several diatremes and has been offset and elongated by post-emplacement faulting. Recent geological studies have recognised at least five distinct volcaniclastic lamproite lithofacies with differing diamond grades. The new data suggest that the centre of the southern (main) diatreme is occupied by well-bedded, olivine lamproite lapilli tuff with very high diamond grades (>10 ct/t). Characteristic features include a clast-supported fabric and high modal abundance of densely packed lamproite lapilli and coarse-grained, likely mantle-derived olivine now replaced by serpentine and/or talc. The persistence of small-scale graded and cross-bedding in this lithofacies to depths of ~1.5 km below the original surface prior to erosion suggests phreatomagmatic volcanism forming the diatreme was syn-eruptively accompanied by subsidence of the tephra, maintaining a steep-walled diatreme in the water-saturated country rock sediments.

     

Mineralogy,geochemistry and petrogenesis of the Metters Bore No. 1 lamproite pipe,Calwynyardah Field,West Kimberley Province,Western Australia

Summary The Metters Bore No. 1 lamproite (MB1) is a small unexposed pipe located in the Calwynyardah field of the Miocene West Kimberley lamproite province. Microdiamonds have been recovered from bulk sampling of the pipe but no macrodiamonds (>0.8 mm) have been found. The pipe contains both volcaniclastic and magmatic (i.e. non-fragmental, extrusive-to-hypabyssal facies) lamproite. The latter rock is dominantly olivine-leucite-diopside lamproite and comprises phenocrysts and microphenocrysts of diopside, altered olivine ( Fo₉₁), and rare phlogopite, together with phenocrysts and glomeroporphyritic aggregates of altered leucite. These are set in an altered, fine-grained to glassy groundmass including diopside, leucite, priderite, apatite, less abundant chrome-spine', perovskite, interstitial richterite with minor calcic amphibole, ilmenite, sphene and wadeite. Mineral compositions are complex and variable: for example: five compositionally distinct fields can be recognizedamong the diopsides, and three among the phlogopites. The Ti-rich, Al-poor diopsides, Ti-F-rich, Al-poor phlogopites, and potassium titanian richterites all have apparent tetrahedral site deficiencies which can best be explained by tetrahedral substitution of Ti⁴⁺ and/or Mg²⁺; no substitution of Fe³⁺ is indicated. Three major types of spinel are recognized: olivine-included titaniferous magnesiochromite (TMC), xenocrystic aluminous magnesiochromite (AMC) and leucite-included pleonaste. Spinel-olivine-melt oxygen barometry indicates that the TMC spine's crystallized from evolving lamproite magma under low oxygen fugacity conditions (MW to IW). Manganiferous groundmass ilmenite has low calculated Fe₂O₃ (< 1 wt%), also consistent with reduced conditions. The maintenance of a low oxidation state during magmatic crystallization, a feature shared with the Argyle olivine-lamproite, is considered a significant factor in preservation of the MB1 microdiamond population. Xenocrystic minerals encountered in heavy mineral concentrates (HMC) indicate that the MB1 lamproite sampled upper mantle spinel ±garnet lherzolite from >60 km depth and crustal mafic rocks. Geochemically, MB1 is typical of West Kimberley leucite-lamproites, which are characterized by high TiO₂ (> 4 wt%), low CaO (< 5 wt%), MgO < 10wt%, and enrichment in incompatible elements (Rb, Sr, Ba, LREE, etc.). Although MB1 is an olivine-bearing lamproite, it has source-related geochemical features, e.g. mantle-normalized Sc/V and Zr/Nb ratios of < 0.75 and > 0.6, respectively, that are similar to other West Kimberley leucite-lamproites and distinct from olivine-lamproites. Petrogenetically, the bulk composition and low magmatic oxidation state of MB1 supports an origin by melting of phlogopite-bearing harzburgitic source under reduced fO₂ (< MW) conditions.

