Flow-top Breccias Research Articles

Interactions of pāhoehoe and ‘a‘ā lavas and fluvial sediments on an alluvial plain (the Cretaceous Gyeongsang Basin, Republic of Korea)

Two lava units of basaltic composition named the Cheongryongsa and Hakbong basalts are intercalated in the Cretaceous nonmarine backarc basinfill of Korea. The Cheongryongsa Basalt is 5 to 6 m thick and laterally extends for about 12 km. It is underlain by massive sandy mudstone and overlain erosionally by a basaltic conglomerate, suggesting lava emplacement on a floodplain, followed by fluvial erosion and deposition. The basalt comprises flow-base breccia throughout the 12 km-long outcrop extent and is overlain by flow-top breccia at one locality. These breccias are composed mainly of irregular/fluidally shaped clasts in a massive mudstone matrix, and are interpreted as fluidal peperite produced by molten lava and wet sediment mingling. The Hakbong Basalt comprises three lava units, a few meters to several tens of meters thick, and intervening stratified sandstones. Three breccia types occur beneath or above the basalt: 1) irregular-clast breccia, 2) angular-clast breccia, and 3) fluidal-clast breccia. They are interpreted to be clinker, blocky peperite, and fluidal peperite, respectively, associated with ‘a‘ā lavas. The clinker could be distinguished from the peperite based on undeformed sedimentary structures in the interstitial sediment and the contrast in matrix grain size with the substrate sediment. Peperite formation was possible when the lava was emplaced upon very fine sandy to muddy sediment, which was probably under-consilidated and susceptible to liquefaction. The liquefied sediment needed to penetrate the carapace of clinker to come into contact with the coherent lava and generate peperite. Blocky peperite was produced by the interaction of semi-solidified lava with very fine sand, whereas fluidal peperite was produced by the mingling of molten lava and mud. These basalts show that various breccia types can be produced along subaerial lavas, involving autobrecciation, quench fragmentation, and peperitic fragmentation of lava. Many factors are interpreted to have been involved in forming these breccias, including the sediment type and consolidation state, the viscosity and emplacement setting of lava, and even a coincidental depositional event in the basin.

A simple recipe for red bole formation in continental flood basalt provinces: weathering of flow-top and flow-bottom breccias

Continental flood basalt (CFB) provinces such as the Deccan Traps show numerous inter-flow horizons called boles, most commonly red or brown. These have been variably interpreted as altered glassy bases of basalt flows, paleosols developed on exposed flow tops, altered volcanic ash beds, or local inter-flow sediments. We believe that each of these mechanisms may apply, singly or in combination, to specific red boles, and propose an additional mechanism, namely the weathering of flow-top and flow-bottom breccias. The breccias are inherently highly prone to weathering, and evidence for the mechanism is a close observed correspondence between breccia zones and red boles. The mechanism explains the absence of red boles in the compound pāhoehoe flows which characteristically lack breccias, and their abundance in rubbly pāhoehoe and ‘a’a lava flows. The latter observation has been misinterpreted as indicating significant eruptive hiatuses and a low eruption frequency. Importantly, even all breccia-derived boles do not have the same significance: those derived from flow-top breccias are paleosols, formed by subaerial weathering during eruptive breaks. Those derived from flow-bottom breccias, thus not in contact with the atmosphere, were formed due to groundwater ingress along flow contacts and fractures. These boles have developed slowly over long time periods. Significant development of all bole types must also have occurred in post-volcanism time and is arguably ongoing even today in the exposed and unexposed parts of CFB sequences. Recognizing multiple and complex origins of superficially similar-looking red boles is of critical importance in CFB volcanology and flow stratigraphy, and a warning sign to straightforward application of mineral chemical, mineral magnetic, geochemical, and similar data on boles to interpreting paleoweathering, paleoclimates, and paleoenvironments.

Breccia-cored columnar rosettes in a rubbly pahoehoe lava flow, Elephanta Island, Deccan Traps, and a model for their origin

Rubbly pahoehoe lava flows are abundant in many continental flood basalts including the Deccan Traps. However, structures with radial joint columns surrounding cores of flow-top breccia (FTB), reported from some Deccan rubbly pahoehoe flows, are yet unknown from other basaltic provinces. A previous study of these Deccan “breccia-cored columnar rosettes” ruled out explanations such as volcanic vents and lava tubes, and showed that the radial joint columns had grown outwards from cold FTB inclusions incorporated into the hot molten interiors. How the highly vesicular (thus low-density) FTB blocks might have sunk into the flow interiors has remained a puzzle. Here we describe a new example of a Deccan rubbly pahoehoe flow with FTB-cored rosettes, from Elephanta Island in the Mumbai harbor. Noting that (1) thick rubbly pahoehoe flows probably form by rapid inflation (involving many lava injections into a largely molten advancing flow), and (2) such flows are transitional to ‘a’ā flows (which continuously shed their top clinker in front of them as they advance), we propose a model for the FTB-cored rosettes. We suggest that the Deccan flows under study were shedding some of their FTB in front of them as they advanced and, with high-eruption rate lava injection and inflation, frontal breakouts would incorporate this FTB rubble, with thickening of the flow carrying the rubble into the flow interior. This implies that, far from sinking into the molten interior, the FTB blocks may have been rising, until lava supply and inflation stopped, the flow began solidifying, and joint columns developed outward from each cold FTB inclusion as already inferred, forming the FTB-cored rosettes. Those rubbly pahoehoe flows which began recycling most of their FTB became the ‘a’ā flows of the Deccan.

