Iron isotopic compositions of adakitic and non-adakitic granitic magmas: Magma compositional control and subtle residual garnet effect

Abstract

Here we present iron (Fe) isotopic compositions of 51 well-characterized adakitic and non-adakitic igneous rocks from the Dabie orogen, Central China and Panama/Costa Rica, Central America. Twelve I-type non-adakitic granitoid samples from the Dabie orogen yield δ56Fe ranging from −0.015‰ to 0.184‰. The good correlations between δ56Fe and indices of magma differentiation (e.g., SiO2, FeOt, Mg#, and Fe3+/ΣFe) suggest Fe2+-rich silicate and oxide minerals dominated fractional crystallization with Δ56Femelt-crystal∼0.06‰ may account for the δ56Fe variation in these samples. One A-type granite sample from the Dabie orogen has δ56Fe as high as 0.447‰, likely indicating less magnetite crystallization and an increase in 103lnβmelt with magma (Na+K)/(Ca+Mg). Combined with the literature data, most high silica (SiO2⩾71wt.%) granitic rocks define a good positive linear correlation between δ56Fe and (Na+K)/(Ca+Mg): δ56Fe=0.0062‰×(Na+K)/(Ca+Mg)+0.130‰ (R2=0.66). Given that fractional crystallization also tends to increase δ56Fe with (Na+K)/(Ca+Mg), this correlation can serve as the maximum estimate of the magma compositional control on Fe isotope fractionation. Low-Mg adakitic samples (LMA) have δ56Fe ranging from 0.114‰ to 0.253‰. The melt compositional control on LMA δ56Fe could be insignificant due to their limited (Na+K)/(Ca+Mg) variation. Except for one sample that may be affected by late differentiation, 14 out of 15LMA have δ56Fe increasing with (Dy/Yb)N, reflecting a subtle but significant effect of residual garnet proportion. This serves as evidence for that source mineralogy may play an important role in fractionating Fe isotopes during partial melting. Dabie and Central America high-Mg adakitic samples have homogeneous Fe isotopic compositions with mean δ56Fe of 0.098±0.038‰ (2SD, N=11) and 0.085±0.045‰ (2SD, N=11), respectively. These samples have undergone melt-mantle interaction, which may neutralize the possible δ56Fe variation of their pristine magmas derived by partial melting of eclogitic rocks.