---

Mineralogie, geochemie und petrogenese der lamproit-pipe Metters Bore No. 1, Kalwynyardah Field, West Kimberley Provinz, West-Australien

Zusammenfassung Der Lamproit Metters Bore No. 1 (MB1) ist eine kleine, nicht an der Oberfläche aufgeschlossene Pipe im Kalwynyardah Gebiet der miozänen Lamproit-Provinz von West Kimberley. Mikrodiamanten sind bei der Untersuchung von Proben aus der Pipe gefunden worden, jedoch keine Makrodiamanten (> 0.8 mm). Die Pipe enthält sowohl vulkanoklastischen wie magmatischen Lamproit (nicht-fragmentierte extrusive bis hypabyssische Fazies). Bei dem magmatischen lamproit handeltes sich um einen Olivin-Leuzit-Diopsid-Lamproit mit Kristallen und Mikrokristallen von Diopsid, umgewandeltem Olivin ( Fo₉₁), seltener Phlogopit, zusammen mit Kristallen und glomeroporphyritischen Aggregaten von umgewandeltem Leuzit. Diese sitzen in einer umgewandelten, feinkörnigen bis glasigen Grundmasse mit Diopsid, Leuzit, Priderit, Apatit, seltener Chromspinell, Perovskit, Richterit mit geringen Mengen an Kalziumamphibol, Ilmenit, Titanit und Wadeit. Die Mineralzusammensetzungen sind komplex und variabel: so können z.B. fünf der Zusammensetzung nach eindeutig definierte Felder für die Diopside nachgewiesen werden und drei solche für die Phlogopite. Die Ti-reichen Al-armen Diopside, Ti-F-reiche Al-arme Phlogopite und Kalium-Titan-Richterite haben alle reduzierte Besetzungen von Tetraederstellen, die am besten durch tetraedrische Substitution von Ti⁴⁺ und/oder Mg` erklärt werden können. Es gibt keine Hinweise für Substition von Fe³⁺. Drei Haupttypen von Spinellen kommen vor: Titan-führender Magnesiochromit (TMC) als Einschlüsse in Olivin, aluminiumführender Magnesiochromit (AMC) und Pleonast, der in Leuzit eingeschlossen ist. Sauerstoffbarometrie (Spinell-Olivin-Schmelze) zeigt, daß die TMC Spinelle von einem fraktionierten lamproitischen Magma bei niedriger Sauerstofffugazität (MW bis IW) kristallisiert sind. Manganführender Ilmenit der Grundmasse hat niedrige berechnete Fe₂O₃ Gehalte (< 1 %), und auch das entspricht reduzierenden Bedingungen. Die Erhaltung eines niedrigen Oxydationsstatus während der magmatischen Kristallisation ist eine Eigenschaft, die auch im Olivin-Lamproit der Argyle Pipe zu beobachten ist. Dies wird als ein signifikanter Faktor für den Erhalt der Mikrodiamanten in MBI gesehen. Xenokristalle die in Schwermineral-Konzentraten (HMC) vorkommen, weisen darauf hin, daß der MB1 Lamproit Material des oberen Mantels (Spinell ± Granatlherzolit) aus mehr als 60 km Tiefe, sowie mafische Gesteine der Kruste aufgenommen hat. Geochemisch gesehen ist MB1 typisch für die Leuzit-Lamproite von West Kimberley, welche durch hohe TiO₂ (> 4 wt.%), niedrige CaO (< 5 wt.%), MgO (< 10 wt.%) und Anreicherung von inkompatiblen Elementen (Rb, Sr, Ba, LSEE, etc.) charakterisiert werden. Obwohl MB1 ein Olivin-führender Lamproit ist, zeigt er geochemische Eigenschaften, wie Mantel-normalisierte Sc/V und Zr/Nb Verhältnisse von < 0.75 und > als 0.6, die ähnlich anderen Leuzit-Lamproiten von West Kimberley sind und sich von Olivin-Lamproiten unterscheiden. Petrogenetisch gesehen weisen sowohl die Gesamtzusammensetzung als auch der niedrige magmatische Oxydationsstatus von MBI auf eine genese durch Aufschmelzen von Phlogopit-führendem Harzburgit unter reduzierenden f0₂ (< MW) Bedingungen hin.

