Insights into the petrogenesis of the West Kimberley lamproites from trace elements in olivine

The Miocene lamproites of the West Kimberley region, Western Australia include olivine-leucite lamproites (≤10 wt% MgO) containing olivine and leucite microphenocrysts, and diamondiferous olivine lamproites (20–30 wt% MgO) containing olivine phenocrysts and larger (1–10 mm) olivine as mantle xenocrysts and dunite micro-xenoliths. Olivine phenocrysts and thin (<100 μm) magmatic rims define trends of decreasing Cr and Ni, and increasing Ca and Mn, with decreasing olivine Mg#, consistent with fractional crystallisation of olivine (and minor chromite). Many phenocrysts are zoned, and those with cores of similar Mg# and trace element abundances to the mantle xenocrysts may be xenocrysts overgrown by later olivine crystallised from the lamproite magma. Magmatic olivines Mg#_91–92 are estimated to have been in equilibrium with olivine lamproite magma(s) containing ~22–24 wt% MgO. The xenocrystic mantle olivines Mg_90–92.5 in the olivine lamproites are inferred from trace element abundances to be mostly derived from garnet peridotite with equilibration temperatures estimated from the Al-in-olivine thermometer (Bussweiler et al. 2017) to be ~1000–1270 °C at depths of 115–190 km. Olivines from the deeper lithosphere are less depleted (lower Mg#, higher Na, Al, P, Ti, Zr etc) than those at shallower depths, a feature suggested to reflect the combined effects of metasomatic re-enrichment of the craton roots (Ti, Fe, Zr etc) and increasing temperature with depth of origin (Na, Al, Ca). The West Kimberley lamproite olivines are not enriched in Li, as might be expected if their source regions contained continental sedimentary material as has been previously inferred from lamproite large-ion-lithophile trace elements, and Sr and Pb isotopes.

     

A high pressure experimental study on a magnesian-rich leucite-lamproite from the West Kimberley area,Australia: petrogenetic implications

Liquidus and subliquidus phase relations of a leucite-lamproite (wolgidite) from the West Kimberley area, Australia have been studied experimentally under the volatile conditions of 3.22 wt.% H₂O ( \(X_{CO_2 }\) =0.11) and 13.0 wt.% H₂O ( \(X_{CO_2 }\) =0.03) between 10 to 40 kbar. Under these conditions, liquids are vapour undersaturated. In experiments with 13.0 wt.% H₂O, olivine is the liquidus phase up to 24 kbar and orthopyroxene above 24 kbar. Phlogopite and rutile occur close to the liquidus above 16 kbar. Crystallization temperatures of clinopyroxenes are 50–120° C below the liquidus. Based on these results, wolgidite magma is unlikely to be a partial melt of a garnet- or spinel-lherzolite mantle but could be derived from phlogopite+rutile±olivine±or-thopyroxene assemblages occurring as metasomatized mantle.     

The McIntosh Layered Troctolite-Olivine Gabbro Intrusion, East Kimberley, Western Australia

