Introduction - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 6

Edgar et al. (1996) that the amount of F increases relative to K 2 O in the following
sequence: Alkali basalt kamafugite kimberlite lamprophyre lamproite

→ → → →

(lamproites having highest F content, also see Aoki et al. 1981).

1.3 Mineralogical Peculiarities

High K 2 O content and silica-undersaturated nature of these rocks, are re ected in

their exotic mineralogy in that they often contain phlogopite and leucite (sometimes
kalsilite). In sodium-rich potassic rocks, nepheline is common, and then melilite
appears as a reaction product between nepheline and clinopyroxene (Bowen 1928;
Schairer et al. 1962), but plagioclase is then absent. Sodalite group of minerals are
rare, but reported from some localities (Chap. 2 ) . Phlogopite present in these rocks
is usually titanium-rich. K-richterite is quite a common mineral in the lamproitic
rocks. High concentration of titanium as silicate and oxide minerals is a hallmark of
these rocks. Thus, sphene, ilmenite, rutile, perovskite and a roedderite-like phase
often appear as accessory minerals. Such K-rich minerals like wadeite and priderite
are typically found in ultrapotassic lamproites. Olivine is a common mineral in the
ultrama c variety, but diopside and salite are common clinopyroxenes. K-feldspar

is a ubiquitous mineral in trachytes, minettes and phonolites, whereas plagioclase
feldspar is a common mineral in tephrites, basanites and tephri-phonolities. Pla-
gioclase is however, totally absent in kamafugitic rocks and lamproites.

1.4 Scope of This Volume

These ultrapotassic silica-undersaturated melts of mantle origin intrigued such
eminent earlier petrologists as von Rath (1864), Hussak (1890), Judd (1887), and
Osann (1906). Washington (1906) however, was the

first geochemist, who made
detailed studies on these rocks form the Roman comagmatic province of Italy.
Holmes and Harwood (1937) made extensive studies on such rocks from the east
African rift valley in a memoir. They have been treated as a special group by Shand
(1943), and Turner and Verhoogen (1960) in their topics on Igneous Petrology.
Geochemical and petrological reviews on potassic rocks have also been made by
(Gupta and Yagi 1980), Foley et al. (1987), Bergman (1987), Mitchell and Berg-
man (1991) and Gupta and Fyfe (2003). Since the publication of these reviews, we
have learnt more about a number of new localities (Chap. 4 ). Systematic age
determination and study of more than forty new complex synthetic systems relevant
to their genesis, are now available. Synthesis of all these data is very important.
Different types of tectonism (Chap. 14 ) are basically outcome of plate tectonic
processes (Ninovitch and Hays 1972; Beccalova et al. 1991; Bianchini et al. 2008),
which are driven by convective cooling processes in our planet. The earth cools by
heat conduction at very speci c sites by transport of energy and molten materials. The