Geology, geochronology and geochemistry of a basanitic volcano, White Island, Ross Sea, Antarctica
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Lapilli
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Basaltic andesite
Fractional crystallization (geology)
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Abstract The island of Rarotonga in the southern Pacific is the emergent summit of a Pliocene‐Pleistocene volcanic complex built by effusive and pyroclastic eruptions of mainly mafic magma. Petrographically the basaltic rock types are ankaramite and basalt, which range in chemical composition from alkali basalt to nephelinite. Phenocryst assemblages suggest two igneous series, one with a relatively simple equilibrium assemblage of olivine, titanian augite and magnetite, and one with olivine, diopside‐augite and titanian augite in which the phenocrysts show disequilibrium textures. These variations reflect fractionation, assimilation, and recharge processes in the upper part of the magmatic system that produced the volcano. The final stages of volcanism at Rarotonga were pyroclastic eruptions of phonolites and effusive eruptions of foidal phonolites, both representing late stage fractionation products. Detailed mapping, together with geochemical work, has prompted a revision of the stratigraphy of the island based on the concept of a single cycle of magmatic activity rather than the Hawaiian style multi‐phase evolution favoured by earlier workers.
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Breccia
Phlogopite
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Peridotite
Ultramafic rock
Amphibole
Caldera
Country rock
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Ultramafic rock
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Abstract Previous studies of the Prairie Creek occurrence have identified three main rock types namely: “volcanic breccias”, “tuffs and fine-grained breccias” and “hypabyssal kimberlite or peridotite”. Our investigation confirms the presence of three distinct rock groups which include both magmatic and crater-facies types. The so-called “volcanic breccias” and “tuffs” are both considered to be predominantly of pyroclastic origin. Many features of these rocks are atypical of kimberlite and indicate a complex intrusion history. The magmatic rocks contain two generations of relatively abundant olivine in a fine-grained matrix composed of phlogopite, clinopyroxene, amphibole, perovskite, spinel, serpentine and glass. Although some petrographic features of these rocks are similar to those of kimberlites, the form of the euhedral olivine, presence of abundant glass and occurrence of potassic richterite are uncharacteristic of kimberlite but typical of lamproitic rocks. Both the groundmass phlogopite and the bulk rock have compositions intermediate between known lamproites and kimberlites. The data presented here shows that the Prairie Creek intrusion is not a kimberlite. Although in many respects Prairie Creek appears to be transitional between kimberlite and lamproite, it is considered that these rocks form an extension of the lamproite field.
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Pyroclastic deposits of the Holocene Igwisi Hills kimberlite volcanoes, Tanzania, preserve unequivocal evidence for rapid, syn-eruptive agglutination. The unusual pyroclasts are composed of ash-sized particles agglutinated to each other by thin necks. The textures suggest the magma was disrupted into droplets during ascent. Collisions between particles occurred within a volcanic plume and on deposition within the conduit to form a weakly agglutinated, porous pyroclastic deposit. Theoretical considerations indicate that agglutination occurred over short timescales. Agglutinated clasts were entrained into weak volcanic plumes and deposited around the craters. Our results support the notion that agglutination can occur during kimberlite eruptions, and that some coherent, dense rocks in ancient kimberlite pipes interpreted as intrusive rocks could instead represent agglutinated pyroclastic rocks. Differentiating between agglutinated pyroclastic rocks and effusive or intrusive rocks in kimberlite pipes is important because of the potential effects that pyroclastic processes might have on diamond concentrations in deposits.
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A 1918‐m‐deep bore hole in a late Miocene shield basalt/central volcano mixed sequence contains 58 stratigraphically distinct Volcaniclastic interbeds, constituting 8.8% of the extrusive part of the section. Textural, mineral chemical and bulk chemical criteria are applied to identify the original nature, composition, and origin of these rocks which have been metamorphosed by hydrothermal alteration between 100°C (top) to ca. 300°C (base of the section). Units range from a few centimeters to 18 m, excluding a 30‐m‐thick ignimbrite, with 75% of the units being less than 1 m thick. Formerly vitric to tachylitic shards and lapilli, lithic fragments, and crystals are the main components. Most thin Volcaniclastic units are basaltic vitric to tachylitic tuffs. Thin felsic air fall tuffs, a few millimeters to a few centimeters thick, occur as separate layers and interbedded with basaltic tuffs. Nearly all thicker units are made up dominantly of felsic vitric to lithic clasts and crystals. Phenocrysts, common only in felsic air fall tuffs, are clinopyroxene, ranging from augite to ferrohedenbergite, and plagioclase, ranging from An 80–20 , both of which span the entire range of compositions previously known from Iceland. Bulk chemical analyses of major and 11 trace elements of 65 samples allow texturally indistinct rocks to be compositionally identified, even though chemical changes due to mixing and alteration are pronounced in many rocks. The rhyolitic ignimbrite cooling unit is composed of four flow units, the upper most being slightly more mafic in composition because of admixture of a basaltic component. The two central flow units show extensive, high‐temperature devitrification with lensoid, tridymite‐rimmed cavities (now laumontite‐filled), representing collapsed pumice lapilli. The composition of the clastic units corresponds in a general way to that of the interbedded lava flows. These data and evidence for weathering and reworking in some units are used to reconstruct the volcanic history of the area: Phase A (1918–1306 m), dominantly mafic shield basalts, contains only few and mostly basaltic tuffs except for sparse rhyolitic air fall tuff from distant central volcanoes; phase B (1306‐653 m), composed dominantly of evolved basalts and icelandites, culminating with the rhyolitic ignimbrite (920–950 m), probably represents the eastern flank of a nearby central volcanic complex, whose center may be situated less than 10 km westward downdip. The upper extrusive 500 m of the section (phase C) are basalt lavas of average tholeiitic composition, containing abundant chiefly felsic and reworked clastic units in the interval between 286 and 410m. These are tentatively interpreted to reflect a phase of structural basin‐forming not compensated by extrusive volcanic activity. Allanite phenocrysts were found for the first time in Icelandic rocks, and the host pumice tuffs may constitute an important stratigraphic marker horizon that may help to tie the drilled section to the subaerial volcanic series in this part of eastern Iceland. The alteration of the clastic layers is discussed in a companion paper.
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