The well-preserved ?lower Proterozoic McIntosh intrusion consistsof 96 macro-layers with a total stratigraphic thickness of about6 km. The lowermost rocks in this possible cone-shaped intrusionare hidden, and the roof and the upper layers were removed byerosion. The layered sequence is dominated by 40 bimodal cyclicunits of troctolite and olivine gabbro. Minor gabbronorite layersoccur throughout the sequence, and are more abundant and morefractionated higher in the sequence. Six imperfect megacycicunits are developed in the upper 2700 m, each unit consistingof several troctolite-olivine gabbro cyclic units followed bya Fe-Ti oxide-bearing gabbronorite. The overall cumulus crystallizationorder in each megacyclic unit was plagioclase first, closelyfollowed by olivine, then augite, orthopyroxene, and magnetitesuccessively. Cryptic composition data for troctolites and olivine gabbrosshow a slight overall decrease of 10 mol per cent An and Fofrom the base to the top of the layered sequence (approximateranges An_80–70 and Fo_78–68). Several major fluctuationsoccur however, and are generally associated with the oxide gabbronorites,which are significantly more fractionated than the adjacentlayers (plagioclase An_53–60, orthopyroxene Mg_52–69Each fluctuation comprises a marked progressive discontinuity(rapid normal fractionation) followed by a gradual to rapidregressive discontinuity (or reversal) in the overlying troctolitesand olivine gabbros. Apparently, such marked progressive discontinuitieshave not been described in layered intrusions. A chilled margin and the overall composition of the intrusionsuggest an olivine tholeiite parent magma, inferred to havecrystallized at P 6 kb, relatively low _PH2O and high f_O2 (>NNO buffer). The troctolite-olivine gabbro cyclic units areinferred to have formed by fractional crystallization of periodicadditions of new magma. However, the oxide gabbronorites seemtoo fractionated relative to the underlying layers to have formedby conventional crystal fractionation mechanisms, and they couldhave resulted from a ‘liquid fractionation’ processin which fractionated residual magma, instead of rising, periodicallybecame denser and ponded on the temporary floor (a density crossover).Gradual, reversed cryptic trends in the cyclic units above theoxide gabbronorite layers may reflect mixing of this fractionatedmagma with successive magma additions.     

The Salt Lick Creek layered intrusion,East Kimberley region,Western Australia

The Lower Proterozoic Salt Lick Creek intrusion, East Kimberley region, Western Australia, is a layered intrusion divisible into two well-defined zones, the Basal and Main Zones, whose combined stratigraphic thickness, as now exposed, is approximately 1000 metres. The Basal Zone, 360 metres thick, contains three members, two of which (Members 1 and 3) are dominated by olivine, plagioclase cumulates (including harrisites and allivalites); Member 2, near the middle of the Basal Zone, consists substantially of more olivine-rich cumulates, including plagioclase-bearing dunites. The Main Zone, commencing with Member 4 plagioclase, orthopyroxene cumulates, is composed largely of anorthositic cumulates of Member 5. Mild but nevertheless measurable rhythmic layering is superimposed upon the three members comprising the Basal Zone. Electron probe microanalyses of the primary phases across some 500 metres of cumulates indicate limited cryptic variation with stratigraphic height. Olivine ranges in composition from Fo₈₁ to Fo₈₄, orthopyroxene from Ca₂Mg₈₃Fe₁₅ to Ca₂Mg₇₈Fe₂₀, clinopyroxene from Ca₄₈Mg₄₆Fe₆ to Ca₄₄Mg₄₈Fe₈, and plagioclase from An₈₄ to An₈₈ but mineral compositions are not a simple function of stratigraphic height. It is inferred that the parental magma(s) was high-alumina mafic, intrinsically subalkaline, strongly olivine- and plagioclase-normative and in all likelihood tholeiitic in its affinities. The olivine-free cumulates of the Main Zone display a higher level of normative saturation than the cumulates of the Basal Zone but mineral and host rock chemistries, particularly 100 Mg/ (Mg+Fe²⁺) atomic ratios, are not favourable to proposals which would relate the origin of the Main Zone or the several members of the intrusion to the differentiation of a single pool of magma. It is suggested that the Main Zone, at least, derived from a separate pulse of relatively more saturated magma and that the lateral replenishment by more or less undifferentiated magma was also a fundamental and critical factor in the genesis of the Basal Zone cumulates.     

