The origin of large varioles in flow-banded pillow lava from the Hooggenoeg Complex, Barberton Greenstone Belt, South Africa (original) (raw)
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Bulletin of Volcanology, 2010
Well-preserved pillow lavas in the uppermost part of the Early Archean volcanic sequence of the Hooggenoeg Formation in the Barberton Greenstone Belt exhibit pronounced flow banding. The banding is defined by mm to several cm thick alternations of pale green and a dark green, conspicuously variolitic variety of aphyric metabasalt. Concentrations of relatively immobile TiO2, Al2O3 and Cr in both varieties of lava are basaltic. Compositional differences between bands and variations in the lavas in general have been modified by alteration, but indicate mingling of two different basalts, one richer in TiO2, Al2O3, MgO, FeOt and probably Ni and Cr than the other, as the cause of the banding. The occurrence in certain pillows of blebs of dark metabasalt enclosed in pale green metabasalt, as well as cores of faintly banded or massive dark metabasalt, suggest that breakup into drops and slugs in the feeder channel to the lava flow initiated mingling. The inhomogeneous mixture was subsequently stretched and folded together during laminar shear flow through tubular pillows, while diffusion between bands led to partial homogenisation. The most common internal pattern defined by the flow banding in pillows is concentric. In some pillows the banding defines curious mushroom-like structures, commonly cored by dark, variolitic metabasalt, which we interpret as the result of secondary lateral flow due to counter-rotating, transverse (Dean) vortices induced by the axial flow of lava towards the flow front through bends, generally downward, in the tubular pillows. Other pillows exhibit weakly-banded or massive, dark, variolitic cores that are continuous with wedge-shaped apophyses and veins that intrude the flow banded carapace. These cores represent the flow of hotter and less viscous slugs of the dark lava type into cooled and stiffened pillows.
Lithos, 2018
Globular structures in volcanic and hypabyssal rocks called varioles (or spherulites) have been described from a wide variety of places and rock types including komatiites, basalts, dacites, rhyolites as well as alkaline dykes, but the processes leading to their formation are still debated. Here, we present a field, petrological and geochemical study of variole-bearing lavas (variolites) in the Paleoproterozoic volcanic Hekpoort Formation in the southern part of the Transvaal sub-basin of South Africa. Variolites were only observed in a restricted area, corresponding to a ≥ 3.5 km long basin or depression. The variolites are locally up to 50 m thick and correspond to a succession of one-meter-thick lava flows. Most of the flows consist of two distinct layers, with an upper layer rich in spherical varioles (1-32 cm in diameter), overlying a layer generally devoid in varioles but often showing vertical jointing. Both layers show randomly oriented acicular textures of centimetre-sized skeletal clinopyroxene crystals (Al-rich augite) that precede the formation of varioles. The varioles themselves are mainly composed of radiating acicular augite (1µm in diameter), the crystallisation of which caused diffusion of incompatible elements, which became enriched in the groundmass around the varioles. The whole rock geochemistry of the Hekpoort variolites is indistinguishable from that of non-variolitic Hekpoort lavas with the same MgO content. This excludes liquid immiscibility as the origin of the varioles. The acicular texture (of Alrich augite) in combination with the incompatible element enrichment around the varioles suggests that both skeletal and acicular augite are related to undercooling rather than devitrification. Sub-aqueous deposition is thought to be the cause of undercooling.
Geological Magazine, 2011
Some large metabasaltic pillows in the uppermost part of the Palaeoarchaean Hooggenoeg Complex in the Barberton Greenstone Belt, South Africa, exhibit flow-banded margins and homogeneous cores that have a different texture and compositional variation. The margins consist of millimetre to several centimetre thick alternating bands of pale green spherulitic and darker, conspicuously variolitic varieties of non-vesicular and aphyric metabasalt, previously inferred to be due to mingling of two different types of lava. The dark cores have sharp, aphanitic contacts with the flow-banded carapaces. They lack flow banding, have coarse-grained interiors and exhibit wellpreserved primary textures with pseudomorphs after prismatic pyroxene set in a groundmass containing skeletal plagioclase. The compositional range of samples from these cores is unlike that of the flowbanded metabasalt but is similar to a 19 m thick lobate metabasalt flow ∼150 m stratigraphically further up, at the local top of the Hooggenoeg volcanic sequence. The pillow cores are inferred to result from the later refilling of drained hollow pillows.
