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Volcaniclastic Rocks of the Orton-Bradley Formation, Banks Peninsula, New Zealand.
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Chapter 3: Terminology and Stratigraphy Chapter 5: Physical Volcanology Chapter 6: Interpretations and Lithofacies Analysis Chapter 7: Discussion and Geological History Acknowledgements and References
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Chapter 4
Petrography Many of the units that make up the Mt. Bradley Volcaniclastic Member are relatively friable and with some of the epiclastic deposits containing water absorbing clays, requiring treatment with epoxy resin before thin sections could be cut. The sections chosen cover a cross section of the units making up the volcaniclastic deposit, giving a more detailed picture of their composition. 4.1 Petrography of the Orton Bradley Formation Lavas There two major types of lava that make up the Orton Bradley Formation, these are defined throughout the text by the terms: aphyric hawaiite and porphyritic basalt. These two types of flow are easily distinguished in the field. 4.1.1 Aphyric Hawaiite Sewell (1985) notes that aphyric hawaiite may be either nepheline or hypersthene normative. In hand specimen, the hawaiite is a hard, dark grey to black rock, with no visible phenocrysts. In thin sections it is aphanitic, and predominantly composed of small (<1mm long), lath-shaped plagioclase microlites, with a moderate flow texture. There are some sparse plagioclase laths, about 1mm in diameter. Abundant roughly cubic, opaque minerals (titanomagnetite according to Sewell, 1985) scattered throughout the section. Some small microlites (also very scarce phenocrysts) of olivine and pyroxene are visible, with amounts varying between different flows. There is a noticeable amount of alteration of the groundmass, due to weathering of the outcrop (see Fig. 4.1). 4.1.2 Porphyritic Basalt According to Sewell (1985), this basalt is a nepheline normative, alkali basalt. The basalt is generally a lighter grey colour than the hawaiite, with large visible phenocrysts of predominantly black pyroxene (up to 1cm across), are yellow-brown olivine (weathered) and some plagioclase. These samples are holocrystalline and highly porphyritic, with a groundmass composed predominantly of fine plagioclase laths with a moderate flow texture and numerous olivine and clinopyroxene microlites. The phenocrysts consist predominantly of clinopyroxene and olivine, with minor amounts of plagioclase. These phenocrysts aregenerally large and anhedral, though some are euhedral (see Fig. 4.2). 4.2 Petrography of the Volcaniclastic Deposits Rocks of both the Mt. Bradley Volcaniclastic Member and the Tablelands Volcaniclastic Member can be described in terms of their constituents. The major variations between the two volcaniclastic members are due to the composition of the melt forming the pyroclastic deposits and the presence of lacustrine and water saturated flow deposits in the Mt. Bradley Volcaniclastic Member. 4.2.1 Blocks The blocks found in the Mt. Bradley Volcaniclastic Member are predominantly aphyric hawaiite, while those of the Tablelands Volcaniclastic Member are mainly porphyritic basalt. The hawaiite blocks found in the lower unit, are generally very similar to the samples taken from hawaiite lava flows in the area, though the plagioclase laths appeared to be coarser, with the flow texture being less defined (see Fig. 4.3). The porphyritic basalt blocks tended to be very similar to samples from lava flows, with no discernable differences. The blocks (and the smaller hawaiite/basalt clasts) are probably derived from one of two sources, and either represent juvenile material erupted as part of the surge clouds or contemporaneous ballistics. The other possible origin is that they represent older material carried out from the vent walls, with the erupted material. The major volume of material is most likely to be juvenile, with only the anomalous clasts (such as porphyritic basalt clasts in a predominantly hawaiite sample) being distinguished as accidental material. The actual amount of accidental material cannot be calculated due to lack of identifying features, observed accidental clasts consisted of porphyritic basalt, hawaiite and pieces of tuff, some of which had been highly altered. 