Physical volcanology and geochemistry of Palaeoarchaean komatiite lava flows from the western Dharwar craton, southern India: implications for Archaean mantle evolution and crustal growth

ABSTRACTPalaeoarchaean (3.38–3.35 Ga) komatiites from the Jayachamaraja Pura (J.C. Pura) and Banasandra greenstone belts of the western Dharwar craton, southern India were erupted as submarine lava flows. These high-temperature (1450–1550°C), low-viscosity lavas produced thick, massive, polygonal jointed sheet flows with sporadic flow top breccias. Thick olivine cumulate zones within differentiated komatiites suggest channel/conduit facies. Compound, undifferentiated flow fields developed marginal-lobate thin flows with several spinifex-textured lobes. Individual lobes experienced two distinct vesiculation episodes and grew by inflation. Occasionally komatiite flows form pillows and quench fragmented hyaloclastites. J.C. Pura komatiite lavas represent massive coherent facies with minor channel facies, whilst the Bansandra komatiites correspond to compound flow fields interspersed with pillow facies. The komatiites are metamorphosed to greenschist facies and consist of serpentine-talc ± carbonate, actinolite–tremolite with remnants of primary olivine, chromite, and pyroxene. The majority of the studied samples are komatiites (22.46–42.41 wt.% MgO) whilst a few are komatiitic basalts (12.94–16.18 wt.% MgO) extending into basaltic (7.71 – 10.80 wt.% MgO) composition. The studied komatiites are Al-depleted Barberton type whilst komatiite basalts belong to the Al-undepleted Munro type. Trace element data suggest variable fractionation of garnet, olivine, pyroxene, and chromite. Incompatible element ratios (Nb/Th, Nb/U, Zr/Y Nb/Y) show that the komatiites were derived from heterogeneous sources ranging from depleted to primitive mantle. CaO/Al2O3 and (Gd/Yb)N ratios show that the Al-depleted komatiite magmas were generated at great depth (350–400 km) by 40–50% partial melting of deep mantle with or without garnet (majorite?) in residue whilst komatiite basalts and basalts were generated at shallow depth in an ascending plume. The widespread Palaeoarchaean deep depleted mantle-derived komatiite volcanism and sub-contemporaneous TTG accretion implies a major earlier episode of mantle differentiation and crustal growth during ca. 3.6–3.8 Ga.

Morphology and Textures of Komatiite Flows of J.C. Pura Schist Belt, Dharwar Craton

Abstract: Study of komatiites for their structures and textures in cratonic blocks could provide more insights into the early Archaean volcanism, mantle processes and associated metallogeny. Jayachamarajapura (J.C.Pura) belt in Western Dharwar craton is a komatiitc milieu, where outcrop features display several flow characteristics and sub-volcanic emplacement features typical of well known komatiitic areas of the world. In spite of deformation, metamorphism and alterations the komatiites still preserve many of the primary cooling structures, which stand testimony for their extrusive volcanic nature. Distinct features like pillows, flow-top polyhedral joints, ocelli, vesicular, flow-top breccia and cumulate segregations and crude layering are observed. However, massive, undifferentiated nature of komatiitic flows is more predominant. Because of serpentinisation, carbonitization and chloritization, the original mineralogy and textures are obliterated and scantily preserved. Still, these observed features provide vital clues to imply the formation of komatiite sequences in a submarine to subaerial conditions when episodic pulses of komatiite lava piled up (about 3.35 Ga ago) to form the ultramafic milieu of J.C.Pura belt.

