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Geology 200A - Lab 3 (Volcanic rocks) Tephra: ashes are less than 4 mm; lapilli 4mm-3.2 cm; blocks > 3.2 cm Intrusive 3-3a: 2: dark grey, medium grained, gabbro;faint alignment of lath shaped apatite; intrusive core to the volcano. 3-3b1, 3-3b2, 3-3b3: tan, pale brown to grey brown weathered, coarsely quartz feldspar pophyritic dacites (QFP); note typical conchoidal and anhedral, glassy habit of quartz, and oscillatory zoning in large, equant feldspar crystals, probably K-feldspar;large phenos indicate long residency time of magma chambers typical of island arcs and which contrasts to mid-ocean ridge spreading centres size of phenocrysts reflects cooling/crystallization rate; finer grained porphyritic samples may be from somewhat "chilled" margins of dykes or small stocks, while coarse-grained rocks may be from dyke or stock interiors.
Basalt/Andesite flows 3-1a1, 3-1a2: 2 samples: aphanitic basalt flow breccia; monomictic, "unsorted", angular clasts of same composition as matrix; can occur at base or top of a sheet flow, or comprise all of a flow; basal flow dominated part of a strato-volcano. 3-1b: 2 samples: pale to dark green weathered, dark green on fresh, feldspar porphyritic basalt; even phenocryst distribution, single crystals and glomerocrysts, lath and feathery forms; vesicular; some autoclastic fragments. 3-1c1, 3-1c2, 3-1c3: pale grey weathered, dark green on fresh, aphanitic basalt with abundant white weathered spherulites; rapidly crystallized feldspar aggregates. 3-1d: pale grey or green weathered, dark green on fresh, pyroxene-phyric basalt; phenocrysts evenly disseminated: cpx = very dark green as opposed to black colour, 2 cleavages at 90, not as evident as in hornblende; large phenocrysts implies long residency time of magma chamber 3-1e: dull green weathered and fresh, hornblende porphyritic basalt; hornblende indicated by black colour, and two cleavages at 120; the presence of amphibole indicates formation in an island arc. 3-1-5: 3 samples: dirty grey weathered, dull green on fresh, calcite amygdaloidal basalt; amygdules from 1 mm to > 1 cm in size, are filled with calcite; amygdules indicate lava was extruded above the hydrostatic compensation depth, where pressure of the water column is insufficient to prevent volatile escape and hence vesiculation. 3-2a2b: felsic (light coloured) andesite - chalky feldspar crystals, phenocrysts of quartz, hornblende and biotite.
Aphanitic silicic flows 3-4a1: massive, featureless, hard, dense, subconchoidal, aphanitic rhyolite flow; felsic magmatism generally at the latter part of volcanic cycles, rhyolites usually in domes and flows emplaced and extruded in the upper part of the volcanic succession; this sample could be an intrusive dome or extrusive lava. 3-4a2: 2: milky white, dense, hard, wavy compositionally laminated rhyolite; rhyolite flow or dome lobe, central core or dome of volcano, uplift and unroofing often the final stage of volcano growth; platy fabric, tabular layers, due to segregation in a laminar flow regime. 3-4b: black, glassy, hard, brittle, conchoidal obsidian; quenched lava, probably rhyolitic but not necessarily so; likely rapid extrusion of lava during dome growth and unroofing at the end of the volcanic cycle; proximal setting. 3-4c1: 2 samples: black, glassy, compositionally laminated and bedded (tabular and lensoidal; soupy), dense, hard subconchoidal obsidian or volcanic glass; sharp bounded laminae from compositional segregation during flow of lava and/or differential devitrification of glass during cooling and burial; quenched, probably rhyolitic but not necessarily so; lava flow extruded on flanks of a dome uplifted and unroofed at the end of the volcanic cycles; proximal setting. 3-4d1: 2 samples: black, glassy, compositionally layered obsidian; white, spherical grains <5mm in diameter are lithophysae, rapidly nucleated feldspar crystal aggregates, formed early and then concentrated in tabular layers during laminar flow of lava; dense, glassy, conchoidal obsidian in lithophysae-poor layers; quenched, probably rhyolitic lava or dome during dome uplift and unroofing, commonly the final stage of volcano growth, proximal setting. 3-4e: aphanitic, dull brown-grey weathered and fresh, aphanitic, hard, dense, siliceous? volcanic rock with abundant, mm-scale, concentrically layered spherules called lithophysae, some still with a black, glassy, obsidian-like core; the rock was probably all obsidian but is now completely devitrifed to minerals like chlorite, calcite, quartz; early formation of lithophysae since concentrated into very crudely defined tabular layers during laminar flow of the lava.