4.2.2 Tuff and Lapilli Tuff Deposits The tuff is composed predominantly of four major constituents: crystals, lithic clasts, tachylite and sideromelane (see fig 4.4). Most of the tuff beds sampled are fine grained, varying from fine lapilli tuff to fine tuff, in order to give a more representative composition of the unit, due to the greater number of grains per slide. Free crystals are generally scarce in the tuffs in the Mt. Bradley Volcaniclastic Member but occurred as a higher percentage in the Tablelands Volcaniclastic Member as might be expected (being predominantly composed of porphyritic basalt). In the Mt. Bradley Volcaniclastic Member, the crystal assemblage consisted of olivine and plagioclase. Magnetite grains are very common both in the sideromelane clasts and as free crystals, these are generally small (<0.25mm) but some larger (approximately 0.5mm) grains were noticed. The Tablelands Volcaniclastic Member tuffs have higher amounts of clinopyroxene phenocrysts, with these crystals being quite large and very common in the tuffs around the tuff cone on the northeastern flanks of Mt. Herbert. Thin section of samples taken from locations 5 and 6 tend to have higher amounts of olivine phenocrysts than other samples from Mt. Bradley Volcaniclastic Member deposits and perhaps indicate that these locations should be grouped in to the Tablelands Volcaniclastic Member. Other factors such as the locations elevation and stratigraphy might also indicate the that the deposits were derived from the later eruptive phase. However the distance from the assumed source vent, the lack of porphyritic basalt clasts and the small size of the phenocrysts do not support this change of grouping and outweigh the evidence supporting change. There are also very minor (<<1%) amounts of brown, pleochroic hornblende, nepheline and some green chlorite. The amounts and types of lithic clasts varied from sample to sample, generally being dependent on the location (samples taken from locations 5 and 6 have little or no lithic clasts) and the type of bed being sampled (lithic fragments are more predominant in the coarser surge deposits). The type of lithic fragments being found is generally related to the unit the sample is taken from, i.e. porphyritic basalt lithics are generally confined to the Tablelands Volcaniclastic Member. The amounts of tachylite present in the samples varies, but is generally low. Tachylite is a black volcanic glass and in most of the samples examined contains numerous plagioclase laths and roughly oval vesicles (see Fig. 4.5). Most of the tachylite consists of homogeneous grains, however some clasts are composed of both tachylite and sideromelane, either as tachylite merged with sideromelane or tachylite with a rim of sideromelane. Sideromelane is a light brown volcanic glass and is one of the major products of basaltic phreatomagmatic volcanism. Most sideromelane consists of fresh, subrounded clasts of a clear, light brown glass. These clasts generally contain numerous flow textured, fine plagioclase laths and some titanomagnetite crystals, with a few elongated (in the flow direction) vesicles. Some clasts of sideromelane contain occasional single large phenocrysts of olivine or plagioclase, especially in the samples from the Mt. Bradley Volcaniclastic Member. Samples taken from locations 5 and 6 tend to have much more angular, blocky clasts of sideromelane, with no vesicles and fewer plagioclase laths within the grains. The two types of sideromelane correlate to the Type A (blocky, see Fig. 4.6) and Type B (vesiculated, see Fig. 4.7) sideromelane noted by Smith (1990). According to Wohletz (1983) the blocky type A sideromelane is attributed to the wetter surtseyan type of eruption, probably due to the complete quenching of the glass. It was noted than samples taken from the fine tuff portions of unit 1A (location 1) tend to consist predominantly of these blockier sideromelane grains. Many sideromelane clasts are weathered, primarily by the formation of palagonite. Alteration generally begins with alteration of the groundmass and of the rims of the grains, followed by the formation of roughly elliptical devitrification features called spherulites (see Fig. 4.5). The early spherulites are probably formed in and around vesicles, with later ones forming around other nucleating points within the glass. The `glassy' texture of some of the units, appears to be due to palagonite cementation of the grains. Thin sections of samples with this texture, are generally composed of sideromelane clasts set in a matrix of palagonite cement. 4.2.3 Volcanic Siltstones In these samples the glass has been totally altered and now only the grain outlines distinguish the altered glass from the fine grained matrix. The grains are predominantly subrounded, with free crystals (plagioclase) and slightly altered glass present only in very minor amounts. These samples tended to contain water absorbing clay, the result of the weathering of the glass and plagioclase. Volcanic siltstones are absent from the Tablelands Volcaniclastic Member, being restricted to certain units of the Mt. Bradley Volcaniclastic Member. 4.3 Summary The high percentages of fresh glass in the tuff/lapilli tuff beds indicate that these deposits are primarily pyroclastic, with the exception of the epiclastic units (indicated by the high percentage of altered glass). The major petrographic difference between the Mt. Bradley Volcaniclastic Member and the Tablelands Volcaniclastic Member, relate to the differences in magma forming the deposits. The Mt. Bradley Volcaniclastic Member has fewer crystals present in the tuffs and the blocks and lapilli are primarily composed of aphyric hawaiite. The Tablelands Volcaniclastic Member is formed from porphyritic basalt magma, giving a higher percentage of phenocrysts in the samples taken from its deposits. |