Geology Reference
In-Depth Information
Chapter 1
Introduction
1.1 Potassium-Rich Silica-Poor Igneous Rocks: A Distinct
Group, Different from Basaltic Rock Series
Basaltic magmas have been puncturing through the earth
flooding its
surface from time immemorial. They have been derived experimentally by partially
melting of mantle peridotites (Green 1973; Takahashi and Kushiro 1983; Hirose and
Kushiro 1993). Basaltic liquids thus, provide the most direct contact with the nature of
the earth at depth and the processes that occur there. For this reason and because
basaltic magmas are considered to be parental to other magma types, they have been
subjected to a number of experimental and
s crust and
'
field studies, and consequently two types of
basaltic rock series have been recognised by Yoder and Tilley (1962): (1) tholeiite-
rhyolite series, and (2) alkali basalt-trachyte series. In the convergent plate boundaries,
another basaltic rock series called the calc-alkaline rocks, have also been recognised.
There is however, a distinct group of potassium-rich rocks, which is characterised by
high K
2
Ocontent(
3wt%),andK
2
O/Na
2
O ratio (always >1). In contrast, average
basaltic rocks not only contain <1 wt% K
2
O (Yoder and Tilley 1962), but their K
2
O/
Na
2
O ratio (Fig.
1.1
a, b) is also <1. The SiO
2
/K
2
O ratio of silica-undersaturated
potassic rocks is normally below 15, but in alkali basalts it is >50.
These potassic rocks sometime may have even more than 7
≥
8 wt% K
2
O, and in a
few localities it may go up to 13 wt% [e.g. Leucite Hills and Highwood Mountains
(U.S.A.), West Kimberley (Australia), Latera Caldera, Roccamm
-
na (Italy) and
other localities (Fig.
1.1
a, b). A plot of K
2
O/Na
2
O ratio versus SiO
2
(Fig.
1.2
a, b
and c), shows that the composition of basaltic rocks once again plots at the bottom,
but the same ratio in the K-rich rocks from Leucite Hills (U.S.A.), Gaussberg
(Antarctica), West Kimberley and New South Wales (Australia), Vico and Roc-
camon
na (Italy) and other areas exhibits extreme enrichment of K
2
O over Na
2
O.