Pyroclastic flows 3-2a1a, 3-2a1b1, 3-2a1b2, 3-2a1c: pale grey weathered lapilli tuff or lapilli ash flow tuff; note flattened aspect of many light colored pumice lapilli in two large samples (not tectonic as there are no associated pressure shadows), and the flame-like nature of dark green chloritic wisps in the small sample (glass fragments are "squeezed" and contorted during burial, compression and devitrification to chlorite); note abundance of quartz and feldspar crystals, and overall monomictic composition of clasts and matrix; flattened fragments suggest that the rock is an ash flow.
Air Fall Tuffs 3-2b1, 3-2b2: pale grey weathered, very light and vesicular, quartz rich pumice; rounded perhaps as an aerodynamic response during flight after explosion; pyroclastic units are predominant in the upper parts of volcanoes, proximal setting (tephra this size don't travel too far), and in the latter part of the volcanic cycle.
Felsic tuffs - indeterminate origin
3-2a2a: pale pinkish, poorly sorted, fragmental rock; lithic volcanic fragments and abundant quartz phenocrysts. 3-2c1: laminated tuff. 3-2c2: quartz rich crystal tuff, dacitic in composition; mottled grey-white-pale yellow colour likely from hydrothermal alteration of ash groundmass, therefore the tuff probably accumulated in a setting proximal to the volcanic source area. 3-2d: white, soft, faintly laminated tuff, or ash tuff; light; pyroclastic unit predominant in the upper parts of volcanoes; proximal setting, in the latter stages of the volcanic cycle; sharp, wavy laminae suggest waterlain deposition; silica layers possibly of biogenic or hot spring origin. 3-2g: (in kleenex box): beach cobbles (St Vincent, Carribean), tephra ejected from an adjacent volcano; lithologies range from clinopyroxenite to feldspar porphyritic basalt to quartz eye dacite tuff; cognate (deep magmatic) and accidental (volcano flanks ripped off during explosion) sources; pyroclastic unit which commonly is in the latter stages of the volcanic cycle, high in the volcanic section, in a proximal setting; these fragments would settle out of the eruptive column quickly, near the eruptive center.
Sulphides 3-5a: 3 samples: grey, silver and brass coloured, metallic iron sulphide in compositional bands or beds with quartz and calcite veins and veinlets; sulphide is fine grained, rocks are locally vuggy; hydrothermal fluids common during felsic volcanic center growth in subaqueous settings; bedded fabric to these rocks indicates sulfide was exhaled from vents and preserved as a "chemical sediment" (note there is no non- sulfide sediment, just sulfide with any open space filled by quartz and calcite; black smokers may be a modern analogue. 3-5b: dark grey, faintly foliated mudstone with layer parallel, brassy iron sulfide; sulfide layer parallel, fine grained and euhedral; "bedded" sulfide as in samples 3-5a, but here sulfide subordinate to a "normal" clastic sediment, hence more of a distal setting here, removed from most intense hydrothermal and/or vent (smoker) activity; fabric and mud composition suggest deposition in a low energy environment; hydrothermal activity generally associated with felsic, subaqueous centers in the latter stages of volcanism. 3-5c: 2 samples: pale grey and brown, earthy alteration from chlorite, carbonate and clay in an aphanitic mafic volcanic rock with abundant veinlets of iron sulfide; sulfide as narrow veinlet stringers and more massive veins, and disseminated blebs; note how one vein extends from a more massive "bed" in one sample; hydrothermal activity near volcanic centers can produce sulfide occurrences by fracturing rock followed by pervasive open space precipitation, or direct replacement of volcanic rocks, by sulfide. 3-5d: steel grey and brass colored, massive and semi-massive iron sulfide; note dense, heavy nature of rocks; brassy sulfide is chalcopyrite (copper); it is softer, and lacks the striated cubic habit of pyrite (e.g. see sample 3-2f), blue sulfide is bornite (copper); semi-massive to massive sulfide like these samples can form from primary sedimentation of sulfide exhalation from vents like black smokers, or from complete replacement of volcanic rock by sulphide-bearing hydrothermal fluids; you need to see occurrences at outcrop scale to decide, although relict quartz eyes in one suggests replacement of a dacite or rhyolite; mineralization commonly in proximal settings, associated with subaqueous felsic centers, late in the volcanic cycle. 