ALBERT

Notes: Abstract A sequence of ultramafic rocks in the Lac Guyer Archean greenstone belt exhibit brecciated flow tops, pillow structures, and spinifex textures testifying to their volcanic origin. Massive, spinifex-textured and differentiated flows in the sequence have the chemical characteristics of peridotitic komatiite, with MgO ranging from 19–25 wt.%. Associated pillowed flows have compositions that straddle the conventional boundary between komatiite and komatiitic basalt with MgO contents ranging from 16 to 19 wt.% MgO and are best termed pyroxenitic komatiites. Unlike other komatiitic occurrences, the peridotitic and pyroxenitic komatiites at Lac Guyer constitute a continuous chemical spectrum with no evidence of population minimum near 18 wt.% MgO. The contrasting behaviour of highly compatible elements, such as Ni and Cr, versus incompatible elements, such as Zr, indicate that this compositional spectrum was produced by a variation in the extent of partial melting (10–40%) of a garnet lherzolite source in the Archean mantle. The pyroxenitic komatiites represent liquids produced during lower (10–20%) degrees of melting during which garnet remained in the mantle residue. However, a change in slope in the distribution of Zr vs. Y between the pyroxenitic and the peridotitic komatiites indicates that garnet was completely consumed at the more extensive degrees of melting which produced the peridotitic komatiites. The Lac Guyer volcanic rocks display a population minimum at 15 wt.% MgO separating komatiitic magmas whose compositions are controlled by partial melting from basalts whose composition is controlled by crystal fractionation. The population minimum near 18 wt.% MgO which is taken as the boundary between komatiite and komatiitic basalt may have a similar origin.

Notes: Abstract A comparison of mantle xenolith suites along the northern Canadian Cordillera reveals that the xenoliths from three suites exhibit bimodal populations whereas the xenoliths from the other four suites display unimodal populations. The bimodal suites contain both fertile lherzolite and refractory harzburgite, while the unimodal suites are dominated by fertile lherzolite xenoliths. The location of the three bimodal xenolith suites correlates with a newly discovered P-wave slowness anomaly in the upper mantle that is 200 km in width and extends to depths of 400–500 km (Frederiksen AW, Bostock MG, Van Decar JC, Cassidy J, submitted to Tectonophysics). This correlation suggests that the bimodal xenolith suites may either contain fragments of the anomalously hot asthenospheric mantle or that the lithospheric upper mantle has been affected by the anomalously hot mantle. The lherzolite xenoliths in the bimodal suites display similar major element compositions and trace element patterns to the lherzolite xenoliths in the unimodal suites, suggesting that the lherzolites represent the regional lithospheric upper mantle. In contrast, the harzburgite xenoliths are highly depleted in terms of major element composition, but their clinopyroxenes [Cpx] have much higher incompatible trace element contents than those in the lherzolite xenoliths. The major element and mildly incompatible trace element systematics of the harzburgite and lherzolite xenoliths indicate that they could be related by a partial melting process. The lack of textural and geochemical evidence for the former existence of garnet argues against the harzburgite xenoliths representing actual fragments of the deeper anomalous asthenospheric mantle. Furthermore, the calculated P-wave velocity difference between harzburgite and lherzolite end-members is only 0.8%, with the harzburgites having higher P-wave velocities. Therefore the 3% P-wave velocity difference detected teleseismically cannot be produced by the compositional difference between the lherzolite and harzburgite xenoliths. If temperature is responsible for the observed 3% P-wave velocity perturbation, the anomalous mantle is likely to be at least 200 °C higher than the surrounding mantle. Taken together these data indicate that the refractory harzburgite xenoliths represent the residue of 20–25% partial melting of a lherzolite lithospheric mantle. The incompatible trace element enrichment of the harzburgites suggests that this melting was accompanied by the ingress of fluids. The association of the bimodal xenolith suites with the mantle anomaly detected teleseismically suggests that anomalously hot asthenospheric mantle provided both the heat and volatiles responsible for the localized melting and enrichment of the lithospheric mantle.

Notes: Zusammenfassung Der nordwestaustralische Kontinentalrand ermöglicht als sehr alter, sedimentverhungerter Kontinentalrand Nordost-Gondwanas die Untersuchung der frühen tektonischen und faziellen Entwicklung während der triassischen bis jurassischen Riftphase (»Neo-Tethys«), der früh-unterkretazischen Rift-Drift-Übergangsphase, sowie der frühen tektonischen, vulkanischen und paläozeanographischen »Post-breakup-Entwicklung« des Indischen Ozeans. Während des 122. und 123. Fahrtabschnittes des Ozeanbohrprogrammes (ODP) wurden 8 Bohrungen auf dem Exmouth Plateau und in den benachbarten Tiefseebenen niedergebracht. Die Bohrungen 759-761 und 764 untersuchten die triassische bis känozoische Entwicklung des Wombat-Plateau, eines kleinen Sub-Plateaus des nördlichen Exmouth-Plateaus. Bohrung 765 durchbohrte eine fast 1 km dicke Folge von kretazischkänozoischen Sedimenten der nahegelegenen Argo-Tiefseebene und 270 m der ältesten Ozeankruste des Indischen Ozeans; die Bohrungen 762 und 763 liegen im südlichen Teil des zentralen Exmouth-Plateaus, während Bohrung 766 in der nahegelegenen Gascoyne-Tiefseebene intrusive MORB-ähnliche Vulkanite nahe dem westlichen Steilrand des Exmouth-Plateaus erbohrte. Wesentliche Erkenntnisse wurden durch beide Kontinentalrand-Ozeanbecken-Bohrtraversen gewonnen. Diese führten zu einem besseren Verständnis der frühen Rift-Geschichte dieses Kontinentalrandes vom Oberperm bis in die Obertrias (Dehnungs- und Blocktektonik, fluviodeltaische/Flachwasserkarbonat-Paläoenvironments, bedeutender rhätischer Aufbau einer Karbonatplatform, Früh-Rift-Vulkanismus); der Prozesse während der Rift-Drift-Übergangsphase (bedeutende Blocktektonik während des Juras mit lokaler Hebung,Riftflanken-Kippung und subaenscher, erosiver Post-Rift-Diskordanz); des Alters und Paläoenvironments des ältesten Teils des Indischen Ozeans (Aufbrechen möglicherweise erst im Ober-Berrias/Unter-Valendis, mindestens 20 Ma Jahre später als erwartet); der »jugendlichen Ozeanphase« (transgressive, kondensierte unterneokome Belemnitensande mit CalcisphärenKreidekalken und Bentonitlagen auf dem Wombat-Plateau); der »reifen Ozeanphase« (post-mittelkretazische eupelagische Kreidekalke); der geochemischen Beschaffenheit und Entstehung alter ozeanischer Kruste und der sie überlagernden Ozeansedimente in einer »geochemischen Referenzbohrung«; sowie der mesozoischen Chrono- (Bio-Magneto-)Stratigraphie und des Alters sowie der Ursachen der bedeutenden relativen Meeresspiegelschwankungen, besonders während der Obertrias und Unterkreide.