Flow-top Breccias Research Articles

Geochemistry of colloidal silica precipitates in altered Grande Ronde Basalt, northeastern Oregon, U.S.A.

Opal-CT-rich fillings in tree molds and interstitial to breccia clasts in flow-top breccia reduce porosity and contain textures indicative of colloidal deposition. The opal-CT occurs in uniformly textured layers and porous laminated layers of particles and agglutinates cemented by chalcedony. Celadonite, smectite, clinoptilolite, quartz, chalcedony and minor calcite are associated with the opal-CT. These minerals occur in two settings: (1) within interstitial spaces between particles of opal-CT; and (2) within particles and agglutinates of opal-CT. Concentrations of trace and major elements are closely related to the textural setting of the secondary minerals within the layers of opal-CT. Layers of opal-CT that are enriched relative to other layers in REE and Th are red-brown in color, uniformly cryptocrystalline with some microcrystallinity, and lack secondary minerals interstitial to opal-CT particles.

Variation in peperite textures associated with differing host-sediment properties

Peperites formed by mixing of magma and wet sediment are well exposed along Punta China, Baja California, Mexico, where two sills intrude a section of lava flows, limestones, and volcaniclastic rocks. Irregular lobes and dikes extend from the sills several meters into host sediments, including highly comminuted flow top breccias (lithic lapilli tuff breccias) and shelly micrites, whereas intrusive contacts with lava flows are sharp and planar. Where one sill intruded both coarse-grained volcaniclastic rock and fine-grained limestone, textural differences between the hosts produced strikingly different styles of peperite. Blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias; the blocks consequently underwent in situ fragmentation as they were rapidly quenched. The high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse-grained permeable host. Elutriation of fine-grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter-scale fluidization of the host. The contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous, immiscible fluids (fluidal peperite). Intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off “grain by grain” as magma advanced into the host. Thin-section-scale elutriation pipes formed. Microglobular peperite represents a “frozen” example of a fuel-coolant interaction (FCI) between basaltic magma and fluidized micrite host. The intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma-vapor-host interface. Such intimate intermixing of magma and water-bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism.

Alteration of Late Middle Proterozoic Volcanics and its Relation to Stratabound Copper-Silver-Gold Mineralization Along the Margin of the Kalahari Craton in Swa/Namibia and Botswana

Summary Metavolcanic and metasedimentary rocks of late Middle Proterozoic age are preserved in basins along the northern margin of the Kalahari Craton. Three broad stratigraphic divisions are recognized: a lower acid extrusive unit, a basic extrusive/red bed unit and a shallow marine unit. The basalts of the basic extrusive/red bed unit are interpreted to be continental within-plate basalts. Copper mineralization occurs as native copper in amygdaloidal altered basalt, and as sediment-hosted, stratiform sulphide deposits at the base of the shallow marine unit. The rocks have been strongly altered during regional metamorphism which reached temperatures up to 350°C. Massive domains were metamorphosed to oligoclase-chlorite-epidote-haematite rocks. In permeable zones such as flow top breccias, amygdaloidal zones, and regions of secondary, tectonically induced fracturing, the basalts have been altered to epidote-quartz-chlorite-haematite-muscovite rocks. During this epidote alteration copper, zinc and cobalt were leached from the basalts, thus providing a possible metal source for the sediment-hosted deposits. A model for multiphase ore genesis is presented in which basalt alteration is the main metal source for epigenetic mineralization.

The nature of the Golden Mile Dolerite south‐east of Kalgoorlie, Western Australia

In places, the Golden Mile Dolerite (GMD) contains volcanic breccias, tuffs and chilled flow top breccias indicating that it is extrusive. The presence of fragments of GMD in the overlying Black Flag Beds is further evidence that the GMD pre‐dates the Black Flag Beds and does not intrude them. The GMD may have been extruded subareally or in extremely shallow water.

Secondary mineralization and hydrothermal alteration in the Reydarfjordur drill core, eastern Iceland

Deep crustal drilling in eastern Iceland has allowed study of a fossil hydrothermal system at a constructive plate margin. The drilled sequence consists of partly to completely altered subaerial lava flows, basaltic dikes, and minor clastic material. Alteration and secondary mineralization are most intense in the flow top breccias where water/rock ratios are presumed to have been the highest. In the upper portion of the cored sequence (to a depth of about 1200 m) alteration is characterized by the deposition of clay minerals ± calcite ± quartz ± laumontite into open spaces such as vugs and vesicles. Low‐temperature zeolites, such as stilbite, epistilbite, mordenite, and heulandite, are also present but are restricted to the upper 500 m of the drill core. Below 1200 m, alteration is characterized by the dissolution and replacement of both primary minerals and earlier authigenic minerals, followed by partial filling of dissolution cavities. Early mineral assemblages consist of epidote ± quartz ± prehnite ± chlorite ± albite, and a later superimposed assemblage consists of calcite + laumontite ± anhydrite. Authigenic sphene, pyrite, chalcopyrite, pumpellyite, actinolite, and wairakite also occur sporadically in the cored sequence. Secondary mineral assemblages and temperature measurements of fluid inclusions suggest a maximum temperature of alteration of about 300°C. Fluid inclusion compositions indicate that the geothermal fluid was meteoric water with very low salinities and high calcium activities. Iron activities and oxygen fugacities were highest in the deeper portions of the systems. The mineral paragenesis suggests that the fluid composition, temperature, and PCO2 varied significantly with time. The thermal energy for the geothermal system was probably derived from a high‐level magma chamber associated with nearby Thingmuli volcano. Local contact metamorphism, indicated by the formation of garnet, occurred during late stage emplacement of dikes into the lava pile. Chlorite ± calcite ± laumontite assemblages were also deposited along subvertical fractures at this time.

