Geology Reference
In-Depth Information
Chapter 15
Genesis of Ultrapotassic Rocks
Different hypotheses, invoked during the
first half of the last century, involved, (1)
assimilation between different magma types and rocks (Daly 1910, 1933; Shand
1931), (2) Subtraction of Eclogite from a Picrite or Peridotite Magma (Holmes
1932; O
'
Hara and Yoder 1967), (3) gaseous transport (Kennedy 1955), (4) zone
re
ning process (Harris 1957), and (5) partial melting of phlogopite
richterite-
-
bearing peridotitic mantle (Gupta and Yagi 1980).
Different types of assimilation hypotheses as invoked by different earth scientists
during the
first half of the last century are described below. This would show that
most petrologists recognized that some processes of mixing between mantle
components and crustal segments were required to explain the genesis of ultra-
potassic rocks. Before the view of this author on the genesis of ultrapotassic
magmas is discussed, different hypotheses are described brie
y.
15.1 Assimilation Processes
Any hypothesis related to the genesis of K-rich magmas must explain the following:
(1) extreme enrichment of potassium over sodium,
(2) high concentration of LREE,
(3) high abundance of HREE (6
20 times more than that of chondrites),
(4) high Ni, Co and Cr concentration in these rocks particularly with high mg-
number,
(5) high
87
Sr/
86
Sr ratio (sometimes as high as 0.725
-
0.730) and their negative
ε
Nd
-
content.
Because of these peculiarities, most serious investigators (see Chap.
1
), who
studied these rocks thought that some mixing processes between a mantle-derived
magma and crustal materials were involved in the generation of ultrapotassic mag-
mas. Following the suggestion of Shand (1910, 1931), the process of assimilation of
some primary magmas with different types of crustal components were invoked by
many. A brief critical evaluation of these assimilation hypotheses are given below:
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