Magma granitic

Figure 4. Radiogenic Ca enrichments measured on whole rocks as reported hy Marshall and DePaolo (1989), shown here as 8c, values with 1-sigma uncertainties and plotted versus (a) K/Ca and (h) Enj value. The inferred value for seawater is also plotted in (a). Mid-ocean ridge basalts have no measurable enrichment of Ca/ Ca relative to the initial value for the Earth (151.016), as expected for magma derived from the Earth s mantle. Many granitic rocks, especially those with high K contents and low e j values, have significantly elevated Ec, values.

For instance, a quite marked effect on the structure of the silicate melt is produced by the basic oxide H2O. Addition of water causes a drastic decrease in the viscosity of silicate melts adding 6.4 weight % of H2O to a granitic melt at T = 1000 °C causes a decrease in viscosity of about six orders of magnitude (i.e., from about 10 to about 10 poise cf Burnham, 1975). The dissolution mechanism of H2O is important in magma rheology and will be discussed more extensively in section 9.6.1. 

Petford, N. 2003. Rheology of granitic magmas during ascend and emplacement. Annual Review of Earth and Planetary Sciences, 31, 399-427. 

Blevin, P.L. Chappell, B.W. 1992. The roie of magma sources, oxidation states and fractionation in determining the granite metaiiogeny of eastern Austraiia, Transactions of the Royal Society of Edinburgh, 83, 305-316. 

The rocks of the TCZ are interpreted to have erupted in a transtensional continental rifting environment in the Late Siiurian to Middle Devonian. In this tectonic environment A-type granites are expected, and indeed the reiativeiy high Zr, Nb, and Y content in these rocks support this. However, many other geochemicai aspects of these rocks, i.e., high Ba, Rb, Th and K, are more typical of the I-type magmas that commonly occur in collisional settings. 

Granitic rocks with 5 0-values lower than 6%c cannot be derived by any known differentiation process from basaltic magmas. Excluding those low gran-... 

Shaw H.R. (1974) Diffusion of H2O in granitic liquids, I experimental data II mass transfer in magma chambers. In Geochemical Transport and Kinetics, Vol. 634 (ed. A.W. Hofmann, B.J. Giletti, H.S. Yoder, andR.A. Yund), pp. 139-170. Washington, DC Carnegie Institution of Washington Publ. 

Watson E.B. and Jurewicz S.R. (1984) Behavior of alkalies during diffusive interaction of granitic xenoliths with basaltic magma. /. Geol. 92, 121-131. 

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