Early Paleoproterozoic rift volcanism in the eastern Fennoscandian Shield related to the breakup of the Kenorland supercontinent

The widespread early Paleoproterozoic (ca. 2.5–2.2Ga) volcanic and associated sedimentary rocks represent an episode of initial rifting of the late Archean craton in the eastern Fennoscandian Shield. These continental margin rocks cover cratonized Archean basement of the Fennoscandian Shield, in eastern-northern Finland and northwest Russia, and represent a typical Sumian-Sariolian volcanic-sedimentary succession. Physical volcanology and major element geochemistry of the subaerially erupted calc-alkaline basaltic to andesitic lavas of the Matinvaara Formation in north-eastern Finland, provide globally important characteristics for this craton-wide event. The lavas are generally massive or amygdaloidal, but flow-top breccias and variolitic lavas are also found. Laterally, different lava flows of this formation can be followed up to several hundred meters. Diagnostic pseudoforms after primary minerals and volcanic glass in the lavas are used in identifying crystallizing phases. Additionally, the primary structures of lava flows give indication of accumulation and settling processes. Specific internal mineral textures also relate to cooling history of these lavas. Flow inflation is suggested by accumulation of multiple vesicle-rich horizons. Thick massive flows may be analogues of tube pahoehoe flows produced by subaerial flow inflation. The vesicular units are, however, directly evidence for devolatilization of lava, whereas varioles, in the Matinvaara Formation, represent a form of plagioclase spherulite. Varioles consist of radiating crystals of plagioclase and pyroxene, presently identified as albite-actinolite pseudoforms. Under near equilibrium crystal growth conditions, kinetics favored euhedral, compositionally homogenous crystals. Continued undercooling facilitated the formation of compositionally zoned crystals and under greater undercooling anisotropic, skeletal and dentritic crystals of plagioclase and pyroxene rapidly developed. Tectono-magmatic evolution of the Matinvaara Formation suggests subalkaline magmatism in a intracratonic setting, which was related to the breakup of the Kenorland supercontinent ca. 2.5–2.1Ga. Geochemistry indicates that the volcanism was affected by felsic continental crustal contamination with plume-driven rifting. This volcanism likely occurred in distinct pulses with total duration of tens of millions of years.

Archean subqueous high-silica rhyolite coulées: Examples from the Kidd-Munro Assemblage in the Abitibi Subprovince

Subaqueous high-silica rhyolite in Prosser Township, southern Abitibi Subprovince, Ontario, consists of intercalated, thin (<15 m; low aspect ratio < 0.3) aphyric lavas with brown felsic tuffs, overlain by a pyroclastic pumice- and spatter-rich tuff breccia and are capped by quartz–porphyritic rhyolite. The quartz–porphyritic flows contain ∼10% phenocrysts whereas ‘aphyric’ flows have only sparse phenocrysts, typically 1%. The aphyric flows are characterized by features that are typical of subaqueous rhyolite flows, including well developed flow-top breccia, basal peperite, flow-banding, and hyaloclastite dominated breccia. Mass balance calculations and textural analysis demonstrate that the high-silica aphyric rhyolites were originally glassy having been recrystallized to a pseudo-spherulitic quartz–albite mixture which was subsequently sericitized and chloritized. The REE were moderately mobile (10–40%) whereas the high field strength elements were less mobile (<10%). The major elements within the flow interior are shown to have been relatively immobile and represent a reasonable estimate of the protolith composition. The viscosity of the aphyric lava protolith was estimated to be 7.8 × 10 6 Pa s based on an eruption temperature of ∼850 °C calculated from measurements using the Ti-in-zircon saturation thermometer. This viscosity estimate is similar to the calculated viscosity (∼4 × 10 7 Pa s) of the subaerial Námshraun rhyolite flow, Iceland. The Prosser and Námshraun rhyolites have comparable dimensions and eruption temperature. It is concluded that the low aspect ratio of the Prosser Township flows may have been controlled by a number of environmental factors including paleo-slope, magma rheology, eruption rate and the insulative effects of the flow breccia.

´A´ā lava flows in the Deccan Volcanic Province, India, and their significance for the nature of continental flood basalt eruptions

Newly identified ´a´ā lava flows outcrop intermittently over an area of ~110 km2 in the western Deccan Volcanic Province (DVP), India. They occur in the upper Thakurvadi Formation in the region south of Sangamner. The flows, one of which is compound, are 15–25 m thick, and exhibit well-developed basal and flow-top breccias. The lavas have microcrystalline groundmasses and are porphyritic or glomerocrystic and contain phenocrysts of olivine, clinopyroxene or plagioclase feldspar. They are chemically similar to compound pāhoehoe flows at a similar stratigraphic horizon along the Western Ghats. Petrographic and geochemical differences between ´a´ā flows at widely spaced outcrops at the same stratigraphic horizon suggest that they are the product of several eruptions, potentially from different sources. Their presence in the DVP could suggest relative proximity to vents. This discovery is significant because ´a´ā lavas are generally scarce in large continental flood basalt provinces, which typically consist of numerous inflated compound pāhoehoe lobes and sheet lobes. Their scarcity is intriguing, and may relate to either their occurrence only in poorly preserved or exposed proximal areas or to the flat plateau-like topography of flood basalt provinces that may inhibit channelization and ´a´ā formation, or both. In this context, the ´a´ā flow fields described here are inferred to be the products of eruptions that produced unusually high-effusion-rate lavas compared to typical flood basalt eruptions. Whether these phases were transitional to lower intensity, sustained eruptions that fed extensive low effusion rate pāhoehoe flow fields remains unclear.