---

---

Deceased

---

With 12 Figures     

华北克拉通东部含金刚石金伯利岩的侵位年龄和Sr-Nd-Hf同位素地球化学特征

本文首次报道了我国华北克拉通内部金伯利岩中金云母巨晶的Ar-Ar同位素年龄和含金刚石金伯利岩的Hf同位素组成特征,并结合金伯利岩的Sr-Nd同位素组成特征对华北金伯利岩的岩石成因及其构造背景进行了探讨.金伯利岩中金云母巨晶的年代学分析显示华北克拉通内部的蒙阴和复县金伯利岩具有一致的侵位年龄,约465±2 Ma.这与以前发表的金伯利岩中钙钛矿的U-Pb年龄和金云母的Rb-Sr等时线年龄一致.Sr-Nd同位素结果表明蒙阴和复县金伯利岩皆具有很小的Nd同位素变化范围（εNd分别为-0.4～0.2和-3.4～-2.3）.然而,这些金伯利岩却具有极大的Sr同位素组成范围（ISr分布为-0.2～28.1和0～75）.其中复县比蒙阴金伯利岩具有较低的Nd同位素和更宽的Sr同位素组成.Hf同位素分析结果表明蒙阴和复县两岩区内部各金伯利岩岩管具有非常一致的铪同位素组成,类似于Nd同位素组成特征,其中蒙阴金伯利岩的^176Hf/^177Hf初始值为0.282474～0.282416,相应的εHf值为-2.37～-0.30;复县金伯利岩的^176Hf/^177Hf初始值为0.282305～0.282369;相应的εHf值为-6.29～-4.04.这说明蒙阴金伯利岩与复县金伯利岩相比具有较高的Hf同位素组成.结合Sr-Nd同位素组成,暗示蒙阴和复县金伯利岩的Hf同位素组成的不同可能反映金伯利岩岩浆形成时软流圈、岩石圈地幔以及俯冲的洋壳物质参与比例的不同.Sr-Nd-Hf同位素特征说明我国华北古生代金伯利岩的形成同样与华北周边的古大洋岩石圈向华北内部俯冲作用有关.     

Origin and age of bornhardts,Southwest Western Australia

Most of the granitic residuals of the Wheat Belt of southwestern Western Australia are bornhardts, with some nubbins developed at the western margin and occasional poorly developed castellated forms. Their origin and age can be deduced from their structure and their relationship to a weathered (lateritic) land surface and various palaeochannels. The bornhardts are massive and most stand lower than local palaeosurface remnants. They are best interpreted as having formed by differential fracture density controlled weathering beneath the weathered land surface in pre‐Eocene times. They were exposed by the stripping of the regolith beginning in the Eocene. Many are clearly stepped, indicating that their exposure took place not all at once, but episodically. A few bornhardts stand higher than the weathered land surface. They pre‐date the Eocene and the stepped morphology preserved on some suggests that their crests are much older.     

Geology and age of the Glikson impact structure,Western Australia

The Glikson structure is an aeromagnetic and structural anomaly located in the Little Sandy Desert of Western Australia (23°59'S, 121°34′E). Shatter cones and planar microstructures in quartz grains are present in a highly deformed central region, suggesting an impact origin. Circumferential shortening folds and chaotically disposed bedding define a 19 km-diameter area of deformation. Glikson is located in the northwestern Officer Basin in otherwise nearly flat-lying sandstone, siltstone and conglomerate of the Neoproterozoic Mundadjini Formation, intruded by dolerite sills. The structure would not have been detected if not for its strong ring-shaped aeromagnetic anomaly, which has a 10 km inner diameter and a 14 km outer diameter. We interpret the circular magnetic signature as the product of truncation and folding of mafic sills into a ring syncline. The sills most likely correlate with dolerites that intrude the Boondawari Formation ～25 km to the north, for which we report a SHRIMP U?–?Pb baddeleyite and zircon age of 508?±?5 Ma, providing a precise older limit for the impact event that formed the Glikson structure.     

Emplacement and metamorphism of Archaean mafic volcanics at Kambalda,Western Australia—geochemical and isotopic constraints

Extensive exploration and mining at Kambalda, Western Australia has revealed a conformable mafic-ultramafic succession consisting of a sulphide ore bearing ultramafic layer located between metabasalt units. Geochemical and Rb-Sr, U-Pb isotopic analyses have been carried out on two 70–140 m sections of drill core from the metabasalts and U-Pb isotopic analyses have been made on sulphide ore samples. Greenschist metamorphism of the metabasalts is dated at 2610 ± 30 m.y. and was approximately isochemical except for addition of H₂O and CO₂ and a differential mobilization of K and Rb within the units. The addition of H₂O and CO₂ is believed to have taken place shortly after extrusion of the metabasalts in a submarine environment resulting in an early greenschist metamorphism. The later 2600 m.y. event is correlated with a high thermal regime also detected at Kalgoorlie 50 km to the north. A Pb-Pb isochron age of 2720 ± 105 m.y. may represent the metamorphic event or the time of emplacement of the metabasalts. New Sm-Nd data (Chauvelet al.; claqué--Longet al.) suggest the mafic volcanics are 3200 m.y. old. If true large scale homogenization of Pb isotopes is required. Alternatively the mafic sequence may be only slightly older than 2800 m.y. and the Sm-Nd data does not have time significance.     

