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<title>UTas ePrints - Textures formed during emplacement and cooling of a Palaeoproterozoic, small-volume rhyolitic sill</title>
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<meta content="Orth, K." name="eprints.creators_name" />
<meta content="McPhie, J." name="eprints.creators_name" />
<meta content="karin.orth@utas.edu.au" name="eprints.creators_id" />
<meta content="J.McPhie@utas.edu.au" name="eprints.creators_id" />
<meta content="article" name="eprints.type" />
<meta content="2007-10-09 17:04:58" name="eprints.datestamp" />
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<meta content="Textures formed during emplacement and cooling of
a Palaeoproterozoic, small-volume rhyolitic sill" name="eprints.title" />
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<meta content="260100" name="eprints.subjects" />
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<meta content="rhyolite; textural zonation; spherulite; Koongie Park Formation; amygdale; crystallisation; sill" name="eprints.keywords" />
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<meta content="The 52-m-thick rhyolitic sill at Onedin, in northwestern Australia, intruded wet, unconsolidated sediment of the
Koongie Park Formation during the Palaeoproterozoic. The sill is composed of five main zones. A thin (1-2 m thick)
basal peperitic contact zone with sparse large (61 cm) amygdales and abundant spherulites, grades up over 1 m into
the lower zone. The lower zone is 10m thick and made up of radiating feldspar laths surrounded by granophyric
quartz and feldspar and has less than 5% amygdales. In the overlying middle zone (20m thick), amygdales increase in
abundance from 5% near the base to 20% near the top. This zone is dominated by spherulitic rhyolite. The 18-m-thick
upper zone is complex, with alternating, 10-cm-thick layers containing abundant spherulites, lithophysae, amygdales
and perlite. Amygdales make up to 40% of the rock. A pumiceous, 2-3-cm-thick layer occurs at the upper contact.
Fluidal and blocky peperite marks the 1-2-m-thick upper contact zone. The five zones formed in response to different
cooling rates within the sill. The lower zone experienced the slowest cooling rate, whereas the upper and basal contact zones, with abundant glass, cooled rapidly. There is an overall upward increase in the amygdale abundance. In many cases, early-formed bubbles became nucleation sites for lithophysae. Spherical bubbles formed in melt above the glass transition temperature as crystallisation proceeded. Irregularly shaped bubbles formed relatively late in crystalline zones. Textural zones in the rhyolitic sill at Onedin are most similar to those in large-volume rhyolitic lavas, even though it is a small-volume intrusion. This similarity may be related to efficient heat retention beneath covering sediments for the sill, or beneath a thick crust for large-volume lavas. The presence of the crystalline lower zone immediately above the base and a high proportion of crystalline rhyolite, and the lack of autobreccia and sub-vertical flow banding near the top, distinguish this rhyolite as a sill rather than a lava." name="eprints.abstract" />
<meta content="2003-12" name="eprints.date" />
<meta content="published" name="eprints.date_type" />
<meta content="Journal of Volcanology and Geothermal Research" name="eprints.publication" />
<meta content="128" name="eprints.volume" />
<meta content="4" name="eprints.number" />
<meta content="341-362" name="eprints.pagerange" />
<meta content="10.1016/S0377-0273(03)00200-2" name="eprints.id_number" />
<meta content="TRUE" name="eprints.refereed" />
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<meta content="Orth, K. and McPhie, J. (2003) Textures formed during emplacement and cooling of a Palaeoproterozoic, small-volume rhyolitic sill. Journal of Volcanology and Geothermal Research, 128 (4). pp. 341-362. ISSN 0377-0273" name="eprints.citation" />
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<meta content="Textures formed during emplacement and cooling of
a Palaeoproterozoic, small-volume rhyolitic sill" name="DC.title" />
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<meta content="The 52-m-thick rhyolitic sill at Onedin, in northwestern Australia, intruded wet, unconsolidated sediment of the
Koongie Park Formation during the Palaeoproterozoic. The sill is composed of five main zones. A thin (1-2 m thick)
basal peperitic contact zone with sparse large (61 cm) amygdales and abundant spherulites, grades up over 1 m into
the lower zone. The lower zone is 10m thick and made up of radiating feldspar laths surrounded by granophyric
quartz and feldspar and has less than 5% amygdales. In the overlying middle zone (20m thick), amygdales increase in
abundance from 5% near the base to 20% near the top. This zone is dominated by spherulitic rhyolite. The 18-m-thick
upper zone is complex, with alternating, 10-cm-thick layers containing abundant spherulites, lithophysae, amygdales
and perlite. Amygdales make up to 40% of the rock. A pumiceous, 2-3-cm-thick layer occurs at the upper contact.