Morphology of rubbly pahoehoe (simple) flows from the Deccan Volcanic Province: Implications for style of emplacement

Lava flows with preserved bases and brecciated upper crusts constitute a morphological type that differs in character from typical pahoehoe and a'a: such flows have been reported from many provinces around the world. Previous studies had referred to these flows informally as ‘pahoehoe flows with rubbly tops’, ‘broken-top pahoehoe’ and ‘rubbly pahoehoe’. Recent studies have formally applied the latter term to describe parts of the well-studied Laki flow in Iceland as well as flows from the Columbia River Basalt province. Rubbly pahoehoe flows are abundant in the upper stratigraphic formations of the Deccan Volcanic Province (DVP), and are more commonly known as simple flows. This study presents detailed observations of such flows from various parts of the DVP and discusses their implications for understanding flow emplacement. These flows, which appear to be single units at the outcrop-scale, are generally much thicker and significantly more extensive than individual pahoehoe lobes that dominate the lower formations of the Deccan stratigraphy. They are characterised by preserved, gently undulating tachylitic bases but variably disrupted crustal zones that grade into flow-top breccias. The breccias are constituted of highly vesicular and oxidised fragments of varying sizes that appear to have been derived from previously formed pahoehoe crusts. Previous work has indicated that the morphology of these flows might be related to initial inflation, accompanied by rapid volatile exsolution and an increase in effusion rate and/or viscosity with time. This agrees reasonably well with the qualitative and quantitative models of emplacement developed for the Laki flow. The abundance of such flows in the upper formations of the Deccan stratigraphy clearly hints at a significant shift in the nature of the Deccan eruptions; this could be indicative of higher eruption rates during this period. This, in turn, raises the possibility of hazardous impact on the climate during the eruption of these flows, which is also discussed in the paper.

Morphological and textural diversity of the Steens Basalt lava flows, Southeastern Oregon, USA: implications for emplacement style and nature of eruptive episodes

This study focuses on Middle Miocene tholeiitic flood basalt lava flows from the Oregon Plateau, northwestern USA (Steens Basalt), and is the first to comprehensively document and evaluate their morphology. Field observations of flows from several sections within and proximal to the main exposures at Steens Mountain have been supplemented with textural and geochemical data, and are used to offer preliminary insights into their emplacement. Compound pahoehoe flows of variable thickness appear to be common throughout the study area, laterally and vertically. These tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as Hawaii and the Snake River Plain. Classic a’a flows with brecciated upper and basal crusts are not abundant, but by no means uncommon. Flows with characters different from typical pahoehoe and a’a are also common. Such flows display a range in morphology; flows with preserved upper crusts but brecciated basal crusts, as well as those displaying well-developed flow-top breccias and preserved basal crusts (rubbly pahoehoe) are observed. The Steens Basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces. The abundant compound pahoehoe flows (often rich in plagioclase phenocrysts) were likely emplaced during slow but sustained eruptive episodes; their constituent lobes show clear evidence for endogenous growth. The relatively aphyric flows with brecciated surfaces (including a’a) hint at higher strain rates and/or higher viscosity, probably caused by higher effusion rates. A couple of sections are characterized by compositionally similar, but morphologically different flows that were possibly part of the same eruption. While differences in pre-eruptive topography could explain this, it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system. It is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems, and deserve closer scrutiny.

Atypical stratiform sulfide-poor platinum-group element mineralisation in the Agnew – Wiluna Belt komatiites, Wiluna, Western Australia

A new style of komatiite-associated sulfide-poor platinum-group element (PGE: Os, Ir, Ru, Rh, Pt, Pd) mineralisation has been identified at Wiluna in the strongly nickel sulfide (NiS) mineralised Agnew – Wiluna Greenstone Belt, Western Australia. The komatiite sequence at Wiluna is ∼200 m thick and comprises a basal pyroxenite layer, a thick ortho-to-mesocumulate-textured peridotite core, which is overlain by rhythmically layered wehrlite, oikocrystic pyroxenite and thick upper gabbroic margins. Pegmatoid and dendritic (harrisitic) domains are common features, whereas spinifex-textured horizons and flow-top breccias are absent. The presence of anomalous PGE-enriched horizons (ΣPt – Pd = 200 – 500 ppb) in the oikocrystic pyroxenite and in the layered melagabbro and gabbronorite horizons directly overlying the wehrlite unit is due to the presence of fine-grained (1 – 10 μm) platinum-group minerals (PGMs). More than 70 PGM grains were identified, and a considerable mineralogical variability was constrained. However, only Pd – Pt-bearing phases were identified, whereas no Ir – Ru-bearing PGMs were found in any of the sections examined. Interestingly, all PGMs are not in paragenetic association with sulfides, and only sulfide-poor/free intervals contain significant PGM concentrations. The whole-rock PGE sequence largely reflects the PGM distribution. It is hypothesised that the Pd – Pt enrichment in the oikocrystic pyroxenite and melagabbros and the overall Ir – Ru depletion in the upper mafic section of the sequence are the result of extensive olivine and chromite crystallisation in the basal ultramafic section. PGE saturation was driven by extensive crystallisation of silicate and oxide phases in a sulfide-undersaturated environment. The crystallisation of clinopyroxene in the oikocrystic pyroxenite horizon may have triggered the formation of Pt – Pd-bearing alloys and arsenides, which were the first PGMs to form. Stratiform sulfide-poor PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich sulfide-poor mineralisation zones in thick differentiated intrusions, rather than to other PGE-enriched zones in komatiite-hosted systems, where PGE enrichment is directly associated with accumulations of magmatic sulfides.

