Minor and Rare Earth Element Geochemistry of K-Rich Silica-Undersaturated Igneous Rocks - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 241

generally comparable trace element and Sr and Nb isotopic characteristics to those
of orogenic potassic magmas, but many of them have unusual Pb isotopic com-
positions with respect to unradiogenic 206 Pb/ 204 Pb ratio. He pointed out that
modern marine sediments characteristically have low U/Pb ratios and the unradi-
ogenic 206 Pb/ 204 Pb ratio of anorogenic potassic magmas may have evolved during
long-term storage of subduction sediments (or components derived from them)
within the subcontinental lithosphere. According to him, these unusual Pb isotopic
compositions require substantial time periods (>1 Ga) to have elapsed between the
fractionation events lowering the U/Pb ratio through erosion and sedimentation at
the earth

is surface and subsequent potassic magmatism and it is therefore, not
surprising that most examples of anorogenic potassic magmatism are not associated
with recent subduction process. This idea seems to be correct. He concludes that the
eruption of potassic magmas is commonly related to rifting and hotspot activity,
and these processes do not necessarily play an important role in the genesis of the
unusual sources from which potassic magmas are derived.

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