The Ferric-Ferrous Ratio in Granitic Rocks and the Interpretation of Relative Oxidation States

P. L. Blevin1 and P. A. Candela2

1GEMOC, Dept Geology, Australian National University, Canberra, ACT, 0200, Australia; 2Geology Department, University of Maryland, Maryland, 20742-4211, USA

The reliability of Fe3+/Fe2+ as an indicator of the intrinsic oxidation state of felsic plutonic rocks has been investigated using bulk rock Fe2O3/FeO data, presence and chemistry of redox sensitive minerals, magnetic susceptibility, and alkali feldspar colour on a large (~2000) granite dataset from SE Australia (B. W. Chappell, unpubl. data). Oxygen fugacity values were calculated from their Fe redox state (700_C and 100MPa) using a modified form of the equation of Kress and Carmichael (1991) which corrects for the alkali-ferric iron effect. S-type granite have calculated DFMQ in the range 0 to -2, while metaluminous to mildly peraluminous I-types range from DFMQ -1 to +2. Correlation between DFMQ and the molecular Al2O3/(K2O+Na2O+CaO) ratio for granites is poor indicating controls other than simple melt compositional considerations. Variation of DFMQ within igneous suites is small with DFMQ values either remaining constant or increasing slightly with increasing bulk rock SiO2. Values of DFMQ calculated from mineralogical data (presence of "primary" magnetite, titanite, ilmenite-biotite equilibria, spinel-cordierite equilibria etc) generally agree with DFMQ values determined from Fe redox state and mineralogical data for the same samples within ±1 log unit (at 700_C and 100MPa). Both S- and I-type granites within some granite provinces have similar relative oxidation states, indicating source region control. The above results suggest bulk rock Fe redox states in felsic plutonic rocks should reflect the initial Fe3+/Fe2+ ratios in a model superliquidus melt (melt ± suspended crystals). This method is best applied to suites, producing linear arrays of ƒO2, which can account for crystal accumulation, or show the effects of alteration. Broad scale oxidation relationships are present throughout the Tasman Orogen of SE Australia and correlate well with the broad scale ore metal ratios (Sn:W:Mo:Cu:Au) in associated mineralisation (Blevin et al., 1996).

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