Anomalous Reef-Type Platinum-Group Element Mineralisation in the Wiluna Domain, Agnew-Wiluna Greenstone Belt, Western Australia

The ultramafic horizons of the Wiluna domain (Agnew-Wiluna Greenstone Belt, Western Australia) form two main tabular and sheet-like fractionated units, which generally comprise a 5-10m-thick basal pyroxenite layer, a 50-100m-thick meso-to-orthocumulate-textured peridotite horizon, which is overlain by a core of wehrlite, ranging 10-50m in thickness, grading upwards into oikocrystic pyroxenites (30-50m-thick) and upper gabbroic margins (30-50m-thick). Primary textures are exceptionally well preserved: pegmatoidal and dendritic (harrisitic) olivine domains are common in all lithologies, whereas spinifex-textured horizons and flowtop breccias are absent. The fractionation sequence and the geochemical profile of the two ultramafic units are almost indistinguishable, suggesting that the belts may have been structurally duplicated. The ultramafic units display a steady trend towards increasing Fe content up stratigraphy, above a narrow basal zone showing a reversed trend. Calculated Ni contents of cumulus olivines indicate that the magma remained sulfur-undersaturated up to about the middle of the unit, where fractionation of a small proportion of sulfide (R>1000) occurred and depleted the upper fractionated levels in Ni. The presence of various anomalous PGE-enriched horizons (SPt-Pd= 200-300 ppb) in the fractionated pyroxenite layer directly overlying the basal ortho-mesocumulate-textured unit indicates the occurrence of localised weak reef-style PGE mineralisation. Reef-style PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich disseminated Fe-Cu sulfide mineralisation zones within thick differentiated intrusions (e.g. Platinova reef in the Skaergaard intrusion) rather than to other zones in komatiite-hosted systems.

GENESIS OF NATIVE COPPER LODES IN THE KEWEENAW DISTRICT, NORTHERN MICHIGAN: A HYBRID EVOLVED METEORIC AND METAMOPHOGENIC MODEL

The Keweenaw Peninsula of northern Michigan is well known for early diagenetic sulfide-dominant sediment-hosted stratiform copper mineralization hosted mainly by basal siltstone-shale units of the Nonesuch Formation in the White Pine district, and later native copper lodes hosted mainly by flow-top breccias and amygdaloidal portions of flood basalts and interbedded conglomerates of the stratigraphically lower Portage Lake Volcanics. The native copper mineralization is generally attributed to downdip burial metamorphism of the host strata. An alternative model is proposed here, involving a complementary influx of evolved meteoric water (elevated in salinity and progressively equilibrating with its aquifers), similar to the fluid which formed the White Pine mineralization. This model has several advantages over the metamorphogenic model alone. It would provide the salinity (by dissolution of evaporites) and moderately oxidizing conditions (as a result of hematitization, i.e., reddening, of coarse continental rift sediments) necessary for copper transport, and the evolved meteoric water would have been an important cupriferous brine owing to the leaching of trace amounts of copper from the voluminous aquifers. Further reduction of the evolved fluid would also explain the updip deposition of native copper in association with dehematitized pumpellyite- and chlorite-dominant (i.e., reduced) hydrothermal mineral assemblages. At the same time, this influx of meteoric water from highlands adjacent to the Keweenaw rift basin would have provided a mechanism to move the deep-seated hot, dense ore brine upward along the aquifers of the Portage Lake Volcanics. On its ascent from deep portions of the rift basin, the gravity-driven evolved meteoric brine would have mixed with the fluid generated by burial metamorphism to form a hybrid evolved meteoric-metamorphogenic oreforming hydrothermal brine.

