Petrologic Conclusions - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 467

Chapter 16
Petrologic Conclusions

Silica-de cient potassic rocks are characterized by high K 2 O content (>3), high

K 2 O/Na 2 O ratio (>1) and low Si 2 O/K 2 O ratio (<15). They have high LREE con-
centration, sometimes 1,000

1,400 higher than that of chondrites. The LILE/HREE
ratios are usually low, but 87 Sr/ 86 Sr ratios are usually high (0.707 - 0.730). The ε Nd
content of these rocks is usually negative compared to average basaltic rocks. If we
exclude some exceptional cases, these rocks are relatively young (<25

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30 Ma) in

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the evolutionary history of the earth.
Potassic rocks can be classi ed into three series: (1) low-K, (2) medium-K and

(3) high-K. All kinds of tephrites, tephriphonolites, phonotephrites and trachytes
can be classi ed with respect to A (alkali feldspar) P (plagioclase) F (feldspathoid)

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contents. The classi cation with respect to total alkali SiO 2 (TAS) is inadequate.

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Classi cation with respect to modal minerals is preferable.

On the basis of mineralogy, K-rich lavas can be classi ed into (1) plagioclase-

free melilite-bearing varieties and (2) plagioclase-bearing types. Tephrites, phon-
olites, phonolitic tephrites and tephri phonolites are the most abundant rock types.
The plagioclase-free ultrapotassic rocks can be plotted in a kalsilite forsterite

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larnite SiO 2 tetrahedron (Yoder 1986). When H 2 O is added, bulk composition of

many of the lamproitic rocks can be plotted in this system.
The K-rich rocks are associated with either rift valleys formed by the ascent of
the mantle plumes or subduction related volcanism. Those well-known localities,
where K-rich rocks were produced by rift-related systems include, (1) East and
equatorial Africa, Damodar Valley coal

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fields, Aldan province (Russia), the Eifel
region (Germany), Gaussbergh and Alto-Paranaiba province (Brazil). Those K-rich
rocks, which were generated by the process of subduction include Aeolean Arc
region (Italy), Southern Spain, Indonesia, Manchuria (China) and the Western part
of the USA. There is a debate about Roman province volcanism. There are some,
who believed that Roman Province volcanism is rift-related (Beccaluva et al. 1991),
but there are others (Alverez 1972; Ellam et al. 1989), who thought that the
volcanism to be subduction related.