Minor and Rare Earth Element Geochemistry of K-Rich Silica-Undersaturated Igneous Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 208

Chapter 5
Minor and Rare Earth
Element Geochemistry of K-Rich
Silica-Undersaturated Igneous Rocks

The silica undersaturated K-rich igneous rocks are highly enriched in the lighter

REE. Their La abundance for example, is often 1,000

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1,400 times greater than that of
chondrites (Mitchell et al. 1987). Abundance of the heavy REE is not as high, but still
6

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cantly high and
range between 160 and 280, average being 209. In those varieties with high mg-
number, Ni, CO and Cr contents are also high. As a consequence of high K content,
the rocks are naturally enriched in Rb, Ba and Sr contents. The LILE/LREE ratios in
these rocks are in general low, but their 87 Sr/ 86 Sr ratios are high. In lamproites from
Spain, this ratio could be as high as 0.725

20 times that of chondrites. The La/Yb ratio of these rocks is signi

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0.730. In contrast, the same ratio in basaltic
rocks varies between 0.703 and 0.705. The

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ε Nd content of K-rich rocks is always
negative, compared to that of average basaltic rocks, which are characterized by
positive values. In the following sections the trace element and REE characteristics of
the K-rich silica

undersaturated lavas from various localities are described.

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5.1 The Minor and Rare Earth Element Characteristics
of Lamproites from Damodar Valley Coal Fields

Five intracratonic basins, developed in eastern India, during rifting in the Damodar
valley coal fields region (from west to east), are: (1) Karanpura, (2) Bokaro, (3)

Ramgarh; (4) Jharia; and (5) Raniganj (Fig. 4.3 ). The basal unit in each basin is the
Talchir boulder bed of Carboniferous to Asselian age, succeeded by thick coal-
bearing silica-rich clastic sequences (Ghosh and Mukhopadhyay 1985). The basins
are characterized by synsedimentary extension faulting, and are demarcated to the
south by the southern boundary fault. Precambrian gneisses are present to the north
and south. Large-scale intrusions of lamproites and lamprophyres are present as
east-west-trending dykes and sills con

ned to the basins. Contemporaneous dolerite
dykes also occur within and outside the basins.
Fromgeochemical data on ultrapotassic rocks in the Jharia basin, Rock et al. (1992)
concluded that these were a comagmatic suite of olivine lamproites, lamproites,
lamprophyres, and minettes. Jia et al. (2003) analysed K-rich intrusive rocks from the