Basaltic Trachyandesite Research Articles

Iron oxidation ratios, norms and the classification of volcanic rocks

Normative minerals are an integral part of most chemical classifications of volcanic rocks; however, when calculating the norms of these rocks the Fe 2 O 3 FeO ratio has a powerful influence on the abundance and nature of these minerals. This arises because when the norms of the common, non-peralkaline rocks are computed at low Fe 2 O 3 FeO ratios (< 1.1), every molecular unit of Fe 2O 3 uses a molecular unit of FeO to form magnetite, thus releasing a molecular unit of SiO 2. At higher Fe 2 O 3 FeO ratios rocks eventually contain an excess of Fe 2O 3 relative to FeO, and normative haematite forms. One of the basalts examined in the present study is nepheline and olivine normative when its Fe 2 O 3 FeO ratio is 0.2, olivine and hypersthene normative when the ratio is 0.5, and hypersthene and quartz normative when the ratio is 1.2. The precise nature of such changes in both normal and peralkaline rocks is examined; and the concept of a set of standard Fe 2 O 3 FeO ratios is evaluated. The following standard ratios are recommended: foidite 0.15–0.40, picrobasalt 0.15, basanite/tephrite 0.20–0.30, basalt 0.20, trachybasalt 0.30, basaltic andesite 0.30, phonotephrite 0.35, basaltic trachyandesite 0.35, andesite 0.35, tephriphonolite 0.40, trachyandesite 0.40, dacite 0.40, phonolite 0.50, trachyte/trachydacite 0.50 and rhyolite 0.50. After examining many erroneous, and a few bizzare, norms that have been published in recent times, it is recommended that a standard igneous norm (SIN) should be developed and used. This programme should use a set of standardized iron oxidation ratios, and also be able to correct the anomalies inherent in the original CIPW system of calculation.

A Chemical Classification of Volcanic Rocks Based on the Total Alkali-Silica Diagram

This contribution summarizes and brings up to date the recommendations made by the IUGS Subcommission on the Systematics of Igneous Rocks for the classification of volcanic rocks when modal analyses are lacking. The classification is on a non-genetic basis using the total alkali-silica (TAS) diagram, and is as nearly consistent as possible with the QAPF modal classification. The diagram is divided into 15 fields, two of which contain two root names which are separated according to other chemical criteria, giving the following 17 root names: basalt, basaltic andesite, andesite, dacite, rhyolite, trachybasalt, basaltic trachyandesite, trachyandesite, trachyte, trachydacite, picrobasalt, basanite, tephrite, phonotephrite, tephriphonolite, phonolite and foidite. Using Na-K criteria, trachybasalt may be further divided into the sub-root names hawaiite and potassic trachybasalt, basaltic trachyandesite into the sub-root names mugearite and shoshonite, and trachyandesite into the sub-root names benmoreite and latite.