A transition from strombolian to phreatomagmatic activity induced by a lava flow damming water in a valley

The Golan Heights is a Plio-Pleistocene volcanic plateau. Cinder cones of Late Pleistocene age are very common in the eastern and northern Golan, while phreatomagmatic deposits are relatively rare and occur just in two structures — the maar of Birket Ram and the tuff ring of Mt. Avital. The complex of Mt. Avital includes two large cinder cones, a tuff ring with an elongated central depression and several basaltic flows, some of them breach the cinder cones. The (exposed) eruptive history of the complex includes (1) an early stage of basaltic lava flows, (2) strombolian activity and the buildup of the southern cinder cone, (3) a second stage of basaltic flows and the buildup of the northern cinder cone, and then a transition to (4) phreatomagmatic explosions. The phreatomagmatic deposits include surges, lapilli fallout deposits and coarse-grained lithic tuff breccias, which were found up to 200 m above the central depression. Basaltic and scoriaceous clasts are the main component of all deposits, while juvenile material is usually a minor component, almost absent in the lapilli deposits. It is suggested that the phreatomagmatic events in Mt. Avital were induced by the infiltration of water from a lake that existed in a nearby topographic low (Quneitra Valley). The lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west. The interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity. It is suggested that the dry–wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone, which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system. The minimum age of the phreatomagmatic events is determined by a 54 ka Musterian site which lies directly on top of the tuff in the Quneitra Valley.

The Concentration of the Platinum-Group Elements in South African Komatiites: Implications for Mantle Sources, Melting Regime and PGE Fractionation during Crystallization

We have analysed 18 samples of komatiite from five consecutive lava flows of the Komati Formation at Spinifex Creek, Barberton Mountain Land. Our samples include massive komatiite, various types of spinifex-textured komatiite, and flow-top breccias. The rocks have low platinum-group element (PGE) contents and Pd/Ir ratios relative to komatiites from elsewhere, at 0·45–2 ppb Os, 1–1·4 ppb Ir, <1–5 ppb Ru, 0·33–0·79 ppb Rh, 1·7–6 ppb Pt, 1·6–6·1 ppb Pd, and Pd/Ir 3·3. Pt/Pd ratios are c. 1·1. Platinum-group elements are depleted relative to Cu (Cu/Pd = 15 300). They display a tendency to increase in the less magnesian samples, suggesting that the magmas were S-undersaturated upon eruption and that all PGE were incompatible with respect to crystallizing olivine. Komatiites from the Westonaria Formation of the Ventersdorp Supergroup and the Roodekrans Complex near Johannesburg have broadly similar PGE patterns and concentrations to the Komati rocks, suggesting that the PGE contents of South African ultrabasic magmas are controlled by similar processes during partial mantle melting and low-P magmatic crystallization. Most workers believe that the Barberton komatiites formed by relatively moderate-degree batch melting of the mantle at high pressure. Based on the concentration of Zr in the Komati samples, we estimate that the degree of partial melting was between 26 and 33%. We suggest that the low PGE contents and Pd/Ir ratios of all analysed South African komatiites are the result of sulphides having been retained in the mantle source during partial melting. The difference in Pd/Ir between our samples and Al-undepleted komatiites from elsewhere further suggests that the PGE are fractionated during progressive partial melting of the mantle. Thus, our data are in agreement with other recent studies showing that the PGE are hosted by different phases in the mantle, with Pd being concentrated by interstitial Cu-rich sulphide, and the IPGE (Os, Ir, Ru) and Rh resting in monosulphide solid solution included within silicates. Pt is possibly controlled by a discrete refractory phase, as Pt/Pd ratios of most komatiites worldwide are sub-chondritic.

Morphology and emplacement of flows from the Deccan Volcanic Province, India

The present study is probably the first of its kind in the Deccan Volcanic Province (DVP) that deals in detail with the morphology and emplacement of the Deccan Trap flows, and employs modern terminology and concepts of flow emplacement. We describe in detail the two major types of flows that occur in this province. Compound pahoehoe flows, similar to those in Hawaii and the Columbia River Basalts (CRB) constitute the older stratigraphic Formations. These are thick flows, displaying the entire range of pahoehoe morphology including inflated sheets, hummocky flows, and tumuli. In general, they show the same three-part structure associated with pahoehoe flows from other provinces. However, in contrast to the CRB, pahoehoe lobes in the DVP are smaller, and hummocky flows are quite common. 'Simple' flows occur in the younger Formations and form extensive sheets capped by highly vesicular, weathered crusts, or flow-top breccias. These flows have few analogues in other provinces. Although considered to be a'a flows by previous workers, the present study clearly reveals that the simple flows differ considerably from typical a'a flows, especially those of the proximal variety. This is very significant in the context of models of flood basalt emplacement. At the same time, they do not display direct evidence of endogenous growth. Understanding the emplacement of these flows will go a long way in determining whether all extensive flows are indeed inflated flows, as has recently been postulated.

