Compositional evolution granites

If the model outlined above is valid, sihcic magmatism in the LFB involved net crustal growth, as juvenile mantle-derived liquids, or their differentiates were instrumental in the formation and compositional evolution of hornblende granites, and, to a lesser extent, the cordierite granites. The amount of new crust generated is estimated by determining the overall mantle component present within both granitic types, and this is best done isotopically, since the trace-element ratios are poorly constrained for the potential basaltic end-members. 

Figure 3 Mantle evolution defined by the initial snf of juvenile granites and their zircon separates. The snf compositions (closed symbols) have been recalculated using the most recently proposed Lu decay constant of Bizzarro et al. (2003). For comparison are shown the initial values (open symbols) calculated using the Blichert-Toft and Albarede (1997) decay constant. Note that the change of decay constant results in an increase in calculated initial SHf compositions of some Early Archean samples. The current database suggests rapid Lu/Hf fractionation in the early Earth and the possibility of episodic changes in mantle evolution similar to that seen in SNd evolution (sources Vervoort and Blichert-Toft, 1999 Amelin et al., 1999, 2000 ...

The compositional parallels between Archean granitic rocks and their Proterozoic to Phanerozoic counterparts suggest that the style of cmstal differentiation was similar during these periods. The implications for crustal evolution are therefore discussed in the next section. 

Wyborn L. A. I., Wyborn D., Warren R. G., and Drummond B. J. (1992) Proterozoic granite types in Austraha impheations for lower crust composition, structure and evolution. Trans. Roy. Soc. Edinburgh Earth Sci 83, 201-209. 

The familiar continental crust of the Earth on which most of us live is of unique importance because it formed the platform above sea level on which the later stages of evolution occurred leading to the appearance of Homo sapiens. The conditions for the production of massive granitic cmsts are probably unique to the Earth and require three or more stages of derivation from a primitive mantle composition. The Earth has transformed less than 0.4% of its volume to continental cmst of intermediate composition and less than 0.2% of its volume into granitic continental crust (i.e., upper continental cmst) in over 4000 million years, so that the process is inefficient. The highland feldspathic crust of the Moon, about 12% of lunar volume, formed in contrast within a few million years. 

Gavrilova, L. K., Bedrinov, V. P. Evolution of the isotopic composition of lead in Precambrian granitic rocks. Vopr. Prikladn. Radiogeol. 228—243 (1963a). 

IWATSUKI, T. Yoshida, H. 1999. Ground-water chemistry and fracture mineralogy in the basement granitic rock in the Tono uranium mine area, Gifu Prefecture, Japan - Groundwater composition. Eh evolution analysis by fracture filling minerals. Geochemical Journal, 33, 19-32. 

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