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     

Mineralogy and Petrology of the Aries Diamondiferous Kimberlite Pipe, Central Kimberley Block, Western Australia

Aries is a deeply weathered micaceous kimberlite pipe (820 Ma)consisting of four lobes: South, Central, North, and North Extension.It is the largest ( 18 ha) and most diamondiferous of the fewkimberlites currently known on the Australian continent, andis rich in country-rock (dolerite and quartzite) xenoliths.Three textural varieties of Aries kimberlites can be recognized,together with autoclastic breccias: (1) macrocrystalmedium-grained;(2) aphanitic (5 vol. % olivine macrocrysts); and (3) macrocrystalsegregated. The kimberlites contain two generations of olivinepseudomorphs (30–40 vol. %), and two of phlogopite (upto 60 vol. %), in a groundmass of apatite, calcite, diopside,sphene, spinels, serpentine, talc, and accessory groundmassminerals including aeschynite [(Ce, Ca) (Ti, Nb)₂O₆], barite,ilmenite, monazite, rutile, siderite, and unidentified Nb-Fe-titanates.Phlogopite zoning is complex and differs from lobe to lobe,but general compositions and trends resemble phlogopites fromkimberlites (TiO₂ 0–5–4 wt. %, A1₂O₃ 9–16%);tetraferriphlogopite substitution is indicated by low Al insome grains. Diopside is low in Cr, Al, Na, and Ti, with highmg-number [molecular Mg/(Mg + Fe²⁺) 93]. Apatite contains upto 17–5% SrO, calcite up to 1–7% SrO but littleMgO or FeO, sphene up to 1.5% Nb₂O₅, and ilmenite 2.6% Nb2O5and 16% MnO but no detectable MgO. Extremely complex moqftiological, textural, and compositionalvariations are present in spinels. They can be divided intofive textural-genetic types: cognate Groundmass chromian spinels(Type G); Inclusions of chromian spinels in olivine macrocrysts(Type I), probably representing either early phenocrysts ormantle xenocrysts: Macrocryst chromian spinels (Type M), probablyrepresenting xenocrysts; late-stage groundmass Fenian spinels(Type F), derived from serpentinization of olivine; Alterationferrian spinels (Type A), found as inclusions associated withsiliceous melt inclusions, in Types I and M, and probably representinginteraction of these earlier types with late-stage melts. Someof these, particularly Types M and F, show further texturalsub-types with no obvious genetic significance. The pipe formed from several magma-pulses. All four lobes maycontain at least one pulse in common, but Central and SouthLobes include additional pulse(s) which yielded distinctivephlogopite zoning, whereas North Lobe and North Extension includepulsc(s) which may have originated at higher mantle levels andyielded more evolved phlogopites. Aries most resembles South African Group II kimberlites mineralogically,certain West African micaceous kimberlites geochemically, andGroup I kimberlites isotopically. A distinctive mantle source-regionis implied by high Nb/U, Ce/Sr, Ce/P, Rb/Ba, and especiallyNb/Zr ratios. Similar anomalous geochemical signatures are sharedwith two other contemporaneous (800 Ma) lampro-phyric intrusionsin the east Kimberley (at Maude Creek and Bow Hill), suggestingthat a scattered alkaline province exists in the Kimberley Block,generated from a regionally anomalous mantle source.     

Palaeomagnetic results from the Hart dolerite of the Kimberley Block,Australia

A routine palaeomagnetic study of the 1800 Ma old Hart dolerite shows that sample directions of magnetism give good within-site grouping at 12 sites after magnetic cleaning. However, the site mean directions are scattered over one-quarter of the sphere, a feature that apparently does not arise from poor structural control. A baked contact test confirms that the magnetization is original. Comparison of bulk NRM, TRM, ARM and saturation IRM properties has been made with the Modipe gabbro of southern Africa from which a palaeointensity has previously been estimated. These measurements are consistent with the hypothesis that the main dipole field intensity temporarily was much reduced with respect to the non-dipole field 1800 m.y. ago resulting in increased directional scatter from secular variation. The average of the site VGPs at 29N 46E with A₉₅ = 24° thus is still a viable estimate of the palaeomagnetic pole at that time bearing in mind the larger than usual statistical error associated with it.     

