Minor and Rare Earth Element Geochemistry of K-Rich Silica-Undersaturated Igneous Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 229

Fig. 5.16 La/Nb and Sm/ Nd
ratios in Bearpaw rocks
plotted against ε Nd . Close
circles: high K-minettes (after
Macdonald et al. 1992)

4

Zortman
alnoite

3

Minettes

2

Latites

Missouri Breaks

1

0

0.20 Minettes

Latites

0.16

Missouri
Breaks

Zortman alnoite

0.12

-20 -10 0 10

ʵ Nd

5.5 The REE and Trace Element Geochemistry of K-Rich
Volcanic Rocks of Smoky Butte

The chondrite-normalized REE distribution pattern for the Smoky Butte rocks were
determined by Mitchell et al. (1987, Fig. 5.17 ). All the samples show similar
distribution patterns. The rocks are strongly enriched in the LREE with La abun-
dance being 1,000 - 1,400 times more than that of chondrite. The concentration of
heavy REE is low 6 20 times that of chondrite values and hence La/Yb ratios are

high, and range between 162 and 280, average being 209 and there is no Eu
anomaly. The Smoky Butte rocks have higher La/Yb ratios compared to lamproitic
rocks from Western Australia (La/Yb = 83

-

246) and wyomingites from Leucite

−

Hills, Wyoming (121 127) but similar La/Yb ratio as that of madupites from

Leucite Hills and Hillspond (Kansas) (La/Yb = 165

-

377). Spanish rocks have
relatively low La/Yb ratio and lower total REE contents. Smoky Butte lamproites
are richer in Sm content (27

−

38 ppm) for a given La/Yb ratio and compared to other
lamproites. The 87 Sr/ 86 Sr ratios are low (0.70587

-

0.70633) and δ Nd values vary

-

between 21.6 and 25.9.

− −