Stratigraphy and flow morphology of Archean tholeiites, northeastern Ontario

Archean (2.7 Ga old) subaqueous basalt flows of the lower Blake River group, at Lava Flow Mountain, Ontario, are dominantly of a thick, massive flow morphology with hyaloclastic flow-top breccias. Pillow lavas are a subordinate component (< 20%) of the stratigraphy. A 3050 m, vertically-dipping, south-facing sequence consists of alternating and interfingerling units, 300 to 500 m thick, of light green low-Fe lava flows and dark green to black high-Fe flows. High-silica variolites occur within the latter. These three lithologies occur in the approximate proportions 40:50:10. Rare indicators of paleoflow direction suggest an eastern provenance for both low- and high-Fe types. Massive, pillowed and hyaloclastic flow morphologies may be laterally equivalent and probably represent separate facies of compound lava flows. Thick massive flows with relatively thin flow-top breccias form in the main lava channel with high rates of extrusion and/or steeper paleoslopes. Pillow lavas form from less voluminous flows or degassed magma where it is ponded or on gentler paleoslopes. Hyaloclastites and pillow breccias form on the top, sides and at the flow front increasing in thickness away from the vent source. The Fe-rich tholeiites were probably erupted rapidly, in deep water on the Archean ocean floor during an early shield-building stage of volcanism.

Volcanism on a Proterozoic continental margin in northwestern Queensland

Abstract The oldest rocks exposed in northwestern Queensland are metamorphosed calc-alkaline volcanics (Leichhardt Metamorphics), which are intruded by elongate tonalitic to granitic batholiths (Kalkadoon Granite). These rocks are overlain by a less metamorphosed sequence containing basic lavas (Magna Lynn Metabasalt) overlain by extensive ignimbritic rhyolite and dacite (Argylla Formation). Sequences of basalt and psammite overlie the rhyolite unconformably and are overlain in turn unconformably by psammitic, pelitic, carbonate and possibly evaporitic sediments and minor volcanics. Younger granites intrude these rocks. The mineral assemblages of the Leichhardt Metamorphics, Magna Lynn Metabasalt and Argylla Formation indicate greenschist and lower amphibolite facies of metamorphism. The rocks contain no glass and some are obviously recyrstallized; however, phenocrysts, lithic fragments, spherulites, amygdales and flow-top breccias are still recognizable. Sixty-nine of the least deformed volcanic and sub-volcanic rocks were analysed for major elements and up to twenty trace elements. Element dispersion in these analyses indicated that metasomatism was probably of limited extent. The Magna Lynn Metabasalt is similar to low-potassium tholeiite, as it has less than 0.5% potash, high normative hypersthene, some normative quartz and typical Ti/Zr/Y ratios. The acid volcanics have calc-alkaline affinities although andesite is not common and the alumina content is relatively low. They have high K/Na ratios and their trace elements (especially Ba, Sr, Rb, Zr and Ce) are similar to Andean volcanics. The tholeiitic sequences that overlie the calc-alkaline volcanics in northwestern Queensland resemble the basaltic sequences of western U.S.A. that also overlie calc-alkaline volcanics. The predominantly calc-alkaline volcanics of northwestern Queensland are believed to have formed at a continental margin similar to that in the Andean region. The younger tholeiitic lavas and minor continental acid volcanism possibly accompanied crustal tension. Later regional metamorphism and intrusion of large granite batholiths stabilized the region.

Thick, layered peridotite–gabbro lava flows in Munro Township, Ontario

Two unusually thick (120 m) lava flows in Munro Township, Ontario, are differentiated from peridotite at the base to gabbro near the top. One flow is komatiitic and contains the following units (from top to bottom): ultramafic flow-top breccia (19.5% MgO); olivine and pyroxene spinifex-textured rocks (17–10% MgO); gabbro (7% MgO); pyroxene cumulates (10–25% MgO); olivine cumulates (32–35% MgO); and a basal border zone. The composition of the lava at the time of eruption is believed to be represented by the ultramafic flow-top breccia. Following eruption, this lava differentiated to produce ultramafic cumulate rocks, and liquid of quartz-normative basaltic composition that solidified as gabbro.The other flow is tholeiitic and has a picritic hyaloclastite flow top (14% MgO) underlain successively by an aphanitic pyroxenite zone (but no spinifex texture), a gabbro zone, a thick clinopyroxene-rich cumulate unit, and a thin basal veneer of cumulate olivine.