Neoproterozoic (Cryogenian) stromatolites from the Wolfe Basin,east Kimberley,Western Australia: Correlation with the Centralian Superbasin

Linella avis, an early to middle Neoproterozoic (Tonian to Cryogenian) stromatolite, occurs in the Eliot Range Dolomite, part of the Ruby Plains Group in the Wolfe Basin, east Kimberley. Previously, this dolomite was assigned to the Mesoproterozoic Bungle Bungle Dolomite in the Osmond Basin, which contains a different suite of stromatolites. Linella avis, which also occurs in the Neoproterozoic Bitter Springs Formation of the Amadeus Basin, central Australia, appears to be restricted to rocks aged around 850 to 800 Ma. The presence of L. avis indicates that the Ruby Plains Group is a probable correlative of the Heavitree Quartzite and Bitter Springs Formation, and is probably much younger than the Bungle Bungle Dolomite. If the correlation suggested here is correct, the Wolfe Basin, together with the Amadeus and Ngalia Basins, formed part of the Centralian Superbasin.     

'Cap carbonates' and Neoproterozoic glacigenic successions from the Kimberley region, north-west Australia

The term ‘cap carbonate’ is commonly used to describe carbonate units associated with glacigenic deposits in Neoproterozoic successions. Attempts to use carbonate units as stratigraphic markers have been counfounded by inconsistent identification of ‘cap carbonates’ and a somewhat broad use of the term. Systematic sedimentological and geochemical analysis of carbonate rocks (mostly dolomite) associated with glacigenic deposits from the Neoproterozoic succession of the Kimberley region, north‐western Australia, shows that it is possible to characterize such units by their specific mineralogical, sedimentological, petrographic, geochemical and stratigraphic features. Hence, it is possible to differentiate true ‘cap carbonates’ from other carbonate units that are associated with glacigenic deposits. In the Kimberley successions two broad carbonate types are identified that reflect two stratigraphically distinct depositional realms. Carbonate rocks from the Egan Formation and Boonall Dolomite (the youngest carbonate units in the succession) are characterized by sedimentary components and features that are consistent with deposition on shallow platforms or shelves, analogous to Phanerozoic warm‐water carbonate platform deposits. In contrast, dolomite from the Walsh, Landrigan and Moonlight Valley Tillites preserves a suite of sedimentary and geochemical characteristics that are distinctly different from Phanerozoic‐like carbonate rocks; they are thin (ca 6 m), laterally persistent units of thinly laminated dolomicrite/dolomicrospar recording δ¹³C fluctuations from −1‰ to −5‰. These latter features are consistent with a ‘Marinoan‐style cap‐carbonate’ rock described from other Neoproterozoic successions. The similarity and broad distribution of these rocks in Australia, when considered within the context of genetic models suggesting a global oceanographic–atmospheric event, support their use as a lithostratigraphic marker horizon for the start of the Ediacaran Period at ca 635 Ma.     

Nature and genesis of a carbonatite-associated fluorite deposit at Speewah, East Kimberley region, Western Australia

