Whole-Rock Studies, Precambrian and Paleozoic

Knowledge of the isotopic composition of lead in Precambrian and Paleozoic upper crustal rocks is important in answering many questions of genesis, such as the source of igneous rocks and ores (for a compilation lation of data see Appendix B). Fig. 16 shows a wide spectrum of present day isotopic compositions in these rocks. All North American data are enclosed in the area shown these data are from Patterson (1953), Peterman et ai (1967), Doe et al (1967), Zartman (1965), and 

Pb/ Pb and -08p 204p versus Pb/ Pb for whole-rock samples of Precambrian and Paleozoic crystalline rocks and composites o Ukrainian Shield Baltic Shield and Scotland x Aldan Shield (AS) + Altai Mountains (AM) North America (NA) (References to data are given in Section 111, part 5 — Whole rock studies, Precambrian and Paleozoic) 

Murthy and Patterson (1961). (All decompositions were by fusions or HF-HCIO4 with concentrations checked by fusions.) The average minus the two most radiogenic samples is 19.14 for Pb/ Pb, 15.55 for ° Pb/ Pb, and 39.89 for ° Pb/ ° Pb. Lines are drawn on the diagram to show the approximate position of a terrestrial lead that lies on the meteorite geochron (a model lead of zero age). The rocks of the Altai Mountains in Asia (Sobotovich and Grashchenko, 1965) also appear to be radiogenic. 

Such radiogenic rocks are not necessarily characteristic for some large segments of the earth s crust as may be seen (Fig. 16) from the data on the Precambrian crystalline rocks of the Baikalian block of the Aldan Shield in Asia (Sobotovich et al, 1965). (The Aldan Shield rocks were decomposed in HF-COOH2, which may not decompose some radioactive minerals such as zircon however, other studies have shown that only minor amounts of lead in the whole rock are held in such phases.) 

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