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c rocks, of the Alto Paranaiba Province
from southwestern Minas Gerais, Brazil. They not only studied petrography and
mineralogy of the whole-rock but also determined isotope geochemistry with the
objective of better understanding the Cretaceous magmatism and characteristics of
the magma sources. Because of the variety and complexity of lithotypes examined in
this research and the paucity of detailed studies of these Brazilian rocks in the
literature, this study also attempted to establish parameters that allow for a clear
distinction between kimberlite and kamafugite. They described
Araujo et al. (2001) studied K-rich me
fifty-two occur-
rences, and classi
ed the rocks as kimberlites and kamafugites. Among the kama-
fugites, both ugandite (characterized by the presence of leucite) and mafurite (with
kalsilite) end members have been characterized. Mineral compositions were found to
be ef
cient in distinguishing between kimberlites, mafurites and uganditesoccurring
in the Alto Paranaiba petrographic province, primarily on the basis of phlogopite
composition. The Re
Os isotope systematics permitted a better understanding related
to the sublithospheric mantle source and associated magmatism in that region.
Kimberlites, mafurites and ugandites have different Os
187
/Os
188
ratios (0.117
-
-
0.129,
0.127
-
0.145 and 0.142
-
0.147, respectively). The Rb
-
Sr and Sm
-
Nd isotope sys-
tematics failed to indicate
first-order differences between kamafugites and kimber-
lites, whilst Pb
206
/Pb
204
ratios for the kimberlites are higher than those for the other
rock types. Kimberlite and kamafugite isotopic compositions appear to be related, to
the mixture of at least two dominant mantle components: one with an isotopic sig-
nature similar to that of lithospheric peridotites, i.e., with Os
187
/Os
188
ratios of the
order of 0.118, similar to those observed in mantle-derived xenoliths entrained in
kimberlites intruded in the Kaapvaal, Wyoming, and Siberian cratons; another with
higher Os
187
/Os
188
ratios of the order of 0.135, within the range of ratios reported for
pyroxenite veins in alpine-type peridotites and ocean island basalts. Different melting
depths of heterogeneous lithospheric sources by a mantle plume are suggested to
explain the isotopic characteristics of the Alto Paranaiba Alkaline Province rocks.
4.7 Silica-Undersaturated Potassic Lavas from Canada
4.7.1 Kirkland Lake, Ontario
Alkaline lavas and tuffs have been described from the Timiskaming Group of the
Abitibi volcanic belt, which is one of the largest greenstone belts of the world
(Cooke and Moorhouse 1969; Basu et al. 1984). Cooke and Moorhouse (1969)
described four volcanic sequences, each separated by a considerable thickness of
sedimentary rocks, each containing lavas and pyroclastic rocks in the Kirkland Lake
area (48
°
10
′
N, 80
°
00
′
W). The rock types include andesite, basalt, trachyte and
'
leucite
'
-bearing rocks. Fresh leucites are not
reported, but pseudoleucites
(0.5
9.5 vol% of the lavas, have been described.
Mineralogically these rocks can be classi
2 cm across) constituting 5
-
-
ed as leucite tephrites, through trachytes
to phonolites.
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