Basin setting and age of the Late Palaeoproterozoic Capricorn Formation,Western Australia

The Palaeoproterozoic Capricorn Formation near Ashburton Downs in northwestern Australia formed during the latter stages of the convergence of the Pilbara and Yilgarn Cratons. Palaeocurrent and facies analyses show that the southwesterly derived sediments were deposited in terrestrial environments and in a lake or shallow sea with a shoreline trending southeast. Intraformational debris flows suggest instability during sedimentation. Zircon grains from an accretionary lapilli tuff, dated at 1804 ± 7 Ma by the SHRIMP U—Pb method, show that the Capricorn Formation was deposited at the same time as granitic plutons were intruded in the Gascoyne Complex to the south and west. Although the Capricorn Formation was deposited with marked angular unconformity over the turbiditic Ashburton Formation, both formations could have been deposited in a foreland basin on the northeast flank of the growing Ashburton Fold Belt.     

Multi-stage metasomatism of diamondiferous eclogite xenoliths from the Udachnaya kimberlite pipe,Yakutia, Siberia

The primary garnet (pyrope-almandine)-omphacite (Cpx 1, 6.5–7 wt% Na₂O)-sulfide (Fe-Ni-Co mss) assemblage of the two diamondiferous eclogite xenoliths studied (U33/1 and UX/1) experienced two mantle metasomatic events. The metasomatic event I is recorded by the formation of platy phlogopite (~ 10 wt% K₂O), prior to incorporation of the xenoliths in the kimberlite. The bulk of the metasomatic alteration, consisting of spongy-textured clinopyroxene (Cpx 2A, 1–3 wt% Na₂O), coarser-grained clinopyroxene (Cpx 2B, 2–5 wt% Na₂O), pargasitic amphibole (~ 0.8 wt% K₂O; 3–3.5 wt% Na₂O), kelyphite (Cpx 3, mostly <1 wt% Na₂O; and zoned Mg-Fe-Al spinel), sodalite, calcite, K-feldspar, djerfisherite (K_5.95Na_0.02Fe_18.72Ni_2.36Co_0.01Cu_4.08S₂₆Cl ) and a small amount of K-Ca-Fe-Mg glass, is ascribed to the metasomatic event II that occurred also in the upper mantle, but after the xenoliths were incorporated in the kimberlite. A pervasive chloritic alteration (mainly clinochlore + magnetite) that overprints earlier assemblages probably took place in the upper crustal environment. The diamonds are invariably associated with secondary clinopyroxene and chlorite, but the diamonds formed before the entrainment of the xenoliths in the Udachnaya kimberlite.Editorial Responsibility: T.L. Grove     

Patterns and controls on the distribution of diamondiferous intrusions in Australia

The distribution of kimberlite, lamproite and related alkaline volcanism in Australia can be broadly related to the structure of the Australian continent and lithosphere. Diamondiferous kimberlites and lamproites, with the apparent exception of the weakly diamondiferous Orrorro kimberlites in the Adelaide Fold Belt, lie within the large Precambrian shield where seismic tomographic models and heat flow data indicate the presence of relatively cold, high seismic wave speed lithosphere (tectosphere) typically some 200 km thick or more beneath the Archaean cratons and up to 300 km in parts of central Australia. Many of the diamondiferous intrusions appear to lie at the margins rather than in the centre of the lithosphere domains. The highest concentration of diamondiferous intrusions (kimberlites and lamproites) is on and around the Kimberley Craton where seismic data indicate crustal thicknesses of 35–40 km and a lithosphere up to 275 km thick that is distinct from Proterozoic northern Australia.

Many, but clearly not all, of the intrusions show evidence of regional and local structural controls. Some are spatially associated with known crustal structures, especially regional faults. Others are aligned, either singly or in clusters, along or near discontinuities and/or gradients evident in regional scale potential field data, especially the total horizontal gradients of gravity data continued upward tens to hundreds of kilometres. Many of these features are not evident in the original datasets as their signatures are masked by shorter wavelength (near surface) anomalies. In some cases, the kimberlites and associated rocks lie within crustal blocks and domains defined by discontinuities in the potential field data rather than at domain boundaries.

Our overview suggests that analysis of potential field data, especially horizontal gradients in upwardly continued potential field data, at all scales can assist definition of crustal and, potentially, lithospheric structures that may influence the distribution of diamond pipes. However, more definitive mapping of Australia's diamond prospective regions requires the integration of data on crustal structures, especially trans-lithospheric faults, and geodynamic settings with high resolution tomographic models and other geophysical, petrologic, and isotopic information on the nature of the lithosphere beneath the Australian continent.     

