Aspects of the mineralogy and chemistry of the intermediate-silicic cainozoic volcanic rocks of eastern Australia. Part 2: Mineralogy and petrogenesis

Ewart A. (1985) Aspects of the mineralogy and chemistry of the intermediate-silicic cainozoic volcanic rocks of eastern Australia. Part 2: Mineralogy and petrogenesis. Australian Journal of Earth Sciences, 32 4: 383-413. doi:10.1080/08120098508729340


Author Ewart A.
Title Aspects of the mineralogy and chemistry of the intermediate-silicic cainozoic volcanic rocks of eastern Australia. Part 2: Mineralogy and petrogenesis
Journal name Australian Journal of Earth Sciences   Check publisher's open access policy
ISSN 1440-0952
Publication date 1985-01-01
Sub-type Article (original research)
DOI 10.1080/08120098508729340
Volume 32
Issue 4
Start page 383
End page 413
Total pages 31
Subject 1900 Earth and Planetary Sciences
1901 Art Theory and Criticism
Abstract The most characteristic trachytic phenocryst assemblage is calcic pyroxene + olivine+ titaniferous magnetite+feldspar ranging between potassic oligoclase—calcic anorthoclase—sodic sanidine; the most significant aspect is the complete trend towards Fe-enrichment of the pyroxenes and olivines, with the development of pure ferrohedenbergite and fayalite in the most silicic trachytes and trachyrhyolites. There is also a tendency towards the late development of Na-enriched phenocryst rims and groundmass phases, especially in certain phonolites. Hornblende (hastingsite) and biotite are of rare occurrence, the former occurring most notably in the more siliceous trachytes of the Minerva Hills in Central Queensland. Ilmenite coexists with magnetite in only a few silicic trachytes. Microprobe analysis indicate complex feldspar zoning patterns, with sharp compositional changes, and in some trachytes, alternation of plagioclase and alkali feldspar. The comendites and peralkaline trachytes are characterized by Ca-poor anorthoclase-sodic sanidine, fluor-arfvedsonite, aegirine-augite and aegirine, and less commonly aenigmatite and an Fe-Ti oxide. The averaged feldspars are most typically Or27.7-4o.8, DUt some marginally peralkaline comendites Ctransitional comendites1) contain more potassic compositions (Or47 6-501), which overlap the feldspar compositions occurring within the fayalite-bearing rhyolites and thus possibly indicating a genetic link. The rhyolites, most extensively developed in S Queensland, are divided into several mineralogical types, characterized by the following phenocryst assemblages: (a) Fayalite+ferrohedenbergite + ilmenite + quartz + sanidine + chevkinite. (b) Ferrohypersthene + ilmenite + quartz + sanidine + sodic plagioclase. (c) Biotite + ilmenite ± magnetite+quartz+sanidine + sodic sanidine+allanite; the biotites are Fe-rich and fluor-bearing in the various rhyolites. Minor accessory phases identified within the various types of volcanic rocks include chevkinite, allanite, sodalite, fluor-apatite, carbonate apatite, and zircon. The latter mineral exhibits good correlation between its Zr/Hf ratios and the whole rock K/Rb ratios. The combined available chemical, mineralogical, and isotopic data (including new Sr and δ18O data) indicate that the most evolved trachytic, comenditic, and rhyolitic magmas have developed via very efficient crystal-liquid fractionation processes. These are interpreted in terms of the types of convective fractionation models which have been proposed, which seem to provide a framework for interpreting the complexity observed within the mineralogical and chemical characteristics of the various rock types, and the separate paths of development of the rhyolitic and trachytic magmas.
Keyword Convective fractionation
Feldspar
Isotopes
Mineralogy
Petrogenesis
Pyroxenes
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Unknown

Document type: Journal Article
Sub-type: Article (original research)
Collection: Scopus Import
 
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