Were intercalated komatiites and dacites at the Black Swan nickel sulphide mine, Yilgarn Craton, Western Australia, emplaced as extrusive lavas or intrusive bodies? The significance of breccia textures and contact relationships

Intercalated Archean komatiites and dacites sit above a thick footwall dacite unit in the host rock succession at the Black Swan Nickel Mine, north of Kalgoorlie in the Yilgarn Craton, Western Australia. Both lithofacies occur in units that vary in scale from laterally extensive at the scale of the mine lease to localized, thin, irregular bodies, from > 100 m thick to only centimetres thick. Some dacites are only slightly altered and deformed, and are interpreted to post-date major deformation and alteration (late porphyries). However, the majority of the dacites display evidence of deformation, especially at contacts, and metamorphism, varying from silicification and chlorite alteration at contacts to pervasive low grade regional metamorphic alteration represented by common assemblages of chlorite, sericite and albite. Texturally, the dacites vary from entirely massive and coherent to partially brecciated to totally brecciated. Strangely, some dacites are coherent at the margins and brecciated internally. Breccia textures vary from cryptically defined, to blocky, closely packed, in situ jig-saw fit textures with secondary minerals in fractures between clasts, to more apparent matrix rich textures with round clast forms, giving apparent conglomerate textures. Some clast zones have multi-coloured clasts, giving the impression of varied provenance. Strangely however, all these textural variants have gradational relationships with each other, and no bedding or depositional structures are present. This indicates that all textures have an in situ origin. The komatiites are generally altered and pervasively carbonate veined. Preservation of original textures is patchy and local, but includes coarse adcumulate, mesocumulate, orthocumulate, crescumulate-harrisite and occasionally spinifex textures. Where original contacts between komatiites and dacites are preserved intact (i.e. not sheared or overprinted by alteration), the komatiites have chilled margins, whereas the dacites do not. The margins of the dacites are commonly silicified, and inclusions of dacite occur in komatiite, even at the top contacts of komatiite units, but komatiite clasts do not occur in the dacites. The komatiites therefore were emplaced as sills into the dacites, and the intercalated relationships are interpreted as intrusive. The brecciation and alteration in the dacites are interpreted as being largely due to hydraulic fracturing and alteration induced by contact metamorphic effects and hydrothermal alteration deriving from the intrusion of komatiites into the felsic pile. The absence of autobreccia and hyaloclastite textures in the dacites suggest that they were emplaced as an earlier intrusive (sill?) complex at a high level in the crust.

Distribution, genesis and age of the Schlagwasser breccia (Warstein anticline, Rhenish massif)

The name "Schlagwasser breccia" is a synopsis of several debris flows in the Warstein area, which can be derived from the Warstein carbonate platform and the Scharfenberg reef. Though only locally developed, the breccia is important for the understanding of paleogeography and sedimentology in the Eastern Sauerland. Considering this breccia some gravitational-resedimentary slide movements between a high, consisting of reef carbonates, and a basin with flinz beds can be pointed out. From the uppermost Middle Devonian to the lowermost Lower Carboniferous several slides yielded the sedimentary components building up the 30 to 50 m thick polymict breccia. Some breccias were redeposited repeatedly as can be verified by different conodont maxima in single samples. Supplying area was the western part of the Warstein high, from which the slide masses glided off to the East and Southeast, more seldom to the West and Westsouthwest. All conodont zones from the upper Middle Devonian up to the lowermost Carboniferous could be identified in the Schlagwasser breccia. Therefore, an uninterrupted continuous sedimentation must have been prevalent in the supplying area; today this area nearly is denuded of flinz beds and cephalopod limestones. The slide masses spread transgressively to the East up to a substratum consisting of different units as massive limestone, flinz beds and cephalopod limestone; they are overlapped by Hangenberg beds, alum schists and siliceous rocks of the Lower Carboniferous. Parts of the substratum were transported during the progress of the slide masses. Proximal and distal parts of the flow masses can be distinguished by the diameter of the pebbles. Graded bedding and banking structures are marked only rarely. Way of transport was up to 3 km. Differently aged slide masses do not always overlap, but are placed side by side, too. Usually the slide masses do not spread out upon a greater area during sedimentation, but form closely limited debris flows. Synsedimentary fracturing and tilting of the reef platform, epirogenetic movements and seaquakes caused the slides. The entire formation period of the breccia includes about 20 millions of years. The longevity of the events points to solid paleomorphological situations around the eastern margin of the carbonate platform.

La invención de Breccia

No hay arte que como la historieta enfrente en su misma constitución la conciliación imposible de dos lenguajes antitéticos: lo lingüístico y lo visual (o el arte de las palabras, la literatura, y el de las imágenes, la pintura). Y no hay artista que como Alberto Breccia -por lo menos en Argentina- haya plasmado con tanta crudeza las contradicciones de este pasaje interrumpido entre un sistema de signos y otro. El propósito de este trabajo es analizar tres etapas de la producción historietística de Alberto Breccia desde el punto de vista de la transformación progresiva de un dibujo puesto al servicio de la historia, donde se trata de figurar un mundo y de comunicar un mensaje determinado; hacia una imagen que no busca decir nada más allá de sí misma y que se torna entonces "símbolo de lo no-comunicable"

La invención de Breccia

No hay arte que como la historieta enfrente en su misma constitución la conciliación imposible de dos lenguajes antitéticos: lo lingüístico y lo visual (o el arte de las palabras, la literatura, y el de las imágenes, la pintura). Y no hay artista que como Alberto Breccia -por lo menos en Argentina- haya plasmado con tanta crudeza las contradicciones de este pasaje interrumpido entre un sistema de signos y otro. El propósito de este trabajo es analizar tres etapas de la producción historietística de Alberto Breccia desde el punto de vista de la transformación progresiva de un dibujo puesto al servicio de la historia, donde se trata de figurar un mundo y de comunicar un mensaje determinado; hacia una imagen que no busca decir nada más allá de sí misma y que se torna entonces "símbolo de lo no-comunicable"

La invención de Breccia

No hay arte que como la historieta enfrente en su misma constitución la conciliación imposible de dos lenguajes antitéticos: lo lingüístico y lo visual (o el arte de las palabras, la literatura, y el de las imágenes, la pintura). Y no hay artista que como Alberto Breccia -por lo menos en Argentina- haya plasmado con tanta crudeza las contradicciones de este pasaje interrumpido entre un sistema de signos y otro. El propósito de este trabajo es analizar tres etapas de la producción historietística de Alberto Breccia desde el punto de vista de la transformación progresiva de un dibujo puesto al servicio de la historia, donde se trata de figurar un mundo y de comunicar un mensaje determinado; hacia una imagen que no busca decir nada más allá de sí misma y que se torna entonces "símbolo de lo no-comunicable"