Summary The Speewah fluorite deposit (>2.28Mt at 25.5% CaF₂) is sited adjacent to the crustal-scale Greenvale Fault on the western side of the Halls Creek Orogen, in the East Kimberley region of Western Australia. Host rocks are Palaeoproterozoic sedimentary rocks, dolerite and granophyre, Early Cambrian basalt, and the Yungul carbonatite. The deposit comprises mainly fluorite–quartz veins associated with lesser barite, sulfides and calcite, controlled by NNE–SSW and N–S brittle faults and fractures. Cross-cutting field relationships indicate that the fluorite veins were deposited post Early Cambrian.Fluorite–quartz vein textures, including colloform banding and comb texture, combined with microthermometric data from primary fluid inclusions, indicate that fluorite was deposited by the incremental infill of open-space structures in an epizonal, and probably epithermal, environment (<160°C) from complex, Li–Ca–Mg-rich, highly saline ore-fluids.The Yungul carbonatite and intimately-associated replacement-type fluorite have similar levels of REE enrichment and identical chondrite-normalised HREE patterns. Samarium and neodymium isotopic analyses of the fluorite indicate extreme differentiation of the ¹⁴⁷Sm/¹⁴⁴Nd ratio, from 0.0709 to 0.6918. These Sm–Nd isotopic data correspond to a mineral isochron with an age of 122±24Ma, interpreted to represent the age of fluorite deposition.Based on the potentially magmatic fluid composition, the replacement-type fluorite within the carbonatite, the similar HREE patterns of fluorite and carbonatite, and direct, if imprecise, isotopic dating of the fluorite, which confirms that fluorite mineralization is younger than the Early Cambrian basalts, the Speewah fluorite deposit is interpreted to be genetically related to the Yungul carbonatite. The large fluorite resource cannot have been derived from the exposed, low-volume carbonatite dyke. Rather, it must have been sourced from a larger carbonatite body at depth, whose presence is implied from basement-derived xenocrystic zircons in the Yungul carbonatite.     

Geochemical and Sr-Nd-Pb isotopic characteristics of Late Cenozoic leucite lamproites from the East European Alpine belt (Macedonia and Yugoslavia)

In the East European Alpine belt, leucite-sanidine-phlogopite-olivine-bearing volcanic rocks of Late Cenozoic age occur at eight localities within the Vardar suture zone and at one locality in the Southern Carpathian fold-and-thrust belt. Most of these volcanics are characterized by high Mg# (66.6–78.6), high abundances of Ni (117–373 ppm) and Cr (144–445 ppm) as well as high primary K₂O contents (5.63–7.01 %) and K₂O/Na₂O values (1.93–4.91). Rocks with more differentiated compositions are rare. A lamproite affinity of these rocks is apparent from their relatively low contents of Al₂O₃ (9.9–14.3 wt%) and CaO (6.2–8.3 wt%) in combination with high abundances of Rb (85–967 ppm), Ba (1,027–4,189 ppm), Th (18.9–76.5 ppm), Pb (19–54 ppm), Sr (774–1,712 ppm) and F (0.16–0.52 wt%), and the general lack of plagioclase. Although eruption of the magmas took place in post-collisional extensional settings, significant depletions of Nb and Ta relative to Th and La, low TiO₂ contents (0.92–2.17 %), low ratios of Rb/Cs, K/Rb and Ce/Pb as well as high ratios of Ba/La and Ba/Th suggest close genetic relationships to subduction zone processes. Whereas Sr and Nd isotope ratios show relatively large variations (⁸⁷Sr/⁸⁶Sr = 0.7078–0.7105, ¹⁴³Nd/¹⁴⁴Nd = 0.51242–0.51215), Pb isotope ratios display a very restricted range with ²⁰⁶Pb/²⁰⁴Pb = 18.68–18.88 and variable but generally high 7/4 (11–18) and 8/4 (65–95) values. The observed petrographic, geochemical and isotopic characteristics are best explained by a genetic model involving preferential melting of phlogopite-rich veins in an originally depleted lithospheric mantle source, whereby the metasomatic enrichment of the mantle source is tentatively related to the addition of components from subducted sediments during consumption of Tethyan oceanic lithosphere.Editorial responsibility: J. Hoefs     

Detrital zircon geochronology of the Speewah Group,Kimberley region,Western Australia: evidence for intracratonic development of the Paleoproterozoic Speewah Basin