187 Os-enriched domain in an Archean mantle plume: evidence from 2.8 Ga komatiites of the Kostomuksha greenstone belt, NW Baltic Shield

The Re^Os data on Archean komatiites from the Kostomuksha greenstone belt in the Baltic Shield are presented. This greenstone belt has been previously interpreted to represent a former oceanic plateau formed by the emplacement of an ancient plume head [Puchtel et al., Earth Planet. Sci. Lett. 155 (1998) 57^74]. Samples of flowtop breccia, spinifextextured and cumulate komatiites and a chromite separate, all collected from the core of a 300 m deep diamond drill hole, yielded a Re^Os isochron with an age of 2795 ˛ 40 Ma and an initial 187 Os/ 188 Os of 0.1117 ˛ 0.0011 (Q 187 Os = +3.6 ˛ 1.0). The high positive Q 187 Os(T) implies that the komatiites were derived from a mantle source with a time-integrated suprachondritic Re/Os ratio. Recycling of oceanic lithosphere to produce the enriched 187 Os isotope signature is considered unlikely, as 15^25% crustal component is required to be incorporated into the plume source as early as 3.5^4.3 Ga. Such a substantial proportion of mafic material in the source would likely destroy the major and trace element characteristics of the komatiites. Our tentative interpretation is that the 187 Os-enrichment in the Kostomuksha plume represents an outer core signature. If confirmed by the ongoing Pt^Os isotope studies, the results would provide evidence for the existence of whole-mantle convection in the late Archean, and might place constraints on the timing of core differentiation in the early Earth. fl 2001 Elsevier Science B.V. All rights reserved.

Platinum group elements in Kostomuksha komatiites and basalts: implications for oceanic crust recycling and core-mantle interaction

We report precise PGE (Os, Ir, Ru, Pt, and Pd) concentrations for komatiite-basalt lava sequences of the Kostomuksha greenstone belt, and for the komatiite standards KAL-1 (Abitibi) and WITS-1 (Barberton). The flowtop breccia komatiites (MgO = 27.5%) from the Kostomuksha sequence, which represent the composition of the primary komatiite liquid, have Os = 1.5 ± 0.1 ppb, chondritic (Os/Ir) N = 1.06 ± 0.05, and moderately fractionated PGE patterns, (Pd/Os) N = 6.5 ± 0.4. Based on the KAL-1 standard, the Alexo primary liquid had Os = 1.8 ± 0.1 ppb, (Os/Ir) N = 1.05 ± 0.05, and (Pd/Os) N = 8.5 ± 1.0. PGEs in the Kostomuksha komatiites exhibit incompatible behavior during magmatic differentiation, with bulk D values of Os, Ir = 0.75, Pt = 0.52 and Pd = 0. PGE abundances from Kostomuksha komatiites and basalts were used to calculate the Pt/Os and Re/Os composition of a magnesian Archean oceanic crust. The Pt-Re-Os isotopic evolution of this crust was modeled, indicating that coupled 186,187Os enrichments observed in some modern plumes require a non-crustal source. A previously proposed model for core-mantle interaction by physical admixture was evaluated and was shown to produce mantle sources with high PGE abundances (Os = 6 to 14 ppb, Pd = 140 ppb for 1% outer core addition) and (Os/Ir) N ≥ 1, depending on the assumptions made. These features were not observed in the Kostomuksha komatiites (γ 187Os(T) = +3.6), shown here to be derived from a reservoir with ∼25% lower absolute PGE abundances than primitive mantle. If the radiogenic initial 187Os/ 188Os in the Kostomuksha komatiite is an outer core signature, then core-mantle interaction cannot have involved physical admixture of outer core material into mantle plume sources. Instead, it must have occurred in the form of isotopic equilibration between liquid outer core and solid mantle at the core-mantle boundary.

Fluid and mass transfer during metabasalt alteration and copper mineralization at Mount Isa, Australia