3-2f: flesh or pink coloured with a distinctive pearly luster, greasy or waxy pyrophyllite phyllite; phyllite because the rock has a pervasive foliation formed by a microscopic mineral; abundant disseminated euhedral pyrite cubes; pyroclastic unit predominant in the upper parts of volcanoes, distal setting (all coarse material dropped out in proximal setting), latter stages of volcanic cycle; pyrophyllite from leaching of all minerals but Al, Si. Barite vein deposits 3-5f-1: 3 samples: beige and white vein deposit of quartz and barite; open space vein deposit; precipitation from hydrothermal fluids related to volcanism; open space textures include large vugs and cockscomb texture- also note wall rock rip up fragments in some veins. 3-5f-2: coarse-grained vein deposit; white barite, heavy, milky or cloudy and softer than a knife, and semi-translucent blue fluorite with 4 cleavages. Volcaniclastic rocks 3-6a: 3 samples: drill core of a coarse-grained volcanic breccia; dark green matrix with abundant chalky feldspar crystals, unsorted, angular fragments of volcanic and sedimentary rocks; volcaniclastic unit which forms from mass-wasting and erosion of unstable flanks of volcanoes in the latter stages of, or following construction of the edifice; proximal setting based on abundance of crystals and angularity of fragments; no evidence of far-traveled transport by means such as turbidity currents. 3-6b: unsorted, muddy volcanic conglomerate; well rounded but low sphericity pebbles generally <2 cm; mostly pale green andesite or dacite fragments, but some pumice, some mudstone; grain supported; very dense fragmental unit from erosion of the unstable flanks of a volcano in the latter stage of, or following construction of the volcano; dense pebble concentration and dense mud matrix may be indicative of debris flow origin, therefore a proximal setting, supported by the abundance of pumice fragments. 3-6c: dull green, feldspar crystal rich volcanic conglomerate with mm; scale fragments of porphyritic volcanic rocks and mudstone; unsorted, no bedding, disseminated, large pyrite euhedra; volcaniclastic unit which forms from the erosion of unstable flanks of volcanoes during the later stages of, or following construction of, the volcano; overall small size of fragments and occurrence of mudstone fragments suggest deposition in a distal basinal setting; sulfide probably early diagenetic product. 3-7a: dark green, well laminated volcanic turbidite; less than 10 cm thick couplet of fining-upward mafic ash, well sorted crystals and lithic fragments; Bouma a-c-d-e present; note sharp, undulatory top of uppermost unit of mud, note all contacts within each individual turbidite are gradational; fine-fraction turbidites in distal parts of dispersal aprons which form from erosion and redeposition of volcano flank material by turbidity currents which most commonly move into back-arc basin settings. 3-7b: 2 samples: tan or grey, quartz pebble conglomerate; well sorted clasts of varied composition including jasper, quartz, aphanitic mafic volcanics; grain supported; high energy depositional setting near an active source of terrigenous clastic detritus; this lithology would be a key to making a tectonic tie of a back arc basin setting to a continental margin, as in the modern day China Sea, where basins are filled from one side by detritus from arc volcanoes, and on the other from continental margins with a river fed clastic input into the basin. 3-8: polymictic conglomerate containing a variety of fragments derived from a boninitic island arc obducted over slope and rise sandstones.
Jaspers, mudstones 3-5e-1a, 3-5e-1b:red, hard, featureless jasper with faint compositional laminae; in hydrothermal settings, fluids often iron rich, hence sediments in associated basins are iron rich; if exhaled hydrothermal solutions are Si rich, as they often are, the jasper you see here might be an example of sedimentation directly from chemical precipitation. 3-5e-2 (no photo): drill core of steel grey, well foliated (chlorite and/or graphite) mudstone with layer parallel calcite-quartz veinlets; basinal setting, low energy, could be proximal or distal; chlorite could be the result of direct chemical precipitation from Mg-rich hydrothermal fluids.
Island Arc Model
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