Oceanic Island Volcanism I: Mineralogy and Petrology
Oceanic islands tend to occur at the young ends of hotspot trails because they record the passage of oceanic plates over rising convection cells (plumes) in the mantle, or the propagation of cracks in the lithosphere. Basaltic volcanism on oceanic islands is generally unexplosive and, although potentially destructive, poses less threat to human life than volcanism in other tectonic environments. However, the possibility of giant tsunamis from the catastrophic gravitational collapse of islands is of real concern for major cities surrounding the ocean basins. Two series of magmas are recognized in oceanic islands. Tholeiites form at lower pressures than alkali basalts, from higher percentages of decompression melting. The former contain a low-Ca pyroxene and the latter can crystallize nepheline. Furthermore, minerals common to both series (chromite, olivine, augite, plagioclase, magnetite and ilmenite) are compositionally distinct reflecting fundamental chemical differences between the two magma series. Mineral compositions vary as magmas evolve in sub-volcanic, lithospheric magma chambers by assimilation and differentiation. Magmas assimilate wall rocks in these chambers. Timescales for differentiation (mostly crystal fractionation) are generally less than a few thousand years. Early olivine, pyroxene, chromite and immiscible sulfide formation cause compatible elements (e.g., Ni, Co, Cr) to decline rapidly as differentiation proceeds. Plagioclase dramatically removes Sr at intermediate stages and alkali feldspars sequester Ba and Rb as late-stage trachytes and phonolites form in alkaline magmas. The high-field-strength elements are generally incompatible but locally decline reflecting apatite (P) and Fe-Ti oxide (Ti, Nb, Ta) removal. Studies of layering in individual lava flows suggest that rising volatiles may effect mass transfer of complexed ions during differentiation in magma chambers.
Mineralogy and Petrology, 1997
Suites of coarse-grained clasts ("plutonic nodules") within the scoria of two separate marie post-caldera parasitic vents, on Sete Cidades volcano, are described. These involve dunites, wehrlites, olivine clinopyroxenites, clinopyroxenites and olivine gabbros in the Eguas ankaramitic cone and pyroxene hornblendites, kaersutite gabbros and diorites in the Pico das Camarinhas basaltic-hawaiitic cone. The plutonic nodules are inferred to be cumulates, crystallised within the crust from magmas co-genetic with the post-caldera marie lavas erupted on the flanks of Sete Cidades.
Precambrian Research, 2002
Many intriguing crystallization textures of Archaean volcanic rocks are 'supercool' in the sense that they are visually striking, and because they owe their origin to substantial undercooling of the silicate liquids from which they grew. Under conditions of high undercooling kinetic effects arise such that spherulitic, acicular or platy mineral habits are stabilized. The term variole refers to globular and spherical centimetre-scale, generally leucocratic masses visible on the weathered surfaces of mafic rock. At many localities the varioles can be shown to be plagioclase spherulites that grew near the quench margins of aphyric mafic lavas. Elsewhere, the varioles are droplets of felsic magma frozen into mafic magma as a result of magma mingling. Platy olivine spinifex in komatiites arises from a coupled process of hydrothermal cooling and constrained crystallization. Shrinkage of the komatiite crust during submarine cooling causes fracturing and sea water ingress such that a self-propagating vertically directed cooling/cracking-front develops. Within the magma below this front, forsterite crystals transport heat from the melt upwards to the hydrothermally cooled crust. Because of their higher thermal conductivity and greater transparency to near infrared thermal radiation, Mg-rich olivine crystals transfer heat more efficiently than the surrounding relatively Fe-rich liquid. The olivine forms vertically oriented platy crystals that cool the liquid directly in front of their tips and are thus self-propagating.