Field observations integrated with new petrographic and sensitive high-resolution ion microprobe (SHRIMP) U–Pb age data for detrital zircons from the Paleoproterozoic Speewah Group of northern Western Australia provide evidence of depositional conditions, source of detritus, timing and evolution of the sedimentary rocks in the Speewah Basin. The Speewah Group is a 1.5 km-thick succession of poorly outcropping, predominantly siliciclastic rocks that preserve a fluviatile to marine, transgressive and regressive event. The Speewah Group unconformably overlies crystalline rocks of the Lamboo Province that were stabilised by the 1870–1850 Ma Hooper Orogeny, then accreted as the Kimberley region onto the North Australian Craton during the 1835–1810 Ma Halls Creek Orogeny. Unconformably overlying the Speewah Group is about 4 km of predominantly siliciclastic marine sedimentary rocks of the Kimberley Group in the Kimberley Basin. This study has detected a detrital zircon component within the Speewah Basin at 1814 ± 10 Ma, with a youngest zircon at 1803 ± 12 Ma (1σ) in fluviatile sandstones located beneath a volcaniclastic rock with magmatic zircons that have been dated at ca 1835 Ma. Previous studies proposed that the Speewah Basin developed as a retro-arc foreland basin during accretion of the North Australian Craton. We interpret the ca 1835 Ma zircons in the volcaniclastic rocks to be xenocrystic in origin. This new 20 million years younger maximum depositional age indicates that the Speewah Group in the Speewah Basin, similarly to the overlying Kimberley Group in the Kimberley Basin, developed in a post-orogenic setting on the North Australian Craton rather than in a syn-orogenic setting associated with the 1835–1810 Ma Halls Creek Orogeny.     

The phlogopites from the Jacupiranga carbonatite intrusions

Summary Electron microprobe analyses of phlogopites from five carbonatite intrusions (C₁ oldest, to C₅ youngest) constituting the carbonatite plug in the Jacupiranga complex support previous conclusions based on magnetite analyses that C₂ to C₅ represent a trend of differentiation, and that C₁ has different chemical characteristics. All analyzed grains are zoned but no systematic zoning trends occur. The FeO/(FeO + MgO) ratio is very low (0.065 to 1.2), MgO is high (22.5 to 28.2%), TiO₂ is very low (up to 0.44w%), BaO may be very high (up to 10.3 w%), and Na₂O may be as high as 2.77 w%. With decreasing age from C₂ to C₅, MgO increases. Phlogopites from a banded reaction rock between carbonatite and jacupirangite are similar to, but not related to the phlogopites of the adjacent carbonatite. Compared with other carbonatite micas, the MgO-rich, TiO₂-poor Jacupiranga phlogopites present one of the less evolved compositions, similar to those in mantle peridotites. Carbonatite micas are generally lower in TiO₂ than BaO-rich micas from other rocks.

---

Die Phlogopite der Karbonatitintrusionen von Jacupiranga und ihre petrogenetische Bedeutung

Zusammenfassung Mikrosondenanalysen von Phlogopiten aus fünf verschiedenen Karbonatitintrusionen (C₁ am ältesten, C₅ am jüngsten) des Jacupiranga-Komplexes unterstützen die früheren Schlüsse aus Magnetitanalysen, daß C₂ bis C₅ durch eine kontinuierliche magmatische Differentiation voneinander abgeleitet werden können, während C₁ andere geochemische Charakteristika aufweist. Alle analysierten Phlogopite sind zoniert, jedoch ohne jede systematische Tendenz in der Zonierung. Das FeO/(FeO + MgO)-Verhältnis ist sehr niedrig (0.065 bis 0.12), TiO₂ ist niedrig (unter 0.44 Gew.-%), BaO kann mit bis zu 10.3 Gew.-% sehr hoch sein, und Na₂O kann 2.77 Gew.% erreichen. Mit abnehmendem Alter von C₂ nach C₅ steigt der Gehalt an MgO an, während BaO abnimmt. Phlogopite aus einem gebänderten Reaktionsgestein zwischen Karbonatit und Jacupirangit sind ähnlich zusammengesetzt wie die benachbarten Karbonatite, mit diesen jedoch nicht verwandt. Verglichen mit Glimmern anderer Karbonatitvorkommen zählen jene von Jacupiranga zu den am wenigsten entwickelten auf Grund ihrer hohen MgO-Gehalte bei gleichzeitiger TiO₂-Armut. Sie weisen eine ähnliche Zusammensetzung wie Glimmer aus Mantelperidotiten auf. Bei gleichen BaO-Gehalten sind Glimmer aus anderen Karbonatiten generell ärmer an TiO₂.