Petrology of two diamondiferous eclogite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Diamondiferous Group A eclogites constitute a minor portion of the mantle-derived xenoliths in the eastern Finland kimberlites. They have been derived from the depth interval 150–230 km where they are inferred to occur as thin layers or small pods within coarse-grained garnet peridotites. The chemical and isotopic composition of minerals suggest that they represent (Proterozoic?) mantle-derived melts or cumulates rather than subducted oceanic lithosphere. During magma ascent and emplacement of the kimberlites, the eclogite xenoliths were mechanically and chemically rounded judging from the types of surface markings. In addition, those octahedral crystal faces of diamonds that were partially exposed from the rounded eclogite xenolith became covered by trigons and overlain by microlamination due to their reaction with the kimberlite magma. The diamonds bear evidence of pervasive plastic deformation which is not, however, evident in the eclogite host. This suggests that annealing at ambient lithospheric temperatures has effectively recrystallised the silicates while the diamond has retained its lattice imperfections and thus still has the potential to yield information about ancient mantle deformation. One of our samples is estimated to contain approximately 90,000 ct/ton diamond implying that some diamonds occur within very high-grade pods or thin seams in the lithospheric mantle. To our knowledge, this is one of the most diamondiferous samples described.     

Constraints on the age of the Warrawoona Group, eastern Pilbara Block, Western Australia

Zircons from mafic and felsic volcanic rocks in the type area of the Warrawoona Group, the basal Archaean greenstone succession of the eastern Pilbara Block, have been dated precisely using the ion-microprobe SHRIMP. The results allow two alternative time-frames for the duration of the Warrawoona Group, dependent on how the dated zircons are considered to relate to the volcanic rocks. Our favoured interpretation requires a hiatus of 135±5 Ma between the Duffer Formation at 3.46 Ga and the overlying felsic volcanic rocks of the Wyman Formation, and a hydrothermal or later magmatic origin for zircons of age 3.33 Ga within one Duffer Formation sample and the underlying metabasalts. The alternative time-frame requires a short time for deposition of the entire Group, less than 15 Ma at 3.33 Ga, and a xenocrystic origin for the 3.46 Ga zircons of the Duffer Formation. Outside the type area of the Warrawoona Group, the age of an intrusive granodiorite requires that greenstones be older than 3.43 Ga and the Group formed over an interval of > 120 Ma.Visibly different zircons within one of the Duffer Formation samples were found to be Palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock at low temperature. Similar zircons were found within samples from other rock units but with a spread of Proterozoic ages.     

SHRIMP baddeleyite age for the Fraser Dyke Swarm,southeast Yilgarn Craton,Western Australia

Recent high‐resolution aeromagnetic data have delineated an extensive swarm of undeformed northeast‐trending dolerite dykes in the southeastern Yilgarn Craton, known previously only from isolated exposures in surface mining operations. Owing to parallelism of the dykes to the Fraser Mobile Belt, the eastern segment of the Albany‐Fraser Orogen, the swarm is referred to here as the Fraser Dyke Swarm. Ion‐microprobe dating of baddeleyite from a granophyric segregation in the centre of one dyke yields a mean ²⁰⁷Pb/²⁰⁶Pb age of 1212 ± 10 Ma (95% confidence limits). The location of the Fraser Dyke Swarm, adjacent and parallel to the Fraser Mobile Belt, suggests that the dykes may have been emplaced into lines of weakness that originated during tectonic loading and downwards flexure of the craton margin. This is the first evidence of ca 1210 Ma mafic dykes and associated crustal‐scale extension in the southeast Yilgarn Craton, although the age is similar to those reported recently for dolerite and quartz diorite dykes in the central and southern part of the craton, suggesting that a genetic relationship may exist between intrusions in the two areas.     

Emplacement age of the Markagunt gravity slide in southwestern Utah,USA

The Markagunt gravity slide (MGS) is a large-volume landslide in southwestern Utah that originated within the Oligocene-Miocene Marysvale volcanic field. Gravity slides are single emplacement events with long runout distances and are now recognized as a new class of volcanic hazard. Accumulation of volcanic material on a structurally weak substrate along with voluminous shallow intrusive events led to collapse. Here, ⁴⁰Ar/³⁹Ar data for landslide-generated pseudotachylyte, the landslide-capping Haycock Mountain Tuff and the deformed Osiris Tuff are combined with a Bayesian age model to determine an emplacement age of 23.05 + 0.22/−0.20 Ma for the MGS. The results suggest a lag time of <200 kyr between the caldera-forming eruption of the Osiris Tuff, additional buildup of the unstable volcanic pile and subsequent mass movement.     