Metabasalts surrounding the metasediment-hosted base metal deposit at Mount Isa have been modified by metamorphism, polyphase deformation, and several stages of hydrothermal alteration. This study aims to understand the large-scale hydrothermal processes that determined the chemical composition of syn to postmetamorphic ore brines, prior to being focused into chemicaly anomalous metasediments that acted as a chemical trap for localized copper precipitation. Field mapping of the metabasalts provided the framework to determine the relative timing of district-scale alteration events, which were then studied by mineralogic observations and bulk mass balance measurements. Microthermometry, Raman microspeetrometry and Br/Cl analyses of fluid inclusions, and H, O, C, and S stable isotope measurements on vein minerals were used to characterize the hydrothermal fluids associated with the alteration events. All fluids are moderately to highly saline, variably Ca enriched with high Br/Cl compared to modern seawater, and isotopiealy most akin to modern evolved basin or low-grade metamorphic brines. The timing and fluid-tracer observations in conjunction with thermodynamic mass transfer modeling permit the following correlation between district-scale alteration processes and the mine-scale hydrothermal events associated with copper deposition at Mount Isa.District-scale epidote-sphene alteration of metabasalts temporally overlapped with the peak of greenschist facies metamorphism and deformation and is widely distributed in flow-top breccias and as halos around synmetamorphic quartz-calcite veins. It is associated with calcic brines with 12 to 38 wt percent CaCl 2 + NaCl equiv, and thermodynamic modeling suggests that the observed mineralogy and bulk mass changes can be explained by focusing of metabasalt-equilibrated fluids and by reaction with similar rocks at somewhat higher temperature. Similar timing and fluid characteristics are associated with dolomitic alteration at the Mount Isa mine, where mixing of metabasalt-derived calcic brine with metamorphic CO 2 from the metasediments led to massive dolomite precipitation.Kilometer-scale fracture zones associated with carbonate-Fe oxide alteration (calcite + albite + magnetite + or - hematite + or - biotite + or - chlorite) cut all penetrative foliations in the metabasalts east of Mount Isa. They are locally associated with uranium mineralization and are characterized by medium-salinity NaCl-rich fluid inclusions (8-93 wt % NaCl equiv). Carbonate-Fe oxide alteration is interpreted to mark channelways of infiltration by oxidized (sulfate-bearing) brines from outside the metabasalt sequence, possibly from an overlying evaporitic cover sequence. Fluid characteristics, evidence for extreme copper mobility, and timing criteria of alteration relative to deformation suggest a temporal, and possibly genetic, correlation with the main copper and silica deposition stage at the Mount Isa mine.Mg chlorite-rutile alteration is restricted to metabasalts immediately below the copper deposit and the vicinity of the Mount Isa fault zone, where higher grade amphibolite facies metabasalts were upthrust against the greenschist facies metabasalts and metasediments now hosting the copper ore bodies. Mg chlorite-rutile alteration of metabasalts, and copper mineralization in the adjacent reduced sediments, can both be explained by fluid focusing along an actively extending contact zone between the oxidized metabasalts and the reducedmetasediments. The brines focused into this contact zone were initially oxidized as a result of infiltration through the carbonate-Fe oxide fracture zones and less altered metabasalts, and became reduced by reaction with the metasediments. The unusual geochemical and isotopic (low 13 C and high 34 S) composition of Mg chlorite-rutile alteration can be explained as a result of back flow of reduced fluids from the metasediments into the metabasalts. Although Mg chlorite-rutile alteration of metabasalts probably liberated some of the copper now present in the orebodies, this alteration may be a relatively localized effect associated with the site of ore deposition rather than an essential sourcing mechanism for the copper.The regional feature of greatest significance to ore formation at Mount Isa, and to exploration for similar high-grade copper deposits elsewhere, is not the somewhat elevated initial copper content of the metabasalts nor their locally significant copper depletions, but their moderately high oxidation state evident from the ubiquitous occurrence of Fe-rich epidote and the large-scale Fe oxide-altered fracture zones. Oxidation of the basalts (or any other silicate rocks) favored high copper solubility along extensive transport paths and provided the essential chemical contrast to the depositional environment, which was anomalously reduced and sulfide rich as a result of an earlier process of sedimentary to diagenetic Pb-Zn-Fe-S mineralization.

Differentiation processes in an unusual iron-rich alumina-poor Archean ultramafic/mafic igneous body, Ontario

Detailed field and petrographic observation and geochemical study of the Boston Creek ferropicrite (BCF), a layered Fe-rich Al-poor Archean ultramafic igneous body, has yielded important insight into the controls on differentiation processes in ferropicrite liquids. Presence of flow top breccia, spinifex, vesicles (amygdules), and altered glass together with chert on its top, and the fact that it is in direct contact with pillowed and massive basalt flows, strongly suggest an effusive origin. Anomalous thickness (30 m) of the spinifex-textured layer compared to thick and differentiated komatiitic basalt and typical tholeiitic basalt flows of similar magnesium contents results from the great abundance of Ca, Fe and Mg relative to Al and high contents of volatiles in the parental liquids and supercooling-induced rapid crystallization, which together increased the interval of clinopyroxene crystallization. Relatively anomalous lithological (clinopyroxenite to diorite), textural (pegmatitiic to fine-grained), and geochemical (Mg-numbers between 45 and <20) complexity in the gabbroic layer results from the high volatile content of the parental liquids and the crystallization of Fe-Ti oxide prior to plagioclase, and consequent displacement, mixing, and entrapment of low-density, volatile-enriched residual liquids. Variations in the development of pegmatite, liquid segregation pockets, and PGE mineralization in the gabbroic layer along strike reflect variations in the extent of within-flow displacement, migration, and mixing of residual liquids.