---

---

With 7 figures     

Granny Smith diopside megacrysts from the kimberlites of the Kimberley area and Jagersfontein,South Africa

Calcic diopside megacrysts, called Granny Smith nodules, in the Kimberley area and Jagersfontein kimberlites are sheared, commonly contain lenticles of ilmenite and intergrowths of phlogopite, and have a distinctive apple-green color. These diopsides have $\text{Ca}\text{(Ca + Mg)}\text{> 0.45,}\text{Mg}\text{(Mg + Fe)}\text{> 0.90, 0.2?0.4}$ wt% TiO₂ and 0.5–3 wt% Cr₂O₃. They have Na > (Al + Cr), in contrast to diopsides in peridotite xenoliths and those that form subcalcic discrete nodules, but in this respect are similar to diopsides in amphibole-bearing MARID nodules and mica-rich glimmerites. Granny Smith megacrysts are not cognate (Kramers, 1979); their parental magmas (in an igneous or metasomatic sense) may have been parts of the spectrum between kimberlites and lamproites.     

Possible animal body fossils from the Late Neoproterozoic interglacial successions in the Kimberley region, northwestern Australia

New specimens of the enigmatic Ediacara-type fossil Palaeopascichnus have been identified from the upper part of the Neoproterozoic Ranford Formation in the Kimberley region, northwest Australia. New material is morphologically similar to Palaeopascichnus and represents the largest species of this genus. They resemble the present-day xenophyophore protists in chamber morphology and growth patterns, supporting the interpretation that Palaeopascichnus is possibly a xenophyophore body fossil rather than a trace fossil. Stratigraphic correlation reveals that the new Palaeopascichnus specimens are preserved in the interglacial successions between the Landrigan/Marinoan and Egan/Ediacaran glaciations. If correlation with the early Ediacaran formations of South Australia is accepted, this represents the earliest known identifiable member of the Ediacara biota. New fossil record fills the evolutionary gap between the Cryogenian and Ediacaran animal assemblages and well-known Ediacaran biota. The new Palaeopascichnus specimens represent the first record of Ediacara-type fossils in Kimberley, and suggest the probability that additional Ediacaran fossils may be found in northwestern Australia.     

Spider impact structure,Kimberley Plateau,Western Australia: interpretations of formation mechanism and age based on integrated map-scale data

The centre of the 13?×?11 km Spider impact structure, Western Australia, displays an unusual system of eroded folds and imbricated thrusts surrounding a sandstone dome. As inferred from GIS-integrated remote sensing, geological and digital elevation data, the structural setting of the original crater was influenced by, and hence post-dates, the formation of the Mt Barnett Syncline, the east?–?west-oriented axis of which runs through the Spider structure. The syncline formed during the regional Yampi Orogeny (ca 900 Ma), thus constraining the maximum age of the impact event. The sandstone dome in the centre of Spider formed prior to the imbrication, as interpreted from the present setting that indicates a deflection of the southward moving material during the crater collapse. Two modes of formation are discussed in order to explain the south-directed shortening in the Spider impact structure: (i) impact into the bottom of a syncline-controlled palaeovalley leading to uplift of the central crater floor followed by gravity-driven asymmetric sliding preferentially from the northern crater wall and valley slope, respectively; and (ii) moderately oblique (～10?–?30°) impact from the north onto the axis of the syncline, producing a central uplift under the influence of downrange residual momentum and, thus, asymmetric deformation inside the uplift and farther downrange. Neither model alone explains all the observations, and only a combination of both may provide a satisfactory solution.