Precambrian crystalline rocks

In Africa, Precambrian crystalline rocks are exposed over about one quarter to one third of the arid/semi-arid land surfaces (Dregne, 1976). While mountain chains are located in the northwest, east and south, much of the rest of the arid and semi-arid land consists of sandstone, limestone, loose... 

Somewhat unusual isotopic compositions have been observed in highly saline deep waters from Precambrian crystalline rocks as well as in deep drill holes, which plot above or to the left of the Meteoric Water Line (Frape et al. 1984 Kelly et al. 1986 Frape and Fritz 1987). There are two major theories about the origin of these Ca-rich brines ... 

Kelly W. C., Rye R. O., and Livnat A. (1986) Saline minewaters of the Keweenaw Peninsula, northern Michigan their nature, origin, and relation to similar deep waters in Precambrian crystalline rocks of the Canadian shield. Am. J. Sci. 286, 281-308. 

This region is located in the Canadian Shield, composed of massive Precambrian crystalline rocks, mainly granite-gneisses and granites. In the syncline of Hudson Bay, on the south shore, and on the lowlands of the River Mackenzie, there occur sedimentary deposits of Silurian and Devonian dolomites, limestones and sandstones. 

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.)... 

Precambrian cherty iron sediments. Data on diagenetic processes that went on in ancient rocks which have not retained their original form can be obtained only indirectly, on the basis of a number of assumptions. In any version of the chemogenic hypothesis, the original cherty iron sediment must have been amorphous-dispersed, and the processes of diagenesis were an important step in the formation of the crystalline rock. However, it is quite unclear whether essential mineral transformations occurred, or whether diagenesis was limited to recrystallization of the original sedimentary miner-... 

McNutt R. H. (1987) Sr/ Sr ratios as indicators of water/rock interactions applications to brines found in Precambrian age rocks from Canada. In Saline Water and Gases in Crystalline Rocks, Special Paper 33 (eds. P. Fritz and S. K. Frape). Geological Association of Canada, Memorial University, Newfoundland, pp. 81-88. 

The Aral Sea is positioned in the zone where geological structures of the Urals join those of Tien-Shan. The Aral depression is bordered on the west by Precambrian crystalline basement, and on the northeast - by the Central Kazakhstan massif [9]. Pre-Mesozoic rocks are highly metamorphosed, heavily distorted and broken by faults into mosaic block systems of varying altitudes. 

Fig, 16. 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)... 

Table 19. Igneous rocks formed in Precambrian crystalline terranes tend to be unradiogenic in their initial lead isotope ratios relative to oceanic ridges and rises which may be due to ancient metamorphism of the source rocks which reduced the U/Pb (Hamilton, 1966). In North America, the unradiogenic leads are concentrated in the Rocky Mountain region and farther east. Present data on igneous rocks indicate...

In order to overcome these problems, interest was focussed on that portion of the organic matter trapped in mineral precipitates which formed synchronously with sedimentation. In these cases, the material is hermetically sealed in the crystalline matter and may survive with relatively little subsequent alteration. Such preservation is common in cherts which are chemical precipitates of silica and now comist of fine grained quartz. These rocks offer the best chance for successful preservation of truely Precambrian molecular fossils. Modem microprobes and spectrophotometer microscopes allow the non-destructive analysis of organic matter enclosed in mineral crystals. Laser bombardment of microscopic... 

Caledonides, with elevations up to 2470 m, and the surface of crystalline Precambrian rock of the Fennoscandian Shield were shaped by glacial erosion, whereas the lowlands of the Russian Plate and West European Platform are covered by glacial sediments. 

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