Fluidal and blocky peperite marks the 1-2-m-thick upper contact zone. The five zones formed in response to different
cooling rates within the sill. The lower zone experienced the slowest cooling rate, whereas the upper and basal contact zones, with abundant glass, cooled rapidly. There is an overall upward increase in the amygdale abundance. In many cases, early-formed bubbles became nucleation sites for lithophysae. Spherical bubbles formed in melt above the glass transition temperature as crystallisation proceeded. Irregularly shaped bubbles formed relatively late in crystalline zones. Textural zones in the rhyolitic sill at Onedin are most similar to those in large-volume rhyolitic lavas, even though it is a small-volume intrusion. This similarity may be related to efficient heat retention beneath covering sediments for the sill, or beneath a thick crust for large-volume lavas. The presence of the crystalline lower zone immediately above the base and a high proportion of crystalline rhyolite, and the lack of autobreccia and sub-vertical flow banding near the top, distinguish this rhyolite as a sill rather than a lava." name="DC.description" />
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<h1 class="ep_tm_pagetitle">Textures formed during emplacement and cooling of a Palaeoproterozoic, small-volume rhyolitic sill</h1>
<p style="margin-bottom: 1em" class="not_ep_block"><span class="person_name">Orth, K.</span> and <span class="person_name">McPhie, J.</span> (2003) <xhtml:em>Textures formed during emplacement and cooling of a Palaeoproterozoic, small-volume rhyolitic sill.</xhtml:em> Journal of Volcanology and Geothermal Research, 128 (4). pp. 341-362. ISSN 0377-0273</p><p style="margin-bottom: 1em" class="not_ep_block"></p><table style="margin-bottom: 1em" class="not_ep_block"><tr><td valign="top" style="text-align:center"><a href="http://eprints.utas.edu.au/2061/1/Orth.McPhie.JVGR.2003.pdf"><img alt="[img]" src="http://eprints.utas.edu.au/style/images/fileicons/application_pdf.png" class="ep_doc_icon" border="0" /></a></td><td valign="top"><a href="http://eprints.utas.edu.au/2061/1/Orth.McPhie.JVGR.2003.pdf"><span class="ep_document_citation">PDF</span></a> - Full text restricted - Requires a PDF viewer<br />1302Kb</td><td><form method="get" accept-charset="utf-8" action="http://eprints.utas.edu.au/cgi/request_doc"><input accept-charset="utf-8" value="2600" name="docid" type="hidden" /><div class=""><input value="Request a copy" name="_action_null" class="ep_form_action_button" onclick="return EPJS_button_pushed( '_action_null' )" type="submit" /> </div></form></td></tr></table><p style="margin-bottom: 1em" class="not_ep_block">Official URL: <a href="http://dx.doi.org/10.1016/S0377-0273(03)00200-2">http://dx.doi.org/10.1016/S0377-0273(03)00200-2</a></p><div class="not_ep_block"><h2>Abstract</h2><p style="padding-bottom: 16px; text-align: left; margin: 1em auto 0em auto">The 52-m-thick rhyolitic sill at Onedin, in northwestern Australia, intruded wet, unconsolidated sediment of the
Koongie Park Formation during the Palaeoproterozoic. The sill is composed of five main zones. A thin (1-2 m thick)
basal peperitic contact zone with sparse large (61 cm) amygdales and abundant spherulites, grades up over 1 m into
the lower zone. The lower zone is 10m thick and made up of radiating feldspar laths surrounded by granophyric
quartz and feldspar and has less than 5% amygdales. In the overlying middle zone (20m thick), amygdales increase in
abundance from 5% near the base to 20% near the top. This zone is dominated by spherulitic rhyolite. The 18-m-thick
upper zone is complex, with alternating, 10-cm-thick layers containing abundant spherulites, lithophysae, amygdales
and perlite. Amygdales make up to 40% of the rock. A pumiceous, 2-3-cm-thick layer occurs at the upper contact.
Fluidal and blocky peperite marks the 1-2-m-thick upper contact zone. The five zones formed in response to different
cooling rates within the sill. The lower zone experienced the slowest cooling rate, whereas the upper and basal contact zones, with abundant glass, cooled rapidly. There is an overall upward increase in the amygdale abundance. In many cases, early-formed bubbles became nucleation sites for lithophysae. Spherical bubbles formed in melt above the glass transition temperature as crystallisation proceeded. Irregularly shaped bubbles formed relatively late in crystalline zones. Textural zones in the rhyolitic sill at Onedin are most similar to those in large-volume rhyolitic lavas, even though it is a small-volume intrusion. This similarity may be related to efficient heat retention beneath covering sediments for the sill, or beneath a thick crust for large-volume lavas. The presence of the crystalline lower zone immediately above the base and a high proportion of crystalline rhyolite, and the lack of autobreccia and sub-vertical flow banding near the top, distinguish this rhyolite as a sill rather than a lava.</p></div><table style="margin-bottom: 1em" cellpadding="3" class="not_ep_block" border="0"><tr><th valign="top" class="ep_row">Item Type:</th><td valign="top" class="ep_row">Article</td></tr><tr><th valign="top" class="ep_row">Additional Information:</th><td valign="top" class="ep_row">Definitive version is available online at http://www.sciencedirect.com/</td></tr><tr><th valign="top" class="ep_row">Keywords:</th><td valign="top" class="ep_row">rhyolite; textural zonation; spherulite; Koongie Park Formation; amygdale; crystallisation; sill</td></tr><tr><th valign="top" class="ep_row">Subjects:</th><td valign="top" class="ep_row"><a href="http://eprints.utas.edu.au/view/subjects/260100.html">260000 Earth Sciences > 260100 Geology</a></td></tr><tr><th valign="top" class="ep_row">ID Code:</th><td valign="top" class="ep_row">2061</td></tr><tr><th valign="top" class="ep_row">Deposited By:</th><td valign="top" class="ep_row"><span class="ep_name_citation"><span class="person_name">Mrs Katrina Keep</span></span></td></tr><tr><th valign="top" class="ep_row">Deposited On:</th><td valign="top" class="ep_row">10 Oct 2007 04:04</td></tr><tr><th valign="top" class="ep_row">Last Modified:</th><td valign="top" class="ep_row">09 Jan 2008 02:30</td></tr><tr><th valign="top" class="ep_row">ePrint Statistics:</th><td valign="top" class="ep_row"><a target="ePrintStats" href="/es/index.php?action=show_detail_eprint;id=2061;">View statistics for this ePrint</a></td></tr></table><p align="right">Repository Staff Only: <a href="http://eprints.utas.edu.au/cgi/users/home?screen=EPrint::View&eprintid=2061">item control page</a></p>
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