Genesis of Ultrapotassic Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 449

15.5 Phlogopite Richterite-Bearing Peridotitic Mantle

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A brief review of all the hypotheses (Chap. 14 ) shows that all the petrologists
realized that the generation of these ultrapotassic magmas could not be derived by a
simple process of partial melting from a chondritic mantle. A magma produced
from a peridotitic upper mantle needed to undergo some kind of mixing process to
account for the extreme enrichment of LIL elements and the LREE over HREE in
the mantle compatible elements. Before discussing further on the genesis of
potassium-rich magmas it is important to know

first what should be the appropriate

parent materials in the mantle for these magmas.
In Chap. 13 , experimental studies by various petrologists on different types of
K-rich rocks have been discussed. The subsolidus assemblages obtained by dif-
ferent investigators are summarized in Table 15.1 .
The above table suggests that different types of K-rich rocks under mantle P-T

T
conditions are represented either by a phlogopite pyroxenite, a phlogopite-bearing
websterite, a phlogopite-bearing harzburgite, a phlogopite-bearing lherzolite or a
phlogopite-bearing wherlite. Garnet is an accompanying phase in the high pressure
subsolidus assemblage, if plagioclase is present in the starting composition.
Experimental study of Luth (1997) and Tronnes (2002) on the stability of phlog-
opite shows that at pressures above 7 Gpa, richterite and a K-bearing X-phase
appear as a break down product. The X-phase has a structure similar to that of
wadeite. Appearance of K-richterite at moderate pressure has also been noted in
many lamproitic rocks by different petrologists. In these high pressure subsolidus
assemblages accessory phases present, are various combination of the following
phases: richterite, apatite, chromian spinel, rutile, priderite, armalcolite, wadeite and
different types of carbonates.
Presence of phlogopite-bearing lherzolites, harzburgites and pyroxenites as
mantle xenoliths in ultrapotassic lavas (Menzies and Hawkesworth 1987) suggests
that different

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c rocks, containing various
accessory minerals like K-richterite, apatite, crichtonite (Haggerty et al. 1983),
wadeite, priderite, rutile, perovskite, ilmenite and armalcolite) should represent the
mantle source material for ultrapotassic magma.
Crichtonite group of minerals are characterized by a variety of end members rich
in K, Ba, Ca, Pb, Sr, U, and REE (Hagerty et al. 1983; Hagerty 1983). Such
minerals may coexist in equilibrium with Nb and Cr-bearing rutile, Cr- and Mg-
bearing ilmenite and armalcolite, rich in such elements as Nb, Zr, Ca. These
minerals are stable under mantle P-T

types of phlogopite-bearing ultrama

T condition in peridotite, which is rich in Mg
and Cr and low in alumina content (Haggerty 1983, also see Foley et al. 1987).
Tanton and McKenzie (1994) thought that the source rock for K-rich magmas
was initially a garnet peridotite containing 0.6

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5 % clino-
pyroxene. In the second stage, partial melting (about 20 %) of such a source rock in
the garnet stability

6.7 % garnet and 0.8

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field, should result in the depletion of such elements as Na, Ca,
Al etc. In the third stage the depleted mantle source is metasomatised by addition of