Evaporitic sediments of Early Archaean age from the Warrawoona Group, North Pole, Western Australia

Chemical sediments are common and diverse in the c. 3500 Myr old North Pole chert-barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi-permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea-shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be incorrect.     

Paleomagnetic age of ferruginous weathering beneath the Hamersley Surface,Pilbara, Western Australia,and the Cenozoic apparent polar wander path

During the Mesozoic and Paleogene, the Precambrian rocks in the Pilbara, Western Australia, underwent erosion and deep weathering that produced an undulating landform now represented by the duricrusted and partly eroded Hamersley Surface. A reddened, ferruginous weathering zone occurs immediately beneath this duricrusted surface. Oriented block samples of ferruginised strata of the Neoarchean–early Paleoproterozoic Hamersley Group exposed within approximately 15 m below the duricrust were collected at 20 sites in roadcuts along the Great Northern Highway between Munjina and Newman and exposures along the adjoining Karijini Drive. Stepwise thermal demagnetisation of cored specimens revealed a stable, high-temperature (680°C) component carried by hematite, with a mean direction (n = 55 specimens) of declination D = 182.0°, inclination I = 52.9° (α₉₅ = 3.6°), indicating a pole position at latitude λ_p = 77.6°S, longitude ?_p = 113.2°E (A₉₅ = 4.3°) and a paleolatitude λ = 33.5 +3.6/–3.3°S. Both normal and reversed polarities are present, indicating that the remanent magnetism was acquired over an interval of at least two polarity chrons (say 10⁵–10⁶ years). Chi-square tests on the determined pole position and three different sets of Cenozoic poles, namely those for dated volcanic rocks in eastern Australia supplemented by poles for Australian Cenozoic weathering horizons, and inferred poles from Pacific Ocean and Indian Ocean hotspot analyses and North American Cenozoic poles rotated to Australian coordinates, yielded a mean age of ca 24 ± 3 Ma, i.e. late Oligocene to early Miocene, interpreted as the time of formation of hematite in the sampled ferruginous zone. The ferruginous weathering occurred under globally warm conditions and was followed during the early to middle Miocene climatic optimum by the deposition of channel iron deposits, which incorporated detrital hematitic material derived from erosion of the ferruginous weathering zone beneath the Hamersley Surface.     

Emplacement age of the Songshugou ultramafic massif in the Qinling orogenic belt,and geologic implications

The Songshugou ultramafic massif is located to the north of the Shang‐Dan fault, the Palaeozoic suture between the North and South China blocks. It is the largest Apline‐type ultramafic body in the Qinling orogenic belt of central China, consisting mainly of dunite with a small amount of harzburgite and minor pyroxenite. We present new LA‐ICP‐MS U?Pb dating and trace element results for zircon from two garnet amphibolite samples in the contact metamorphic zone surrounding the massif. One was sampled ～1 m from the massif, the other ～5 m away. The studied zircon grains are small, anhedral, and display typical metamorphic characteristics of low Th/U values (<0.1). The U and Th concentrations of zircon range from several hundred ppm to less than 10 ppm. Cathodoluminescence images show two apparent generations of zircon, with lighter cores and darker rims. Core and rim ages however, are identical within error. These two samples yield identical concordant ages of 506±7 and 510±7 Ma, suggesting that the Songshugou ultramafic massif was emplaced at ～510 Ma. Low HREE concentrations and the absence of Eu anomalies in most analysed zircons suggest that the studied grains most likely formed during garnet amphibolite metamorphism induced by emplacement of the ultramafic massif.

To better understand the cooling history of the massif, ⁴⁰Ar/³⁹Ar ages of amphibole from three garnet amphibolite specimens in the contact metamorphic zone and one amphobolite sample about 20 m away from the massif were determined. The ⁴⁰Ar/³⁹Ar ages increase from 372±15 Ma (JSM‐01) near the massif to disturbed, unreliable ‘plateau’ ages of 474±8 Ma (JSM‐03) and 781±146 Ma (JSM‐04) with increasing distance from the ultramafic massif, showing limited heating during exhumation of the massif, followed by slow cooling. Therefore, the Songshugou ultramafic massif does not reflect the Jining orogeny at ～1 Ga. Instead, it was emplaced into the Proterozoic, Qinling Group during the Palaeozoic, probably due to the subduction along the Shang‐Dan fault.