The origin of basaltic lava flow textures
2006
This study addresses the links between the surface morphologies, internal structure, and microtexture of basaltic lava flows. The AD 1783-84 Laki eruption in south Iceland produced a 600 km 2 basaltic lava flow-field dominated by rubbly pahoehoe surface morphology. Field observations and aerial photograph interpretation show that the lava surface gradually changed from spiny to slabby and then to rubbly along single flows through repetitive crust disruption at the active front. The rubble was compressed into ridges when lava advance was obstructed and during large lava surges that coincided with the opening of new eruptive fissures. Fluid lava was transported in an extensive network of tubes that formed within the flows. Petrological study of Laki near-vent tephra and lava surface samples shows that, during the eruption, the magma lost-1 wt. % of water during ascent, which induced melt undercooling and triggered groundmass crystallization. This caused an-10% anorthite gap across plagioclase phenocrysts and drove considerable microlite formation (up to 30 vol.%) in the early stages of flow. It is estimated that fluid lava was transported from the vent to the most distal active front, 60 km from the vent, with cooling rates of < 0.5°Clkm. Young lava flow-fields in the Reykjanes Peninsula (Iceland), and some flows from the flood basalts of the Columbia River Province (USA), have surface and internal structures intermediate between rubbly pahoehoe and 'a'a. The increasing size and decreasing number density of plagioclase microlites with increasing depth in these flows, as in the Laki lavas, indicate that solidification rates decreased sharply inwards. Differences between sections are attributed to variations in lava bulk composition and the transport mode, duration of emplacement, and interaction with surface water of the fluid lava. Pahoehoe and 'a'a have low and high plagioclase number densities respectively, with an inverse correlation with the average size and aspect ratio of plagioc1ases. Rubbly pahoehoe lavas have intermediate characteristics. This correlation between lava surface morphologies and plagioclase textural characteristics provides a tool that may be useful for inferring eruption and emplacement processes from textural measurements of flow interiors in ancient basaltic lava flow-fields. A rna mere et mon pere qui m'ont encouragee tout au long de ce travail. A rna sreur pour nos eclats de rire (te souviens-tu de mon volcan de sable?). A Antoine pour nos balades en France et en Angleterre. A tous mes fantastiques amis fran9ais (et belges) pour leur soutien outre-manche (Anne &
Canadian Mineralogist, 2002
The Jurassic (201 Ma) quartz-normative continental tholeiitic basalts of the North Mountain Basalt (NMB) Formation of southern Nova Scotia, Canada, record evidence of pervasive silicate-liquid immiscibility. The basalts, up to 400 m thick, typically with phenocrysts of plagioclase (An 50-70 ) and clinopyroxene (Wo 40 En 40 Fs 20 ), are subdivided into lower, middle, and upper units on the basis of the nature of the flows and petrographic features. Petrographic observations combined with image analysis indicate that the middle and upper units are characterized by an abundance of mesostasis material (i.e., quenched residual melt) that contains skeletal clinopyroxene, acicular plagioclase, skeletal Fe-Ti oxides, Fe-Ti-P-rich globules, and andesitic to rhyolitic glass. Raster analysis of the mesostasis tracks an in situ fractionation that culminated in formation of an interstitial felsic glass (i.e., 74 wt.% SiO 2 ) that contains skeletal apatite and Fe-rich clinopyroxene. The composition of the skeletal clinopyroxene, ca. Wo 20 En 15 Fs 65 , acicular plagioclase (An 35-55 , ≤2.5 wt.% FeO), Fe-Ti-P-rich globules, and intergranular glass of the mesostasis are consistent with formation from an interstitial melt that evolved in a disequilibrium environment promoted by rapid cooling of the lavas. The mineralogical and chemical features are consistent with the process of silicate-liquid immiscibility within the residual liquids of basaltic systems, in this case late-stage intergranular melts. The presence within the NMB of (1) Fe-rich clinopyroxene-bearing mafic pegmatite, (2) thin (i.e., 1-2 cm) seams of rhyolite associated with the mafic pegmatites, and (3) locally abundant segregation pipes of mixed mafic-felsic composition, is considered to reflect mobilization of the Fe-and silica-rich immiscible melts. SOMMAIRE Les basaltes tholéiitiques continentaux de la Formation de North Mountain, du sud de la Nouvelle-Ecosse, au Canada, d'âge jurassique (201 Ma) et à quartz normatif, démontrent de l'évidence répandue d'une immiscibilité liquide. Les basaltes, atteignant une épaisseur de 400 m, possèdent typiquement des phénocristaux de plagioclase (An 50-70 ) et de clinopyroxène (Wo 40 En 40 Fs 20 ), et sont subdivisés en séquences inférieure, moyenne, et supérieure selon des critères pétrographiques et les aspects des coulées.