Two main lineages are apparent for the potassic rock-types (Figure 5 and inset (II)): (1) a silica undersatured lineage (B-P) ranging from basanite to phonolite and peralkaline phonolite and (2) a silica-saturated lineage (AB-T) ranging from alkali basalt to trachyphonolite and
trachyte [13,42,44].
Three reference suites from Iceland, Moorea and Tristan da Cunha illustrate (respectively) the tholeiitic series containing basalt to rhyolite, the alkaline series with alkali basalts to trachytes and a strongly alkaline series, beginning with basanite and ultimately yielding phonolite; few oceanic islands contain tholeiiteseries rocks.
However, Renzulli and Santi (2000) show that under high f[O.sub.2] conditions, slightly Ne-normative basalts can yield highly evolved peralkaline, Hy- and Q-normative trachytes through a two-stage fractionation process occurring in deep and then shallow-level magma chambers.
Rhyolite has the lowest Th/U ratio (3.1, one analysis) followed by tephrite (3.3, two analyses), latites and trachytes (3.4 to 3.7, three analyses), diabase porphyry (4.3, one analysis), olivine diabase porphyry (4.7, one analysis) and fine-grained diabase (4.2 to 4.8, three analyses).
Separation of clinopyroxene and plagioclase phenocrysts resulted in phenocryst-poor magma enriched in Na, K, and Si and depleted in Mg and Fe relative to the parental magma, such as the phenocryst-poor latites and trachytes. Assuming that the clinopyroxene and possibly olivine settled somewhat toward the bottom of the chamber or chambers, the latite-trachyte magma would be in the upper part of the chamber and would contain sparse plagioclase phenocrysts as its only phenocryst phase.
Twenty-nine sediment samples and twenty-nine rock samples were collected along creek valleys that transect the metamorphites, phonolites,
trachytes, and pegmatites (outside the study area) in compliance with the principles of geochemical prospecting.
(3) Trachytes. Trachytes have more K-feldspar than plagioclase and biotite and no quartz compared to dacites.
The Sabalan Quaternary volcanism includes a sequence of trachyandesites, latites, trachytes with huge bodies of ignimbrites, and pyroclastic rocks.
They are classified as alkaline rhyolites and
trachytes and high-K calcalkaline rhyolites.
Those rocks are modal alkaline-feldspar
trachytes with anorthoclase and sanidine crystals in the matrix (Pardo, 2004).
(1990) stated that the late-stage
trachyte porphyries are the most consistently mineralized igneous rocks in the Golden Reward mine area.