Rhyolitic Volcanics

Ewart A. and Taylor S. R. (1969). Trace element geochemistry of rhyolitic volcanic rocks. Central North Island, New Zealand. Phenocryst data. Contrib. Mineral Petrol, 22 127-138. 

Chaco-Pampean plain, Argentina 1.2 million <1-11 500 Holocene and earlier loess with rhyolitic volcanic ash Oxidizing, neutral to high pH, high alkalinity, groundwaters often saline. Arsenic mainly as As(V), accompanied by high B, V, Mo, and U. Also high arsenic in some river waters 1955... 

Keller et al. (1971) reported on the occurrence of halloysite formed by the action of hot spring waters on rhyolitic volcanic rock in Michoacan, Mexico and suggested that high concentrations of Si and Al in solution, low pH (about 3.5) and sulfate as the solvent anion allows the formation of halloysite rather than other kaolinite minerals. 

Keller, W.D., Hanson, R.F., Huang, W.H. and Cervantes, A., 1971. Sequential active alteration of rhyolitic volcanic rock to endellite and a precursor phase of it at a spring in Michoacan, Mexico. Clays Clay Miner., 19 121-127. 

Compositionally, the cave sediments match the stream sediments in the bed of Fairy Creek, particularly the pebbles and cobbles, which are predominantly rhyolitic volcanics. 

Jorrasson K (1994) Rhyolite volcanism in the Krafla central volcano, north-east Iceland. Bitll Volcanology... 

If elemental concentration rather than oxide concentrations are required, then the oxide conversion can be removed at this point. For obsidian (rhyolitic volcanic glass), using silicon as an internal standard yields results in very close agreement with the Gratuze approach, as shown in Table 37.1. 

After 1950 the application of such methods by workers in Japan (Aomine and Yoshinaga [1955]) and New Zealand (Birrell and Fieldes [1952]) showed that certain ando soils from volcanic ash in Japan and in New Zealand all classes of both yellow-brown pumice soils derived from rhyolitic volcanic ash and yellow-brown loams derived from andesitic and from rhyolitic volcanic ash had clay fractions composed predominantly of allophane (Fieldes [1953, 1955]). The ability to obtain allophane relatively free from crystalline material and the high proportions in Japan and New Zealand of agriculturally important land derived from volcanic ash encouraged the study and elucidation of the properties of allophane in both countries at an early date. Subsequent work in other countries confirmed that allophane was a common constituent of volcanic ash soils. Allophane was also shown to be present in high proportions in clay fractions of weakly developed soils derived from basalt (Fieldes [1953]) or from other basic rocks (Mitchell and Farmer [1962]). In addition, in recent years serious attention has been given to the more difficult problem of assessing amounts of allophane in the presence of crystalline clay constituents in soils of many different kinds (Mitchell and Farmer [1962]). 

Table 2. Typical Feldspar, Glass, and Allophane Contents of New Zealand Rhyolitic Volcanic Ash and of a Young Yellow-brown Pumice Soil and an Older Yellow-brown Loam Derived from Rhyolitic Ash...

Figure 1.199. Plot of K2O versus Si02 for volcanic rocks thought to be genetically related to Au mineralization. High sulfidation (HS) deposits appear to be associated with a narrow range of igneous rock composition, dominated by dacite and andesite. Low sulfidation (LS) deposits are largely associated with andesite and rhyolite, but major deposits such as Ladolam, Porgera and Cripple Creek are associated with shoshonitic and alkalic rocks (Hedenquist et al., 1996).

Volcanism Bi-modal (rhyolite, basalt) Uni-modal (basalt) ... 

Three U-Th isochrons are available from the Nevados de Payachata volcano (Central Volcanic Zone of Chile) (Bourdon et al. 2000) one rhyolite sample gives an age similar to the eruption age, while two other give older ages, in particular a dacite whose crystallization age is 100 ka older than the eruption age. 

In the Taupo volcanic zone of New Zealand, the 26.5 ka Oruanui eruption was studied by Charlier and Zellmer (2000). Three fractions of zircons (sub 63 pm 63-125 pm 125-250 pm) were extracted from the rhyolitic pumice, which together with the whole rock respectively define three ages from 5.5 to 12.3 ka before eruption (Fig. 12b). Microscopic observation of the zircons showed that they are composed of a core surrounded by euhedral rims, and the preferred explanation of the authors is that zircons represent mixtures in variable proportions of old crystal cores crystallized 27 ka before eruption and crystal rims crystallized just before eruption. 

Figure 12 continued, (d)-(g) Mineral isochrons for rhyolites from the Olkaria volcanic center (Kenya), (d) and (f) alpha spectrometry resnlts from Black et al. (1997). (e) and (g) TIMS resnlts from Henmaim and Davies (2002). All the rhyolites have eraption ages between 3.3 and 9.2 ka. Note that the same sample (570) analyzed in both stndies gives rather different ages (f and g). Same abbreviations of mineral names as in Fignre 10 + Qz qnartz KF alkali feldspar Amph amphibole Bt biotite. 

The Coso Volcanic Field (CVF) in California, USA (Fig. 1) contains at least 38 high-silica rhyolite domes (Duffield Bacon 1981), many of which contain obsidian glass that has been quarried for tools by the indigenous population for more than 12,000 years. CVF obsidian... 

Bacon, C.R., MacDonald, R., Smith, R.L., Baedecker, P.A. 1981. Pleistocene high-silica rhyolites of the Coso Volcanic Field, Inyo County, California. Journal of Geophysical Research, 86, 10223-10241. Draucker, A.C. 2007. Geochemical... 

Bulk rock geochemistry of the Lac Cinquante deposit shows that the Archean volcanic rocks (both the mafic volcanic flows and the bimodal volcanic suite) are dominantly tholeiitic high-Fe basalts and basaltic andesites. The alternating felsic units in the bimodal suite are dacitic and rhyolitic in composition (Fig. 2). 

Fig. 2. Interpretative volcanic facies mapping of the Cap d Ours segment of the Glenwood rhyolite (Genna Moore 2008).

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